HIGH PERFORMANCE POLYMERS
INDUSTRIAL POLYMERS TECHNOLOGY AND APPLICATIONS The purpose of this series is to provide practicing scientists and engineers with a set of authoritative, comprehensive, and dynamic reference volumes. With a focus on industry and applications, each book will enable readers to quickly access in-depth information, data, and knowledge on a specialized theme. The content of each volume will be designed and presented in such a way that professionals beyond chemists and engineers, such as materials managers, executives, lawyers, will find the material extremely helpful, easy to use and pertinent to their needs. For more information about the book series and new book proposals please contact William Andrew at
[email protected] or visit http://www.williamandrew.com/
HIGH PERFORMANCE POLYMERS
Johannes Karl Fink Montanuniversität, Leoben, Austria
N o r w i c h , N Y, U S A
Copyright © 2008 by William Andrew Inc. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the Publisher. ISBN: 978-0-8155-1580-7 Library of Congress Cataloging-in-Publication Data Fink, Johannes Karl. High performance polymers / Johannes Karl Fink. p. cm. ISBN 978-0-8155-1580-7 (acid-free paper) 1. Polymer engineering. 2. Polymers--Industrial applications. I. Title. TP1087.F56 2008 620.1’92--dc22 2008001312 Printed in the United States of America This book is printed on acid-free paper. 10 9 8 7 6 5 4 3 2 1 Published by: William Andrew Inc. 13 Eaton Avenue Norwich, NY 13815 1-800-932-7045 www.williamandrew.com
NOTICE To the best of our knowledge the information in this publication is accurate; however the Publisher does not assume any responsibility or liability for the accuracy or completeness of, or consequences arising from, such information. This book is intended for informational purposes only. Mention of trade names or commercial products does not constitute endorsement or recommendation for their use by the Publisher. Final determination of the suitability of any information or product for any use, and the manner of that use, is the sole responsibility of the user. Anyone intending to rely upon any recommendation of materials or procedures mentioned in this publication should be independently satisfied as to such suitability, and must meet all applicable safety and health standards.
Contents
Preface
xvii
1 Carbazole Polymers 1.1 Monomers . . . . . . . . . . . . . . . . . . . . . 1.1.1 N-Vinylcarbazole . . . . . . . . . . . . . 1.1.2 Nonlinear Optical Side Chain Monomers 1.1.3 Molecular Glasses . . . . . . . . . . . . 1.2 Polymerization and Fabrication . . . . . . . . . . 1.2.1 Polymerization . . . . . . . . . . . . . . 1.2.2 Other Vinylcarbazole Compounds . . . . 1.3 Properties . . . . . . . . . . . . . . . . . . . . . 1.3.1 Liquid Crystalline Phases . . . . . . . . 1.3.2 Optical Properties . . . . . . . . . . . . . 1.4 Applications . . . . . . . . . . . . . . . . . . . . 1.4.1 Electrophotographic Films . . . . . . . . 1.4.2 Polymeric Light-Emitting Diodes . . . . 1.4.3 Organic Photorefractive Materials . . . . 1.4.4 Photovoltaic Devices . . . . . . . . . . . 1.4.5 Amplified Spontaneous Emission . . . . 1.4.6 Optical Elements . . . . . . . . . . . . . 1.4.7 Antistatic Polymer . . . . . . . . . . . . 1.4.8 Other Applications . . . . . . . . . . . . 1.5 Suppliers and Commercial Grades . . . . . . . . 1.6 Safety . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . v
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1 2 2 3 5 5 5 17 18 18 22 22 22 25 37 47 51 51 52 52 52 53 53
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High Performance Polymers Poly(p-xylylene)s 2.1 Monomers . . . . . . . . . . . . . . 2.2 Polymerization and Fabrication . . . 2.2.1 Chemical Vapor Deposition 2.2.2 Solution Polymerization . . 2.3 Properties . . . . . . . . . . . . . . 2.3.1 Mechanical Properties . . . 2.4 Applications . . . . . . . . . . . . . 2.4.1 Coatings . . . . . . . . . . 2.4.2 Medical Applications . . . . 2.5 Suppliers and Commercial Grades . 2.6 Safety . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .
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69 70 73 73 75 76 76 77 77 82 83 84 84
Poly(arylene vinylene)s 3.1 Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Polymerization and Fabrication . . . . . . . . . . . . . . . 3.2.1 Sulfonium Precursor Route . . . . . . . . . . . . . 3.2.2 Transition Metal-catalyzed Cross-coupling Process 3.2.3 Chemical Vapor Deposition . . . . . . . . . . . . 3.2.4 Ring-Opening Metathesis Polymerization . . . . . 3.2.5 Electropolymerization . . . . . . . . . . . . . . . 3.2.6 Knoevenagel Polycondensation . . . . . . . . . . 3.2.7 Gilch Reaction . . . . . . . . . . . . . . . . . . . 3.2.8 Dehydrohalogenation Phase Transfer Catalysis . . 3.2.9 Anionic Polymerization . . . . . . . . . . . . . . 3.2.10 Others . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Mechanical Properties . . . . . . . . . . . . . . . 3.3.2 Thermal Properties . . . . . . . . . . . . . . . . . 3.3.3 Electrical Properties . . . . . . . . . . . . . . . . 3.3.4 Optical Properties . . . . . . . . . . . . . . . . . . 3.4 Special Additives . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Dopants . . . . . . . . . . . . . . . . . . . . . . . 3.5 Applications . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Electroluminescent Devices . . . . . . . . . . . . 3.5.2 Photovoltaic Devices . . . . . . . . . . . . . . . . 3.5.3 Poly(p-phenylene vinylene) Nano Fibers . . . . .
89 89 91 91 93 93 94 95 97 97 98 98 100 100 100 100 105 105 106 106 107 107 113 118
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Contents 3.5.4 Poly(p-phenylene vinylene) Nanotubes 3.5.5 Sensors . . . . . . . . . . . . . . . . . 3.6 Suppliers and Commercial Grades . . . . . . . 3.7 Safety . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .
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4 Poly(phenylene ether)s 4.1 Monomers . . . . . . . . . . . . . . . . . . . 4.2 Polymerization and Fabrication . . . . . . . . 4.2.1 Functionalized Poly(phenylene ether) 4.2.2 Copolymers . . . . . . . . . . . . . . 4.2.3 Blends . . . . . . . . . . . . . . . . 4.2.4 Thermosetting Resins . . . . . . . . 4.2.5 Other Related Types . . . . . . . . . 4.3 Properties . . . . . . . . . . . . . . . . . . . 4.3.1 Mechanical Properties . . . . . . . . 4.3.2 Thermal Properties . . . . . . . . . . 4.3.3 Electrical Properties . . . . . . . . . 4.4 Special Additives . . . . . . . . . . . . . . . 4.4.1 Impact Modifiers . . . . . . . . . . . 4.4.2 Fibers . . . . . . . . . . . . . . . . . 4.4.3 Flame Retardants . . . . . . . . . . . 4.4.4 Blowing Agents . . . . . . . . . . . 4.5 Applications . . . . . . . . . . . . . . . . . . 4.5.1 Automotive Components . . . . . . . 4.5.2 Adhesives . . . . . . . . . . . . . . . 4.5.3 Membranes . . . . . . . . . . . . . . 4.6 Suppliers and Commercial Grades . . . . . . 4.7 Safety . . . . . . . . . . . . . . . . . . . . . 4.8 Environmental Impact and Recycling . . . . . References . . . . . . . . . . . . . . . . . . . . . .
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139 139 141 145 149 149 151 152 152 153 153 154 154 154 154 155 156 156 157 157 157 163 163 163 168
5 Poly(phenylene sulfide) 5.1 Monomers . . . . . . . . . . . . . . . . . 5.2 Polymerization and Fabrication . . . . . . 5.2.1 Standard Procedure . . . . . . . . 5.2.2 Other Methods of Preparation . . 5.2.3 Oxidized Poly(phenylene sulfide) 5.2.4 Copolymers . . . . . . . . . . . .
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High Performance Polymers 5.2.5 Thermosets . . . . . . . . . . . . . . . . . . . . . 5.2.6 Blends and Composites . . . . . . . . . . . . . . . 5.2.7 Poly(arylene ether sulfide)s . . . . . . . . . . . . . 5.2.8 Poly(phenylene sulfide phenyleneamine) . . . . . 5.2.9 Poly(dithiathianthrene)s . . . . . . . . . . . . . . 5.2.10 Poly(aryl ether thianthrene)s. . . . . . . . . . . . . 5.3 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Mechanical Properties . . . . . . . . . . . . . . . 5.3.2 Thermal Properties . . . . . . . . . . . . . . . . . 5.3.3 Electrical Properties . . . . . . . . . . . . . . . . 5.3.4 Optical Properties . . . . . . . . . . . . . . . . . . 5.3.5 Solubility . . . . . . . . . . . . . . . . . . . . . . 5.4 Special Additives . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Decolorants . . . . . . . . . . . . . . . . . . . . . 5.4.2 Corrosion Inhibitors . . . . . . . . . . . . . . . . 5.4.3 Adhesion Reduction . . . . . . . . . . . . . . . . 5.5 Applications . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.1 Solder Friendly Thermoplastic Blends . . . . . . . 5.5.2 Abrasion-resistant Poly(tetrafluoroethylene) Blends 5.5.3 Electrically Conducting Polymers . . . . . . . . . 5.5.4 Proton Exchange Membrane Materials . . . . . . . 5.5.5 Ozone Filter Materials . . . . . . . . . . . . . . . 5.6 Suppliers and Commercial Grades . . . . . . . . . . . . . 5.7 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Environmental Impact and Recycling . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
183 184 187 187 189 190 190 190 191 191 191 192 192 192 192 193 193 194 195 196 197 197 197 199 199 202
Poly(aryl ether ketone)s 6.1 Monomers . . . . . . . . . . . 6.2 Polymerization and Fabrication 6.2.1 Nucleophilic Process . 6.2.2 Electrophilic Process . 6.2.3 Blends . . . . . . . . 6.2.4 Modification . . . . . 6.3 Properties . . . . . . . . . . . 6.3.1 Mechanical Properties 6.4 Special Additives . . . . . . . 6.4.1 Melt Stabilizers . . . .
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Contents 6.4.2 Fillers and Reinforcing Materials Applications . . . . . . . . . . . . . . . . 6.5.1 Nonadhesive Coating . . . . . . . 6.5.2 Porous Membranes . . . . . . . . 6.5.3 Rechargeable Batteries . . . . . . 6.5.4 Coatings . . . . . . . . . . . . . 6.6 Suppliers and Commercial Grades . . . . 6.7 Safety . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .
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220 220 220 222 226 227 227 228 232
7 Poly(arylene ether sulfone)s 7.1 Monomers . . . . . . . . . . . . . . . . . . . . 7.1.1 4,4 -Biphenol . . . . . . . . . . . . . . 7.1.2 Bisphenol A . . . . . . . . . . . . . . 7.1.3 Bis-(4-hydroxyphenyl)-sulfone . . . . . 7.1.4 Bis-(4-chlorophenyl)-sulfone . . . . . . 7.2 Polymerization and Fabrication . . . . . . . . . 7.2.1 Step-growth Polycondensation . . . . . 7.2.2 Chain-growth Polycondensation . . . . 7.2.3 Copolymers from Telechelic Monomers 7.2.4 Macrocyclic Polymers . . . . . . . . . 7.2.5 Friedel-Crafts Polymerization . . . . . 7.2.6 Sulfonation . . . . . . . . . . . . . . . 7.2.7 Blends . . . . . . . . . . . . . . . . . 7.2.8 Varieties of PES . . . . . . . . . . . . 7.2.9 Modification . . . . . . . . . . . . . . 7.3 Properties . . . . . . . . . . . . . . . . . . . . 7.3.1 Thermal Properties . . . . . . . . . . . 7.3.2 Chemical Properties . . . . . . . . . . 7.3.3 Electrical Properties . . . . . . . . . . 7.4 Applications . . . . . . . . . . . . . . . . . . . 7.4.1 Membranes . . . . . . . . . . . . . . . 7.4.2 Medical Applications . . . . . . . . . . 7.4.3 Optical Waveguide Applications . . . . 7.5 Plumbing Materials . . . . . . . . . . . . . . . 7.6 Suppliers and Commercial Grades . . . . . . . 7.7 Safety . . . . . . . . . . . . . . . . . . . . . . 7.8 Environmental Impact and Recycling . . . . . .
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High Performance Polymers References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
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Poly(arylene ether nitrile)s 8.1 Monomers . . . . . . . . . . . . . . . . 8.1.1 Halogenated Benzonitriles . . . 8.1.2 Aromatic Hydroxy Compounds 8.2 Polymerization and Fabrication . . . . . 8.2.1 Electrophilic Route . . . . . . . 8.2.2 Nucleophilic Route . . . . . . . 8.3 Properties . . . . . . . . . . . . . . . . 8.3.1 Mechanical Properties . . . . . 8.3.2 Thermal Properties . . . . . . . 8.3.3 Solubility . . . . . . . . . . . . 8.4 Applications . . . . . . . . . . . . . . . 8.4.1 Reinforced Resins . . . . . . . 8.4.2 Filter Materials . . . . . . . . . 8.4.3 Resin-bonded Magnets . . . . . 8.4.4 Proton Exchange Membranes . 8.5 Suppliers and Commercial Grades . . . 8.6 Safety . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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283 283 284 286 287 288 290 291 291 291 293 293 293 293 294 294 295 297 297
Triazole Polymers 9.1 Monomers . . . . . . . . . . . . . . . . . . . . . 9.2 Polymerization and Fabrication . . . . . . . . . . 9.2.1 Reaction of Dinitriles with Dihydrazides 9.2.2 Aromatic Nucleophilic Displacement . . 9.2.3 Poly(bis-1,2,4-triazole)s . . . . . . . . . 9.2.4 Poly(1-vinyl-1,2,4-triazole) . . . . . . . 9.2.5 1,2,4-Triazole Dendrimers . . . . . . . . 9.3 Properties . . . . . . . . . . . . . . . . . . . . . 9.3.1 Thermal Properties . . . . . . . . . . . . 9.3.2 Electrical Properties . . . . . . . . . . . 9.3.3 Optical Properties . . . . . . . . . . . . . 9.4 Special Additives . . . . . . . . . . . . . . . . . 9.4.1 Degradation Inhibitors . . . . . . . . . . 9.5 Applications . . . . . . . . . . . . . . . . . . . . 9.5.1 Blocked Isocyanates . . . . . . . . . . . 9.5.2 Crosslinking Rubbers . . . . . . . . . . .
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301 301 303 303 303 305 305 306 306 306 308 308 315 315 315 315 316
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Contents 9.5.3 Coatings . . . . . . . . . . . . 9.5.4 High-Temperature Adhesives . . 9.5.5 Polymeric Corrosion Inhibitors 9.5.6 Gas-Generating Compositions . 9.5.7 Biocidal Polymers . . . . . . . 9.6 Suppliers and Commercial Grades . . . 9.7 Safety . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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329 329 332 332 337 337 338 339 339 340 340 340 341 341 343 343
11 Poly(naphthalates) 11.1 Monomers . . . . . . . . . . . . . . . . . . 11.1.1 Naphthalenedicarboxylic acid . . . 11.2 Polymerization and Fabrication . . . . . . . 11.2.1 Poly(ethylene naphthalate) . . . . . 11.2.2 Copolymers . . . . . . . . . . . . . 11.2.3 Blends . . . . . . . . . . . . . . . 11.2.4 Poly(1,3-propylene 2,6-naphthalate) 11.3 Properties . . . . . . . . . . . . . . . . . . 11.3.1 Mechanical Properties . . . . . . . 11.3.2 Thermal Properties . . . . . . . . . 11.3.3 Electrical Properties . . . . . . . . 11.3.4 Optical Properties . . . . . . . . . .
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347 347 347 349 349 358 362 367 369 369 370 370 370
10 Poly(oxadiazole)s 10.1 Monomers . . . . . . . . . . . . . 10.2 Polymerization and Fabrication . . 10.2.1 Polycondensation . . . . . 10.2.2 Anionic Polymerization . 10.2.3 Sulfonation . . . . . . . . 10.3 Properties . . . . . . . . . . . . . 10.4 Applications . . . . . . . . . . . . 10.4.1 Fibers . . . . . . . . . . . 10.4.2 Membranes . . . . . . . . 10.4.3 Sensors . . . . . . . . . . 10.4.4 Light-Emitting Devices . . 10.4.5 Graphite Precursors . . . . 10.5 Suppliers and Commercial Grades 10.6 Safety . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . .
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High Performance Polymers 11.3.5 Gas Permeability . . . . . . . 11.3.6 Chemical Resistance . . . . . 11.4 Special Additives . . . . . . . . . . . 11.4.1 Flame Retardants . . . . . . . 11.4.2 Protective Coatings . . . . . . 11.5 Applications . . . . . . . . . . . . . . 11.5.1 Poly(ethylene naphthalate) . . 11.5.2 Films . . . . . . . . . . . . . 11.5.3 Fibers . . . . . . . . . . . . . 11.6 Suppliers and Commercial Grades . . 11.7 Safety . . . . . . . . . . . . . . . . . 11.8 Environmental Impact and Recycling . References . . . . . . . . . . . . . . . . . .
12 Poly(phthalamide)s 12.1 Monomers . . . . . . . . . . . . . 12.2 Polymerization and Fabrication . . 12.2.1 Conventional Route . . . . 12.2.2 Instant or Aerosol Process 12.2.3 Batch Processes . . . . . . 12.2.4 Continuous Routes . . . . 12.2.5 Interfacial Condensation . 12.2.6 Ester Recycling Route . . 12.2.7 Side Reactions . . . . . . 12.2.8 Blends and Copolymers . 12.2.9 Fabrication Techniques . . 12.3 Properties . . . . . . . . . . . . . 12.3.1 Mechanical Properties . . 12.3.2 Thermal Properties . . . . 12.3.3 Chemical Properties . . . 12.4 Special Additives . . . . . . . . . 12.4.1 Fillers . . . . . . . . . . . 12.4.2 Antioxidants . . . . . . . 12.4.3 Impact Modifiers . . . . . 12.4.4 Flame Retardants . . . . . 12.5 Applications . . . . . . . . . . . . 12.5.1 Transparent Types . . . . 12.5.2 Compositions for Welding
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370 373 374 374 374 375 375 376 379 380 380 383 383
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391 392 394 394 396 396 397 398 398 399 399 401 402 403 403 403 403 404 404 408 408 409 409 410
Contents 12.5.3 Electroplated Articles . . . . . 12.5.4 Hot-melt Adhesives . . . . . . 12.6 Suppliers and Commercial Grades . . 12.7 Safety . . . . . . . . . . . . . . . . . 12.8 Environmental Impact and Recycling . References . . . . . . . . . . . . . . . . . .
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411 413 414 414 414 418
13 Aramids 13.1 Monomers . . . . . . . . . . . . . . . . 13.2 Polymerization and Fabrication . . . . . 13.2.1 Acid Chloride Route . . . . . . 13.2.2 Acid Route . . . . . . . . . . . 13.2.3 Carbon Monoxide Route . . . . 13.2.4 Partially Aromatic Poly(amide)s 13.2.5 Fibers . . . . . . . . . . . . . . 13.2.6 Aramid Paper . . . . . . . . . . 13.2.7 Honeycombs . . . . . . . . . . 13.2.8 Aramid Films . . . . . . . . . . 13.3 Properties . . . . . . . . . . . . . . . . 13.3.1 Mechanical Properties . . . . . 13.3.2 Thermal Properties . . . . . . . 13.3.3 Optical Properties . . . . . . . . 13.4 Special Additives . . . . . . . . . . . . 13.4.1 Ultraviolet Stabilizers . . . . . 13.4.2 Electrically Conductive Modifier 13.5 Applications . . . . . . . . . . . . . . . 13.5.1 Friction Materials . . . . . . . . 13.5.2 Gaskets . . . . . . . . . . . . . 13.5.3 Reinforcing Materials . . . . . 13.5.4 Catalyst Supports . . . . . . . . 13.5.5 Carbon Fiber Precursors . . . . 13.5.6 Cryogenic Fuel Tanks . . . . . 13.5.7 Hyperbranched Aramids . . . . 13.6 Suppliers and Commercial Grades . . . 13.7 Safety . . . . . . . . . . . . . . . . . . 13.8 Environmental Impact and Recycling . . References . . . . . . . . . . . . . . . . . . .
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423 423 425 427 428 428 428 429 430 431 431 432 432 434 434 434 434 435 435 436 436 437 438 439 439 439 440 442 442 442
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High Performance Polymers
14 Poly(amide imide)s 14.1 Monomers . . . . . . . . . . . . . . . . 14.2 Polymerization and Fabrication . . . . . 14.2.1 Isocyanate Route . . . . . . . . 14.2.2 Acid Chloride Route . . . . . . 14.2.3 Direct Polymerization Route . . 14.2.4 Microwave Polymerization . . . 14.2.5 End Capped Poly(amide imide) 14.2.6 Unsaturated Poly(amide imide) 14.2.7 Blends . . . . . . . . . . . . . 14.2.8 Foams . . . . . . . . . . . . . . 14.3 Properties . . . . . . . . . . . . . . . . 14.3.1 Mechanical Properties . . . . . 14.4 Applications . . . . . . . . . . . . . . . 14.4.1 Membranes . . . . . . . . . . . 14.4.2 Coatings and Adhesives . . . . 14.4.3 Fibers . . . . . . . . . . . . . . 14.4.4 Optical Applications . . . . . . 14.5 Suppliers and Commercial Grades . . . 14.6 Safety . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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15 Poly(imide)s 15.1 Monomers . . . . . . . . . . . . . . . . . . . . . . 15.2 Polymerization and Fabrication . . . . . . . . . . . 15.2.1 Conventional Route . . . . . . . . . . . . . 15.2.2 Isocyanate Route . . . . . . . . . . . . . . 15.2.3 Aqueous Route . . . . . . . . . . . . . . . 15.2.4 Nucleophilic Displacement Polymerization 15.2.5 Transimidization . . . . . . . . . . . . . . 15.2.6 Chemical Vapor Deposition . . . . . . . . 15.2.7 Hindered Biphenols . . . . . . . . . . . . 15.2.8 Poly(isoimide)s . . . . . . . . . . . . . . . 15.2.9 Functionalized Poly(imide) . . . . . . . . . 15.2.10 Bis(maleimide)s . . . . . . . . . . . . . . 15.2.11 Poly(imide sulfones) . . . . . . . . . . . . 15.3 Properties . . . . . . . . . . . . . . . . . . . . . . 15.4 Special Additives . . . . . . . . . . . . . . . . . .
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449 449 453 453 453 454 455 456 456 456 456 459 459 460 460 462 464 465 468 468 470
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475 476 479 479 481 482 483 483 484 485 485 486 488 488 489 489
Contents 15.5 Applications . . . . . . . . . . . . . . . . . . . . . . . 15.5.1 Foams . . . . . . . . . . . . . . . . . . . . . . 15.5.2 Membrane Technology . . . . . . . . . . . . . 15.5.3 Sensor Technology . . . . . . . . . . . . . . . 15.5.4 Polymer Matrix Electrolytes . . . . . . . . . . 15.5.5 Films and Coatings for Electronic Applications 15.5.6 Photosensitive Compositions . . . . . . . . . . 15.6 Suppliers and Commercial Grades . . . . . . . . . . . 15.7 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . 15.8 Environmental Impact and Recycling . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Liquid Crystal Polymers 16.1 Monomers . . . . . . . . . . . . . . . . . . . 16.1.1 Acetyletion . . . . . . . . . . . . . . 16.1.2 Functionalized Monomers . . . . . . 16.2 Polymerization and Fabrication . . . . . . . . 16.2.1 Copoly(ester)s . . . . . . . . . . . . 16.2.2 Poly(ester amide)s . . . . . . . . . . 16.3 Properties . . . . . . . . . . . . . . . . . . . 16.3.1 Mechanical Properties . . . . . . . . 16.3.2 Thermal Properties . . . . . . . . . . 16.3.3 Electrical and Optical Properties . . . 16.4 Applications . . . . . . . . . . . . . . . . . . 16.4.1 In Situ Composites . . . . . . . . . . 16.4.2 Optical Data Storage . . . . . . . . . 16.4.3 Stationary Phases . . . . . . . . . . . 16.4.4 Liquid Crystal Displays . . . . . . . 16.4.5 Electrically Conductive Compositions 16.5 Suppliers and Commercial Grades . . . . . . 16.6 Environmental Impact and Recycling . . . . . References . . . . . . . . . . . . . . . . . . . . . . Index Tradenames . Acronyms . . Chemicals . . General Index
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xvi
High Performance Polymers
Preface
This book focuses on high performance polymers. The text is arranged according to chemical constitution of the polymers. The most recent developments in the art spanning roughly the last ten years are reviewed. Each chapter follows the same template. In the introductory comments of each chapter a brief introduction to the polymer type is given and earlier monographs and reviews dealing with the topic are listed for quick reference. The text continues with monomers, polymerization and fabrication techniques and discusses aspects of application. After this, suppliers and commercial grades are collected, as well as safety aspects. Nowadays, economics is changing very quickly, companies are frequently reorganized and change their names. For this reason, the information about the manufacturers may not be state of the art, even after 6 months of finishing the text. However, usually the trademarks given are more persistent. Even when the material is ordered according to chemical structure, a great variety of individual materials belonging to the same polymer type is discussed. For this reason, the properties and safety data reproduced should be considered rather as examples. The reader who is actively engaged with the materials presented here should consult the untold technical data sheets and material safety data sheets provided by the individual manufacturers. The book belongs to the series Industrial Polymers Technology and Applications. For this reason the reader may not find all polymer types belonging to high performance polymers, for instance fluoropolymers. These polymer types are dealt with in other volumes of the series. xvii
xviii
High Performance Polymers
How to Use this Book Utmost care has been taken to present reliable data. Because of the vast variety of material presented here, however, it cannot be complete in all relevant aspects, and it is recommended that the reader study the original literature for complete information. Therefore, the author cannot assume responsibility for the completeness and validity of, nor for the consequences of, the use of the material presented here. Every attempt was made to identify trademarked products in this volume; however, there were some that the author was unable to locate, and we apologize for any inadvertent omission.
Index There are four indices: an index of trademarks, an index of acronyms, an index of chemicals, and a general index. Unfortunately the acronyms presented in the literature are not always consistent. This means that in a few cases the same acronym stands for different terms. Further, in the literature the acronyms are sometimes expanded in a different way, in particular for chemical names. The author has not unified the system of chemical names, even when the same compound appears with different names, because otherwise back tracing in the original literature would be difficult. I apologize here for this somewhat unsatisfactory situation. In the index of chemicals, compounds that occur extensively, e.g., “styrene”, are not included at every occurrence, but rather when they appear in an important context.
Preface
xix
ACKNOWLEDGEMENTS I am indebted to our local library, Dr. Lieselotte Jontes, Dr. Christian Hasenhüttl, Dr. Johann Delanoy, Franz Jurek, Friedrich Scheer, and Christian Slamenik for support in literature acquisition. I express my gratitude to all the scientists who have carefully published their results concerning the topics dealt with here. The book could not have been compiled otherwise. I would like to thank Dr. Sina Ebnesajjad for his review and comments on the manuscript. The publisher, Martin Scrivener and the editorial staff of William Andrew, Inc. have been most supportive of this project, especially Valerie Haynes, Jane Higgins, and Linda Mohr. J.K.F. 29th April 2008
xx
High Performance Polymers
1 Carbazole Polymers
Carbazole polymers can be roughly subdivided into polymers that contain the carbazole group as a pendent group and those that have the carbazole group in the backbone. A special variety is a carbazole polymer with conjugated groups. The most important class belongs to polymers based on N-vinylcarbazole (NVK). Carbazole polymers are reviewed in the literature.1–3 The interest in poly(N-vinylcarbazole) (PVK) and related polymers originates because these polymers are: 1. Photoconductive and have thus found applications in electrophotography, in 2. Polymeric light-emitting diodes, 3. Organic photorefractive materials, and 4. Photovoltaic devices. Early uses in the 1940s were in electric capacitors and other electric applications because of the good dielectric properties. In 1970, a charge transfer complex of PVK with 2,4,7-trinitro-9-fluorenone (TNF) was commercially introduced in electrophotography by IBM, based on a patent of Shattuck and Vahtra.4 1
2
High Performance Polymers Table 1.1: Comonomers for Poly(N-vinylcarbazole) Monomer
Comonomer
N-Vinylcarbazole
Styrene, vinyl acetate, divinylbenzene, methacrylates, N-vinyl-2-pyrrolidone 2-Phenyl-5-4-[(4-vinylphenyl)methoxy]phenyl-1,3,4-oxadiazole5 Acrylic acid, methacrylic acid, fumaric acid, maleic anhydride6 Methyl methacrylate (for electroluminescence microcapsules)7 Methyl methacrylate, europiummethacrylate complex8, 9 N-Vinylphthalimide10 Methyl methacrylate11
N-Vinylcarbazole N-Vinylcarbazole N-Vinylcarbazole N-Vinylcarbazole 9-(2,3-Epoxypropyl)carbazole Methacrylic acid 6-[3-(2-cyano2-(4-nitrophenyl)-vinyl)-carbazol-9yl]hexyl ester Methacrylic acid 6-[3-[2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester Methacrylic acid 6-[3-(diphenyl-hydrazonomethyl)-carbazol-9-yl]hexyl ester
Methyl methacrylate11
Methyl methacrylate11
1.1 MONOMERS Carbazole is obtained as a byproduct from the residues of coal-tar distillation. The carbazole unit can be varied in a variety of ways.11
1.1.1 N-Vinylcarbazole The vinyl group can be introduced into carbazole by the reaction with acetylene. NVK is a slightly brown crystalline solid with a melting point of 63°C. Often PVK does not consist of a single monomer, i.e., NVK itself, but other comonomers are added. Comonomers are shown in Table 1.1 and in Figure 1.1.
Carbazole Polymers
3
O N CH CH2
N CH CH2
O
N-Vinylcarbazol
N-Vinylphthalimide
N CH2 O N-Epoxypropyl-carbazole
Figure 1.1: Monomers Used for N-Vinylcarbazole Polymers and Copolymers
1.1.2 Nonlinear Optical Side Chain Monomers Multifunctional monomers have been synthesized whose polymers are exhibiting both photoconductivity and nonlinear optical (NLO) properties.11 Second-order optical nonlinearity requires the total system does not possess a center of symmetry. Therefore, the monomers have a rather complicated structure. The functionalities can be introduced by standard reactions in organic chemistry. Examples for this type of monomers are methacrylic acid 6-[3-(2cyano-2-(4-nitrophenyl)-vinyl)-carbazol-9-yl]hexyl ester, methacrylic acid 6-[3-[2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester, and methacrylic acid 6-[3-(diphenyl-hydrazonomethyl)-carbazol-9-yl]hexyl ester. The monomers are shown in Figure 1.2. The polymers from these monomers can be obtained by free radical polymerization using 2,2 -azobisisobutyronitrile (AIBN) as an initiator. Investigation of the photoconductivity showed that some of the polymers are photoconductive without any addition of sensitizer and charge transporting agent.
4
High Performance Polymers
H2C C CH3 C O (CH2)6O N
NC NO2 2-Methyl-acrylic acid 6-[3-[2-cyano-2-(4-nitrophenyl)-vinyl]carbazol-9-yl]hexyl ester H2C C CH3 C O (CH2)6O N
NO2 2-Methyl-acrylic acid 6-[3-[2-(4-nitrophenyl)-vinyl]carbazol-9-yl]hexyl ester H2C C CH3 C O (CH2)6O N
N
N
2-Methyl-acrylic acid 6-[3-(diphenyl-hydrazonomethyl)carbazol-9-yl]hexyl ester
Figure 1.2: Monomers for Non Linear Optics Applications11
Carbazole Polymers
5
1.1.3 Molecular Glasses Amorphous, film-forming photoconductive and charge transporting materials are addressed as molecular glasses .12 For many years, it was believed that polymers, but not low-molecular-weight compounds might exhibit vitrification.13 Amorphous organic materials are divided into two categories: compounds with low molecular weights, and polymers. Devices made from low-molecular-weight compounds are generally fabricated by vacuum deposition. Devices made from polymers are fabricated by the casting or spin-coating methods. Triarylamine and carbazole-based low-molar-mass compounds and polymers have been extensively studied for the different applications due to their good hole transport and luminescent properties.12 Polymers with 3,3 -dicarbazolyl units in the main chain can be synthesized.14 In the first step, adducts with pendent iodine groups are synthesized as shown in Figure 1.3. Polymers with di(carbazol-3-yl)phenylamine and N,N -di(carbazol3-yl)-N,N -diphenyl-1,4-phenylenediamine units in the main chain can be synthesized by a modified Ullmann condensation.15 The mechanism is shown schematically in Figure 1.4. The glass transition temperatures range from 102 to 216 °C.
1.2 POLYMERIZATION AND FABRICATION 1.2.1 Polymerization 1.2.1.1
Poly(N-vinylcarbazole)
PVK is the oldest known and most widely characterized polymeric photoconductor. Classically it is used in combination with TNF in photocopiers. New materials have been proposed, because the PVK/TNF system has a comparatively low photosensitivity. TNF has a high toxicity and the films of PVK have a poor mechanical strength. The nitrogen atom attached to the vinyl group lowers the electron density in the vinyl group by its inductive effect, but the mesomeric effect of the nitrogen electron pair overwhelms the inductive effect so that a conjugated π -electron system is formed. Therefore, NVK can be polymerized readily by cationic initiators. Anionic polymerization is not possible with
6
High Performance Polymers
KJ, KJO3, CH3COOH N
N
I
FeCl3,CHCl3 N
R = (CH2)n KOH, BrRBr
N R
N
I
N
I
Figure 1.3: Synthesis of Adducts Suitable for Ullmann Condensation
Carbazole Polymers
N R
NH
HN
N
7
+
I
I
N R
N
N
N
Figure 1.4: Ullmann Condensation of Carbazole Units with N,N -Diphenyl-1,4phenylenediamine
8
High Performance Polymers
NVK, but related carbazole compounds with the vinyl group bonded to the aromatic kernel can undergo anionic polymerization. NVK polymerizes with radical initiators. Further, NVK can be polymerized by Ziegler-Natta catalysts, by charge transfer polymerization, radiation polymerization, and electrochemical polymerization. Free Radical Polymerization. NVK can be polymerized with free radical initiators, such as AIBN or peroxides. The thermal polymerization is also possible, but the products obtained are not reproducible. Ultra-high-molecular-weight PVK is obtained by the heterogeneous solution polymerization in methanol/tert-butyl alcohol with a low-temperature free radical initiator, such as 2,2 -azobis-(2,4-dimethylvaleronitrile) (ADMVN). In this solvent system, the polymerization rate of NVK, is in a nearly proportional concentration of ADMVN, thus suggesting a heterogeneous nature for the polymerization.16 Otherwise, the overall rate of polymerization would be proportional to the square root of the initiator concentration. At room temperature, a weight average molecular weight of 3,230,000 Dalton is obtained. Since the monomer is prone to form cations, in cases where such cations are not trapped, say by a protic solvent, both radical and cationic polymerization may occur concomitantly. The presence of cyclic ethers, such as cyclohexene oxide (CHO) and trioxane do not influence the radical polymerization of NVK with AIBN. However, the polymerization is remarkably promoted in the presence of PH2 I+ PF− 6 , which is prone to accepting an electron. In fact, the presence + of PH2 I PF− 6 , CHO and NVK form a block copolymer. This is explained as the propagating NVK radical can be transformed into the corresponding cation by an electron transfer reaction.17 The free radical polymerization in bulk and in water suspension is used for technical processes. Radical Graft Polymerization on Glass Fibers NVK has been shown to be grafted on glass fibers. In the first step, an azo compound is chemically attached to the surface of the fibers.18 This is achieved by the reaction of the pendant hydroxyl groups with an excess of a bifunctional isocyanate, such as toluene diisocyanate. In the second step, an acid group or hydroxyl group containing an azo initiator is fixed on the modified surface. Two azo initiators, 4,4 -azobis-(4-cyanopentanoic acid) and 2,2 -azobis-(2-cyanopropanol) are bearing the desired
Carbazole Polymers CH3 Glass OH + OCN
9
CH3
NCO
NCO
HN O O Glass HO H3C H3C HO
N N
CN
CN
Figure 1.5: Attaching Functional Azo Compounds on a Glass Surface Using TDI
reactive functional groups. The process of attaching azo compounds on a glass surface is shown schematically in Figure 1.5. Polymerization and Grafting onto Nanotubes Supercritical fluids are widely used in the manipulation of porous materials. Supercritical carbon dioxide can be used for the impregnation of carbon nanotubes with NVK and AIBN with subsequent polymerization.19 Functionalized carbon nanotubes can be also obtained by electrochemical polymerization of the monomer, NVK.20, 21 The materials are suitable as they can be used as electrodes in rechargeable lithium batteries and for electrical capacitors. PVK can be grafted onto multi-walled carbon nanotubes.22 The grafting occurs by a radical reaction using AIBN in 1,2-dichlorobenzene at 70°C. Carbon nanotubes can be activated by free radical initiators as they open their π -bonds. In this way, they participate in polymerization reactions. The materials have a potential for optical applications. Oxidative Matrix Polymerization The polymerization of NVK in a matrix of poly(ethylene glycol) (PEG) has been described. As an oxidant, Ce4+ has been used.23 PEG proved to be a more suitable matrix in order to obtain a stable homogeneous ternary complex solution in comparison to poly(acrylic acid) and poly(N-vinyl-2-pyrrolidone). Purification The purity of the polymer has a decisive influence on various electrophotographic characteristics, such as photoconductivity and photosensitivity. A content of less than 100 ppm is required with respect
10
High Performance Polymers
to the monomer. Regardless of the method of preparation, the polymer is contaminated with up to 6% of NVK, up to 500 ppm carbazole, anthracene, and with sulfur compounds in the ppm range.24 The purification of PVK can be achieved by a precipitation process. The polymer material is dissolved in a suitable solvent, such as N,N-dimethylformamide, tetrahydrofuran (THF), benzene, toluene or methylene chloride, and is precipitated by adding methanol. The process of precipitation must be repeated several times. The precipitation process needs large kettle units because the material readily coagulates. Another process uses inorganic acid to bind basic components. The residual level of NVK in PVK can be reduced to an amount less than 25 ppm by treatment of the PVK in a solution with a strong acid. The polymer can be recovered in pure form by precipitation of the polymer from a solution with a non-solvent.25 Extraction An extraction process in order to remove the impurities has been claimed to be superior to the precipitation method from organic solutions.24 An apparatus based on the Soxhlet principle, is used for the purification. Suitable extracting agents are non-solvents that do not cause pronounced swelling, such as ethyl acetate. The solvent loaded with the impurities extracted can be recovered by distillation. In comparison to conventional precipitation techniques, the extraction process has the following advantages: 1. Only 10 to 20% of solvent is needed. 2. The time required is substantially shorter. A PVK sample originally contained 4.2% of NVK, 0.1% of carbazole and anthracene in the ppm range as impurities. After extraction with ethyl acetate at ca. 70°C, the sample contained only from 150 to 200 ppm of vinylcarbazole after 8 hours, and less than 50 ppm of vinylcarbazole were determined after 72 hours. The content of carbazole was below the detection limit. Electroluminescent Materials Radical polymerization is used in the preparation of electroluminescent materials that are composed of alternate copolymerizates of hole transfer monomers and electron transfer monomers.26
Carbazole Polymers
11
Table 1.2: Arrhenius Parameters for the Propagation Constant of NVK A/l mol−1 s−1
Ea /kJ mol−1
References
2.20 × 108 3.60 × 107
27.4 22.8
27 28
Conventionally, as organic material possessing a hole transfer capability, diamine derivatives, which include low molecule organic materials, such as aryl amine compounds are used. Examples are N,N -bis-(3-methylphenyl)-N,N -diphenylbenzidine (TPD), and macromolecule organic materials, such as PVK. As an electron transfer material, 2-(4-biphenylyl)5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) is well known. However, polymers incorporating both functionalities have certain advantages. As hole transfer monomers, carbazole derivatives, such as NVK, and 3,6-dimethyl-9-vinylcarbazole can be used. As electron transfer monomers, oxadiazole derivatives, preferably 2-α -naphthyl-5-(4-vinylphenyl)1,3,4-oxadiazole, can be used. As catalyst systems, 1,1 -azobis-(1-acetoxy-1-phenylethane) and AIBN have been preferred over others. Photopolymerization. The free radical solution polymerization of NVK in THF at temperatures in the range of −20°C, to 20°C with photoinitiation of ADMVN as the radical initiator showed an overall rate proportional to the square root of the initiator concentration. At low temperatures and small concentrations of the initiator, weight average molecular weights of 510,000 Dalton were obtained.29, 30 The same is true, when 1,1,2,2-tetrachloroethane is used as a solvent.31 The rate constant for the propagation reaction was determined by the pulsed laser method.27 In order to avoid interference with the excitation of the monomer, a photoinitiator, which absorbs up to ca. 410 nm was selected, among others: 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1 (Irgacure™ 369, Ciba). Using 2,2-dimethoxy-2-phenylacetophenone as photoinitiator, the propagation rate constants were not accessible for temperatures exceeding 20°C. It is suggested that the monomer is excited by pulsed laser light of 355 nm, contributing to the initiation by the formation of free radicals.28 For this reason, the results may be different from those published before. The Arrhenius parameters are shown in Table 1.2.
12
High Performance Polymers
Nanocomposite Materials The techniques to fabricate nanocomposite materials have been reviewed by Biswas and Ray.32 These consist of: 1. Monomer impregnation and intercalation into the clay, 2. Polymerization, 3. Clay exfoliation. Polymer-clay nanocomposite materials can be prepared by intercalation of NVK into montmorillonite followed by the photoinitiated polymerization with a triarylsulfonium salt.33 Nanocomposites based on aluminum oxide, poly(pyrrole) (PPY), and PVK can be prepared by precipitating PVK in a suspension of PPY-coated aluminum oxide particles.34, 35 A PPY/Al2 O3 composite is added to an aqueous slurry of aluminum oxide powder, pyrrole, and anhydrous FeCl3 . Afterwards, a solution of PVK in THF is added. The PVK precipitates out on the preformed PPY/Al2 O3 particles. The electrical conductivity is significantly higher than the conductivity of the polymer alone. Ziegler-Natta Polymerization. NVK and other monomers, such as vinyl ethers, 1,5-hexadiene, dihydrofuran, and dihydropyrans can be polymerized using a titanium, hafnium or zirconium pentamethylcyclopentadienyl complex as an initiator in the presence of a borane co-initiator, e.g.,36 B(C6 F5 )3 or B(C6 H5 )(C6 F5 )2 . The polymerization takes place at −78 °C. In fact, it is not clear, whether the polymerization takes place via a coordinate mechanism, because a concomitantly cationic mechanism, which is known to be fast, may take place. Cationic Polymerization. NVK is highly reactive to cationic polymerization initiators, such as proton acids, Lewis acids, metal salts, etc. To illustrate the reactivity for polymerization, it has been shown that even carbon whiskers can initiate a cationic polymerization.37 As an initiation mechanism, both the addition and electron transfer can take place, since many of the cationic initiators are electron acceptors. The rate constant of propagation is by a factor of 105 higher than observed in radical polymerization. The rate of polymerization depends on
Carbazole Polymers
13
whether free ions are involved, or ion pairs are involved in the mechanism. Ion pairs are somewhat less reactive than free ions. NVK is used as a sensitizer of cationic photopolymerization. Cationically photopolymerizable, or photocurable, compositions typically contain monomers or oligomers having the epoxy or the ether functionality, and a photoinitiator. The most commonly used photoinitiators employed for photoinduced cationic ring-opening polymerizations are diaryliodonium salts, or triarylsulfonium salts. To increase the cure, photosensitizers have been used, which increase the response of photoinitiators to longer wavelengths. Electron rich polynuclear aromatic compounds, such as anthracene pyrene, perylene, coronene, 9,10-diphenylethynylanthracene, and carbazole compounds have been used. Most of these polynuclear aromatic compounds are acutely toxic, as well as potentially carcinogenic. To circumvent these obstacles, polymerizable photosensitizers, or their polymers have been used, e.g., 9-(2,3-epoxypropyl)carbazole (= N-glycidylcarbazole) or 9-(2-vinyloxyethyl)carbazole.38 Poly(N-epoxypropyl)carbazole (PEPC) films are photosensitive only in a near UV range. However, composites from the poly(imide)s with PEPC and its dichloro derivatives and dibromo derivatives exhibit an appreciable photoelectric sensitivity in the near UV and visible range.39 It has been shown that for NVK and PVK as photosensitizers, both the monomolecular and the polymeric photosensitizer behave similarly. Copolymers of NVK with diethylfumarate show excellent solubility in these monomers. A marked improvement of the photo-response of the photosensitized polymerization using broadband UV light was observed in comparison to experiments in the absence of a photosensitizer. The copolymers can be prepared with AIBN. NVK is suitable, among other cationically polymerizable monomers, for inorganic/organic host-guest hybrid materials. These hybrid materials are prepared by the polymerization in the pores of zeolites.40, 41 Well-defined polymers are formed under the conditions of constricted geometry in the pores. Triblock copolymers using NVK, 4-(1-pyrenyl)butyl vinyl ether and 2-chloroethyl vinyl ether have been synthesized in an sequential cationic polymerization technique.42 The block copolymers were further functionalized with 2-(4-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole, by reaction of
14
High Performance Polymers
O
Cl
O
+
OH
N N
O
O
N N
O
Figure 1.6: Modification of the Pendent Chlorine Groups with 2-(4-Hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole
the chlorine in the 2-chloroethyl ether moiety. The modification procedure is illustrated in Figure 1.6. Light-emitting diodes (LED)s that were fabricated from the material show a low electroluminescence. Charge Transfer Polymerization. The polymerization by a charge transfer mechanism is often dealt with as a special mechanism, although it can be interpreted as a cationic mechanism where a zwitterion is involved. In a charge transfer polymerization, side reactions often occur that produce low-molecular-weight compounds. Charge transfer agents for NVK to initiate a charge transfer polymerization include electron acceptors, such as tetracyanoethylene,43 and with nitrogen dioxide and sulfur dioxide in dichlorethane solutions.44 The polymerization is fast with nitrogen dioxide but slow with sulfur dioxide. The mechanism of initiation is shown in Figure 1.7. A radical mechanism can be excluded, since 2,2-diphenyl-1-picrylhydrazyl, which is a free radical inhibitor, does not inhibit the polymerization initiated by nitrogen dioxide. RAFT Polymerization. The carbazole unit in poly(N-ethyl-3-vinylcarbazole) is directly bound to the polymer main chain. It can be synthesized by
Carbazole Polymers
15
NO2 N
N
CH CH2
CH CH2 NO2
Figure 1.7: Cationic Polymerization of NVK
reversible addition-fragmentation chain transfer (RAFT) polymerization.45 The polymers have low polydispersities of Mw /Mn of 1.15–1.29. In RAFT polymerization, the monomer N-ethyl-3-vinylcarbazole (E3VC) is polymerized with AIBN in the presence of benzyl-1-pyrrolecarbodithioate as the chain transfer agent (CTA). The kinetics of polymerization follows a linear first order, and the molecular weight increases proportionally to the conversion. Further, there is a linear relationship between the molecular weight and the ratio of monomer to CTA. Dithiocarbamate-terminated poly(E3VC) can be chain extended to form block copolymers with poly(styrene) (PS) blocks. These block copolymers exhibit excimer emission at 454 nm with blue fluorescence. The polymers are thermally stable above 350°C in a nitrogen atmosphere.45 Electrochemical Polymerization. Electrochemical polymerization conveniently enables the deposition of a conducting film of PVK onto the surface of a working electrode.46 Electrochemical polymerization of NVK in organic solvents such as dichloromethane and acetonitrile yields both a white-colored nonconducting PVK and a green-colored conducting PVK. The nonconducting PVK precipitates in electrolyte solution, whereas the conducting PVK covers the electrode surface as a green-colored film. In dioxane H2 SO4 , no conducting PVK is formed on the electrode surface in electrolysis experiments done with acid concentrations below 2.0 M H2 SO4 . However, conducting PVK is formed at lower acid concentrations when the solvent is changed from dioxane to ethanol. The fact that polymerization occurs at a lower acid concentration in ethanol than in dioxane is attributed to the greater dissociation of the acid in ethanol. Polymer films on a surface can be characterized by Raman spectroscopy spectra removing the film from the surface. The formation of conducting PVK is suggested as an anodic oxidation
16
High Performance Polymers
CH CH2
CH CH2
N
N H
CH CH2 N
H
H
N - 2H
CH CH2
CH CH2 N
N CH CH2
Figure 1.8: Anodic Oxidation and Crosslinking of PVK46
of PVK via a carbazyl radical cation, which dimerizes to form a crosslinked material as shown in Figure 1.8. The temperature dependence of the electrical resistance values of the polymer in the range of 30–125°C is such that it can be used as a thermal sensor.47 When solid carbazole crystals are immobilized on an electrode surface, oxidative dimerization and polymerization can also be achieved in solid state.48 By means of electrochemical nanolithography, conducting nanopatterns due to the selective oxidative crosslinking of PVK can be produced.49
Carbazole Polymers 1.2.1.2
17
NVK/Pyrrol Composites
Flexible films of composites prepared by the laser-electrochemical polymerization of a mixture of NVK and pyrrole in methylene chloride with tetrabutylammonium perchlorate as electrolyte showed a conductivity of 10 S cm−1 at room temperature.50 It is suggested that the degree of polymerization in composites synthesized by laser-electrochemical polymerization could be higher than that of composites synthesized by pure electrochemical polymerization. The conductivity of the composites varies with laser energy density by one order of magnitude. On the other hand, for a fixed laser energy of 7 mJ cm−2 , the conductivity of the composites is not very dependent on the ratio of the monomers. Core-shell nanoparticles with a poly(pyrrole) as core and PVK as shell have been fabricated by nanoparticle-seeded dispersion polymerization.51 The thickness of the PVK shell can be easily tuned by varying the total amount of NVK monomer added. These polymers exhibit superior conductivity and fluorescence.
1.2.2 Other Vinylcarbazole Compounds 1.2.2.1
Grignard Coupling
3-Halo-6-halomagnesio-9-alkyl-9H-carbazoles can be coupled with palladium catalysts, in the manner of a Grignard reaction.52 The structural analysis of the polymers showed that carbazole repeating units are linked exclusively at the 3,6-positions. 1.2.2.2
Anionic Polymerization
It is common opinion that NVK itself does not polymerize when treated with n-butyllithium in THF,53 however, it is not completely inert. Carbazole and ethene are formed as reaction products. In contrast to this view, the anionic copolymerization of NVK with C60 fullerenes has been recently reported. The polymerization is initiated with lithium naphthalene. However, fullerene polyanion salts do not initiate the polymerization of NVK and other monomers.54 E3VC can be prepared by the Wittig reaction of 9-ethyl-3-carbazolecarboxaldehyde in THF. This monomer can be anionically polymerized
18
High Performance Polymers
using n-butyllithium as an initiator. Further, a block copolymer with styrene can be prepared. The polymers exhibit a blue photoluminescence.55
1.3 PROPERTIES Due to their use in electrical applications, interest in polymers focuses on their electrical properties.
1.3.1 Liquid Crystalline Phases PVK can form liquid crystalline polymers. The lowest degree of polymerization for PVK that may form a stable liquid crystalline phase is in the range of 150 to 200 Dalton, which is significantly higher than 50 Dalton for most conventional side chain liquid crystalline polymers.56 1.3.1.1
Charge Transporting Materials
Polymers based on the carbazole moiety are attractive as photoconductors or charge transporting materials because the carbazole moiety forms comparatively easy stable radical cations, Moreover, the carbazole ring can be modified by various substituents. Further, the substance class exhibits a high thermal and photochemical stability. Charge transporting materials include a positive hole-transporting material and an electron transporting material. Examples of the electron transporting material include electron acceptors, such as: • • • • • •
Chloroanil, bromoanil, Tetracyanoethylene, tetracyanoquinodimethane, 2,4,7-Trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone, 2,4,5,7-Tetranitroxanthone, 2,4,8-trinitrothioxanthone, 1,3,7-Trinitrodibenzothiophene-5,5-dioxide, and Benzoquinone derivatives.
Examples of the positive hole-transporting material are PVK and its derivatives.57 Further materials are: • Poly-γ -carbazolylethylglutamate, pyrene-formaldehyde condensation products, • Poly(vinyl pyrene), poly(vinyl phenanthrene), poly(silane),
Carbazole Polymers • • • • •
19
Oxazole and oxadiazole derivatives, Imidazole derivatives, Arylamine derivatives, Stilbene derivatives, and Arylmethane derivatives.
The materials may be used in binder resins. On the use of PVK, which is common as the hole-transporting material, there is a known problem that the adjacent carbazole groups are liable to stack up and form an excimer. Formation of the excimers causes some problems that the excimers serve as a trap for charge to obstruct the transport of charge, or that the excimers serve as a quenching center when using it in combination with a lightemitting material to obstruct the light emission.58 Therefore, copolymers of the compounds shown in Figure 1.9 have been suggested. 1.3.1.2
Principle of Xerography
Electrophotography, more commonly known as xerography, is based on the formation of a latent electrostatic image on an imaging surface by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light-struck areas of the imaging layer are then rendered conductive and the electrostatic charge is selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image-bearing surface is made visible by development with a finely divided colored electroscopic material, i.e., the toner. The toner will be attracted to those areas on the image-bearing surface that retain the electrostatic charge and thus form a visible powder image. The developed image can then be permanently fixed to the photoconductor where the imaging layer is not to be reused. In plain paper copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. In plain paper copying systems, the materials used in the photoconductive layer should preferably be capable of rapid switching from nonconductive to conductive state and back, in order to permit cyclic use of the imaging surface.
20
High Performance Polymers
CH CH2
N
N
9-[4′-(Carbazol-9-yl)[1,1′-biphenyl]-4-yl]-3-vinylcarbazole CH CH2
CH CH2 N
O
N
O N-Vinylcarbazol
O CH3
Figure 1.9: Comonomers for Hole-Transporting Materials58
Carbazole Polymers
21
The failure of a material to return to its nonconductive state prior to the succeeding charging sequence will result in a decrease in the maximum charge acceptance of the photoconductor. This phenomenon is addressed as fatigue. It can be avoided by the selection of photoconductive materials having a rapid switching capacity. Suitable materials for the use in such systems include anthracene, sulfur, and selenium. In addition to anthracene, other organic photoconductive materials most attractive to PVK, have been the focus of interest in electrophotography. Poly(vinylcarbazole)s, when sensitized with TNF exhibit a good photo-response and discharge characteristics and a low dark decay. However, the dark decay depends upon the polarity of the surface charge. The maximum concentration of sensitizer is limited by some constraints. High loadings of sensitizer can cause bad mechanical or photoconductive properties of the sensitized composition. For example, the excessive addition of sensitizer can result in crystallization of the sensitizer. Some sensitizers, even when present in low concentrations can result in over-sensitization of the composition, in that the photocurrent generated upon exposure will persist comparatively long after the illumination ceases. As an alternative to the sensitization by additives, intramolecular charge transfer complexes have been proposed, where the electron donor and electron acceptor functions are located along a common vinyl backbone. Examples are nitrated vinyl polymers of poly(acenaphthylene), poly(9-vinylcarbazole) and poly(1-vinylnaphthalene),59 copolymers from 3,6-diphenyl-vinylcarbazole and 3,6-dinitro-9-vinylcarbazole.60 and copolymers from NVK and N-vinylphthalimide.10 In the intramolecular transfer of triplet excitation energy from a donor chromophore consisting of phthalimide and carbazole to a naphthalene acceptor chromophore, a complete transfer of triplet excitation energy occurred, when the chromophores are separated by methylene groups.61 It is thought that the spacial constraint placed upon the electron donor and electron acceptor functions enhances the probability of charge transfer interaction. In addition, certain conformational and steric requirements must also be satisfied in order to facilitate efficient overlap of donor and acceptor electron orbitals required of this type of charge transfer interaction.
22
High Performance Polymers
1.3.2 Optical Properties An alternative to poly(N-vinylcarbazole) is poly(1-hexyl-3,4-dimethyl-3,5pyrrolylene) (PHDP). PHDP is completely soluble in common organic solvents. The luminescence of PHDP is comparable to that of PVK. However, the quantum efficiency of PHDP is 2.5 times higher than that of PVK.62 UV light effects dramatic modifications of the physical and photophysical properties of PVK. The modifications are concomitantly with modifications of the chemical structure of the polymer.63 In the initial stage of irradiation, mainly crosslinking occurs. The polymer becomes insoluble. Subsequently, a decrease in the molecular weight is observed that indicates degradation.
1.4 APPLICATIONS 1.4.1 Electrophotographic Films 1.4.1.1
Photoconductivity
The process of photoconductivity can be broken down into several steps: 1. Absorption of radiation 2. Formation of excitons. An exciton is an excited state where an electron is still bound to the matrix, but the matrix is going to form a gap of charge. In other words, an exciton is a pair of electron and a hole of charge, 3. Formation of movable charges. The excitons dissociate by the help of donor acceptor sites present in the material into movable charges, and 4. Charge recombination. The carbazole group absorbs light in the UV range. Therefore, polymers of this type can become photoconductive as such only in the UV range. However, colored sensitizers can be added to shift the photoconductivity into the visible region. After the formation of excitons, the charges become more separated. Important for the final yield of charges is the efficiency by which the excitons are converted into free charges. The yield can be improved by doping with an electron acceptor or by tailoring the molecule as electron acceptors are introduced as substituents or side chains. Both an intermolecular and
Carbazole Polymers
23
an intramolecular mechanism and electric fields can effect the charge separation process.64 The transport of charges can be imagined, as a positive hole is formed whence the electron is removed. The hole can be filled with another electron from a neighboring site, which in turn generates anther positive hole ate this site. In this way, the hole is obviously moving, thus effecting a transport of charge. The efficiency of photoconductivity is hampered by recombination reactions. Further, the charges may be trapped on sites of suitable structure. In the course of trapping, they are not really destroyed, but can be released after some time. Trapping is comparable with chromatographic reversible adsorption processes. It is responsible for the decrease of photoconductivity.
1.4.1.2
TNF-PVK complexes
The TNF-PVK complex is formed by direct reaction of the two materials in a suitable solvent, e.g., THF.65 In order to produce films free of surface crystals, THF has been found to be the best solvent. Chlorobenzene gives good results when the films are applied hot. The usual procedure is to make up a stock solution of PVK in THF. The TNF is then added in suitable amounts.
1.4.1.3
Photoconductive Copolymers
A photoconductive copolymer containing ca. 10 mol-% of N-vinylphthalimide and the rest NVK can be prepared by free radical polymerization using AIBN in benzene solution. The polymer is precipitated by hexane, then purified by dissolution in a benzene/THF mixture, and then re-precipitated from hexane. This process is repeated several times. The number average molecular weight of such a copolymer is in the range of 100,000 Dalton.10 The copolymers can be formed into photoconductive films useful in electrophotography by simple solvent casting and coating techniques. Typical solvents are THF and mixtures of toluene/cyclohexanone. The film thickness can be controlled by adjustment of the viscosity of the coating solution.
24
High Performance Polymers
1.4.1.4
Copolymers
A photoconductive sol-gel material based on an organic/inorganic interpenetrating network has been described. PVK acts as the charge transporting matrix and TNF acts as the sensitizer.66
Hole Transport Electron Acceptor Systems. NVK as the hole transport monomer can be copolymerized with electron transport monomers. Examples for such monomers are 2-phenyl-5-4-[(4-vinylphenyl)methoxy]phenyl-1,3,4-oxadiazole and 2-(4-tert-butylphenyl)-5-4-[(4-vinylphenyl)methoxy]phenyl-1,3,4-oxadiazole.5 The polymerization proceeds as free radical copolymerization and statistical copolymers are obtained. Thus, the composition of the copolymers can be varied and the conductive properties can be tailored. The copolymers are transparent in the visible region and form good films. In comparison to blends of low-molecular-weight oxadiazole compounds with PVK, the glass transition temperatures of these copolymers remain high. Since the oxadiazole units are fixed by the copolymerization reaction, there is no change for phase separation and recrystallization. The electrical properties of the copolymers are somewhat reduced in comparison to pure PVK. The copolymers show a good efficiency in dye doped devices emitting blue, green, and orange light.
Sensitizers. The photoinduced discharge rate of a PVK film increases dramatically when the film is doped with a metallofullerene, dysprosium fulleride (DyC-82), or the fullerenes C-84 and C-60. A PVK film doped with DyC-82 displayed better photoconductivity than a film doped with C-84. This explains that DyC-82 is a better electron acceptor than C-84. However, the film doped with C-60 shows the best photoconductivity.67
Photoconductive Lithographic Printing Plate Assembly. NVK has been copolymerized with olefinic monomers possessing carboxylic acid, such as acrylic acid, methacrylic acid, fumaric acid, and, maleic anhydride, or carboxylic anhydride.6 The acid functionality yields copolymers that are soluble in aqueous alkaline media. The copolymers are intended to be part of a photoconductive lithographic printing plate assembly.
Carbazole Polymers
25
1.4.2 Polymeric Light-Emitting Diodes An organic light-emitting diode consists of polymer film in between two electrodes. One electrode, such as gold or indium tin oxide (ITO), should have a high work function, to achieve the injection of positive hole charges. The other electrode consists of a metal with low work function in order to inject electrons. Suitable metals with low work functions are aluminum, calcium, or magnesium. Both hole charges and electrons that are introduced into the polymer film combine there into excitons, as either singlet or triplet states. An exciton is a pair, consisting of an excited electron and a hole. The electroluminescence occurs by the transition from the excited excitons into the ground state by emission of radiation. In conventional systems, the excited state is the singlet state. However, there are systems, in which from both singlet and triple state electroluminescence is observed. These systems are superior to the conventional ones. Many conjugated molecules have photoluminescence (PL) efficiencies of more than 50%. In contrast, the electroluminescence (EL) efficiencies are often less than 5%. One of the reasons is thought to be the formation of triplet excitons. Namely, when electrons and holes combine to form an exciton, the exciton can either be in a singlet or triplet state. In conjugated molecules, only singlet excitons can generate light, the triplet excitons are lost by non-radiative processes. Due to spin statistics, only 25% of the excitons have singlet character in conjugated molecules, thus the efficiency of a conjugated polymeric material cannot be more than 25%. So it is promising, to use lanthanide ions, attached to organic ligands, as emitters in organic EL materials.8, 9 In these systems, singlet states and triplet states can be transferred to the f levels of the lanthanide ion in generating electroluminescence. Further, in contrast to conjugated molecules, rare earth ions have sharp emission spectra. We provide some definitions used in the subsequent text. An excimer is a simple an excited molecule. In contrast, an exciplex stands for an excited complex that consists of different molecules forming together an excited state. By the decomposition of an exciplex, PL may occur. In some cases, long-wavelength emissions in electroluminescence occur that are not observed in the photoluminescence spectra. These longwavelength emissions are postulated to arise from electromers and elec-
26
High Performance Polymers
troplexes, respectively. The mechanism of the latter type of emission is by phosphorescence or direct radiative recombination of holes and electrons attached at two neighboring molecules.68 The terms electromers and electroplexes, are complete analogies to excimers and exciplexes. In blends of PVK with PBD and in random copolymers with carbazole and oxadiazole groups attached as side chains, the active groups have different mobility, or are subjected to different topological constraints, respectively. In the blends, exciplexes emerge, and in the copolymers, electroplexes are effective. Both types of complexes shift the EL spectra to red in comparison to pure PVK homopolymer. The red-shift is significantly greater for the electroplex. Therefore, the different complexes affect the external quantum efficiency of dye doped organic light-emitting diodes. This arises, because the efficiency of Förster energy transfer from the matrix to the dye is dependent on the degree of overlap between the EL spectrum of the matrix material and the absorption spectrum of the dye.69 1.4.2.1
Förster Energy Transfer
The Förster energy transfer70 is a non-radiative transfer of electronic excitation from a donor molecule D to an acceptor molecule A, according to the equation D∗ + A A D + A∗ .
(1.1)
The transfer arises from dipole-dipole coupling. Förster energy transfer is an important mechanism in order to transfer the energy from a donor molecule to an acceptor molecule that eventually emits light at another frequency, as the donor would do. Important for effective energy transfer is that the absorption spectrum of the acceptor and the emission spectrum of the donor overlaps sufficiently. An indicator for effective energy transfer is the observation of substantial red-shifts in photoluminescence. Förster energy transfer is sometimes an undesired effect. The theory of Förster treats only allowed transitions. Dexter has extended this theory to include a transfer mechanism by means of forbidden transitions.71 The rate constant of energy transfer kT between the donor molecule and the acceptor molecule in the Förster energy transfer mechanism can be described by the following equation:
Carbazole Polymers
kT ∝ κ Qd n Na τd R J
κ 2 Qd J . π 5 n4 Na τd R6
27
(1.2)
Orientational factor for the dipole-dipole interaction Fluorescence quantum yield of the donor molecule without the acceptor molecule Refractive index of the medium Avogadro’s number Fluorescence lifetime of the donor molecule in the absence of acceptor Distance between the centers of the donor and acceptor molecules Normalized spectral overlap integral
The equation indicates that the rate of energy transfer is directly proportional to the square of the orientational factor, κ , and inversely proportional to the sixth power of the distance between the center of the molecules R. Thus, by increasing the distance between the molecules by a factor of 1.6, the rate of energy transfer is reduced by more than one order of magnitude. In the solution state, the molecules are free to rotate and sample most of the orientational possibilities during the excited state lifetime. In the solid state, the molecules are effectively frozen in place with little or no rotation allowed. This leads to a further decrease in the energy transfer. By changing the distance between the molecules and decreasing the orientational factor, quenching is reduced due to Förster energy transfer.72 In functionalized polymers, the distance between donor and acceptor can be altered by the structure, in particular by the side chain length of the pendant functionality.73 The energy transfer depends rather on the distance than on the spectral overlap integral, in accordance to Eq. 1.2. 1.4.2.2
Service Time
It is thought that there are five causes of degradation to light-emitting materials made from organic compounds, and to organic light-emitting elements:74 1. Chemical degradation of the organic compound in the excited state, 2. Melting of the organic compound due to heat generated during driving, 3. Dielectric breakdown originating in macro faults, 4. Degradation of the electrodes or the organic layer interfaces, and
28
High Performance Polymers 5. Degradation due to instabilities in the amorphous structure of the organic compound.
The first to third causes of degradation are due to driving of the organic light-emitting element. The generation of heat is inevitable because electric current flowing within the element is converted into heat. If the melting point of the organic compound, or the glass transition temperature, is low, it is assumed that melting will occur. The existence of pinholes or cracks within the organic compound will concentrate the electric field in those locations and cause dielectric breakdown. Degradation proceeds even if the light-emitting element is maintained at room temperature according to items four and five. The fourth cause creates dark spots and is due to cathode oxidation and reactions with moisture. The fifth cause is because all organic compounds used in the organic light-emitting element are amorphous materials. It is thought that crystallization occurs during long-term storage, changes over a long time, and the generation of heat, and that there are almost no materials with a stable amorphous structure can be maintained. In organic LEDs, the heat, which is caused by a non-emissive site, is a crucial factor that affects mainly the degradation process of the organic material. The effect of heat treatment of PVK on the performance of the organic electroluminescent devices has been investigated. The degree of degradation was increased, as the exposure time and temperature were increased.75 Since the layer formed with the organic material in the light-emitting diode is as thin as from several 10–100 nm, the voltage applied per unit thickness is extremely high and the device is driven at a high current density of several mA cm−2 , a great amount of heat is generated. Often, the hole-transporting low-molecular-weight compound or the fluorescent organic low-molecular compound are formed as films by vapor deposition in an amorphous glass state. Then it is gradually crystallized and finally melted. These procedures may lower the brightness or favor dielectric breakdown. Consequently, the lifetime of the device is lowered. Further, it may suffer from aging changes and deterioration caused by oxygen-containing atmospheric gas or moisture during long-time use. A protective layer may be disposed for preventing degradation of the device due to moisture or oxygen. Specific materials for the protective layer can include metals, metal oxides and resins, such as poly(ethylene) resin, poly(urea) resin and poly(imide) resin.76
Carbazole Polymers
29
Further, a poly(ethylene terephthalate) as the transparent plastic substrate is widely used. Poly(ethylene naphthalate) derivatives exhibit still better oxygen and moisture permeability.77 For forming a protective layer, dependent on the nature of the material, a vacuum vapor deposition method, a sputtering method, a plasma polymerization method, a chemical vapor deposition method or a coating method, can be applied.76 1.4.2.3
Methods of Fabrication of LEDs
Spin-Coating. A widely used technique to fabricate thin layers is spincoating. In the first step, the material is poured or sprayed to the center of a disk. Next, the disk starts rotating in an axis normal to the surface of the disk. Thereby the material moves outward. In the stationary state, the viscous forces in the fluid dominate the thinning of the layer. Since the layer is very thin, moving interference colors can be observed. Excess material may leave the disk at the edge as droplets. If the material contains a solvent, the solvent may evaporate. Spin-coating may be done at elevated temperatures and in vacuo. Various designs are in use.78, 79 Color Displays. There are several methods to fabricate color displays. The conventional patterning method of an organic thin film layer includes: • • • • • •
A masked vacuum deposition method, A screen-printing method, A stamping method, A masked dye diffusion method,80 An ink-jet printing method,81 and A micro gravure method.
White organic electroluminescent devices have been fabricated using an ink-jet printing method.81 Copolymers of NVK and methyl methacrylate have been used as microcapsules for LEDs.7 Process without Photolithography. To optimize the performance of a polymeric light-emitting diode, devices with holes injected through an ITO/poly(aniline) (PANI) electrode into the polymer are much more efficient than devices fabricated with the anode made only by ITO.
30
High Performance Polymers
By using doped PANI as a hole injection layer in a polymeric lightemitting diode, the manufacturing process can become simpler. Then, the pattern of a conductive layer can be produced without ITO photolithography by UV exposition.82 1.4.2.4
Poly(fluorene)s
Poly(fluorene)s are an important class of semiconducting conjugated polymers. They are efficient emitters of blue light. However, the first blue light LED was based on poly(p-phenylene).83 Energy transfer mechanisms enable the emission of light of other colors, when used as host materials. In poly(fluorene)s a low energy emission band in the range of 2.2–2.4 eV appears in the course of time and destroys the color. It was suspected that the low energy emission band results from keto defects that were introduced either during synthesis or by photo-oxidation during service. Experiments with poly(9,9-dioctylfluorene-co-fluorenone) with 1% fluorenone as a model compound demonstrated that fluorenone defects are generated by photo-oxidation and by thermal-oxidation.84 Moreover, the formation of these defects is catalyzed by the metals with a low work function that are used as cathode materials in light-emitting diodes. Copolymers with fluorene and 1,3,4-oxadiazole show highly efficient photoluminescence.85 A double layer device consisting of PVK and an alternating copolymer of 9,9 -didodecylfluorene-2,7-diyl and (1,4-bis-(1,3,4oxadiazole)-2,5-di(2-ethylhexyloxy)phenylene)-5,5-diyl exhibits a narrow blue electroluminescence with a maximum at 430 nm. Electrochemical analysis of the polymers using cyclic voltametry suggests that they can be used both as electron transport materials and as blue emission materials for LEDs. Blends of polymers are frequently incompatible. Consequently, compatibility problems are a concern in the field of polymeric light-emitting diodes composed from polymer blends. However, this effect is sometimes an advantage in the development of light-emitting devices. White light emission was obtained from a device made of a ternary polymer blend, comprised of PVK, poly(9,9 -dihexylfluorene-2,7-divinylene-m-phenylene vinylene-stat-p-phenylene vinylene) (CPDHFPV). Another common material is poly(2-methoxy-5-(2-ethylhexyloxy)1,4-phenylene vinylene) (MEH-PPV).86 PVK and CPDHFPV are compat-
Carbazole Polymers
31
ible, whereas CPDHFPV and MEH-PPV are only poorly compatible. This leads to a simultaneous emission of two colors. Consequently, a pure white color can be obtained. In a fluorene-spirobifluorene alternating copolymer P(OF-SBF), in the spiro-segment, the two fluorene rings are orthogonally arranged and connected through a tetrahedral bonding carbon atom. This forces a special rigid structure and prevents the π -stacking of the polymer backbone. Both thermal and spectroscopic stabilities are improved. The polymer can serve as a host matrix to effectively transferring its excitation energy to a derivatized perylene dopant, yielding an efficient blue light-emitting layer.87 In spirobifluorene-based pyrazoloquinolines, two identical luminophores have been connected via a sp3 -hybridized carbon atom and are orthogonally arranged. The rigid spirobifluorene linkage significantly increases in the glass transition temperatures, which are in the range of 246–280°C. The luminophores show the characteristic absorptions and photoluminescence of ordinary mono-pyrazoloquinoline derivatives, in the blue region.88 Oxadiazole (OXD) groups were linked to the fluorene unit by attaching 4-tert-butylphenyl-1,3,4-oxadiazole groups onto the 9 position of the alternating fluorene unit to form a three dimensional cardo-structure (PF-OXD). The polymer has a high glass transition temperature Tg of 213°C and a good thermal stability. The commonly observed aggregate/ excimer formation in poly(fluorene)s is essentially suppressed in this polymer. Usage as the emitting layer shows a bright blue emission at a turn-on voltage at 5.3 V with a brightness of 2770 cd m−2 at a drive voltage of 10.8 V. The improved performance of the device in comparison to that of poly(9,9-dioctylfluorene) is attributed to its better electron injection and transport in PF-OXD and the efficient energy transfer from the OXD side chain to the poly(fluorene) main chain.89 1.4.2.5
NVK/PBD
The combination of PBD as an electron transporting compound with PVK finds frequent application. The direct attachment of PBD to the backbone of poly(p-phenylene vinylene) (PPV) enhances the electroluminescent efficiency. This is attributed to a more facile electron injection and an enhancement of the electron transporting properties of the polymer.90
32
High Performance Polymers
Multilayer devices use PBD and tris-(8-hydroxyquinoline)-aluminum (Alq3 ) with 1-benzothiazol-3-phenyl-pyrazoline as blue dye, which has an emission peak at 445 nm. Alq3 enhances the electron injection and luminous efficiency.91 1.4.2.6
Functionalized Polymers
Cyano-groups. Polymers made from 2,5-bis-(2-thienyl-1-cyanovinyl)1-(2 -ethylhexyloxy)-4-methoxybenzene (α -TPT) and 2,5-bis-(2-thienyl2-cyanovinyl)-1-(2-ethylhexyloxy)-4-methoxybenzene (β -TPT) and then blended into poly(methyl methacrylate) (PMMA) and PVK show highly different optical properties, even when the structures are very similar.92 The PL emission maximum of α -TPT is blue-shifted in comparison to that of β -TPT. The intensity of PL of β -TPT is stronger than that of α -TPT. Anthracene Groups. A homopolymer from 9-(4-vinylphenyl)anthracene emits green light originating from the excimer of the anthracene units. Fluorescent vinyl polymers containing 9-phenylanthracene pendants were synthesized and examined as an emitter layer in organic electroluminescent devices. The single layer polymer EL device uses the homopolymer. On the other hand, a blue emission can be observed from a copolymer with vinylcarbazole.93 Side Chain Carbazole Groups. Poly[2-(carbazol-9-yl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] has both pendant carbazole and 2-ethylhexyloxy groups. Poly[2-(carbazol-9-yl)-1,4-phenylene vinylene] has one pendant carbazole group. LEDs from these materials emit yellow-green light and green light, respectively. In comparison to PPV, the efficiency is drastically enhanced.94 Copolymers from 2-(carbazol-9-yl)ethyl methacrylate and 3-phenyl7-methacryloyloxyethoxy-1-methyl-1H-pyrazolo[3,4-b]-quinoline can be fabricated into LEDs that emit blue light.95 1.4.2.7
Metal Complexes
Europium Complexes. The europium complex can be built via a methacrylate unit into the backbone of a polymer chain,8, 9 as shown in Figure 1.10. Similarly, a europium complex can be attached to vinylbenzoate,
Carbazole Polymers
33
CH3 CH2
C C
N
O
O Eu
N
O O CH3 2
Figure 1.10: Europium Complex8, 9
which in turn can be copolymerized with NVK, however, intended for the use of memory devices.96 PS films containing the electron transporting organic molecule PBD and small amounts of TPD exhibit energy transfer to europium complexes, but not to samarium complexes.97 Iridium Complexes. PVK can be doped with a phosphine cyano iridium(III) complex. The complex shows a blue emission at both 467 and 496 nm caused by triplet transitions of states built between metal and ligand and those built in the ligand only.98 An iridium complex with the ligands of N,N-di(4-tert-butylphenyl)-4-(2-pyridyl) phenylamine and acetylacetone shows green phosphorescence at 533 nm in a blend of PVK and PBD.99 A maximum external quantum efficiency 10% photons per electron at a current density of 32 mA cm−2 is achieved. Another green dye is tris(2-phenylpyridine)iridium,100 whereas tris(1-phenylisoquinoline) iridium is a red emitting dye.101 High efficiency white light-emitting devices can be constructed from PVK as the hole conductor, utilizing multiple doping.102, 103 As an electron transporting compound, 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-phenylene has been used. The dyes are bis-((4,6-difluorophenyl)-pyridinato-N,C-2)(picolinato)Ir(III) for the blue light emission, and for the red light emission bis(2-(2 -benzothienyl)-pyridinato-N,C-3)(acetylacetonate)Ir(III).102 When multiple dyes are present in the matrix, interactions may occur.
34
High Performance Polymers
Blending green emitting fac-tris(2-phenylpyridyl)Ir(III) (Ir(ppy)3 )∗ and red emitting bis[2-(2 -benzothienyl)-pyridinato-N,C-3](acetylacetonate)Ir(III) simultaneously in a PVK host, causes a nearly two-fold efficiency of the red emission due to resonant energy transfer.104 With Ir(ppy)3 as dye, in a layered structure, using water soluble materials, the performances are improved. Further, these materials facilitate the fabrication of LEDs utilizing a wet process.105 Ruthenium Complexes. Tris(2,2 -bipyridyl-4,4 -diphenyl) ruthenium(II), tris(2,2 -bipyridyl-4,4 -dimethyl) ruthenium(II), and tris(1,10-phenanthroline) ruthenium(II) are red fluorescent dyes.106 They can be incorporated in a PVK a matrix as a hole-transporting material and PBD as electron transporting material. Bright red is emitted when the concentration of the dye in the PVK matrix is appropriately adjusted in a LED device with the layers ITO/(dye PVK PBD)/Mg/Ag. The ruthenium complex dyes show their electroluminescence in regions of higher wavelength, than they absorb (Stokes’ shift). Because of the large Stokes’ shift, there is only a minimum peak overlap area between the absorption and the PL emission. Thus, the phenomenon of emission light reabsorbed by the dye is avoided. Light-emitting devices using ruthenium complexes, such as tris-4,7diphenyl-1,10-phenanthroline ruthenium(II) (c.f. Figure 1.11) as dopant in a PVK-based matrix, have been studied. The device was built up of several layers. First, a PVK doped with the ruthenium complex was spin-coated onto ITO. Then a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline layer was applied for hole blocking. Then, a buffer layer of Alq3 was applied. Finally, a bilayer of LiF and Al was used. The ruthenium complex dopant shows an efficient improvement in device brightness and efficiency in comparison to other devices.107 Further, a tunable device, based on the same ruthenium complex has been described. Osmium complexes. Red light phosphorescence108, 109 emitting devices have been reported using osmium complexes. Efficient red emission was achieved using an in situ polymerized tetraphenyldiaminobiphenyl-containing polymer as the hole-transporting layer and a blend osmium complexes of PVK and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (BPD) ∗ fac:
3 groups occupying the corners of the same face of an octahedron
Carbazole Polymers
N
N
H3C
CH3
N
N
2+
Ru N
35
BCP N N
N N
O O N Ru(dphphen)3
Al
N
O
Alq3
Figure 1.11: Tris-4,7-diphenyl-1,10-phenanthroline ruthenium(II) (Ru(dphphen3 ), 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline , Tris-(8-hydroxyquinoline) aluminum (Alq3 )
as the emitting layer. The emission peaks can be tuned by modifying the nature of the ligands because the emission results from a triplet metalto-ligand charge transfer excited state. Still more efficient is the use of a poly(fluorene) derivative with hole-transporting triphenylamine moieties and electron transporting OXD groups as side chains and a blend of BPD in PVK.109, 110 Platinum complexes. From di[2,5-diphenyl-1,3,4-oxadiazole-C-2 N-3]platinum(II) doped PVK devices with greenish-yellow electrophosphorescence could be obtained.111 A blend of PVK and PBD was used as the host matrix and a bicyclic platinum complex containing the 1,3,4-oxadiazole moiety in 2% doping concentration was added. No emission from either PVK or PBD was observed in the devices. Other Complexes. We summarize other metal complexes that are not extensively dealt with in this section in Table 1.3. Copper Complexes A copper complex is a triplet emitting material. It has been used with PVK and PBD for LEDs. Both Förster and Dexter energy transfer are involved in the device.112
36
High Performance Polymers Table 1.3: Metal Complexes as Dyes
Complex
Color References
Tris(1-phenylisoquinoline) Ir(III) Bis-(2-(2 -benzothienyl)-pyridinato-N,C-3 )(acetylacetonate)Ir(III) Tris(2-phenylpyridine)Ir(III) N,N-di(4-tert-butylphenyl)-4-(2-pyridyl) phenylamine2 acetylacetone Ir(III) Tris(2-phenylpyridyl)Ir(III) Tris-{9,9-dihexyl-2-[phenyl-4 -(-pyridin-2 -yl)]fluorene} Ir(III) Phosphine cyano Ir(III) Bis-((4,6-difluorophenyl)-pyridinato-N,C-2 )(picolinato)Ir(III) Tris-[9,9-dihexyl-2-(pyridinyl-2 )fluorenyl Ir(III) Tris-[2,5-bis-2 -(9,9 -dihexylfluorene) Ir] Copper complex Copper phthalocyanine 4,4 -Dimethyl-2,2 -bipyridine)Re(CO)3 Cl Tris(acetylacetonato)(1,10-phenanthroline) erbium
red
101
red green
102, 113, 114
green green
99
green blue
116
blue various red green blue green IR
102, 113
1.4.2.8
100, 115
113
98
116 116, 117 112 118 119 120
Mixed Dyes
White light-emitting electroluminescent LED can be obtained from devices that are constituted by a simple mixture of green light-emitting and orange light-emitting dyes in blue light-emitting PVK films. Intramolecular proton-transfer (ESIPT) dyes can be combined independently with limited energy transfer by the special properties of the ESIPT system.121 1.4.2.9
Multilayer Light-emitting diode
Bilayer LEDs with two blue light-emitting materials, PVK and poly(2-dodecyl-p-phenylene) (C12O-PPP), can emit blue or white light, depending on the solvent used in the fabrication of the second layer, C12O-PPP. When hexane, which is a non-solvent for PVK is used, the device emits blue light as a single layer device with C12O-PPP. However, if toluene, which is a solvent for both polymers is used, the device emits white light originating from an exciplex emission at the bilayer interface in addition to the exciton emission from C12O-PPP.122 The efficiency varies with
Carbazole Polymers
37
the temperature. At low temperatures, the intensity of the exciton emission becomes dominant over the intensity of the exciplex emission. White light emission can be obtained with multilayer devices of different colors.123 A multilayer device consisting of ITO, PVK 1,1,4,4tetraphenyl-1,3-butadiene/8-(quinolinolate)-aluminum that is doped with 5,6,11,12-tetraphenylnaphthacene emits white light in high brightness and with high efficiency.124 1.4.2.10
Other Chromophores
Pyrazolo-based organic materials belong to the most promising blue electroluminescent and transporting materials. A series of pyrazoloquinoline derivatives have been synthesized for use in LEDs. Their optical properties can be tuned by the modification of the side groups.125 4-Methyl-pyrazolo[3.4-b]quinoline emits at 440–460 nm.126 Phthalocyanine and naphthalocyanine are guest dye dopants suitable for the near infrared (IR) region.127 PVK is used as usual, as the hole transport polymer, Alq3 , or a sulforamide derivative (Al(qs)3 ) is used as the host dye. The absorbance spectra of the guest dyes are significantly different from the emission spectra of the host dyes. However, the high molar absorption of the host dye dopants result in such efficiencies of energy transfer that are comparable to quinacridone or rubrene dopants. Rubrene is 5,6,11,12-tetraphenylnaphthacene. A rubrene layer is inserted between PVK and Alq3 .128
1.4.3 Organic Photorefractive Materials Photorefractivity was discovered by Ashkin and coworkers129 in 1966 with Lithium niobate LiNbO3 and other compounds. Photorefractive materials change their index of refraction when irradiated by light. The change of index of refraction continues from milliseconds to years. The basic aspects of photorefractivity are treated in textbooks.130, 131 The change of refractive index is achieved by a series of steps, including:132 1. Charge generation by laser irradiation, 2. Charge transport, resulting in separation of positive and negative charges, and 3. Trapping of one type of charge (charge delocalization),
38
High Performance Polymers 4. Formation of a non-uniform internal electric field (space-charge field) as a result of charge delocalization, and 5. Refractive index change induced by the non-uniform electric field.
Therefore, good photorefractive properties can be seen only for materials that combine good charge generation, good charge transport, or photoconductivity, and good electro-optical activity. Photorefractive materials have many promising applications, such as high density optical data storage, dynamic holography, optical image processing, phase conjugated mirrors, optical computing, parallel optical logic, and pattern recognition. In 1990, the first organic photorefractive crystal and polymeric photorefractive materials were reported.133 Organic photorefractive materials offer many advantages over the original inorganic photorefractive crystals, such as large optical nonlinearities, low dielectric constants, low cost, lightweight, structural flexibility, and the ease of device fabrication. Other important issues include long shelf life, optical quality, and thermal stability. These types of organic polymers are now emerging as key materials for advanced information and telecommunication technology. Good photorefractive properties depend upon:132 1. Good charge generation, 2. Good charge transport, also known as photoconductivity, and 3. Good electro-optical activity. The photoconductivity is frequently provided by incorporating materials containing carbazole groups or phenylamine groups. NLO properties are provided by including chromophore compounds, such as an azo-type dye, which can absorb photon radiation. The chromophore may also provide adequate charge generation. Alternatively, a sensitizer may be added to provide or boost the mobile charge required for photorefractive properties. Many materials, including a wide range of dyes and pigments, can serve as sensitizers. A photorefractive composition can be made by mixing the molecular components that provide the individual properties required into a host polymer matrix. However, most compositions prepared in this way are not stable over time, because of phase separation or crystallization effects. Therefore, the substitution of low-molecular components
Carbazole Polymers
39
Table 1.4: Chromophores for Photorefractive Formulations Compound
References
1-(2-Ethylhexyloxy)-2,5-dimethyl-4-(4-nitrophenylazo)benzene 2,5-Dimethyl-4-(p-nitrophenylazo)anisole 4-Butoxy-3-propyl-1-(4 -nitrophenylazo)benzene 2-Dicyanomethylene-3-cyano-2,5-dihydrofuran (2,4,7-Trinitrofluorene-9-ylidene)-malononitrile β , β -Diacetyl-4-methoxystyrene
134, 135 136 136 137 138 139
by polymers that bear the active components in the polymer structure is straightforward. A major improvement was the usage of the photoconductive polymer PVK. This allowed the concentration of the charge transport agent to be increased, while completely excluding crystallization of the carbazole groups.140 As chromophore, an azo dye 2,5-dimethyl-4-(p-nitrophenylazo)anisole (DMNPAA) was used, and TNF as a sensitizer. The compositions showed almost 100% diffraction efficiency at a laser intensity of 1 W cm−2 and 90 V μ m−1 biased voltage. However, the response time was slow, >100 ms. To achieve good photorefractivity, materials are typically doped with large concentrations of chromophore, such as 25% or more. The large concentration in turn makes the material prone to crystallization and phase separation because the chromophore is highly polar. To eliminate instabilities caused by phase separation, fully functionalized photorefractive polymers, i.e., polymers, in which both the photoconductivity and the NLO capability are unified, have been proposed. Functionalized photorefractive polymers are collected in Table 1.5. 1.4.3.1
Photorefractive Formulations
Chromophores for photorefractive formulations in PVK are summarized in Table 1.4. Acrylics and Methacrylics. Acrylic and methacrylic polymers with pendent (tricyanovinyl)carbazole groups141 or dicyanovinylidenephenylamine groups142 have been synthesized. TPD acrylate can be polymerized with ethyl-2-bromo-2-methylpropionate as an initiator.132 The polymer serves as a charge transport homo-
40
High Performance Polymers Table 1.5: Functionalized Photorefractive Polymers
Main Backbone
Functional Group
References
Methacrylic Acrylic Dodecyl methacrylate N-Methacryloxypropyl carbazole
Tricyanovinylcarbazole Dicyanovinylidenephenylamine Carbazole methycrylates N-Methacryloxypropyl-3-(p-nitrophenyl)azo carbazole
141 142 143 144
F
+ CHO
N
CHO
N H
N
CN CN 7-DCST
Figure 1.12: Synthesis of 2-(4-azepan-1-yl-benzylidene)-malononitrile
polymer. 2-(4-Azepan-1-yl-benzylidene)-malononitrile (7-DCST) is prepared and serves as a chromophore. The synthesis of 7-DCST is shown in Figure 1.12. From TPD acrylate, 7-DCST and N-ethylcarbazole as a plasticizer and a sensitizer, a photorefractive composition has been prepared by dissolving the components in toluene, mixing and eventually removing the solvent. Functionalized photorefractive poly(methacrylate)s were synthesized consisting of 2-(carbazol-9-yl)ethyl methacrylate and 6-(carbazol-9yl)hexyl methacrylate with infrared sensitivity and different spacer lengths. A series of photorefractive poly(methacrylate)s, containing a Disperse Red-type chromophore and carbazole as the charge transport agent with various spacer lengths, was synthesized and characterized. The photorefractive effect of these materials was studied by four-wave mixing and two beam coupling at 780 nm after sensitization with 1% (2,4,7-trinitroflu-
Carbazole Polymers
41
orene-9-ylidene)-malononitrile (TNFDM). A gain coefficient of Γ = 140 cm−1 at an applied electric field of 60 V μ m−1 and complete internal diffraction at an applied electric field of 52 V μ m−1 . For fully functionalized photorefractive polymers, these values rank among the highest reported up to date.145 A bifunctional methacrylate copolymer with as pendant side chains consisting of N-methacryloxypropyl-3-(p-nitrophenyl)azo carbazole and N-methacryloxypropyl carbazole has a high stability.146 Functionalized methacrylate momomers are obtained, e.g., from methacryloyl chloride and 9-(3-hydroxypropyl)carbazole. Potential applications could be memory devices. Vinyl Compounds. Photorefractive polymers can be prepared by living radical polymerization. 4-Vinyltriphenylamine can be polymerized by a conventional radical catalyst or a 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) like catalyst, such as N-(α -methylbenzyloxy)-2,2,6,6-tetramethylpiperidine.147 A polymer composite with a low glass transition temperature has been described as based on layered photoconductive polymers, namely, poly(p-phenylene terephthalate carbazole)s.148 These polymers consist of a rigid backbone of poly(pentylene terephthalate) with pendant oxyalkyl carbazole groups. When the host polymers are mixed with various dopants, the layers are preserved and their layer distance increases, indicating that all the guest molecules are confined to the nanoscale interlayer space. The compounds are doped with C-60 as the photosensitizer and NLO chromophores diethylaminodicyanostyrene. No plasticizers are added. These composites exhibit very low glass transition temperature, despite the absence of a plasticizer. The materials show excellent photorefractive properties. A photorefractive material for a three dimensional bit optical data storage device in the near infrared range consists of either PVK or PMMA as matrix with DMNPAA and TNF and N-ethylcarbazole (ECZ) in various amounts.149 The two-photon excitation technique was used to achieve rewritable bit data storage in the photorefractive polymer. This results in a quadratic dependence of the excitation on the incident intensity producing an excitation volume that is confined to the focal region in both the transverse and axial directions.
42
High Performance Polymers
Ultrashort-pulsed lasers are effective, but are not a practical solution for an optical data storage system. It is possible to achieve a three dimensional rewritable bit data storage using continuous wave illumination. With this technology an information density of 88 G bit cm−3 could be achieved. A polymer composite of PVK/TNF, doped with DMNPAA can still be improved by modifying the structure of DMNPAA. DMNPAAs modified with certain alkyl substituents have fast orientational response to an external electric field and keep large anisotropy in polarizability.150 4-Butoxy-3-propyl-1-(4-nitrophenylazo)benzene has the shortest reorientation time constant of 19 ms and photorefractive time, which are 2,300 times and 63 times faster than those of a simple DMNPAA composite. The fast reorientational response results from the improvement of the dispersivity in the polymer composites and the decrease of the glass transition temperature. Fast response photorefractive materials have been described that are based on a bis-triarylamine side chain polymer matrix with a low ionization potential.151 In comparison to PVK-based composites, composites based on poly[1,4-phenylene-1,2-di(4-benzyloxyphenyl)vinylene] show superior steadystate performances.152 It is believed that this performance is caused by the comparatively larger internal free volume. Further, conjugated PPV homopolymers show a higher hole-drift mobility than PVK.
Poly(siloxane)s. An alternative to PVK is the hole-transporting polymer poly(methyl-bis-(3-methoxyphenyl)-(4-propylphenyl)amine)siloxane. It has been doped with the photorefractive chromophore 4-di(2-methoxyethyl) aminobenzylidene malononitrile. The low intrinsic glass transition temperature of the siloxane polymer allows the preparation of samples without additional plasticizers. The composites exhibit good chromophore orientational mobility and exhibit photorefractive response times in the millisecond range.153
CdSe-Based Nanoparticles. 1-Hexadecylamine capped CdSe nanoparticles or CdSe/ZnS core-shell nanoparticles in a polymer composite comprising PVK and 1-(2-ethylhexyloxy)-2,5-dimethyl-4-(4-nitrophenylazo)benzene as chromophore serve as sensitizers.134, 135
Carbazole Polymers
43
Colloidal Gold. A photorefractive polymer composite composed from PVK, TNF, 4-(dicyanovinyl-N,N-diethylaniline), and gold particles, exhibited an effective enhancement on the photorefractivity.154 It is suggested that the enhancement on the photorefractivity is due to the increment of the density of the effective trap center by doping with gold particles. Fullerenes. Fullerenes exhibit a high optical nonlinearity. This is caused by the difference in polarizabilities of fullerene molecules and their anion radicals, which are formed upon absorption of photons and charge transfer by PVK molecules.155 Trinitrofluorenone attached to C-60 fullerenes acceptors has in PVK composites a similar photorefractive performance than that of C-60, but exhibits shorter response times, with slightly lower gain coefficients. However, larger voltages can be applied, resulting in larger gain coefficients.156 Functionalization. PVK can be functionalized with cyanoacetylated Disperse Red 1 (DR-1). In this way, PVK-based NLO polymers with a high density of chromophores and improved comprehensive properties are obtained. In the first step, PVK is formulated up to 52%. Then the pendent formyl groups are condensed with cyanoacetylated DR-1.157 Plasticizers. PVK polymers can be modified with ECZ as a plasticizer to reduce the glass transition temperature. However, they may suffer from a thermal instability due to the crystallization of the additives. It has been found that the addition of a dimeric analog of PVK, 1,3-biscarbazolyl propane (BisCzPro), is very effective.158, 159 By the replacement of ECZ with BisCzPro or ECZ/BisCzPro mixtures, the glass transition temperature and the temperature dependence of the diffractivity can adjusted by changing the amount and the ratio of the additives. The film remains transparent at high temperatures. Stability. Polymer composites doped with 2,5-dimethyl-4-(4 -nitrophenylazo)phenyl benzyl ether are phase stable, while composites doped with 2,5-dimethyl-4-(4-nitrophenylazo)phenyl octyl ether show a critical concentration of 47%. Above this concentration, the composite rapidly degrades due to the crystallization of the dye.160
44
High Performance Polymers
1.4.3.2
Holograms
Erasable holograms can be produced utilizing the photorefractive effect by forming a light-induced charge redistribution in a NLO material. Local changes in the index of refraction are produced so that dynamic, erasable holograms can be formed, which diffract visible light. The photorefractive effect is achieved by exposing the material to an optical intensity pattern consisting of bright and dark regions, such as formed by interfering two coherent laser-writing beams. Mobile charge generated in the material migrates to the appropriate region to form internal space charge electric fields, which diffract the light during readout with a reading bee in accordance with the electro-optic effect.161 The effect was recognized to be useful for storing volume phase holograms. The mechanism of the photorefractive effect may be explained as follows: crystals are illuminated with a light pattern, e.g., the interference pattern of two laser beams. As a result, charges are excited in the bright areas of the defects in the conduction band or valence band, redistributed, and recaptured preferentially in the darker areas. Space charge patterns are formed, which modulate the refractive index via the electro-optic effect. Charge sources and charge traps often may be transition metal ions, which occur in different valence states. Diffusion, the volume photovoltaic effect, and drift in external fields, in space charge fields and pyroelectric fields are known as drive mechanisms for the charge transfer. The first reports of photorefractive liquid crystals occurred in 1994. Since then, the performance of these materials has dramatically improved. Full-field, retroreflective holographic imaging through turbid media has been achieved using a photorefractive polymer composite as a coherence gate.162 The photorefractive devices used, are based on PVK and TNFDM which is doped with the chromophore 1-(2 -ethylhexyloxy)2,5-dimethyl-4-(4 -nitrophenylazo)benzene. There is certain evidence that TNFDM interacts with chromophores by complexation.163 A recording technique of holograms and the non-destructive readout in a photorefractive polymer, utilizes two-photon absorption. The holograms are formed through the photorefractive effect. The technique uses the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into a PVK matrix. The holograms can be fully erased with a pulsed laser beam. However, they are insensitive to continuous wave laser beams with the same wavelength.164 N,N-Diphenyl-7-(2-(4-pyridinyl)-ethenyl)-9,9-di-n-decyl-9H-fluorene-
Carbazole Polymers
45
N N N R R
R=C10H21
Figure 1.13: N,N-Diphenyl-7-(2-(4-pyridinyl)-ethenyl)-9,9-di-n-decyl-9H-fluorene-2-amine
2-amine (AF-50), c.f. Figure 1.13, has been studied as a chromophore in a PVK matrix for read/write applications.165 Information was written onto a sample using the 325 nm line of a continuous wave He-Cd laser. The PVK/ AF-50 undergoes a chemical change upon laser irradiation causing the blue shift in the PL spectrum. IR investigations suggest the formation of a new conjugated system, such as a ketimine. The chemical change appears to be irreversible. A photorefractive polymer consisting of DMNPAA, TNF, ECZ, and PVK has been used for erasable/rewritable three dimensional bit optical data storage under two-photon excitation.166 A three dimensional bit density of 5 Gbits/cm(3) is achieved by pulsed beam illumination at an infrared wavelength of 800 nm in the recording process. Complete erasing of the recording information can be completely erased by ultraviolet illumination. Dual-use chromophore molecules allow to write both irreversible photochromic and erasable photorefractive holographic gratings into the same storage volume.167 At 675 nm, the chromophore undergoes a photochemical reaction in creating irreversible holographic gratings. Later, at longer wavelengths, the storage of erasable photorefractive holograms in the same location can be achieved. The photochemical gratings have a diffusion-limited dark half-life of about two weeks, depending on the glass transition temperature of the composite. The composites consist of PVK. As plasticizers, butyl benzyl phthalate, or tricresyl phosphate are used. The sensitizer and charge generator consists of fullerene C-60 or TNF. The chromophores consist of moieties with the basic structure of 2-(5,5-dimethyl-3-styryl-cyclohex-2enylidene)-malononitrile, c.f. Figure 1.14. The chromophores serve for the formation of efficient photorefractive
46
High Performance Polymers
CN NC
H 3C
CH3 DCPT
CN NC CH2CH3 H 3C
N CH2CH3
CH3 4NEt2DCPT
Figure 1.14: 2-(5,5-Dimethyl-3-styryl-cyclohex-2-enylidene)-malononitrile and Related Chromophores167
gratings and they are photochemically active, probably by 2+2 photochemical reactions, when triplet sensitized. The photorefractive external diffraction efficiency is highly dependent on the glass transition temperature of the composites. A low glass transition temperature favors the efficiency. The following seems to be a general rule: The response times of photorefractive polymer composites are strongly dependent on both the glass transition temperature and the electro-optical chromophore.168 Composites with a glass transition temperature below the temperature of measurement, with varying chromophore content respond in comparable response times of 200–500 ms. However, significant differences occur in composites with a glass transition temperature above the temperature of measurement. In this case, the composites with the highest chromophore content show the best steadystate performance. However, their response time is much slower than that for those containing lower chromophore content. Reversible photorefractive grating and irreversible local photoinduced aggregation grating could be established in a low glass transition temperature polymer composite of PVK, TNF, ECZ, and N-(4-nitrophenyl)-1-prolinol.169
Carbazole Polymers
47
1.4.4 Photovoltaic Devices The photovoltaic process is just the reverse process utilized in light-emitting devices. Thus, the experience gained in LEDs is useful in the development of photovoltaic devices. By light absorption, molecules are excited from the ground state in the excited state, an exciton. The exciton may decay by several mechanisms that are not useful to generate electric power. However, an exciton may dissociate into a charge pair. This process is responsible for the photovoltaic effect. The photocurrent occurs by the discharge of the electric charge at the electrodes. The transport of charges is affected by recombination reactions during the migration to the electrodes, particularly if the same material serves as transport medium for both electrons and holes. Interactions with atoms or other charges may slow down the migration and thus limit the current.170 There are several reasons to use organic materials for photovoltaic solar cell applications. Most important is the peculiar advantages of organic materials:170 1. Organic materials can be processed easily by spin-coating, by doctor blade techniques, or by evaporation through a mask. 2. The amounts needed are comparatively, relatively small. The production process of organic materials is easier than the production of inorganic materials. 3. Organic materials can be tuned chemically in order to adjust the desired properties, such as the band gap, solubility, etc. A vast variety of chemical structures and functionalities of organic materials is basically available. Polymer photovoltaic cells are interesting not only for traditional applications, such as electric power generation, but also for large area, digital image sensing.171 Certain conjugated polymers, dyes, and molecular organic glasses are known, which show semiconducting properties. The absorption coefficient of organic materials is much higher in comparison to silicon so that only about 100 nm are necessary to absorb between 60 and 90%, if a reflecting backside is used. The high absorption coefficient opens the possibility for the production of very thin solar cells
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High Performance Polymers
with a minimum need of material. Furthermore, organic materials can be tailored to a great extent. It is expected that organic photovoltaic devices can be manufactured in large scale amounts utilizing low-temperature processes at low costs.172 In contrast to inorganic photovoltaic devices, the penetration depth of organic devices is small. A major disadvantage of polymer photovoltaic cells is the low photoinduced current, due to the low carrier mobility and short exciton migration distance. However, the electrical current for polymer photovoltaic cells can be significantly enhanced by adding a small amount of ionic solid electrolyte.173 It is well known that the solar emission spectrum is not monochromatic. One of the major challenges is to optimize photovoltaic devices to match the solar emission spectrum. The internal photon to current efficiency is defined as
ηi = Imax G λ h c e
Imax hc . Gλ e
(1.3)
Short circuit current Illumination Wavelength Plank’s constant Velocity of light Elemental electric charge
The external photovoltaic yield ηe is defined as the ratio of the maximum electric power Pmax collected, to the illumination G times the surface S of the device. Pmax ηe = (1.4) GS The external photovoltaic yield can be expressed in terms of the photogeneration yield φ , fill factor f , as ηe = φ A f eVmax /hν . From these data the maximum possible efficiency can be estimated. The photogeneration yield φ can be assumed to be approximately 1, as is also the fill factor f . The ratio of created electron energy and absorbed radiation energy eVmax /hν is ca. 1/4. Assuming that the sunlight absorption A is 0.5, a maximum external photovoltaic yield can be estimated in the range of 10%. Important parameters to achieve this efficiency are the exciton diffusion length and the charge mobility.170
Carbazole Polymers
49
Molecules that exhibit donor functions include chlorophyll, phthalocyanine complexes, and perylene. Examples for molecules with acceptor functionality are fullerenes and MEH-PPV. Fullerenes are efficient electron acceptors for photo-excited polymers. They mimic some of the electron transfer steps photosynthesis. Conversion efficiencies of almost 4% have been achieved by blending polymers with fullerene derivatives, cadmium selenide, and titanium dioxide.174 Fullerenes suffer from a photochemical reaction in the presence of oxygen.175 Interpenetrating networks with modified fullerenes have been used.176 Interpenetrating networks can be produced by co-evaporation of fullerenes and molecular dyes such as zinc phthalocyanine. Comparably small efficiencies of 1.05% for solar cells have been reported.177 Dyad systems are addressed as electron acceptor molecules covalently linked to photoactive donors. Fullerenes as electron accepting units in combination with phthalocyanines as electron donors appear particularly promising. In organic solar cells consisting of zinc phthalocyanine and a perylene pigment it was possible to raise the short-circuit current by a factor of 1.5 when increasing the partial pressure of oxygen by a factor of three in comparison to that of ordinary atmosphere.178 The structure of a metal phthalocyanine complex is shown in Figure 1.15. The perylene pigment, e.g., perlenetetracarboxylic-bis-benzimidazole is added to extend the absorption spectrum.179 An energy conversion efficiency under standard illumination (100 mW cm−2 ) up to 1.9% could be achieved. Long living photoinduced charge separation with a lifetime of 200 ns) in the solid state have been reported.180 The long lifetime of the chargeseparated state suggests to collect the charges at suitable electrodes and to use such electron transfer systems in organic photovoltaic applications. A low band gap copolymer consists of poly(2,7-(9,9-dioctyl)fluoreneco-5,5 -(4,7-diselenophenyl)-2,2-yl-2,1,3-benzothiadiazole). The optical band gap of this type of copolymer is very low, e.g., 1.87 eV for a copolymer obtained from substituted fluorene and 4,7-diselenophen-2 -yl2,1,3-benzothiadiazole, and 1.77 eV for a similar copolymer from 4,7-diselenophen-2 -yl-2,1,3-benzoselenadiazole. The efficient fast energy transfer from fluorene segments to narrow band gap sites was observed.181 Studies indicate that PFSeBT is a potential polymer functioning as
50
High Performance Polymers
CH3 H3C C CH3
N CH3 H 3C C
N
CH3 N
N
N CH3
Zn N
N N
C CH3 CH3
H3C C CH3 CH3
Figure 1.15: Zinc Phthalocyanine Complex182
an electron donor in polymer photovoltaic cells.183, 184 The devices have a spectral response up to 680 nm. An open-circuit voltage of 1.00 V and a short-circuit current density of 4.42 mA cm−2 is achieved. The energy conversion efficiency is 1.67%. Conjugated copolymers from 9,9-dioctylfluorene and 4,7-di-2-thienyl-2,1,3-benzothiadiazole blended with methano-fullerene [6,6]-Ph C61butyric acid methyl ester show a spectral response up to 650 nm. Further, the open-circuit voltage reaches 0.95 V. The energy conversion efficiency reaches up to 2.24% in a solar simulator. The photovoltaic devices retain their high energy conversion efficiency at high illumination rates. This issue allows the construction of high efficiency modules in combination with a light concentrator.185 Studies of blends of MEH-PPV and fullerene containing conjugated polymers indicate that both polymers contribute to the light absorption in photovoltaic cells.186 The energy conversion efficiency for white light is 0.01%. The efficiency in hyperbranched phenylene vinylene polymers can be improved by blending with a small amount of MEH-PPV.187 The combination of MEH-PPV as electron donor and poly(pyridopyrazine vinylene) as
Carbazole Polymers
51
electron acceptor shows under low-intensity monochromatic light an opencircuit voltage of 900 mV.188
1.4.5 Amplified Spontaneous Emission A low-threshold, blue amplified spontaneous emission (ASE) in a statistical copolymer from 9,9-dihexylfluorene-2,7-divinylene-m-phenylene vinylene and p-phenylene vinylene) and its blend with PVK has been reported.189 PVK and CPDHFPV act as donor and as acceptor of the Förstertype excitation energy transfer, respectively. ASE around 400 nm was observed in polymer films of PS and PVK doped up to 20% with the hole-transporting organic molecule TPD.97, 190 Therefore, these films are promising materials for blue-emitting organic diode lasers.
1.4.6 Optical Elements Conventionally, an optical system comprises a plurality of lenses for refracting a light beam. The chromatic aberration is decreased by combining glass materials with different dispersion characteristics. Objective lenses of a telescope, for example, comprise a positive lens using a low dispersive glass material and a negative lens using a high dispersive glass material. These lenses are combined to correct chromatic aberration appearing on an axis. However, when the lens configuration is restricted or glass materials to be used are limited, sometimes the chromatic aberration cannot be corrected fully. On the other hand, the chromatic aberration can be decreased by mounting a diffraction grating to a lens. The grating can be formed from PVK. PVK as such can be bonded onto a diffraction grating. Since PVK is a very fragile material, even a small load can easily crack it. Moreover, the method is time-consuming and thus not suitable in industrial processes. Another method is to apply PVK by thermoplastic molding. Further, the polymer can be dissolved in a solvent, which is evaporated to form the optical element. Still another method is supplying the monomer, NVK to a mold. Then the monomer is polymerized in the mold by means of a polymerization reaction to convert it into PVK. The polymerization reaction is a thermal polymerization between 70 °C and 130 °C. Photopolymerization is also possible, e.g. by means of 1-hydroxycyclohexyl-phenyl-ketone.191
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High Performance Polymers
Table 1.6: Examples for Commercially Available Poly(N-vinylcarbazole) Polymers Tradename
Producer
Luvican® Polectron®
BASF General Aniline and Film Corporation POLICARB
References 3 3 3
1.4.7 Antistatic Polymer PVK or its nitrate derivatives can be converted into semiconductor materials by inserting photosensitive coloring units, which are capable of dissipating the static charge. The static charge may be generated during manipulations of the materials.192
1.4.8 Other Applications 1.4.8.1
Microscopic Imaging
PVK is used for polymer-coated glass substrates for deoxyribonucleic acid (DNA) stretching and fixation. Thus, the precise gene location on DNA can be obtained utilizing fluorescence microscopy and atomic force microscopy (AFM). Partially stretched and aggregated DNA molecules are observed on uncoated glass. This suggests that DNA interacts rigidly with the surface due to the strong polarity of the glass surface. Only few DNA molecules are fixed on a poly(vinyl butyral) coated glasses. PVK and poly(phenazasiline) coated surfaces sufficiently fix and stretch the DNA molecules. Such coated surfaces provide an adequate AFM observation of the stretched DNA molecules. A specific interaction is attributed to the π -stacking between the aromatic amines in the polymers and the base pairs in the DNA molecules.193
1.5 SUPPLIERS AND COMMERCIAL GRADES Examples for commercially available grades and tradenames are shown in Table 1.6.
Carbazole Polymers
53
1.6 SAFETY NVK is harmful if swallowed or absorbed through the skin. Moreover, NVK is suspected to cause cancer. PVK is a stable substance. However, it is incompatible with strong oxidizing agents. Its toxicology has not been fully investigated.194
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162. P. Dean, M. R. Dickinson, and D. P. West. “Full-field coherence-gated holographic imaging through scattering media using a photorefractive polymer composite device.” Appl. Phys. Lett., 85(3):363–365, July 2004. 163. E. Hendrickx, Y. D. Zhang, K. B. Ferrio, J. A. Herlocker, J. Anderson, N. R. Armstrong, E. A. Mash, A. P. Persoons, N. Peyghambarian, and B. Kippelen. “Photoconductive properties of PVK-based photorefractive polymer composites doped with fluorinated styrene chromophores.” J. Mater. Chem., 9(9):2251–2258, September 1999. 164. P. A. Blanche, B. Kippelen, A. Schulzgen, C. Fuentes-Hernandez, G. Ramos-Ortiz, J. F. Wang, E. Hendrickx, N. Peyghambarian, and S. R. Marder. “Photorefractive polymers sensitized by two-photon absorption.” Opt. Lett., 27(1):19–21, January 2002. 165. R. Sivaraman, S. J. Clarson, B. K. Lee, A. J. Steckl, and B. A. Reinhardt. “Photoluminescence studies and read/write process of a strong two-photon absorbing chromophore.” Appl. Phys. Lett., 77(3):328–330, July 2000. 166. D. Day and M. Gu. “Use of two-photon excitation for erasable-rewritable three-dimensional bit optical data storage in a photorefractive polymer.” Opt. Lett., 24(14):948–950, July 1999. 167. K. D. Harris, R. Ayachitula, S. J. Strutz, L. M. Hayden, and R. J. Twieg. “Dual-use chromophores for photorefractive and irreversible photochromic applications.” Appl. Opt., 40(17):2895–2901, June 2001. 168. R. Bittner, C. Brauchle, and K. Meerholz. “Influence of the glass-transition temperature and the chromophore content on the grating buildup dynamics of poly(N-vinylcarbazole)-based photorefractive polymers.” Appl. Opt., 37 (14):2843–2851, May 1998. 169. F. Wang, B. Zhang, Q. H. Gong, P. Wang, Z. M. Feng, and C. Ye. “Reversible and irreversible index gratings in a NPP-doped low-t-g polymer composite.” J. Mod. Opt., 48(1):47–53, January 2001. 170. J.-M. Nunzi. “Organic photovoltaic materials and devices.” Compt. Rendus Phys., 3(4):523–542, 2002. 171. G. Yu, G. Srdanov, J. Wang, H. Wang, Y. Cao, and A. J. Heeger. “Large area, full-color, digital image sensors made with semiconducting polymers.” Synth. Met., 111-112:133–137, 2000. 172. A. Goetzberger, C. Hebling, and H.-W. Schock. “Photovoltaic materials, history, status and outlook.” Mater. Sci. Eng., R, 40(1):1–46, January 2003. 173. F.-C. Chen, Q. Xu, and Y. Yang. “Enhanced efficiency of plastic photovoltaic devices by blending with ionic solid electrolytes.” Appl. Phys. Lett., 84:3181–3183, 2004. 174. K. M. Coakley and M. D. McGehee. “Conjugated polymer photovoltaic cells.” Chem. Mater., 16:4533–4542, 2004. 175. F. C. Krebs, J. E. Carle, N. Cruys-Bagger, M. Andersen, M. R. Lilliedal, M. A. Hammond, and S. Hvidt. “Lifetimes of organic photovoltaics: Photochemistry, atmosphere effects and barrier layers in ITO-
Carbazole Polymers
176.
177.
178.
179.
180.
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184.
185.
186.
187.
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MEHPPV:PCBM-aluminium devices.” Sol. Energ. Mater. Sol. Cells, 86: 499–516, 2005. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger. “Polymer photovoltaic cells: Enhanced efficiencies via a network of internal donor-acceptor heterojunctions.” Science, 270:1789–1791, December 1995. J. Rostalski and D. Meissner. “Monochromatic versus solar efficiencies of organic solar cells.” Sol. Energ. Mater. Sol. Cells, 61(1):87–95, February 2000. H. R. Kerp and E. E. van Faassen. “Effects of oxygen on exciton transport in zinc phthalocyanine layers.” Chem. Phys. Lett., 332(1-2):5–12, December 2000. D. Gebeyehu, M. Pfeiffer, B. Maennig, J. Drechsel, A. Werner, and K. Leo. “Highly efficient p-i-n type organic photovoltaic devices.” Thin Solid Films, 451-452:29–32, March 2004. M. A. Loi, P. Denk, H. Hoppe, H. Neugebauer, C. Winder, D. Meissner, C. Brabec, N. S. Sariciftci, A. Gouloumis, and a. Vazquez et. “Long-lived photoinduced charge separation for solar cell applications in phthalocyanine-fulleropyrrolidine dyad thin films.” J. Mater. Chem., 13(4):700–704, April 2003. R. Yang, R. Tian, J. Yan, Y. Zhang, J. Yang, Q. Hou, W. Yang, C. Zhang, and Y. Cao. “Deep-red electroluminescent polymers: Synthesis and characterization of new low-band-gap conjugated copolymers for light-emitting diodes and photovoltaic devices.” Macromolecules, 38:244–253, 2005. H. Neugebauer, M. A. Loi, C. Winder, N. S. Sariciftci, G. Cerullo, A. Gouloumis, P. Vazquez, and T. Torres. “Photophysics and photovoltaic device properties of phthalocyanine-fullerene dyad:conjugated polymer mixtures.” Sol. Energ. Mater. Sol. Cells, 83(2-3):201–209, June 2004. R.-Y. Tian, R.-Q. Yang, Q.-M. Zhou, J.-B. Peng, and Y. Cao. “Performances of photovoltaic cells based on Se-containing low-bandgap polymer.” Huanan Ligong Daxue Xuebao, Ziran Kexueban (Journal of South China University of Technology), 33:6–9, 18, 2005. R.-Y. Tian, R.-Q. Yang, J.-B. Peng, and Y. Cao. “Efficient photovoltaic cells from low band-gap fluorene-based copolymer.” Chin. Phys., 14:1032–1035, 2005. Q. Zhou, Q. Hou, L. Zheng, X. Deng, G. Yu, and Y. Cao. “Fluorenebased low band-gap copolymers for high performance photovoltaic devices.” Appl. Phys. Lett., 84:1653–1655, 2004. C. Yang, H. Li, Q. Sun, J. Qiao, Y. Li, Y. Li, and D. Zhu. “Photovoltaic cells based on the blend of MEH-PPV and polymers with substituents containing C60 moieties.” Sol. Energ. Mater. Sol. Cells, 85:241–249, 2004. J. Qiao, C. Yang, Q. He, F. Bai, and Y. Li. “Hyperbranched conjugated polymers for photovoltaic applications.” J. Appl. Polym. Sci., 92:1459–1466, 2004.
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188. F. Zhang, M. Jonforsen, D. M. Johansson, M. R. Andersson, and O. Inganas. “Photodiodes and solar cells based on the n-type polymer poly(pyridopyrazine vinylene) as electron acceptor.” Synth. Met., 138:555–560, 2003. 189. T. W. Lee, O. O. Park, D. H. Choi, H. N. Cho, and Y. C. Kim. “Low-threshold blue amplified spontaneous emission in a statistical copolymer and its blend.” Appl. Phys. Lett., 81(3):424–426, July 2002. 190. M. A. Diaz-Garcia, S. F. De Avila, and M. G. Kuzyk. “Dye-doped polymers for blue organic diode lasers.” Appl. Phys. Lett., 80(24):4486–4488, June 2002. 191. H. Ukuda and M. Ohgane. Optical element and method of manufacturing the same. EP Patent 1 342 557, assigned to Canon KK (Jp), September 10, 2003. 192. G. F. Zamora, C. M. C. Gonzalez, and M. A. D. Franco. Process for obtaining an antistatic polymer based on poly(N-vinylcarbazole). ES Patent 2 092 427, assigned to Univ Pais Vasco; Diputacion Foral De Guipuzcoa, November 16, 1996. 193. H. Nakao, H. Hayashi, T. Yoshino, S. Sugiyama, K. Otobe, and T. Ohtani. “Development of novel polymer-coated substrates for straightening and fixing DNA.” Nano Lett., 2(5):475–479, May 2002. 194. The physical and theoretical chemistry laboratory. Chemical and other safety information. Safety web pages, Oxford University, Oxford, 2006. [electronic] http://ptcl.chem.ox.ac.uk/MSDS/.
2 Poly(p-xylylene)s The discovery of poly(p-xylylene)s (PPX)s is attributed to M. Szwarc at around 1947.1 He found that the pyrolysis of p-xylene produces the p-xylyl radical. This radical disproportionates into a more stable p-quinodimethane diradical. The diradical is somehow stable in the gas phase, but not in the liquid phase. An insoluble polymer is formed with a softening point at 175°C. Superficially, Szwarc was interested in the bond strength of aromatic hydrogens,2 however, he wrote a review on the topic,3 although he probably became more famous for living polymers. The deposition process was then improved and commercialized by William Gorham at Union Carbide.4, 5 Gorham used cyclophanes to increase the yield of polymers. In 1968 the licence was transferred to Para Tech Coating, Inc. which developed the process further. Meanwhile, Parylene is a trademark used by several companies. The history of Parylene is given in the internet.6 Basically, PPX can be regarded as a special case of poly(p-phenylene alkylene) polymers. However, polymers with other alkyl spacers have not been investigated very much and have been termed as a forgotten class of polymers.7 We emphasize that PPX contains the phenylene group Φ and the ethylene group in the backbone, i.e. −Φ−CH2 −CH2 −, which is identical to −CH2 −Φ−CH2 −. A few papers deal with the phenylene group and the methylene group as a repeating unit of the backbone, namely −Φ−CH2 −, which is an incomplete notation. The latter polymer is referred to as poly(p-phenylene methylene). Rarely, PPX polymers are abbreviated as poly(p-phenylene ethylene) (PPE). However, the acronym 69
70
High Performance Polymers Table 2.1: Monomers for PPX Monomer
References
p-Xylene [2.2]Paracyclophane 4-Vinylbiphenyl 4-Ethyl[2.2]paracylophane (Parylene E) Amino[2.2]paracyclophane Dicyano[2.2]paracyclophane 4-Carboxyl[2.2]paracyclophane α , α , α α -Tetrafluoro-p-xylylene Dichloro-[2.2]paracyclophane 3-(5-Phenylpentyl)-4-methylbenzyl chloride Dichloro tetrafluoro-[2.2]paracyclopane 1,4-Bis(trifluoromethyl)benzene (Parylene F) 1,1,2,2,9,9,10,10-Octafluoro[2.2]paracyclophane 1,4-Bis(phenoxymethyl)benzene 1,4-Bis[(phenylmethoxy)methyl]benzene p-Xylylene diacetate p-Xylylene dipropionate
1 8, 9 10 11 12, 13 14 15 16 17 18 19 20 21 22 22 23 23
PPE is used prevalently for poly(phenylene ether).
2.1 MONOMERS Monomers for PPX are summarized in Table 2.1 and in Figure 2.1. Common precursor monomers belong to the class of cyclophanes. The chemistry of cyclophanes has been reviewed recently24 . [2.2]Paracyclophane can be prepared by the Hofmann elimination of p-methylbenzyltrimethylammonium hydroxide in the presence of 2-imidazolidinone as co-solvent and crown ethers, dimethyl sulfoxide (DMSO) and other compounds as reaction promoters.8, 9 Yields greater than 70% are reported. The synthesis of α , α , α α -tetrafluoro-p-xylylene (TFPX) is relatively costly, time-consuming and not suitable for commercial products16 . TFPX can be obtained from the preparation by mixing the chlorinated analogue with potassium fluoride, and allowing it to react for 12 hours at 260–280°C. Thereby the danger of gelation arises. This issue is reduced, when the reaction is conducted in a solvent such as sulfolane. A quaternary phosphonium
Poly(p-xylylene)s
F CH3
H 3C
F
H C
C H
F F α,α,α′,α′-Tetrafluoro-p-xylylene
p-Xylene H 2C
CH2
F2 C
CF2
H 2C
CH2
F2 C
CF2
[2,2]Paracyclophane ClH2C
Octafluoro-[2,2]paracyclophane CH3 (CH2)5
3-(5-Phenylpentyl)-4-methylbenzyl chloride O H 2C
CH2
O
1,4-Bis(phenoxymethyl)benzene
Figure 2.1: Monomers Used for PPX
71
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High Performance Polymers
NO2 H 2C
CH2
H 2C
CH2
HNO3
H 2C
CH2
H 2C
CH2
Fe3(CO)12
NH2
H 2C
CH2
H 2C
CH2
Figure 2.2: Synthesis of Amino[2.2]paracyclophane12
salt is added as a phase transfer catalyst. A lower temperature of 160°C is applied. However, the reaction time increases up to 48 h. Octafluoro[2.2]paracyclophane is prepared in refluxing tetrahydrofuran (THF) and hexamethylphosphoramide or DMSO solution from 1,4bis(bromodifluoromethyl)benzene. Trimethylsilyltributyltin is used as a reducing agent.25 Cesium fluoride as a catalyst gives superior yields (40%) in comparison to potassium fluoride. To favor the formation of rings, the reaction must be accomplished in highly diluted systems. Octafluoro[2.2]paracyclophane and dodeca-fluoro[2.2]paracyclophane can be alternatively prepared by the treatment of 1,4-bis(halodifluoromethyl)benzene with PbBr2 /Al in N,N-dimethylformamide at room temperature via the a cyclocoupling reaction.26 Amino[2.2]paracyclophane and diamino[2.2]paracyclophane can be accessed by the nitration with trifluoromethanesulfonic acid/nitric acid and reduction of the nitro group with strategy incorporating triiron dodecacarbonyl. F33 (CO)12 is a powerful reductant even under mild reaction conditions.12 Crown ethers are used as a phase transfer catalyst. The reaction scheme is shown in Figure 2.2. Functionalized monomers are used as anchor groups for surface modification in medical applications. Besides cyclophanes, ethers of xylene are suitable for the formation of PPX by chemical vapor deposition (CVD).
Poly(p-xylylene)s
73
Suitable ethers are 1,4-bis(phenoxymethyl)benzene and 1,4-Bis[(phenylmethoxy)methyl]benzene.22 These ethers are more readily available than cyclophanes.
2.2 POLYMERIZATION AND FABRICATION A wide variety of PPX have been synthesized, however only a few materials are commercially sold.
2.2.1 Chemical Vapor Deposition One of the earliest methods for the preparation of polymeric p-xylylene coatings is a high-temperature pyrolysis of p-xylene at 800–1,000°C and subatmospheric pressures, followed by cooling the pyrolysis vapors to a polymerization temperature by condensing the vapors on a cold surface.2, 27 During pyrolysis, reactive radicals are formed that polymerize when cooled down again. The early preparation methods of PPX described in the patent literature in fact followed the method by Szwarc. A process for preparing PPX has been disclosed wherein the vapors of p-xylene were pyrolyzed in the presence of chlorine gas.28 The Gorham process uses [2.2]paracyclophanes as a precursor for deposition.5 The precursor is evaporated at 150°C in vacuo. In the next stage, the vapors are conducted into a pyrolysis chamber at 700°C. Here the monomeric diradical is formed. Then the reactive vapor reaches a deposition chamber at an ambient temperature. The vapor condenses and polymerizes at the cold surfaces. The technique of CVD for polymer has been reviewed by several authors.29, 30 The basic mechanism of polymerization is shown in Figure 2.3. A continuous vapor deposition apparatus for coating objects has been described.31 2.2.1.1
Fluor containing Parylenes
The conventional fabrication process for poly(tetrafluoro-p-xylylene) (Parylene F) is difficult, involving many process steps, and is more expensive than that for Parylene N. Typically, this process first involves the formation of a dimer, 1,1,2,2,9,9,10,10-octafluoro[2.2]paracyclophane. The dimers are cracked at 720–730°C to get the monomer TFPX.
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High Performance Polymers
H 2C
CH2
H 2C
CH2
H 2C
H 2C
CH2
H 2C
CH2
CH2
Figure 2.3: Basic Mechanism of CVD Polymerization32
1,4-Bis(trifluoromethyl)benzene can be used together with traces of α , α -dibromo-α , α , α , α -tetrafluoro-p-xylene as halogen initiator for the production of Parylene F.20, 33 Notably, 1,4-bis(trifluoromethyl)benzene is commercially available. It is synthesized from terephthalaldehyde, in a two step reaction. In the first step, terephthalaldehyde is reacted with sulfur tetrafluoride at 150°C under pressure to give tetrafluoro-p-xylene. In the second step, this compound is brominated by a photolysis reaction in the presence of N-bromo succinimide. Films from this precursor can be produced with a conventional reaction system. It has been found that the optimum surface temperature for deposition is around −15°C. 2.2.1.2
Copolymers
The properties of Parylenes can be tailored by forming copolymers. The fabrication consists of the simultaneous co-evaporation of two precursors with subsequent vapor deposition.10 Suitable comonomers are p-xylylene, chloro-p-xylylene, perfluorooctyl methacrylate, and 4-vinylbiphenyl, and in general [2.2]paracyclophanes functionalized with hydroxy, methoxy, amino, triflate, or trifluoroacetyl groups.34 For example, copolymerization allows the adjustment of the dielectric constant of the product. In light-emitting devices, copolymers can be used to tune the wavelength of the emitted light.35
Poly(p-xylylene)s
CH3
ClH2C
(CH2)n
CH2
H 2C -
75
HCl
(CH2)n
Figure 2.4: Dehydrochlorination of Xylyl Chloride
Table 2.2: Melting and Glass Transition Temperatures of Substituted PPXs18, 36 Substituent
Tm /[°C]
– Phenyl Benzyl Ethylphenyl Propylphenyl Butylphenyl Pentylphenyl tert-Butyl Norbornenyl Trifluoromethyl
Tg /[°C] 420
106 51 32 5 −3 −8 177 136 89
137 86
2.2.2 Solution Polymerization The synthesis of most PPX types by wet chemistry results in intractable materials, since the materials are insoluble. However, by a modification of the monomers by suitable side chains, tractable PPX types can be obtained in liquid state. The substitution of the aromatic backbone ring by alkylphenyl groups is helpful to get soluble PPX types.36, 37 The polymer synthesis using these monomers can proceed in a THF, dioxane, or benzene solution. The polymer is formed by a dehydrochlorination reaction using tert-butyl oxide. The synthesis is shown in Figure 2.4. The dehydrohalogenation reaction is also referred to as the Gilch route. Alkyl-aryl substituted PPX is soluble at ambient temperatures and has a comparatively low melting temperature. The glass transition temperatures of alkylphenyl substituted PPX is shown in Table 2.2.
76
High Performance Polymers Table 2.3: Properties of Parylene N 38
Property Density Water Absorption 24 hrs Coefficient of friction Secant Modulus MD Tensile Strength Yld MD Tensile Strength Brk MD Elongation Yield MD Elongation Break MD Rockwell Hardness (R-Scale) Melting point Specific Heat (20°C) Thermal Conductivity Surface Resistivity Volume Resistivity Dielectric Strength
Value 1.11 0.1 0.25 2410 42.1 58.6 2.5 140 85 420 837 0.13 1.0E+13 1.4E+17 276
Unit
Standard
g cm−3
ASTM D1505 ASTM D570 ASTM D1894 ASTM D882 ASTM D882 ASTM D882 ASTM D882 ASTM D882 ASTM D785
% MPa MPa MPa % % °C J kg−1 K−1 W m−1 K−1 Ω Ω cm kV mm−1
ASTM C351 ASTM C177 ASTM D257 ASTM D257 ASTM D149
A norbornenyl substituted PPX has been successfully prepared. The norbornenyl group is suitable for further functionalization. PPX types with pendent CF3 groups are still soluble, but reach in their thermal stability the neat PPX. In comparison to polymers prepared by CVD, polymers prepared in solution exhibit a much lower polydispersity.36
2.3 PROPERTIES By attaching one chlorine atom to the xylylene ring (Parylene C), the permeability to moisture and other gases, can be significantly reduced. Polymers of dichloro-p-xylylene have better electrical and thermal properties as ordinary PPXs. The introduction of fluorine atoms still improves the thermal resistance.
2.3.1 Mechanical Properties Some properties of a PPX are summarized in Table 2.3. Parylene E, with 69% diethylated and 25% monoethylated p-xylylene groups shows unusual properties among the Parylene group. It is nearly optically isotropic. Therefore this Parylene type is a candidate for optical waveguides.39 It
Poly(p-xylylene)s
77
has a low degree of crystallization after annealing at 150°C. Further, it is soluble in methylene chloride, chloroform, and toluene. Parylene N is stable at temperatures up to 130°C. Parylenes have dielectric constants of 2.35 to 3.15. The dielectric constant decreases as the quantity of fluorine atoms increases within the polymer, therefore, octafluoro[2.2]paracyclophane is a valuable monomer from this aspect.16 PPX (Parylene N) has a low dielectric constant, which is independent of frequency, and has a low dissipation factor, which makes it ideal for high-frequency applications. Parylene coatings are inert and transparent. However, Parylenes exhibit a poor photostability, which makes outdoor applications problematical. UV irradiation in the presence of air results in the oxidation of the Parylene.40 The degradation starts with the formation of aldehyde at low doses. At higher doses, additional carboxylic acid groups are formed. A kinetic model has been set up to explain the gross reaction of photo-degradation.41
2.4 APPLICATIONS The properties and applications of Parylenes have been summarized.42 The major fields of application can be classified into: 1. Electronics, and 2. Medical. In both fields the polymers fulfill coating and protective tasks.
2.4.1 Coatings Parylene is widely used as a protective coating in the field of electronics, aerospace and medical applications. Parylene is formed on surfaces from the gas phase, in contrast to most other coating techniques that use liquid precursor materials. The resulting film is thin and conformal, has no pinholes at sufficient thickness, and is chemically resistant. The coatings impart several properties concomitantly, including electrical insulation, moisture and chemical isolation, mechanical protection, enhanced lubricity. The poor adhesion of the surface rejects dust and soil.
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High Performance Polymers
However, the poor adhesion of the surface does not allow a further painting stage. The surface of Parylene coatings can be modified by a special plasma coating. In this way, an interlayer is placed to provide good adhesion to the substrate as well as to a subsequent primer. After this procedure, the hydrophobic Parylene polymer again becomes paintable with both solventborne and waterborne spray primers.43 Pinholes that are permeable to ions can be detected by cyclovoltametric measurements. Films deposited on iron surfaces by the Gorham process show significant ion permeability for film thicknesses below 500 nm, but no permeability at all for films equal to or thicker than 700 nm. The tendency of the formation of pinholes is related to the surface roughness.44 2.4.1.1
Coatings for Solder Powders
Solder powders coated with a thin layer of Parylene exhibit a high degree of resistance to oxidation and to reaction with the flux contained in the solder paste without substantially interfering with the reflow characteristics of the solder. The powders are used as such or as solder paste. The preferred Parylene type is Parylene E, made from 4-ethyl[2.2]paracylophane since it melts below the melting point of the solder, i.e., about 180°C.45 The formulation of a typical solder pastes are shown in Table 2.4. 2.4.1.2
Waveguide Coatings
Waveguides are designed to confine and direct the propagation of light waves. Significant gains in performance can be made when highly reflective materials are used in combination with optically transmissive materials. Silver has a reflectance of about 98% over the entire visible light spectrum at normal incidence. In comparison, aluminum, a more commonly used reflective layer material, possesses a reflectance of about 93% at normal incidence. Therefore, silver is superior as a reflective material in such devices. Although silver possesses excellent optical characteristics, there are several problems associated with the use of the reflective metal. Silver has a tendency to undesirably tarnish when exposed to the atmosphere, especially in the presence of corrosive gases and contaminants, including sulfur dioxide, hydrogen sulfide, nitrogen dioxide, ozone, hydrogen chloride, chlorine, and organic acids.
Poly(p-xylylene)s
Table 2.4: Formulations of Solder Pastes45 Component
% by weight
Paste 1 Coated solder powder Dimerized resin Lauric acid Triethanol amine 2-Methyl-1-butanol Hydrogenated castor oil
90.0 3.5 1.0 0.5 4.5 0.5
Paste 2 Coated solder powder Polyethylene glycol 2-Methyl-1-butanol Betaine hydrochloric acid Ethyl cellulose Hydrochloric acid
90.0 4.0 4.8 0.5 0.5 0.2
Paste 3 Coated solder powder Rosin derived ester resin Terpineol 2-Methyl-1-butanol Lauric acid Monoethanolamine Hydrogenated castor oil
90.0 4.0 2.0 2.0 0.5 0.5 1.0
79
80
High Performance Polymers
Parylene protective layer Reflective layer Adhesive-promoting layer
Cladding Core
Figure 2.5: Fiber Optic Waveguide (Schematically)46
To protect the reflective layer from exposure to the ambient atmosphere, corrosive substances, salt, humidity, etc., a Parylene polymer protective layer is used.46 A cross section through a fiber optic waveguide is shown in Figure 2.5. The fiber optic waveguide comprises a: • Core composed of an optically transmissive glass or polymer material, • Cladding composed of an optically transmissive glass or polymer material with a lower refractive index than the core, • Optional adhesive-promoting layer, • Reflective layer, and • Parylene polymer protective layer overlaying the reflective layer. 2.4.1.3
Reinforcement Layers
Micromachining techniques originate from microelectronics, where small features in a silicon wafer are placed. Membrane particle filters can be fabricated by micromachining technologies. The filters are fabricated using a substrate membrane that is perforated with holes. The holes can have different shapes, such as circular, hexagonal, and rectangular. The dimensions range from 6–13 μ m. In order to improve the mechanical properties of the filter, a layer of Parylene material is uniformly coated on the filters and on
Poly(p-xylylene)s
81
Silicone nitride Silicone substrate
Parylene Coating
Figure 2.6: Fabrication of a Parylene Coated Filter by Micromachining Techniques
the inner surfaces of the holes.47 The basic filter material is made of silicone nitride. This material is placed on a silicone substrate for fabrication purposes. The steps of fabrication are shown in Figure 2.6. 2.4.1.4
Printed Circuit Boards
Printed circuit boards that are operated in an aggressive environment need special corrosion protection. A particularly acute need exists in commercial utility meters, such as natural gas meters, water meters, or electric meters. In addition, there may be the demand for repair and reapplication
82
High Performance Polymers Table 2.5: Parylene Coatings in Medical Devices42 Application
References
Catheter mandrels Laproscopic and endoscopic devices Prosthetic components Wound closure devices Stents Blood-handling components Catheter balloons Needles
48, 49 50 51, 52 53–55
or recovering of the board, components, and component leads. A coating method has been described that consists of two steps. The first deposition step comprises depositing Parylene and the second deposition step comprises a corrosion-inhibiting viscous fluid.56 Parylene is applied commonly by vacuum deposition.
2.4.2 Medical Applications In a wide variety of medical devices, Parylenes are used, mainly as protective coatings. In Table 2.5 some devices are summarized.
2.4.2.1
Surface Modification
The variability in functional groups that can be prepared by CVD polymerization opens the field of surface engineering of microfluidic devices.57 Insulin has been immobilized on CVD-coated surfaces in order to enhance the attachment and the growth of cells under in vitro conditions. Copolymers with amino-p-xylylene and p-xylylene moieties bear pendant amino groups that may act as anchors for functionalization. These amino groups can be used for the immobilization of thrombin inhibitors such as R-hirudin. The functionalization is useful for devices that are in contact with native blood. A potential field of applications is stents with reduced restenosis, i.e., a reduced tendency of blocking the blood flow.13
Poly(p-xylylene)s
83
Table 2.6: Examples for Commercially Available Parylene Polymers Tradename
Producer
Remarks
Parylene C
Specialty Coating Systems (SCS) Specialty Coating Systems (SCS) Specialty Coating Systems (SCS) Specialty Coating Systems (SCS) Para Tech Coating, Inc. Advanced Surface Technology, Inc. Daisan Kasei Co., Ltd. Alpha Metals, Inc. Solvay
Chlorinated type
Parylene D Parylene N Parylene HT Parylene (dimers) Parylene (dimers) [2.2]Paracyclophane Parylene (dimers) Primospire
2.4.2.2
Dichlorinated type Standard polymer Fluorinated type
Benzoyl-substituted
Drug Release
A poly(amino-p-xylylene) coated functional surface was used to immobilize a polymeric drug release system consisting of poly(N-isopropylacrylamide)-co-poly(acrylic acid). Into the drug release system, the thrombin inhibitor R-hirudin was incorporated.58 The fabrication of implantates, such as stents, combined with a bioactive material using Parylene as a coating material has been described in detail.59
2.5 SUPPLIERS AND COMMERCIAL GRADES Since the polymer is created mostly on the fly, the precursor monomers are rather sold than the polymers as such. Often when talking about Parylene, the real meaning refers to the precursor dimers. Various types of Parylenes are sold. There are four forms of Parylene. Parylene N is the polymer of p-xylylene. Parylene C or Galaxyl Parylene C is poly(chloro-p-xylylene). Parylene D is poly(dichloro-p-xylylene). Parylene HT is poly[(2,3,5,6-tetrafluoro-1,4-phenylene)(1,1,2,2tetrafluoro-1,2-ethanediyl)]. Suppliers and commercial grades are shown in Table 2.6. Tradenames appearing in the references are shown in Table 2.7.
84
High Performance Polymers Table 2.7: Tradenames in References
Tradename Description
Supplier
Eulexin® Schering Corp. Nonsteroidal antiandrogen51, 52 Hytrin® Abbott Laboratories Corp. Cardiovascular preparation51, 52 Proscar® Merck Medicinal preparation for treatment of the prostate gland51, 52 ReoPro® Eli Lilly and Co. Glycoprotein IIb/IIIa inhibitor51 Taxol® Bristol-Myers Squibb Co. Antiproliferative preparation51, 52
2.6 SAFETY Not much is known about the toxicity of cyclophanes. p-xylylenes, cyclophanes and halogen substituted p-xylylenes show minor toxic effects.60 However, the polymer class is extensively used in medical applications. The cytotoxicity of poly(2(3)-(4-phenylbutyl)-1,4-phenyleneethylene) has been tested by in vitro experiments and showed a mild toxicity.18
REFERENCES 1. M. Szwarc. “The p-quinodimethane molecule.” Discuss. Faraday Soc., pages 46–49, 1947. 2. M. Szwarc. “The C-H bond energy in toluene and xylene.” J. Chem. Phys., 16:128–136, 1948. 3. M. Szwarc. “Poly-para-xylelene: Its chemistry and application in coating technology.” Polym. Eng. Sci., 16:473–479, 1976. 4. W. F. Gorham. Alkylated di-p-xylylenes. US Patent 3 117 168, assigned to Union Carbide Corp., January 07, 1964. 5. W. F. Gorham. Para-xylylene polymers. US Patent 3 342 754, assigned to Union Carbide Corp, September 19, 1967. 6. Specialty Coating Systems. Parylene knowledge. [electronic] http://www.scscoatings.com/parylene_knowledge/history.cfm, 2007. 7. D. Steiger, T. Tervoort, C. Weder, and P. Smith. “Poly(p-phenylene alkylene)s – a forgotten class of polymers.” Macromol. Rapid Commun., 21:405–422, 2000.
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8. H. Härtner. Process for the preparation of [2,2]-paracyclophane. US Patent 4 532 369, assigned to Merck Patent Gmbh (DE), July 30, 1985. 9. C. Lee and D. R. Bassett. Process for the preparation of the parylene dimer. US Patent 4 769 505, assigned to Union Carbide Corporation (Danbury, CT), September 6, 1988. 10. J. F. Gaynor and S. B. Desu. “Room temperature copolymerization to improve the thermal and dielectric properties of polyxylylene thin films by chemical vapor deposition.” J. Mater. Res., 9:3125–3130, 1994. 11. C. E. King and A. J. Gulino. Preformed solder parts coated with parylene in a thickness effective to exhibit predetermined interference colors. US Patent 5 789 068, assigned to Fry’s Metals, Inc. (Jersey City, NJ), August 4, 1998. 12. J. Lahann, H. Höcker, and R. Langer. “Synthesis of amino[2.2]paracyclophanes – beneficial monomers for bioactive coating of medical implant materials.” Angew. Chem. Int. Ed., 40:726–728, 2001. 13. J. Lahann, D. Klee, and H. Höcker. “CVD-polymerization of a functionalized poly(p-xylylene). a generally applicable method for the immobilization of drugs on medical implants.” Materialwiss. Werkstofftech., 30:763–766, 1999. 14. S. Y. Park, J. Blackwell, S. N. Chvalun, A. A. Nikolaev, K. A. Mailyan, A. V. Pebalk, and I. E. Kardash. “The structure of poly(cyano-p-xylylene).” Polymer, 41(8):2937–2945, April 2000. 15. H. Pu, Y. Wang, and Z. Yang. “Chemical vapor deposition copolymerization of 4-carboxyl-[2,2] paracyclophane and 4-amino-[2,2] paracyclophane.” Mater. Lett., 61(13):2718–2722, May 2007. 16. C.-Y. Ho, T.-F. Lin, C.-H. Lin, and S.-J. Wang. Method for synthesizing TFPX. US Patent 7 173 159, assigned to Yuan-Shin Materials Technology Corp. (Taipei, TW), February 6, 2007. 17. H. Maruyama. Process for the preparation of dichloro-(2,2)-paracyclophane. US Patent 5 679 874, assigned to Daisan Kasei Kabushiki Kaisha (Tokyo, JP), October 21, 1997. 18. O. Schäfer, F. Brink-Spalink, B. Smarsly, C. Schmidt, J. H. Wendorff, C. Witt, T. Kissel, and A. Greiner. “Synthesis and properties of ω -phenylalkyl-substituted poly(p-xylylene)s prepared by base-induced 1,6dehydrohalogenation.” Macromol. Chem. Phys., 200:1942–1949, 1999. 19. H. Maruyama. Dichloro tetraflouro-{2,2}-paracyclopane, a process for manufacturing thereof and poly-α , α -difluoro-chloro-p-xylylene film prepared therefrom. US Patent 6 194 620, assigned to Daisan Kasei Kabushiki Kaisha (Tokyo, JP), February 27, 2001. 20. L. You, G.-R. Yang, T.-M. Lu, J. A. Moore, and J. F. P. McDonald. Vapor deposition of parylene-F using 1,4-bis (trifluoromethyl) benzene. US Patent 5 268 202, assigned to Rensselaer Polytechnic Institute (Troy, NY), December 7, 1993. 21. W. R. Dolbier and W. F. Beach. “Parylene-AF4: A polymer with exceptional dielectric and thermal properties.” J. Fluorine Chem., 122(1):97–104, July
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2003. 22. N. T. Tung, Y. J. Yu, K. Kim, S.-H. Joo, and J.-I. Jin. “Synthesis of poly(p-xylylene) from α ,α -bis(alkoxy or aryloxy)-p-xylenes by chemical vapor deposition polymerization.” Bull. Korean Chem. Soc., 25:31–32, 2004. 23. P. Simon, S. Mang, A. Hasenhindl, W. Gronski, and A. Greiner. “Poly(p-xylylene) and its derivatives by chemical vapor deposition: Synthesis, mechanism, and structure.” Macromolecules, 31:8775–8780, 1998. 24. R. Gleiter and H. Hopf, editors. Modern Cyclophane Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2004. 25. W. R. Dolbier, Jr. and X. X. Rong. Process for the preparation of octafluoro(2,2) paracyclophane. US Patent 5 849 962, assigned to Specialty Coating Systems, Inc. (DE), December 15, 1998. 26. S.-z. Zhu, Y.-y. Mao, G.-f. Jin, C.-y. Qin, Q.-l. Chu, and C.-m. Hu. “A convenient preparation of octafluoro[2,2]paracyclophane and dodecafluoro[2,2]paracyclophane.” Tetrahedron Lett., 43(4):669–671, January 2002. 27. T. E. Baker, G. L. Fix, and J. S. Judge. Process for forming polymeric paraxylylene coatings and films possessing improved oxidation resistance. US Patent 4 176 209, assigned to Raytheon Corporation (Lexington, MA), November 27, 1979. 28. L. A. R. Hall. Production of p-xylene polymers. US Patent 2 719 131, assigned to Du Pont, September 27, 1955. 29. S. Iwatsuki. “Polymerization of quinodimethane compounds.” Adv. Polym. Sci., 58:93–120, 1984. 30. R. Vedula., S. Kaza, and S. B. Desu. Chemical vapor deposition of polymers: Principles, materials and applications. In J.-H. Park and T. S. Sudarshan, editors, Chemical Vapor Deposition, volume 2 of Surface engineering series, chapter 8, pages 243–285. ASM International, Materials Park, OH, 2001. 31. R. A. Olson, F. W. Kopitzke, III, and J. P. O’Connor. Continuous vapor deposition apparatus. US Patent 5 424 097, assigned to Specialty Coating Systems, Inc. (Indianapolis, IN), June 13, 1995. 32. B. Ratier, Y. S. Jeong, A. Moliton, and P. Audebert. “Vapor deposition polymerization and reactive ion beam etching of poly(p-xylylene) films for waveguide applications.” Opt. Mater., 12(2-3):229–233, June 1999. 33. P. K. Wu, G. R. Yang, L. You, D. Mathur, A. Cocoziello, C. I. Lang, J. A. Moore, T. M. Lu, and H. Bakru. “Deposition of high purity parylene-F using low pressure low temperature chemical vapor deposition.” J. Electron. Mater., 26(8):949–953, August 1997. 34. J. Lahann and R. Langer. “Novel poly(p-xylylenes): Thin films with tailored chemical and optical properties.” Macromolecules, 35:4380–4386, 2002. 35. K. M. Vaeth and K. F. Jensen. “Blue electroluminescent copolymers by parylene-based chemical vapor deposition.” Macromolecules, 33:5336–5339, 2000.
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36. M. Ishaque, S. Agarwal, and A. Greiner. Synthesis and properties of novel poly(p-xylylene)s with aliphatic substituents. e-polymers 031/2002 [electronic] http://www.e-polymers.org/, 2002. 37. O. Schäfer, A. Greiner, M. Antonietti, and M. Zisenis. “Synthesis and properties of a soluble, rigid poly(p-xylylene) with high molecular weight.” Acta Polym., 47:386–390, 1996. 38. IDES Integrated Design Engineering Systems. The Plastics Web®. IDES Inc. 209 Grand Avenue Laramie, WY 82070 USA [electronic] http://www.ides.com/prospector/, 2006. 39. J. J. Senkevich, C. J. Mitchell, A. Vijayaraghavan, E. V. Barnat, J. F. McDonald, and T.-M. Lu. “Unique structure/properties of chemical vapor deposited parylene E.” J. Vac. Sci. Tech. A, 20:1445–1449, 2002. 40. K. G. Pruden, K. Sinclair, and S. Beaudoin. “Characterization of parylene-N and parylene-C photooxidation.” J. Polym. Sci., Part A: Polym. Chem., 41 (10):1486–1496, 2003. 41. K. G. Pruden and S. P. Beaudoin. “Model for the photooxidation of parylenes.” J. Polym. Sci., Part A: Polym. Chem., 42(11):2666–2677, 2004. 42. R. Wood. “To protect and preserve.” Mater. World, 8:30–32, 2000. 43. Q. Yu, J. Deffeyes, and H. Yasuda. “Engineering the surface and interface of parylene C coatings by low-temperature plasmas.” Prog. Org. Coat., 41(4): 247–253, May 2001. 44. P. Hanefeld, F. Sittner, W. Ensinger, and A. Greiner. Investigation of the ion permeability of poly(p-xylylene) films. e-polymers 026/2006 [electronic] http://www.e-polymers.org/, 2006. 45. M. W. Sowa and R. D. Jenkinson. Solder pastes. US Patent 5 328 522, assigned to Union Carbide Chemicals & Plastics Technology Corporation (Danbury, CT), July 12, 1994. 46. R. J. Saccomanno and G. A. West. Metallic coated dielectric substrates. US Patent 6 906 257, assigned to Honeywell International Inc. (Morristown, NJ), June 14, 2005. 47. Y.-C. Tai and X. Yang. Micromachined membrane particle filter using parylene reinforcement. US Patent 6 622 872, assigned to California Institute of Technology (Pasadena, CA), September 23, 2003. 48. C. J. Hess, M. F. Clem, G. W. Knight, K. L. Jambor, and G. L. Long. Vessel harvesting retractor with bilateral electrosurgical ligation. US Patent 6 740 102, assigned to Ethicon, Inc. (Somerville, NJ), May 25, 2004. 49. J. R. Morris. Electrosurgical instrument having a parylene coating. US Patent 5 380 320, assigned to Advanced Surgical Materials, Inc. (Littleton, CO), January 10, 1995. 50. J. G. Furst. Sutures and surgical staples for anastamoses, wound closures, and surgical closures. US Patent 2 005 038 472, assigned to Icon Interventional Systems In (US), February 17, 2005.
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51. A. O. Ragheb, B. L. Bates, N. E. Fearnot, T. G. Kozma, W. D. Voorhees, III, and A. H. Gershlick. Coated implantable medical device. US Patent 6 774 278, assigned to Cook Incorporated (Bloomington, IN) MED Institute, Inc. (West Lafayette, IN), August 10, 2004. 52. B. L. Bates, N. E. Fearnot, T. G. Kozma, T. A. Osborne, A. O. Ragheb, J. W. Roberts, and W. D. Voorhees, III. Silver implantable medical device. US Patent 6 530 951, assigned to Cook Incorporated (Bloomington, IN) MED Institute Inc. (West Lafayette, IN), March 11, 2003. 53. A. K. Khair and D. A. Anderson. Coated sleeve for wrapping dilatation catheter balloons. US Patent 5 425 710, assigned to Medtronic, Inc. (Minneapolis, MN), June 20, 1995. 54. D. J. Zarbatany, R. Pintor, and M. Verbeek. Dilatation catheter with varied stiffness. US Patent 6 030 405, assigned to Medtronic Inc. (Minneapolis, MN), February 29, 2000. 55. S. E. Boatman and K. D. Brummett. Flexible stent having a pattern formed from a sheet of material. US Patent 6 409 752, assigned to Cook Incorporated (Bloomington, IN), June 25, 2002. 56. R. L. McCullough, J. L. Wayt, and J. N. Butch. Method of coating printed circuit board. US Patent 6 389 690, assigned to American Meter Company (Scott Depot, WV), May 21, 2002. 57. D. Klee, N. Weiss, and J. Lahann. Vapor-based polymerization of functionalized [2.2]paracyclophanes: A unique approach towards surface-engineered microenvironments. In R. Gleiter and H. Hopf, editors, Modern Cyclophane Chemistry, chapter 18, pages 463–484. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2004. 58. A. Roos, D. Klee, K. Schuermann, and H. Höcker. “Development of a temperature sensitive drug release system for polymeric implant devices.” Biomaterials, 24(24):4417–4423, November 2003. 59. N. E. Fearnot, T. G. Kozma, A. O. Ragheb, and W. D. Voorhees. Coated implantable medical device. US Patent 5 609 629, assigned to MED Institute, Inc. (West Lafayette, IN), March 11, 1997. 60. A. D. Frolova, N. A. Minkina, V. G. Vasil’kovskii, O. A. Kuznetsova, E. L. Dolgopolova, E. S. Shaposhnikova, and T. G. Martinson. “Evidence for control of dichlorodi-p-xylylene, dibromodi-p-xylylene, and di-p-xylylene.” Gigiena Truda i Professional’nye Zabolevaniya, pages 30–32, 1991.
3 Poly(arylene vinylene)s Poly(p-phenylene vinylene) (PPV) belongs to the class of electroluminescent conjugated polymers. These materials emit light when electric current is passed through them. For this reason, a substantial interest emerges in the field of organic semiconductors. A lot of papers have appeared dealing with this topic, often focused on the physical aspects, however there are reviews dealing with chemical aspects, such as synthesis of these polymeric types.1 To the family of π -conjugated polymers, besides PPV, other types belong, such as poly(p-pyridyl-vinylene), poly(p-phenylene ethynylene), poly(p-thienyl vinylene), poly(3-hexylthiophene), and, poly(9,9-dihexylfluorene). The basic structure of these polymers is shown in Figure 3.1.
3.1 MONOMERS Monomers for PPV are listed in Table 3.1 and shown in Figure 3.2. Table 3.1: Monomers for Poly(p-phenylene vinylene) Monomer
Remarks
1,4-Bis-(dichloromethyl)-benzene α , α -Dibromo-p-xylene Chlorinated cyclophanes
Standard procedure For several reaction pathes Chemical vapor deposition2
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N
Poly(p-phenylene vinylene)
Poly(p-pyridyl vinylene)
S
Poly(p-thienyl vinylene)
Poly(p-phenylene ethinylene)
C6H13
C6H13
S
C6H13 Poly(3-hexylthiophene)
Poly(9,9-dihexylfluorene)
Figure 3.1: π -Conjugated Polymers
Br Cl
CH2
CH2
Cl
C H
CH3
Br 1,4-Bis(dichloromethyl)benzene
α,α-Dibromo-p-xylene
Cl Cl 1,9-Dichloro[2.2]-paracyclophane
Figure 3.2: Monomers Used for Poly(p-phenylene vinylene)
Poly(arylene vinylene)s
91
3.2 POLYMERIZATION AND FABRICATION Because the polymer cannot be processed as such due to its properties, a precursor polymer is synthesized, which is then processed, e.g., into films. In this form the final polymer is obtained by a post treatment. However, varieties of PPV with bulky substituents exhibit solubility in organic solvents. A commercial variety is poly(2-methoxy-5-(2-ethylhexyloxy)1,4-phenylene vinylene) (MEH-PPV). A variety of methods of preparation have been described, such as the preparation of the precursor polymer via:1 • • • • •
Sulfonium precursor (Wessling route), Ring opening metathesis polymerization, Chemical vapor deposition, Electropolymerization, Dehydrohalogenation condensation polymerization (Gilch Reaction), • Dehydrohalogenation phase transfer catalysis, • Anionic Polymerization. The reactions and their mechanisms have been discussed and reviewed in detail.3 For the application in the field of electronics, it is desirable that the polymer formed is perfectly π -conjugated. Defects are responsible for a reduced performance of light-emitting devices.
3.2.1 Sulfonium Precursor Route The standard procedure of the preparation of PPV involves the reaction of 1,4-bis-(dichloromethyl)-benzene with tetrahydrothiophene in the first step, c.f. Figure 3.3. Instead of tetrahydrothiophene, other related sulfur compounds may be used. The polymerization of the monomeric salt takes place at 0–5°C by adding alkali. The precursor polymer thus obtained is still soluble. It is purified by dialysis to remove low-molecular-weight impurities. Eventually, the final polymer is obtained by heating. Molecular weights greater than 100 k Dalton can be obtained. It is believed that the polymeric intermediate is formed by a radical mechanism, since the presence of oxygen lowers the molecular weight. The precursor polymer can be fabricated, e.g., by spin-coating, dipcoating or by Langmuir-Blodgett techniques into thin films. The pyro-
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S Cl
CH2
CH2
S+ ClCH2
Cl CH2 S+ Cl
NaOH
Δ
+S
Cl-
Figure 3.3: Synthesis of Poly(p-phenylene vinylene)
Poly(arylene vinylene)s
93
lysis of the materials are performed in high vacuum at temperatures of 180–300°C for 12 h. In the final step, hydrogen chloride and tetrahydrothiophene will be removed. Functionalized PPV precursor polymers with ester and carboxyl side groups have been prepared.4 These polymeric tetrahydrothiophenium salts can be converted via the sulfonium route into PPVs. After conversion, the pendent carboxyl functionalities can be further exchanged by others, such as nitro, amino, and aldehyde functionalities.
3.2.2 Transition Metal-catalyzed Cross-coupling Process PPVs are accessible by polycondensation methods based on transition metal-catalyzed cross-coupling processes. Several individual reactions are known. For example, using palladium catalysts, divinylbenzene yields with diiodobenzene derivates directly from PPV derivates. Another reaction, the Suzuki coupling, uses 1,4-aryldiboron acids and 1,2-dibromoethene with palladium catalysts to arrive at PPV. The monomers should be substituted, otherwise an insoluble polymer is obtained immediately. With bulky substituents, polymers can be obtained that are soluble in common solvents, such as chloroform, N,N-dimethylformamide (DMF), and methanol.5 The transition metals, which are used as a catalyst, cannot by removed completely from the polymer. Residues of transition metals may cause problems with regard to service time of the final products.
3.2.3 Chemical Vapor Deposition The chemical vapor deposition (CVD) techniques with respect to the synthesis of π -conjugated polymers have been reviewed in the literature.6 The CVD method can be very useful for the preparation of a wide variety of insoluble PPV in desired dimensions and shapes. The advent of the technique used in this particular field arose in 1994.7, 8 At the moment, poly(p-xylylene)s are the only polymers that are commercially fabricated by CVD. PPV can be also obtained by CVD techniques using 1,4-bis-(dichloromethyl)benzene, α , α -dibromo-p-xylene, or chlorinated cyclophanes. 1,9-Dichloro[2.2]paracyclophane can be deposited via an intermediate on a surface. In a second step, hydrogen chloride is eliminated to yield the PPV. The process is shown in Figure 3.4.
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Cl
Cl
Cl
Figure 3.4: Chemical Vapor Deposition of 1,9-Dichloro[2.2]paracyclophane
Films with a thickness of 10–100 μ m can be manufactured. A disadvantage of the process is that the cyclophanes are not easily accessible. Therefore, the use of 1,4-bis-(dichloromethyl)-benzene is more favorable. In addition, CVD allows the production of copolymers, when two monomers are deposited simultaneously.9 In this way, the electroluminescence (EL) can be tuned. On the other hand, it has been demonstrated that copolymers with different monomeric units can be prepared from a single monomer, when the temperature conditions are varied. Thus, copolymers consisting of 1,4-phenylene vinylene and 1,4-phenylene-1,2-ethanediyl units can be obtained from p-(methoxymethyl)benzyl chloride, via CVD.10
3.2.4 Ring-Opening Metathesis Polymerization Originally, the conversion of substituted bicyclo[2.2.2]octa-2,5-dienes into a precursor polymer of PPVs by ring opening metathesis polymerization (ROMP) has been shown by Grubbs11 using a molybdenum-based metathesis catalyst. The precursor polymer aromatizes in an inert atmosphere at 280°C, or in the presence of trioctylamine at 200°C. Later, siloxy substituted cyclophanes used as starting material for ROMP to get PPV.12 The mechanism is sketched in Figure 3.5. The precursor polymer that is formed by ROMP is transformed by pyrolysis into the final polymer. Related to ROMP is the acyclic diene metathesis (ADMET) procedure. Here, the starting monomer is a substituted p-divinylbenzene. During the polymerization, ethene is ejected. However, only low-molecularweight polymers can be obtained by ADMET.
Poly(arylene vinylene)s
95
OCOR ROCO
ROCO
OCOR
R O Si
R
R R R
Si O R
Figure 3.5: Synthesis of PPV via ROMP11, 12
A variant of metathesis is the Grignard metathesis polymerization. The polymerization of substituted thiophenes proceeds by a living chaingrowth mechanism. Thiophene modified MEH-PPV can be synthesized from a precursor monomer with Grignard reagents.13 The procedure is shown in Figure 3.6.
3.2.5 Electropolymerization In contrast to pyrolytic and chemical routes, in electrochemical methods a wide variety of substituents can be introduced. However, the method is not suitable for monomers with substituents that can be easily reduced. The formation of PPV by electropolymerization is sketched in Figure 3.7.14 The reaction proceeds via quinodimethane intermediates. In general, the reaction is carried out in an aprotic solvent, such as DMF. However, for the preparation of the soluble poly(2,5-dimethoxy-1,4-phenylene vinylene), an undivided flow cell with constant current at a lead cathode and aqueous DMF can be used. Co-electrolysis of different 1,4- bis-(halomethyl)-arenes results in random copolymers.15 Carboxyl functionalities yield water soluble polymer salts.
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OCH3 O
O O H
C
+ (C2H5O)2PH2C Br
S
CH2P(OC2H5)2 O
OCH3 Br
S
S
Br
O
CH3MgCl OCH3 S
S O
Figure 3.6: Grignard Metathesis Polymerization13
Poly(arylene vinylene)s
R
R
R
Br
97
R + eR
R
R
R
-
- Br R
Br
R
R= H or Br R
R
Figure 3.7: Synthesis of Poly(p-phenylene vinylene) by Electropolymerization
3.2.6 Knoevenagel Polycondensation Typically, in the Knoevenagel polycondensation, a terephthalic aldehyde is condensed with a bis-(cyanomethyl)-benzene compound. Since two monomers are used, PPVs with a strictly alternating structure can be synthesized. The Knoevenagel polycondensation allows the convenient synthesis of compounds in that the backbone double bond is substituted. PPV modified with cyano groups show a high EL activity.16 The synthesis is shown in Figure 3.8. In a quite similar way, PPV can be formed by the Wittig reaction.17
3.2.7 Gilch Reaction In the Gilch reaction, bis-(halogen methyl)benzene derivates are condensed with an excess of a strong base, such as potassium-tert-butyl alcoholate to yield a PPV. A precursor polymer with pendant chlorine groups is formed, as those from the cyclophane polymerization, c.f. Figure 3.4, however, this precursor polymer is converted immediately into PPV, if excess of the base is present. On the other hand, the precursor polymer can be isolated when the base is not given in excess. Originally, an ionic mechanism was postulated, which is suggestive, as a strong base used to initiate the polymerization reaction. However, some experimental results do not support the concept of an ionic mechanism. It is rather believed that the reaction is initiated by a radical process
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R
R H CH2
NC CH2
CN +
H C
C O
O R
R
R R CN R
NC R
Figure 3.8: Synthesis of PPV with Pendent Cyano Groups16
in which diradicals are formed. The radical mechanism is supported by the finding that 2,2,6,6-tetramethylpiperidine-N-oxyl acts as a scavenger.18
3.2.8 Dehydrohalogenation Phase Transfer Catalysis MEH-PPV can be synthesized by a liquid solid two-phase reaction.19 The liquid phase consists of 1,4-bis-(chloromethyl)-2-methoxy-5-(2-ethylhexyloxy)benzene dissolved in tetrahydrofuran (THF) and tetrabutylammonium bromide as a phase transfer catalyst. The solid phase consists simply of small-sized potassium hydroxide particles. The reaction is sketched in Figure 3.9. The polymer exhibits a high molar mass of 10 k Dalton and a narrow polydispersity. It can be spin-coated. In a quite similar procedure, copolymers of MEH-PPV and poly(1,5-naphthylene vinylene) (PNV) have been synthesized.20 PNV is insoluble in toluene and THF, due to the lack of bulky side chains pending from its backbone. The introduction of MEH-PPV blocks into PNV improves the solubility of the copolymer.
3.2.9 Anionic Polymerization In the Gilch reaction, a strong base is added to the monomer yielding polymers with high molecular weight. MEH-PPV can be obtained in the pres-
Poly(arylene vinylene)s
99
OCH3
OCH3 C8H17Br
O
OH
HCl, HCHO
OCH3
OCH3 CH2Cl
KOH ClH2C O
O
Figure 3.9: Synthesis of MEH-PPV19
ence of small amounts of an anionic initiator, e.g., deprotonated p-methoxyphenol, after reversing the order of addition. This means that the monomer is added to the base. The p-methoxyphenol is deprotonated by the addition of potassium tert-butoxide. Because the pKa of p-methoxyphenol allows a full deprotonation by potassium tert-butoxide, it is suggested that the propagation results from nucleophilic attack of the phenoxide on an intermediate quinodimethane formed by dehydrohalogenation of the monomer. Polymerization under these conditions is found to yield polymers with very low polydispersity values.21 Both green and red light-emitting polymers can be produced in this process by using aromatic monomers, for example, poly(2-dimethyloctylsilyl)-phenylene vinylene (DMOS-PPV), a yellow-green light-emitting polymer and MEH-PPV, a red emitting polymer. Block copolymers with units of a polydispersity index of less than 1.5 have been synthesized. In addition, diblock copolymers, triblock copolymers, star homopolymers, and block copolymers can be obtained via anionic polymerization methods using difunctional and trifunctional initiators. Suitable initiators include sulfonyldiphenol bisphenol A, and phloroglucinol.
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3.2.10 Others Because neat PPV is hard to process, a variety of related or modified polymers, in particular those with certain side groups, have been described. These varieties are summarized in Table 3.2. Amino substituted PPV, namely, poly(2-(N,N-dimethylamino) phenylene vinylene) can be made by reacting a bis-cycloalkylene sulfonium salt with sodium hydroxide at about 0°C. A cycloalkylene sulfonium salt precursor polymer is formed, which is heated to get the amino substituted PPV.22 Hyperbranched PPV with a partially conjugated structure can be synthesized from 1,1,1-tri-(p-tosyloxymethyl)-propane and 4,4 -(p-phenylenedi-1,2-ethenediyl)-diphenol.23 The synthesis route is shown in Figure 3.10. The hyperbranched PPV is formed as the tosylate groups are replaced by an ether group of the conjugated monomer. The hyperbranched polymers exhibit a good solubility and processability, a high glass transition temperature and a high fluorescence.
3.3 PROPERTIES 3.3.1 Mechanical Properties Neat PPV is insoluble, intractable, and infusible. The polymer is obtained from synthesis just in this form. Some derivates with appropriate side chains are soluble.
3.3.2 Thermal Properties PPV exhibits a higher thermal stability than other related polymers. Thermal degradation temperatures of some π -conjugated polymers are summarized in Table 3.3. Thermogravimetry indicates a beginning degradation of PPV around 500°C. This conforms with vapor deposition experiments.24 In situ mass spectrometry suggests that in the range of 500–600°C, products of degradation that contain toluene and xylene moieties are ejected. Block copolymers consisting of PPV and poly(methyl methacrylate) (PMMA) blocks, can be obtained by atom transfer radical polymerization. The thermal stability is slightly improved in comparison to neat PPV derivates by the introduction of PMMA blocks.25 The onset of thermal degradation starts around 200°C.
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Table 3.2: Varieties of Poly(p-phenylene vinylene) Acronym BAMH-PPV
Polymer
Referencesa
Poly(2,5-bis-(N-methyl-N-hexylamino)phenylene vinylene) BB-PPV Dendritic PPV BCHA-PPV Poly(2,5-bis-(cholestanoxy)1,4-phenylene vinylene) BDA-PPV Poly(2,15-dioxabicyclo(14.2.2)icosa1(19),16(20),17-trien-17,19-ylenevinylene) BDMO-PPV Poly(2,5-bis-(3 ,7 -dimethyl-octyloxy)1,4-phenylene vinylene) BDMOS-PPV Poly(2,5-bis-(dimethyloctylsilyl)l,4-phenylene vinylene) BDMP-PPV Phenyl substituted PPV BDP-PPV Phenyl substituted PPV BeCHA-PPV Poly(bis-2,5-epi-cholestanoxy-1,4-phenylene vinylene) BEH-PPV Poly(2,5-bis-(2 -ethylhexyloxy)1,4-phenylene vinylene) BTEM-PPV Poly(2,5-bis-(trisethoxymethoxy)1,4-phenylene vinylene) BuEH-PPV Poly(2-butyl-5-(2 -ethyl-hexyl)1,4-phenylene vinylene) C8-PPV Poly(2,5-bis-(octoxy)-1,4-phenylenevinylene) CN-Si-Carb-PPV Disilyl substituted PPV CN-Si-Ph-PPV Disilyl substituted PPV CH3O-PPV Poly(2,5-dimethoxy-1,4-phenylene vinylene) CN-PPV Cyano substituted PPV CzEH-PPV Poly[2-(carbazol-9-yl)-5-(2-ethylhexyloxy)1,4-phenylene vinylene] DD-PPV Poly(2,8-dibenzothiophene-5,5-dioxidevinylene-alt-1,4-phenylene vinylene) Dimethoxy-PPV Poly(2,5-dimethoxy-1-4-phenylene vinylene) DMeO-PPV Poly(2,5-dimethoxy-1,4-phenylene vinylene) DMOS-PPV Poly(2-dimethyloctylsilyl-p-phenylene vinylene) DOO-PPV Poly(2,5-dioctyloxy-1,4-phenylene vinylene) a Most recent references b Often investigated, see text
26 27 28 29 30 31 32 32 33 34 35 36 b 37 38 38 39 40 b b 41 42 43 44 45
102
High Performance Polymers Table 3.2 (cont): Varieties of Poly(p-phenylene vinylene)
Acronym DPOP-PPV
Polymer
Referencesa
Poly(1,4-phenylene-1,2-di(4-phenoxyphenyl) vinylene) DPO-PPV Poly(2-phenyl-3-phenyl-4-(3 ,7 -dimethyloctyloxy)-1,4-phenylene vinylene) DP-PPV Poly(2,3-diphenylphenylene vinylene) DPSP-PPV Poly(1,4-phenylene-1,2-di(4-phenylthiophenyl) vinylene) DPS-PPV Poly(2,5-dipropoxy sulfonato1,4-phenylene vinylene) EO-PPV Ethylene oxide substituted PPV HCN-PPV Poly(1,6-hexanedioxy1,4-phenylene1,2-ethenylene-(2,5-dicyano1,4-phenylene)-1,2-ethenylene1,4-phenylene) HPA-10-PPV Poly(2-hexyloxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy)-1,4-phenylene vinylene) MCHE-PPV Poly(1-methoxy-4-cyclohexylethyloxy-2,5phenylene vinylene) MDMO-PPV Poly(2-methoxy-5-(3 ,7 -dimethyloctyloxy)1,4-phenylene vinylene) MEH-PPV Poly(2-methoxy-5-(2 -ethylhexyloxy)-1,4phenylene vinylene) M3EH-PPV Poly(2-methoxy-5-(2 -ethylhexyloxy)-1,4phenylene vinylene 2,5-dimethoxy-1,4-phenylene vinylene) MH-PPV Poly(2-methoxy-5-(n-hexadecyloxy)-1,4phenylene vinylene) MN-PPV Poly(2-methoxy-5-nonyloxy-1,4-phenylene vinylene) MO-PPV Poly(2,5-dimethoxy-1,4-phenylene vinylene) m-PPV-DP Poly-(1,3-phenylene diphenylvinylene) MPS-PPV Poly((2-methoxy-5-sulfopropoxy)1,4-phenylene vinylene) MTEO-PPV Poly(2-methoxy-5-triethoxy-1,4-phenylene vinylene) OC1C10-PPV Poly(2-(3 ,7 -dimethyloctyloxy)-5-methoxy1,4-phenylene vinylene) OO-PPV Poly(2,5-dicotyloxy-1,4-phenylene vinylene) a Most recent references b Often investigated, see text
46 47 48 49 50 51 52
53 54 55 b 56
57 58 59 60 50 61 62 63
Poly(arylene vinylene)s
103
Table 3.2 (cont): Varieties of Poly(p-phenylene vinylene) Acronym
Referencesa
Polymer
O-PPV
Poly(2,5-diphenylene-1,3,4-oxadiazolyle4,4 -vinylene) OxdEH-PPV Poly(2-(4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazolyl)-phenyl)-5-(2-ethylhexyloxy)1,4-phenylene vinylene) PEO-PPV Poly(2-methoxy-5-(triethoxymethoxy)-1,4phenylene vinylene) PEO-OC9-PPV Poly(2-(n-nonyloxy)-5-(triethoxymethoxy)1,4-phenylene vinylene) PO-PPV Poly(2-phenoxy-1,4-phenylene vinylene) PPE-PPV Phenylene-ethynylene/phenylene vinylene hybrid polymers PTVMEH-PPV Poly((1 ,4 -bis-(thienyl-vinyl))-2-methoxy5-(2 -ethylhexyloxy)1,4-phenylene vinylene) RO-PPV Poly(2,5-dialkoxy-1,4-phenylene vinylene) SiPhOPPV Poly(2-4 -dimethyldodecylsilylphenyloxy1,4-phenylene vinylene) a Most recent references b Often investigated, see text
64 65
66 66 67 68 13 69 70
Table 3.3: Degradation Temperatures of π -Conjugated Compounds Polymers Poly(acetylene) 2,5-Dicyano-1,4-phenylene vinylene-based PPV Poly(p-xylene) Poly(p-phenylene) Poly(p-phenylene vinylene)
Tg /°Ca
References
200 304
24
420 450 500
24
52
24 24
Oligomers 2-Hexadecyloxy-5-methoxybenzene-1,4bis-(4-dimethylaminophenylene vinylene) 1,4-Bis-(2-methylstyryl)-benzene Benzene-1,4-bis-(phenylene vinylene) a Onset of degradation
200
71
200 300
71 71
104
High Performance Polymers
HO +
CH3
CR3
R=
CH2OSO2
OH CH3
CH3 O C O O
Figure 3.10: Hyperbranched PPV from 4,4 -(p-Phenylenedi-1,2-ethenediyl)-diphenol and 1,1,1-Tri-(p-tosyloxymethyl)-propane23
Poly(arylene vinylene)s
105
At temperatures above 800°C, PPV can be converted into graphite.72, 73 The electrical conductivity of PPV that is pyrolyzed at 3000°C is strongly influenced by stretching. By doping with sulfur trioxide a conductivity of 105 S cm−1 is reached, which is comparable to a highly oriented pyrolytic graphite or natural graphite. Another study74 in that PPV and other related polymers are pyrolyzed up to 1700°C reports an electric conductivity of 0.26 103 S cm−1 . However, the electric conductivity of graphite obtained from poly(1,4-phenylene ethynylene) is still higher, namely, 0.80 103 S cm−1 .
3.3.3 Electrical Properties Ideally, PPV should exhibit a thoroughly π -conjugated structure for electronic applications. In practice, this cannot be achieved. Saturated defects interrupt the π -conjugated structure and thus the length of conjugation. On the other hand, ethynylene moieties instead of vinylene moieties may act as traps of charge carriers. The trapping may occur at either radiative or non-radiative trap states. In fact, there are two types of defects:75 1. Structural defects, and 2. Chemical defects. Structural defects include grain boundaries, crystallographic defects, chain ends and oxidative defects. In contrast, chemical defects may either be due to impurities incorporated during material processing, or in the polymer backbone itself. Photoluminescence (PL) and EL spectroscopy can be used to determine the presence of traps. Other techniques include current voltage measurements, capacitance voltage measurements, capacitance transient spectroscopy, and admittance spectroscopy.75, 76 Under favorite conditions, the identification of the nature of the trap is possible.
3.3.4 Optical Properties PPV is a bright yellow fluorescent polymer. The emission maxima are in the yellow-green region of the visible spectrum, at 551 nm and 520 nm. The EL of PPV was discovered in 1990.77 Since PPV shows a good holetransporting capability, within the group of electrical conducting polymers, it has been used not only as a light-emitting layer but also as a hole-transporting material in the EL devices.78
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The PL spectra of MEH-PPV exhibit a maximum around 640 nm.79 The maximum is red-shifted with increasing annealing temperature. However, the PL emission from MEH-PPV and blend films with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole becomes blue-shifted with increasing temperature.80 In PPV, with increasing static pressure a red-shift of the maximum in the PL spectra is observed.81 In addition, the intensity of the main peak is decreased. This phenomenon becomes evident as the increasing pressure of the interaction of polymeric chains becomes more pronounced. The structure tends to become more planar and the conjugated length becomes longer.
3.4 SPECIAL ADDITIVES 3.4.1 Dopants Doping creates structural and electronic modifications in the polymer backbone. Usually, doping enhances the electrical conductivity. Doping can be achieved by initiating oxidation or reduction reactions in the backbone of the polymer. Oxidation is addressed as p-doping, whereas reduction is addressed as n-doping. Dye doping consists of the addition of a fluorescent dye to the polymer.82 The symmetry of electrons and holes is not maintained in doped PPV. In this way, the difference between electron and hole intra-chain mobility in PPV can be explained.83 Dopants and doping methods are listed in Table 3.4.
3.4.1.1
Reactive Doping
Arsenic pentafluoride effects a Friedel-Crafts chain extension and crosslinking in PPV.84 The electrical conductivity takes place by a variablerange hopping mechanism. The modification is reflected by infrared techniques, as new bands emerge. For example, this can be attributed by the formation of quinoid structures.85
Poly(arylene vinylene)s
107
Table 3.4: Dopants and Doping Methods for Poly(p-phenylene vinylene) Compound Arsenic pentafluoride Iodine Ferric chloride Coumarin 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylamino-styryl)-4H-pyran 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole Tetra-n-butylammonium tetrafluoroborate Dye vapor transportation
3.4.1.2
References 84 85 85 86 87 80 78 82
Dye Doping
Since PPV is insoluble, it is not possible to dope PPV with dyes utilizing the solution method. Vapor transportation is a dye doping method, where the dye molecules are introduced into the polymers via the gas phase in vacuo.82 Besides fabrication of photoelectric devices, the method of vapor transportation has been used in the fields of optical memories and waveguides.
3.5 APPLICATIONS PPVs are important π -conjugated polymers for electronic and luminescent devices.
3.5.1 Electroluminescent Devices Simple light-emitting devices are essentially built according to the principle shown in Figure 3.11. The scheme in Figure 3.11 refers to a lightemitting diode, which is most common. However, organic light-emitting transistors88 and organic field effect transistors89 based on PPV materials have also been described. Next, we explain the basic principle of a lightemitting diode. When a voltage is applied to the electrodes, negative charges are injected from the cathode and positive charges are injected from the anode into the polymer. When the charges are recombining, their energy is transferred to the polymer in that singlet or triplet sites are formed. The singlet
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Aluminum cathode Polymer Indium-tin oxde anode Glass substrate
Light
Figure 3.11: Basic Elements of a Light-Emitting Device
sites are the same as formed in PL. PL takes place by an excitation from the ground state followed by fluorescence. Therefore, the light emission process induced by charges follows the same path of mechanism as occurs in PL. Usually, the triplet stage that is also formed beside the singlet stage has not sufficient energy to emit light in the visible region. For this reason, the material development focuses to enhance the yield of singlet stages. In general, the light-emitting efficiency is improved by using a polymer with a high electron affinity and a cathode with a low work function. The polymeric film is very thin so that electric field strengths in the range of grater than 105 V m−1 occur. The quantum efficiency, expressed as the number of photons related to the number of electrons injected is in the range of 0.1–5%. Other performance indicators are the luminous efficiency and the power efficiency. The indium tin oxide (ITO) anode layer is kept as thin as 15 nm to allow passage of the light emitted in the polymer. The cathode is fabricated from a material that has a small energy barrier with respect to electron emission, e.g., aluminum, calcium, or barium. To improve the operational stability, low work function metals such as Mg and Li are alloyed with more stable metals with higher work function, such as Al or Ag, and used as cathode material.90 Most simply, the polymeric layer consists of a single layer. Multilayer structures of different polymers are more common. Multilayer organic devices, such as are conventionally constructed in a sequential manner: 1. A transparent electrode, usually ITO is vacuum sputtered on a glass substrate. 2. A hole transport layer, such as poly(3,4-ethylenedioxythiophene)
Poly(arylene vinylene)s
109
(PEDOT), is coated onto the layer. 3. A layer of light-emitting polymer, such as MEH-PPV, is coated onto the uppermost layer. 4. A top electrode, such as barium, is thermally evaporated on the uppermost layer. 5. Finally, a protective layer, such as aluminum, is deposited. The fabrication steps pointed out above yield a standard polymeric light-emitting device with a layer structure of Glass/ITO/PEDOT/MEH − PPV/Ba/Al. The polymer is commonly applied by spin-coating or ink-jet printing, while the electrodes are usually constructed by vacuum deposition or sputtering. In this sequential fabrication process, both wet processes and dry processes are required. In order to reduce the number of sequential layers to be deposited one by one, devices can be fabricated from two separate parts, each part formed on a substrate with different functional layers. By fabricating the specific layers on each substrate before combining the two parts to form the organic semiconductor devices or circuits, the difficulties arising from the integration of wet and dry processes can be overcome and the costs of fabrication can be reduced. In the last step, the two parts can be assembled to form the final device. The parts are aligned permanently by a thermosetting reaction.91 This combinational method will ultimately provide the flexibility of varying combination possibilities of the final device. For example, if 5 different first parts and 5 different second parts are produced, up to 25 different devices configurations can be constructed.91 Thin film devices can be fabricated by an electrophoretic deposition technique.92 In the electrophoretic deposition method, the materials are applied as colloidal particles in a non-solvent. By subjecting the particles to an electrophoretic force, a nanostructured film is formed. Drying of the film is done under non-solvent conditions in order to keep the structure. When residual solvents and other impurities, such as monomers and polymeric precursors, remain in the material layer, a deterioration of the device could occur. Between the individual processing steps, these impurities can be removed by extraction with supercritical fluids. When the process can be performed under comparatively low-temperature and lowpressure conditions, the impurities can be removed, suppressing chemical
110
High Performance Polymers
physical changes of the layer.93 For example, while the supercritical point of ethanol is 241° C at 6.1 MPa, the cleaning process can be performed at 260°C and 8.0 MPa. Using carbon dioxide, a temperature of 80°C and a pressure of 15 MPa can be used. Bright blue electroluminescent devices have been fabricated using poly(9,9-dioctylfluorene) (POF) as an emissive layer, PPV as a hole-transporting layer, tetra-n-butylammonium tetrafluoroborate as a dopant, and a lithium-aluminum alloy as a cathode.78 Modified polymers of PPV with pendent carbazol groups and oxadiazole groups have been prepared by direct polymerization of the respective α , α -dibromo-p-xylene monomers.94 These polymers are then doped with 4-(dicyanomethylene)-2-methyl6-(4-dimethylamino-styryl)-4H-pyran (DCM). The PL results mostly from the dopant. Namely, the optical spectra indicates that Förster energy transfer from the modified polymers to the dopant may occur and effect the light emission.87 Functional dyes can be dispersed into neat PPV, by a vacuum process. It is possible to change the color of PPV from yellow to green with a blue dye, 1,4-(N,N -diethylamino)anthraquinone. Further, the fluorescent color of PPV can be changed from green to red by doping with DCM. A pattern doping with DCM results in the formation of a multicolored luminescent medium.95 In alternating copolymers bearing the m-phenylene vinylene unit and the p-phenylene vinylene unit, the green light emission is caused by the m-phenylene vinylene unit.96 Multilayer green light-emitting devices have been fabricated from poly(N-vinylcarbazole) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4oxadiazole. The EL is highly dependent on the thickness of the PPV layer in such devices. The performance is strongly dependent on the polymeric structure, which can be tuned by the introduction of side groups and comonomers. In polymers with bulky side groups, the intermolecular interactions are reduced. This effects an increased performance.97, 98 However, there is an optimum length of the side chains.99 Long side chains may tilt or fold back, attempting to crystallize. For this reason, the intermolecular distance may be reduced again in a certain region of the length of the side chain. Interchain interactions can be suppressed by blending MEH-PPV with small amounts of an inert polymer. Still another route in order to reduce interchain interactions is to fix the units by some extent due to crosslink-
Poly(arylene vinylene)s
111
ing.100 In contrast, in PPV types that are capable of photo crosslinking or of thermal crosslinking, the performance decreases due to the existence of residual radicals in the material.101 Silicon modified PPV with PEDOT exhibits an EL maximum around 550 nm.102 The degree of phase separation between the different chromophores is an important parameter for the efficiency of the device. In multi-component chromophore blends, silicon modified PPV is highly phase separated. The extensive phase separation and the resulting morphology prevents efficient energy transfer. MEH-PPV clay hybrid nanocomposite materials prepared by in situ polymerization in organically modified montmorillonite show a comparatively higher current and a lower turn-on voltage.103 By controlling the ratio of monomeric precursors to montmorillonite, exfoliated nanocomposites can be obtained. In intercalated structures, the polymer chains are merely inserted into the interlayer spaces of the montmorillonite. However, in the exfoliated structure, two-dimensional nanospaces will no longer be present. Intercalation refers to the inclusion of molecules between the layers. Exfoliation refers to the separation of the layers. In other words, intercalated polymer chains are sandwiched in between clay layers. In exfoliated nanocomposites the individual layers are separated, and somewhat uniformly dispersed in the polymeric matrix. This class of materials has been widely investigated because of their unique properties.104 The efficiency of PPV-related electroluminescent devices is summarized in Table 3.5. Luminescent layers can be fabricated from a pattern formed by an ink-jet method105 . A composition with a viscosity of 0.002–0.004 Pa s and the surface tension in the range of 0,025–0,040 N m−1 is used and placed by means of an ink-jet printer. Screen-printable electroluminescent polymeric inks contain a variety of additives in order to control the viscosity of the electroluminescent polymer ink, to decrease the solvent evaporation rate, and to improve the ink consistency and working time.106 In addition, the additives can improve the charge injection and power efficiency of light-emitting devices manufactured from the screen-printable electroluminescent polymer ink. A formulation of a polymeric electroluminescent ink is shown in Table 3.6. The device consists of four layers: substrate, transparent electrode, polymeric ink, and top electrode. The electroluminescent ink is screen-
112
High Performance Polymers
Table 3.5: Efficiency of PPV Related Electroluminescent Devices Device Structure
lm/W
References 107
MEH-PPV DEN-PPV 0.12 BDMO-PPV 0.13 ITO/MEH-PPV/LiF/Al 0.42 EHDVP-PPVa 0.73 BDMO-PPV phenyleneethynylene compound 1.2 DMOS-PPV-co-DMOS-PPDFVb 1.31 ITO/PEDOT/MEH-PPV:PFc /Ba/Al 1.8 MEH-PPV PFc 1.83 MEH-PPV 3 2-Butyl-5-(2-ethyl-hexyl)-PPV 5.6 POF doped with MEH-PPV 8 a PPV with (2,2-diphenylvinyl)phenyl side group b Fluorinated PPV c Poly(alkyfluorene)
108 109 110 108 111 112 113 114 115 116
Table 3.6: Polymeric Electroluminescent Ink106 Component MEH-PPV Poly(ethylene oxide) Tetra-n-butylammonium tetrafluoroborate Chlorobenzene a mg g−1 Chlorobenzene
[mg g−1 ] a 12 4 1.3
Poly(arylene vinylene)s
113
printed using multiple passes to result in a dry film thickness between about 100 nm and 1 μ m. In the ink-jet method, since the material is jetted and is scattered, if a distance between a coated surface and a nozzle of a head for ink-jet is not made suitably, there can occur the problem of a so-called flying curve in which a droplet falls to a position other than intended. To overcome this undesired behavior, an improved thin film-forming apparatus has been constructed.117 The coating unit of this apparatus is equipped with a suck-back mechanism. Three kinds of luminescent layers emitting lights of the respective colors of red, green, and blue can be formed at the same time so that the luminescent layers can be formed at a high throughput. It is possible to apply coating in a stripe shape without a gap in one pixel line, making the throughput extremely high.
3.5.2 Photovoltaic Devices Photovoltaic devices can be classified in two types:118 • The regenerative type converts light into electrical power leaving no net chemical change behind. The current-carrying electrons are transported to the anode, and the external circuit and the holes are transported to the cathode where they are oxidized by the electrons from the external circuit. • In the photosynthetic type, two redox systems are present; one reacts with the holes at the surface of the electrode, and one reacts with the electrons entering the counter-electrode. For example, water is oxidized to oxygen at the photoanode and reduced to hydrogen at the cathode. Here, we are dealing with the regenerative type. Photovoltaic cells were first developed in the 1950’s as p-n junctions of inorganic materials. A wide variety of cells since then have been fabricated using homojunction, heterojunction and tandem architectures with inorganic materials, most commonly silicon. The devices convert solar radiation directly into direct-current electrical power.119 Organic solar cells have several potential advantages compared with conventional inorganic solar cells, including light weight, flexibility, and the potential for low-cost fabrication of large areas by using printing tech-
114
High Performance Polymers
niques.120 However, there is still a lack of both power conversion efficiency and long-term stability, which is needed for practical device applications. The efficiency of photovoltaic devices depends on both the photoinduced charge generation, which is based on the electron transfer efficiency and the transport of charges created to the electrodes, i.e., the charge carrier mobility. These two issues must be fulfilled simultaneously. It is possible to construct a structural arrangement of the device materials in order to enhance both demands in a microscopic region separately. The term heterojunction in semiconductors science refers to a connection of layers of material with different electric properties, such as band gaps. The term bulk heterojunction has been introduced in organic semiconductors, and refers to a polymeric composite that is microscopically phase separated, e.g., by an interpenetrating network. These materials are characterized by a high interfacial area. As common in semiconductors, the material or the composite material, respectively, must provide both electron donor and electron acceptor properties. In photovoltaic devices, the absorption of light effects the separation of electric charges that are flowing to the electrodes and are building up a difference of electric potential. A significant resistance against a charge carrier crossing occurs in the transition region between the photoactive layer and the electrode, which may be attributed to reactions between the metallic electrode and the organic photoactive layer. Therefore, if these indirect influences may be suppressed, then an improvement of the charge crossing, which leads to an increase of the efficiency, must be expected if the other conditions remain identical. By providing an electrically insulating transition layer, these indirect reactions between the photoactive layer and the electrode may be largely interrupted. However, the thickness of the electrically insulating transition layer must be restricted to at most 5 nm, so that the high electric resistance of this transition layer does not hinder the easier crossing of the charge carrier between the photoactive layer and the electrode.121 In devices with transitions layers made from lithium fluoride by vapor deposition, an improved efficiency has been demonstrated. The most commonly used polymeric electron donors are PPV and poly(alkyl thiophene)s. Polymeric electron acceptors are cyano substituted PPV and poly(p-pyridyl vinylene). There are also low-molecular-weight electron acceptors, which include fullerenes and perylene derivatives, such as tetrabenzyl perylene-3,4,9,10-tetracarboxylate.122
Poly(arylene vinylene)s
Current Density /[mA cm-2]
2
115
light dark
1.5 1 0.5 0 -0.5 -1 -1.5
-1
-0.5
0
0.5
1
1.5
Voltage /[V]
Figure 3.12: Current viz. Voltage of a MDMO-PPV-Based Photovoltaic Cell55
Photovoltaic devices based on conjugated polymers receive considerable attention.55 Most commonly, MEH-PPV-based devices have been investigated. Under favorite circumstances, prototypes with an energy conversion efficiency of 3% power have been achieved. Mixtures of conjugated polymers with fullerenes have been found to be suitable for photovoltaic devices. The photovoltaic effect arises from the photoinduced electron transfer from conjugated polymers onto the fullerene. Figure 3.12 shows the current viz. voltage diagram of a poly(2-methoxy-5-(3,7 -dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV) fullerene-based photovoltaic cell. Phase separated devices show improved photovoltaic performance. The phase separation is controlled as the blends separate into an interpenetrating network. During the spin-coating process, the extent of phase separation from the well mixed state, which exists in the solution is limited by the evaporation time of the volatile solvent. When the solvent has fully evaporated, a further rearrangement is impossible and the morphology of the blend is locked in place. Thermal annealing the device allows the polymers used to rearrange into a state with lower energy. In this way, the desired molecular alignment can be built up. Certain poly(thiophene)s are known to aggregate during annealing, leading to the formation of ordered regions in which charge transport is facilitated. The aggregation of the polymers in blends allows improved transport of the charges formed on illumination to the contacts, thus reducing the probability of recombination before the charges are ex-
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High Performance Polymers
tracted.123 Flexible photovoltaic devices have been designed by coating the ITO onto poly(ethylene terephthalate) films. The effect of morphology can be demonstrated, if the same materials are fabricated in the same manner, however, using a different solvent of spin-coating. If a MDMO-PPV fullerene photovoltaic cell is produced from a toluene solution or chlorobenzene solution, it makes a big difference in the final performance. Photovoltaic devices based on DCM-doped PPV have been fabricated.82 DCM absorbs light at a longer wavelength than PPV. Glass plates with ITO are used as substrates. An interfacial layer of PEDOT doped with poly(styrene sulfonic acid) is used, and then the DCM-doped PPV is applied. Eventually, aluminum electrodes are deposited by vacuum evaporation. The devices are tested using a xenon lamp with a AM1.5 filter, 100 mW cm−2 . The short-circuit current of this particular device is reported to be 2.77 μ A cm−2 and the open-circuit voltage is 1.15 V. The energy conversion efficiency is 0.00055% under AM1.5 illumination.82 In spiro segments, the bifluorene moieties are orthogonally arranged. Thus, the resulting polymer chains are twisted, which causes a reduction of intermolecular interactions. Further, the packing of the polymer chains is less dense. On the other hand, the increased stiffness of the chains increases the glass transition temperature. The glass transition temperatures of the polymers are in the range 170–222°C. For this reason, poly[3,6-bis-(3,7-dimethyloctyloxy)-9,9-spirobifluorenyl-2,7-vinylene] (OC10)2-spiro-PFV) and other related copolymers have been synthesized and examined as photovoltaic devices.124 The photoactive layer thickness is in the range of 70–75 nm and LiF/Al is used as the cathode. The copolymer feed ratio has a considerable effect on the power conversion efficiency. It increases with increasing MEH-PPV content. A maximum power conversion efficiency of 1.30% for a copolymer of (OC10)2-spiro-PFV and MEH-PPV 50:50 has been achieved. However, with a pristine MEH-PPV, a power conversion efficiency of 2.10% is found. The use of materials with higher glass transition temperature results in a significant improvement of the thermal stability of the photovoltaic performance.125 This is explained by a more stable bulk morphology of the materials. Glass transition temperatures of PPV polymers are shown in Table 3.7.
Poly(arylene vinylene)s
117
Table 3.7: Glass Transition Temperatures of PPV Polymers Polymer MDMO-PPV MEH-PPV M3EH-PPV PPV with C10 side chains PPV modified with spirobifluorenyl monomers
Tg /°C
References
45 68 108 150 177
125 56 56 126 124
O O(CH2)10O N N
Figure 3.13: Oxadiazole Units Used to Enhance Solar Cell Performance127
In MEH-PPV modified with tetrabenzyl perylene-3,4,9,10-tetracarboxylate, it has been shown that an annealing process effects the formation of crystal networks within the polymer. This network considerably increases the external quantum efficiency and the energy conversion efficiency.122 The incorporation of oxadiazole moieties, which are highly electron deficient, into PPV/MEH-PPV as side chains, increases the exciton dissociation rate and promotes the electron transport.127 The oxadiazole moieties, c.f. Figure 3.13, are attached to the backbone via C10 alkyloxy links. The exciton dissociation rate is assumed to follow an exponential decay law. The decay constant (obviously the mean lifetime) τ for oxadiazolecontaining PPV/MEH-PPV is 0.4 ns, whereas the decay constant for pure MEH-PPV is around 0.65 ns. As already mentioned, the thiophene unit shows good donor properties, but poly(thiophene)-based devices show a low open-circuit voltage. On the other hand, PPV-based solar cell devices exhibit higher open-circuit voltages. For this reason, it is suggestive to combine both structural units into a polymer.13, 128 An open-circuit voltage of 900 mV and a power conversion efficiency of 1.2% can be obtained by such a combination. By means of the ink-jet technique, somewhat ordered lattices of at least two materials with different electron affinities can be placed.129 The method has been demonstrated with MEH-PPV and cyano substituted PPV.
118
High Performance Polymers
The close proximity of photoresponsive materials having differing electron affinities ensures efficient charge separation when an exciton is formed within the photoresponsive region upon exposure of the photoresponsive region to light. The thickness of the spots is preferably made as small as possible to minimize the lateral diffusion length, i.e., the distance that a charge carrier needs to travel before collection in an area of high or low electron affinity. The special arrangement in space of the different materials enhances the device efficiency. In general, photovoltaic devices are sensitive to atmospheric oxygen and humidity, which reduces their service times. With MDMO-PPV based solar cells, it was demonstrated that the encapsulation with poly(ethylene naphthalate) (PEN) increases the service time from a few hours up to more than four months.130 In general, PEN is used as an ultra-high barrier material.
3.5.3 Poly(p-phenylene vinylene) Nano Fibers PPV nano fibers can be produced by electrospinning a PPV precursor polymer from an alcohol solution. Electrospinning uses an electrical charge to form fine fibers. Electrospinning is similar to electrospraying. A polymer solution is ejected through a fine needle that is electrically charged. The charges cause the droplet to stretch. If the viscosity of the droplet is sufficiently high, the droplets are not sprayed as it occurs in the electrospray technique, but a liquid jet is formed. Voltages in the range of 10 kV are applied. After annealing at 180°C for 2 h, the PPV precursor polymer is transformed into PPV fibers. The morphology can be controlled using poly(vinyl alcohol) (PVA)/ PPV precursor polymers.131, 132 The morphology of fibers can be characterized by scanning electron microscopy and fluorescence microscopy. The fluorescence spectra of PVA/PPV nano fibers and of composite nano fibers made from PPV/MEH-PPV exhibit an appreciable blue shift, a stronger intensity of fluorescence, and a higher surface photovoltage in comparison to bulk material.132, 133 Thus, it is possible to fabricate nano fibers with a fluorescence from yellowish green to blue. The nano fibers have a potential application in optical and electronic devices.
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3.5.4 Poly(p-phenylene vinylene) Nanotubes Nanotubes made from PPV exhibit markedly different fluorescence decay times in comparison to bulk samples.6 Carbon nanotubes are of potential interest as electron guns, in flat panel displays.6 Carbon nanotubes exhibit many desirable properties for improved field emission: • • • •
Good electrical and thermal conductivity, Good mechanical strength, High aspect ratio, and Satisfactory low work function.
Instead of carbon nanotubes, graphitic carbonized PPV nanotubes prepared in an alumina membrane have been used. Further, nanotubes embedded with gold nanoparticles were used for the fabrication of field emitting devices.134 The performance of these devices is comparable or better than the performance of pure carbon nanotube-based devices. The work function and turn-on field are reduced significantly by the presence of gold nanoparticles. The field amplification factor is doubled by the embedding of the gold nanoparticles.
3.5.5 Sensors 3.5.5.1
pH-Sensor
A pH sensitive photoconductor based on PPV has been reported.135 The detection of local changes in pH by small-scale sensors is of particular interest for a variety of medical, biological, and environmental applications. In thin sheets of 25–30 nm of PPV, an increase in dark current and photocurrent is observed upon exposure to aqueous solutions. The change in photocurrent is a function of the pH. A fairly linear dependence of the photocurrent with pH is observed in the range of 4.5–9.5. The dark current is in the range of 10−14 A. Upon illumination, the current increases sharply and becomes stationary after some 10 to 100 s. The photocurrent is in the range of nA. The results are reversible and reproducible. The dependence of the current on pH is shown in Figure 3.14.
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Device 1 Device 2 Device 3
Photocurrent /[nA]
0.2
0.15
0.1
0.05 4
5
6
7
8
9
10
pH Figure 3.14: Photocurrent viz. pH Dependence135
3.5.5.2
Gas Sensors
A bridged PPV type, BDA-PPV is suitable for use in an interdigitated electrode system, c.f. Figure 3.17, for the elective detection of NO2 .29 The repeating units of the polymer are shown in Figure 3.15. The bulky bridged structure reduces intermolecular interactions in comparison to other PPVs. Multilayer films are deposited onto silicone substrates with interdigitated gold electrodes. The sensing properties of the device have been tested with NO2 , CO, NH3 , SO2 , at different temperatures. Among the gases mentioned, the electrical conductivity responds only to NO2 , thus showing a selectivity to NO2 . The relative change in electric current with the concentration of NO2 is shown in Figure 3.16. PPV-based block copolymers with phenylene oxide units are suitable to detect organic vapors, such as acetone, ethanol, ethyl acetate, hexane, toluene, acetic acid, methanol, and diethyl ether.136 The device is constructed as gold-coated copper interdigitated electrodes with a gap of 1 mm. No response is detected when the sensors are exposed to air saturated with water. Thus, humidity does not affect the performance.
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O
121
O
Figure 3.15: Repeating Units of Poly(2,15-dioxabicyclo(14.2.2)icosa1(19),16(20),17-trien-17,19-ylenevinylene) (BDA-PPV)29
Relative Increase of Current /%]
T = 75 C 1000
100
10
100
1000
ppm NO2 Figure 3.16: Electrical Response of the Device Against NO2 Concentration29
122 3.5.5.3
High Performance Polymers Impedimetric Sensors
The function of impedimetric sensors consists of the electrical properties of an electrode change, with its surface properties. The electric properties of surface modified electrodes can be probed by means of impedance spectroscopy.137, 138 It is in order to review a few technical terms. The electrical resistance R, is related to the electric current I and the voltage U by Ohm’s law as U . (3.1) I In systems with alternating voltage and current, these quantities can be represented as complex numbers. The voltage for a sinusoidal time (t) dependence is R=
u(t) = Umax cos(2πν t + φ ), where ν is the frequency, and φ is the phase. u(t) is a real quantity. For easier calculations, electrotechnicians prefer to use the associated complex quantity and express the voltage as U(t) = Umax exp(ı2πν t) exp(ıφ ). U(t) is a complex quantity. In the complex plane, it can be represented as a rotating pointer in time. The same is true, mutatis mutandis for the electrical current. In a system with alternating voltage and current, the electrical resistance in Eq. 3.1 is identified as the impedance Z. The impedance reads as Z(t) =
U(t) . I(t)
(3.2)
The impedance can be calculated for several types of electrical basic devices. For example, for an Ohm-type resistor, the impedance is Z = R, for an ideal capacitor, the impedance is Z = −ı/(2πν C), and for an ideal inductor, the impedance is Z = −ı2πν L. C is the electric capacity, and L is the electric induction. The real part of the impedance is addressed as the resistive part of the impedance. or by short resistance. The imaginary part of the impedance is addressed as the reactive part of the impedance, or by short reactance.
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Figure 3.17: Schematic Layout of an Interdigitated Capacitor139, 140
Impedance spectroscopy measures the frequency response of the electric impedance. In an impedimetric sensor, the molecules to be characterized are adsorbed on the electrodes, or on a surface between a pair of electrodes. This process of absorption changes the electrical performance of the device. By measuring the impedance between the electrodes, some conclusions can be established.141 The underlying principle of impedimetric techniques is electrical impedance spectroscopy. These techniques are used in biochemical and environmental applications; because of their sensitivity, however, with varying success. A wide variety of designs of the devices is known for the analysis of biomaterials.138, 141 A possible schematic layout of an electrode system is shown in Figure 3.17. The electrodes are placed on a substrate that selectively absorbs the molecules to be analyzed. This effects a change in the dielectric properties in the space between the electrodes that can be detected. Basic modules with the design shown in Figure 3.17 are frequently used in sensor technology. By the way, such as device has been used for the detection of the curing behavior of coatings.142 The trend goes to nanoscale fabrication in order to improve the performance of impedimetric biosensors. Electrode widths and spacings from 500 to 250 nm can be achieved.139 In practice, devices with complementary layers are used so that certain antibodies are adsorbed physically onto thin polymer films as they are fabricated.143
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Table 3.8: Examples for Commercially Available Poly(arylene vinylene) Polymers PPV-polymers
Supplier
BEHP-co-MEH-PPV PSS PPV-co-MEH PPV BEHP-PPV MDMO-PPV MPS-PPV, potassium salt PSS PPV BTEM-PPV BEH-PPV PSS PPV-co-MEH PPV MEH-PPV MDMO-PPV copolymers
Sigma-Aldrich a Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich H. W. Sands Covion (Merck) a
Precursors 2,5-Bis-(bromomethyl)-1-methoxy-4-octyloxybenzene 2,5-Bis-(chloromethyl)-1,4-bis-(octyloxy)-benzene 2,5-Bis-(chloromethyl)-1-methoxy-4-(3 ,7 -dimethyloctyloxy)benzene 2,5-Bis-(octyloxy)-benzene-1,4-diacetonitrile 2,5-Bis-(octyloxy)-terephthalaldehyde 2-Methoxy-5-(3 ,7 -dimethyloctyloxy)benzene-1,4-diacetonitrile Poly((m-phenylene vinylene)-alt-(2,5-dibutoxy-p-phenylene vinylene)) Poly(p-xylene tetrahydrothiophenium chloride) solution Poly(2,5-dihexyloxy-1,4-phenylene vinylene) a Several other PPV types available
Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich
3.6 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 3.8 Tradenames appearing in the references are shown in Table 3.9.
3.7 SAFETY For MEH-PPV and MDMO-PPV no special exceptional hazards have been reported.
Poly(arylene vinylene)s
Table 3.9: Tradenames in References Tradename Description
Supplier
Baytron® P Bayer AG Complex of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonic acid)123 Luxprint® 7144 DuPont Carbon conductor ink118 Luxprint® 7145L DuPont 118 Silver conductor ink Luxprint® 7151 Dupont Electroluminescent phosphor paste118 Luxprint® 7153E DuPont Barium titanate paste118 Tegoglide™ 410 Goldschmidt Chemical Corp. Poly(siloxane) surfactant118 Tegowet™ Goldschmidt Chemical Corp. Poly(siloxane)-poly(ester) copolymer surfactant118 Zonyl® 7950 DuPont Fluorinated surfactant118 Zonyl® FSO 100 DuPont Ethoxylated nonionic fluorosurfactant118
125
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111. Y. Jin, J. Jee, K. Kim, J. Kim, S. Song, S. H. Park, K. Lee, and H. Suh. “Synthesis and electroluminescent properties of copolymers based on PPV with fluoro groups in vinylene units.” Polymer, 48(6):1541–1549, March 2007. 112. F. Shen, F. He, D. Lu, Z. Xie, W. Xie, Y. Ma, and B. Hu. “Bright and colour stable white polymer light-emitting diodes.” Semicond. Sci. Tech., 21:L16– L19, 2006. 113. R.-H. Lee, K.-T. Lin, and C.-Y. Huang. “High red, green, and blue color purity electroluminescence from MEH-PPV and polyalkyfluorenes-based bright white polymer light emitting displays.” J. Polym. Sci., Part B: Polym. Phys., 45:330–341, 2006. 114. S. Sohn, K. Park, D. Lee, D. Jung, H. M. Kim, U. Manna, J. Yi, J.-h. Boo, H. Chae, and H. Kim. “Characteristics of polymer light emitting diodes with the LiF anode interfacial layer.” Jpn. J. Appl. Phys., Part 1, 45:3733– 3736, 2006. 115. G. Yu, Y. Cao, M. Andersson, J. Gao, and A. J. Heeger. “Polymer lightemitting electrochemical cells with frozen p-i-n junction at room temperature.” Adv. Mater., 10:385–388, 1998. 116. J. Huang, G. Li, E. Wu, Q. Xu, and Y. Yang. “Achieving high-efficiency polymer white-light-emitting devices.” Adv. Mater., 18:114–117, 2006. 117. S. Yamazaki, K. Yamamoto, M. Hiroki, and T. Fukunaga. Method for precisely forming light emitting layers in a semiconductor device. US Patent 7 115 434, assigned to Semiconductor Energy Laboratory Co., Ltd. (JP), October 3, 2006. 118. H. Andriessen. Layer configuration comprising an electron-blocking element. US Patent 6 977 390, assigned to Agfa Gevaert (Mortsel, BE), December 20, 2005. 119. J. Salafsky. Solid-state electric device. US Patent 6 991 958, assigned to The Trustees of Columbia University in the City of New York (New York, NY), January 31, 2006. 120. K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D. D. C. Bradley, and J. R. Durrant. “Degradation of organic solar cells due to air exposure.” Sol. Energ. Mater. Sol. Cells, 90(20):3520–3530, December 2006. 121. S. Shaheen, C. Brabec, T. Fromherz, F. Padinger, S. Sariciftci, and E. Gloetzl. Photovoltaic cell. US Patent 6 933 436, assigned to Konarka Austria Forschungs und Entwicklungs GmbH (AU), August 23, 2005. 122. S. Lu, J. Niu, W. Li, J. Mao, and J. Jiang. “Photophysics and morphology investigation based on perylenetetracarboxylate/polymer photovoltaic devices.” Sol. Energ. Mater. Sol. Cells, 91(4):261–265, February 2007. 123. J. J. Halls and R. H. Friend. Photoresponsive devices. US Patent 6 872 970, assigned to Cambridge Display Technology Limited (Cambridge, GB), March 29, 2005.
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124. S. Chul Kim, B. Vijaya Kumar Naidu, S.-K. Lee, W.-S. Shin, S.-H. Jin, S.-J. Jung, Y.-R. Cho, J.-M. Shim, J. Kook Lee, J. Wook Lee, J. Hyeon Kim, and Y.-S. Gal. “Synthesis and photovoltaic properties of novel PPV-derivatives tethered with spiro-bifluorene unit for polymer solar cells.” Sol. Energ. Mater. Sol. Cells, 91(6):460–466, March 2007. 125. S. Bertho, I. Haeldermans, A. Swinnen, W. Moons, T. Martens, L. Lutsen, D. Vanderzande, J. Manca, A. Senes, and A. Bonfiglio. “Influence of thermal ageing on the stability of polymer bulk heterojunction solar cells.” Sol. Energ. Mater. Sol. Cells, 91(5):385–389, March 2007. 126. H. Becker, H. Spreitzer, W. Kreuder, E. Kluge, H. Schenk, I. Parker, and Y. Cao. “Soluble PPVs with enhanced performance - a mechanistic approach.” Adv. Mater., 12(1):42–48, 2000. 127. S.-P. Huang, J.-L. Liao, H.-E. Tseng, T.-H. Jen, J.-Y. Liou, and S.-A. Chen. “Enhanced photovoltaic cells efficiency via incorporation of high electrondeficient oxadiazole moieties on side chains of poly(phenylene vinylene)s and poly(fluorene)s.” Synth. Met., 156(14-15):949–953, July 2006. 128. D. A. M. Egbe, L. H. Nguyen, B. Carbonnier, D. Muhlbacher, and N. S. Sariciftci. “Thiophene-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene)s: Synthesis, characterisation and optical properties.” Polymer, 46(23):9585–9595, November 2005. 129. J. H. Burroughes. Optoelectronic devices. US Patent 7 091 516, assigned to Cambridge Display Technology Limited (Cambridge, GB), August 15, 2006. 130. G. Dennler, C. Lungenschmied, H. Neugebauer, N. S. Sariciftci, M. Latreche, G. Czeremuszkin, and M. R. Wertheimer. “A new encapsulation solution for flexible organic solar cells.” Thin Solid Films, 511-512:349– 353, July 2006. 131. W. Zhang, Z. Huang, E. Yan, C. Wang, Y. Xin, Q. Zhao, and Y. Tong. “Preparation of poly(phenylene vinylene) nanofibers by electrospinning.” Mater. Sci. Eng., A, 443(1-2):292–295, January 2007. 132. W. Zhang, E. Yan, Z. Huang, C. Wang, Y. Xin, Q. Zhao, and Y. Tong. “Preparation and study of PPV/PVA nanofibers via electrospinning PPV precursor alcohol solution.” Eur. Polym. J., 43(3):802–807, March 2007. 133. Q. Zhao, Z. Huang, C. Wang, Q. Zhao, H. Sun, and D. Wang. “Preparation of PVP/MEH-PPV composite polymer fibers by electrospinning and study of their photoelectronic character.” Mater. Lett., 61(11-12):2159–2163, May 2007. 134. K. Kim, S. H. Lee, W. Yi, J. Kim, J. W. Choi, Y. Park, and J.-I. Jin. “Efficient field emission from highly aligned, graphitic nanotubes embedded with gold nanoparticles.” Adv. Mater., 15:1618–1622, 2003. 135. P. Pistor, V. Chu, D. M. F. Prazeres, and J. P. Conde. “pH sensitive photoconductor based on poly(para-phenylene-vinylene).” Sens. Actuators, B, 123(1):153–157, April 2007.
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4 Poly(phenylene ether)s Around 1956, the oxidative coupling of 2,6 substituted phenols to yield high molecular products was discovered.1, 2 Poly(phenylene ether) (PPE), also addressed as poly(phenylene oxide), was commercialized in 1964 by General Electric3 and AKZO,4 and eventually by several other companies. Remarkably, the oxidative coupling of phenols plays a role in certain biological reactions, e.g., in the formation of lignin or melamine.5
4.1 MONOMERS Preferred monomers for PPE are shown in Table 4.1 and in Figure 4.1. Alkylphenols are oxidized by air. They must be stored under nitrogen to prevent oxidation reactions. The oxidative coupling reaction is a general reaction, suitable for 2,6 disubstituted phenols.6 However, with bulky subTable 4.1: Monomers for Poly(phenylene ether)s Monomer
Remarks
2,6-Xylenol 2,3,6-Trimethylphenol 2,4,6-Trimethylphenol p-Phenylphenol 2,6-Diphenylphenol 4-Bromo-4 ,4 -dihydroxytriphenylmethane 2-Allyl-6-methylphenol
Standard
139
Chain stopper7 Chain stopper8 Hyperbranchend types9 Thermosetting types10
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High Performance Polymers
CH3
CH3 OH
OH
CH3 2,6-Xylenol
H 3C
CH3
2,3,6-Trimethylphenol
H 3C
CH3
O
O
H 3C
CH3
Tetramethyldiphenylquinone Br
HO
C
OH
H 4-Bromo-4′,4′′-dihydroxytriphenylmethane
Figure 4.1: Monomers Used for Poly(phenylene ether)s
Poly(phenylene ether)s
141
stituents only the C−C coupled product, the diphenoquinone, is formed. The oxidation potential of phenols is reduced by alkyl substitution and increased by electron-withdrawing substituents. 2,6-Dimethylphenol is also addressed as 2,6-xylenol. Typical industrial processes for preparing 2,6-dimethylphenol involve the reaction of phenol and methanol in the presence of a metal oxide catalyst. The major byproduct of this reaction is 2,4,6-trimethylphenol. 2,4,6-Trimethylphenol is then dealkylated into 2,6-dimethylphenol. The process can run via: • A catalytic steam dealkylation with zinc oxide catalysts,11 or • A selective oxidation into the corresponding p-hydroxy benzaldehyde with subsequent deformylation.12, 13 For the selective alkylation to the o-position of phenol, with methanol, catalysts based on ammonium metavanadate NH4 VO3 and ferric nitrate Fe(NO3 )3 × 9H2 O have been suggested.14 In this process a raw product of 2,6-dimethylphenol of 64% yield is recovered. Phenol and o-cresol are recycled in this process, as starting materials. 2,6-Diphenylphenol can be obtained from cyclohexanone.15 The reaction is shown in Figure 4.2. The oxidative polymerization of 2,6-diphenylphenol yields poly(2,6-diphenyl-1-4-phenylene oxide), which is a completely aromatic analogue of PPE. 4-Bromo-4 ,4 -dihydroxytriphenylmethane is obtained from the reaction of p-bromobenzaldehyde with two moles of phenol.9
4.2 POLYMERIZATION AND FABRICATION There are three methods known for polymerization: • Oxidative coupling, • Radical polymerization, and • Ullmann reaction. The various aspects of oxidative polymerization of phenols have been thoroughly reviewed.5 Most commonly PPEs are produced by the self-condensation of a monovalent phenol in the presence of oxygen and a metalamine-complex catalyst. Manganese, copper and cobalt can be used as the metal in the catalyst. Cu+ is most commonly utilized. For example, the preparation of the catalyst can be achieved by stirring cuprous bromide and di-n-butyl amine in toluene.16
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O
O
O NaOH
O
OH
Figure 4.2: Synthesis of 2,6-Diphenylphenol from Cyclohexanone via the Three Intermediate Isomers15
Poly(phenylene ether)s
CH3
143
CH3 CuCl
OH
O O2
CH3 H 3C O H 3C
CH3 CH3 O CH3
Figure 4.3: Oxidative Coupling of 2,6-Dimethylphenol
Besides alkali metal bromides, quaternary ammonium salts can be employed as promoters, e.g., methyltri-n-octylammonium chloride.17 The nature of the metal bromide promoter is not particularly critical. Other promoters, such as diphenylguanidine may be employed in combination with the bromide or quaternary ammonium salt. The polymerization takes place by oxidative coupling. The basic mechanism is shown in Figure 4.3. At temperatures around and above 100°C, significant amounts of a diphenoquinone, i.e., tetramethyldiphenyl quinone (TMDQ) are produced. The processes run in the presence of an organic solvent. Aromatic solvents, such as benzene, toluene, xylene and o-dichlorobenzene are the preferred solvents, although tetrachloromethane, trichloromethane, dichloromethane, 1,2-dichloroethane and trichloroethylene may also be used. The polymerization takes place slightly above room temperature, from 35°C to about 55°C, with the higher reaction temperature near the end of reaction. The reaction is terminated by the removal of the catalyst from the reaction mixture. To the reaction mixture, an inorganic alkali metal bromide or an alkaline earth metal bromide is added as a promoter. High-molecular-weight poly(phenylene ether) with an intrinsic viscosity (IV) of 0.70 dl g−1 can be produced in a methylene chloride solution without precipitation of the catalyst complex. The catalyst is then removed by extraction with either aqueous acid, which removes both the amine and metal ion catalyst components, or by treatment with a chelating agent, such as ethylenediamine tetraacetic acid.18
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High Performance Polymers Table 4.2: Rate of Polymerization of PPE with Promoters17 mol-% Xylenola
Intrinsic viscosity [dl g−1 ], after 60 min 90 min 120 min
0.00 0.25 0.44 0.55 b b 0.12 1.20 0.16 0.27 0.58 0.72 0.08 0.27 0.52 0.67 0.04 0.26 0.51 0.68 0.02 0.29 0.51 0.62 b b b 2.00 Xylenol concentration 17%, at 30°C Molar ratio Xylenol:CuCl2 :NaBr:dibutylamine = 200:1:2:12 a Moles per mole of copper salt, Methyltri-n-octylammonium chloride b No polymer
The combined effect of tetraalkylammonium salts and diphenylguanidine on the polymerization rate is illustrated in Table 4.2. An environmentally friendly process for the preparation of PPE consists of the reduction of Cu2+ and recovery for the regeneration of the complex Cu+ amine catalyst.16 The use of the copper catalysts greatly increases the rate of oxygen utilization in the early stages of the polymerization reaction. The lower oxygen concentration in the head space of the reactor helps in reducing the risk of fire or explosion in the reactor. The faster initial reaction rate with the copper catalyst also results in less accumulation of the unreacted monomer and a reduction of the amount of TMDQ produced. In some cases, namely for functionalization, the formation of TMDQ is desirable. Water soluble catalysts have been developed for polymerization techniques in an aqueous solution.19 This topic is detailed in Section 4.8. The molecular weight can be controlled by the addition of chain stoppers. In general, para substituted phenols, often being present as impurities, lower the molecular weight. 4,4 -Bis-(4-hydroxy-3,5-dimethylphenyl)pentanoic acid acts in this way. Also 2,4,6-trimethylphenol acts as a chain stopper.7 In contrast, a free ortho position tends toward branching reactions.
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In the course of the oxidative coupling, the amine catalyst may frequently become chemically bound to the poly(phenylene ether). These amino groups cause a negative effect on the impact strength of the final polymer.20 The compositions can be improved in respect to impact strength, by removing or inactivating a substantial proportion of the amino compounds. Polymers treated in this way are sometimes referred to as inactivated PPE.
4.2.0.4
Hyperbranched Poly(phenylene ether)
Hyperbranched PPEs with phenolic terminal groups can be prepared from 4-bromo-4 ,4 -dihydroxytriphenylmethane via a modified Ullmann reaction.9 The monomer is treated with potassium carbonate or sodium hydroxide as a base. Copper chloride is used as a catalyst in an aprotic solvent, namely, dimethyl sulfoxide or sulfolane. The degrees of branching reach from 48–71%. The phenolic end groups can be easily modified. With triphenylphosphine and diisopropyl azodicarboxylate, various ether end groups can be attached, e.g., methoxy groups with methanol, 2-methoxyethoxy end groups with ethylene glycol mono methyl ether, etc. Hyperbranched polymers find applications in rheology modifiers, processing additives, and coating applications as bulk materials, and as functional materials in catalysts, and sensors.21
4.2.1 Functionalized Poly(phenylene ether) Functionalization serves to improve the compatibility of the PPE with other polymers and to attach reactive groups onto the backbone. Functionalization can be achieved in various ways, namely: 1. 2. 3. 4. 5.
Redistribution reactions, Treatment with peroxide compounds, Treatment with vinyl compounds, Treatment with a capping agent, and Melt functionalization in an extruder.
Compounds used for compatibilization are summarized in Table 4.3.
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High Performance Polymers Table 4.3: Compounds Used for Compatibilization Compound Maleic anhydride Fumaric acid Citraconic anhydride Methacrylic anhydride Epichlorohydrin Epibromohydrin Glycidyl tosylate Epoxychlorotriazine Benzoyl chloride Trimellitic anhydride acid chloride
4.2.1.1
References 22 22 22 23 24 24 24 25 25 26
Redistribution with Phenols
One obstacle to blending PPE with other resins is the lack of compatibility between the resins. This lack of compatibility often manifests itself as delamination or poor physical properties, such as poor ductility. One useful method to improve the compatibility between resins is to introduce suitable comonomers in the chain. Another method is to modify the end groups of the polymer. Functionalized poly(phenylene ether) resins can be obtained through a redistribution reaction with a functionalized phenolic compound in the polymerization reaction.27 In the redistribution reaction of PPE with phenolic compounds, the PPE is split into shorter chains with the phenolic compound incorporated in the PPE. To initiate the redistribution reaction, TMDQ or diphenyl quinone is added. TMDQ is made from 2,6-dimethylphenol by oxidative coupling. TMDQ is also addressed as the backward dimer. However, it is preferably generated in situ during the oxidative coupling process. For this reason, reaction conditions are chosen that favor the formation of TMDQ. When the level of monomer is increased during the early stages of oxidation, higher levels of TMDQ are produced. In addition, a slower initial reaction rate with the copper catalyst results in an increased accumulation of the unreacted monomer and in an increase of the amount of TMDQ. The redistribution reaction is done at a temperature of up to 150°C. In this way, low-molecular-weight resins with an IV between about 0.08 dl g−1 and 0.16 dl g−1 are obtained. If no additional functionalized phenolic compound is added at the redistribution step, the TMDQ is incorporated
Poly(phenylene ether)s
CH3
CH3 O
O
OH +
O CH3
C
CH3 C CH2
CH3 O C
O
C O
CH2
CH3
CH3
O
C
H 3C
147
O
O
CH3 C CH2
CH3
CH3
Figure 4.4: Capping Reaction of Poly(phenylene ether) with Methacrylic Anhydride
into the PPE, which leads to a PPE with a high hydroxyl content. 4.2.1.2
Treatment with Vinyl Compounds
Functionalized PPE may be prepared by allowing PPE to react with a species that contains both a C=C bond and a reactive moiety, such as hydroxyl, acid, anhydride, amine, imide, epoxy, etc. Examples of species of the acid and anhydride type include maleic anhydride, fumaric acid, and citraconic anhydride.22 For blending PPE with resin systems that involve curing or polymerization reactions, including radical reactions, it is desirable to operate with a PPE that contains residual aliphatic unsaturation and capped phenolic end groups at the same time. In order to achieve convenient distribution in another matrix, the PPE-type should exhibit a low viscosity. Unsaturated moieties can be introduced by the reaction of the hydroxyl groups of PPE with methacrylic anhydride. The reaction is conducted in a toluene solution. 4-Dimethylaminopyridine or 4-dimethylbutylamine serves as a catalyst.23 The capping reaction is shown in Figure 4.4. The ester formation occurs between a phenolic end group of the PPE and a carboxylic group of the methacrylic anhydride. A highly efficient
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High Performance Polymers
Table 4.4: Backbone and Surface Modification of Poly(phenylene ether) Method
Reagent
Sulfonation Chloromethylation Bromination, alkynylation Surface grafting Enzyme immobilization Silylation Nitration/amination Etherification
Chlorosulfonic acid Methyl chloromethyl ether28 Various Reagents29 Various enzymes Trimethylchlorosilane30 31
Methanol, ethylene glycol monomethyl ether9
capping with respect to aliphatic unsaturation can be readily achieved. Instead of toluene, the capping reaction can be performed in a styrene monomer. The mixture can be used for further thermosetting. When the capped PPE does not need to be isolated by a further process, a reduction in color is observed.32
4.2.1.3
Other Functionalization Methods
Functionalized PPE may be prepared with an oxidizing agent, e.g., dibenzoylperoxide, or a mixture of a peroxide and a phenol, such as bisphenol A.22 The process is similar to a redistribution process. Benzoate end capped PPE can be readily prepared by allowing the terminal hydroxyl groups of PPE to react with benzoyl chloride in chloroform.25 Trimellitic anhydride groups can be introduced by the reaction with trimellitic anhydride acid chloride.26 This reagent is used for the compatibilization of PPE with poly(amide) (PA). Several other related acid chlorides have been proposed for the functionalization of PPE.33 PPE is a versatile material for performing chemical modification on its backbone, either on the phenyl ring or on the methyl group. The backbone of PPE can be modified with various methods. Most common is the introduction of sulfonic acid groups, which can be done with chlorosulfonic acid. Other backbone modification methods are shown in Table 4.4. The methods are used mainly in the preparation of membranes and will be discussed in this particular section. The modification by bromination and subsequent alkynylation leads to polymers that contain substituted alkynes on the aromatic ring.29
Poly(phenylene ether)s
149
4.2.2 Copolymers Low-molecular-weight PPE, functionalized with methacrylic groups may serve as a macromonomer with pendent vinyl groups.34 The PPE chains may have two end caps to provide two reaction sites for forming crosslinks. Mixtures of mono capped PPE and PPE chains bearing two or more end caps are also useful. PPE with such end caps can be copolymerized with styrene and acrylonitrile monomers. Suspension polymerization has been demonstrated to be a suitable technique of polymerization. With only one end cap, the PPE is integrated with the styrene and acrylonitrile units to form a comb-type structure with the PPE segments forming the teeth of the comb. Without end caps, the hydroxyl groups on the PPE chains tend to inhibit the reaction of the styrene monomers. Crosslinking can be expected to increase with multiple functionalized PPE. Typically, the amount of end capped PPE introduced to the reaction medium is 10–15%. The glass transition temperatures of the copolymers increase with increasing levels of PPE and range from 144°C to 170°C. The copolymers offer the advantage of being miscible with styrenic resins such as styrene acrylonitrile copolymer, resulting in a composition retaining the positive qualities of PPE, particularly flame retardancy and a comparatively high glass transition temperature.
4.2.3 Blends Neat PPE is difficult to process by extrusion or injection molding. At a temperature of 300–350°C, the viscosity is still very high. Special precautions must be taken to minimize oxidation reactions. This inherent problem can be solved by blending with other polymers. Table 4.5 lists the very blends described in the literature. 4.2.3.1
Miscible Blends
The miscibility can be studied by various techniques, such as differential scanning calorimetry, electron microscopy, thermal mechanical analysis, and viscometry. Miscible blends exhibit a single glass transition temperature. Viscometry is an effective, quick, and inexpensive technique to elucidate polymer/polymer interactions.
150
High Performance Polymers Table 4.5: Blends including Poly(phenylene ether) Component
References
Poly(styrene) Poly(amide) Poly(phenylene sulfide) Poly(ethylene) Silicone Brominated poly(styrene) (PS)
35, 36 37 38 26 36 39
Blends of PPE with PS are homogeneous blends. Blending lowers the temperature of fabrication. Noryl® is such a blend, which was introduced in 1966. PPE is miscible with brominated PS above 75% PPE.39 Poly(styrene), saturated polyalicyclic resins and terpene phenol reduce viscosity and impart high flow to the resulting composition. For this reason, the polymers added are sometimes addressed as flow promoters. The materials have some adverse effect as they reduce the heat deflection temperature of the product and increase the flammability. High flow polyphenylene ether resin compositions have been described.40 Dendritic polymers based on polyesters with multifunctional hydroxy compounds offer the advantage to be effective in smaller amounts in comparison to common flow improvers.41 In addition, heterogeneous blends of PPE with PA have been introduced. 4.2.3.2
Compatibilization
Improved compatibility between PPE and other components in a blend may be achieved by treatment with maleic anhydride, fumaric acid, N-phenylmaleimide and other compounds. Mixtures of PPE and PS, or styrene buadiene styrene block copolymers have been extruded with maleic anhydride of fumaric acid to obtain compatibilized blends.35 In the same way, PPE with pendent glycidyl groups can be co-extruded with a poly(olefin) having anhydride groups.25 PPE can be grafted onto poly(ethylene) by melt kneading both modified polymers in the presence of a binder such as phenylenediamine. Both polymers are modified with maleic anhydride. The grafting takes place in situ. Amines may enhance the improvement of certain physical properties when used in combination with various compatibilizers.26 Ethylene/acrylic acid random copolymers are suitable for grafting.25
Poly(phenylene ether)s
151
A catalyst, which catalyzes the esterification reactions between the OH end groups of PPE and the COOH groups of the pendent acrylic acid moieties, is magnesium acetate tetrahydrate. If the functionalization of PPE with maleic anhydride is done, using a special evaporator for maleic anhydride, so that the maleic anhydride is fed in the gaseous state and under inert conditions, materials with improved color tones will be obtained.42
4.2.4 Thermosetting Resins Curable thermosetting resin compositions containing PPE are useful in electrical applications because they exhibit low dielectric constants. Such compositions are used as fiber reinforced prepregs, for example as copperclad laminates suitable for printed circuit boards (PCB)s In order to take part in the thermosetting reaction, functionalized PPE is used. The curable part is an epoxide resin. For PCBs, a flame retardant composition is preferred, such as a bisphenol A diglycidyl ether tetrabromobisphenol A-based epoxy resin.22, 43 Glycidyl groups can be attached to PPE in a first step by increasing the amount of pendent hydroxyl groups, e.g., by a redistribution reaction. In a second step, these hydroxyl groups are allowed to react with epichlorohydrin.24 Other reagents that have been claimed to be useful are epibromohydrin and glycidyl tosylate. Functionalized PPE can be used as thermosetting resins in combination with epoxide resins as powder coatings.7 Sealing resins for electric parts is another field of application.44 A variant of thermosetting has been described in that PPE resins are dissolved in epoxy resins. A variety of polymers can be dissolved in epoxy resins.45 In order to facilitate the processability of PPE, the PPE is dissolved in an epoxy resin as processing aid. After processing by kneading, the epoxy resin is cured. In contrast to other approaches where the thermoplastic polymer acts as a toughener for the epoxy matrix, the amount of epoxy resin added can be adjusted so that the PPE will form the continuous phase in the final state. The oxidative polymerization of 2-allyl-6-methylphenol with 2,6-dimethylphenol yields thermosets capable of thermal curing. The copolymerization yields high-molecular-weight copolymers with a number average Mn around 50,000 Dalton The polymers have broad molecular weight
152
High Performance Polymers Table 4.6: Properties of Neat PPE46 Property Density Tensile Modulus Tensile Elongation Break Flexural Strength Glass Transition Temperature Melting Temperature
Value 1.06 2.7 20–40 114 205 267
Unit
Standard
g cm−3 GPa % MPa °C °C
ASTM D638 ASTM D638 ASTM D790
distributions of Mw /Mn around 35. Crosslinking of the precursor polymers can be archived by thermal treatment with optionally 2,5-dimethyl2,5-di(tert-butylperoxy)-3-butane).10 The materials are expected to be useful in high-speed and high-frequency PCB applications.
4.2.5 Other Related Types Poly(2,6-diphenyl-1,4-phenylene oxide) (P3O) is obtained from the polymerization of 2,6-dimethylphenol. In comparison to 2,6-dimethylphenol, the oxidation potential of 2,6-diphenylphenol is higher. Higher temperatures of polymerization are necessary.2 P3O shows a glass transition temperature of 230°C and a melting temperature of 480°C. In contrast to PPE it readily crystallizes from the melt.47 It shows and excellent thermal stability both in nitrogen and in air. Due to its high melting point, P3O cannot be processed by injection molding or extrusion techniques. However, P3O can be fabricated by solution casting techniques into films or by wet spin techniques into fibers. On orientation or heat treatment, these materials become insoluble.
4.3 PROPERTIES Commonly, PPE has a number average molecular weight of 3,000–40,000 Dalton. PPE is essentially an amorphous polymer, when cooled from the melt. It is soluble in chloroform, benzene, and toluene. Some properties of neat PPE are shown in Table 4.6. However, PPE is used mostly as a blend for the sake of processability. Properties of a PPE/PS are given in Table 4.7.
Poly(phenylene ether)s
153
Table 4.7: Properties of a PPE Blend a 48 Property
Value
Density 1.06 9.2 Melt Mass-Flow Rate (MFR)b Mold Shrink, Linear-Flow c 7E-3 Water Absorption, 24 h 0.06 Tensile Modulus 2.86 Tensile Strength Break 49.6 Tensile Elongation Yield 7.2 Tensile Elongation Break 28 Flexural Strength 90.0 Notched Izod Impact (23°C) 214 Dielectric Constant (60 Hz) 2.65 Oxygen index 22 a Noryl® 731, General Electric b (289°C/5.0 kg) c 3.18 mm
Unit
Standard
g cm−3
ASTM D792 ASTM D1238 ASTM D955 ASTM D570 ASTM D638 ASTM D638 ASTM D638 ASTM D638 ASTM D790 ASTM D256 ASTM D150 ASTM D2863
g/10 min cm/cm % GPa MPa % % MPa J m−1 %
4.3.1 Mechanical Properties Molded parts show a good dimensional stability. This arises from the comparatively small coefficient of thermal expansion. Other mechanical properties of moldable PPE are shown in Table 4.7.
4.3.2 Thermal Properties Because of the high glass transition temperature, Tg , elevated temperatures are required for processing. Pure PPE has a glass transition temperature of 205°C and a melting temperature of 267°C. The crucial issue is to avoid oxidative degradation during melt processing. Therefore, blends of PPE, preferably with PS are used. In blends composed from syndiotactic PS and PPE, it was discovered that the PPE destabilizes the PS somewhat in the thermal degradation region.49 Infrared (IR) spectroscopy indicates that PPE undergoes a rearrangement in which the ether link is broken and the chain is regenerated by the methyl group before mass loss is visualized. Since the degradation temperatures of syndiotactic PS are in the same region as this rearrangement, it is likely that syndiotactic PS interferes with
154
High Performance Polymers
the rearrangement of PPE by a cross termination process. Thus, the rearrangement of PPE is hindered and the stability of the blend is reduced.
4.3.3 Electrical Properties PPE exhibits a high dielectric strength. Thus, it can be used as a polymeric electret.50 Neat PPE shows an excellent charge storage behavior, which is better than that of cellular poly(propylene) and poly(ether imide).
4.4 SPECIAL ADDITIVES 4.4.1 Impact Modifiers As mentioned extensively, PPE is not mainly used as such, but in polymeric blends and copolymers to facilitate the fabrication. Some of these copolymers act also as impact modifiers; for example, block copolymers built from styrene, ethylene, butylene, and propylene.51 Naturally, the impact can be improved by using high impact poly(styrene) (HIPS) instead of ordinary PS in blends. Other impact modifiers include rubbery materials, such as poly(octenylene), and ethylene propylene diene monomer rubber.52
4.4.2 Fibers Many attempts have been made to increase the rigidity of PPE molding compositions by admixing reinforcing fibers composed of inorganic or organic material in the resin. Already by 1974, glass fibers had been suggested as reinforcing material.53, 54 In order to improve the fiber-matrix adhesion in the composition, unsized fibers with siloxanes that contain the Si−H bond have been reported.55 The treatment of the fibers with vinylsilanes or γ -glycidoxypropyl-trimethoxysilanes is advantageous to improve the adhesion.56 Another commonly used surface modification method of the reinforcing fibers consists in the treatment with aminoalkylsilanes, for example γ aminopropyltriethoxysilane. Glass fibers, which have been sized in this manner, are incorporated in numerous PPE-containing compositions. In addition, it is always necessary to modify the composition of the thermoplastic matrix to bond the fibers to the matrix. In addition to glass fibers, carbon fibers, e.g., epoxy resin-sized carbon fibers have been used for reinforcement.57
Poly(phenylene ether)s
155
4.4.3 Flame Retardants Compositions of PPE and PS are not normally flame retardant. There are instances when it is desirable to impart a degree of flame retardancy to the compositions so that the molded articles are better able to resist burning or melting when exposed to elevated temperatures or placed near an open flame. Originally, halogenated flame retardants have been preferred over phosphate-based flame retardants because the former exhibit less stresscracking. The situation has been changed, because halogens are dammed. However, there is an ongoing investigation on halogen containing flame retardants. For this reason, we are discussing the state of the art with respect to halogen containing flame retardants, before we cross over to phosphorous-based flame retardants. Brominated PS is combined with antimony trioxide as a flame retardant.58 In addition, brominated poly(phenylene ether) has been used. Other flame retardants are tetrabromocyclooctane, tetrabromovinylcyclohexene, or bis-(allyl ether) tetrabromobisphenol A. For the latter flame retardants, dicumyl peroxide is added as a synergist59 and the additives are used together with blowing agents. The halogen groups can be incorporated directly in the backbone of the PPE. Consequently, tribromophenols have been condensed to get brominated polyphenylene oxide.60 Obviously, the mechanism of polymer formation is different from that of oxidative coupling. The condensation takes place in the presence of NaOH, initially below room temperature. After condensation, a decolorization treatment is necessary. Hydrazine is used for decolorization. Corrosion problems in molding have been attributed to brominated PPE-based flame retardants when the process temperature is high. Further, they may have a negative influence on the color tone and the thermal stability of the moldings. Flame retardants based on phosphorous show certain disadvantages. Some of them have to be included in large amounts in order to achieve the desired grade of flame retardancy, however, undesired side effects then emerge. For example, resorcinol diphosphate plasticizes the composition and significantly reduces the heat deflection temperature of the formulation.61 Examples for phosphorous-based flame retardants for PPE include resorcinol diphosphate, bisphenol A diphosphate, tetraxylyl piperazine diphosphoramide, etc. Organoclay additives show synergistic effects with phosphates with respect to flame retardancy. For this reason, the amount
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High Performance Polymers
R
O Si
R Si
O O R
O Si
Si
Si R
O
O
O
O
R
Si O
R
O O
R
Si
Si O
R
Figure 4.5: Cage Silsesquioxane
of organophosphate flame retardant can be reduced, but will still be in the range of 5–30%. The flame retardancy can be influenced by the nature of the blend.36 The melt flowability and flame resistance of PPE resin compositions are remarkably improved by the addition of a specific cage silsesquioxane compound. The basic structure of a cage silsesquioxane is shown in Figure 4.5.
4.4.4 Blowing Agents Blowing agents may be introduced, in order to generate foams. Suitable blowing agents are low boiling halogenated hydrocarbons and those that generate volatile gases by chemical decomposition. The latter class includes azodicarbonamide (ADC), metal salts of ADC, 4,4 -oxybis-(benzenesulfonylhydrazide).34 Sodium bicarbonate, or ammonium carbonate are used to advantage with citric acid.58
4.5 APPLICATIONS Since poly(phenylene ether)-based resins have a lightweight and are excellent in impact resistance as compared with metal or glass, the resins have been employed in a variety of fields including automobile parts, household electric appliance parts, and office automation equipment parts. However, poly(phenylene ether) resins have a bad moldability. Therefore, the resins
Poly(phenylene ether)s
157
are not used singly but rather as a mixture with a PS-based resin, which is completely compatible.36 However, the incorporation of the poly(styrene)-based resin, which is more flammable than the poly(phenylene ether)-based resins, lowers the heat resistant temperature of the mixed resin of poly(phenylene ether)/ poly(styrene) and makes the resin flammable. Therefore, a novel method that enables molding a poly(phenylene ether)-based resin without incorporating poly(styrene) has been desired. Moreover, it has also been desirable to develop a method of achieving both moldability and flame resistance at the same time.
4.5.1 Automotive Components Highly compatible polymer blends of PPE and linear polyester resins provide beneficial improvements in the chemical resistance required for automotive applications. Such automotive applications include molded thermoplastic body panels.20 Foamable compositions of PPE resins are particularly suited as sources of lightweight structural substitutes for metals, especially in the automotive industry.
4.5.2 Adhesives Melt blending poly(phenylene ether), poly(styrene) and curable epoxy resins yields materials that are suitable for use as adhesives in electronics applications.62 In particular, the composition is useful in laminating films for electronic applications. Preferably, the PS polymer is a HIPS type. As curatives of photocatalysts, e.g., N-methyl-4-picolinium hexafluorophosphate or thermal curing agents are used. Photocatalysts include two general types: onium salts and cationic organometallic salts. Both are useful in the curing of the adhesive composition. When photocuring of the composition is desirable immediately after extrusion, i.e., before the thermoplastic polymer cools and solidifies, UV irradiation of the heated extrudate can take place directly at the die orifice.
4.5.3 Membranes PPE is widely used in such membrane separation processes, such as:63
158
High Performance Polymers • • • • •
Low-pressure reverse osmosis, Nanofiltration, Membrane gas separation Membrane vapor separation, and Polymeric electrolytes.
Literature differentiates between dense membranes and porous membranes. The latter type is subdivided into macroporous, microporous, and nanoporous membranes according to their pore sizes. Apart from the separation properties, the mechanical, thermal, and chemical stability of membranes are important parameters for estimating the performance of the membranes. In general, the efficiency of a membrane depends on the structure of the polymer and the modifiers used. PPE is particularity suitable for membranes, because it has a high free volume. The ease of the rotational motion of the phenyl rings effect a high gas diffusivity and permeability.64 PPE has comparatively hydrophobic properties. Therefore, its performance is not as sensitive to water vapor as is the case for more hydrophilic materials.65 4.5.3.1
Gas Separation Membranes
The permeation of a gas through a membrane follows Eq. 4.1. A Q˙ = k p Δp l Q˙ kp Δp A l
(4.1)
Rate of permeation as volume flow rate Permeation coefficient Pressure drop across the membrane Area of the membrane Thickness of the membrane
The selectivity α1,2 is the ratio of the permeation coefficients of two gases (1) and (2). The interaction of gases with the PPE can be studied by means of IR spectroscopy.66 The shape of some absorption regions changes, when gases, such as methane and other hydrocarbons, permeate through the membrane. The original spectrum is restored by purging with helium. Dense membranes are cast from solution, such as from trichloroethylene solution, and other solvents. It has been demonstrated that the temperature of evaporation of the solvent has a significant effect on the surface
Poly(phenylene ether)s
159
Table 4.8: Permeation Coefficients and Selectivity in Carbon Dioxide Methane Mixtures67 Membrane Material PPE PPE/HPA Sulfonated PPE
Permeability/[Barrer]a CO2 CH4 43.7 28.2 18.4
3.6 1.36 0.67
Selectivity αCO2 ,CH4 12.1 20.6 27.2
1 Barrer = 10−10 cm3 cm cm−2 s−1 cmHg−1 = 7.5 × 10−18 m2 s−1 Pa−1 Feed pressure 30 at, 30°C a
roughness and on the performance of the membranes.68 The permeability decreases with increasing solvent evaporation temperature. Sulfonated PPE membranes have been modified by exchanging the proton of the sulfonic groups with metal cations of varying valence. In this way, the polarity and density of the polymer can be tailored. Carbon dioxide, methane, oxygen, and nitrogen were used for testing the performance. Replacing the hydrogen by a metal cation increases the permeability of all the gases. It is believed that this effected by a decrease in the packing density of the membranes. However, within a series of monovalent cations with increasing ionic radius, such as Li+ , K+ , Cs+ , the permeability decreases in this order for nonpolar gases. The same is true for earth alkaline cations in the series Mg2+ , Mg2+ , Ca2+ . Therefore, it is concluded that both the size of the cation plays a role in increasing the void volume, and the change in polarity is another factor for the performance.69 In brominated PPE, the permeability of gases increases with the degree of bromination.70 A potential large scale application for gas separation emerges in natural gas purification. For this reason, the carbon dioxide/methane system is subject to extensive research. Sulfonated PPE and a blend of PPE with heteropolyacids (HPA)s has been compared with respect to their separation efficiency for mixtures of carbon dioxide and methane.67 The permeation coefficients and the selectivity are shown in Table 4.8. The permeability is dependent on the nature of the membrane. In general, in modified membranes a reduction of the permeability rate is observed, however, coupled with an increase of selectivity. In Table 4.8, the permeability values and the selectivity obtained from pure gases are reproduced. In gas mixtures the permeation rate is lower, due to the reduced partial pressure of the respective gas.
160
High Performance Polymers Table 4.9: Sorption Capacities of Poly(phenylene ether)71 Compound Ethanol Acetic acid Water Ethyl acetate
4.5.3.2
PPE
Sorption [%] PPE + 2% C60
13.0 23.6 0 23.4
15.0 24.2 0 26.4
Pervaporation Membranes
The mass transport in pervaporation is can be described by the solution diffusion model, which explains the mechanism of transport by a process consisting of: 1. Sorption of the permeants of the liquid at the upstream side of the membrane, 2. Diffusion of the permeants through the membrane, and 3. Desorption at the low-pressure side of the membrane. Therefore, the rate of permeation depends on the solubility and diffusivity. Also, the selectivity of the membrane is governed by the solubility and diffusivity.64 The separation of methanol from ethylene glycol is an important industrial process in the synthesis of poly(ethylene terephthalate).64 The methanol/ethylene glycol system has been extensively studied using various analytical methods and pervaporation experiments as well. The methanol selectivity reaches up to 250 at low methanol concentrations, however, the total flux decreases in this region. Fullerene C60 improves the transport properties of PPE membranes. Homogeneous C60 PPE membranes containing up to 2%w fullerene can be prepared by mixing PPE and C60 solutions in toluene with subsequent casting and drying.71 Table 4.9 shows the sorption capacities of PPE. The properties of such membranes can be advantageously used in esterification reactions, since water does not sorb to the polymer. Methyl-tert-butyl ether (MTBE) has received great attention as an octane enhancer, to replace tetraethyl lead. MTBE is produced by the reaction of methanol and isobutene with a strongly acidic ion exchange resin catalyst. Using an excess of methanol in the reaction results in prob-
Poly(phenylene ether)s
161
lems with respect to separation because methanol forms an azeotrope with MTBE. For this reason, alternative separation methods are of interest. PPE membranes filled with silica and silane-modified silica nanoparticles can be used for the pervaporation separation of methanol MTBE mixtures. In comparison to an unfilled PPE membrane, the filled membranes exhibit higher methanol selectivity and lower permeability.72 4.5.3.3
Membrane Catalysts
Membrane catalysts use a polymeric support. Polymer composites based on PPE as polymeric support with HPAs are used as a catalysts.73 The HPA-polymer composites are prepared by blending HPA with the polymer in a methanol/chloroform solvent mixture. These composites are active in the synthesis of tert-butyl alcohol, ethyl tert-butyl ether and in some reactions involving ethanol and MTBE. A PPE-based membrane reactor showed the best performance, among some other polymer membranes tested. 4.5.3.4
Ultrafiltration Membranes
Besides chemical treatment with chlorosulfonic acid, sulfonic groups can be introduced by plasma-initiated surface grafting using sodium styrene sulfonate.74 The introduction of ionic groups increases the surface polarity and thus the water permeation. Sulfonated membranes are an excellent base material for bipolar membranes. The deposition of allylamine plasma polymer on the surface results in bipolar, amphoteric membranes with improved ultrafiltration properties. Anchor sites can be introduced into PPE by chloromethylation using methyl chloromethyl ether in the presence of a Friedel-Crafts catalyst like SnCl4 . The chloromethylated PPE serves for the preparation of 1,2-diaminoethane modified porous membranes,28 which in turn enzymes, such as papain, can be immobilized. Papain is a proteolytic enzyme. It exhibits a proteolytic activity towards various ester and amide links, as they occur in proteins, or peptides. The performance has been tested with ultrafiltration experiments of casein solutions. The modified membranes show a self-cleaning effect. Whereas the nonenzymatic membranes are completely blocked during ultrafiltration, the enzyme-functionalized membranes do not clog.
162 4.5.3.5
High Performance Polymers Carbon Molecular Sieve Membranes
Carbon molecular sieve membranes are used in gas separation technology, for example, to recover CO2 and H2 O from natural gas, and other purification steps. A variety of polymeric precursors for carbon molecular sieve membranes are available, such as poly(imide), poly(acrylonitrile) phenolic resins, and poly(furfuryl alcohol). PPE can be modified in various ways, which procedure is advantageous for tailoring the selectivity.75 Derivatives from PPE have been functionalized with sulfonic acid groups, carboxylic acid groups, bromine, trimethylsilyl groups, and other groups in a one-step reaction. The introduction of metal cations into sulfonated PPE affects the structures of the resulting carbon membranes.76 The trimethylsilyl group is introduced by the lithiation of PPE with n-butyllithium followed by a treatment with chlorotrimethylsilane. Hollow fiber precursor membranes can be prepared by a dry/wet spinning process. After drying, the precursor membranes are preheated to 280°C and the carbonization takes place at programmed heating with 10°C/min from 550–750°C in vacuo. The properties of carbon membranes obtained from PPE by pyrolysis can be modified by a treatment in air at elevated temperatures.77, 78 Thereby an oxidation process takes place that increases the size of the pores. Also, blends of PPE as a thermally stable polymer with a thermal less stable polymer poly(N-vinyl-2-pyrrolidone) are possible. By the blending ratio of these polymers and by the conditions of pyrolysis, the permeation properties can be tailored.79 4.5.3.6
Polymer Electrolyte Membranes
The temperature of operation of polymer electrolyte membrane fuel cells tends to get higher, because certain advantages are faced, such as improved tolerance of carbon monoxide, the improved ease of water and heat management, and increased energy efficiency. However, several commonly used polymeric membranes cannot withstand the high temperatures. Therefore, there is a need to look for alternative materials. Sulfonated PPE can be treated with imidazole to get proton conducting polymer electrolytes.80 The PPE is sulfonated in chloroform solution with chlorosulfonic 81 acid. After isolation of the sulfonated PPE, the imidazole complex is formed in N,N-dimethylformamide solution by adding sulfonated PPE and
Poly(phenylene ether)s
163
imidazole in the desired amount. The mixture is homogenized in an ultrasonic bath. Membranes are cast onto glass by evaporation of the solvent. The temperature dependence of the proton conductivity suggests that the mechanism of charge transport is not achieved by vehicular transport, but rather by a concerted series of hydrogen bond formation and breaking, which is termed a Grotthus mechanism. Obviously, from this behavior, a high proton conductivity originates. Mechanical stretching of sulfonated PPE shows a great effect on electric properties.82 Under favorite conditions, electrical conductivities increase up to 10 times that of the original membranes. Composite membranes prepared by casting Nafion® 15 on films made from PPE and phosphomolybdic acid show a lower methanol permeability in comparison to pure Nafion® 15.83 The composite membranes have a potential application as electrolytes in direct methanol fuel cells.
4.6 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 4.10. Commercial grades are mostly blends with PS. Some types are fiber reinforced, or formulated as flame retardant. Most of the tradenames listed in Table 4.10 are available in different grades. Tradenames appearing in the references are shown in Table 4.11.
4.7 SAFETY Phenols are toxic materials and must be handled with caution. Inhalation and dermal exposure to phenols is highly irritating to the skin and eyes. No known acute or chronic health hazards are associated with exposure to PPE resins. However, the gases of degradation are toxic, but ranging in the rear ranks, as tested with mice84 Therefore, fabrication devices and areas where gases from melted polymer may be produced should be adequately vented.
4.8 ENVIRONMENTAL IMPACT AND RECYCLING The oxidative polymerization of 2,6-dimethylphenol proceeds near room temperature. Moreover, the polymerization reaction does not create any leaving groups. From the aspect of green chemistry, water as a solvent for
164
High Performance Polymers
Table 4.10: Examples for Commercially Available PPE Polymers48 Tradename
Producer
Remarks
ACCUGUARD™ ACCUTECH™ ACNOR™ ASHLENE® Noryl® Norylux™ PPO PMC EP PX1000 PRL PPX QR Resin QR4000 Styvex TARONYL Thermocomp TYNELOY® Vestoran Xyron®
ACLO Compounders Inc. ACLO Compounders Inc Aquafil Technopolymers S.p.A Ashley Polymers, Inc. General Electric Westlake Plastics Company PMC Engineered Plastics, Inc. Polymer Resources Ltd. QTR, Inc.
Pellets Glass fiber reinforced PPE+PS General Purpose
Ferro Corporation Taro Plast S.p.A. LNP Engineering Plastics Inc. Tyne Plastics LLC. Degussa AG Asahi Kasei Corporation
PPE+PS PPE+PS+PA PPE+PS
PPE+PS+PA PPE+PS PPE+PS PPE+PS+PA
the oxidative polymerization would be desirable. The oxidation of 2,6-dimethylphenol suffers from producing mainly TMDQ. However, by using an excess of oxidant, the formation of TMDQ can be suppressed, even in an alkaline aqueous solution.19 Water soluble copper catalysts, which is a complex of CuCl2 × 2H2 O and diethylenetriamine-N,N,N ,N ,N -pentaacetic acid and other related complexes have been developed. Surfactants increase the yield of polymer. The products from the thermal degradation of Noryl GTX poly(phenylene oxide)-polyamide in air and in nitrogen have been identified and quantified. Ecotoxicologic testing of the products of pyrolysis with aquatic organisms indicated that in a fire, no greater harm than burned beech wood is to be expected when the fire-fighting water reaches aquatic ecosystems.85 Artificially prepared mixtures of poly(vinyl chloride), poly(carbonate), poly(oxymethylene) and PPE can be separated by a flotation technique using common wetting agents like sodium lignin sulfonate, tannic acid, poly(oxyethylene), and saponin.86 The order of flotation response is given in Table 4.12.
Poly(phenylene ether)s
Table 4.11: Tradenames in References Tradename Description
Supplier
Black Pearls® Cabot Corp. Carbon black51 Blendex™ General Electric Poly(2,6-dimethylphenylene ether)62 Boltorn® (Series) Perstorp Specialty Chemicals Dendritic poly(ester)s41 Buna® AP 437 Bunawerke Hüls Gmbh 52 EPDM Cabelec® Cabot Corp. Conductive carbon black masterbatch in PA 651 Calprene® Repsol Styrene-(ethylene-butylene)-styrene triblock copolymer51 Cariflex® Shell Triblock copolymer34, 35, 40, 41 Conductex® Columbian Chemical Corp. Carbon black51 Disflamoll® DPK Bayer AG Diphenylcresyl phosphate57 DYLARK® Nova Chemicals S.A. (Arco Chemical Co.) Copolymers of styrene with maleic anhydride62 Epon® (Series) Resolution Performance Products LLC. Corp. (Shell) Diglycidyl ethers of bisphenol A62 ERL™ Union Carbide Corp. Alicyclic epoxides62 Fortafil® Fortafil Fibers, Inc. Carbon fiber51 Geloy® resin General Electric ASA copolymer35 Grafil® fibers Courtaulds Advanced Materials Carbon fiber51, 57
165
166
High Performance Polymers
Table 4.11 (cont): Tradenames in References Tradename Description
Supplier
Irganox® 1010 Ciba Geigy Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), phenolic antioxidant51, 57 Irganox® 1076 Ciba Geigy Octadecyl-3-(3 ,5 -di-tert-butyl-4 -hydroxyphenyl) propionate51 Irganox® 1098 Ciba Geigy N,N -hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide)51 Kapton® DuPont-Toray Co., Ltd. Poly(imide)62 Ketjenblack® Akzo Conductive carbon black51 Kevlar® DuPont Aramid34, 41 Kraton® Shell Styrenic block copolymer34, 35, 41, 51, 62 Luran® BASF AG SAN copolymer62 Nirez® 2150/7042 Arizona Chemical Co. Terpene phenol flow modifier41 Noryl® General Electric PPE PS Blend2, 62 Sandostab® 4020 Clariant GmbH Pentaerythritol tetrakis(3-laurylthiopropionate)51 Sapron™ S DSM Engineering Plastics SMA copolymer62 Septon® Kuraray Co., Ltd. 51 Hydrogenated styrenic block copolymer Sniamid® ASN 32 Rhodia Inc. Poly(amide)51 Solprene® Philips Petroleum Co. (Industrias Negromex, S.A.) Styrenic block copolymer51 Surlyn® DuPont Ionomer resin51
Poly(phenylene ether)s
Table 4.11 (cont): Tradenames in References Tradename Description
Supplier
Tenax® Carbon fiber57 Torayca® Carbon fiber51 Ultramid® (Series) Poly(amide)52 Valox® 315 Poly(butylene terephthalate)20 Vector® Styrenic block copolymer51 Vestamid® Poly(amide)52 Vestenamer® 8012 Poly(octenylene)52 Vulcan® XC72 Carbon black51 Zoltek® HT Carbon fiber51 Zytel® Poly(amide)51
Akzo Toray Industries, Inc. BASF AG General Electric Dexco Polymers LP Hüls Hüls Cabot Corp. Zoltek Corp. DuPont
Table 4.12: Order of Flotation Response of Polymers86 Wetting agent Tannic acid Lignin sulfonate Poly(oxyethylene)
Polymer PPE = POM > PVC = PC PPE = POM > PC PVC PPE > POM = PC PVC
167
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High Performance Polymers
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Polym. Sci., Part A: Polym. Chem., 32(7):1361–1369, May 1994. 30. J. Zhang and X. Hou. “The gas permeation property in trimethylsilyl-substituted PPO and triphenylsilyl-substituted PPO.” J. Membr. Sci., 97(1):275– 282, January 1994. 31. Y. S. Bhole, P. B. Karadkar, and U. K. Kharul. “Nitration and amination of polyphenylene oxide: Synthesis, gas sorption and permeation analysis.” Eur. Polym. J., 43(4):1450–1459, April 2007. 32. A. J. F. M. Braat, H. S.-I. Chao, H. Guo, J. Liska, and G. W. Yeager. Compositions comprising functionalized polyphenylene ether resins. US Patent 6 780 959, assigned to General Electric Company (Pittsfield, MA), August 24, 2004. 33. A. J. F. M. Braat, R. de Jongh, and J. Liska. Process for the manufacture of functionalized polyphenylene ether resins. US Patent 6 417 274, assigned to General Electric Co. (Schenectady, NY), July 9, 2002. 34. H. Guo, N. Devanathan, and C. Lewis. Copolymers of functionalized polyphenylene ether resins and blends thereof. US Patent 6 620 885, assigned to General Electric Company (Pittsfield, MA), September 16, 2003. 35. R. van der Meer and J. B. Yates, III. Functionalized polyphenylene ether from polyphenylene ether chain terminated with phenoxy moiety containing amino and hydroxy group. US Patent 4 888 397, assigned to General Electric Company (Schenectady, NY), December 19, 1989. 36. H. Saito and M. Ikeda. Polyphenylene ether-based resin composition containing silicon compound. US Patent 7 122 591, assigned to Asahi Kasei Kabushiki Kaisha (Osaka, JP), October 17, 2006. 37. N. A. McGaughan, G. F. Lee, Jr., R. J. Wroczynski, J. B. Yates, C. H. J. Koevoets, J. P. Keulen, and J. K. Gianchandani, deceased. Compositions of poly(phenylene ether) and polyamide resins, which exhibit improved beard growth reduction. US Patent 5 981 656, assigned to General Electric Company (Pittsfield, MA), November 9, 1999. 38. S. B. Brown, C.-F. R. Hwang, H. Ishida, J. J. Scobbo, Jr., and J. B. Yates, III. Functional poly(phenylene ether)/poly(arylene sulfide)/epoxy function alpha olefin elastomer/elastomeric block copolymer/metal salt compositions and process for making thereof. US Patent 6 303 708, assigned to General Electric Company (Pittsfield, MA), October 16, 2001. 39. A. Z. Aroguz and B. M. Baysal. “Miscibility studies on blends of poly(phenylene oxide)/brominated polystyrene by viscometry.” Eur. Polym. J., 42(2): 311–315, February 2006. 40. N. Patel. High flow polyphenylene ether formulations. US Patent 7 056 973, assigned to General Electric (Pittsfield, MA), June 6, 2006. 41. A. Adedeji. High flow polyphenylene ether formulations with dendritic polymers. US Patent 6 809 159, assigned to General Electric Company (Pittsfield, MA), October 26, 2004.
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42. T. Tokiwa. Functionalized polyphenylene ether resin. US Patent 6 835 795, assigned to Asahi Kasei Kabushiki Kaisha (Osaka, JP), December 28, 2004. 43. E. W. Walles, J. H. Lupinski, M. Markovitz, R. E. Colborn, J. R. Presley, M. J. Davis, M. G. Minnick, S. J. Kubisen, Jr., J. E. Hallgren, D. A. Bolon, V. J. Eddy, and P. C. Irwin. Printed circuit board from fibers impregnated with epoxy resin mixture, halogenated bisphenol and polyphenylene ether. US Patent 4 975 319, assigned to General Electric Company (Schenectady, NY), December 4, 1990. 44. K. Ishii, Y. Norisue, K. Hiramatsu, M. Miyamoto, M. Yamazaki, and D. Ohno. Polyphenylene ether oligomer compound, derivatives thereof and use thereof. US Patent 6 835 785, assigned to Mitsubishi Gas Chemical Company, Inc. (Tokyo, JP), December 28, 2004. 45. B. J. P. Jansen, H. E. H. Meijer, and P. J. Lemstra. “Processing of (in)tractable polymers using reactive solvents. Part 5: Morphology control during phase separation.” Polymer, 40(11):2917–2927, May 1999. 46. D. Aycock, V. Abolins, and D. M. White. Poly(phenylene ether). In H. F. Mark, N. Bikales, C. G. Overberger, and G. Menges, editors, Encyclopedia of Polymer Science and Engineering, volume 13, pages 1–31. Wiley Interscience, New York, 2nd edition, 1988. 47. A. S. Hay. “Poly(2,6-diphenyl-1,4-phenylene oxide).” Macromolecules, 2: 107–108, 1969. 48. IDES Integrated Design Engineering Systems. The Plastics Web®. IDES Inc. 209 Grand Avenue Laramie, WY 82070 USA [electronic] http://www.ides.com/prospector/, 2006. 49. S. Stack, O. O’Donoghue, and C. Birkinshaw. “The thermal stability and thermal degradation of blends of syndiotactic polystyrene and polyphenylene ether.” Polym. Degrad. Stabil., 79(1):29–36, 2003. 50. D. Lovera, H. Ruckdaschel, A. Goldel, N. Behrendt, T. Frese, J. K. Sandler, V. Altstadt, R. Giesa, and H.-W. Schmidt. “Tailored polymer electrets based on poly(2,6-dimethyl-1,4-phenylene ether) and its blends with polystyrene.” Eur. Polym. J., 43(4):1195–1201, April 2007. 51. J. H. P. Bastiaens, G. J. C. Doggen, and J. G. M. van Gisbergen. Conductive polyphenylene ether-polyamide composition, method of manufacture thereof, and article derived therefrom. US Patent 7 022 776, assigned to General Electric (Pittsfield, MA), April 4, 2006. 52. W. Neugebauer. High impact strength thermoplastic molding compositions based on polyphenylene ether graft copolymers and polyamides and process for producing them. US Patent 5 115 044, assigned to Huels Aktiengesellschaft (Marl, DE), May 19, 1992. 53. A. Katchman. Glasverstärkte Zusammensetzungen aus Polyphenylenaethern und kristallinen Styrolharzen. DE Patent 2 364 901, assigned to General Electric, July 04, 1974.
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54. A. Katchman. Glass reinforced compositions of polyphenylene ethers and crystal styrene resins. GB Patent 1 445 605, assigned to General Electric, August 11, 1976. 55. R. van der Meer. Polymer mixture comprising polyphenylene ether and reinforcing fibres. US Patent 4 749 737, assigned to General Electric Company (Selkirk, NY), June 7, 1988. 56. A. Nakanishi, S. Izawa, and K. Toyama. Reinforced polyphenylene ether compositions. US Patent 3 708 455, January 2, 1973. 57. T. Grosse-Puppendahl, C. Baron, and F. G. Schmidt. Fiber-reinforced polyphenylene ether molding compositions and process for their preparation. US Patent 5 338 789, assigned to Huels Aktiengesellschaft (Marl, DE), August 16, 1994. 58. E. Pressman. Modified flame retardant polyphenylene ether resins having improved foamability and molded articles made therefrom. US Patent 4 791 145, assigned to General Electric Company (Selkirk, NY), December 13, 1988. 59. J. M. Joyce and D. J. Kelley. Polyphenylene ether-alkenyl aromatic polymer blends having organobromine additives. US Patent 4 927 858, assigned to Huntsman Chemical Corporation (Salt Lake City, UT), May 22, 1990. 60. H. Onishi. Brominated polyphenylene oxide and flame retardant employing the brominated polyphenylene oxide. US Patent 6 864 343, assigned to Dai-Ichi Kogyo Seiyaku Co., Ltd. (Kyoto, JP), March 8, 2005. 61. N. Patel. Fire retardant polyphenylene ether-organoclay composition and method of making same. US Patent 6 579 926, assigned to General Electric Company (Pittsfield, MA), June 17, 2003. 62. R. S. Clough and M. A. Perez. Melt blending polyphenylene ether, polystyrene and curable epoxy. US Patent 6 518 362, assigned to 3M Innovative Properties Company (Saint Paul, MN), February 11, 2003. 63. G. Chowdhury, B. Kruczek, and T. Matsuura, editors. Polyphenylene oxide and modified polyphenylene oxide membranes: gas, vapor and liquid separation. Kluwer Academic Publishers, Dordrecht, The Netherlands, 2001. 64. M. Khayet, J. P. G. Villaluenga, M. P. Godino, J. I. Mengual, B. Seoane, K. C. Khulbe, and T. Matsuura. “Preparation and application of dense poly(phenylene oxide) membranes in pervaporation.” J. Colloid Interface Sci., 278(2): 410–422, October 2004. 65. M. Pourafshari Chenar, M. Soltanieh, T. Matsuura, A. Tabe-Mohammadi, and K. C. Khulbe. “The effect of water vapor on the performance of commercial polyphenylene oxide and cardo-type polyimide hollow fiber membranes in CO2 /CH4 separation applications.” J. Membr. Sci., 285(1-2):265– 271, November 2006. 66. F. Hamad, K. C. Khulbe, and T. Matsuura. “Interaction of gaseous hydrocarbons with poly(phenylene oxide) membranes by infrared spectroscopic technique.” J. Membr. Sci., 204(1-2):27–36, July 2002.
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67. S. Sridhar, B. Smitha, M. Ramakrishna, and T. M. Aminabhavi. “Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane.” J. Membr. Sci., 280(1-2):202–209, September 2006. 68. K. C. Khulbe, F. Hamad, C. Feng, T. Matsuura, T. Gumi, and C. Palet. “Characterization of the poly(phenylene oxide) dense membrane prepared at different temperatures.” Sep. Purif. Tech., 36(1):53–62, April 2004. 69. F. Hamad, G. Chowdhury, and T. Matsuura. “Effect of metal cations on the gas separation performance of sulfonated poly (phenylene oxide) membranes.” Desalination, 145(1-3):365–370, September 2002. 70. F. Hamad, K. C. Khulbe, and T. Matsuura. “Characterization of gas separation membranes prepared from brominated poly (phenylene oxide) by infrared spectroscopy.” Desalination, 148(1-3):369–375, September 2002. 71. G. A. Polotskaya, A. V. Penkova, and A. M. Toikka. “Fullerene-containing polyphenylene oxide membranes for pervaporation.” Desalination, 200(1-3): 400–402, November 2006. 72. M. Khayet, J. P. G. Villaluenga, J. L. Valentin, M. A. Lopez-Manchado, J. I. Mengual, and B. Seoane. “Filled poly(2,6-dimethyl-1,4-phenylene oxide) dense membranes by silica and silane modified silica nanoparticles: Characterization and application in pervaporation.” Polymer, 46(23):9881–9891, November 2005. 73. I. K. Song and W. Y. Lee. “Heteropolyacid (HPA)-polymer composite films as heterogeneous catalysts and catalytic membranes.” Appl. Catal., A, 256 (1-2):77–98, December 2003. 74. G. Pozniak, I. Gancarz, and W. Tylus. “Modified poly(phenylene oxide) membranes in ultrafiltration and micellar-enhanced ultrafiltration of organic compounds.” Desalination, 198(1-3):215–224, October 2006. 75. M. Yoshimune, I. Fujiwara, and K. Haraya. “Carbon molecular sieve membranes derived from trimethylsilyl substituted poly(phenylene oxide) for gas separation.” Carbon, 45(3):553–560, March 2007. 76. M. Yoshimune, I. Fujiwara, H. Suda, and K. Haraya. “Gas transport properties of carbon molecular sieve membranes derived from metal containing sulfonated poly(phenylene oxide).” Desalination, 193(1-3):66–72, May 2006. 77. H.-J. Lee, M. Yoshimune, H. Suda, and K. Haraya. “Effects of oxidation curing on the permeation performances of polyphenylene oxide-derived carbon membranes.” Desalination, 193(1-3):51–57, May 2006. 78. H.-J. Lee, D.-P. Kim, H. Suda, and K. Haraya. “Gas permeation properties for the post-oxidized polyphenylene oxide (PPO) derived carbon membranes: Effect of the oxidation temperature.” J. Membr. Sci., 282(1-2):82–88, October 2006. 79. H.-J. Lee, H. Suda, K. Haraya, and S.-H. Moon. “Gas permeation properties of carbon molecular sieving membranes derived from the polymer blend
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High Performance Polymers of polyphenylene oxide (PPO)/polyvinylpyrrolidone (PVP).” J. Membr. Sci., 296(1-2):139–146, June 2007. Y. Liu, Q. Yu, and Y. Wu. “More studies on the sulfonated poly(phenylene oxide)+imidazole bronsted acid-base polymer electrolyte membrane.” J. Phys. Chem. Solids, 68(2):201–205, February 2007. Y. Liu, Q. Yu, J. Yuan, L. Ma, and Y. Wu. “Bronsted acid-base polymer electrolyte membrane based on sulfonated poly(phenylene oxide) and imidazole.” Eur. Polym. J., 42(9):2199–2203, September 2006. C. Li, J. Liu, R. Guan, P. Zhang, and Q. Zhang. “Effect of heating and stretching membrane on ionic conductivity of sulfonated poly(phenylene oxide).” J. Membr. Sci., 287(2):180–186, January 2007. J. Sauk, J. Byun, and H. Kim. “Composite nafion/polyphenylene oxide (PPO) membranes with phosphomolybdic acid (PMA) for direct methanol fuel cells.” J. Power Sources, 143(1-2):136–141, April 2005. C. J. Hilado and P. A. Huttlinger. “Polymer structure and relative toxicity of off-gases.” J. Elastomers Plast., 13:177–182, 1981. G. Schmaus, B. Beck, G. Matuschek, and A. Kettrup. “Thermolysis of new plastics and ecotoxicological evaluation.” J. Therm. Anal., 47:485–491, 1996. J. Shibata, S. Matsumoto, H. Yamamoto, E. Kusaka, and Pradip. “Flotation separation of plastics using selective depressants.” Int. J. Miner. Process., 48 (3-4):127–134, December 1996.
5 Poly(phenylene sulfide) Poly(phenylene sulfide) (PPS) is a polymer composed of a series of alternating aromatic rings and sulfur atoms. More correctly it should be addressed as poly(p-phenylene sulfide). PPS is a high quality engineering polymer. Compounds containing the −S− group are called thioethers or sulfides. Compounds containing the −S−S− group are called dithioethers. Poly(thioether)s should not be confused with poly(sulfide)s, in that the term poly refers directly to the sulfide linkage, i.e., −Sn −, but at the same time to a polymer. These types of polymers are used in a completely different field of application, e.g., additives for elastomers, antioxidants for lubricating oils, intermediates for the production of organic chemicals, insecticides, germicides, and as an additive to diesel fuels to improve the octane number and ignition qualities of these fuels.1 These polymeric types are not dealt with in this chapter. PPS was discovered by Charles Friedel and James Mason Crafts in 1888.2 In 1967, Edmonds and Hill of Phillips Petroleum Company developed a method for producing PPS through the synthesis of p-dichlorobenzene and sodium sulfide.3 The commercial production of PPS started in 1972. In addition, varieties of PPS have been described,4 which are shown in Figure 5.1. Poly(arylene thioether ketone)s have an excellent heat resistance, but they have poor heat stability upon melting (melt stability). Poly(arylene thioether ketone ketone)s, are not suitable for industrial production because particular polymerization solvents and monomers must be used.4 Poly(arylene thioether ketone ketone) has a melting point as extremely 175
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High Performance Polymers
O C
S
Poly(arylene thioether-ketone) O
O
C
C
S
S
Poly(arylene thioether thioether ketone ketone) O
O
C
C
S
Poly(arylene thioether ketone ketone)
Figure 5.1: Varieties of Poly(phenylene sulfide)4
Table 5.1: Monomers for Poly(phenylene sulfide) Monomer
Remarks
p-Dichlorobenzene 4,4 -Dibromobiphenyl 4,4 -Difluorobiphenyl 1,3,5-Trichlorobenzene p-Nadimidochlorobenzene Bis-(pentafluorophenyl)-sulfide
Standard Comonomer Comonomer Branched polymers5 Thermosets5 For poly(aryl ether sulfide)s, optical applications6 For poly(aryl ether sulfide)s Laboratory preparation7
Bis-(4-fluorophenyl)-sulfide Copper 4-bromobenzenethiolate
high at about 410°C. Accordingly, their melt processing temperature are high so that they tend to loose their crystallinity or to undergo crosslinking or carbonization, resulting in a rapid increase in melt viscosity, upon their melt processing.4
5.1 MONOMERS Monomers for the synthesis of PPS are shown in Table 5.1 and Figure 5.2. Most common is the use of p-dichlorobenzene.
Poly(phenylene sulfide)
Cl
Cl
F
F
1,4-Dichlorobenzene Cl
4,4′-Difluorobiphenyl O N
Cl Cl 1,3,5-Trichlorobenzene
177
Cl
O p-Nadimidochlorobenzene
Figure 5.2: Monomers Used for Poly(phenylene sulfide)
Cl + Na2S
Cl
S
Figure 5.3: Synthesis of Poly(phenylene sulfide)
5.2 POLYMERIZATION AND FABRICATION 5.2.1 Standard Procedure The conventional process for manufacturing PPS is by reacting a halogenated aromatic compound, such as p-dichlorobenzene and sodium sulfide (Na2 S × 9H2 O) in an aprotic organic solvent; for example, N-methyl2-pyrrolidone (NMP) under a nitrogen atmosphere. The synthesis of PPS is shown in Figure 5.3. Originally, in a preliminary heating, up to 160°C, the water in the sodium sulfide is removed. Then, over a period of 88 h the temperature is gradually raised up to 260°C.3 The material obtained by this procedure can be molded to a hard film at 290°C. However, it is less time-consuming to conduct the reaction in a stainless steel bomb. 5.2.1.1
Dehydration
It is important to eliminate the water of the hydrated sodium sulfide for producing a proper sulfidizing agent. Sodium sulfide nonahydrate is dehydrated by means of distillation in a large amount of an organic amide solvent. In practice, the water content is 0.8–1.2 mol mol−1 of alkali metal
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sulfide. If the water content exceeds 2.5 mol mol−1 , the reaction rate is low and byproducts, such as phenols are formed that increase the final purification step. Further, the degree of polymerization is low. If it is less than 0.5 mol mol−1 , the reaction rate is too fast to obtain sufficiently high-molecular-weight one, and unfavorable reactions, such as side reactions, may occur.8 In the course of the dehydration step, hydrogen sulfide also escapes. This may cause a problem, since the amount of the alkali metal sulfide varies in an uncontrolled manner. In order to reach a high degree of polymerization, the alkali metal sulfide and the dichloro aromatic compound should be there in a stoichiometric ratio. Further, the hydrogen sulfide vaporized off in the dehydration step is a harmful substance. Therefore, the amount of hydrogen sulfide escaped in the dehydration step should be analyzed precisely to get the amount of sulfur still present in the reaction vessel. The hydrogen sulfide lost should be captured for further reuse due to environmental demands.9 The escaping hydrogen sulfide can be captured in NMP outside the reaction vessel. The absorption temperature is below room temperature at pressure. The sulfide may be regenerated by alkali. 5.2.1.2
Catalyst
The time of synthesis of the polymer can be shortened by reducing either the time of water elimination of the sulfide compound and by the acceleration of the rate of polymerization by the choice of proper polymerization catalysts. Suitable polymerization catalysts are organic metal carboxylates. Lithium salts are highly soluble in the reaction system and large in the catalytic effect, but expensive. Potassium, rubidium, cesium, and alkaline earth metal salts are considered poorly soluble in the reaction system. Sodium salts can preferably be used since they are cheap and moderately soluble in the polymerization system. Thus, sodium acetate is a preferred catalyst.10 Another class of catalysts are cyclic amine compounds, N-heteroaromatic compounds, and organic sulfoxide compounds. The effect of lithium as a catalyst is illustrated in Table 5.2. Both polymers in Table 5.2 are prepared in a nearly similar manner. The dehydration is done for 50 min to a final temperature of 210–220°C. The molar ratio of catalyst to sulfur compound is adjusted to 0.60. A final temperature
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Table 5.2: Effect of a Lithium Catalyst11 Molar Ratio H2 O/S
a
Catalyst
Melt Flow Ratea
0.93 Lithium benzoate 30 1.19 Sodium benzoate 100 Melt flow rate of polymer g/10 min, ASTM D 1238-86,
of the end of the 265°C is reached.11 A problem in the conventional process resides in the difficulty in removing the byproduct sodium chloride from resins by washing, since sodium chloride is insoluble in the solvents, such as NMP, and incorporated in the resins. A process using a lithium salt instead of the sodium salt is attracting attention as a process for overcoming this problem. Lithium chloride is produced as a side product in the polymerization reaction. It is soluble in many of the aprotic organic solvents, such as NMP. Therefore, it is relatively easy to reduce the lithium content in the resin. However, lithium is far more expensive than sodium, so it is essential to recover and to reuse the lithium. Methods to recycle the lithium have been proposed.12, 13 In the early stage of the poly(arylene sulfide) (PAS) production, it was possible only to obtain a high-molecular-weight polymer by preparing a polymer of a low degree of polymerization and then heating the polymer in the presence of air to subject it to partial oxidative crosslinking. Meanwhile, the production process has been improved to develop a process for obtaining a high-molecular-weight PAS by a polymerization reaction. These processes have permitted the provision of linear, high-molecularweight PAS types. The problems arising in the production of high-molecular-weight types can be summarized as follows:9 1. The ratio of the alkali metal sulfide to monomer varies due to the vaporization of hydrogen sulfide. This causes the degree of polymerization of the polymer to vary. 2. A reproducible process for the production of PAS with a high degree of polymerization is cumbersome due to the vaporization of hydrogen sulfide. A high-molecular-weight PPS can be obtained when the polymerization process is conducted in two steps. In the first step, a prepolymer is obtained. In the second step, the ratio of water to sulfide is increased, as well as the polymerization temperature is increased, and the process
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is completed under these conditions.14 A process and equipment for the continuous polymerization of PPS has been described.15 5.2.1.3
Modifiers
As molecular weight modifier, monohalogenated compounds can be used. In contrast, for forming a branched or crosslinked polymer, it is possible to use a branching or crosslinking agent; for example, 1,3,5-trichlorobenzene or 1,2,4-trichlorobenzene. Also, halogenated aromatic compound with an active hydrogen are functional, such as the various dichloroanilines.10 The sodium salt of benzosulfimide, i.e., sodium saccharinate, the cyclic imide of o-sulfobenzoic acid, increases the molecular weight of the polymer. The mechanism is not explained.16 5.2.1.4
Cleanup
When the polymerization step is complete, it is necessary to recover of the solvent, and to wash and dry the resin. The work-up consists basically of the following steps:17 • Cooling the reaction mixture to form particulate PAS, • Separation of the high molecular polymer from linear and cyclic oligomers, unreacted reactants, and water, and • Vacuum drying. Several variations of this process have been described.17 By the addition of NMP, the reaction mixture separates into a more dense polymer-rich liquid phase, and into a less dense phase, containing the oligomers, and unreacted reactants. The less dense phase can be used for further recovery of a high molecular fraction or reuse in a further polymerization step.18, 19 A process for continuous washing with a countercurrent flow of the washing liquid has been designed. The washing liquid should be miscible with water and the solvent used in a polymerization reaction, notably NMP. For recycling by distillation, it should have a lower boiling point than water. In the washing process, NMP, the salty byproducts and the oligomers are extracted and washed out from the particulate PPS. The washing process consists of two stages, where the washing fluids are acetone and water, respectively.20 The wet cake is then dried for 13 h in an oven at 105°C in order to recover the polymer. The dried particles exhibit a NMP concentration of 200
Poly(phenylene sulfide)
181
ppm, an Na+ concentration of 1,300 ppm. The amount of PPS recovered by a two-fold continuous washing operation is 95% of the PPS originally in the polymer slurry. The process does not include a commercially problematic sieving step as essential, but can effectively recover PPS particles from the polymer slurry. As a method for raising the crystallization temperature, the polymer is treated in a strong acid solution of a pH less than 2.8 PPS with high whiteness and high melt viscosity are obtained by adding small amounts of a zinc compound to the polymerization feed.8 The zinc compound should be soluble in a PPS slurry after the reaction and is preferably zinc chloride. The particle size of the PPS formed in the reactor can be controlled by adjusting the reflux in the final stage of the polymerization reaction. This effect is explained as follows: Upon cooling or heating the upper part of the reactor, the amount of a reflux from the gaseous phase in the reactor increases or decreases, respectively. The reflux composition is rich in water, compared to that in the liquid phase bulk. Accordingly, the distribution of water near the upper surface of the liquid phase in a reactor is influenced and, in turn, a water content in the liquid phase bulk changes, which makes a particle size of PAS finer or coarser.21, 22 The molecular weight of PPS can be increased by a heat treatment. This process is termed as curing. Additionally, in the course of the heat treatment, the powder is compacted. PPS powders and compacted powders are generally cured before being used to form final products.23 Originally, the curing processes have been conducted as a batch process. However, a semi-continuous process, whereby the uncured polymer is added intermittently and the cured polymer is removed intermittently, from the curing vessels, has been described.24 Additives to effect curing can be used. These include o-dihydroxybenzene, p-quinone and quinhydrone.25
5.2.2 Other Methods of Preparation Copper 4-bromobenzenethiolate can be polymerized at 200°C in quinoline solution.7, 26 Electron spin resonance spectroscopy shows the existence of organic free radicals throughout the polymerization. In addition, Cu2+ are observed. These phenomena suggest a radical mechanism of polymerization. It was early suspected that the reaction mechanism should not be a
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normal polycondensation.27 In addition, PPS can be prepared by oxidation of thiophenol with thionyl chloride in the presence of superacids.28
5.2.3 Oxidized Poly(phenylene sulfide) The sulfide moiety of PPS can be oxidized to a sulfoxide group. This effects a higher temperature stability. The oxidation has been performed as a polymer analogue reaction using acetic acid and concentrated nitric acid for 24 h at 0–5°C.29 However, in this process, long reaction times are required, and the attack of the aromatic rings by nitric acid may occur. The oxidation of PASs with ozone leads to the formation of poly(arylene sulfoxide)s with a high selectivity. This reaction takes place although ozone is an extremely strong oxidant. Thus, an appropriate amount of ozone enables either to partially or completely convert the sulfur bond into the sulfoxide bond.30 The oxidation is carried out in an ozone/inert gas stream in which the ozone is present in a concentration of 2–6% by volume. The selection of the suspension medium has a decisive influence. In methylene chloride suspension, complete oxidation to the sulfoxide is achieved in a short time. The sulfide group in PPS can be oxidized in a mixture of glacial acetic acid, concentrated sulfuric acid, and concentrated hydrogen peroxide at 55°C within 3 h to yield a polymer with sulfone groups.31 This modified polymer is used with poly(tetrafluoroethylene) (PTFE) for composites with improved abrasion resistance.
5.2.4 Copolymers In an essentially similar way as in the conventional process, copolymers with phenylene and biphenylene units separated by sulfide groups have been prepared. The biphenylene unit is made up from 4,4 -diflurobiphenyl or 4,4 -dibromobiphenyl and the phenylene unit is formed from p-dichlorobenzene.32 Sodium acetate and sodium hydrogen sulfide are charged in an autoclave and NMP is used as a solvent. At the end of the heating period of 150 min up to 310°C, a pressure of around 25 bar develops in the autoclave. A two-step process uses the synthesis of a PPS oligomer of a degree of polymerization of ca. 5 in the first step. This oligomer is coupled to 4,4 -dichlorobenzophenone to give a copolymer bearing ketone and sulfide
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linkages in the backbone.4 The copolymers have melt a stability sufficient to permit the application of conventional melt processing techniques.
5.2.5 Thermosets 5.2.5.1
Nadimide Units
When p-nadimidochlorobenzene as a monofunctional dichloro compound is added to the reaction mixture of sulfide and aromatic dichloro compound, an end capped oligomer with a pendant double bond is obtained.5 Further, a trifunctional compound, such as trichlorobenzene, may be added to compensate the monofunctional compound with respect to molecular weight. Oligomers with a molecular weight of 1,000–6,000 Dalton can be readily obtained. The oligomerization is achieved at 225–260°C. At this temperature, the double bonds are still not reactive. A solution of the oligomer in tetrahydrofuran (THF) is used to impregnate fabrics. The fabrics are dried and consolidated under 7 bar. Curing is achieved at 330°C for 1–2 h. The final composites exhibit superior solvent resistance, resistance to delamination, shear strength, thermo-oxidative, and thermomechanical properties. The method of end capping is not restricted to PPS as an oligomer and nadimide as an end cap. According to this principle described above, a variety of thermosets with different backbones can be created.33
5.2.5.2
Ethynyl Units
A fluorinated poly(arylene ether sulfide) (PSI) is prepared from the condensation of bis-(pentafluorophenyl)-sulfide with 4,4 -(trifluoromethylphenylisopropylidene)diphenol, which is a methyl fluorinated bisphenol A.34 In an analogous reaction, bis-(pentafluorophenyl)-sulfone is used for the preparation of poly(arylene ether sulfone) (PAES). Bis-(pentafluorophenyl)-sulfone is prepared by the oxidation of bis-(pentafluorophenyl)sulfide. In the next step, 3-ethynylphenol or 4-(phenylethynyl)phenol is attached to the PSI, or PAES.35 These groups are capable of thermal crosslinking. Fluorinated PSI6 or PAES36 can be used for optical waveguide applications.
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Modifier
Remark/Reference
Glycidyl methacrylate copolymers Ethylene/butyl acrylate/maleic anhydride copolymers Styrene/ethylene-butylene/styrene block copolymer Poly(amide) (PA), MgO
Improved impact strength37–39 Improved impact strength40
Silicone rubber and aminosilane Liquid crystalline polymers
Improved impact strength39 Improved electrical properties, in glass fiber applications41 Improved mechanical properties42 Viscosity reduction43
5.2.6 Blends and Composites For certain applications, the properties of PPS can be adapted or improved by the fabrication of compositions. Compositions based on PPS are summarized in Table 5.3. 5.2.6.1
Impact Strength Modification
Poly(phenylene sulfide) exhibits a low impact strength and hence is brittle. Attempts to improve the impact strength go back to 1983, when ethylene/glycidyl methacrylate copolymers were incorporated into PPS as impact modifiers.37 However, the adhesion of impact modifiers to PPS at the interface is not satisfactory, and an improvement was suggested in the treatment of the PPS by aqueous acid before use, to improve the adhesion properties.38 The modification of PPS with silicone rubber and aminosilane improves the mechanical properties.42 It is believed that the aminosilane functions as a type of compatibilizer between the silicone rubber and PAS. It has been found that PAS resins lacking an aminosilane additive and containing a functionalized silicone rubber have different impact and elongation characteristics compared to PAS resins containing a non-functionalized silicone rubber and an aminosilane. It is believed that the aminosilane, non-functionalized silicone rubber, and PAS components undergo a reaction in the melt.42 Another method is to improve the impact strength by using terpolymers composed from ethylene, ethyl acrylate, and glycidyl methacrylate and terpolymers composed from ethylene, butyl acrylate, and maleic an-
Poly(phenylene sulfide)
185
hydride as modifiers.40 The impact strength of the composition of PPS can be improved by the particular process of incorporating the modifiers. The modifiers should be dispersed homogeneously in the PPS matrix, whereby partial crosslinking should occur. Thus, PPS is melt-blended with epoxy elastomer modifiers that can be crosslinked during melt blending. The composite is prepared by dry mixing one of the terpolymers with PPS, then melting, kneading, and adding the other terpolymer. Other procedures of preparation have been tried out, but this way of preparation yields the best results. The kneading temperature is 290–330°C. Temperatures that are too low do not permit a sufficient melting of the PPS for a homogeneous dispersion. Temperatures that are too high can result in degradation of the PPS and the other compounds as well. During kneading, measuring the torque is the best control to access the dispersion and crosslinking. In the initial stage, when the polymeric mixture starts melting and homogenizes, the torque decreases. After the addition of the second polymer, a partial crosslinking is indicated by an increase of the torque. After some 5 min, the torque stabilizes at the increased value. However, when no crosslinking takes place, the torque remains stationary after the addition of the second compound. When the polymer containing the maleic anhydride is introduced in the kneading device 3 min after mixing PPS and polymer, an increase in the torque is observed, which is attributed to a crosslinking reaction. When all the compounds are simultaneously mixed together, no crosslinking is observed.40 The torque in dependence of time is illustrated in Figure 5.4. In this way, it possible to obtain a good dispersion of an elastomeric phase in a PPS matrix, with good interfacial cohesion. The compositions exhibit a greatly increased flexibility in comparison to pure PPS. In electrical and electronic applications, such as circuit breakers, multipole rods, and breaker bulbs, a material must be available which combines heat resistance with good electrical properties. Namely, tracking current resistance and arc resistance are necessary. PPS as such exhibits good heat resistance. The electrical properties can be improved by a formulation containing PA and magnesium hydroxide. In addition, the composite is reinforced by glass fibers (GF)s.41 Blends of PAS, poly(phenylene ether), with a polyester resin and a compatibilizer have been described. The polyester should be aromatic, e.g., poly(ethylene terephthalate), poly(cyclohexylenedimethanol terephthalate) and poly(butylene terephthalate) (PBT). Because of the ten-
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No crosslinking Crosslinking
0.55
Torque [kg m]
0.5 0.45 0.4 0.35 0.3 0.25 0.2 0
2
4
6
8
10
Time [Min]
Figure 5.4: Torque in Dependence of Time40
dency of polyesters to undergo hydrolytic degradation at the high extrusion and molding temperatures encountered by the compositions, it is preferred that the polyester be substantially free of water. Orthoesters or epoxidefunctionalized compounds are suitable compatibilizers.44
5.2.6.2
Viscosity Reduction
Blends of liquid crystal polymer (LCP) polyester, LCP poly(ester amide) and PAS exhibit a reduced melt viscosity.43 LCP polyesters are made by polymerizing aromatic diacids with diols or by polymerizing aromatic hydroxy acids, e.g. 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. In LCP poly(ester amide)s, some of the hydroxyl groups in the monomers are replaced with amino groups. The blends are prepared by melt blending, in an extruder. If an inorganic filler, such as glass, is also included, it is preferably added in a later mixing zone after the polymers are mixed to minimize crushing or fragmentation of the filler. The polymers are dried before mixing by heating them in a dry atmosphere for several hours. These blends are particularly useful in the manufacture of electronic connectors. The reduction in melt viscosity that results from the addition of LCP poly(ester amide) is advantageous in the molding of intricate parts with small cavities that are difficult to fill during molding with polymers having higher viscosity.
Poly(phenylene sulfide)
F
O + S Cl Cl
187
O F
S
F O
O C C
Cl F
S
Cl F
Figure 5.5: Synthesis of Bis-(4-fluorophenyl)-sulfide45
5.2.7 Poly(arylene ether sulfide)s A variety of poly(ethersulfone) are PSI. These compounds are accessible e.g., by the polymerization of the bis-(4-fluorophenyl)-sulfide and bisphenol A.45 The sulfide group is already in the monomeric unit present, and the polymerization involves the formation of the ether group in the backbone. The synthesis of bis-(4-fluorophenyl)-sulfide is shown in Figure 5.5. The reaction is fundamentally a Friedel-Crafts reaction utilizing thionyl chloride followed by the reduction of the sulfoxide to the sulfide with oxaloyl chloride. The sulfide group activates the fluorine atom for the nucleophilic aromatic substitution polymerization. Thus, the ether linkage in the backbone of the polymer can be readily produced by a nucleophilic aromatic substitution. The reaction with bisphenol A proceeds at 150°C.
5.2.8 Poly(phenylene sulfide phenyleneamine) Aromatic copolymers bearing the sulfide linkage and the amine linkage in the backbone have been synthesized, i.e., poly(1,4-phenylene sulfide-1,4phenyleneamine) (PPSA). The reaction is sketched in Figure 5.6. PPSA can also be obtained by the condensation of methyl-(4-anilinophenyl) sulfide with antimony pentachloride.46 The reaction proceeds in chloroform at −68°C. The process is shown in Figure 5.7. After polymerization, a demethylation step follows as shown in Figure 5.6. Poly(aniline) (PANI) is an electric conducting polymer and is used in organic semiconductor technology. The copolymers are of interest, be-
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O
O
S CH3 + CH3 S OH
NH
O
CH3 S+
NH
CH3SO3-
N
S
NH
Figure 5.6: Aromatic Condensation of the Methylsulfinyl Group Followed by Demethylation47
NH
S CH3
SbCl6
CH3 NH
S
+
SbCl6-
CH3 NH
S+ SbCl6-
Figure 5.7: Condensation of Methyl-(4-anilino-phenyl) Sulfide with Antimony Pentachloride
Poly(phenylene sulfide)
189
cause they might combine the good thermal properties of PPS with the unique properties of PANI.47, 48 In the same way, poly(phenylene sulfide-phenyleneamine-phenyleneamine) (PPSAA) has been synthesized via a soluble precursor polymer through an acid-induced polycondensation reaction of N-(4-methylsulfinyl)phenylene-N -phenyl-1,4-phenylenediamine.49 The polymerization is performed by treating the monomer with methanesulfonic acid for 24 h at room temperature. The demethylation is performed in refluxing pyridine. Precipitation in methanol yields the PPSAA as a light purple solid in 91% yield. Experiments with model compounds for PPSAA revealed that the nature of the acid is important for success. Methanesulfonic acid is a good choice. If stronger acids, such as perchloric acid or trifluoromethanesulfonic acid are used, the yield is drastically reduced. This behavior has been explained by an intermolecular redox process, being dominant over the coupling reaction in the presence of very strong acids. In the presence of strong acids, the nitrogen is oxidized to form a −N= structure.49 This behavior is in contrast to PPSA, where the molecular weights generally increase with increasing acid strength. This highmolecular-weight compound is exceptionally soluble in common organic solvents, such as THF, N,N-dimethylformamide (DMF) and NMP. Thermogravimetric analysis of PPSAA shows that the polymer is stable under nitrogen up to 340°C.
5.2.9 Poly(dithiathianthrene)s Poly(dithiathianthrene) belongs to the ladder-type polymers. It is synthesized by the intramolecular cyclization of a methylsulfinyl substituted poly(m-phenylene sulfide). Under strongly acidic conditions the demethylation results in a poly(dithiathianthrene).50 The basic reaction is the same as shown in Figure 5.6. Poly(imide)s with the dithiathianthrene group are accessible from the reaction of thianthrene-2,3,7,8-tetracarboxylic dianhydride aromatic diamines.51 Thianthrene-2,3,7,8-tetracarboxylic dianhydride can be synthesized via a nucleophilic aromatic substitution of N-phenyl-4,5-dichlorophthalimide with thiobenzamide, thioacetamide, and sodium sulfide. The polymers obtained have a good thermal stability in air and nitrogen. The polymers are amorphous and have been found to be soluble only in H2 SO4 .
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S F
F
S
Figure 5.8: 2,7-Difluorothianthrene
5.2.10 Poly(aryl ether thianthrene)s. 2,7-Difluorothianthrene, i.e., 2,7-difluoro-9,10-dithiaanthracene, c.f. Figure 5.8, is a monomer for poly(aryl ether thianthrene)s. This monomer is a high melting crystalline solid that sublimes under reduced pressure, simplifying the purification for polymer synthesis. It can be condensed with a bisphenol to yield poly(aryl ether thianthrene)s.45, 52 The poly(aryl ether thianthrene)s are flame resistant and exhibit a high refractive index of 1.61–1.70, depending on the wavelength and the corresponding bisphenol. For this reason, the polymers are interesting for optical applications.
5.3 PROPERTIES PPS is a semi-crystalline material. It exhibits an excellent balance of properties, including high-temperature resistance, chemical resistance, flowability, dimensional stability, and electrical characteristics. PPS is brittle. Therefore, it must be filled with fibers and fillers. Because of its low melt viscosity, PPS can be molded with high loadings of fillers or reinforcing materials. The fillers and reinforcing materials enhance the strength, dimensional stability, and other properties. PPS exhibits flame retardancy and thus is ideal for high-temperature electrical applications.
5.3.1 Mechanical Properties PPS is brittle and notch sensitive. A significant improvement in the notched Izod impact toughness can be obtained by the addition of a rubber modifier.53
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191
5.3.2 Thermal Properties PPS shows a glass transition temperature Tg of 85–90°C and melts around 290°C. PPS is comparatively highly thermally stable. Differential scanning calorimetry and thermogravimetry indicate that the weight loss on thermal degradation starts at 430°C.54 Below 450°C, extensive crosslinking takes place.55 At pyrolysis temperatures up to 550°C, the major volatile pyrolysis products are the cyclic tetramers and the linear trimers and dimers. These products are formed by a random scission followed by depolymerization and cyclization. In the range of 550–800°C, as can be expected, the products of pyrolysis shift to smaller moieties such as benzene, benzenethiol, and hydrogen sulfide.56, 57
5.3.3 Electrical Properties PPS is a good electrical insulator, even at high temperatures. The volume resistivity and the dielectric strength remain high up to 200°C. The polymers have a low dielectric constant and a low loss factor throughout a wide temperature range.
5.3.4 Optical Properties Unstabilized PPS tends to degrade in sunlight. This is caused by the overlap of the absorption spectrum of PPS and the spectrum of the sun in the ultraviolet light. A systematic study has been performed to find suitable light stabilizers.58, 59 A wide number of UV absorbers, quenchers, and antioxidants have been added in bulk to PPS. The effectiveness of the additives in suppressing UV degradation was evaluated. Metal acetylacetonates and thio-organic complexes show a strong interaction during processing as they yield dark colored samples. These materials are not acceptable as UV stabilizers. Hindered amine light stabilizers (HALS)s are commonly used polymer photostabilizers. HALSs are ineffective in imparting photostability to the polymer. Successful photostabilization of PPS can be achieved by using UV absorbers rather than quencher-type additives.
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5.3.5 Solubility The polymer has a broad compatibility with chemicals and solvents, offering a better chemical resistance than stainless steel. No solvent for PPS is known below 200°C. Above this temperature, 1-chloronaphthalene is a suitable solvent. Molecular mass, characterization e.g., by size exclusion chromatography at moderate temperatures can be done only when derivatives are formed prior to analysis in order to enhance the solubility.60 The sulfide groups are oxidized, by a mixture of nitric acid and methanesulfonic acid resulting in a poly(p-phenylene sulfoxide) (PPSO) polymer. Actually, methanesulfonic acid is a solvent for PPSO.
5.4 SPECIAL ADDITIVES 5.4.1 Decolorants In most cases, PPS exhibits a pale yellow color. Even when PPS of high whiteness is obtained, it is easily colored during melt molding. Accordingly, in order to obtain PPS articles of high whiteness, special means are needed. Various means for decreasing discoloration have been used.8 The simplest procedure is to add a white pigment to the PPS resin to whiten it. However, a decrease in mechanical strength may occur. The decrease in mechanical strength can be compensated by adding an epoxy resin. Other discoloring additives are organic phosphorus compounds.
5.4.2 Corrosion Inhibitors PAS resins tend to form corrosive gases, such as sulfur dioxide when heated to an elevated temperature, and hence involve problems that metallic portions of processing machines, molds, etc., are corroded upon their molding and processing. The usual processing machine is made of an iron-based material and hence tends to suffer from chemical corrosion when coming into contact with a PAS resin melted upon its molding. On the other hand, the molded products also become liable to color. The corrosion of a mold causes a great economical loss because the mold is expensive. Moreover, it is then difficult to precisely mold. In order to solve corrosive problems involved in the PAS resins, it has been proposed to blend various kinds of corrosion inhibitor.
Poly(phenylene sulfide)
193
However, certain corrosion inhibitors, such as γ -alumina, calcium carbonate, zinc oxide, sodium oxalate, etc., show insufficient corrosion inhibition. For some corrosion inhibitors, the mechanical strength of the PAS resin is deteriorated. More satisfactory corrosion inhibitors are nickel compounds, such as nickel carbonate, nickel hydroxide, and nickel citrate.61 The anticorrosive effect is already satisfactory when the corrosion inhibitor is added in amounts of 0.1%.
5.4.3 Adhesion Reduction Undesirable adhesion to metal surfaces may cause a problem in injection molding, melt spinning, and thermoforming. In the melt spinning of PAS a variety of adhesion-related problems have been observed. These problems include the build up of polymer on spinneret surfaces resulting from both initial filament extrusion and lick-back from filament breaks. These adhesion-related problems lead to premature spin pack failure and equipment down-time to replace spent spinneret packs, and can limit the use of PAS in certain melt spinning operations. A variety of lubricants and mold release agents, such as fatty acid esters and amides, have been suggested as additives to reduce the adhesion of the polymer to metal surfaces. These materials function to increase the polymer lubricity at its point of contact with a surface. In the case of PAS, conventional lubricants alone have not been found significant in reducing the adhesion problems associated with melt spinning operations. It has been demonstrated that PAS in melt spinning operations can be better processed by the addition of barium hydroxide, together with conventional lubricants. In fact, the barium treatment has improved the melt strength of PAS in the production of fine denier fibers. This improvement in melt strength translates to the ability to melt-spin lower molecular weight polymer.62
5.5 APPLICATIONS PPS are useful materials for electrical and electronic parts and appliances, and as high rigidity materials for various applications. In particular, PPS have found applications in technical parts such as pumps, automotive parts, printer components, and liquid crystalline display projectors.
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High Performance Polymers
5.5.1 Solder Friendly Thermoplastic Blends In order to miniaturize products and to improve productivity in the electronics industry, a method of soldering resinous electronic parts has been developed for affixing parts, such as connectors, switches, relays and coil bobbins to printed circuit board (PCB). This method is addressed as surface-mount technology. This refers to a mounting system wherein electronic parts are affixed to a PCB. A creamy lead-free solder is used to facilitate the adhesion of the electronic parts to the PCB. Thermoplastic compositions are used as insulating materials for electronic parts. The PCB is then passed through a heating oven (reflowing oven), thereby melting the solder to fix the electronic parts to the wiring board. The surface-mount technology permits mounting to be conducted on both surfaces of the PCB, thereby reducing production costs. However, the thermoplastic compositions exposed to a lead-free solder that are used as insulators begin to fail. Loss of insulating ability, which generally occurs after failure, renders the thermoplastic composition unreliable for these types of applications. A solder resistant high-temperature composition that does not suffer from this drawback has been developed. The blend is composed of poly(arylene ether) (PAE), PPS, and GFs. The PAE has an intrinsic viscosity (IV) less than or equal to about 0.15 dl g−1 as determined in chloroform at 25°C. The use of the low IV PAE permits improved blending, which leads to improved high-temperature properties. Homopolymers of PAE are those containing 2,6-dimethylphenylene ether units. Suitable copolymers include random copolymers containing, for example, 2,6-dimethylphenylene ether units. In combination with 2,3,6-trimethyl-1,4-phenylene ether units or alternatively, copolymers derived from the copolymerization of 2,6-dimethylphenol with 2,3,6-trimethylphenol. Partially crosslinked PPS, as well as mixtures of branched and linear PPS, may be used in the hightemperature compositions. The composition advantageously displays a thermal resistance effective to withstand the high temperatures encountered in a reflowing oven. Further, the high-temperature compositions more closely match the thermal shrinkage of PBT, which is presently used in solder connector applications. The thermal performance is improved without any changes to existing processing equipment, such as molding machines, dies, molds, or extrud-
Poly(phenylene sulfide)
195
ers. The compositions can also be molded into various shapes and forms, such as connectors, circuit boards, pipes, rods, films, sheets and bearings, which renders them useful in electrical applications that might result in contact with lead-free solder.63
5.5.2 Abrasion-resistant Poly(tetrafluoroethylene) Blends PTFE tape is used in many applications, including sealing joints, insulating conductive wires, and protecting materials from corrosive elements. PTFE demonstrates a good chemical and heat resistance, electrical insulation characteristics, as well as a low coefficient of friction. However, in general, it has less than desirable mechanical properties, in particular with respect to abrasion resistance and compression strength. One commonly used approach is to make composites, which incorporate poly(imide) resins along with the fluoropolymers. Typically such insulation is based on poly(imide) films, such as Kapton®, which are coated or laminated-over with tetrafluoroethylene polymers.64 The addition of PPS that contain oxidized sulfide groups to fluoropolymers improves the tendency to creep and abrasion while substantially retaining the chemicals resistance and heat resistance. The sulfide group of PPS can be oxidized either to the stage of sulfoxide or to the stage of sulfone.31 In practice, at a higher stage of oxidation, both sulfide groups, sulfoxide groups and sulfone groups will be present. The effect of abrasion resistance of a PPS modified fluoropolymer is shown in Table 5.4. In addition, polymer blends of this type exhibit good mechanical properties in the tensile test. These mixtures are particularly suitable for applications in the form of extruded tubes, as used for sheathing Bowden cables.65 PTFE-filled unsintered tapes, which are made from a uniform dispersion of non-thermally cycled PPS milled to an average particle size of about 1 to about 20 μ m provide a good degree of flexibility and durability, when sintered. The properties are significantly better than PTFE alone, or PTFE compositions containing milled PPS, which is heat-treated prior to milling. A significant improvement in durability and insulation properties is further observed by the addition of 1 to 3% of poly(p-oxybenzoate) to the PTFE/PPS composition.64
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Table 5.4: Abrasion Resistance of Fluoropolymer/oxidized Poly(phenylene sulfide) Composites31 Material
Abrasion [mm3 Nm−1 /10−7 ]
Fluoropolymera 958 Fluoropolymera /oxidized PPS, 95:5 (w/w) 180 Fluoropolymera /oxidized PPS, 80:20 (w/w) 110 a 75.5% tetrafluoroethylene, 21.0% ethylene, 3.5% perfluorobutylethylene Test specimens: diameter 10 mm, height 20 mm Pressure: 0.42 N mm−2 Sliding speed: 4 m s−1 Sliding partner: gray iron 30 steel X20Cd13 Test time: 100 h Test temperature: 100°C Test distance: 1436 km
5.5.3 Electrically Conducting Polymers Various copolymers with the basic backbone of −Ar−NH−Ar−S−, i.e., aromatic amino sulfide copolymers have been synthesized. This type of copolymer is semiconducting and can be used in many electronic and electrooptical applications. Examples of such applications are antistatic layers, electromagnetic-shielding layers, anticorrosion layers, batteries, electroluminescent devices, and in electronic circuits, such as conductor tracks of transistors. In general, semiconducting polymers should have a continuous, conjugated chain of conjugated repeating units. They are also referred to as conductive or conjugated polymers. An example for a copolymer that contains sulfide groups is PPSA.48 PPSA is prepared from poly(1,4-phenylene-methylsulfonium-1,4-phenyleneamine)methylsulfonate, by heating in dry pyridine. The latter polymer is prepared from 4-methylsulfoxy-diphenylamine and methanesulfonic acid. The synthesis of PPSA is shown in Figure 5.6. Polymers with molecular weights greater then 105 Dalton can be dissolved, up to 20%, in DMF, THF, NMP and dimethylacetonitrile, and in particular in dimethyl sulfoxide. The polymer is stable at temperatures up to 380°C. Optically clear, self-supporting layers having a modulus of elasticity of 1.3 GPa can be prepared from a solution. Layers of the polymer adhere very well to metals, in particular to gold. By means of oxidation agents, PPSA can be doped to form a p-type material. Doping of a self-supporting layer of PPSA with
Poly(phenylene sulfide)
197
SbCl5 results in an electric conductivity of 0.18 S cm−1 , while doping with FeCl3 leads to a conductivity of 0.8 S cm−1 .
5.5.4 Proton Exchange Membrane Materials Most common membranes for severe conditions are made from perfluorinated polymers bearing sulfonic acid groups in their side chains, such as Nafion®. These membranes are expensive. For this reason, research is focused to develop alternative materials. Among a series of other materials, PPS has been tested as proton exchange membrane material, because of its promising properties.66 Heterogeneous membranes based on medium-sulfonated PPS are made by dispersing the PPS in a poly(olefin) matrix. A commercial fuel cell has been used for membrane testing. It turned out that membranes with high amounts of sulfonated particles are almost as conductive as Nafion®117. However, these membranes exhibit considerably lower diffusive permeabilities to methanol. Further, the membranes are less oxidatively stable.
5.5.5 Ozone Filter Materials In electrophotographic copier processes and in printing processes, small amounts of ozone are formed. The resulting ozone is constantly given off into the surrounding air by the apparatuses. The removal of ozone from the air-stream released by such apparatuses can be achieved by a polymerbased filter composed of a PPS. When the PPS is contacted with the ozonecontaining medium, the sulfide units are oxidized to a sulfoxide.67 The polymers can be used as powder, fibers, films, or moldings for the production of a filter. The polymers are applied in shapes having a large surface area, e.g., a lattice structure or honeycomb structure. The removal of ozone proceeds quantitatively, the reaction times being dependent on the flow velocities, and the surface area of the filter material or the bed height in the case of powders.
5.6 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 5.5. Most PPS compounds are used for their combination of high-temperature stability, chemical resistance, dimensional reliability, and flame retardancy. It is suitable
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Table 5.5: Examples for Commercially Available Poly(phenylene sulfide) Polymers Tradenamea
Producer
Albis PPS Celstran® PPS
Albis Plastics Corp. Celanese Ticona
Comshield® PPS Edgetek™ PPS Emi-X* PPS Fortron® PPS HiFill® PPS Hiloy® PPS Konduit* PPS Lubriblend® PPS Lubricomp* PPS Lubrilon® PPS Lubri-Tech™ PPS NORYL* PPS+PPE NORYL* PPS+PPE PRIMEF® PPS RTP PPS (Series) Ryton® PPS Schulatec® PPS Statiblend® PPS Stat-Kon* PPS SUPEC* PPS TEDUR® PPS Therma-Tech™ PPS Thermocomp* PPS Thermotuf* PPS Torelina® Verton* PPS Xtel® PPS Xyron® PPS+PPE
A. Schulman Inc. PolyOne Corp. LNP Engineering Plastics Inc. Celanese Ticona TP Composites, Inc. A. Schulman Inc. LNP Engineering Plastics Inc. TP Composites, Inc. LNP Engineering Plastics Inc. A. Schulman Inc. PolyOne Corp. GE Plastics Asia Pacific LNP Engineering Plastics Inc. Solvay Advanced Polymers RTP Company Chevron Phillips Chem. Co. A. Schulman Inc. TP Composites, Inc. LNP Engineering Plastics Inc. GE Plastics Asia Pacific Albis Plastics Corp. PolyOne Corp. LNP Engineering Plastics Inc. LNP Engineering Plastics Inc. Toray LNP Engineering Plastics Inc. Chevron Phillips Chem. Co. Asahi Kasei Corp.
a
More details can be found in the internet68
Remarks Reinforced PPS
Linear PPS
Film
Poly(phenylene sulfide)
199
for structural applications in corrosive environments or as a replacement for poly(ether ether ketone) for application at a lower temperature. The majority of the PPS types are reinforced with fibers. Reinforcing with long GFs is the most common. A wide range of injection molding grades of PPS is available. Unreinforced PPS resins are available as powders for slurry coating and electrostatic spraying. The coatings are suitable in food industries and for equipment used in chemical processing. Types of PPS resin include both the crosslinked and linear type. A wide variety of grades is offered. Most of the grades are GF and/or inorganic filler reinforced, but also alloys with PTFE are available as a nonabrasive type, or alloys with elastomers as a high impact type. Tradenames appearing in the references are shown in Table 5.6.
5.7 SAFETY In the internet, a series of data sheets and material safety data sheets can be found.69
5.8 ENVIRONMENTAL IMPACT AND RECYCLING During the production of PPS, waste material is obtained as byproduct. This waste material may consist of:19 • • • • •
Linear and cyclic oligomers, Byproducts in polymerization, Unreacted reactants and polymerization modifiers, Fine particle sized materials, and Polymer, which is not within the specifications for the desired product.
A portion of the waste material resulting from the manufacture of PAS polymer can continuously be reused in subsequent polymerization processes, thus avoiding the necessity of disposing of at least a portion of such waste material.19
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Table 5.6: Tradenames in References Tradename Description
Supplier
Aqua-Cleen® Ethoxylated mercaptan, surfactant1 Bondfast®
Philips Petroleum Co.
Epoxy functional poly(olefin)39 Ekonol®
Sumitomo Chemical Co., Ltd. Norton Performance Plastics Corp.
Poly(p-oxybenzoate)64 Forton® (Series) Hoechst Celanese Corp. Poly(phenylene sulfide)39, 43, 61, 63, 64 Glycolube® (Series) Lonza Inc. Fatty esters, flow promotor, mold release agent63 Igetabond® Sumitomo Chemical Co., Ltd. Epoxy functional poly(olefin)39 Kapton® DuPont-Toray Co., Ltd. Poly(imide)64 Kraton® Shell Styrenic block copolymer39, 63, 64 Lotader® Elf Atochem (Arkema) Epoxy functional poly(olefin), Adhesive39, 40 Ryton® (Series) Philips Petroleum Co. Poly(phenylene sulfide)11, 17, 43, 64 Septon® Kuraray Co., Ltd. Hydrogenated styrenic block copolymer63 Solprene® Philips Petroleum Co. (Industrias Negromex, S.A.) Styrenic block copolymer63 Supec® General Electric Poly(phenylene sulfide)43 T-4 Tohpren Co., Ltd. Poly(phenylene sulfide)40 Tedur® Mobay Corp. Poly(phenylene sulfide)43
Poly(phenylene sulfide)
201
Table 5.6 (cont): Tradenames in References Tradename Description
Supplier
Vector® Styrenic block copolymer63 Vectra® (Series)
Dexco Polymers LP
Hoechst Celanese Corp. (Ticona) Liquid Crystal Polymer, composed from mainly 4-hydroxybenzoic acid or 6hydroxy-2-naphthoic acid, further, depending on type: p-acetaminophenol, terephthalic acid, and biphenol43
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27. W. Koch and W. Heitz. “Models and mechanism of the formation of poly(thio-1,4-phenylene).” Makromol. Chem., 184(4):779 – 792, 1983. 28. M. Wejchan-Judek. “Synthesis of poly(1,4-phenylene sulphide) by oxidation of thiophenol with thionyl chloride in the presence of superacid.” Polym. Degrad. Stabil., 37(1):7–10, 1992. 29. H. A. Smith. Laminates comprising a linear polyarylene sulfoxide adhesive interlaver. US Patent 3 303 087, assigned to Dow Chemical Co., February 07, 1967. 30. D. Fleischer, H. Strutz, J. Kulpe, and A. Schleicher. Oxidized polyarylene sulfides. US Patent 6 020 442, assigned to Ticona GmbH (DE), February 1, 2000. 31. H. Scheckenbach, A. Schleicher, J. Kulpe, and B. Jansen. Mixtures of fluoropolymers and oxidized polyarylene sulfides. US Patent 5 708 089, assigned to Hoecht AG (Frankfurt, DE), January 13, 1998. 32. M. C. Yu and J. J. Straw. Preparation of poly (biphenylene/phenylene) sulfide. US Patent 5 219 983, assigned to Phillips Petroleum Company (Bartlesville, OK), June 15, 1993. 33. H. R. Lubowitz and C. H. Sheppard. Oligomers with multiple chemically functional end caps. US Patent 5 969 079, assigned to The Boeing Company (Seattle, WA), October 19, 1999. 34. J.-P. Kim, W.-Y. Lee, J.-W. Kang, S.-K. Kwon, J.-J. Kim, and J.-S. Lee. “Fluorinated poly(arylene ether sulfide) for polymeric optical waveguide devices.” Macromolecules, 34:7817–7821, 2001. 35. J. S. Lee, J. J. Kim, J. P. Kim, J. W. Kang, and W. Y. Lee. Poly (arylene ether sulfide) and poly (arylene ether sulfone) for optical device and method for preparing the same. US Patent 6 512 076, assigned to Kwangju Institute of Science and Technology (Kwangju, KR), January 28, 2003. 36. J.-P. Kim, J.-W. Kang, J.-J. Kim, and J.-S. Lee. “Fluorinated poly(arylene ether sulfone)s for polymeric optical waveguide devices.” Polymer, 44(15): 4189–4195, July 2003. 37. S. Inoue, M. Okamoto, and M. Yanagi. Polyarylene sulfide resin composition. JP Patent 58 154 757, assigned to Toray Industries, September 14, 1983. 38. H. Kobayashi and A. Kishimoto. Polyphenylene sulfide resin composition and a process for producing it. US Patent 4 889 893, assigned to Toray Industries (Tokyo, JP), December 26, 1989. 39. C.-F. R. Hwang, J. J. Scobbo, Jr., and S. B. Brown. High flow, high ductility poly(arylene sulfide) resin blends. US Patent 5 723 542, assigned to General Electric Company (Pittsfield, MA), March 3, 1998. 40. M. Lambla, R. Mestanza, D.-J. Lin, E. Vandevijver, and M.-P. Collard. Polyphenylene sulphide-based compositions with improved impact strength and process for preparing them. US Patent 6 849 697, assigned to Solvay Polyolefins, S.A. (Brussels, BE), February 1, 2005.
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54. Y. Cohen and Z. Aizenshtat. “Isothermal fluidized-bed studies on the kinetics and pyro-products of linear and branched poly(p-phenylene sulfide) and proposed mechanisms.” J. Anal. Appl. Pyrolysis, 27(2):131–143, December 1993. 55. O. A. Peters and R. H. Still. “The thermal degradation of poly(phenylene sulphide)–part 1.” Polym. Degrad. Stabil., 42(1):41–48, 1993. 56. D. R. Budgell, M. Day, and J. D. Cooney. “Thermal degradation of poly(phenylene sulfide) as monitored by pyrolysis–GC/MS.” Polym. Degrad. Stabil., 43(1):109–115, 1994. 57. L. H. Perng. “Thermal decomposition characteristics of poly(phenylene sulfide) by stepwise Py-GC/MS and TG/MS techniques.” Polym. Degrad. Stabil., 69(3):323–332, September 2000. 58. P. K. Das, P. J. DesLauriers, D. R. Fahey, F. K. Wood, and F. J. Cornforth. “Photostabilization of poly (p-phenylene sulfide).” Polym. Degrad. Stabil., 48(1):1–10, 1995. 59. P. K. Das, P. J. DesLauriers, D. R. Fahey, F. K. Wood, and F. J. Cornforth. “Photodegradation and photostabilization of poly(p-phenylene sulfide). Part 2. UV induced physicochemical changes.” Polym. Degrad. Stabil., 48(1):11– 23, 1995. 60. D. Daoust, S. Bebelman, P. Godard, J. M. Coisne, and C. Strazielle. “Molecular characterization of poly(p-phenylene sulfide) (PPS) from size exclusion chromatography of a modified PPS and dilute solution properties.” Polymer, 37(17):3879–3888, August 1996. 61. Y. Satake and T. Ono. Poly (arylene sulfide) resin composition. US Patent 5 650 459, assigned to Kureha Kagaku Kogyo K.K. (Tokyo, JP), July 22, 1997. 62. B. B. Gupta, A. B. Auerbach, and B. L. Davies. Poly(arylene sulfide) compositions having improved processability. US Patent 5 824 767, assigned to Hoechst Celanese Corporation (Warren, NJ), October 20, 1998. 63. B. Liu. Lead free solder friendly thermoplastic blends and methods of manufacture thereof. US Patent 7 037 986, assigned to General Electric Company (Pittsfield, MA), May 2, 2006. 64. P. E. Sarkis and D. Delgado. Abrasion-resistant polytetrafluoroethylene tape. US Patent 7 008 989, assigned to Coltec Industrial Products, Inc. (Charlotte, NC), March 7, 2006. 65. H. Scheckenbach, A. Schleicher, J. Kulpe, W. Neumann, and B. Jansen. Abrasion-resistant fluoropolymer mixtures. US Patent 5 864 095, assigned to Ticona GmbH (DE), January 26, 1999. 66. J. Schauer and L. Brozova. “Heterogeneous ion-exchange membranes based on sulfonated poly(1,4-phenylene sulfide) and linear polyethylene: Preparation, oxidation stability, methanol permeability and electrochemical properties.” J. Membr. Sci., 250(1-2):151–157, March 2005.
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67. H. Strutz, D. Fleischer, J. u. Kulpe, and A. Schleicher. Filter material and process for removing ozone from gases and liquids. US Patent 5 593 594, assigned to Hoechst Aktiengesellschaft (DE), January 14, 1997. 68. IDES Integrated Design Engineering Systems. The Plastics Web®. IDES Inc. 209 Grand Avenue Laramie, WY 82070 USA [electronic] http://www.ides.com/prospector/, 2006. 69. Ryton® PPS data sheets. MSDS and data sheets, Chevron Phillips Chemical Company LLC, The Woodlands, TX, 2005. [electronic] http://www.cpchem.com/enu/ryton_pps_p_pps.asp.
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6 Poly(aryl ether ketone)s Poly(aryl ether ketone)s have aromatic groups and both the ether group and the keto group are in the backbone. Figure 6.1 illustrates the basic repeating structures of this class of substances. Of course, there are several varieties of those structures shown in Figure 6.1, resulting from the use of comonomers, etc. A special related class is that of poly(ethersulfone). Poly(aryl ether ketones) belong to the class of engineering polymers. In early to mid-1970s, Raychem Corp. commercially introduced a poly(aryl ether ketone) called Stilan®. In this polymer, each ether and keto group is separated by 1,4-phenylene units. In 1978, Imperial Chemical Industries PLC (ICI) commercialized a poly(aryl ether ketone) under the trademark Victrex® PEEK.1
6.1 MONOMERS Monomers for poly(ether ether ketone)s (PEEK)s and poly(ether ketone)s (PEK)s are shown in Table 6.1. Varieties of PEEK are shown in Figure 6.2. PEEK-WC is poly(oxa-p-phenylene-3,3-phthalido-p-phenyleneoxa-p-phenylene-oxy-phenylene). In addition to PEEK, the carbonyl group is partly modified with phthalide units. It is an amorphous PEEK. Still other somewhat refined varieties include the poly(ether ketone ketone) (PEKK), poly(ether ether ketone ketone) (PEEKK) and poly(ether ketone ether ketone ketone) (PEKEKK) polymer type.2 209
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(R1 O)n
(R1 C O)n
(R1 C)n O
Poly(ether)
O
Poly(ketone)
(R1 O R2 C)n
(R1
Poly(ester) O R2 O R3 C)n
O
O
Poly(etherketone) ((R1 O)m R2 C)n
Poly(etheretherketone) ((R1 O)m R2 C O)n
O Poly(poly(ether)ketone)
O Poly(poly(ether)ester)
Figure 6.1: Basic Backbone Structures of Ether Ketone and Ether Ester Polymers
Table 6.1: Monomers for PEK and PEEK Nucleophilic Route Hydroxy Functional Monomer Hydroquinone Bisphenol A, and 4 -hydroxy phenyl4-hydroxybenzoate 4,4 -Dihydroxybenzophenone PEEK oligomer Catechol (4-(4 -Trifluoromethyl)phenoxyphenyl)hydroquinone and hydroquinone Electrophilic Route Ether Functional Monomer 1,4-Diphenoxybenzene Diphenyl ether Multi Functional Monomer p-Phenoxybenzoyl chloride p-Phenoxyphenoxybenzoyl chloride
Halogen Functional Monomer 4,4 -Difluorodiphenyl ketone 4,4 -Difluorodiphenyl ketone3 4,4 -Dichlorodiphenyl sulfone4 1,5-Bis-(4-(4 -fluorobenzoyl)-phenoxy)-naphthalene (1,5-BFPN)5 4,4 -Difluorodiphenyl ketone6 4,4 -Difluorodiphenyl ketone7
Acid Halogenide Terephthaloyl chloride8 Terephthaloyl chloride and 1,4-diphenoxybenzene9
Poly(aryl ether ketone)s
C
211
O
O
O PEEK
C
O
C
O
O
O
O PEEK-WC
Figure 6.2: Varieties of PEEK10
F
F
C
+ NaO
ONa
O
C
O
O
O
Figure 6.3: Condensation of 4,4 -Difluorodiphenyl ketone with Hydroquinone
6.2 POLYMERIZATION AND FABRICATION Two major processes for the polycondensation are in use, namely, nucleophilic and electrophilic reaction type.
6.2.1 Nucleophilic Process The nucleophilic route employs hydroquinone and 4,4 -dihalobenzophenone with a base as a catalyst, in solvents, such as N-methyl-2-pyrrolidone (NMP) or sulfolane. For example, PEEK is manufactured by the reaction of 4,4 -difluorodiphenyl ketone with the potassium salt of hydroquinone, as shown in Figure 6.3. The reaction temperatures are about 200–250°C. The PEEK so pro-
212
High Performance Polymers
duced, however, exhibits a low-molecular weight with an intrinsic viscosity (IV) smaller than 0.7 dl g−1 and comparatively low mechanical properties. The nucleophilic route can be improved, by using diphenyl sulfone as a high-boiling solvent.4 In this process, hydroquinone is transformed into its dipotassium salt by heating with an equivalent amount of potassium carbonate or potassium bicarbonate, with simultaneous removal of the water at 150–200°C, followed by the addition of the second monomer, namely, 4,4 -difluorobenzophenone. The polymerization reaction is carried out at 320–350°C to obtain a polymer of an IV in the range of 0.8 to 1.4 dl g−1 with a melting point of 335–350°C. The polymers by this process are claimed to be very useful for wire coating.4 6.2.1.1
Poly(ether ketone)s with Ester Groups
It is difficult to prepare composite polymer materials with liquid crystalline polyesters, because the liquid crystalline molecules aggregate and do not mix with the other component. Therefore, block copolymers are prepared. The polymer can be synthesized from 4,4 -difluorobenzophenone bisphenol A and 4 -hydroxy phenyl-4-hydroxybenzoate.3 NMP and toluene are used as a solvent and for azeotropic water removal. The condensation is conducted at 155°C for 8 h using a Dean-Stark trap. In the final stage of condensation, the toluene is drained and the temperature is increased to 190°C. Liquid crystalline poly(aryl ether ketone) polyesters are then prepared by copolymerization of the ester group containing poly(aryl ether ketone) with a liquid crystalline polyester. The crystalline polyester is synthesized from phenylhydroquinone tert-butylhydroquinone, 2-chloroterephthaloyl chloride and isophthaloyl chloride.3 6.2.1.2
Powder Preparation
In a variant of the process, instead of the ketone monomer, a ketimine monomer is used to prepare a poly(ether ketimine).11 The ketimine polymer can be dissolved in NMP. The ketimine groups in the polymer can then be hydrolyzed by means of a dilute aqueous acid solution. Hydrolysis gives back the ketone polymer that precipitates out as a fine powder. Particles of the size of 0.5–5 μ m are produced by this procedure. This technique is claimed to be far superior to grinding because it
Poly(aryl ether ketone)s
213
is not as expensive. Moreover, grinding produces particles that are much larger in diameter, namely 15–80 μ m.
6.2.2 Electrophilic Process The electrophilic route for the production of aromatic polyether ketones involves the use of Friedel-Crafts catalysts. AlCl3 is used as a catalyst for the polymerization of p-phenoxybenzoyl chloride as such, or p-phenoxybenzoyl chloride or terephthaloyl chloride and 1,4-diphenoxybenzene to give a PEK. A PEEK is obtained by the use of p-phenoxyphenoxybenzoyl chloride, respectively.8 The process is carried out at low temperatures, such as 0–30°C. Due to the heterogeneous nature of this reaction, generally undesirable lower molecular weight polymers are produced. Capping agents, are added to the polymerization reaction medium to cap the polymer on at least one end of the polymer chain. This terminates continued growth of that chain and controls the resulting molecular weight of the polymer, as shown by the inherent viscosity of the polymer. Judicious use of the capping agents results in a polymer within a selected narrow molecular weight range, decreased gel formation during polymerization, and decreased branching of the polymer chains and increases melt stability. Both nucleophilic and electrophilic capping agents are used to cap the polymer at each end of the chain. Preferred nucleophilic capping agents are 4-chlorobiphenyl, 4-phenoxybenzophenone, 4-(4-phenoxyphenoxy)benzophenone, biphenyl, and 4benzenesulonylphenyl phenyl ether. The PEEK obtained by this process shows a high degree of branching. These structural defects lead to a lowering of the melting point from greater than 330°C to 315–320°C.12 Condensation of terephthaloyl chloride with diphenyl ether and diphenoxybenzene yields a copolymer of PEK and PEEK.9 Aluminum trichloride and lithium chloride are used as Friedel-Crafts catalysts and the process is carried out in a slurry of dichloromethane. Polymers prepared from diphenyl ether and terephthaloyl chloride or mixtures of terephthaloyl and isophthaloyl chlorides contain xanthydrol end groups, which tend to make the polymer melt unstable. The addition of an appropriate comonomer, such as diphenoxybenzene, suppresses the xanthydrol end group content somewhat and improves the melt stability. Thus, higher diphenoxybenzene contents increase the thermal stability. It has been found that not only is the nature of the repeat unit critical,
214
High Performance Polymers
in order to obtain good thermal and mechanical properties, but the nature of the end group is also critical for attaining desired thermal stability.12 By manipulating the end groups, it is possible to prepare PEEK structures that show still better thermal stability. Non-reactive end groups effect a better thermal stability and melt processing. End capping is achieved with an aromatic compound like benzene, toluene, xylene, phenol, anisole, diphenyl ether.
6.2.3 Blends Blends of poly(aryl ether ketone)s and certain poly(amide imide)s and poly(imide)s (PI)s are highly compatible. They tend to form one phase in the amorphous state, and thus are miscible systems. As a result, such blends significantly improve the processability of the poly(amide imide) or the PI material. Further, by increasing its Tg , the ultimate use temperature of the poly(aryl ether ketone) is significantly increased.13 Due to the miscibility of these blends, injection molded samples of the blends are transparent even though the poly(aryl ether ketone) is opaque under normal injection molding conditions. However, with proper annealing, opacity due to crystallization of the poly(aryl ether ketone) can be accomplished. PEEK is miscible with poly(ether imide) (PEI). PEI is less expensive than PEEK; it is used as an amorphous thermoplastic. The kinetics of crystallization and other properties of such blends have been presented in the literature.14
6.2.4 Modification PEEK can be sulfonated to achieve a certain solubility in concentrated sulfuric acid.15 6.2.4.1
Surface Functionalization
The surface properties of PEEK can be controlled by functionalization of the surface. For example, surface modified PEEK films can be used in the field of cell cultivation. Therefore, there is a certain interest in surface modification. Hydroxyl groups can be introduced by the treatment with sodium borohydride.16 The hydroxyl groups (PEEK-OH) can be further reacted
Poly(aryl ether ketone)s
215
with 4-aminobenzoic acid or succinamic acid to give carboxyl modified PEEK surfaces. Moreover, PEEK can be aminated.17 Functionalized arylazides can be readily grafted on the PEEK film surfaces by UV irradiation.18 Aromatic azides, such as 4-azido-tetrafluorobenzoic acid, or N-butyl-N -(4-azidophenyl)thiourea belong to the class of photoactivable reagents. These compounds can be photo-grafted onto PEEK. Another method of surface modification is oxygen plasma treatment. Oxygen plasma treatment also affects surface topography, by unveiling the spherulitic structure of PEEK.19
6.3 PROPERTIES The physical properties of PEEK are shown in Table 6.2. Unfilled PEEK has a light brownish color. Thermoplastic aromatic polyether ketones, such as PEEK, have melting points greater than 330°C, and their service temperatures may exceed 260°C. They exhibit high mechanical strengths, such as tensile strength greater than 85 MPa.12 PEEK can be used permanently up to 250°C, even in hot water or steam. The chemical resistance of PEEK is shown in Table 6.3. PEEK exhibits a remarkable chemical resistance, comparative to fluoropolymers. PEEK is approved by the FDA. PEEK undergoes crosslinking by irradiation in vacuum under stress. The tensile properties of PEEK sheets after UV radiation show a tendency to embrittlement. This is caused not only by crosslinking but also by the orientation of molecular chains resulting from the temperature rise of the specimens. Furthermore, the tensile stress applied during exposure accelerates molecular scission and disturbs the crosslinking.20
6.3.1 Mechanical Properties The wear properties of PEEK-based composites filled with 5% nanometer or micron Al2 O3 against the medium carbon steel are improved by the addition of Al2 O3 . In contrast, the friction properties are not improved. However, the filling of 10% poly(tetrafluoroethylene) (PTFE) into pure PEEK results in a simultaneous decrease of the friction coefficient and the wear coefficient of the filled composite.21 For this reason, PEEK-filled PTFE composites are attractive as solid lubricants. It is hypothesized that a synergistic effect shuts down the dom-
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Table 6.2: Properties of Poly(ether ether ketone)a22 Physical Properties
Value
Unit
Density Water Absorption Moisture Absorption at Equilibrium Moisture Vapor Transmission Oxygen Transmission
1.3 0.5 0.5 6.5 55
g cm−3 % % cm3 mm m−2 d−1 cm3 mm m−2 d−1
Mechanical Properties Tensile Strength, Yield Elongation at Break Modulus of Elasticity Flexural Modulus Flexural Yield Strength Compressive Yield Strength Poisson’s Ratio Shear Modulus Shear Strength Izod Impact, Notched
97 >60 3.5 4.1 170 118 0.4 1.3 53 0.63
MPa % GPa GPa MPa MPa
2.16 0.25 340 315 160 −65 143 V-0 35
J g K−1 W m−1 K−1 °C °C °C °C °C
GPa MPa J cm−1
Thermal Properties Heat Capacity Thermal Conductivity Melting Point Maximum Service Temperature, Air Deflection Temperature at 1.8 MPa Brittleness Temperature Glass Temperature Flammability, UL94 Oxygen Index a Victrex® PEEK 450G
%
Poly(aryl ether ketone)s
Table 6.3: Chemical Resistance of Poly(ether ether ketone)22 Chemcial
20°C
60°C
100°C
Acetaldehyde Acetic acid (glac./anh.) Acetone Acetylene Alcohols Aliphatic esters Aqua regia Aromatic solvents Benzene Brines, saturated Bromine Chlorine, wet Chloroform Detergents, synthetic Emulsifiers, concentrated Fluorine, dry Formaldehyde (40%) Formic acid Hydrochloric acid (conc.) Hydrogen peroxide (30%) Lime (CaO) Methanol Naptha Nitric acid (50%) Nitric acid (fuming) Oils, diesel Oils, essential Perchloric acid Phenol Sea water Sulfur dioxide, dry Sulfur trioxide Sulfuric acid (70%) Sulfuric acid (95%) Sulfuric acid, fuming R: Resistant NR: Not recommended
NR NR R R R NR R R R R R R R R R R R R R R ND R R R R R R R R R NR NR NR R R R R R R R R R R R R R R R R R R R R R R R ND R R R R R R R R R R R R R R R NR NR NR R R R R R R R R R R R ND R R R R R R R R R R R R NR NR NR NR NR NR LR: Limited resistant ND: No data
217
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Table 6.4: Time Averaged Friction Coefficients as a Function of PEEK Content in PTFE23 PEEK in PTFE %
Friction Coefficient μ¯
0 50 100
0.135 0.111 0.353
inant wear mechanism of each constituent of the composite.23, 24 The time averaged friction coefficients as a function of compositions are shown in Table 6.4.
6.4 SPECIAL ADDITIVES 6.4.1 Melt Stabilizers In the course of melt processing, unstabilized poly(aryl ether ketone)s show a strong tendency to crosslink. This behavior is highly undesirable since the melt viscosity increases and the polymer properties suffer. As the exposure time in the melt lengthens, melt fabrication becomes progressively more difficult. Non-hydrolyzable divalent metal oxides or sulfides, such as zinc oxide or zinc sulfide, are good stabilizers for poly(aryl ether sulfone)s. Organic aromatic phosphite and diphosphonite compounds were used as stabilizers. These compounds are shown in Figure 6.4. Other compounds are triphenyl phosphate and substituted derivates, such as tricresyl phosphate.25 However, it has been objected that, due to their high vapor pressure, these compounds might escape from the melt at the processing temperature customary for poly(aryl ether ketone)s, which can lead to odor nuisance and, in the case of injection molding, to coatings forming on the mold surface. For this reason, less volatile stabilizers have been proposed.26 The phosphonite type is much less effective than the phosphite type in the stabilization of poly(aryl ether ketone)s. However, the addition of organic phosphorus compounds in conjunction with an organic acid, such as oxalic acid or acetic acid results in a very significant reduction in melt viscosity, even where the phosphorus compound alone has a detrimental effect. The results using the stabilizers can be summarized as follows:1
Poly(aryl ether ketone)s
219
tBu tBu
tBu
tBu = C CH3
tBu
O P O
CH3
O tBu
CH3
tBu Tris(2,4-di-tert-butyl phenyl) phosphite tBu
tBu tBu
tBu
O tBu
tBu
O P
O
P O
tBu
tBu
Tetrakis(2,4-di-tert-butyl phenyl)-4,4,-biphenylene diphosphonite
Figure 6.4: Phosphorous-based Stabilizers. Mark 2112: Tris(2,4-di-tert-butyl phenyl) phosphite, Sandostab®-P-EPQ: Tetrakis(2,4-di-tert-butyl phenyl)-4,4-biphenylene diphosphonite1
220
High Performance Polymers 1. While the phosphite shows little effect on the melt flow, the phosphonite drastically reduces it and thus has a detrimental effect. 2. Oxalic acid has a major detrimental effect on the melt flow. 3. The combination of either the phosphonite or the phosphite with oxalic acid or acetic acid significantly improves melt flow in comparison to unstabilized samples.
6.4.2 Fillers and Reinforcing Materials In a variety of applications for engineering materials, PEEK is used as a composite with reinforcing materials. These include: • Glass fibers, • Carbon fibers, and • Poly(amide) fibers. Other filler materials are hydroxyapatite, aluminum oxide and aluminum nitride.27, 28 In addition, nanofillers are used. PEEK polymer filled with nano sized silica or alumina fillers of 15–30 nm exhibit an improvement of the mechanical properties by 20–50%.29 The agglomeration tendency can be somewhat diminished by a modification of the surface of the fillers with stearic acid.30
6.5 APPLICATIONS PEEK derivatives have significant commercial utility as plastics, especially as molded articles and as composites with glass, carbon, and Kevlar fibers for a variety of structural applications, including the aerospace and general engineering industries. PEEK also finds applications as extruded rods and profiles for manufacture of bushings, seals, etc. In general, they are processed using extruders and injection molding machines in the temperature range of 360–400°C, thus requiring extremely high thermal stability.12 Examples of uses are summarized in Table 6.5. Subsequently, we will discuss some selected topics in detail.
6.5.1 Nonadhesive Coating It is well known that cooking pans can be coated with PTFE in order to get surfaces that are nonadhesive to the food being fried. The addition of
Poly(aryl ether ketone)s
Table 6.5: Fields of Use of Poly(ether ether ketone) Materials Field of Use
References
Slides and Sealing Applications Sliding materials Valve seat sealings Analytical flow cell sealing High vacuum sealings
31, 32 33 34 35, 36
Coatings Cooking pans Coated wear surfaces in pumps
37 38
Electrical Applications Cable ties Cable insulation Rechargeable batteries
39 40 41
Medical Applications Catheter body materials Arthroeresis prosthesis systems Compression bone plates Bone substitutes
42, 43 44 45 46
221
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PEEK enhances the scratch resistance of the coating. Moreover, PEEK can be used without being mixed with a fluorocarbon resin to constitute a hard undercoat.37 The coats are sintered at a temperature in the range of 400–420°C. The grain size of the PEEK powder has a mean grain size of about 20 μ m. Suitable inert fillers are from the group of metal oxides, silica, mica particles, and flaked fillers.
6.5.2 Porous Membranes Microporous polymeric membranes are used widely for filtration and purification processes, such as filtration of wastewater, preparation of ultrapure water, and in medical, pharmaceutical or food applications, including removal of microorganisms, dialysis and protein filtration. These membranes have found broad utility for a variety of purposes. However, they suffer from several disadvantages, such as broad and frequently non-uniform pore size distribution, and limited chemical, solvent and thermal resistance. Poly(aryl ether ketone)s show outstanding chemical resistance and thermal properties. They are virtually insoluble in all common solvents at room temperature. These properties make poly(aryl ether ketone)s attractive materials for porous membrane preparation. However, the application of poly(aryl ether ketone)s in membrane fabrication has been limited owing to their intractability, which prevents the use of conventional solvent-based methods of membrane casting. Several routes are known for the preparation of poly(aryl ether ketone) membranes. Porous PEEK membranes are obtained from solutions in concentrated sulfuric acid. In this process, PEEK can undergo sulfonation in the concentrated sulfuric acid media and looses some of its desirable sought after properties. It has been suggested to get non-sulfonated porous PEEK membranes from less concentrated sulfuric acid to prevent sulfonation. The membranes are formed by casting a PEEK solution to form a film followed by coagulation.47 This process produces large amounts of waste acid. Sulfonation can be avoided, by using low temperatures, e.g. 15°C.48 However, only dilute PEEK solutions can be formed in the concentrated sulfuric acid at low temperatures. Another approach is to use non-sulfonating acid solvents, such as methanesulfonic acid and trifluoromethane-
Poly(aryl ether ketone)s
223
Table 6.6: Solubility of PEEK in Various Solvents49 Compound
MW [D]
mp. [°C]
bp. [°C]
Solubility [%]
at T [°C]
Triphenylmethanol Triphenylmethane Triphenylene 1,2,3-Triphenylbenzene 4-Biphenylcarboxylic acid Diphenyl carbonate Bibenzyl Diphenyl methyl phosphate 1-Bromonaphthalene N,N-Diphenylformamide Phenyl benzoate 1-Phenyldecane 1-Methoxynaphthalene 4-Bromodiphenyl ether 4-Bromodiphenyl ether Benzophenone Diphenyl ether Dioctyl phthalate Pentachlorophenol 4,4 -Dihydroxybenzophenone
260 244 228 306 198 214 182 264 207 197 198 218 158 249 249 182 170 391 266 214
161 93 196 158 225 79 51 – -1 71 69 – – 18 18 50 27 -50 189 214
360 359 438 – – 301 284 389 280 337 298 293 269 305 305 305 259 384 310 –
< 50.1 < 50.2 < 50.0 < 50.1 > 50.1 > 10.1 > 10.3 > 10.0 > 9.8 > 25.2 > 9.8 > 10.2 > 10.0 > 24.8 > 5.4 > 24.9 > 10.1 > 10.8 > 50.6 > 50.0
349 349 350 349 349 302 274 349 274 302 274 274 240 302 241 302 241 349 302 319
sulfonic acid.10 An alternative to acid-based solvent systems for PEEK membrane preparation involves the use of high-boiling point solvents and plasticizers that dissolve the PEEK at elevated temperatures. The solubility of 104 different solvents are presented in the literature.49 A few data are reproduced in Table 6.6. Examples of high-boiling organic polar solvents are benzophenone and 1-chloronaphthalene.49 The final porous material is formed by removing the organic polar solvents or the plasticizers by dissolution into a low boiling solvent. Another method to prepare membranes utilizes the thermally induced phase inversion (TIP) process.50 TIP refers to a process whereby the polymer is dissolved in a solvent in which the solubility of the polymer in the solvent is temperature dependent. The polymer/solvent blend is extruded or cast at elevated temperatures. As the temperature approaches an ambient temperature, a polymerrich phase separates from the solvent. The solvent is subsequently removed
224
High Performance Polymers
O
S
O
O
O H
H O
S
O
O H2SO4
+
S O
Figure 6.5: Crosslinking of Pendant Sulfonate Groups51
from the phase separated blend by leaching. Also, polymers such as poly(sulfone), are suitable for leaching.52 In a similar concept in a first step, PEEK/PI blends are prepared. The PI is removed from the blend by selective decomposition.53 In particular, the PI can be decomposed by contacting the shaped article with a primary aliphatic amine, e.g., monoethanolamine and thus forms into easily removable low-molecular-weight fragments. PEEK can be sulfonated to form proton conducting membranes. These membranes are used as electrolytic membranes in fuel cells.51 The sulfonated polymer is soluble in mixtures of organic solvents with water, which is not desired. For this reason, the sulfonic acid groups are partly crosslinked, by a condensation reaction shown in Figure 6.5. The crosslinking is achieved by heating the sulfonated material to 120°C in vacuo. The retaining sulfonic acid groups still provide electric conductivity. The proton conductivity of sulfonated poly(ether ether ketone) can be improved by using special types of PEEK, namely, block copolymers consisting of a hydrophobic and a hydrophilic block.54 PEEK-WC is a modified PEEK, as shown in Figure 6.2. Ultrathin asymmetric gas separation membranes of modified PEEK can be prepared by a dry/wet phase inversion technique.55, 56 Under optimized conditions, membranes with an open cellular morphology and an ultrathin dense skin
Poly(aryl ether ketone)s
225
of about 50 nm can be obtained. The membranes are prepared by casting a film of a solution of PEEK-WC on a glass plate. The films are then coagulated, dried and removed from the glass plate. In addition, PEEK-WC membranes have been prepared by using a phase inversion process with supercritical fluids. The supercritical fluid acts as a non-solvent. In comparison to the dry/wet phase inversion method, the supercritical fluid allows the cell size and the membrane morphology to modulate by changing the experimental conditions, such as polymer concentration, temperature, and pressure. A dry membrane can be obtained rapidly and without additional post-treatments.57 Sulfonated PEEK membranes exhibit proton conductivity and are thus candidates for fuel cell applications. Various sulfonated poly(ether ketone) types has been described in literature: • • • • •
Sulfonated poly(ether ketone) (SPEK), Sulfonated poly(ether ether ketone) (SPEEK) Sulfonated poly(ether ketone ketone) (SPEKK), Sulfonated poly(ether ether ketone ketone) (SPEEKK), and a Modified sulfonated poly(ether ether ketone) (SPEEK-WC).
The reason for the interest in these materials is that the membranes based on poly(ether ketone)s show a good chemical and mechanical stability, high proton conductivity, a reduced methanol permeability, and a lower cost with respect to a Nafion membrane.58, 59 The proton conductivity is affected by the degree of sulfonation of the polymer. The glass transition temperature increases with an increasing degree of sulfonation.60 In order to improve the proton conductivity, in particular for membranes with low degrees of sulfonation, amorphous zirconium phosphate sulfophenylenphosphonate Zr(HPO4 )(O3 PC6 H4 SO3 H) can be incorporated into the polymeric matrix.59 Sulfonated PEEK-WC membranes with a degree of sulfonation of 15–40%, have been tested as dense membranes for fuel cell applications. A solvent evaporation technique, with dimethylacetamide as the solvent was used to prepare dense membranes. A water uptake of up to 15% has been found for the dense membranes at 80°C. The membranes exhibit electrochemical performances comparable to Nafion membranes.61 The performance of such membranes can be improved by entrapping hetero poly(acid)s in the polymeric matrix. Tungstophosphoric acid
226
High Performance Polymers
(H3 PW12 O40 ), silicotungstic acid (H4 SiW12 O40 ), and phosphomolybdic acid (H3 PMo12 O40 ) are used as additives in the composite membranes.62, 63 Microporous membranes that are blended from PEEK-WC and poly(urethane) are of interest in medical science, e.g., to support long-term maintenance and differentiation of human liver cells.64
6.5.3 Rechargeable Batteries Coin-type nonaqueous rechargeable batteries are used in the field of backup power source for electronic devices, because of their advantages, such as high energy density and light weight. In the case of a battery based on lithium, lithium has to be pressure-welded or electrochemically deposited to the negative electrode. In these batteries, the selection of the material for the gasket, which assures the air tightness and liquid tightness of the battery, as well as the insulation of the positive electrode and negative electrode cans is particularly important. Poly(propylene) (PP) is conventionally used as the gasket material because of its advantages in resistance against chemicals, elasticity, creep resistance, and moldability which enables injection molding, and because it is cheap. Batteries for memory backup power sources are often soldered onto printed wiring boards together with memory devices. This is achieved by passing the mounted printed wiring board through a furnace at 200–230°C, thereby allowing the solder to melt and accomplish the soldering. This process is addressed as reflow soldering. If the device is not designed to use a heat resistant material, a loss of its functionality during reflow soldering may occur. The lithium alloy may react with the electrolytic solution and other components of the battery to cause abrupt bulging or explosion. Therefore, materials resistant to the reflow temperature must be used for the electrolytic solution, separator, or gasket. Taking these problems into account, special materials are selected as components for battery devices. We do not mention electrode and electrolyte composition in detail, but rather focus on the suggested polymers. As the separator, an insulating membrane having a large ion transmittance and a predetermined mechanical strength is used. As a material for use in the reflow soldering, glass fibers are the most stable. However, a resin having a thermal deformation temperature of 230°C or higher, such as
Poly(aryl ether ketone)s
227
poly(phenylene sulfide), poly(ethylene terephthalate), poly(amide), poly(imide), etc., may also be used. The pore size of the separator is set in a range generally used in the batteries, i.e., pores of 0.01 to 10 μ m. For the gasket, instead of PP, poly(phenylene sulfide), poly(ethylene terephthalate), poly(amide), liquid crystal polymer, tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer resin, PEEK, and poly(ether nitrile) have been found to be free of explosions and other drawbacks at the reflow temperature.41
6.5.4 Coatings PEEK coatings can be prepared on aluminum or stainless steel substrates using a flame spraying technique. However, the coating obtained in this way is simply an accumulation of the powder with a high porosity and bad mechanical properties. A more dense coating can be obtained by a laser treatment. In the course of this treatment, the polymer coating is remelted. Several laser types have been tested, the most suitable laser is the carbon dioxide laser to get more compact coatings. The laser-treated coating shows an amorphous structure. However, no obvious chemical modification is observed after the flame spraying process and the laser treatment.65 The sintering of PEEK coatings can be achieved by a microwave sintering technique. It has been demonstrated that silicon carbide distributed in the polymer matrix, is a good absorber for the microwave radiation, because of its high dielectric loss factor66 . The contact area of polymeric matrix and inorganic filler increases, as the particle diameter decreases. For this reason, nanoparticles are superior to microparticles. The danger of overheating is reduced for small particles and thus the sintering can be done more rapidly.
6.6 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 6.7. Other manufacturers and data sheets for poly(ether ether ketone) can be found in the internet.67 Tradenames appearing in the references are shown in Table 6.8.
228
High Performance Polymers
Table 6.7: Examples for Commercially Available Poly(arylene ether sulfone)s Tradename
Producer
PEEK-OPTIMA® Granular Victrex®PEEK Ketron®PEEK Ensinger PEEK Vestakeep® Edgetek™-PK Avaspire™ Ketaspire™ Gatone™ RTP Compounds 22XXa Larpeek a X=0. . . 9
Invibio Inc. Victrex Quadrant Engineering Plastic Products Ensinger Inc. Degussa AG PolyOne Corp. Solvay Advanced Polymers Solvay Advanced Polymers Gharda Chemicals Ltd. RTP Compounds LATI S.p.A.
6.7 SAFETY Heath hazards originating from PEEK are rather minor. As for other polymer classes, fumes created by overheating are considered harmful.
Poly(aryl ether ketone)s
Table 6.8: Tradenames in References Tradename Description
Supplier
DC® -704 Silicone oil49 DC® -710 Silicone oil49 Flemion® Fluoropolymer ion-exchange membrane2 Fluorinert®
Dow Dow Asahi Glass Company Minnesota Mining and Manufacturing Co. (3M)
Fluorinated oil34, 53 Freon® 113 DuPont 1,1,2-Trichloro-1,2,2-trifluoroethane49, 53 Gatone™ Gharda Chemicals Ltd. PEEK27 Gore-Select® W. L. Gore Microporous expanded PTFE membrane"(ePTFE), ion conductive membrane2 Grafoil® Advanced Energy Technology Inc. Flexible graphite33 HB® -40 Monsanto Co. Hydrogenated terphenyl49 Kevlar® DuPont Aramid2 Krytox® DuPont Fluorinated oil34 Kynar® Arkema, Inc. Poly(vinylidene fluoride)33 Lenzing® P84 Lenzing AG Benzophenone tetracarboxylic dianhydride-MDI-2,4-TDI copolymer, poly(imide)53 Mark 2112 Argus Chemical Corp. Tris(2,4-di-tert-butyl phenyl) phosphite1 Matrimid® Ciba Geigy Poly(imide)53
229
230
High Performance Polymers
Table 6.8 (cont): Tradenames in References Tradename Description
Supplier
Mobiltherm® (Series) Mobil Oil Corp. Heat transfer oil49 Nafion® 1100 EW DuPont Sulfonated PTFE, Nafion membrane, of equivalent weight (EW) of 11002 Nafion® DuPont Sulfonated PTFE, for membrane applications2, 61 Radel® R Solvay Poly(biphenyl sulfone)2 Sandostab® -P-EPQ Sandoz AG Tetrakis(2,4-di-tert-butyl phenyl)-4,4,-biphenylene diphosphonite1 Santowax® R Monsanto Co. Mixed terphenyls49 Siltem® STM 1500 General Electric Poly(ether imide)53 Stilan® Raychem Corp. Poly(etherketone)1, 13 Sulfan® B General Chemical Co.Corp. Sulfur trioxide2 Teflon® AF 1600 DuPont Amorphous copolymers of perfluoro-2,2-dimethyl-1,3-dioxole with tetrafluoroethylene34 Teflon® AF 2400 DuPont Amorphous copolymers of perfluoro-2,2-dimethyl-1,3-dioxole with tetrafluoroethylene34 Therminol® 66 Monsanto Co. Partially hydrogenated terphenyls49 Therminol® 75 Monsanto Co. Mixed terphenyls and quaterphenyls49 Torlon® (Series) Solvay (Amoco) 33 Poly(amide imide) Ucarsol® Union Carbide (Dow) Amine mixture53 Udel® Polysulfone Solvay Poly(bisphenol A sulfone)2
Poly(aryl ether ketone)s
Table 6.8 (cont): Tradenames in References Tradename Description
Supplier
Ultem® (Series) General Electric Poly(imide), thermoplastic53 Ultem® 6050 General Electric Poly(ether imidesulfone)53 Ultrapek® KR 4176 BASF AG 4,4 -Diphenoxybenzophenone-terephthaloyl chloride copolymer10 Vespel® DuPont Poly(imide), thermosetting33 Victrex® 381G Victrex PLC Poly(etheretherketone), cable coating42 Victrex® PEEK 450 Victrex PLC Poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene), poly(etheretherketone)42 Victrex® PEK Victrex Manufacturing Ltd. Poly(oxy-1,4-phenylenecarbonyl-1,4-phenylene)10
231
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REFERENCES 1. J. E. Harris. Stabilized poly(aryl ether ketone) compositions. US Patent 5 063 265, assigned to Amoco Corporation (Chicago, IL), November 5, 1991. 2. R. M. Formato, R. F. Kovar, P. Osenar, N. Landrau, and L. S. Rubin. Composite solid polymer electrolyte membranes. US Patent 7 052 793, assigned to Foster-Miller, Inc. (Waltham, MA), May 30, 2006. 3. R. J. Kumpf, D. A. Wicks, D. K. Nerger, H. Pielartzik, and R. Wehrmann. Poly(arylether)/liquid crystalline polyester block copolymers and a process for their production. US Patent 5 618 889, assigned to Bayer Corporation (Pittsburgh, PA), April 8, 1997. 4. J. B. Rose and P. A. Staniland. Thermoplastic aromatic polyetherketones. US Patent 4 320 224, assigned to Imperial Chemical Industries Limited (London, GB2), March 16, 1982. 5. Y. Niu, S. Zhang, X. Zhu, L. Liu, G. Wang, and Z. Jiang. “New poly(aryl ether ketone) copolymers containing 1,5-napthalene rings.” J. Macromol. Sci. Pure Appl. Chem., 42(5):641–648, 2005. 6. A. Ben-Haida, H. M. Colquhoun, P. Hodge, and D. J. Williams. “Synthesis of a catechol-based poly(ether ether ketone) ("o-PEEK") by classical step-growth polymerization and by entropically driven ring-opening polymerization of macrocyclic oligomers.” Macromolecules, 39(19):6467–6472, September 2006. 7. Y. M. Niu, X. L. Zhu, L. Z. Liu, Y. Zhao, G. B. Wang, and Z. H. Jiang. “Synthesis and properties of poly(aryl ether ketone) copolymers with trifluoromethyl-substituted benzene in the side chain.” J. Macromol. Sci., Pure Appl. Chem., 43(9):1459–1467, September 2006. 8. V. Jansons, H. C. Gors, S. Moore, R. H. Reamey, and P. Becker. Preparation of poly(arylene ether ketones). EP Patent 0 174 207, assigned to Raychem Corp. (US), March 12, 1986. 9. K. J. Dahl, V. Jansons, and S. Moore. Aryl ether ketone copolymers. US Patent 4 808 693, assigned to Raychem Corporation (Menlo Park, CA), February 28, 1989. 10. L. C. Costa. Asymmetric semipermeable poly(aryletherketone) membranes and method of producing same. US Patent 5 089 192, assigned to Ionics, Incorporated (Watertown, MA), February 18, 1992. 11. J. E. McGrath, K. R. Lyon, R. M. Davis, A. Texier, and A. Gungor. Fine powders of ketone-containing aromatic polymers and process of manufacture. US Patent 5 357 040, assigned to The Center for Innovative Technology (Herndon, VA) Virginia Polytechnic Institute & State University (Blacksburg, VA) Virginia Tech Intellectual Properties, Inc. (Blacksburg, VA), October 18, 1994. 12. K. H. Gharda, P. D. Trivedi, V. S. Iyer, U. M. Vakil, and S. C. Limaye. Melt processible polyether ether ketone polymer. US Patent 6 881 816, assigned
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to Gharda Chemicals Ltd. (Mumbai, IN), April 19, 2005. 13. J. E. Harris, L. M. Robeson, M. D. Cliffton, B. H. Eckstein, and M. Matzner. Miscible blends of a poly(aryl ether ketone) and an imide containing polymer. US Patent 5 171 796, assigned to Amoco Corporation (Chicago, IL), December 15, 1992. 14. M. J. Jenkins. “Crystallisation in miscible blends of PEEK and PEI.” Polymer, 42(5):1981–1986, March 2001. 15. P. Zschocke and D. Quellmalz. Integral asymmetric, solvent-resistant ultrafiltration membrane made of partially sulphonated, aromatic polyether ether ketone. DE Patent 3 321 860, assigned to Berghof Forschungsinst (DE), December 20, 1984. 16. C. Henneuse, B. Goret, and J. Marchand-Brynaert. “Surface carboxylation of PEEK film by selective wet-chemistry.” Polymer, 39(4):835–844, February 1998. 17. C. Henneuse-Boxus, T. Boxus, E. Duliere, C. Pringalle, L. Tesolin, Y. Adriaensen, and J. Marchand-Brynaert. “Surface amination of PEEK film by selective wet-chemistry.” Polymer, 39(22):5359–5369, October 1998. 18. C. Henneuse-Boxus, E. Duliere, and J. Marchand-Brynaert. “Surface functionalization of PEEK films using photochemical routes.” Eur. Polym. J., 37 (1):9–18, January 2001. 19. S.-W. Ha, R. Hauert, K.-H. Ernst, and E. Wintermantel. “Surface analysis of chemically-etched and plasma-treated polyetheretherketone (PEEK) for biomedical applications.” Surf. Coat. Tech., 96(2-3):293–299, November 1997. 20. H. Nakamura, T. Nakamura, T. Noguchi, and K. Imagawa. “Photodegradation of PEEK sheets under tensile stress.” Polym. Degrad. Stabil., 91(4):740– 746, April 2006. 21. H.-B. Qiao, Q. Guo, A.-G. Tian, G.-L. Pan, and L.-B. Xu. “A study on friction and wear characteristics of nanometer Al2 O3 /PEEK composites under the dry sliding condition.” Tribol. Int., 40(1):105–110, January 2007. 22. K-Mac Plastics. Data sheets. K-Mac Plastics 3821 Clay Ave. SW, Wyoming, MI 49548 [electronic] http://k-mac-plastics.net, 2006. 23. D. L. Burris and W. G. Sawyer. “A low friction and ultra low wear rate PEEK/PTFE composite.” Wear, 261(3-4):410–418, August 2006. 24. D. L. Burris and W. G. Sawyer. “Tribological behavior of PEEK components with compositionally graded PEEK/PTFE surfaces.” Wear, 262(1-2): 220–224, January 2007. 25. E. Reske and A. Schneller. A mixture containing stabilised aromatic polyether ketones, and its use in the preparation of moulded products. EP Patent 0 308 803, assigned to Hoechst AG (DE), March 29, 1989. 26. J. Koch, G. Schuermann, and G. Heinz. Stabilized polyaryl ether ketone molding compositions containing a phosphorus compound. US Patent 5 145 894, September 8, 1992.
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27. R. K. Goyal, A. N. Tiwari, U. P. Mulik, and Y. S. Negi. “Effect of aluminum nitride on thermomechanical properties of high performance PEEK.” Composites Part A, 38(2):516–524, February 2007. 28. R. K. Goyal, A. N. Tiwari, U. P. Mulik, and Y. S. Negi. “Novel high performance Al2 O3 /poly(ether ether ketone) nanocomposites for electronics applications.” Compos. Sci. Tech., 67(9):1802–1812, July 2007. 29. M. C. Kuo, C. M. Tsai, J. C. Huang, and M. Chen. “PEEK composites reinforced by nano-sized SiO2 and Al2 O3 particulates.” Mater. Chem. Phys., 90(1):185–195, March 2005. 30. Y. H. Lai, M. C. Kuo, J. C. Huang, and M. Chen. “On the PEEK composites reinforced by surface-modified nano-silica.” Mater. Sci. Eng., A, 458(1-2): 158–169, June 2007. 31. H.-D. Sturm, F. Sehr, and A. Dwars. Slide bearing for a centrifugal pump. US Patent 6 981 799, assigned to KSB Aktiengesellschaft (Frankenthal, DE), January 3, 2006. 32. W. Bickle and F. Haupert. Plain bearing composite material. US Patent 7 056 590, assigned to KS Gleitlager GmbH (St. Leon-Rot, DE), June 6, 2006. 33. B. A. Hotton, C. R. Brown, G. A. Carlson, J. K. Iveljic, J. S. Timko, and K. J. Mracek. Ball valve seat seal. US Patent 6 695 285, assigned to Swagelok Company (Solon, OH), February 24, 2004. 34. T. A. Dourdeville, A. C. Gilby, and D. DellaRovere. Flow cell, analyte measurement apparatus and methods related thereto. US Patent 6 526 188, assigned to Waters Investments Limited, February 25, 2003. 35. A. Murari, C. Vinante, and M. Monari. “Comparison of PEEK and VESPEL(R)SP1 characteristics as vacuum seals for fusion applications.” Vacuum, 65 (2):137–145, April 2002. 36. A. Murari and A. Barzon. “Comparison of new PEEK(R) seals with traditional helicoflex for ultra high vacuum applications.” Vacuum, 72(3):327– 334, November 2003. 37. J.-P. Buffard, M. Fontaine, and C. Gardaz. Antiadhesive coating with improved scratch resistance. US Patent 6 596 380, assigned to Seb SA (FR), July 22, 2003. 38. S. C. Kennedy, T. H. F. Tan, M. L. Taylor, and B. H. Tan. Submergible pumping system with thermal sprayed polymeric wear surfaces. US Patent 6 565 257, May 20, 2003. 39. C. G. Hutter, III. Cable tie. US Patent 6 928 701, assigned to Physical Systems, Inc. (Carson City, NV), August 16, 2005. 40. M. W. Orlet, M. M. Darpi, and J. P. Varkey. Dual stress member conductive cable. US Patent 6 960 724, assigned to Schlumberger Technology Corporation (Sugarland, TX), November 1, 2005. 41. S. Watanabe, T. Harada, Y. Kanno, S. Takasugi, T. Sakai, H. Onodera, and T. Tamachi. Non-aqueous electrolyte rechargeable batteries. US Patent
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6 713 215, assigned to Sii Micro Parts Ltd. (Miyagi, JP), March 30, 2004. 42. T. M. Williams and K. C. Gardeski. Deflectable tip catheter for CS pacing. US Patent 6 408 214, assigned to Medtronic, Inc. (Minneapolis, MN), June 18, 2002. 43. D. A. White and A. E. Williams. System and method for intraluminal imaging. US Patent 7 044 915, assigned to Boston Scientific SciMed, Inc. (Maple Grove, MN), May 16, 2006. 44. M. E. Graham. Sinus tarsi implant. US Patent 7 033 398, April 25, 2006. 45. K. Fujihara, Z.-M. Huang, S. Ramakrishna, K. Satknanantham, and H. Hamada. “Performance study of braided carbon/PEEK composite compression bone plates.” Biomaterials, 24(15):2661–2667, July 2003. 46. L. Mastronardi, A. Ducati, and L. Ferrante. “Anterior cervical fusion with polyetheretherketone (PEEK) cages in the treatment of degenerative disc disease. Preliminary observations in 36 consecutive cases with a minimum 12month follow-up.” Acta Neurochir., 148(3):307–312, March 2006. 47. T. Shimoda and H. Hachiya. Porous membrane. US Patent 6 017 455, assigned to Asahi Kasei Kogyo Kabushiki Kaisha (Osaka, JP), January 25, 2000. 48. T. Shimoda and H. Hachiya. Process for preparing a polyether ether ketone membrane. US Patent 5 997 741, assigned to Asahi Kasei Kogyo Kabushiki Kaisha (Osaka, JP), December 7, 1999. 49. H. N. Beck, R. A. Lundgard, and R. D. Mahoney. Process for making microporous membranes from poly(etheretherketone)-type polymers. US Patent 5 064 580, assigned to The Dow Chemical Company (Midland, MI), November 12, 1991. 50. R. E. Kesting. Synthetic Polymer Membranes. A Structural Perspective. Wiley, New York, 2nd edition, 1985. 51. S.-P. S. Yen, S. R. Narayanan, G. Halpert, E. Graham, and A. Yavrouian. Polymer material for electrolytic membranes in fuel cells. US Patent 5 795 496, assigned to California Institute of Technology (Pasadena, CA), August 18, 1998. 52. M. F. Sonnenschein. “Hollow fiber microfiltration membranes from poly(ether ether ketone) (PEEK).” J. Appl. Polym. Sci., 72(2):175–181, 1999. 53. Y. Yuan. Porous poly(aryl ether ketone) membranes, processes for their preparation and use thereof. US Patent 6 887 408, assigned to PoroGen LLC (Wilmington, MA), May 3, 2005. 54. C. Zhao, X. Li, Z. Wang, Z. Dou, S. Zhong, and H. Na. “Synthesis of the block sulfonated poly(ether ether ketone)s (S-PEEKs) materials for proton exchange membrane.” J. Membr. Sci., 280(1-2):643–650, September 2006. 55. J. C. Jansen, M. G. Buonomenna, A. Figoli, and E. Drioli. “Ultra-thin asymmetric gas separation membranes of modified PEEK prepared by the dry-wet phase inversion technique.” Desalination, 193(1-3):58–65, May 2006.
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56. M. Macchione, J. C. Jansen, and E. Drioli. “The dry phase inversion technique as a tool to produce highly efficient asymmetric gas separation membranes of modified PEEK. Influence of temperature and air circulation.” Desalination, 192(1-3):132–141, May 2006. 57. S. Cardea, A. Gugliuzza, E. Schiavo Rappo, M. Aceto, E. Drioli, and E. Reverchon. “Generation of PEEK-WC membranes by supercritical fluids.” Desalination, 200(1-3):58–60, November 2006. 58. E. Drioli, A. Regina, M. Casciola, A. Oliveti, F. Trotta, and T. Massari. “Sulfonated PEEK-WC membranes for possible fuel cell applications.” J. Membr. Sci., 228(2):139–148, January 2004. 59. A. Regina, E. Fontananova, E. Drioli, M. Casciola, M. Sganappa, and F. Trotta. “Preparation and characterization of sulfonated PEEK-WC membranes for fuel cell applications: A comparison between polymeric and composite membranes.” J. Power Sources, 160(1):139–147, 2006. 60. L. Paturzo, A. Basile, A. Iulianelli, J. C. Jansen, I. Gatto, and E. Passalacqua. “High temperature proton exchange membrane fuel cell using a sulfonated membrane obtained via H2 SO4 treatment of PEEK-WC.” Catal. Today, 104 (2-4):213–218, June 2005. 61. A. Basile, L. Paturzo, A. Iulianelli, I. Gatto, and E. Passalacqua. “Sulfonated PEEK-WC membranes for proton-exchange membrane fuel cell: Effect of the increasing level of sulfonation on electrochemical performances.” J. Membr. Sci., 281(1-2):377–385, September 2006. 62. E. Fontananova, A. Regina, E. Drioli, and F. Trotta. “Improving of the performances of sulfonated PEEK-WC membranes by introducing heteropolyacids in the polymeric matrix.” Desalination, 200(1-3):658–659, November 2006. 63. J. Shan, G. Vaivars, H. Z. Luo, R. Mohamed, and V. Linkov. “Sulfonated polyether ether ketone (PEEK-WC)/phosphotungstic acid composite: Preparation and characterization of the fuel cell membranes.” Pure Appl. Chem., 78(9):1781–1791, September 2006. 64. L. De Bartolo, S. Morelli, M. C. Gallo, C. Campana, G. Statti, M. Rende, S. Salerno, and E. Drioli. “Effect of isoliquiritigenin on viability and differentiated functions of human hepatocytes maintained on PEEK-WC-polyurethane membranes.” Biomaterials, 26(33):6625–6634, November 2005. 65. G. Zhang, H. Liao, H. Yu, S. Costil, S. G. Mhaisalkar, J.-M. Bordes, and C. Coddet. “Deposition of PEEK coatings using a combined flame spraying-laser remelting process.” Surf. Coat. Tech., 201(1-2):243–249, September 2006. 66. G. Zhang, S. Leparoux, H. Liao, and C. Coddet. “Microwave sintering of poly-ether-ether-ketone (PEEK) based coatings deposited on metallic substrate.” Scripta Mater., 55(7):621–624, October 2006. 67. IDES Integrated Design Engineering Systems. The Plastics Web®. IDES Inc. 209 Grand Avenue Laramie, WY 82070 USA [electronic] http://www.ides.com/prospector/, 2006.
7 Poly(arylene ether sulfone)s Poly(arylene ether sulfone)s (PAES)s have been known since the 1970’s. They are tough linear polymers and possess a number of attractive features, such as excellent high-temperature resistance, good electrical properties, and good hydrolytic stability. The topic has been reviewed previously in the literature.1, 2 In this text, we prefer the term arylene ether over aryl ether, in the sense that the aryl group is situated in the backbone of the polymer; for example, recall the meaning of the term methylene. There are variants of this type of polymer, for example, poly(ether ketone sulfone)s, or poly(ether ether ketone sulfone)s. Basically, a poly(ether ketone sulfone) can be understood as a copolymer bearing both the poly(ether ketone) moiety, and the poly(ether ketone) moiety in the backbone. This type of polymer could be dealt with either in the poly(ether ketone) chapter or in this chapter; it is a matter of taste that we include this type here. Still other variants are summarized in Table 7.1. The nomenclature is not unique. Sometimes, PAESs are simply addressed as poly(sulfone) resins. When collecting the literature to this text, more then 100 acronyms referring to polymers that containing sulfone groups were encountered. A few acronyms are compiled in Table 7.1. In ordinary organic chemistry, thioether is a synonym for sulfide. A thioether or sulfide is a compound that contains the R−S−R link. This is in contrast to the nomenclature of polymer chemistry.3 A polysulfide is a polymer that contains the −S−S− group in the backbone. However, a poly(sulfide), with sulfide in brackets, such as poly(phenylene sulfide) or 237
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Table 7.1: Aromatic Sulfone and Related Polymers Polymer
Acronym
Poly(sulfone) Poly(ether sulfone) Poly(phenylene ether sulfone) Poly(arylene sulfide sulfone) Poly(arylene thioether ketone ketone sulfone) Poly(arylene ether biphenyl ether sulfone) Poly(ether ether ketone sulfone) Poly(arylene ether ether sulfone) Poly(arylene ether sulfone ether ketone ketone) Poly(hydroxyether sulfone) Poly(phthalazinone ether sulfone) Poly(phthalazinone ether sulfone ketone) Poly(phenylene sulfide) Poly(phenylene sulfide ether) Poly(biphenyl ether sulfone) Poly(phenylene sulfide sulfone) Poly(phenylene sulfone ether ketone) Poly(phenylene thioether ether ketone) Poly(arylene thioether ketone) Sulfonated poly(arylene ether ketone ketone sulfone) Sulfonated poly(arylene ether sulfone) Sulfonated poly(arylene thioether ketone ketone sulfone) Sulfonated poly(ether ether ketone sulfone) Sulfonated poly(ether ether sulfone) Sulfonated poly(ether sulfone) Sulfonated poly(sulfide sulfone)
PSF PES PES PASS PATKKS PEBES PEEKS PEES PESEKK PHES PPES PPESK PPS PPSE PPSF 4 PPSS PSEK PTEK PTK SPAEKKS SPAES SPATKKS SPEEKS SPEES SPES SPSS
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O S
C
S
O
O
O
Sulfone
Ketone
Sulfoxide
O
S
S S
Ether
Thioether
Sulfide O
O
O
S O
Poly (biphenyl ether sulfone) CH3 C
O
O O
CH3
S O
Poly (bisphenol A ether sulfone)
Figure 7.1: Functional Groups and Building Blocks for Poly(ethersulfone) Types
poly(thio-1,4-phenylene) as an alterative name, contains only one sulfur group. Poly(sulfides) are mainly used for adhesives, coatings, and sealants, because they are easily crosslinkable, even at room temperature. Functional groups and building blocks of poly(ethersulfone) (PES) and related compounds are shown in Figure 7.1.
7.1 MONOMERS Monomer combinations for PES are shown in Table 7.2. The most common are 4,4 -Dichlorodiphenyl sulfone as sulfone combined with 4,4 -biphenol (BP) or bisphenol A as hydroxy component.
7.1.1 4,4 -Biphenol BP can be been prepared by:5 1. Alkali fusion of diphenyl disulfonic acid,
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4,4 -Dichlorodiphenyl
sulfone and biphenol sulfone and bisphenol A disodium
salt 4,4 -Dichlorodiphenyl sulfone and 4,4 -dihydroxydiphenyl sulfone 4-Fluorobenzenesulfinate catalyzed by 4,4 -difluorodiphenyl sulfone 4-Fluoro-4 -hydroxydiphenyl sulfone, and (fluorophenyl)(trifluorophenyl) sulfone as initiator 4,4 -Difluorodiphenyl sulfone, and silylated 4-tert-butylcatechol 5-[(4-fluorophenyl)sulfonyl]-2-fluorobenzoic acid and bis-(4-hydroxyphenyl)-sulfone
References 6, 7 8 6 9 10 11 12
2. Hydrolysis of dibromodiphenyl, or 3. Oxidative dimerization of 2,6-di-tert-butyl phenol to 3,3 ,5,5 -tetra-tert-butyl biphenol (TBBPL) and subsequent debutylation. In the first two methods, severe reaction conditions are required. Problems arise by the need of separation of large amounts of inorganic salts used. For this reason, the oxidative dimerization of 2,6-di-tert-butyl phenol is most preferably used to prepare BP on an industrial level. The debutylation rates of the four butyl groups in TBBPL are not equal, and that the rate slows as the number of butyl groups decreases. Therefore, continuous debutylation of TBBPL using a cascade of reactors can produce BP with a high yield and high purity continuously on an industrial level. For the debutylation reaction of TBBPL, an acid catalyst, such as p-toluenesulfonic acid is used at a reaction temperature up to 250°C.13
7.1.2 Bisphenol A Bisphenol A is produced by the condensation reaction of excess phenol with acetone in the presence of an acidic catalyst. Sulfur compounds that may be used as a cocatalyst include alkyl mercaptans, such as methyl mercaptan, ethyl mercaptan and thioglycol acid. Recently, a catalyst composed of an acid-type ion exchange resin, which is modified in part with a sulfur-
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containing amine compound has been disclosed.14 Bisphenol A is used for a wide variety of resins, mostly in the production of epoxy resins.
7.1.3 Bis-(4-hydroxyphenyl)-sulfone Bis-(4-hydroxyphenyl) sulfone is also addressed as 4,4 -dihydroxydiphenyl sulfone, or 4,4 -bisphenol S. It is of great commercial interest for the preparation of PES. It can be prepared by reacting phenol with sulfuric acid, without using a solvent. Such processes give a crude product with 60–70% bis-(4-hydroxyphenyl)-sulfone. In addition, the crude product contains 20–30% of the isomer 2,4-dihydroxydiphenyl sulfone and ca. 10% of 6-hydroxy[1,3-bis-(4-hydroxyphenylsulfonyl]benzene. A major effort is required to isolate the desired bis-(4-hydroxyphenyl)-sulfone in high purity from the mixture. Yield and selectivity of the reaction can be improved by using a solvent. Purification is simplified, when the bis-(4-hydroxyphenyl)-sulfone formed is selectively deposited on a nucleating surface, i.e., a supercooled surface, during or after the reaction, and thus removed from the reaction medium.15 Suitable solvents are either excess phenol or an inert aromatic solvent. Preferred solvents are those in which the 2,4 -isomer is more highly soluble than the bis-(4-hydroxyphenyl)-sulfone, such as chlorobenzene, dichlorobenzene and trichlorobenzene. In a semi-continuous process, in the first step phenol is extensively reacted with sulfuric acid to give phenolsulfonic acid. Any water of reaction thereby formed is removed by distillation. Then, the phenolsulfonic acid is reacted with phenol to give bis-(4-hydroxyphenyl)-sulfone and the water of reaction formed is again removed by distillation.16
7.1.4 Bis-(4-chlorophenyl)-sulfone Bis-(4-chlorophenyl)-sulfone is an important intermediate, which is used mainly for the preparation of aromatic poly(sulfone)s and for the synthesis of bis-(aminophenyl)-sulfone. This compound is required both for the therapy of leprosy and for curing epoxy resins.17 The most well-known method for the preparation of bis-(4-chlorophenyl)-sulfone is the Friedel-Crafts reaction of 4-chlorobenzenesulfonyl chloride with chlorobenzene, the catalyst used, for example, iron(III) chlor-
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ide. The reaction is carried out in chlorobenzene as a solvent at about 140°C. However, iron(III) chloride also acts as a chlorinating agent for chlorobenzene at this temperature. Therefore, considerable amounts of dichlorobenzenes are formed as byproducts. Dichlorobenzenes can react with chlorobenzenesulfonyl chloride to give undesirable sulfones. If the reaction is carried out as a single-stage reaction starting from chlorobenzene, without isolation of the intermediate chlorobenzenesulfonyl chloride, care must be taken to ensure that sulfonic acid, thionyl chloride and sulfur chlorides, as impurities in the thionyl chloride. are no longer present in the reaction mixture, since the free sulfonic acid causes deactivation of the catalyst, and thionyl chloride and sulfur chlorides likewise lead to undesirable byproducts. A further difficulty is that N,N-dimethylformamide is required for complete conversion of chlorobenzenesulfonic acid with thionyl chloride into the corresponding sulfonyl chloride, carcinogenic N,N-dimethylcarbamyl chloride being formed as a byproduct.17 Another synthesis route for bis-(4-chlorophenyl)-sulfone is the reaction of chlorobenzene and chlorobenzenesulfonic acid. At 220–260°C at an elevated pressure of 1.7 bar, bis-(4-chlorophenyl)-sulfone is obtained in a good yield.18 Further, bis-(4-chlorophenyl)-sulfone can be prepared by heating a mixture of chlorobenzene and sulfuric acid to 200–250°C.17 The water formed in the course of the reaction is removed by azeotropic distillation. The process is conducted at a pressure of 4–5 bar. The addition of catalytic amounts of boric acid or trifluoromethanesulfonic acid reduces the reaction time considerably.
7.2 POLYMERIZATION AND FABRICATION The synthesis and the modification techniques concerning PES have been reviewed by Kricheldorf.19 Poly(ethersulfone)s can be obtained either by a conventional step-growth polycondensation, or by a chain-growth polycondensation, which is in fact a living polycondensation.20
7.2.1 Step-growth Polycondensation A broad range of PES can be formed by the nucleophilic aromatic condensation reaction of an aromatic dihydroxy compound and a bis-(halophen-
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yl)-sulfone.21 The condensation can be conducted in several ways. The most convenient way is to prepare the salt of aromatic dihydroxy compound in situ, using a high-boiling solvent that forms an azeotrope with water and then allowing it to react with the bis-(halophenyl)-sulfone. In this way, problems with residual water can be avoided. Limitations on polymer molecular weights may be expected when the aromatic dihydroxy compound or the corresponding alkali metal derivative contain strong electron-withdrawing groups. This may result in lower molecular weight polymers or slow reaction rates. Therefore, the aromatic dihydroxy compound should be a rather weakly acidic phenol, such as 2,2-bis-(4-hydroxyphenyl)-propane, 1,1-bis-(4-hydroxyphenyl)-2phenyl ethane, and bis-(4-hydroxyphenyl)-methane. In the bis-(halophenyl)-sulfone, the sulfone group activates the halogens toward the condensation reaction. For this reason, it is preferred that the aromatic rings do not contain electron supplying groups on the same ring where the halogen is bonded. Several examples of how to prepare PESs are given in the literature.21 Poly(biphenyl ether sulfone)s with improved polydispersity, a lower level of undesirable low-molecular-weight oligomeric components and improved melt flow properties have 4,4 -biphenylene, p-phenylene, 4,4 -diphenyl sulfone and 2,2-diphenyl propane groups in the backbone.7 Poly(biphenyl ether sulfone)s can be prepared, with carbonates or with the alkali metal hydroxides as activators. In the carbonate method, the poly(sulfone)s are prepared by the reaction of equimolar amounts of dihydroxy aromatic compounds and dihalodiaryl sulfones. Dihydroxy aromatic compounds are BP, 4,4 -dihydroxydiphenyl sulfone, hydroquinone, bisphenol A. Dihalodiaryl sulfones, are 4,4 -dichlorodiphenyl sulfone or 4,4 -difluorodiphenyl sulfone. 0.5 to about 1.0 mole of an alkali metal carbonate per mole of hydroxyl group is added. The condensation is conducted as azeotropic condensation, at a temperature of 210–300°C up to 15 hours. In a variant of the procedure, earlier, it has been proposed to add 4,4 -dichlorodiphenyl sulfone not in equimolar quantities, but in a slight excess. When the reaction is essentially complete, 4,4 -difluorodiphenyl sulfone may be added. The effects of the 4,4 -difluorodiphenyl sulfone depend on the proportion added. At not more than about 0.5% molar, relative to the bis-(chloro-
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aryl)-sulfone, it essentially prevents further polycondensation by reacting with hydroxy salt on the polymer chain. At 0.5 to 5%, molar stabilization or a decrease in the molecular weight of the polymer occurs. If the polymer molecular weight is greater than desired, the addition of a suitable proportion of 4,4 -difluorodiphenyl sulfone can decrease it to the desired value.22 While the carbonate method for preparing the polymers is simple and convenient, in some cases products of higher molecular weight can be obtained by the alkali metal hydroxide method. In the alkali metal hydroxide method, a double alkali metal salt of a dihydric phenol is reacted with a dihalobenzenoid compound. Poly(biphenyl ether sulfone)s having a low color, can be manufactured by using an anhydrous potassium carbonate having an average particle size of up to about 100 μ m.23 The low color poly(biphenyl ether sulfone)s have a superior appearance. They are particularly desirable for use in applications where color is unacceptable, such as lenses, filters and other optical goods, for transparent covers or lids, and in containers. The resins may be more readily dyed or pigmented to achieve a desired coloration. A PAES type that contain the biphenylene moiety has a high glass transition temperature of 260°C, which is 35°C higher than that of PES.24 Sulfonated PAES random copolymers can be prepared by the by potassium carbonate mediated direct aromatic nucleophilic substitution polycondensation of disodium 3,3 -disulfonate-4,4-dichlorodiphenyl sulfone, 4,4 -dichlorodiphenyl sulfone and BP.25 The condensation reaction proceeds quantitatively to high molecular weight in N-methyl-2-pyrrolidone (NMP) at 190°C. In addition, a monofunctional monomer, 4-tert-butylphenol, can be used as an end capping reagent. The phenol functional group has a similar reactivity as biphenol.26 In this way, the molecular weight can be controlled.
7.2.2 Chain-growth Polycondensation In chain-growth polycondensation, the polymer end group is activated. This activation changes the reactivity of the substituents attached to the aromatic ring, so that a chain-growth polycondensation takes place.20 This type of polycondensation allows the synthesis of polymers with low polydispersity. However, other mechanisms of polymerization, such as
Poly(arylene ether sulfone)s
245
O KO
Cl
S O
O O
S O
n
Figure 7.2: Condensation of Chlorophenylsulfonyl phenoxide
step-growth polycondensation, as well as side reactions must be effectively suppressed. Perhaps the first indication of chain-growth polymerization was realized in the synthesis of poly(phenylene sulfide) using p-halothiophenol salts.27 For certain monomers, it was realized that the polymer chain end groups were more reactive than the monomers. Later, in the polycondensation of chlorophenylsulfonyl phenoxide, an increased reactivity of the polymer end group was detected.28 The reaction scheme is shown in Figure 7.2. The polymeric end group reacts ca. 20 times faster than the monomer. Essentially the same phenomenon was observed in the polycondensation of 4-chlorobenzenesulfinate or 4-fluorobenzenesulfinate. The addition of a small amount of 4,4 -difluorodiphenyl sulfone greatly increases the yield of polymer. It was explicitly stated that 4,4 -difluorodiphenyl sulfone may act as an initiator for a chain-growth polycondensation.9 The reaction scheme is shown in Figure 7.3. A well-defined poly(ether sulfone) can be synthesized from 4-fluoro-4 -hydroxydiphenyl sulfone, with (fluorophenyl)(trifluorophenyl) sulfone as an initiator, as shown in Figure 7.4. The condensation is performed with 18-crown-6 ether in sulfolane at 120°C. However, when the polymerization is conducted at a higher feed ratio of monomer to initiator, both chain-growth and step-growth polycondensation occurs.10 The copolymerization of 5-[(4-fluorophenyl)sulfonyl]-2-fluorobenzoic acid with bis-(4-hydroxyphenyl)-sulfone results in carboxylated PES. However, during polycondensation, partial decarboxylation occurs. The copolymerization of 2,5-dihydroxybenzoic acid with bis-(4-fluorophenyl)sulfone results in a PES with quantitative decarboxylation.12
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SO2Na
X X = F, Cl
O F
F
S O
O X
F
S O n
Figure 7.3: Condensation of 4-Halobenzenesulfinate Catalyzed by 4,4 -Difluorodiphenyl Sulfone
F
O
F C
S
F
O
F + O KO
F
S O
F
O
F C
S
F
O
O O
F
S O
n
Figure 7.4: Condensation of 4-Fluoro-4 -hydroxydiphenyl Sulfone, with (Fluorophenyl)(trifluorophenyl) sulfone as an Initiator10
Poly(arylene ether sulfone)s
247
HO3S O F
S
F
O SO3H 3,3′-Sulfonyl bis(6-fluorobenzene sulfonic acid) HO3S O HO
S
OH
O SO3H 3,3′-Sulfonyl bis(6-hydroyxbenzene sulfonic acid)
Figure 7.5: Telechelic Sulfonated Hydroxy Functional Monomer and Telechelic Sulfonated Fluoro Functional Monomer29
7.2.3 Copolymers from Telechelic Monomers Telechelic compounds are oligomers or low-molecular-weight polymers carrying monofunctional terminal groups or reactive terminal groups, respectively, on both chain ends. Block sulfone copolymers have been synthesized from hydroxy-telechelic sulfonated PESs and fluorotelechelic PESs. As a monomer for the sulfonated hydroxy-telechelic compound, 3,3 -sulfonyl bis-(6-hydroxybenzene sulfonic acid) disodium salt is used.29 This compound is synthesized from bis-(4-hydroxyphenyl)-sulfone by sulfonation with concentrated sulfuric acid and subsequent neutralization. Similarly, as monomer for the sulfonated fluorotelechelic compound, 3,3 -sulfonyl bis-(6-fluorobenzene sulfonic acid) disodium salt is obtained by sulfonation with concentrated sulfuric acid and neutralization. The compounds are shown in Figure 7.5 The momomers can be chain extended with bis-(4-fluorophenyl)sulfone or bis-(4-hydroxyphenyl)-sulfone, respectively in a next condensation step. Eventually, in a final condensation step, block copolymers containing blocks of unsulfonated aromatic polyether sulfones and blocks of aromatic polyether sulfones sulfonated on the aromatic rings are obtained. The block copolymers provide compounds with both an adjustable degree of sulfonation and a defined length of sulfonated and unsulfonated blocks. The materials are suitable for the preparation of synthetic membranes.
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High Performance Polymers
7.2.4 Macrocyclic Polymers In poly(ether sulfone)s that are prepared by polycondensation of silylated 4-tert-butylcatechol and 4,4 -difluorodiphenyl sulfone, macrocyclic polymers were obtained to some extent.11 The cyclic polymers were detected by means of matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry . Under the conditions of a kinetically controlled polycondensation reaction, cyclization reactions compete with propagation steps. The extent of cyclization depends on the flexibility of the polymer chain and on the concentration of the active species. Blends of PES with its homologous macrocyclic oligomers show greatly lowered melt viscosities in comparison to the corresponding original PES. This may facilitate the production and fabrication of such materials. The macrocycles can undergo a ring-opening polymerization in situ. The ring-opening is governed by entropy.30
7.2.5 Friedel-Crafts Polymerization In fact, the Friedel-Crafts polymerization is a polycondensation, however, the term polymerization is more common. The Friedel-Crafts polymerization is notorious in producing an intractable reaction product, which is difficult to remove from the reaction vessel and to purify. Further, polymers with undesirably low molecular weights or poor thermal stability are obtained, if the reaction conditions are not appropriately chosen.31 In Friedel-Crafts reactions, ortho substitution of the polymer is more likely to occur if the reaction is conducted at elevated temperatures and for a relatively long reaction time. To overcome these problems, it has been proposed to use a boron trifluoride catalyst in anhydrous hydrogen fluoride. Another procedure uses lithium chloride and aluminum chloride to polymerize p-phenoxybenzoyl chloride as the ketone monomer and p-phenoxybenzenesulfonyl chloride as the sulfone monomer.31 High-molecular-weight poly(ketone)s and poly(ketone sulfone)s can be prepared by reacting dicarboxylic acids with aromatic compounds in the presence of trifluoromethanesulfonic acid and phosphorus pentoxide for water binding. The polymerization occurs as water is formed by the elimination of a hydroxyl group from the carboxylic acid, and hydrogen from an aromatic ring. In this way, the polymer chain becomes linked together between
Poly(arylene ether sulfone)s
249
a carbonyl group and an aromatic ring.32 The reaction proceeds sightly below room temperature, at 16–20°C. The electrophilic polymerization of this type is often referred to as Friedel-Crafts polymerization. Examples are polymers obtained from near equimolar feeds of terephthalic acid as dicarboxylic acid and 4-phenoxyphenyl sulfone, or 4-biphenylyl sulfone, respectively. Usage of 4,4 -diphenoxybenzophenone yields a poly(ketone). When hydroquinone as hydroxy compound is substituted by 2,7-dihydroxynaphthalene, the glass transition temperature increases, while the melting temperature and the thermal stability decrease with the increase of the 2,7-naphthalene moieties.33
7.2.6 Sulfonation PES are hydrophobic. For applications in membrane technology, it would be desirable to raise their water affinity. One effective method to increase the water affinity, is sulfonation. The very mechanisms of sulfonation of high polymers have been reviewed by Kuˇcera and Janˇcáˇr.34 The sulfonation reaction proceeds easily in the presence of groups, such as −Cl, −NH2 , −OH, −SH, etc. In fact, the active agent in the sulfonation reaction is the SO+ 3 cation. The sulfonation of aromatic compounds is a reversible reaction. Sulfonating agents can be classified into three groups, namely:34 1. Electrophilic agents: sulfuric acid, chlorosulfonic acid, fluorosulfonic acid, 2. Nucleophilic agents: sulfites, hydrogen sulfites, and sulfur dioxide, which react with halogen derivates and double bonds, and 3. Radically reacting agents: sulfuryl chloride, mixtures of sulfur dioxide and chlorine. The sulfonation of the monomers prior to condensation or direct sulfonation of the polymer results in hydrophilic materials. These materials are used in membrane technology, in particular in the fabrication of fuel cells. When the monomers are sulfonated, the degree of sulfonation of the polymers can be controlled by varying the ratio of feed of unsulfonated monomer to sulfonated monomer. The intrinsic viscosities of the polymers increase with the degree of sulfonation.35 The sulfonation of PES can be
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High Performance Polymers
carried out in chloroform and chlorosulfonic acid at 0°C.36 The reaction is complete after 15 min. For the sulfonation of PAES-b-poly(butadiene) block copolymers, acetyl sulfate, which can be prepared from acetic anhydride concentrated sulfuric acid, is used successfully.37
7.2.7 Blends Immiscible PES blends with improved properties are composed from poly(biphenyl ether sulfone) and poly(1,4-phenylene ether sulfone).38 Ternary resin blends comprising a poly(biphenyl ether sulfone), a poly(ether sulfone) and a poly(sulfone), exhibit very attractive thermal and environmental resistance characteristics together with excellent mechanical properties. Molded articles from such ternary blends can be steam-sterilized repeatedly without stress-cracking. They are not affected by corrosion-reducing additives such a morpholine. Further, they exhibit a good chemical resistance in contact with commonly used hospital cleaners and detergents.4 Super-tough poly(sulfone)s can be produced by blending PES with acrylate rubber and a polycarbonate as an impact modifier.39 From this high impact strength composition, films and molded articles are useful in automotive applications, durable goods and appliances, medical and plumbing applications where resistance to hot, humid environments may be particularly important, and safety equipment and protective gear. A cold-pressed molding of a PAES with poly(phenylene sulfide) as an additive component does not melt even at temperatures of above 360°C, although poly(phenylene sulfide) has a melting temperature of 280–290°C. It has been established that after the processing of the polymer mixtures, only the glass transition temperature of the PAES is detected. This indicates that the originally heterogeneous polymer mixture is being converted into a homogeneous polymer mass by the processing procedure.40 Blends of poly(biphenyl ether sulfone) with poly(tetrafluoroethylene) (PTFE) and titanium dioxide exhibit as filler enhanced flame retardant characteristics. They can be processed by melt extrusion and are useful for a wide variety of applications, such as aircraft interior parts.41
Poly(arylene ether sulfone)s
251
7.2.8 Varieties of PES In this section, a few related varieties of poly(ethersulfone) are briefly described. These varieties include: • Poly(arylene thioether ketone)s, • Poly(arylene thioether sulfone)s, or • Poly(arylene sulfide sulfone)s. 7.2.8.1
Poly(arylene thioether ketone)s
Poly(arylene thioether ketone)s (PTK)s are ultra-high heat resistant aromatic polymers combining a high melting point of about 350°C with a high glass transition temperature of about 135°C. There is a strong demand for the provision of such polymers in the fields of frontier technologies. PTKs can be produced by causing an alkali metal sulfide and a 4,4 -dihalobenzophenone to undergo a dehalogenation and sulfurization reaction in an organic amide solvent.42 However, PTKs involve problems when they are processed by extrusion and subsequent stretching or sheet forming. It is difficult from the technical viewpoint to apply these forming and processing methods to these materials. This problem is attributed to the formation of coarse spherulites in a product formed by the extrusion owing to the high crystallization rate. 7.2.8.2
Poly(arylene thioether sulfone)s
In contrast to PTK materials, the addition of sulfone groups to the polymer minimize the problems. When a 4,4 -dihalobenzophenone as a dihalogenated aromatic compound is combined with a 4,4 -dihalodiphenyl sulfone followed by their reaction with an alkali metal sulfide, an aromatic thioether ketone/thioether sulfone random copolymer can be obtained with a high molecular weight.43 The thioether ketone/thioether sulfone copolymer exhibits a reduced crystallinity and a high melting point, but a glass transition temperature higher than that of the corresponding PTK. When proportion of the 4,4 -dihalobenzophenone to the 4,4 -dihalodiphenyl sulfone is selectively limited to a specific range, a copolymer moderately reduced in crystallization rate can be obtained in the form of granules. For example, such a copolymer is prepared by charging an autoclave with NMP) as an organic amide solvent, alkali metal sulfide, 4,4 -di-
252
High Performance Polymers
O S
S S
S y
x
O
z
Figure 7.6: Poly(ethersulfone)s with −S−S− Moieties44
HOOC O O
C
S
O
O 2
Figure 7.7: 4,4 -Bis-((3-carboxyphenoxy)(p-benzoyl))-phenyl sulfone
chlorobenzophenone, and 4,4 -dichlorodiphenyl sulfone. The autoclave is purged with nitrogen, degassed, and the polycondensation is conducted up to 200°C.43, 45, 46 The materials obtained, have been extensively characterized. The results are detailed in the literature.43 7.2.8.3
Poly(arylene sulfide sulfone)
In this section, polymers that contain the −S−S− group are dealt with. Copolymers that can be described with repeating units corresponding to the structure as shown in Figure 7.6, exhibit an enhanced interfacial adhesion. The copolymer is prepared by the reaction of sulfur, p-diiodobenzene, and p,p -diiododiphenyl sulfone.44 The resulting copolymer is an amorphous, high viscosity material with a Tg of 147°C. 7.2.8.4
Poly(amide)s with Sulfone in the Backbone
Poly(amide)s (PA)s that bear the sulfone group in the backbone have been described.47 These types of PA, which are based on 4,4 -bis-((3-carboxyphenoxy)(p-benzoyl))-phenyl sulfone, c.f. Figure 7.7, or the corresponding 4-carboxy acid component48 is amidized by an aromatic diamine, e.g., p-diaminobenzene. The direct polycondensation of a dicarboxylic acid with aromatic diamines uses triphenyl phosphite and pyridine. The introduction of m-structures increases the solubility of the polymers and leads to
Poly(arylene ether sulfone)s
253
OH
F N
O S
N N
O O
OH
Figure 7.8: 2,4-Bis-(4-hydroxyphenyl)-6-(4-(4-(4-fluorobenzenesulfonyl)phenoxy)phenyl)-1,3,5-s-triazin
better processability. PAs from the m-diacid show greater thermal stability than the p-linked PAs.
7.2.8.5
Hyperbranched PES
In general, hyperbranched polymers are obtained by the polymerization of an AB2 monomer. Thus, in the first step, the hyperbranched PES must be manufactured. For example, the synthesis of 3,5-difluoro-4 -hydroxydiphenyl sulfone can be accomplished by the reaction of 3,5-difluorophenylmagnesium bromide with 4-methoxyphenylsulfonyl chloride, followed by deprotection of the phenol group with HBr in acetic acid.49 The actual formation of hyperbranched material proceeds during the polymerization of 3,5-difluoro-4 -hydroxydiphenyl sulfone in the presence of 3,4,5-trifluorophenylsulfonyl benzene or tris(3,4,5-trifluorophenyl)phosphine oxide as a core molecule. Cyclic oligomers formed during this polymerization contribute to a low-molecular-weight polymer ranging from 3400 to 8400 Dalton. A triazin-based AB2 monomer has also been described.50 This monomer is shown in Figure 7.8. A hyperbranched aromatic poly(ether sulfone) with sulfonyl chloride terminal groups has been prepared by the polycondensation of 4,4 -(m-phenylenedioxy)-bis-(benzenesulfonyl chloride). The polymerization was carried out in nitrobenzene at 120°C for 3 h in the presence of a catalytic amount of FeCl3 .51
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High Performance Polymers
7.2.9 Modification When the surface properties are to be changed, e.g. in membrane technology, grafting techniques are used. However, sometimes it is desirable to modify the building material, in a similar way as surface grafting affects just the surface. Poly(ethersulfone) can be modified by dissolving it in NMP and adding acrylic acid (AA) to the solution. The solution is cast between glass plates and irradiated with UV. The glass plates filter out shorter wavelength ultraviolet light and prevent excessive UV photochemical degradation of the PES. The PES degrades to some extent and initiates a polymerization of AA. In this way, a block copolymer is formed.52 Thiohydroxamic esters, such as N-hydroxypyridine-2-thione, were first used as free radical precursors by Barton.53, 54 The decomposition of such esters by heat or visible light yields acyloxy radicals and pyridine thiol radicals. However, on irradiation at low-temperature, the chain reaction is essentially suppressed. Carboxylated PAES can be synthesized by sequential lithiation and carboxylation of poly(arylene ether sulfone) in tetrahydrofuran. The carboxyl groups are then converted into acid chloride groups by thionyl chloride and treatment with N-oxypyridine-2-thione gives the thiohydroxamic ester.55 A grafting reaction is obtained by adding to the polymer styrene and irradiation. Homopolymerization of the styrene does not take place under these conditions. Other vinyl monomers, including methyl methacrylate and acrylamide can be grafted by this method. The scheme of grafting is shown in Figure 7.9. By end capping with phenylethynyl moieties, PAES thermosets can be obtained.56 The curing process occurs with a free radical mechanism. Curing needs a temperature as high as 370°C for 2 h in a nitrogen atmosphere.
7.3 PROPERTIES Selected physical properties of a transparent injection molding grade type of PES are shown in Table 7.3.
7.3.1 Thermal Properties Poly(sulfone)s have high oxygen indices and low smoke emission on burning. The material extinguishes after removal of the test flame and poly-
Poly(arylene ether sulfone)s
CH3 O
O O
C
S
CH3 O SOCl2
O C OH
O- N+ S-
CH3 O
C
O O
S
CH3 O
O C O N
CH CH2 hν 25˚C
CH3 O
C
S
O O
S
CH3 O
O C O (CH CH2)n S N
Figure 7.9: Grafting of Carboxylated PES55
255
256
High Performance Polymers Table 7.3: Properties of PES a Property
Value
Unit
Density 1.29 g cm−3 Water Absorption, 24 h 0.37 % Water Absorption, equilibrium 1.1 % Tensile Modulus 2.3 GPa Tensile Strength Break 69.6 MPa Tensile Elongation Yield 7.2 % Tensile Elongation Break 60–120 % Flexural Modulus 2.4 GPa Flexural Strength 91.0 MPa Notched Izod Impact b 690 J m−1 b 400 kJ m−2 Tensile Impact Strength Glass Transition Temperature 220 °C Dielectric Constant b (60 Hz) 3.44 Refractive Index 1.672 Oxygen index 38 % Injection Molding 360–390 °C a RADEL®R R-5000, Poly(phenyl sulfone), Solvay b 3.18 mm
Standard ASTM D792 ASTM D570 ASTM D570 ASTM D638 ASTM D638 ASTM D638 ASTM D638 ASTM D790 ASTM D790 ASTM D256 ASTM D1822 ASTM E1356 ASTM D150 ASTM D542 ISO 4589
(sulfone)s are inherently self-extinguishing materials owing to their highly aromatic character.57 A hydroxy terminated copoly(ester sulfone) synthesized by melt polycondensation of the diethylene glycol and 4,4-dihydroxydiethoxydiphenyl sulfone with adipic acid can be used as a flame retardant for poly(ethylene terephthalate). With filament pulse pyrolysis experiments at 800°C, the pyrolysis chamber being coupled with gas chromatography mass spectrometry have been performed with both a poly(1,4-phenylene ether sulfone) and a poly(bisphenol A ether 1,4-phenylene ether sulfone).58 The major products of pyrolysis are shown in Table 7.4. During pyrolysis, the formation of sulfide groups in the condensed phase from PES through the reduction of sulfone groups by hydrogen radicals increases the flame retardancy of PES.59
7.3.2 Chemical Properties Sulfone polymers exhibit varying levels of chemical compatibility, depending upon their polymeric structure. The chemical compatibility is influ-
Poly(arylene ether sulfone)s
257
Table 7.4: Major Products of Pyrolysis of Poly(ether sulfone)s58 Product
Found in
ab Sulfur dioxide ab Benzene b Toluene b Styrene a b Phenol a b Diphenyl ether b Methyldiphenyl ether a Dibenzofuran a Poly(1,4-phenylene ether sulfone) b Poly(bisphenol A ether 1,4-phenylene ether sulfone)
enced by the nature of the reagent, reagent concentration, temperature, exposure time, and whether the polymer is under stress. Tables of chemical compatibility can be found in the literature.60 Sulfone-based polymers show a very good resistance to prolonged chlorine exposure at elevated temperatures. The weight change after an exposure of 6 months to static chlorinated water at 60°C at chlorine levels of up to 30 ppm is essentially zero for Udel®, whereas e.g., poly(acetal) exhibits a weight loss of ca. 5% at 30 ppm chlorine in water after 6 months.61 This property suggests applications in water delivery systems.
7.3.3 Electrical Properties PAESs show good electrical properties and are thus used in electric and electronic applications, c.f. Table 7.3. They exhibit a high dielectric constant and a high dielectric strength of 15 kV mm−1 . The dielectric strength is a measure of a material’s ability to resist high voltage without dielectric breakdown62 .
7.4 APPLICATIONS Due to their interesting properties, PAES have found a wide field of applications. The most widespread use seems to be in membrane technology. Applications are summarized in Table 7.5.
258
High Performance Polymers Table 7.5: Fields of Use of Poly(ethersulfone) Materials Field of Use Membrane technology Composites Stain-resistant microwave cookware63 Electrical applications Medical applications Dental plaque barriers Plumbing materials to replace soldering Coating dispersions64
7.4.1 Membranes Membrane technology is reviewed in the monograph by Mulder65 and more recently by Rikukawa and Sanui.66 7.4.1.1
Ultrafiltration Membranes
The majority of polymer membranes used for microfiltration and ultrafiltration of liquids are prepared by the wet phase inversion process. Such membranes exhibit a typical asymmetric structure characterized by a thin dense surface layer and a thick microporous bulk. Poly(phthalazinone ether sulfone ketone) (PPESK) copolymers, c.f. Figure 7.10, show glass transition temperatures in the range of 263–305°C.67, 68 The polymers show an outstanding chemical stability. They are soluble only in 98% H2 SO4 . Concentrated aqueous solutions of sodium chlorate, hydrogen peroxide, acetic acid, and nitric acid show no effect.67 These copolymers can be sulfonated and fabricated into ultrafiltration and nanofiltration asymmetric membranes.69 Such a membrane formulation exhibited a 98% rejection rate for poly(ethylene glycol) 12000 and a high pure water flux of 867 kg m−2 h−1 . The proton conductivities of the sulfonated materials reach higher than 10−2 S cm−1 at a degree of sulfonation of 1.0. Therefore, the use of the materials in fuel cells has been suggested.70 Surface modification in order to improve the hydrophilicity of membrane surface can be achieved by blending surface modifying macromolecules to the base material. Blends of phenolphthalein poly(ether sulfone) and poly(acrylonitrile-co-acrylamido methylpropane sulfonic acid), a material that contains charged groups, have been prepared.71 It was found
Poly(arylene ether sulfone)s
259
HO3S O
O N N
O
S
O
O
N N
C
O
Figure 7.10: Poly(phthalazinone ether sulfone ketone) Copolymers
that the charged groups tend to accumulate onto the membrane surface using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The anti-fouling properties of ultrafiltration (UF) membranes can be improved by the modification with TiO2 nanoparticles.72 The nanoparticles of a size of 40 nm, or less, with an anatase crystal structure are prepared by the controlled hydrolysis of titanium tetraisopropoxide. By irradiation with UV light, crosslinking and chain scission in the base membrane material occurs.73 Crosslinking increases the hydrodynamic resistance of the membrane. Chain scission is responsible for the loss of membrane selectivity. The presence of a monomer during UV irradiation, such as N-vinyl-2-pyrrolidone (NVP) seems to retard the loss of membrane selectivity. UV treatment of a PES membrane in the presence of NVP and 2mercaptoethanol as a chain transfer agent increases the permeability of the membrane considerably.74 The addition of a chain transfer agent facilitates termination of the monomer at various points to reduce the size of the grafted monomer chain. Thus, the chain transfer agent decreases the degree of grafting. When the modification is conducted with a dip technique rather than
260
High Performance Polymers
an immersion technique, a low energy UV light in the range of 280 to 300 nm is desirable to prevent damage to the membrane, because significantly more UV light reaches the membrane in the dipping technique. The altered dip modification technique using a liquid or solid filter yields modified membranes with lower protein fouling and reduced pore enlargement.75 The dip modification with 2-mercaptoethanol followed by ethanol cleaning produces more permeable, but less retentive membranes. The surface can be modified by a technique, which combines controlled deposition by electrophoresis of charged moieties, with UV grafting.76 Polyelectrolytes, such as methacrylic acid (MA), AA, 2-acryamido glycolic acid (AAG), 2-hydroxyethyl methacrylate (HEMA) and N-vinyl formamide were used for modification. The modified membrane surfaces exhibit more hydrophilic and negative charged features after the treatment. Grafting with MA and AAG decreases the permeability of natural organic matter (NOM) to less than half of the untreated membrane. The modification reduces fouling by foulants such as NOM. In a similar study, best results, with respect to protein retention and protein solution flux, were obtained by grafting NVP, 2-acrylamido-2-methyl-1-propane sulfonic acid, and AA onto a 50 k Dalton PES.77 Membranes modified with the weak acid AA monomer are able to reduce irreversible fouling to zero, in contrast to other strongly hydrophilic monomers, such as HEMA and AAG. These compounds increase irreversible fouling relative to the unmodified membrane.78 7.4.1.2
Hollow Fiber Membranes
Hollow fiber UF membranes have been prepared from PPESK with a dry/ wet phase inversion technique. Ethylene glycol mono methyl ether, diethylene glycol, and methyl ethyl ketone were used as non-solvent additives and NMP was used as a solvent in membrane preparation.79, 80 With the increase of the concentration of PPESK in the casting solution, the viscosity strongly increases and becomes shear-rate dependent. Then the morphology of the hollow fiber membranes changes from a finger-like structure to sponge-like structure. Hollow fiber membranes made from poly(imide)/sulfonated PES, with a phthalide group, exhibit a high selectivity in the vapor permeation of mixtures of methanol and methyl-tert-butyl ether (MTBE) as high as 12,000.81 The structures of the polymers used are shown in Figure 7.11.
Poly(arylene ether sulfone)s
261
O N O
O
O
O N O
CH2
PI
O
O
SO3H
S
CH3
C O
O O SPES
Figure 7.11: Poly(imide) and Sulfonated Poly(ethersulfone)81
The separation of methanol and MTBE is of interest, because MTBE is synthesized from isobutylene and methanol. 7.4.1.3
Carbon Membranes
PPESK can serve as a polymeric precursor for the preparation of carbon membranes.82 The weight loss of the PPESK precursor is about 43.0% at to 800°C pyrolysis temperature. After the heat treatment, the typical chemical structure of the PPESK precursor disappears. At the same time a graphite-like structure with more aromatic rings is formed. The selectivity for H2 /N2 , CO2 /N2 and O2 /N2 gas pairs reach 278.5, 213.8 and 27.5, respectively. 7.4.1.4
Fuel Cell Membranes
The concept of using a polymeric cation exchange membrane as a solid electrolyte in electrochemical cells was first described for a fuel cell by Grubb in 1959.66, 83 The traditional membrane material for fuel cells is Nafion®, a sulfonated PTFE. However, there are attempts to find alternative materials, such as PAES-based membranes.84
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High Performance Polymers
Sulfonated PAESs are good candidates for substitution, due to their good acid and thermal oxidative stabilities, high glass transition temperatures, and excellent mechanical strengths.26, 85 Sulfonated PPESK copolymers with pendant sodium sulfonate groups can be prepared by direct copolymerization of the sulfonated momomers.86 The direct synthesis from a sulfonated monomer is more advantageous than the post-sulfonation method. In comparison to post-sulfonated material, the concentration as well as the positions of the sulfonate groups in the directly synthesized monomers can be more readily controlled. Further, the direct sulfonation method avoids crosslinking and other side reactions. This may result in a better thermal stability and in better mechanical properties.87 Zirconium hydrogen phosphate as an inorganic additive effects an improvement in high-temperature conductivity of PPESK membranes.88, 89 Quaternary nitrogen groups can be introduced into the membrane by using chloromethylated PPESK and immersing the membrane into an aqueous trimethylamine solution.90 These membranes show an increase in the pure water flux and reject dyes and MgCl2 . Quaternized PPESK membranes doped with H3 PO4 show a high proton conductivity of 0.072 S cm−1 at 150°C, and thus suggest the use for high-temperature proton exchange membranes in fuel cells.91 7.4.1.5
Direct Methanol Fuel Cells
Sulfonated PPESK membrane materials have been demonstrated to be useful for various types of fuel cells, such as formic acid fuel cells, and methanol fuel cells.92 The direct methanol fuel cell has certain advantages over the proton exchange membrane fuel cell because it is more suitable for portable applications. Because of the interest in these cells, many papers focus on materials suitable for membranes. The reactions in a direct methanol fuel cell are:93 CH3 OH + H2 O 3 + − 2 O2 + 6H + 6e 3 CH3 OH + 2 O2
A A A
CO2 + 6H+ + 6e− 3H2 O CO2 + 2H2 O
Anode Cathode Brutto reaction
(7.1)
The protons are transported across the proton exchange membrane to the cathode. There they react with oxygen and extract electrons from the cathode to produce water. Besides methanol, also water is consumed at the an-
Poly(arylene ether sulfone)s
263
ode; pure methanol cannot be used. Due to the migration of the hydrogen ions through the membrane from the anode through the cathode and due to the inability of the free electrons to pass through the membrane, the electrons must flow through an external circuit, which produces an electrical current through the external circuit. Methanol permeation is an important parameter for the suitability of the membranes. The other parameter is the proton conductivity. The performance of PPESK-containing membranes can be predicted using structure-property-relationships.94 The structure-property-relationships are useful in the development of new membrane materials of this type. For example, water uptake and ion exchange capacity are impacted by the incorporation of fluorine moiety or polar groups. The incorporation of fluorine groups decreases the water uptake, but increases the proton conductivity at comparable ion exchange capacity, i.e., degree of sulfonation. The introduction of polar groups, such as benzonitrile or triphenylphosphine oxide, on the other hand, decreases the water uptake, conductivity, and methanol permeability. It has been demonstrated that the properties of sulfonated PPESK copolymer membranes are dependent on the copolymer composition, conditions and method of casting, and hydrothermal history. Membrane structures with different morphology can be identified that give rise in the difference of mechanical and electrical properties.95 The important advantage of the development of polymer blend membranes is that the membrane structure or the membrane properties can be optimized in a targeted manner by varying the blend components and the mixing ratio.96 Aminated or nitrated PES, and PAES, respectively, show interactions that improve the stability of the membrane with respect to the swelling behavior. In blends of PPESK and sulfonated poly(ether ether ketone) (PEEK), both methanol permeability and proton conductivity increase nonlinearly with increasing content of PEEK.97 Sulfonated PAES copolymers obtained from sulfonated 4,4 -dichlorodiphenyl sulfone, 4,4 -dichlorodiphenyl sulfone and phenolphthalein have been tested with respect to their use for direct methanol fuel cell application. The proton conductivity increases linearly with the degree of sulfonation, but the methanol permeability increases linearly up to 20 mol-% sulfonated monomer content. Above this level, a sudden increase in permeability is observed.98 This effect is referred to as percolation threshold.
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7.4.2 Medical Applications Alkyl ether/PAES copolymer and alkyl ether/poly(ether ketone) copolymer exhibit excellent blood compatibility. Therefore, these materials can be advantageously used to produce medical materials used to contact blood.99 Sterilizability is of essential importance when the polymer is used in medical applications. Steam sterilization is preferred over chemical sterilization and radiation sterilization. Steam sterilization consists of a treatment of the membrane with superheated steam of >110°C for 30 min. Steam sterilizable membranes include poly(ether imide), PES and poly(vinylidene fluoride). PESs fulfill the mechanical and thermal properties requirements and exhibit an excellent resistance to chemicals. However, a major drawback is the hydrophobicity of the membrane material, which excludes spontaneous wetting with aqueous media. PES can be hydrophilically modified by sulfonation with sulfuric acid. However, this procedure allows just a random distribution of the sulfonic acid groups in the polymer. For regulation of the biocompatibility, it is desirable when the total number of sulfonic acid groups in the polymer, and also their distribution in the polymer chain, can be influenced. By the selective introduction of, domains with high and low degrees of sulfonation, the variational possibilities with respect to the functional polymer groups can be increased and thus, the hydrophilicity properties can be graded even more selectively.29 For this reason, block copolymers containing blocks of sulfonated and unsulfonated polyether sulfones are more suitable than sulfonated PES. 7.4.2.1
Dental plaque barriers
Dental plaque results when cariogenic bacteria aggregate in colonies on the surface of teeth and form a tenacious deposit thereon. The presence of plaque on teeth is believed to be a precursor to the development of gingivitis, dental caries, and periodontal disease. Hydrophilic sulfonic acid and sulfonic acid salt derivatives of certain PES have been synthesized. It was found that these classes of polymers inhibit the deposition of dental plaque onto human teeth. The materials have good film-forming characteristics. Accordingly, they are applied to teeth from various dentifrice formulations, mouth rinses, or other oral hygiene procedures.
Poly(arylene ether sulfone)s
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The sulfonate polymers are anionic in nature and substantially soluble in water, because of the high degree of sulfonation achieved during preparation of these derivatives. It is presumed that films of the anionically charged polymers deposited on teeth effect a mutual repulsion between the negatively charged polymer film and the negatively charged microorganisms in oral fluids responsible for plaque generation. It has been demonstrated by zeta potential measurements that when powdered human dental enamel is dispersed in the aqueous media that contains salts of the polymeric sulfonates, a substantially negative surface charge is built up on the enamel particles.100 Commercially available PES can be sulfonated using sulfur trioxide, triethyl phosphate (TEP) complexes of sulfur trioxide, and chlorosulfonic acid. Due to the high reactivity of sulfur trioxide and its potent dehydration properties, sulfonation reactions with sulfur trioxide sometimes result in the formation of highly insoluble polymer dispersions due to crosslinking caused by inter-polymer chain sulfone formation. In these situations, it is preferable to moderate the sulfonation reactivity by utilization of the sulfur trioxide complexes with TEP. The sulfonation reaction can be effected in solvents, such as methylene chloride, 1,2-dichloroethane, and chloroform. These compounds are generally good solvents for the starting aromatic polymer and poor solvents for the sulfonated polymer, which precipitates directly from the reaction medium and is filtered.100 7.4.2.2
Controlled Release Systems
Blends of PES and poly(N-vinyl-2-pyrrolidone) (PVP) have been tested for the use in the field of controlled release systems.101 The blends are immiscible due to the hydrophobic and hydrophilic nature of the polymers. However, the blends are compatible over the entire range of compositions. They exhibit a highly organized arrangement of both the phases. The PVP is dispersed uniformly in the continuum of the PES even when it is the major component. The performance with respect to controlled drug release has been tested with acetaminophen. After an initial burst, which takes place in the first 30 min, the drug release drops and becomes steady for the rest of the time. The initial burst effect is attributed to the greater concentration drop across the membrane at the start of the process.
266 7.4.2.3
High Performance Polymers Nucleotide Mimetics
Poly(ether thioether)s, poly(ether sulfoxide)s or PES nucleic acids have been suggested as mimics, for natural materials, i.e., with the action of an antisense oligonucleotide. The backbone may bear a plurality of ligands including naturally occurring nucleobases. Antisense oligonucleotides are short-chain molecules that may bind its target nucleic acid either by Watson-Crick base pairing or by Hoogsteen and anti-Hoogsteen base pairing. When a nucleotide using a messenger RNA (single-stranded nucleic acid) regenerates, the antisense oligonucleotide may interfere and block certain ranges of the base sequence to be reproduced. Gene expression involves a few distinct and well-regulated steps.102 The first major step of gene expression involves transcription of a messenger RNA (mRNA) which is a RNA sequence complementary to the antisense deoxyribonucleic acid (DNA) strands, or, in other words, identical in sequence to the DNA sense strand, composing the gene. The second major step of gene expression involves the translation of a protein, e.g., enzymes, structural proteins, secreted proteins, gene expression factors, etc., in which the mRNA interacts with ribosomes and amino acid activated transfer RNA to direct the synthesis of the protein coded for by the mRNA sequence. Initiation of transcription requires specific recognition of a promoter DNA sequence located upstream to the coding sequence of a gene by a RNA polymerase. This recognition is preceded by sequence-specific binding of one or more protein transcription factors to the promoter sequence. Additional proteins, which bind at or close to the promoter sequence may upregulate transcription and are known as enhancers. Other proteins, which bind to or close to the promoter, but whose binding prohibits action of RNA polymerase, are known as repressors. There is also evidence that in some cases gene expression is down regulated by endogenous antisense RNA repressors that bind a complementary mRNA transcript and thereby prevent its translation into a functional protein. Thus, gene expression is typically upregulated by transcription factors and enhancers and down regulated by repressors. Antisense oligonucleotides have the potential to regulate gene reproduction and to disrupt the essential functions of the nucleic acids. Therefore, antisense oligonucleotides have possible uses in modulating a wide range of diseases.102, 103
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For therapeutic or prophylactic treatment, the poly(ether-thioether), poly(ether-sulfoxide) or poly(ether sulfone) nucleic acids can be formulated in pharmaceutical compositions.
7.4.3 Optical Waveguide Applications Ethynyl terminated fluorinated PAES has a potential use for optical waveguide applications. The synthesis of ethynyl terminated fluorinated PAES is shown in Figure 7.12. The polymer is cured at 270°C for 2 h. The key requirements of optical waveguide materials include low optical loss in the infrared region, high thermal stability, refractive index controllability, and low birefringence. The material has a small light absorption at a telecommunication wavelength of 1300 and 1550 nm due to its high fluorine content. The propagation loss at 1550 nm is less than 0.37 dB cm−1 . The birefringence of the copolymers is 0.0021–0.0025. This value is much lower than those of fluorinated poly(imide)s used for optical waveguide applications.104, 105 The optical properties, such as refractive index, birefringence, thermal stability, and optical loss of the fluorinated polymers are related to the molecule structure of the polymers.106 For example, the birefringence is based on the microscopic anisotropic ratio of the polarizability of the molecular repeating unit. It shows a good linear relation dependent on the polymer groups. In a similar way, a crosslinkable polymer can be synthesized from decafluorodiphenyl ketone, 4,4 -(hexafluoroisopropylidene)diphenol and 1,1-bis-(4-hydroxyphenyl)-ethyl-1-phenyl-2,3,5,6-tetrafluoro4-vinylphenyl ether,107 c.f. Figure 7.13. Other monomers are hexafluoroisopropylidene, 4,4 -(hexafluoroisopropylidene)-diphenyl, or 4,4 -isopropylidene diphenyl.108 The refractive index can be tailored over a range of 1.50 to 1.57, allowing the polymers to be used as both core and cladding materials in optical waveguide applications.109 Pentafluorophenyl sulfone is highly reactive even at room temperaThe reaction is catalyzed by potassium fluoride. It activates the phenol group and acts as a base to absorb the hydrogen fluoride, which is a byproduct of the polycondensation. By adjusting the feed ratio of monomers, the refractive index and crosslinking density of the polymers can be readily controlled.111 ture.110
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CF3 C
HO
OH
CF3
+ F
F
F
F
O F
OH
F
S
CH
O F C
F
F
C
F F
CH
F F F
O
O S
F O
O
n
F
F F
F3C C CF3
F F F F
O
O S
F O
O
F
F F
C CH
Figure 7.12: Synthesis of Ethynyl Terminated Fluorinated PAES104, 105
Poly(arylene ether sulfone)s
269
CH3 OH
C
HO
O F
F
F
F C H
CH2
Figure 7.13: 1,1-Bis-(4-hydroxyphenyl)-ethyl-1-phenyl-2,3,5,6-tetrafluoro-4vinylphenyl ether107
7.5 PLUMBING MATERIALS For many years, the standard material used for manufacturing pipes, fixtures, couplings, and other plumbing articles has been metal, primarily copper, and brass. Alternative materials for manufacturing such plumbing articles have been introduced. Plastics offer advantages in that they are generally lighter in weight, and more easily cut and shaped. In addition, during the construction of a home or commercial building, the plumber can connect the plastic pipes using a coupler. It has been found that blends of a poly(biphenyl ether sulfone) and a second poly(arylene ether sulfone) based on bisphenol A are most suitable for plumbing applications. The blend has an outstanding resistance to hot water.112
7.6 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 7.6. Tradenames appearing in the references are shown in Table 7.7.
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Table 7.6: Examples for Commercially Available Poly(arylene ether sulfone)s Tradename
Producer
RADEl® Edgetek™ Westlake PES UTTAP SF 50030 GF Thermocomp® Sumiploy® Sumikaexcel® RTP Compounds ESD CoorsTek Neat PES Gafone™ Udel® Mindel® Epispire
Solvay Advanced Polymers PolyOne Corp. Westlake Plastics Co. Ovation Polymers Inc. LNP Engineering Plastics Inc Sumitomo Chemical America, Inc. Sumitomo Chemical America, Inc. RTP Co. CoorsTek Gharda Chemicals Ltd. Solvay Advanced Polymers Solvay Advanced Polymers Solvay Advanced Polymers
Poly(arylene ether sulfone)s Table 7.7: Tradenames in References Tradename Description
Supplier
Aciplex® Perfluorosulfonic acid membrane66 Diaion® (Series)
Asahi Chemical Industry
Mitsubishi Chemical Industries Ltd. Sulfonic acid type ion exchange resin modified with 2-mercaptoethylamine14 DYLARK® Nova Chemicals S.A. (Arco Chemical Co.) Copolymers of styrene with maleic anhydride63 Flemion® Asahi Glass Company Fluoropolymer ion-exchange membrane66 Gore-Select® W. L. Gore Microporous expanded PTFE membrane"(ePTFE), ion conductive membrane66 Lexan® General Electric Poly(carbonate)39 Makrolon® Bayer AG Poly(carbonate)39 Merlon® Mobay Poly(carbonate)39 Nafion® DuPont Sulfonated PTFE, for membrane applications66, 93, 97 Neosepta® ASTOM Corp. (Tokuyama Soda Co.) Perfluorinated ion exchange membranes66 Paraloid® Rohm & Haas Acrylate rubber, impact modifier39 Polymist® (Series) Solvay Solexis (Ausimont USA, Inc.) Poly(tetrafluoroethylene) lubricant powders41 Radel® A Solvay Poly(ether sulfone)4, 41 Radel® R Solvay Poly(biphenyl sulfone)4, 41 Udel® Polysulfone Solvay Poly(bisphenol A sulfone)4, 39, 100 Ultem® (Series) General Electric Poly(imide), thermoplastic63
271
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7.7 SAFETY Low-molecular-weight PES and poly(sulfone) resins in general may cause irritations of eye, skin, and digestive tract.
7.8 ENVIRONMENTAL IMPACT AND RECYCLING In a diluted solution of dimethylacetamide, the ether linkages of PES are opened and macrocyles are formed. The reaction products show a linear oligomer content of less than 4%. Macrocycles with rings of eight, twelve, sixteen, and twenty aromatic rings have been isolated. On the other hand, the macrocyclic oligomers can undergo ring-opening polymerization in the presence of phenoxide and esp. thiophenoxide initiators to regenerate a high-molecular-weight polymer. Therefore, it is believed that the recovery and recycling of PES is feasible.113, 114
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63. L. A. McKenna. Stain resistant cookware from blends comprising an interpolymer of an unsaturated dicarboxylic acid compound. US Patent 4 772 653, assigned to Amoco Corporation (Chicago, IL), September 20, 1988. 64. U. Eichenauer, E. Neufeld, A. Ludwig, T. Sauer, and A. Ulzhöfer. Production of polyarylene-ether-sulfone useful for coating, especially to produce sliding surface. DE Patent 19 816 955, assigned to Basf AG (DE), October 21, 1999. 65. M. Mulder. Basic Principles of Membrane Technology. Kluwer Academic, Dordrecht, 2nd edition, 1996. 66. M. Rikukawa and K. Sanui. “Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers.” Prog. Polym. Sci., 25(10):1463– 1502, December 2000. 67. X. Jian, Y. Dai, G. He, and G. Chen. “Preparation of UF and NF poly (phthalazine ether sulfone ketone) membranes for high temperature application.” J. Membr. Sci., 161(1-2):185–191, August 1999. 68. Y. Su, X. Jian, S. Zhang, and C. Yan. “Preparation of novel PPES-B UF membrane with good thermal stability: The effect of additives on membrane performance and cross-section morphology.” J. Membr. Sci., 271(1-2):205– 214, March 2006. 69. Y. Dai, X. Jian, S. Zhang, and M. D. Guiver. “Thermostable ultrafiltration and nanofiltration membranes from sulfonated poly(phthalazinone ether sulfone ketone).” J. Membr. Sci., 188(2):195–203, July 2001. 70. Y. Gao, G. P. Robertson, M. D. Guiver, X. Jian, S. D. Mikhailenko, K. Wang, and S. Kaliaguine. “Sulfonation of poly(phthalazinones) with fuming sulfuric acid mixtures for proton exchange membrane materials.” J. Membr. Sci., 227(1-2):39–50, December 2003. 71. M. Wang, L.-G. Wu, X.-C. Zheng, J.-X. Mo, and C.-J. Gao. “Surface modification of phenolphthalein poly(ether sulfone) ultrafiltration membranes by blending with acrylonitrile-based copolymer containing ionic groups for imparting surface electrical properties.” J. Colloid Interface Sci., 300(1): 286–292, August 2006. 72. M.-L. Luo, J.-Q. Zhao, W. Tang, and C.-S. Pu. “Hydrophilic modification of poly(ether sulfone) ultrafiltration membrane surface by self-assembly of TiO2 nanoparticles.” Appl. Surf. Sci., 249(1-4):76–84, August 2005. 73. B. Kaeselev, P. Kingshott, and G. Jonsson. “Influence of the surface structure on the filtration performance of UV-modified PES membranes.” Desalination, 146(1-3):265–271, September 2002. 74. J. Pieracci, J. V. Crivello, and G. Belfort. “Increasing membrane permeability of UV-modified poly(ether sulfone) ultrafiltration membranes.” J. Membr. Sci., 202(1-2):1–16, June 2002. 75. G. Belfort, J. V. Crivello, and J. Pieracci. UV-assisted grafting of PES and PSF membranes. US Patent 6 852 769, assigned to Rensselaer Polytechnic
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Institute (Troy, NY), February 8, 2005. 76. X. Wei, R. Wang, Z. Li, and A. G. Fane. “Development of a novel electrophoresis-UV grafting technique to modify PES UF membranes used for NOM removal.” J. Membr. Sci., 273(1-2):47–57, March 2006. 77. M. Taniguchi and G. Belfort. “Low protein fouling synthetic membranes by UV-assisted surface grafting modification: Varying monomer type.” J. Membr. Sci., 231(1-2):147–157, March 2004. 78. M. Taniguchi, J. E. Kilduff, and G. Belfort. “Low fouling synthetic membranes by UV-assisted graft polymerization: Monomer selection to mitigate fouling by natural organic matter.” J. Membr. Sci., 222(1-2):59–70, September 2003. 79. Y. Yang, D. Yang, S. Zhang, J. Wang, and X. Jian. “Preparation and characterization of poly(phthalazinone ether sulfone ketone) hollow fiber ultrafiltration membranes with excellent thermal stability.” J. Membr. Sci., 280 (1-2):957–968, September 2006. 80. Y. Yang, X. Jian, D. Yang, S. Zhang, and L. Zou. “Poly(phthalazinone ether sulfone ketone) (PPESK) hollow fiber asymmetric nanofiltration membranes: Preparation, morphologies and properties.” J. Membr. Sci., 270 (1-2):1–12, February 2006. 81. B. Shi, Y. Wu, and J. Liu. “Vapor permeation separation of MeOH/MTBE through polyimide/sulfonated poly(ether-sulfone) hollow-fiber membranes.” Desalination, 161(1):59–66, February 2004. 82. B. Zhang, T. Wang, S. Zhang, J. Qiu, and X. Jian. “Preparation and characterization of carbon membranes made from poly(phthalazinone ether sulfone ketone).” Carbon, 44(13):2764–2769, November 2006. 83. W. T. Grubb. “Ionic migration in ion-exchange membranes.” J. Phys. Chem., 63:55–58, January 1959. 84. H. A. Every, M. A. Hickner, J. E. McGrath, and T. A. Zawodzinski, Jr. Nafion versus sulfonated poly(arylene ether sulfone)s. A comparison of the methanol diffusion behavior. In Fuel Cells from Materials to Systems, The Electrochemical Society, Pennington, New Jersey 08534-2839, USA, 2003. 203rd Meeting of the The Electrochemical Society, Paris. 85. S. Wang and J. E. McGrath. Synthesis of poly(arylene ether)s. In M. Rogers and T. E. Long, editors, Synthetic Methods in Step-growth Polymers, chapter 6, pages 327–374. Wiley, New York, 2003. 86. Y. Gao, G. P. Robertson, M. D. Guiver, X. Jian, S. D. Mikhailenko, K. Wang, and S. Kaliaguine. “Direct copolymerization of sulfonated poly(phthalazinone arylene ether)s for proton-exchange-membrane materials.” J. Polym. Sci., Part A: Polym. Chem., 41:2731–2742, 2003. 87. X. Li, C. Zhao, H. Lu, Z. Wang, and H. Na. “Direct synthesis of sulfonated poly(ether ether ketone ketone)s (SPEEKKs) proton exchange membranes for fuel cell application.” Polymer, 46(15):5820–5827, July 2005.
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88. M. L. Hill, Y. S. Kim, B. R. Einsla, and J. E. McGrath. “Zirconium hydrogen phosphate/disulfonated poly(arylene ether sulfone) copolymer composite membranes for proton exchange membrane fuel cells.” J. Membr. Sci., 283(1-2):102–108, October 2006. 89. G. M. Anilkumar, S. Nakazawa, T. Okubo, and T. Yamaguchi. “Proton conducting phosphated zirconia-sulfonated polyether sulfone nanohybrid electrolyte for low humidity, wide-temperature PEMFC operation.” Electrochem. Commun., 8(1):133–136, January 2006. 90. Y. Su, X. Jian, S. Zhang, and G. Wang. “Preparation and characterization of quaternized poly(phthalazinone ether sulfone ketone) NF membranes.” J. Membr. Sci., 241(2):225–233, October 2004. 91. M. Li, H. Zhang, and Z.-G. Shao. “Quaternized poly(phthalazinone ether sulfone ketone) membrane doped with H3 PO4 for high-temperature PEMFC operation.” Electrochem. Solid-State Lett., 9:A60–A63, 2006. 92. H.-J. Kim, N. N. Krishnan, S.-Y. Lee, S. Y. Hwang, D. Kim, K. J. Jeong, J. K. Lee, E. Cho, J. Lee, and J. Han. “Sulfonated poly(ether sulfone) for universal polymer electrolyte fuel cell operations.” J. Power Sources, 160 (1):353–358, September 2006. 93. A. J. Curello, F. Fairbanks, and C. Loonis. Fuel gauge for fuel cartridges. US Patent 7 117 732, assigned to Societe BIC (Clichy Cedex, FR), October 10, 2006. 94. Y. S. Kim, B. Einsla, M. Sankir, W. Harrison, and B. S. Pivovar. “Structureproperty-performance relationships of sulfonated poly(arylene ether sulfone)s as a polymer electrolyte for fuel cell applications.” Polymer, 47(11): 4026–4035, May 2006. 95. Y. S. Kim, L. Dong, M. A. Hickner, B. S. Pivovar, and J. E. McGrath. “Processing induced morphological development in hydrated sulfonated poly(arylene ether sulfone) copolymer membranes.” Polymer, 44(19):5729– 5736, September 2003. 96. W. Cui. Polymer blend membranes for use in fuel cells. US Patent 6 869 980, assigned to Celanese Ventures GmbH (DE), March 22, 2005. 97. H.-L. Wu, C.-C. M. Ma, F.-Y. Liu, C.-Y. Chen, S.-J. Lee, and C.-L. Chiang. “Preparation and characterization of poly(ether sulfone)/sulfonated poly(ether ether ketone) blend membranes.” Eur. Polym. J., 42(7):1688–1695, July 2006. 98. D. S. Kim, K. H. Shin, H. B. Park, Y. S. Chung, S. Y. Nam, and Y. M. Lee. “Synthesis and characterization of sulfonated poly(arylene ether sulfone) copolymers containing carboxyl groups for direct methanol fuel cells.” J. Membr. Sci., 278(1-2):428–436, July 2006. 99. H. Kuwahara, T. Kawaguchi, S. Ohmori, and S. Matsumura. Application of sulfone, ketone and ester containing polyalkyl ether units to medical materials. US Patent 5 969 082, assigned to Teijin Limited (Osaka, JP), October 19, 1999.
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100. C. J. Buck. Sulfonated poly(arylene ether sulfone) polymers as dental plaque barriers. US Patent 4 360 513, assigned to Johnson & Johnson Products, Inc. (New Brunswick, NJ), November 23, 1982. 101. R. Bhattacharya, T. N. Phaniraj, and D. Shailaja. “Polysulfone and polyvinyl pyrrolidone blend membranes with reverse phase morphology as controlled release systems: Experimental and theoretical studies.” J. Membr. Sci., 227(1-2):23–37, December 2003. 102. D. Segev. Nucleic acid derivatives. US Patent 7 034 131, assigned to BioRad Laboratories Inc. (Hercules, CA), April 25, 2006. 103. D. Segev. Poly(ether-thioether), poly(ether-sulfoxide) and poly(ether-sulfone) nucleic acids. WO Patent 0 116 365, assigned to Bio Rad Laboratories (US); Segev David (IL), March 08, 2001. 104. J.-P. Kim, J.-W. Kang, J.-J. Kim, and J.-S. Lee. “Fluorinated poly(arylene ether sulfone)s for polymeric optical waveguide devices.” Polymer, 44(15): 4189–4195, July 2003. 105. J. S. Lee, J. J. Kim, J. P. Kim, J. W. Kang, and W. Y. Lee. Poly (arylene ether sulfide) and poly (arylene ether sulfone) for optical device and method for preparing the same. US Patent 6 512 076, assigned to Kwangju Institute of Science and Technology (Kwangju, KR), January 28, 2003. 106. J.-W. Kang, J.-P. Kim, J.-S. Lee, and J.-J. Kim. “Structure-property relationship of fluorinated co-poly(arylene ether sulfide)s and co-poly(arylene ether sulfone)s for low-loss and low-birefringence waveguide devices.” J. Lightwave Tech., 23:364–373, 2005. 107. Y. Qi, C. L. Callender, J. Jiang, T. Norsten, M. Day, and J. Ding. Cross-linkable highly fluorinated poly(arylene ethers) for optical waveguide applications. CA Patent 2 507 981, assigned to National Research Council (CA), November 19, 2005. 108. J. Ding, F. Liu, M. Zhou, M. Li, M. Day, and P. Vuillaume. Techniques for the preparation of highly fluorinated polyethers. WO Patent 03 099 907, assigned to National Research Council (CA), December 04, 2003. 109. Y. Qi, J. Jiang, C. L. Callender, J. Ding, and M. Day. “Cross-linkable highly fluorinated polymers with tunable refractive index.” Mater. Res. Soc. Symp. Proc., 888:263–268, 2006. 110. J. Ding, Y. Qi, M. Day, J. Jiang, and C. L. Callender. “A low temperature polycondensation for the preparation of highly fluorinated poly(arylene ether sulfone)s containing crosslinkable pentafluorostyrene moieties.” Macromol. Chem. Phys., 206:2396–2407, 2005. 111. Y. Qi, J. Ding, M. Day, J. Jiang, and C. L. Callender. “Cross-linkable highly fluorinated poly(arylene ether ketones/sulfones) for optical waveguiding applications.” Chem. Mater., 17:676–682, 2005. 112. M. J. El-Hibri and B. L. Dickinson. Plumbing articles from poly(aryl ether sulfones). US Patent 6 329 493, assigned to BP Corporation North America Inc. (Chicago, IL), December 11, 2001.
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113. H. M. Colquhoun, D. F. Lewis, P. Hodge, A. Ben-Haida, D. J. Williams, and I. Baxter. “Ring-chain interconversion in high-performance polymer systems. 1. [poly(oxy-4,4 -biphenyleneoxy-1,4-phenylenesulfonyl-1,4-phenylene)] (Radel-R).” Macromolecules, 35:6875–6882, 2002. 114. H. M. Colquhoun, D. F. Lewis, A. Ben-Haida, and P. Hodge. “Ring-chain interconversion in high-performance polymer systems. 2. ring-opening polymerization-copolyetherification in the synthesis of aromatic poly(ether sulfones).” Macromolecules, 36:3775–3778, 2003.
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8 Poly(arylene ether nitrile)s Processes for the manufacture of poly(arylene ether nitrile) (PEN) had already been described by the 1970’s,1 however the commercial production is reported to be in 1986 by Idemitsu.2 PENs belong to high performance semi-crystalline thermoplastic polymers that exhibit a high melting temperature, excellent mechanical properties, and high chemical resistance. Essentially, a PEN has the structure as shown in Figure 8.1.3 The polymer is also addressed as poly(cyano aryl ether). Of course, the nitrile group cannot constitute the backbone but is rather a side chain group. The synthesis routes and the properties of PENs have been reviewed in the literature.2
8.1 MONOMERS Monomers for PEN are summarized in Table 8.1 and in Figures 8.2 and 8.3. Bifunctional monomers are used to build up the polymeric chain, whereas monofunctional monomers are suitable to regulate the molecular weight of
C N O
O
Figure 8.1: Poly(arylene ether nitrile)
283
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High Performance Polymers Table 8.1: Monomers for PEN
Nitrile Monomer
Remarks
2,6-Dichlorobenzonitrile Pentachlorobenzonitrile Pentafluorobenzonitrile 4-Phenoxy-2,3,5,6-tetrafluorobenzonitrile 2-Fluorobenzonitrile
Commonly used4 Branched polymers5 Film types6 Film types7 Chain stopper8
Hydroxy Monomer
Remarks
Resorcinol Hydroquinone Biphenol 2,7-Dihydroxynaphthalene Bisphenol A 4,4 -(Hexafluoroisopropylidene)diphenol 9,9-bis-(4-hydroxyphenyl)-fluorene Phenolphthalein 4,4 -Dihydroxydiphenyl sulfone Hydroxyaryl substituted spirodilactams, e.g., 1,6-Di(3-hydroxyphenyl)-1,6-diazaspiro[4.4]nonane-2,7-dione 2,2-Bis-(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane p-Phenylphenol
Common Common Common Increases Tg 4 Common1 Membranes9 7 7
Increases Tg 10 Film types7 Chain stopper8
the polymeric chain.
8.1.1 Halogenated Benzonitriles 2,6-Difluorobenzonitrile is more reactive than 2,6-dichlorobenzonitrile. In particular, fluorinated monomers enhance film casting, because of an increase in solubility. 8.1.1.1
2,6-Dichlorobenzonitrile
2,6-Dichlorobenzonitrile is produced by ammonoxidation using a vanadium-molybdenum oxide catalyst.11 The reaction temperature is around 360°C, with contact time of the reaction gas with the catalyst of about 7.5 s. Besides in polymers, other uses are as starting material of herbicides and insecticides.
Poly(arylene ether nitrile)s
Cl
Cl CN
Cl
Cl
Cl 2,6-Dichlorobenzonitrile
CN
Cl Cl Pentachlorobenzonitrile F
F
O
CN
F F 4-Phenoxy-2,3,5,6-tetrafluorobenzonitrile
Figure 8.2: Nitrile Monomers for Poly(arylene ether nitrile)s
OH
HO Biphenol HO
OH
HO OH 2,7-Dihydroxynaphthalene 9,9-Bis(4-hydroxyphenyl) fluorene CF3 HO
OH
C
CF3 2,2-Bis(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane O OH
N HO
N
O 1,6-Di(3-hydroxyphenyl)-1,6-diazaspiro[4.4]nonane-2,7-dione
Figure 8.3: Hydroxy Monomers for Poly(arylene ether nitrile)s
285
286 8.1.1.2
High Performance Polymers 2,6-Difluorobenzonitrile
2,6-Difluorobenzonitrile can be prepared by reacting 2,6-dichlorobenzonitrile with potassium fluoride in the presence of a solvent, such as dimethyl sulfoxide and sulfolane at 180°C for several hours. In a solventless process for making 2,6-difluorobenzonitrile, 2,6-dichlorobenzonitrile is reacted with a substantially anhydrous alkali metal fluoride at about 225°C.12 A mixture of 2-chloro-6-fluorobenzonitrile, 2,6difluorobenzonitrile, and unreacted 2,6-dichlorobenzonitrile is produced in this step. The 2,6-difluorobenzonitrile is separated, and the other products are fed back in the reactor. Crown ethers are used as a catalyst. Another route to access 2,6-difluorobenzonitrile consists of fluorinating 2,3,6-trichlorobenzonitrile in a first step to get 3-chloro-2,6-difluorobenzonitrile. In a second step, 3-chloro-2,6-difluorobenzonitrile is hydrogenated to get the final product.13 The starting material, 2,3,6-trichlorobenzonitrile can be derived from p-toluenesulfonic acid quite easily. 8.1.1.3
Pentachlorobenzonitrile and Pentafluorobenzonitrile
Pentachlorobenzonitrile is obtained by the gas phase chlorination of benzonitrile using transitions metals on active carbon.14, 15 Pentafluorobenzonitrile can be synthesized from pentachlorobenzonitrile using a fluorinating agent for a halogen exchange reaction in benzonitrile.16 Pentafluorobenzonitrile has a boiling point of 162°C, which is much less than that of benzonitrile (191°C) and pentachlorobenzonitrile. Therefore, the product can be easily distilled off. Dry potassium fluoride is a suitable fluorinating agent.17 Severe reaction conditions may be needed.
8.1.2 Aromatic Hydroxy Compounds For most of the aromatic hydroxy compounds mentioned in Table 8.1, the synthesis is well known. We discuss in detail only a few less common compounds. Hydroquinone (HQ) is prepared by the oxidation of phenol. Resorcinol can be prepared by the oxidation of m-diisopropylbenzene. Biphenol is prepared by the oxidative dimerization of 2,6-di-tert-butyl phenol and subsequent debutylation.18 4,4 -Dihydroxydiphenyl sulfone can be prepared by reacting phenol with sulfuric acid. Phenolphthalein can be obtained
Poly(arylene ether nitrile)s
287
from phthalic anhydride and phenol in the presence of acid cation exchange resins and aryl phosphites. 8.1.2.1
9,9-Bis-(4-hydroxyphenyl)-fluorene
9,9-Bis-(4-hydroxyphenyl)-fluorene is commercially synthesized by the reaction of phenol with 9-fluorenone, in the same way as the synthesis of bisphenol A proceeds.19 Hydrogen chloride, 3-mercaptopropionic acid or methanesulfonic acid are used as catalysts. The condensation reaction of fluorenone and phenol in the presence of gaseous hydrogen chloride proceeds with sufficient speed already by 30°C.20 A high purity monomer can be obtained by a two-step purification process.21 In the first purification step, the crude 9,9-bis-(4-hydroxyphenyl)-fluorene is refluxed in acetonitrile and recrystallized. In the second step, the product is purified by crystallization from a toluene/isopropanol mixture. 8.1.2.2
Spirodilactams
1,6-Di(4-hydroxyphenyl)-1,6-diazaspiro[4.4]nonane-2,7-dione is prepared from 1,6-dioxaspiro[4.4]nonane-2,7-dione and p-aminophenol, and more basically by the reaction of p-aminophenol and 4-oxoheptanedioic acid.22 The reaction scheme is shown in Figure 8.4. When 1,6-diaza[4.4]spirodilactams with oxyaryl groups are used as dihydroxy compounds, polymers with particularly high glass transition temperatures are formed. For example, a polymer, which is made from 1,6-di(4-hydroxyphenyl)-1,6-diazaspiro[4.4]nonane-2,7-dione and 2,6-dichlorobenzonitrile exhibits a glass transition temperature of 254°C.10 In general, polymers containing the spirodilactam structure in the backbone have a good thermal stability and comparatively high glass transition temperatures. Several poly(ether)s apart from PEN have been prepared and characterized.23 The polymers are soluble in many organic solvents and form clear and flexible films.
8.2 POLYMERIZATION AND FABRICATION Similarly to poly(ether ketone)s (PEK)s, PENs have been prepared according to a nucleophilic route and an electrophilic route.
288
High Performance Polymers
COOH OH
N 2N C NH2
HO
O HOOC -
H 2O
O HO
N N
OH
O
Figure 8.4: Synthesis nonane-2,7-dione22
of
1,6-Di(4-hydroxyphenyl)-1,6-diazaspiro[4.4]-
8.2.1 Electrophilic Route PEKs that are subsequently end capped with nitrile functions have been prepared by Friedel-Crafts polymerization.24 For example, m-hexaphenyl ether are reacted with p-cyanobenzoyl chloride with AlCl3 as catalyst in dry 1,2-dichloroethane to form a nitrile end capped oligomer.25 In another study, 5-cyanoisophthaloyl chloride was used as a nitrile-containing comonomer for the Friedel-Crafts polymerization.26 In the Friedel-Crafts polymerization, the nitrile compound need not necessarily bear a chlorine function, if another aromatic chlorine compound is present. Actually, a Friedel-Crafts polymerization using 2,6-diphenoxybenzonitrile as the nitrile compound with the comonomers shown in Figure 8.5 has been used.27 The resins can be crosslinked by heating with Friedel-Crafts catalysts. It is assumed that the nitrile groups are forming triazines by heating. Even without Friedel-Crafts catalysts crosslinking is possible; however, the reaction proceeds slowly. The materials are intended for use as laminating resins.25
Poly(arylene ether nitrile)s
O CN O
2,6-Diphenoxybenzonitrile O Cl
O C
C
Cl
O O Benzofuro[2,3-b]benzofuran-2,9-dicarbonyl dichloride O
C
C
O
O O 1,4-Bis(p-phenoxybenzoyl)benzene
O
C
C
O
O O 1,3-Bis(p-phenoxybenzoyl)benzene
O
O O O 1,1′-Bis(p-phenoxybezoyl)[2.2]Metacyclophane
Figure 8.5: Monomers for Friedel-Crafts Polymerization27
289
290
High Performance Polymers
8.2.2 Nucleophilic Route The nucleophilic route seems to be favored by industrial processes. Polymers are prepared from dichlorobenzonitrile, aromatic diols such as resorcinol, HQ, and biphenol. The reaction proceeds in polar solvents using a base as a catalyst.3 For example, 2,6-dichlorobenzonitrile, and a mixture of 2,7-dihydroxynaphthalene, and hydroquinone, with a slight stoichiometric excess of the hydroxy compound is condensed at 160–190°C in an inert atmosphere. Potassium carbonate is used as a catalyst and a mixture of sulfolane and toluene is used as a solvent.4 With 1,3-dimethylimidazolidinone as a solvent, higher condensation temperatures can be reached.8 Higher molecular weights are also obtained, therefore 1,3-dimethylimidazolidinone is favored as a solvent. Monovalent nitriles, such as 2-fluorobenzonitrile, act as a molecular weight regulation agent.8
8.2.2.1
Cyclic Oligomers
In molding processes, instead of melting the final polymers, they can be formed during molding in a reactive process. However, during the reactive process, when it is a condensation reaction, low-molecular-weight byproducts are formed. However, when cyclic precursor oligomers are used, which opens the ring upon heating with a catalyst in the mold, and the formation of byproducts is suppressed. So, there are no co-products that must be removed. Cyclic poly(aryl ether) oligomers, exhibit low melt viscosities. A general method to prepare cyclic oligomers has been described.28 Several types of cyclic poly(aryl ether) oligomers have been described, including the preparation of cyclic PEN oligomers. Cyclization is favored by conducting the polymerization reaction in an inert diluted medium. To the refluxing solvent mixture of N-methyl-2-pyrrolidone (NMP) and toluene, with potassium carbonate as catalyst, separate solutions of the monomers resorcinol and 2,6-difluorobenzonitrile in NMP are added continuously by means of a syringe pump.28 Thus, the ingredients are basically the same as in the preparation of high-molecular-weight variants. However, the method of preparation is different.
Poly(arylene ether nitrile)s 8.2.2.2
291
Branched Polymers
Branched, high-molecular-weight, thermoplastic poly(arylene ether)s that contain nitrile groups, are prepared by the use of pentachlorobenzonitrile, tetrachlorophthalodinitrile, or tetrachloroisophthalodinitrile.5 The polymers are obtained by condensation with an aromatic dihydroxy compound, such as bisphenol A. Bis-(4-chlorophenyl)-sulfone as a bifunctional chloro compound acts as a chain extender, thus imparting a sulfone group into the polymer. These compounds result in polymers with very good surface hardness and solubility in organic solvents, such as methylene chloride, acetone, and chlorobenzene. 8.2.2.3
Aryl Carbonate Cyano Arylene Ether Copolymers
Arylene carbonate cyanoarylene ether copolymers can be prepared by the reaction of a solution of a bisphenolic capped cyanoarylene ether oligomer with phosgene in the presence of a base. The reaction with phosgene is carried out in an inert atmosphere. The polymerization reaction is carried out at a subambient temperature so that the reaction proceeds at a controllable rate.29, 30 The materials are useful as gas separation membranes.
8.3 PROPERTIES The family of PEN polymers exhibits a good radiation resistance and low flammability and toxic gas emission. The cyano group, because of its polarity, imparts good adhesion properties to glass fibers, therefore the polymers are suitable as a matrix for composite materials.
8.3.1 Mechanical Properties The mechanical properties of some PENs are shown in Table 8.2. PENs exhibit excellent mechanical and thermal characteristics. However, their crystallization rates are slow. This property prolongs molding cycles in the course of fabrication.8 In contrast, some halogenated types do not suffer from this drawback.
8.3.2 Thermal Properties The thermal properties of some PENs are shown in Table 8.3.
292
High Performance Polymers
Table 8.2: Mechanical Properties of Poly(arylene ether nitrile) Sample
Yield strength [kp cm−2 ]
Breaking strength [kp cm−2 ]
Tensile modulus [kp cm−2 ]
Elongation at break [%]
(DCBN):(R)=1:1a8 1,400 1,320 32,000 60 1,350 1,300 31,000 50 (DCBN):(R)=1:1b8 1,450 1,320 32,000 50 (DFBN):(R)=1:1a8 8 950 900 30,000 20 (DCBN):(R)=1:1 +(PPh)a (DCBN):(S)=1:110 – 1,030 28,500 7 (DCBN) 2,6-dichlorobenzonitrile (DFBN) 2,6-difluorobenzonitrile (PPh) p-phenylphenol as chain stopper (R) resorcin (S) 1,6-di(4-hydroxyphenyl)-1,6-diazaspiro[4.4]nonane-2,7-dione a Prepared in NMP solvent b Prepared in 1,3-dimethylimidazolidinone solvent
Table 8.3: Thermal Properties of Poly(arylene ether nitrile)8 Sample
Reduced viscosity [dl g−1 ]
Glass transition temperature [°C]
Melting point [°C]
Decomp. Temperature [°C]
(DCBN):(R)=1:1a 0.91 148 340 0.86 146 345 (DCBN):(R)=1:1b (DFBN):(R)=1:1a 1.01 148 345 (DCBN):(R)=1:1 0.45 144 343 +(PPh)a (DCBN):(S)=1:110 – 267 – (DCBN) 2,6-dichlorobenzonitrile (DFBN) 2,6-difluorobenzonitrile (PPh) p-phenylphenol as chain stopper (R) resorcin (S) 1,6-di(4-hydroxyphenyl)-1,6-diazaspiro[4.4]nonane-2,7-dione a Prepared in NMP solvent b Prepared in 1,3-dimethylimidazolidinone solvent
484 481 486 475 –
Poly(arylene ether nitrile)s
293
8.3.3 Solubility PEN is soluble e.g., in p-chlorophenol. The solubility in certain solvents is important for casting applications. Conventional PEN exhibits a lack of solubility. Types with improved solubility by keeping the other desired properties, such as heat resistance, hydrolysis resistance, and weatherability, can be designed by replacing the hydrogen atoms in the monomers by fluorine.7 For example, types composed from 2,2-bis-(4-hydroxyphenyl)1,1,1,3,3,3-hexafluoropropane and 4-phenoxy-2,3,5,6-tetrafluorobenzonitrile are soluble in dimethylacetamide, NMP, chloroform, and toluene, at room temperature.
8.4 APPLICATIONS Thermoplastic resinous compositions that contain PEN have found wide uses because of their excellent thermal resistance and mechanical strength. For example, the materials can be used for electrical and electronic instruments or mechanical parts, as laminated and reinforced products.8 Fields of application are aerospace, electrical coatings,31 and in automotive components.
8.4.1 Reinforced Resins A reinforced resinous composition can be obtained by mixing a PEN with a fibrous reinforcing material, followed by kneading. The amount of fibers is in the range of 5–60%. If the amount of fiber exceeds 60%, the PEN will not be desirably distributed through the voids of the reinforcing material. The kneading temperature is preferably 340–360°C and the kneading time is 1–3 min.8
8.4.2 Filter Materials Filters of poly(phenylene sulfide) (PPS) fibers or fluorine resin fibers are widely used as dust filters for city garbage incinerators or coal boilers. The plants run at increasingly high inner temperatures for reducing the amount of dioxin that may be generated in these plants. Unfortunately, the particle size of dust generated becomes increasingly smaller with the increase of the inner temperatures of incinerators. Therefore, the filters
294
High Performance Polymers
must be modified in order to increase their dust-trapping capacity to be run without problems at high temperatures. PPS has a melting point of 270–280°C and its highest temperature for continuous use is around 200°C. For this reason, this type of polymer filter is of limited use for high-temperature applications. However, PPS fibers modified with PEN have been recognized as a filter material having the advantages of excellent long-term, high-temperature heat resistance and easy dust removal.32
8.4.3 Resin-bonded Magnets Traditionally, resin-bonded permanent magnets have mainly been prepared by the compression molding method, or by the injection molding method. In the compression molding method, epoxy resins have been used. Binder resins for the injection molding method are poly(amide)s and ethylene vinyl acetate copolymers, and more recently poly(ether ether ketone) (PEEK) and PPS, because of their superior properties. Crystalline resins such as PPS or PEEK require a high temperature for fusion molding of 350°C or higher, so that there is a disadvantage in that the magnetic powder of the rare earth is likely to be oxidized by the molding process. With soluble polymers, it is possible to disperse the magnetic material in a polymeric solution and precipitate the dispersion in a non-solvent. Or else, the solvent can be evaporated in vacuo. This process of manufacture can be conducted at lower temperatures. PEN types are highly suitable as soluble polymers for preparing such resin-bonded magnets, still having good properties.33 A suitable solvent is NMP, applied at ca. 190°C. When PPS is used as a matrix resin for samarium cobalt, which is a high-temperature magnetic material, it was found that this compound appears to catalyze degradation in the molten state. To prevent degradation, it has been recommended to coat the magnetic material with a potassium silicate/kaolin coating. In this way, the organic material is separated from the magnetic material, thus preventing degradation during manufacture.34
8.4.4 Proton Exchange Membranes The introduction of nitrile groups into proton conductive sulfonated thermoplastics decreases their moisture absorption.9 Nitriles promote the adhesion of the polymers to heteropolyacids (HPA)s in composite membranes or to electrodes. In addition, the dimensional stability of the membrane is
Poly(arylene ether nitrile)s
295
improved. Thus, PEN membranes with higher ion capacity should be possible, operating at lower humidities.35 Nitrile-functional, (hexafluoroisopropylidene)diphenol-based poly(arylene ether) copolymers with pendent sulfonic acid groups can be prepared by the copolymerization of 4,4 -(hexafluoroisopropylidene)diphenol, 2,6dichlorobenzonitrile, and 3,3 -disulfonate-4,4-dichlorodiphenyl sulfone.9 Hexafluoroisopropylidene)diphenol is a fluorinated bisphenol A. HPAs/sulfonated poly(arylene ether nitrile ketone) (SPAENK) composite membranes can be prepared by solution casting of the acid form SPAENK with various contents of phosphotungstic acid.36 The transparency of the composite membranes and scanning electron microscope images indicate that the HPA particles are well dispersed within the polymer matrix. In copolymers consisting of 2,6-difluorobenzonitrile, with 2,8-dihydroxynaphthalene-6-sulfonate sodium salt and 4,4 -biphenol as hydroxy compounds, the sulfonic acid group content can be varied by the ratio of the hydroxy monomers.37 In the polymer, the sulfonic acid group is in m-position to the ether linkage. Therefore, it is believed that sulfonic acid groups are being deactivated, giving membranes with high proton conductivity due to the increased acidity. In addition, the m-position is expected to contribute to hydrolytic stability. The glass transition temperature of the copolymers increase with increasing sulfonic acid group content, They vary from 233 to 336°C in sodium form or the sulfonic acid group and from 230 to 260°C for acid form. Instead of 2,8-dihydroxynaphthalene-6-sulfonate as sulfonated monomer, sulfonated hydroquinone has been used.38 Sulfonated poly(phthalazinone ether ketone nitrile) (SPPEKN) copolymers prepared by the copolymerization of disodium 3,3 -disulfonate4,4 -difluorobenzophenone, 2,6-difluorobenzonitrile, and 4-(4-hydroxyphenyl)-1(2H)-phthalazinone exhibit a tensile strength higher than that of Nafion®117. The proton conductivities of the acid form of SPPEKN copolymers, with a feed ratio or sulfonated to unsulfonated monomer above 0.35, are around 10−1 S cm−1 at 80°C, which is close to that of Nafion®117.39
8.5 SUPPLIERS AND COMMERCIAL GRADES Tradenames appearing in the references are shown in Table 8.4.
296
High Performance Polymers
Table 8.4: Tradenames in References Tradename Description
Supplier
DER® 332 Dow Bisphenol A diglycidyl ether based epoxy resin29 Diaion® (Series) Mitsubishi Chemical Industries Ltd. Sulfonic acid type ion exchange resin modified with 2-mercaptoethylamine19 Dowex® (Series) Dow Anion and cation exchangers19 Lynite® DuPont Japan Poly(ethylene terephthalate)12 PEN™ Idemitsu Poly(arylene ether nitrile)2 Radel® A Solvay Poly(ether sulfone)28 Ryton® (Series) Philips Petroleum Co. Poly(phenylene sulfide)8 Toreca™ Toray Industries, Inc. Carbon fiber8 Udel® Polysulfone Solvay Poly(bisphenol A sulfone)2, 28 Ultem® (Series) General Electric Poly(imide), thermoplastic28 Ultem® 6050 General Electric Poly(ether imidesulfone)28 Ultrapek® BASF AG Poly(arylene ether ketone)28 Victrex® 381G Victrex PLC Poly(etheretherketone), cable coating2 Victrex® PEEK (Series) Victrex PLC Poly(etheretherketone)8, 28 Victrex® PES (Series) Victrex PLC Poly(aryl ethersulfone)28
Poly(arylene ether nitrile)s
297
8.6 SAFETY 2,6-Dichlorobenzonitrile functions as a herbicide. It is also addressed as dichlobenil. It may be absorbed through the dry skin and it is toxic to aquatic organisms. Further, it decomposes on heating or on burning in toxic fumes including hydrogen chloride, hydrogen cyanide, nitrogen oxides, and phosgene. 2,6-Dichlorobenzonitrile is obtained from chlorthiamid by the reaction with bases. For the aromatic hydroxy compounds mentioned, the regulations typical for phenols are valid. Not much special toxicological studies are found in the literature.
REFERENCES 1. R. Heath, Darrel and J. G. Wirth. Process for making cyanoaryloxy polymers and products derived therefrom. US Patent 3 730 946, assigned to General Electric Company (Schenectady, NY), May 1, 1973. 2. V. L. Rao, A. Saxena, and K. N. Ninan. “Poly(arylene ether nitriles).” J. Macromol. Sci., Polym. Rev., C42:513–540, 2002. 3. T. Takahashi, H. Kato, S. P. Ma, T. Sasaki, and K. Sakurai. “Morphology of a wholly aromatic thermoplastic, poly(ether nitrile).” Polymer, 36(20):3803– 3808, 1995. 4. S. Matsuo, T. Murakami, and R. Takasawa. Preparation of cyanoaryl ether copolymer. US Patent 4 703 104, assigned to Idemitsu Kosan Company Limited (Tokyo, JP), October 27, 1987. 5. G. Blinne, H. Bender, and P. Neumann. Branched, high molecular weight, thermoplastic polyarylene ethers containing nitrile groups, and their preparation. US Patent 4 567 248, assigned to BASF Aktiengesellschaft (DE), January 28, 1986. 6. K. Kimura, Y. Tabuchi, A. Nishichi, Y. Yamashita, Y. Okumura, and Y. Sakaguchi. “Synthesis of novel fluorine-containing poly(aryl ether nitrile)s derived from 2,3,4,5,6-pentafluorobenzonitrile.” Polym. J. (Tokyo), 33:290–296, 2001. 7. K. Kimura, Y. Yamashita, Y. Okumura, and S. Ito. Polycyanoaryl ether and method for production thereof. US Patent 6 506 872, assigned to Nippon Shokubai Co., Ltd. (Osaka, JP), January 14, 2003. 8. S. Matsuo, T. Murakami, T. Bando, and K. Nagatoshi. Reinforced resinous composition comprising polycyano arylene ether. US Patent 4 812 507, assigned to Idemitsu Kosan Company Limited (Tokyo, JP), March 14, 1989. 9. M. J. Sumner, W. L. Harrison, R. M. Weyers, Y. S. Kim, J. E. McGrath, J. S. Riffle, A. Brink, and M. H. Brink. “Novel proton conducting sulfonated
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10. 11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22. 23. 24.
High Performance Polymers poly(arylene ether) copolymers containing aromatic nitriles.” J. Membr. Sci., 239(2):199–211, August 2004. P.-C. Wang. Novel polyaryl ethers. US Patent 4 968 769, assigned to Shell Oil Company (Houston, TX), November 6, 1990. Y. Kiyomiya, Y. Yamaguchi, M. Ushigome, and H. Murata. Process for producing 2,6-dichlorobenzonitrile. US Patent 4 883 897, assigned to Nitto Chemical Industry Co., Ltd. (Tokyo, JP), November 28, 1989. M. W. Zettler, R. E. Tobey, and R. B. Leng. Solventless process for making 2,6 difluorobenzonitrile. US Patent 5 502 235, assigned to DowElanco (Indianapolis, IN), March 26, 1996. R. Nishiyama, K. Fujikawa, Y. Tsujii, S. Murai, and H. Jyonishi. Process for producing 2,6-difluorobenzonitrile. US Patent 4 406 841, assigned to Ishihara Sangyo Kaisha Ltd. (Osaka, JP), September 27, 1983. T. Yamada, S. Kimura, T. Hotsuta, and A. Mouri. Preparation of pentachlorobenzonitrile. JP Patent 60 239 452, assigned to Ishihara Sangyo Kaisha, November 28, 1985. H. Zhou, H. Luo, and P. Lin. Benzonitrile chlorination for preparing pentachlorobenzonitrile catalyst and its application. CN Patent 1 213 585, assigned to Dalian Chemical Physics Inst (CN), April 14, 1999. O. Kaieda, K. Hirota, N. Tominaga, and T. Nakamura. Preparation of pentafluorobenzonitrile. JP Patent 60 184 057, assigned to Nippon Catalytic Chem Ind, September 19, 1985. K. Hirota. Method for production of aromatic fluorine compound. US Patent 6 437 168, assigned to Nippon Shokubai Co., Ltd. (Osaka, JP), August 20, 2002. M. Inaba, N. Mine, and M. Mizutani. Method for preparing 4,4’-biphenol. US Patent 5 324 868, assigned to Mitsubishi Petrochemical Company, Ltd. (Tokyo, JP), June 28, 1994. B. Carvill, K. Glasgow, and M. Roland. Process for the synthesis of bisphenol. US Patent 7 132 575, assigned to General Electric Company (Schenectady, NY), November 7, 2006. W. Orth, E. Pastorek, W. Weiss, and H. W. Kleffner. Preparation of 9,9-bis(4-hydroxyphenyl)-fluorene. US Patent 5 169 990, assigned to Rütgerswerke Aktiengesellschaft (DE), December 8, 1992. S. Angiolini and M. Avidano. High purity 9,9-bis-(hydroxyphenyl)-fluorene and method for the preparation and purification thereof. US Patent 6 620 979, assigned to Ferrania, S.p.A. (Ferrania, IT), September 16, 2003. P. C. Wang. Novel spirolactones. US Patent 4 939 251, assigned to Shell Oil Company (Houston, TX), July 3, 1990. H. Zhou, E. Bucio, S. R. Venumbaka, J. W. Fitch, and P. Cassidy. “New spirodilactam polymers.” Polymer, 47(20):6927–6930, September 2006. J. Verborgt and C. S. Marvel. “Aromatic polyethers, polysulfones, and polyketones as laminating resins.” J. Polym. Sci., Polym. Chem. Ed., 11(1):261–
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273, 1973. 25. C. S. Marvel and J. Verborgt. Nitrile terminated aromatic polyethers. US Patent 3 970 681, assigned to University Patents, Inc. (Stamford, CT), July 20, 1976. 26. K. P. Sivaramakrishnan and C. S. Marvel. “Aromatic polyethers, polysulfones, and polyketones as laminating resins. II..” J. Polym. Sci., Polym. Chem. Ed., 12(3):651–662, 1974. 27. A. Banihashemi and B. Akhlaghinia. “Synthesis and crosslinking of poly(ether-ketone)s, and poly(ether-ketone-sulphone)s with pendant nitrile groups (II).” Iranian Polym. J., 11:365–371, 2002. 28. M. J. Mullins, E. P. Woo, K. E. Balon, D. J. Murray, and C.-C. C. Chen. Cyclic poly(aryl ether) oligomers. US Patent 5 264 538, assigned to The Dow Chemical Company (Midland, MI), November 23, 1993. 29. E. S. Sanders, Jr. and T. L. Parker. Novel aryl carbonate cyanoaryl ether gas separation membranes. US Patent 5 034 034, assigned to The Dow Chemical Company (Midland, MI), July 23, 1991. 30. T. L. Parker and T. O. Jeanes. Arylene carbonate cyanoaryl ether copolymer. US Patent 5 124 430, assigned to The Dow Chemical Company (Midland, MI), June 23, 1992. 31. L. M. Maresca, A. G. Farnham, T. H. Schwab, and U. A. Steiner. Crystalline polyarylnitrile ether polymer. US Patent 4 963 643, assigned to Amoco Corporation (Chicago, IL), October 16, 1990. 32. T. Tomura and T. Murakami. Filter material containing a polycyanoaryl ether. US Patent 6 074 449, assigned to Idemitsu Petrochemical Co., Ltd. (Tokyo, JP), June 13, 2000. 33. H. Kawato and T. Tomioka. Methods for preparing magnetic powder material and magnet, process for preparaton of resin composition and process for producing a powder molded product. US Patent 5 350 558, assigned to Idemitsu Kosan Co., Ltd. (Tokyo, JP), September 27, 1994. 34. J. Carlberg and P. R. Nastas. Thermally stable, high temperature, samarium cobalt molding compound. US Patent 6 737 451, assigned to Arnold Engineering Co., Ltd. (Marengo, IL), May 18, 2004. 35. Y. Sakaguchi, K. Kitamura, S. Nagahara, and S. Takase. “Preparation of sulfonated poly(ether sulfone nitrile)s and characterization as proton-conducting membranes.” Polym. Prepr. (ACS), 45:56–57, 2004. 36. H. Zhang, J. h. Pang, D. Wang, A. Li, X. Li, and Z. Jiang. “Sulfonated poly(arylene ether nitrile ketone) and its composite with phosphotungstic acid as materials for proton exchange membranes.” J. Membr. Sci., 264(1-2):56–64, November 2005. 37. Y. Gao, G. P. Robertson, M. D. Guiver, S. D. Mikhailenko, X. Li, and S. Kaliaguine. “Low-swelling proton-conducting copoly(aryl ether nitrile)s containing naphthalene structure with sulfonic acid groups meta to the ether linkage.” Polymer, 47(3):808–816, January 2006.
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38. Y. Gao, G. P. Robertson, M. D. Guiver, S. D. Mikhailenko, X. Li, and S. Kaliaguine. “Synthesis of copoly(aryl ether ether nitrile)s containing sulfonic acid groups for pem application.” Macromolecules, 38:3237–3245, 2005. 39. Y. Gao, G. P. Robertson, M. D. Guiver, G. Wang, X. Jian, S. D. Mikhailenko, X. Li, and S. Kaliaguine. “Sulfonated copoly(phthalazinone ether ketone nitrile)s as proton exchange membrane materials.” J. Membr. Sci., 278(1-2): 26–34, July 2006.
9 Triazole Polymers Poly(1,2,4-triazole)s (PT)s are heterocyclic polymers, which were first synthesized by the reaction of bistetrazoles and a bisimidoyl chloride.1 Meanwhile, several different reaction pathways have been developed to prepare these polymers.2
9.1 MONOMERS Monomers containing the triazole unit are shown in Figure 9.1. The synthetic routes to get 1,2,3-triazole compounds are reviewed in the literature.3 Suitable monomers are summarized in Table 9.1. 1,2,4-Triazole is a white to pale yellow solid with a melting point of 120°C. Synonyms are pyrrodiazole, 1H-1,2,4-triazole, and s-triazole. This compound should not be confused with a sulfanilamide compound that is addressed as triazole. In contrast to a triazole, a triazin refers to a 6-membered ring with three nitrogen atoms. 1,2,4-Triazole is used for the synthesis of agrochemicals, pharmaceutical substances, hydraulic fluids, and photochemical products. Both 1,2,4-triazole and 1,2,4-triazole are effective solvents for proton conducting electrolytes.4, 5 3,5-Dimethyl-1,2,4-triazole is a colorless solid with a melting point of 144°C. 1-Vinyl-1,2,4-triazole can be prepared by the reaction of 1,2,4triazole with acetylene under pressure under alkaline conditions.6 Benzotriazole can exist in two tautomeric forms as shown in Figure 9.1. Substituted 1,2,3-triazoles, can are formed via the cycloaddition of 301
302
High Performance Polymers
NH2 N
N
N N
N N
H
H
1,2,4-Triazole
3-Amino-1,2,4-triazole
CH3 N
N H 3C
N
N N
N
CH CH2
H
3,5-Dimethyl-1,2,4-triazole
1-Vinyl-1,2,4-triazole N
N
N H
N N
N H Benzotriazole
Figure 9.1: Monomers Containing the Triazole Unit
Table 9.1: Monomers for Triazole Polymers Monomer 1,2,4-Triazole 3,5-Dimethyl-1,2,4-triazole 3-Amino-1,2,4-triazole Benzotriazole 1-Vinyl-1,2,4-triazole
Triazole Polymers
303
sodium azide with acetylene in the presence of the organic iodides R−I that will become the adjacent radicals.7 Further, the feasibility of poly(ethylene glycol)-supported azide cycloadditions towards acetylene compounds has been demonstrated. A systematic study on the behavior of these kind of azides towards acetylenes, ethylenes and the cyano group has been published.8
9.2 POLYMERIZATION AND FABRICATION 9.2.1 Reaction of Dinitriles with Dihydrazides Poly(1,2,4-triazole)s can be obtained by the reaction of dinitriles with dihydrazides or by heating cyanocarboxylic acids.9 They are found to be resistant to high temperatures, especially when they contain aromatic compounds in the polymer chain.
9.2.2 Aromatic Nucleophilic Displacement PTs can be prepared by involving the aromatic nucleophilic displacement reaction of di(hydroxyphenyl)-1,2,4-triazole monomers with activated aromatic dihalides or activated aromatic dinitro compounds. The reactions are carried out in polar aprotic solvents, such as sulfolane or diphenyl sulfone, using alkali metal bases, such as potassium carbonate, at elevated temperatures under nitrogen. The di(hydroxyphenyl)-1,2,4-triazole monomers are synthesized by reacting bis-(4-hydroxyphenyl)-hydrazide with aniline hydrochloride at ca. 250°C in the melt or by reacting 1,3 or 1,4-bis-(4-hydroxyphenyl)-phenylene dihydrazide with 2 moles of aniline hydrochloride in the melt. The synthesis is shown in Figure 9.2. Purification of the di(hydroxyphenyl)1,2,4-triazole monomers is accomplished by recrystallization. This synthetic route results in high molecular weight PTs. It is economically and synthetically more favorable than other routes and allows a facile variation of the chemical structure, because a large variety of activated aromatic dihalides are available. The polymers are useful as composite matrix resins for aircraft and dielectric interlayers in electronic devices.2
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High Performance Polymers
O HO
C NHNH2
+
O HO
C O
O HO
OH
O
C NH NH C
OH
NH3Cl N N HO
OH N
Figure 9.2: Synthesis of Di(hydroxyphenyl)-1,2,4-triazole Monomers2
Triazole Polymers
305
NH2 NH N C C C N NH C R C NH2
O
N
R N
O
N N N
N
Figure 9.3: Synthesis of Poly(bis-1,2,4-triazole)9
9.2.3 Poly(bis-1,2,4-triazole)s Poly(bis-1,2,4-triazole)s are a class of polymers in which two triazole rings are immediately adjacent. These polymers can be obtained by the reaction of oxalic acid bis-amidrazone with aromatic dicarboxylic acid dihalides or with fumaroyl chloride and subsequent cyclodehydration of the poly(acyl oxamidrazone)s. The process is shown in Figure 9.3. These polymers have a very good resistance to heat, the decomposition temperatures in general being above 350°C. However, as they are insoluble and infusible, it is impossible to shape the polymers. However, materials from poly(bis-1,2,4-triazole)s can be formed by dissolving poly(acyl oxamidrazone)s, e.g., poly(terephthaloyl oxamidrazone) in dilute aqueous alkali. The solution is then spun or poured into a precipitation bath.9 The filaments or foils that are formed are washed and heated in an inert gas to a temperature of 240–320°C.
9.2.4 Poly(1-vinyl-1,2,4-triazole) Polymers of 1-vinyl-1,2,4-triazole are nontoxic, have a high hydrophilicity, and a high hydrolysis stability. Possible applications are in the food industry and medicine.6 Copolymers with of 2-hydroxyethyl methacrylate (HEMA) are used in biological and medical applications. They can be prepared in ethanol and N,N-dimethylformamide (DMF) in the presence of 2,2 -azobisisobutyronitrile (AIBN) as the initiator. Monomer reactivity ratios with various comonomers are summarized
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Table 9.2: Monomer Reactivity Ratios of 1-Vinyl-1,2,4-triazole (1) with Various Comonomers (2) in DMF6, 10 Monomer (2) 2-Hydroxyethyl methacrylate Methyl methacrylate 2,2,3,3-Tetrafluoropropyl methacrylate 2,2,3,3,4,4,5,5-Octafluoropropyl methacrylate
r1
r2
0.09 0.62 0.23 0.25
2.40 1.48 1.77 2.42
in Table 9.2. For all comonomers, the methacrylate tend to form blocks repeated by a single 1-vinyl-1,2,4-triazole unit.
9.2.5 1,2,4-Triazole Dendrimers Dendrimers are of interest as soluble supports for homogeneous catalysis since their large size enables recycling by membrane separation techniques. Conjugated triazole moieties, e.g., 3-(4-biphenylyl)-4-phenyl-5-(4tert-butylphenyl)-1,2,4-triazole (TAZ) are known for their excellent electron transporting and hole blocking ability. Therefore, this class is used in the construction of organic light-emitting devices either as such or pendent on polymers. The major advantage of dendrimers over analogous heterocyclic polymers is that their electronic and physical properties can be optimized independently. The synthesis of triazole dendrimers starts from 3,5-dichloro-4-(4methoxyphenyl)-4H-1,2,4-triazole, which can be obtained from methoxyphenylisocyanate, as shown in Figure 9.4. This substance has an appropriate AB2 -symmetry. Therefore, it is well-suited for convergent dendrimer synthesis. The reaction proceeds in the way of a Williamson type ether synthesis. To control the branching, 3,5-bis-(tert-butyl)-phenol is added.11
9.3 PROPERTIES 9.3.1 Thermal Properties The incorporation of triazoles to various types of polymers enhances the thermal properties. Poly(amide)s that compose at temperatures of greater than 340°C without melting, can be prepared from various diazoles and triazoles. The materials are shown in Table 9.3. The thermal degrada-
Triazole Polymers
CH3
O
307
NCO NH2NH2COOEt O
CH3
O
N
POCl3
N N
O Cl
CH3
O
N
N N
Cl
Figure 9.4: Synthesis of 3,5-dichloro-4-(4-methoxyphenyl)-4H-1,2,4-triazole11
Table 9.3: Diazoles and Triazoles for Poly(amide)s12 Compound 1,3-Bis[5 -[3 -(p-aminophenoxy)-phenyl]-oxadiazol-2-yl]benzene 4,4 -Bis-(p-aminophenoxy)diphenyl-1,3,4-thiadiazole 3-(3-aminophenyl)-5-[3 -(4-aminophenoxy)phenyl]-1,2,4-triazole 3-(3-aminophenyl)-5-[3 -(4-aminophenylsulfonyl)phenyl]-1,2,4-triazole
308
High Performance Polymers
CH3 CH3
N N
C CH3
N
Figure 9.5: 3-(4-Biphenylyl)-4-phenyl-5-(tert-butylphenyl)-1,2,4-triazole
tion of poly(triazoloquinazoline)s in air results in a weight loss of 10% at 460–540°C.12
9.3.2 Electrical Properties The 4-(4-(hexyloxy)phenyl)-3,5-diphenyl-4H-1,2,4-triazole moiety can be used in electron transporting polymeric segments13, 14 and shows electroluminescence in poly(p-phenylene vinylene) (PPV) derivatives. Polymers with triazole groups act to enhance the electroluminescent efficiency when used in two layer devices with PPV as a hole-transporting emitter.15 PPV-based copolymers bearing an electron-withdrawing triazole unit in the main chain can be synthesized by the Wittig reaction between triazole diphosphonium salt and the corresponding dialdehyde monomers, respectively.16 TAZ, c.f. Figure 9.5, is a low electron mobility material. It can be used as a hole blocking layer, which may limit electron injection and transfer in electroluminescent devices.17, 18 The triazole unit was found to be an effective π -conjugation interrupter and can play the rigid spacer role in determining the emission color of the resulting copolymer.19
9.3.3 Optical Properties 9.3.3.1
Polymeric Light-Emitting Diode
Electroluminescent conjugated polymers can be synthesized by incorporating high electronegative heterocyclic groups, such as 1,3,4-oxadiazole, 1,3,4-thiadiazole and 1,2,4-triazole moieties.20 These electroluminescent polymers are obtained by polymerization of a bis-(halomethyl) aromatic monomer modified with a heterocyclic group.21
Triazole Polymers
309
By a proper selection of the monomers and their ratios in the polymerization, the emissive polymer can be synthesized. The film formed of the electroluminescent polymer or copolymer can be used as a light emissive layer in a single layer polymeric light-emitting diode. 9.3.3.2
Photocuring
Photocuring technology has a major limitation since UV absorbers, which are incorporated into the coating to protect the substrate or to stabilize the coating, compete for the incident actinic radiation, are inhibiting the photocuring process. This increases the energy demand of the curing source, and a slow or insufficient curing rate will take place. However, this obstacle can be circumvented by protecting the UV-absorbing group temporarily, when the polymerization should take place. The in situ development of an ultraviolet absorber can be provided by a compound, such as a hydroxyphenyltriazole, bearing a group, which protects the absorber during actinically activated polymerization by light at a certain frequency. The protective group is formed by replacing the hydrogen of the hydroxyl group with an acyl group. After polymerization, the protective group is removed by the photochemical reaction at a second frequency lower than the first frequency. The basic scheme is shown in Figure 9.6. For example, 2-acetoxy-5-vinylphenyl-benzotriazole was blended in an amount of 0.1-1.0% with methacrylate esters, such as methyl methacrylate, n-butyl methacrylate, and ethyl methacrylate.22 The acetoxy derivatives retain the absorption band of the two original 2-hydroxy derivatives at 300 nm, but lose the absorption band at 340 nm. The photocatalyst was a mixture of diphenyl carbonyl and triethylamine. Other photocatalysts, such as aryl onium salts (benzene iodonium fluoroborate and benzene arsonium fluoroborate) can also be used successfully. The monomer mixture was irradiated at 366 nm to form a polymer. The 2-hydroxy group can be regenerated to have an UV-absorbing group in the polymer by irradiation at 310 nm. 9.3.3.3
Photographic Materials
The detailed chemistry and function of photographic couplers is beyond the scope of this text. It is reviewed in the literature.23 Triazole compounds are
310
High Performance Polymers
O C CH 3 O N N N
hν1 O C CH 3 O N N N
hν2 HO N N N
Figure 9.6: In Situ Formation of a Polymeric UV-absorber22
Triazole Polymers
311
S N
N N
N H 3C
N
N
N
N
N N
H 3C CH2 CH C O CH2 O
CH2 O C CH2 O
N
N N
N
Figure 9.7: Amino(1,2,4)-triazole Compounds for Increased Spectral Sensitivity in Silver Halide Emulsions: Dimethyl-(5-thiomorpholin-4-yl-2H[1.2.4]triazol-3-yl)-amine, 5-Acryloyloxyethoxycarbonylmethyl-7-hydroxy-1,2,4-triazolo[1.5-a]pyrimidine
used as photographic couplers.24 The precipitation of silver halogenides can be performed in the presence of triazole containing polymers.25 Amino(1,2,4)-triazole compounds serve in the production of photographic materials with increased spectral sensitivity.26 They exhibit a good shelf life, particularly when stored under humid climatic conditions. Examples of Amino(1,2,4)-triazole compounds are shown in Figure 9.7. 1,2,4-Triazolo[1,5-a]pyrimidines, for example, 4-hydroxy-6-methyl1,3,3a,7-tetraazaindene, have been extensively used as a stabilizer for silver halide photographic light sensitive materials. Due to diffusion reactions, it is difficult to make a stable layer using low-molecular-weight compounds. To avoid this problem, 1,2,4-triazolo[1,5-a]pyrimidines have been incorporated into polymers. If the compound is attached to the polymer chain through its characteristic active functional group, then the effect is only small. Therefore, the compound should be attached to the polymer via functional groups other than the active group. For example, 5-acryloyloxyethoxycarbonylmethyl-7-hydroxy-1,2,4triazolo[1.5-a]pyrimidine, c.f. Figure 9.7, can be obtained from 7-hydroxy5-carboxymethyl-1,2,4-triazolo[1,5-a]pyrimidine by esterification with 2hydroxyethyl acrylate. This compound has a vinyl group, thus it can be copolymerized with vinylic monomers.27, 28 2-Acrylamido-2-methylprop-
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anesulfonate, N-vinyl-2-pyrrolidone, 2-hydroxyethyl acrylate and 2-hydroxyethyl methacrylate, acrylamide, and methacrylamide are preferred comonomers. A homopolymer can be synthesized using a radical initiator, such as 2,2 -azobis-2,4-dimethylvaleronitrile. 9.3.3.4
UV Absorbers
Polymers, which are used for outdoor applications are often light sensitive and must be protected from ultraviolet light in order to prevent degradation of the polymer in the final product. Triazoles, such as phenylbenzotriazole, naphthylbenzotriazole, and related compounds, such as benzophenone are UV absorbers. A particularly well-known group of ultraviolet stabilizers are hydroxyphenylbenzotriazole-based compounds. However, UV light stabilizers of this type are often lost from the product into which they are incorporated by leaching or by evaporation. To overcome these difficulties, efforts have been directed towards chemically incorporating the ultraviolet light stabilizer into the polymeric backbone of the material to be protected.29 Polymerizable ultraviolet absorbers are highly compatible with plastics in which they can be easily incorporated. The absorbers, can be chemically bound to polymeric materials either by copolymerization techniques, grafting techniques or during processing of the polymer. Thus, the ultraviolet absorber becomes an integral part of the polymer chain. The bonded UV absorbers do not leach out of the polymer matrix by exudation or volatilization during high-temperature processing. Further, the materials are not subject to solvent extraction or physical migration. Polymerizable UV absorbers that contain hydroxyethoxy groups can be incorporated into polymers by condensation polymerization. The monomers may be prepared by the reaction of the 5-(2H-benzotriazole-2-yl)-2,2,4,4 -tetrahydroxybenzophenone and its chloro and methoxy derivatives with allyl bromide, acryloyl chloride, methacryloyl chloride, glycidyl acrylate, and ethylene chlorohydrin.30 Suitable molar ratios of the reactants must be employed to react with the hydroxyl group on the 4 -position by leaving the other hydroxyl groups at the 2,2 and 4 positions untouched. The monomer shown in Figure 9.8 and its methacrylic analogue have been demonstrated to undergo radical homopolymerization and copolymerization with styrene and methyl methacrylate. However, in the homo-
Triazole Polymers
OH
O
313
OH
C O
O O C CH
HO N N
N
CH2
O C CH
HO N N
N
CH2
Figure 9.8: Polymerizable UV Absorbers: 5-(2H-benzotriazole-2-yl)-2,2 ,4trihydroxy-4 -acryloxybenzophenone, 2-(2-hydroxy-7-acryloyloxynaphthyl)-2Hbenzotriazole30, 31
polymerization of naphthyl-2H-benzotriazole containing momomers, only low molecular weights could be obtained, because of the bulkiness of the group.31, 32 The admixture to an unsaturated polyester resin is possible. Further, they can be grafted onto poly(styrene) by hot blending at 180°C.30, 33 The compounds described offer an increase in stabilizer efficiency that is as comparable to monomeric benzotriazoles and benzophenones. Styrene and methyl methacrylate copolymers onto UV stabilizer moieties that were fixed were characterized by UV spectroscopy and size exclusion chromatography. The stability of intramolecular hydrogen bonds is important for the performance of UV stabilizers. The highest stability of the intramolecular hydrogen bonds is obtained for polymers with phenylbenzotriazole unit attached to the backbone. It was shown that the UV stabilizer units were statistically distributed along the polymer backbone.34 A number of polymerizable ultraviolet stabilizers of the 2-(2-hydroxyphenyl)-2H-benzotriazole types have been synthesized. E.g., 5-vinyl and 5-isopropenyl derivatives of 2-(2-hydroxyphenyl)-2H-benzotriazole and 4-acrylates or 4-methacrylates of 2-(2,4-dihydroxyphenyl)-2H-benzotriazole or 2-(2,4-dihydroxyphenyl)-1,3-2H-dibenzotriazole have been prepared and copolymerized with various monomers.29 The 4-acrylates or 4-methacrylates of 2-(2,4-dihydroxyphenyl)-2Hbenzotriazole, although being readily prepared and extremely reactive with other comonomers, are suspect for the possibility of hydrolytic instability because of the presence of the aromatic ester group.
314
High Performance Polymers
CH3O OH
N N
O CH3
O CH2 CH CH2 O C CH CH2
N
Figure 9.9: 2-[2-hydroxy-4-alkoxy-(2-oxypropyl methacrylate)phenyl]2H-4methoxybenzotriazole (MBDHG)29
Therefore, attempts have been undertaken to prepare acrylate and methacrylate esters of 2-(2-hydroxyphenyl)-2H-benzotriazole derivatives where the 2-(2-hydroxyphenyl)-2H-benzotriazole units are connected to the acrylate or methacrylate groups by aliphatic ester linkages. These polymeric reaction products should have good hydrolytic stability. Instead of acryloyl chloride or methacryloyl chloride, the glycidyl esters are used for functionalization with polymerizable compounds. Such a compound is 2-[2-hydroxy-4-alkoxy-(2-oxypropyl methacrylate)phenyl]2H-4-methoxybenzotriazole, as shown in Figure 9.9. It is prepared by the reaction of 4-(5-methoxy-2H-benzotriazole-2-yl)resorcinol with glycidyl methacrylate. Tetrabutylammonium bromide is used as a catalyst and hydroquinone is used as a polymerization inhibitor. Polymerizable 2-(2-hydroxyphenyl)-2H-benzotriazoles that are substituted in the 5-position with reactive hydroxyl and carboxyl groups have been synthesized by reaction with N-hydroxymethylmethacrylamide.35 The acrylamidomethyl-2-(2-hydroxyphenyl)-2H-benzotriazole compounds obtained in the first step can be copolymerized with acrylic and methacrylic monomers. The substitution at the 5 position with long chain acids, long chain hydrocarbons, fluorocarbon or silicon oligomeric alcohols, results in surface active compounds that are also UV stabilizers. Ultraviolet blocking lenses are especially useful for those who have had the natural lens of the eye removed, since the natural lens has UV absorption properties that help to protect the interior of the eye. Hence, UVabsorbing intraocular lenses (IOL) are also highly desirable, since such lenses are implanted in place of the eye’s natural lens. Hydrogels are desirable for use in lenses, particularly in IOL. However, because of their hydrophilic nature and expanded structure, it has been difficult to incorporate low-molecular compounds and UV-absorbing compounds into hydrogels. Polymeric UV absorbers can be used in contact lenses, artificial intraocular lenses, etc. These are made from acrylic polymers.36 One UV-ab-
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315
sorber is a benzotriazole derivative, and the other UV-absorber is a benzophenone derivative. In general, lenses are prepared by the free radical polymerization of the lens forming monomers. The constituents in the formulation can be modified to produce lenses with required water content and other physical properties. Copolymerizable UV absorbers are usually directly incorporated in the lens formulation, such as HEMA. Monofunctional UV absorbers may not be incorporated into the polymeric backbone to 100%. Therefore, multifunctional crosslinkable UV absorbers have been proposed.37
9.4 SPECIAL ADDITIVES 9.4.1 Degradation Inhibitors Halogen containing polymers are sensitive to degradation by ejecting hydrogen chloride. In the presence of zinc compounds, this effect is even enhanced, since zinc chloride is formed, which functions as a Lewis acid and catalyzes the degradation of the polymer. By the addition of aromatic triazoles to halogenated polymeric compositions, the degradation can be prevented. In particular, triazoles are effective in stabilizing halogenated polymers in the presence of zinc oxide. Effective stabilizers are benzotriazole, or tolyltriazole.38
9.5 APPLICATIONS 9.5.1 Blocked Isocyanates The isocyanate moiety is highly reactive towards poly(ol)s. This reaction is utilized in the preparation of poly(urethane)s (PU)s. With catalysts such as amines, the reaction occurs rapidly at room temperature. Consequently, urethane polymers are mostly prepared from two components, one containing the isocyanate component, and the other containing the polyol component. The components are mixed prior to curing. However, there is one component formulations available that can be cured by heating. PU powder coatings consist essentially of a polyol and a poly(isocyanate), whose NCO groups are partially or completely masked with a blocking agent, so that the polyaddition reaction is inhibited at temperatures below 140°C.
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A common blocking agent is ε -caprolactam. In this way, blocked isocyanates require curing temperatures in the range of 170–200°C. Other blocking agents include malonic acid diethyl ester, 2,4-dimethyl-3-pentanone oxime or 2,6-dimethyl-4-heptanone oxime, which cure at lower temperatures. However, the coatings tend toward yellowing. An alternative blocking agent is 1,2,4-triazole.39 Mixtures of diisopropylamine and 1,2,4-triazole may also be applied for blocking. The crosslinking temperature of aliphatic poly(isocyanate)s, based on 1,6-hexane diisocyanate, which are blocked with equimolar quantities of diisopropylamine and 1,2,4-triazole is 139°C, without the addition of any catalyst. Aliphatic poly(isocyanate)s, which are blocked with equimolar quantities of diisopropylamine and malonic acid diethyl ester have a crosslinking temperature of 130°C.40 Triazole blocked isocyanates are stable up to 130–140°C. The blocking reaction can be performed at a temperature of about 15–25°C below the unblocking temperature.39 The storage stability is distinctly improved in comparison to those formulations that are exclusively blocked with CHacidic esters, such as malonic acid diethyl ester.40 Poly(isocyanate)s blocked with 1,2,4-triazole are particularly suitable as crosslinking agents for powder coating compositions. However, they are essentially unsuitable for use in solvent-containing coating compositions because their solutions in organic solvents are relatively highly viscous and are often unstable due to the tendency of the blocked poly(isocyanate)s to crystallize. However, poly(isocyanate)s, which are prepared from 1,6-hexane diisocyanate and blocked with 3,5-dimethyl-1,2,4-triazole are storage stable and have a low viscosity.41 In hybrid blocked poly(isocyanate)s, multiple blocking agents are used together, such as ε -caprolactam, diisopropylamine and 1,2,4-triazole. This type of component of the stoving lacquers provide coatings that have deep-drawability at room temperature and resistance to subsequent tearing after aging of the coatings. Such materials are preferably used to produce primers for coil coatings.42
9.5.2 Crosslinking Rubbers When a butyl rubber, an ethylidene norbornene (ENB)-type ethylene propylene diene monomer (EPDM), or a mixture of desired proportions of a butyl rubber and an ENB-type EPDM is thermally crosslinked using an
Triazole Polymers
317
alkylphenol-formaldehyde resin and a triazole compound, a higher crosslinking rate is obtained. When, a hydrazide compound is also used, even higher crosslinking rates are obtained. 3-(N-Salicyloyl)amino-1,2,4-triazole exhibits a remarkably high rate of crosslinking in an alkylphenol-formaldehyde resin crosslinking for butyl rubber or ENB-type EPDM. 3-Amino-1,2,4-triazole, which has a chemical structure very similar to that of 3-(N-salicyloyl)amino-1,2,4-triazole, shows no crosslinking ability under similar conditions.43
9.5.3 Coatings 3,5-Diamino-1,2,4-triazole is a component in polyester imide coatings with increased hardness that is used for thermosetting insulating varnishes for coating of electric wires.44, 45 Acid components are terephthalic acid esters and trimellitic acid anhydride.
9.5.4 High-Temperature Adhesives The incorporation of pyridine or triazole improves the adhesion between poly(imide)s and copper.46 Poly(3,3 ,4,4 -benzophenone tetracarboxylic dianhydride-3,5-diamino-1,2,4-triazole) (BTDA-DATA) contains the triazole moiety as repeating units. Poly(4,4 -oxydiphthalic anhydride-1,3aminophenoxybenzene-8-azaadenine) (ODPA-APB-8-AA) bears the triazole moieties at the end.47 BTDA-DATA starts to decompose at 350°C. However, ODPA-APB-8-AA starts to decompose at 400°C. The polymers have been tested as adhesives for copper surfaces. The adhesion is increased by the formation of copper complexes. 4,4 -Oxydianiline was separately mixed with 2,6-diaminopyridine and 3,5-diamino-1,2,4-triazole, to form a mixture of diamines. Then poly(imide)s (PI)s were synthesized by reacting the mixture of the diamines and pyromellitic dianhydride. The adhesion strength of sputter-deposited copper to PI films is proportional to the content of functional groups.48 The adhesion strength of epoxy resin copper joints is often very poor. This is caused by the naturally formed copper oxide having a low mechanical strength. In order to improve the adhesion strength of such joints, copper lead frames were created with azole compounds as adhesion promoters. The azole compounds used are benzotriazole (CBTA) and other non triazolic classes.49
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High Performance Polymers
Triazole compounds, such as CBTA and 8-azaadenine, showed excellent adhesion strength, whereas imidazole-based azole compounds did not improve the adhesion strength. However, the adhesion strength of CBTA and 8-azaadenine-treated joints decreased with increasing treatment time, since thick porous copper-azole complexes had a weaker mechanical strength when formed. The polymeric azole compound polybenzimidazole showed the highest adhesion strength, of 785 N m−1 , because of better coverage of the surface.
9.5.5 Polymeric Corrosion Inhibitors The triazole moiety has anticorrosive properties in general. For copper and its alloys, in aqueous medium, benzotriazole is most widely used as a corrosion inhibitor. 5-Aminotriazole is used as such in an anticorrosion lubricating oil composition.50 The corrosion of metals in acidic solutions is inhibited by adding 0.01-1 wt.% of a polymer with a backbone of 1,2,4triazole.51 9.5.5.1
Electrodeposition
In protecting silver from tarnishing, an electrodeposited film of poly(amino triazole) has been tested. The protection by poly(amino triazole) is not reliable for all nuances of silver. In contrast, a film formed with hexadecane thiol shows satisfactory properties.52 To protect copper films on electrical boards, the copper surface can be covered by a polymer film obtained by anodic oxidation of a corrosion inhibitor, such as amine or imidazole monomers. By electro-oxidation not only vinyl monomers, but also compounds with other functionalities, such as phenols, acrolein, benzonitrile, etc., can be polymerized.53, 54 The electro-oxidation of 3-amino-1,2,4-triazole on a copper substrate in an alkaline methanol solution produces a homogeneous and adherent polymer film.55 The thickness of the polymer film is about a few μ m. However, the current efficiency of the electropolymerization process is weak because of the formation of oligomers and the oxidation of the electrolyte. Triphenylphosphine suppresses the formation of the surface polymer complex, because a soluble complex is formed. This leads to a significant decrease of the surface inhibition efficiency.56
Triazole Polymers 9.5.5.2
319
Aqueous Dispersions
In aqueous epoxy resin dispersions, based on poly(glycol)s, bisphenol epoxies and 3-amino-1,2,4-triazole in propylene glycol monobutyl ether, the triazole compound imparts enhanced corrosion protective properties.57 The composites have good shelf lifetimes.58, 59 Triazole particles can be made insoluble in water using a plasma polymerization technique.60 An ultrathin polymer film on the particles is formed, as confirmed by secondary ion mass spectrometry. The encapsulated triazole slowly releases the active triazole and can be used as a paint pigment in a water-based epoxy coating. This technique could replace chromate pigments in paints.
9.5.6 Gas-Generating Compositions Gas-generating compositions are suitable for air bags in cars systems. The gas must be nontoxic. The gas-generating compositions upon combustion must rapidly generate gases. Thermally stable nonazide gas-generating composites are needed that have acceptable burn rates and eject a relatively high gas volume to solid particulate ratio at acceptable flame temperatures.61, 62 For an extrudable pyrotechnic composition poly(5-amino-1-vinyltetrazole), poly(5-vinyltetrazole), poly(2-methyl-5-vinyl)tetrazole, poly(1-vinyltetrazole), poly(3-vinyl-1,2,5-oxadiazole), or poly(3-vinyl-1,2,4triazole), may be used. Preferred vinyltetrazoles include 5-amino-1-vinyltetrazole and poly(5-vinyltetrazole). These compounds exhibit self-propagating thermolysis or thermal decomposition. Poly(5-amino-1-vinyltetrazole) does not show an endothermic reaction before the exothermic decomposition begins. Therefore, the heat-consuming step normally attendant prior to the energy releasing steps of combustion that acts as an energy barrier is not present. The synthesis of a gas-generating composition starts with the synthesis of a poly(vinyltriazole). A substituted triazole salt is added to a free radical brominating reagent, such as N-bromo succinimide, and to a radical initiator to form a brominated triazole. The brominated triazole is then added to triphenylphosphine to form a Wittig salt. Formaldehyde in alkaline medium effects the formation of a vinyltriazole salt, which can be polymerized with AIBN and a catalytic amount of a cationic initiator or a Ziegler-Natta catalyst. The reaction sequence is shown in Figure 9.10.
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CH3 M+N
CH2
Br O
N
N
O
M+N
Br
N
N
N PPh3
Br-PPh3+
CH2
M+N
N
CH2 OH-, H2CO
M+N
N CH 2
N
N
CH
M+N
CH
N
N
CH2
CH
M+N
N
N
Figure 9.10: Formation of Vinyltriazole Polymer Salts61
An oxidizer is combined with the fuel polymer and is made up from phase-stabilized ammonium nitrate, i.e., ammonium nitrate co-precipitated with 10% potassium nitrate. Ammonium nitrate-containing pyrotechnic gas-generating formulations may suffer from phase changes in a crystalline structure associated with volumetric expansion during temperature cycling. Potassium nitrate and cupric oxide have a synergistic effect in stabilizing ammonium nitrate.63
9.5.7 Biocidal Polymers 3-Amino-1,2,4-triazole has herbicidal properties. However, the substance is highly water soluble and is washed out quickly from the soil. The effectiveness can be increased by bounding the active substance to a polymer. In this case, it is only set free slowly and acts over a prolonged period.64 Polymers with pendant herbicide groups have been prepared by free radical polymerization of amino triazoles. For example, 1-(4-vinylbenzoyl)-5-amino-1,2,4-triazole, or 1-(3-carbomethoxyacryloyl)-5-amino-1,2,4-
Triazole Polymers
321
triazole, is copolymerized with methyl methacrylate or styrene. The release of herbicides by hydrolysis depends on the nature of the amide bonds.65 For example, poly[1-(4-vinylbenzoyl)-5-amino-1,2,4-triazole] exhibits a high release, whereas poly[3-(4-vinylbenzoyl)-5-amino1,2,4-triazole] exhibits a low herbicide release.
9.6 SUPPLIERS AND COMMERCIAL GRADES There are essentially no registered trademarks concerning triazole polymers, at least in the United States. In contrast, there are some trademarks with regard to agricultural products. Tradenames appearing in the references are shown in Table 9.4.
9.7 SAFETY 1,2,4-Triazole is suspected to be a neurotoxicant and a respiratory toxicant.
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Table 9.4: Tradenames in References Tradename Description
Supplier
Acronal® 4F BASF AG Poly(n-butylacrylate)42 Alftalat® AN 739 Hoechst/Italy Polyester39 CP-45X Fuji Photo Film Co., Ltd. Developer24 Crylcoat® 2392 UCB Polyester39 Desmodur® W Bayer AG Bis-(4-isocyanatocyclohexyl) methane (H12 MDI)42 Desmophen® 690 Bayer AG Branched lacquer polyester with OH groups42 Hakkol FWA-SF Showa Chemical Triazinylaminostilbene fluorescent brightening agent24 Lupersol® 256 Elf Atochem 2,5-Dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane36 Shieldex® C 303 Grace & Co. Ca ion-exchanged silica, anticorrosion pigment42 Silicone KF351A Shin-Etsu Chemical Co., Ltd. Poly(dimethyl siloxane) surfactant24 Solvesso® Exxon Higher aromatic solvent mixtures40 Tinuvin® 326 Ciba Geigy 2-(2 -Hydroxy-3 -tert-butyl-5 -methylphenyl)-5-chlorobenzotriazole, UV absorber36 Tinuvin® P, Ciba Geigy 2-(2 -Hydroxy-5 -methylphenyl)benzotriazole, UV absorber22 Tronox® R-KB-2 Tronox Inc. (Kerr-McGee Chemical Corp.) Alumina silica treated, rutile titanium dioxide, pigment42 Uralac® P 1460 DSM Polyester polyol39
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14. S. H. Chen and Y. Chen. “Poly(p-phenylenevinylene) derivatives containing electron-transporting aromatic triazole or oxadiazole segments.” Macromolecules, 38(1):53–60, January 2005. 15. D. D. C. Bradley, M. Grell, A. Grice, A. R. Tajbakhsh, D. F. O’Brien, and A. Bleyer. “Polymer light emission: Control of properties through chemical structure and morphology.” Opt. Mater., 9(1-4):1–11, January 1998. 16. Z. Liu, Y. X. Cheng, G. P. Su, L. X. Wang, X. B. Jing, and F. S. Wang. “Novel PPV-based light-emitting copolymers containing triazole moiety.” Synth. Met., 137(1-3):1113–1114, April 2003. 17. J. Kido, C. Ohtaki, K. Hongawa, K. Okuyama, and K. Nagai. “1,2,4-triazole derivative as an electron transport layer in organic electroluminescent devices.” Jpn. J. Appl. Phys., Part 2, 32(7A):L917–L920, July 1993. 18. J. Peng, N. Takada, and N. Minami. “Red electroluminescence of a europium complex dispersed in poly(N-vinylcarbazole).” Thin Solid Films, 405(1-2): 224–227, February 2002. 19. Z. Liu, L. X. Wang, X. B. Jing, and F. S. Wang. “Synthesis and characterization of novel bipolar PPV-based copolymer containing triazole and carbazole units.” Chin. J. Polym. Sci., 19(6):615–621, November 2001. 20. T. Taguchi. Polymer and light emitting element using the same. US Patent 6 803 124, assigned to Fuji Photo Film Co., Ltd. (Kanagawa, JP), October 12, 2004. 21. S.-A. Chen and Y.-Z. Lee. Electroluminiscent conjugated polymers modified with high electronegative. US Patent 6 693 158, assigned to National Science Council (Taipei, TW), February 17, 2004. 22. V. B. McKoy and A. Gupta. Photocurable acrylic composition, and UV curing with development of. US Patent 5 141 990, assigned to California Institute of Technology (Pasadena, CA), August 25, 1992. 23. P. Bergthaller. “Couplers in colour photography - chemistry and function part 2.” Imaging Sci. J., 50(3):187–229, 2002. 24. H. Satoh, T. Nakamine, N. Seto, and H. Yoneyama. 1h-pyrazolo[1,5-b]-1,2,4triazole compound, coupler and silver halide color photographic. US Patent 6 995 273, assigned to Fuji Photo Film Co., Ltd. (Kanagawa-ken, JP), February 7, 2006. 25. D. Brennecke and B. Weber. Production of emulsion based on tabular silver chloride, with increased gradation. DE Patent 19 731 576, assigned to Agfa Gevaert Ag (De), January 28, 1999. 26. P. Bergthaller, J. Siegel, and H.-U. Borst. Photographic silver halide material. US Patent 6 498 003, assigned to Agfa-Gevaert (BE), December 24, 2002. 27. S. Ishiguro, Y. Fuseya, T. Heki, and A. Mitsui. Silver halide photographic light-sensitive material. US Patent 4 397 943, assigned to Fuji Photo Film Co., Ltd. (Kanagawa, JP), August 9, 1983. 28. S. Ishiguro, T. Heki, A. Mitsui, and Y. Fuseya. Light-sensitive photographic silver halide material. DE Patent 3 223 316, assigned to Fuji Photo Film Co.,
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Ltd., Japan, January 05, 1983. 29. O. Vogl and C. Zhang. 2(2-Hydroxyphenyl)2H-benzotriazole compounds and homopolymers or copolymers thereof. US Patent 5 099 027, assigned to PPG Industries, Inc. (Pittsburgh, PA), March 24, 1992. 30. K. Shuhaibar and F. A. Rasoul. Ultraviolet light absorbing benzotriazolylbenzophenone compounds and their copolymerizable derivatives. GB Patent 2 232 667, assigned to Kuwait Institute for Scientific Research, Kuwait, December 19, 1990. 31. F. A. Rasoul and K. Shuhaibar. Naphthylbenzotriazole UV light absorbers for plastics. GB Patent 2 237 567, assigned to Kuwait Institute for Scientific Research, Kuwait, May 08, 1991. 32. K. F. Shuhaibar, F. A. Rasoul, H. Pasch, and A. Mobasher. “Synthesis and characterization of polymers containing naphthyl-2H-benzotriazoles. 1. Monomer synthesis and characterization.” Angew. Makromol. Chem., 193: 147–158, 1991. 33. F. A. Rasoul, H. Pasch, K. F. Shuhaibar, and A. Attari. “Synthesis and characterization of polymers containing naphthyl-2H-benzotriazoles. 2. Polymer synthesis and characterization.” Angew. Makromol. Chem., 193:159– 167, 1991. 34. H. Pasch, K. F. Shuhaibar, and S. Attari. “A comparative study on polymers containing different ultraviolet stabilizer moieties.” J. Appl. Polym. Sci., 42: 263–271, 1991. 35. L. Stoeber, A. Sustic, W. J. Simonsick, Jr., and O. Vogl. “Functional polymers 65. Synthesis and brief characterization of surface active 2(2-hydroxyphenyl)2H-benzo-triazole ultraviolet stabilizers.” J. Macromol. Sci. Pure Appl. Chem., A37(9):943–970, 2000. 36. H. Faubl. UV blocking lenses and material containing benzotriazoles and benzophenones. US Patent 6 244 707, assigned to Wesley Jessen Corporation (Des Plaines, IL), June 12, 2001. 37. S.-G. Hong. Crosslinkable UV absorbing agent for UV absorbing lens. US Patent 6 914 086, July 5, 2005. 38. C. A. Schneider and T. C. Rees. Halogenated polymers stabilized with triazoles. US Patent 4 274 997, assigned to The Sherwin-Williams Company (Cleveland, OH), June 23, 1981. 39. R. Gras. Blocked polyisocyanates, a process for their preparation, and their use. US Patent 6 437 074, assigned to Huels Aktiengesellschaft (Marl, DE), August 20, 2002. 40. E. König, H. Casselmann, F. Kobelka, and K.-A. Foster. At least partially blocked organic polyisocyanates, a process for their preparation and their use in coating compositions. US Patent 5 350 825, assigned to Bayer Aktiengesellschaft (Leverkusen, DE);, September 27, 1994. 41. G. Kurek, E. König, K. Nachtkamp, and T. Engbert. Polyisocyanates blocked with 3,5-dimethyl-1,2,4-triazole. US Patent 6 005 046, assigned to Bayer Ak-
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tiengesellschaft (Leverkusen, DE), December 21, 1999. 42. E. König, B. Baumbach, and C. Füssel. Polyisocyanates blocked with epsilon-caprolactam and either diisopropylamine or 1,2,4-triazole, their preparation and use. US Patent 6 723 817, assigned to Bayer Aktiengesellschaft (Leverkusen, DE), April 20, 2004. 43. M. Onizawa. Method for crosslinking of isoprene-isobutylene rubber, ethylene-propylene-diene rubber containing ethylidenenorbornene as unsaturated component, or mixture thereof; and crosslinked rubber product obtained by said method. US Patent 6 403 713, June 11, 2002. 44. A. Dobbelstein, H. D. Hille, and H. Holfort. Terephthalic trimellitic polyesterimides for electric insulation. DE Patent 1 937 311, assigned to Herberts, Dr. Kurt, und Co., February 18, 1971. 45. A. Dobbelstein, H. D. Hille, and H. Holfort. Thermosetting insulating varnishes for coating electrical conductors. DE Patent 1 966 084, assigned to Herberts, Dr. Kurt, und Co., September 23, 1971. 46. J. Seo, J. Kang, K. Cho, and C. E. Park. “Synthesis of polyimides containing triazole to improve their adhesion to copper substrate.” J. Adhes. Sci. Tech., 16(13):1839–1851, 2002. 47. R.-W. Lee, G. F. Walker, and A. Viehbeck. “Formation of polyimide-Cu complexes: Improvement of direct Cu-on-PI and PI-on-Cu adhesion.” J. Adhes. Sci. Tech., 9:1125–1141, 1995. 48. C. K. Ku, C. H. Ho, and Y. D. Lee. “Synthesis of polyimides containing pyridine or triazole moiety to improve their adhesion to sputter-deposited copper.” J. Adhes. Sci. Tech., 19(11):909–925, 2005. 49. S. M. Song, C. E. Park, H. K. Yun, C. S. Hwang, S. Y. Oh, and J. M. Park. “Adhesion improvement of epoxy resin copper lead frame joints by azole compounds.” J. Adhes. Sci. Tech., 12(5):541–561, 1998. 50. R. L. Sung and B. H. Zoleski. Polyoxyalkylene polyamine triazole complexes. US Patent 4 464 276, assigned to Texaco Inc., USA, August 07, 1984. 51. A. Kotone, T. Hori, M. Hoda, and Y. Nakane. Corrosion inhibition of metals in acidic solutions. JP Patent 48 089 141, assigned to Sakai Chemical Industry Co., Ltd., November 21, 1973. 52. M. C. Bernard, E. Dauvergne, M. Evesque, M. Keddam, and H. Takenouti. “Reduction of silver tarnishing and protection against subsequent corrosion.” Corros. Sci., 47:663–679, 2005. 53. R. V. Subramanian. Electroinitiated polymerization on electrodes. In Electric phenomena in polymer science, volume 33 of Advances in Polymer Science, pages 33–58. Springer-Verlag Berlin, Berlin, 1979. 54. G. Mengoli. Feasibility of polymer film coatings through electroinitiated polymerization in aqueous medium. In Electric phenomena in polymer science, volume 33 of Advances in Polymer Science, pages 1–31. Springer-Verlag Berlin, Berlin, 1979.
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55. B. Trachli, M. Keddam, H. Takenouti, and A. Srhiri. “Protective effect of electropolymerized 3-amino 1,2,4-triazole towards corrosion of copper in 0.5 m nacl.” Corros. Sci., 44(5):997–1008, May 2002. 56. J.-L. Yao, Y.-X. Yuan, and R.-A. Gu. “Negative role of triphenylphosphine in the inhibition of benzotriazole at the Cu surface studied by surface-enhanced raman spectroscopy.” J. Electroanal. Chem., 573(2):255–261, December 2004. 57. H. Matsuki and R. Nishida. Aqueous epoxy resin dispersions and thermosetting coating compositions with good corrosion resistance. JP Patent 2 003 034 713, assigned to Kansai Paint Co., Ltd., Japan, February 7, 2003. 58. H. Matsuki, R. Nishida, and M. Murata. Storage-stable one-liquid waterborne epoxy resin dispersions for coatings with good thermal curability and resistance to corrosion and impact. JP Patent 2 003 253 004, assigned to Kansai Paint Co., Ltd., Japan, September 10, 2003. 59. T. Miyoshi, A. Matsuzaki, K. Sasaki, K. Okai, T. Sakamoto, N. Yoshimi, M. Yamashita, and M. Murata. Environmentally friendly corrosion-resistant precoated steel sheet and its manufacture. JP Patent 2 004 162 097, assigned to JFE Steel Corp., Japan; Kansai Paint Co., Ltd., June 10, 2004. 60. H. Yang and W. J. van Ooij. “Plasma-treated triazole as a novel organic slow-release paint pigment for corrosion control of AA2024-T3.” Prog. Org. Coat., 50(3):149–161, August 2004. 61. G. K. Williams, S. P. Burns, and I. B. Mishra. Gas generating compositions. WO Patent 2 005 035 466, assigned to Automotive Systems Laboratory, Inc., USA, April 21, 2005. 62. G. K. Williams and R. J. Matlock. Gas generant and manufacturing method thereof. WO Patent 2 005 097 711, assigned to Automotive Systems Laboratory, Inc., USA, October 20, 2005. 63. B. K. Hamilton. Phase-stabilized ammonium nitrate. US Patent 6 872 265, assigned to Autoliv ASP, Inc. (Ogden, UT), March 29, 2005. 64. K. Wermann, H. J. Bauer, M. Hartmann, G. Globig, G. Schwarz, and I. Seewald. Polymeric triazole herbicides. DD Patent 224 203, assigned to VEB Fahlberg-List, Ger. Dem. Rep., July 03, 1985. 65. M. Hartmann, D. Kohrs, and K. Wermann. “Biocidal polymers. X. Synthesis and hydrolytic behavior of polymers with pendantly bound 3-amino-1,2,4-triazole.” Acta Polym., 36:185–187, 1985.
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10 Poly(oxadiazole)s Poly(1,3,4-oxadiazole)s (PODA)s were first described in 1961.1 The oxadiazole moieties are mostly linked by aromatic units, although PODA with aliphatic linkages have been described, with considerable lower melting points. Most common is the 1,3,4-oxadiazole isomer; related compounds have the 1,2,4-oxadiazole ring, 1,2,5-oxadiazole ring, thiazole ring, oxazole ring, isothiazole ring, isooxazole ring, and thiophene ring, respectively, in the backbone. Owing to their interesting properties, PODAs find use in advanced applications. The class of PODA has been reviewed.2–4 Besides as polymers, 1,3,4oxadiazole derivatives are used in medical and agricultural applications, e.g., in the field of chemotherapy or as herbicides and insecticides.5, 6
10.1 MONOMERS Monomers are shown in Table 10.1 and in Figure 10.1. Vinyl monomers with the oxadiazole moiety have been described.7 The synthesis of such a monomer is shown in Figure 10.2. Such oxadiazole derivatives are liquid crystalline, and thus can be oriented in one of their mesophases to yield materials with advantageous anisotropic electrical or optical properties. (4,4 -Tetrazolyl-4 -methyl)triphenylamine can be polymerized with bifunctional acid chlorides.8 Additionally, these compounds bear diphenyl silane groups. 2,5-Bis-(4-carboxyphenyl)-1,3,4-oxadiazole (ODCA) is used for the 329
330
High Performance Polymers Table 10.1: Monomers for Poly(1,3,4-oxadiazole)s Monomer
References
p-Phenylene-5,5 -tetrazole 1,4-Benzenedicarboximidic acid dihydrazide Isophthaloyl chloride 4 4 -Diphenyl ether dicarboxylic acid 2,2 -(Oxydi-4,1-phenylene)bis[5-(4-fluorophenyl)-1,3,4oxadiazole] 1,5-Naphthalenediol (4,4 -Tetrazolyl-4 -methyl)triphenylamine 2,5-Bis-(4-carboxyphenyl)-1,3,4-oxadiazole
N N
N
N
N
N
N
O
N
Cl
1,4-Phenylene-5,5´-tetrazole
C
1 1 1 9 10 10 8 11
C
O Cl
Isophthaloyl chloride
HN
NH C
C H2N HN
NH NH2
1,4-Benzenedicarboximidic acid dihydrazide
HOOC
COOH
O
4′4′-diphenylether dicarboxylic acid N N F
Ar
N N Ar
O
O Ar
Ar
F
O
2,2′-(oxydi-4,1-phenylene)bis[5-(4-fluorophenyl)-1,3,4-oxadiazole] Ar =
Figure 10.1: Monomers Used for PODA
Poly(oxadiazole)s
F S
Br
F
F
F
Cl
+
BuLi, CO2
COOH S F
F S F
F
H
O
N H Cl C C Cl H N O H
S F
N N F
F O F
O C O
S
F S
C O O
Figure 10.2: Synthesis of a Crosslinkable Oxadiazole Monomer7
331
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High Performance Polymers
modification of poly(ethylene terephthalate) (PET).11 When small amounts of ODCA are added as an acid component into the monomer feed for PET, the glass transition temperature is increased. Copolymers containing ODCA show somewhat higher solubilities for acetone than that of neat PET.
10.2 POLYMERIZATION AND FABRICATION PODAs can be synthesized, either by:4 • Forming the oxadiazole moiety in the course of synthesis of the polymer, mostly via a precursor polymer, or • Condensing monomers that already contain the oxadiazole moiety.
10.2.1 Polycondensation Initially, PODA has been synthesized by the reaction of a bistetrazole and an aromatic diacid chloride, such as (e.g. p-phenylene-5,5 -tetrazole) and isophthaloyl chloride.1 The reaction is shown in Figure 10.3. However, the polymers obtained by this reaction route suffer from solubility, and high molecular weight.4 Another method reported is the condensation of bis-(amidrazones) with diacid chlorides with subsequent intramolecular elimination of ammonia or condensation of bis-(amidrazones), e.g., 1,4-benzenedicarboximidic acid dihydrazide with dicarboxylic acids in a one-step reaction. However, the bis-(amidrazone) monomers are not easily accessible. For this reason, a two-step reaction of dihydrazides with dicarboxylic acid chlorides to form poly(hydrazide)s in the first step is favored. A wide variety of aromatic moieties can be used.12 In the second step, the cyclization to PODA is achieved by cyclodehydration at 300°C in vacuo, or by the application of dehydrating solvents. Likewise, the intermediate poly(hydrazide) can be cast or spun, and the fabricated material can be converted into the PODA form. The cyclodehydration reaction is strongly dependent on the morphological structure of the precursor polymer.13 A side reaction degradation may occur, which lowers the mechanical properties. PODA, with just one kind of aromatic moiety can be synthesized by the reaction of an aromatic dicarboxylic acid with hydrazine sulfate, as demonstrated with 4 4 -diphenyl ether dicarboxylic acid. In this way,
Poly(oxadiazole)s
N N
N
N
N
N
N N
N
C
C
Cl
N
N
N
O
N
O
+
N
N O
N C
N
333
O Cl
O Cl
N
O
Figure 10.3: PODA from a Bistetrazole and an Aromatic Diacid Chloride
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OH
H
OH
O C
+
C O
H N N
H
H
N CH2
CH3 N N O
N CH2
CH3
Figure 10.4: Synthesis of Ethyl carbazole Oxadiazole Copolymers14
poly(4,4 -diphenyl ether-1,3,4-oxadiazole) (POD-DPE) is obtained. The reaction is carried in poly(phosphoric acid) (PPA). The conditions of the reaction must be carefully controlled. Otherwise, large variations in molecular weight and macromolecular structure of the polymeric materials are encountered.9 The high sensitivity to the reaction conditions is attributed to the high viscosity of the reaction medium, to degradation reactions caused by the acidity of the solvent, and to secondary reactions. The precursor polymer can be synthesized by a polycondensation reaction in the range of −20–0°C.15 N-methyl-2-pyrrolidone is used as a solvent and lithium chloride is used as a co-solvent. The glass transition temperatures of the precursor polymer correlate with their intrinsic viscosity, and thus with the molecular weight. The use of hydrazine sulfate as a reagent effects a partial sulfonation of the polymer.16 Actually, soluble polymers can be obtained. Copolymers having the carbazole group and the oxadiazole group directly in the backbone, i.e., poly[3,6-N-Ethylcarbazole-1,3,4-oxadiazole-2,5-diyl], can be prepared from N-ethylcarbazole-3,6-dicarboxylic acid and hydrazine hydrochloride.14 The reaction runs at 140°C with PPA. The synthesis is shown in Figure 10.4. The oxadiazole moiety can be introduced in a sequence of several steps in the backbone of poly(p-phenylene vinylene) (PPV).17, 18 The re-
Poly(oxadiazole)s
335
N N H 3C
CH3
O Br2, CCl4 N N
BrH2C
CH2Br
O (CH3)2S N N
-
+
Br (H3C)2S CH2
CH2S+(CH3)2Br-
O OHN N O
Figure 10.5: Oxadiazole Phenylene Vinylene Copolymers17
action scheme is shown in Figure 10.5. Oligomers are obtained, which are soluble in both chloroform and tetrahydrofuran. Alternating copolymers of 9,9-dioctylfluorene and oxadiazole can been prepared by the tetrazole route or the Suzuki coupling reaction.19 The tetrazole route offers advantages in the preparation of PODAs with well-defined structures in comparison to other routes. The copolymers exhibit decomposition temperatures around 430°C. Electroluminescent polymers bearing the oxadiazole moiety are represented in Figure 10.6.20 Oligomeric PPV has improved electron transport properties, due to the presence of electron deficient nitrogen in oxadiazole.21 Fluorine containing PODAs show emission of blue light.22 The synthesis of poly((2,5-bis-(5-hexyloxyphenyl)-1,3,4-oxadiazole)-2,2-diylvinylene-alt-1,4-phenylenevinylene) (POOXPV) is shown in Figure 10.6. It is soluble in common organic solvents and has a thermal stability up to
336
High Performance Polymers
CF3
N N O
CF3
Poly(phenylene-1,3,4-oxadiazole-phenylene-hexafluoro isopropylidene) N N O Poly[(2,5-diphenylene-1,3,4-oxadiazole)-4,4′-vinylene] (OPPV) O(CH2CH2O)3
OC6H13
N N N
N
O
O
C12H25O OC12H25 N N
O
C6H13O POOXPV
Alternating oxadiazole--alkoxyphenylene polymer
Figure 10.6: Electroluminescent Oxadiazole-Containing Polymers20
Poly(oxadiazole)s
337
400°C.23 Random copolymers can be obtained by allowing a reaction of a mixture of terephthalic and isophthalic acid and hydrazine.24 The molecular weight of the polymers obtained from this route is higher than the molecular weight of polymers obtained from the respective acid chlorides. The polycondensation of terephthalic acid, dimethyl terephthalate, and hydrazine sulfate results in the formation of p-phenylene oxadiazole/ N-methyl hydrazide copolymers.25 The process is conducted in fuming sulfuric acid. The oxadiazole group is already present in 2,2 -(oxydi-4,1-phenylene)bis[5-(4-fluorophenyl)-1,3,4-oxadiazole], c.f. Figure 10.1. The compound can be condensed with various naphthalenedioles, such as 1,5-naphthalenediol to result in PODA types.10 All polymers are amorphous materials and some of them are soluble in aprotic solvents. Alkylated 2,7dibromo-9H-fluorene compounds, with pendent carbazole and oxadiazole units can be condensed with nickel catalysts.26, 27 This type of polymerization is referred to as Yamamoto coupling.28, 29 Polymers prepared by Yamamoto coupling exhibit a higher degree of polymerization than those prepared by the Heck reaction.30
10.2.2 Anionic Polymerization It has been demonstrated that 2,5-bis-(chloromethyl)-1,3,4-oxadiazole can undergo anionic polymerization. With sodium alcoholate, poly(1,3,4-oxadiazole-2,5-diyl-1,2-vinylene) is formed,31 however the reaction cannot be controlled even at temperatures as low as −40°C. Instead, the exothermic reaction can be controlled by performing the polymerization at a toluene/ water interface with tetrabutylphosphonium bromide as a phase transfer catalyst. The mechanism is shown in Figure 10.7. The mechanism of formation is similar to the Gilch polymerization route.32 The resulting polymers exhibit a significantly higher molecular weight and have less structural defects as those prepared by the polycondensation route.
10.2.3 Sulfonation PODAs can be readily sulfonated using sulfuric acid. In contrast, the sulfonation with chlorosulfuric acid trimethylsilyl ester is not successful. A degree of sulfonation from 1.0 to 4.0 can be achieved. Sulfonated PODAs
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High Performance Polymers
N N Cl H2C
O
N N CH2 Cl
Cl HC
O
CH2
N N H2C HC
O
Figure 10.7: Anionic Polymerization of 2,5-Bis-(chloromethyl)-1,3,4-oxadiazole
with C(CF3 )2 moieties are insoluble in water even with a sulfonation degree of 2.0.33
10.3 PROPERTIES Aromatic PODA show a high thermal stability, which is imparted both by the aromatic unit and by the oxadiazole unit. The thermal stability increases with increasing content of p-phenylene moieties in the polymer backbone.24 Further, they exhibit other desirable properties, e.g., good hydrolytic stability, high glass transition temperatures, low dielectric constants, and good mechanical properties. Flexible PODA films can be obtained only if the inherent viscosities of the polymers are higher than 2.7 dl g−1 .24 PODA can be tailored to impart liquid crystalline properties. Fully aromatic PODAs are not soluble in organic solvents but only soluble in strong acids, such as sulfuric acid, chlorosulfonic acid, or methanesulfonic acid. However, by the introduction of substituents as side chains, the solubility is significantly improved towards organic solvents. The electric conductivity of p-PODA at room temperature is 1022 −1 S cm . This corresponds to common insulating polymers in the glassy state. However, above 440 K, ionic conduction is observed.34 Electrically conductive PODAs can be obtained by doping with electron donors or electron acceptors. Some varieties of PODA are semiconductors and exhibit photoconductive properties. Therefore, they are used in electrical applications. PODA films exhibit exceptional gas separation properties.35
Poly(oxadiazole)s
H3C + N N Ar
XN N
Ar O
H2O, H+
O CH3
N N Ar O Oxadiazole
Ar O
Ar = Arylene
Ar
339
O
C N N C H Hyrazide
Figure 10.8: Hyrdrolysis Reaction and Structure of a p-Phenylene oxadiazole/ N-Methyl hydrazide Copolymer
10.4 APPLICATIONS 10.4.1 Fibers Copolymers with p-phenylene oxadiazole and N-methyl hydrazide moieties exhibit high strength and modulus.25 The structure of the random copolymer is shown in Figure 10.8. The remarkable properties make these materials attractive for the reinforcement of articles, such as tires. Yarn fabricated of these copolymers shows no significant degradation under conditions to which a tire cord material is subjected in tire-building and end-use. The performance in vehicle tires and tire cords is competitive with other reinforcing agents, such as glass fibers, steel, and poly(p-phenylene terephthalamide). The fibers are prepared by a wet spinning process, whereby a precursor copolymer with N-alkyloxadiazolium hydrosulfate moieties in sulfuric acid or oleum is extruded into an aqueous coagulation medium. There, a fiber is formed and the copolymer undergoes a hydrolysis reaction into the hydrazide and oxadiazole.36 For this reason, the process is termed reaction spinning. Details of the spinning process have been disclosed. Since the spinnerets and other devices are in contact with concentrated H2 SO4 , they must be made out of acid resistant materials.37
340
High Performance Polymers
10.4.2 Membranes Fluorene-containing sulfonated PODAs have been considered for the application as proton exchange membranes for polymer electrolyte membrane fuel cells.38 The polymers are highly stable thermally and show improved oxidation stability. However, the proton conductivity is around 10−3 S cm−1 , which is by a factor of 50 lower than that of Nafion® membranes.
10.4.3 Sensors By dispersing carbon black into POD-DPE, conductive composites can be obtained. The presence of carbon black enhances the thermal stability. The resistivity decreases continuously with an increase of pressure. Further, the composite shows a typical semiconductor behavior, characterized by an increase of conductivity with temperature.39 The thermal and electrical properties and pressure sensitivity make this compound a good candidate for the application in manufacture of pressure sensors for high ambient temperatures.
10.4.4 Light-Emitting Devices Several oxadiazole derivatives, not necessarily polymers, find use in lightemitting devices.40 This is justified, because the oxadiazole group is one of the best electron transport structures.20 A common material is 2-(4biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), which acts as electron transporting material. PBD is not a polymer, but rather a low-molecular-weight compound. Poly(N-vinylcarbazole) (PVK) acts as hole-transporting polymer. As guest materials, complexes, such as tris(2-phenylpyridine)iridium can be used.41 In combination with organic dyes, mixtures of PVK to PBD in the ratio of 100:40 are used.42 Examples for organic dyes are coumarin with blue light-emission, coumarin 6 with green light-emission and nile red with red light-emission. PBD may exhibit short operating lifetimes due to recrystallization or aggregate formation. This leads to phase separation and formation of charge carrier traps that inhibit the desired emission. Therefore, it has been proposed to bond the PBD electron transporting structure to a polymer chain, which results in amorphous materials. For example, poly(methyl
Poly(oxadiazole)s
341
methacrylate)s with oxadiazole side chains have been reported.43 The oxadiazole group may be part of the polymeric backbone, together with carbazole moieties or phenylene vinylene moieties.14, 17 Crosslinkable oxadiazole monomers with pending acrylic units have been described.7 The synthesis is shown in Figure 10.2. The molecules can be oriented and this orientation can be frozen by a polymerization or crosslinking process, yielding a material with anisotropic properties. Vinyl monomers with a pendent oxazole moiety can be copolymerized with other monomers, e.g. PVK. Examples are 2-phenyl-5-4-[(4vinylphenyl)methoxy]phenyl-1,3,4-oxadiazole and 2-(4-tert-butylphenyl)5-4-[(4-vinylphenyl)methoxy]phenyl-1,3,4-oxadiazole.44 In this way, the charge transport moieties exist in the side groups of the polymers. The copolymerization yields homogeneously statistical copolymers in a wide range of compositions, and thus tunable carrier transport properties. Since the glass transition temperatures of these copolymers are high, there is no possibility for the oxadiazole units to phase-separate through recrystallization. Single layer dye doped devices have been fabricated that emit blue, green, and orange light. Alternatively, oxadiazole-containing dendrimers and starburst compounds have been synthesized that form stable glasses.45 The general methods of how to synthesize this type of polymers are reviewed in the literature.46
10.4.5 Graphite Precursors High aromatic PODA types, such as poly(p-phenylene-1,3,4-oxadiazole), can be graphitized to yield a high quality graphite.47 The graphitization takes place at 2,800–3,000°C. From a heat-treated PODA film at 3000°C, an electrical conductivity of 1.4–1.8 104 S cm−1 has been obtained. Graphite fibers with excellent mechanical properties have been obtained.48
10.5 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 10.2. Tradenames appearing in the references are shown in Table 10.3.
342
High Performance Polymers
Table 10.2: Examples for Commercially Available PODA Polymers Tradename
Producer
Remarks
Oxalon®
Grodno KHIMVOLOKNO
Poly(oxadiazole) fiber
Table 10.3: Tradenames in References Tradename Description
Supplier
Apical® Poly(imide)42 Kapton® Poly(imide)42 Multiposit® XP-9500 Thermoset epoxy resin42 Mylar® (Series) Poly(ethylene terephtalate)42 Oxalon® Poly(oxadiazole) fibers4 Ultem® 6050 Poly(ether imidesulfone)42 Upilex® Poly(imide)42
Kaneka DuPont-Toray Co., Ltd. Shipley Company Inc. DuPont Grodno KHIMVOLOKNO General Electric Ube Industries, Ltd.
Poly(oxadiazole)s
343
10.6 SAFETY Some monomeric compounds containing the oxadiazole moiety are known to be irritants. On the other hand, this class is used in medical applications.
REFERENCES 1. C. J. Abshire and C. S. Marvel. “Some oxadiazole and triazole polymers.” Makromol. Chem., 44-6:388–397, 1961. 2. P. E. Cassidy and N. Fawcett. “Thermally stable polymers: Polyoxadiazoles, polyoxadiazole-N-oxides, polythiazoles, and polythiadiazoles..” J. Macromol. Sci., Rev. Macromol. Chem., C17(2):209–266, 1979. 3. M. J. Nanjan. Polyhydrazides and polyoxadiazoles. In H. F. Mark, N. Bikales, C. G. Overberger, and G. Menges, editors, Encyclopedia of Polymer Science and Engineering, volume 12, pages 332–339. Wiley Interscience, New York, 2nd edition, 1988. 4. B. Schulz, M. Bruma, and L. Brehmer. “Aromatic poly(1,3,4-oxadiazole)s as advanced materials.” Adv. Mater., 9:601–613, 1997. 5. S. G. Kucukguzel, I. Kucukguzel, E. Tatar, S. Rollas, F. Sahin, M. Gulluce, E. De Clercq, and L. Kabasakal. “Synthesis of some novel heterocyclic compounds derived from diflunisal hydrazide as potential anti-infective and antiinflammatory agents.” Eur. J. Med. Chem., 42(7):893–901, July 2007. 6. Y.-P. Luo and G.-F. Yang. “Discovery of a new insecticide lead by optimizing a target-diverse scaffold: Tetrazolinone derivatives.” Bioorg. Med. Chem., 15 (4):1716–1724, February 2007. 7. P. Kirsch and A. Hahn. Oxadiazole derivative and its use as charge transport and light emitting material. US Patent 6 863 841, assigned to Merck Patent GmbH (Darmstadt, DE), March 8, 2005. 8. R.-H. Lee, H.-F. Hsu, L.-H. Chan, and C.-T. Chen. “Synthesis and electroluminescence properties of a novel tetraphenylsilane-oxadiazole-diphenyl(para-tolyl)amine polymer.” Polymer, 47(20):7001–7012, September 2006. 9. D. Gomes, C. Borges, and J. C. Pinto. “Effects of reaction variables on the reproducibility of the syntheses of poly-1,3,4-oxadiazole.” Polymer, 45(15): 4997–5004, July 2004. 10. F. A. Bottino, G. Di Pasquale, and A. Pollicino. “Synthesis and characterization of new poly(arylene ether 1,3,4-oxadiazole)s based on dihydroxynaphthalene isomers.” Polym. Bull., 45:345–350, 2000. 11. C. C. McDowell, J. M. Partin, B. D. Freeman, and G. W. McNeely. “Acetone solubility and diffusivity in poly(ethylene terephthalate) modified with low levels of 2,6-naphthalene dicarboxylic acid, isophthalic acid, and 2,5-bis(4-carboxyphenyl)-1,3,4-oxadiazole.” J. Membr. Sci., 163(1):39–49, October 1999.
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12. J. W. Connell, P. M. Hergenrother, and P. Wolf. Poly(1,3,4-oxadiozoles) via aromatic nucleophilic displacement. US Patent 5 118 781, assigned to Administrator of the National Aeronautics and Space Administration (Washington, DC), June 2, 1992. 13. E. Leibnitz. “Zur Optimierung der Synthese von Poly(p-phenylen-1,3,4-oxadiazol) in Oleum (Optimization of preparation of poly(p-phenylene-1,3,4oxadiazole) in oleum).” Angew. Makromol. Chem., 204:101–110, 1993. 14. P. Denisevich, Jr., A. H. Schroeder, V. P. Kurkov, and S. Suzuki. Carbazoleoxadiazole electroactive polymers. US Patent 4 597 896, assigned to Chevron Research Company (San Francisco, CA), July 1, 1986. 15. D. Gomes, S. P. Nunes, J. Carlos Pinto, and C. Borges. “Synthesis and characterization of flexible polyoxadiazole films through cyclodehydration of polyhydrazides.” Polymer, 44(13):3633–3639, June 2003. 16. D. Gomes, J. Roeder, M. L. Ponce, and S. P. Nunes. “Characterization of partially sulfonated polyoxadiazoles and oxadiazole-triazole copolymers.” J. Membr. Sci., 295(1-2):121–129, May 2007. 17. S. Yin, J. Peng, C. Li, W. Huang, X. Liu, W. Li, and B. He. “Heterocycle-substituted poly(p-phenylene vinylene) for light-emitting devices.” Synth. Met., 93(3):193–195, March 1998. 18. Z. Wang, S. Yin, X. Yang, Z. Sun, X. Xu, and X. Zhang. “Interchain charge-transfer states in poly[(2,5-diphenylene-1,3,4-oxadiazole)-4,4’-vinylene] (O-PPV) oligomer.” Chem. Phys. Lett., 307(1-2):75–80, June 1999. 19. J. Ding, M. Day, G. Robertson, and J. Roovers. “Synthesis and characterization of alternating copolymers of fluorene and oxadiazole.” Macromolecules, 35(9):3474–3483, April 2002. 20. L. Akcelrud. “Electroluminescent polymers.” Prog. Polym. Sci., 28(6):875– 962, June 2003. 21. X. Yang, Y. Hua, S. Yin, Z. Wang, Y. Hou, Z. Xu, X. Xu, J. Peng, and W. Li. “A novel oligomer poly(phenylene vinylene) derivative containing oxadiazole segment.” Synth. Met., 111-112:455–457, June 2000. 22. Q. Pei and Y. Yang. “Bright blue electroluminescence from an oxadiazolecontaining copolymer.” Adv. Mater., 7(6):559–561, 1995. 23. S.-Y. Song, M. S. Jang, H.-K. Shim, I.-S. Song, and W.-H. Kim. “New soluble light-emitting diode polymer containing oxadiazole unit.” Synth. Met., 102(1-3):1116–1117, June 1999. 24. E. R. Hensema, J. P. Boom, M. H. V. Mulder, and C. A. Smolders. “Two reaction routes for the preparation of aromatic polyoxadiazoles and polytriazoles: Syntheses and properties.” J. Polym. Sci., Part A: Polym. Chem., 32 (3):513–525, 1994. 25. H. C. Bach, F. Dobinson, K. R. Lea, and J. H. Saunders. “High-strength/highmodulus fibers of p-phenylene oxadiazole/n-methyl hydrazide copolymers – a new class of high-performance organic materials.” J. Appl. Polym. Sci., 23 (7):2125–2131, 1979.
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26. Y. Jin, J. Y. Kim, S. H. Park, J. Kim, S. Lee, K. Lee, and H. Suh. “Syntheses and properties of electroluminescent polyfluorene-based conjugated polymers, containing oxadiazole and carbazole units as pendants, for LEDs.” Polymer, 46(26):12158–12165, December 2005. 27. S.-J. Lee, J. R. Gallegos, J. Klein, M. D. Curtis, and J. Kanicki. “Poly(fluorene-oxadiazole) copolymer-based light-emitting devices on a plastic substrate.” Synth. Met., 155(1):1–10, October 2005. 28. T. Yamamoto. “Electrically conducting and thermally stable π -conjugated poly(arylene)s prepared by organometallic processes.” Prog. Polym. Sci., 17 (6):1153–1205, 1992. 29. A.-D. Schlüter and G. Wegner. “Palladium and nickel catalyzed polycondensation - the key to structurally defined polyarylenes and other aromatic polymers.” Acta Polym., 44(2):59–69, 1993. 30. J. A. Mikroyannidis. “Synthesis, characterization and photophysics of novel conjugated polymers with 1,3,4-oxadiazole pendant on a vinylene unit.” Synth. Met., 145(2-3):271–277, September 2004. 31. R. Zhang, R. Jordan, and O. Nuyken. “Preparation of poly(1,3,4-oxadiazole2,5-diyl-1,2-vinylene) via anionic mechanism.” Macromol. Rapid Commun., 24:246–250, 2003. 32. J. Wiesecke. Untersuchungen zum Polymerisationsmechanismus der GilchReaktion. PhD thesis, Technische Universität Darmstadt, Darmstadt, 2004. 33. S. Vetter and S. P. Nunes. “Synthesis and characterization of new sulfonated poly(arylene ether 1,3,4-oxadiazole)s.” React. Funct. Polym., 61(2):171–182, September 2004. 34. T. Tsutsui, Y. Fukuta, T. Hara, and S. Saito. “Electronic conduction in poly(p-phenylene-1,3,4-oxadiazole) films.” Polym. J. (Tokyo), 19:719–725, 1987. 35. B. Gebben. Thermally stable and chemically polymer membranes-aromatic polyoxadiazoles and polytriazoles. PhD thesis, Twente University, Enschede, Holland, 1988. 36. H. C. Bach. Novel process for the preparation of fiber of arylene oxadiazole/arylene N-alkylhydrazide copolymer. US Patent 4 115 503, assigned to Monsanto Company (St. Louis, MO), September 19, 1978. 37. H. C. Bach. Fibers of arylene oxadiazole/arylene N-alkylhydrazide copolymer. US Patent 4 202 962, assigned to Monsanto Company (St. Louis, MO), May 13, 1980. 38. X. Y. Shang, D. Shu, S. J. Wang, M. Xiao, and Y. Z. Meng. “Fluorene-containing sulfonated poly(arylene ether 1,3,4-oxadiazole) as proton-exchange membrane for pem fuel cell application.” J. Membr. Sci., 291(1-2):140–147, March 2007. 39. F. G. Souza, Jr., M. E. Sena, and B. G. Soares. “Thermally stable conducting composites based on a carbon black-filled polyoxadiazole matrix.” J. Appl. Polym. Sci., 93(4):1631–1637, March 2004.
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40. S. H. Chen, A. C.-A. Chen, J. U. Wallace, and L. Zeng. Light-emitting organic materials. WO Patent 2 007 016 454, assigned to Univ. Rochester (US); Chen Shaw H (US); Chen Andrew Chien-an (US); Wallace Jason U (US); Zeng Lichang (US), February 08, 2007. 41. S. Seo, M. Murakami, and S. Yamazaki. Organic light emitting element and light emitting device using the element. US Patent 7 199 515, assigned to Semiconductor Energy Laboratory Co., Ltd. (Kanagawa-ken, JP), April 3, 2007. 42. A. R. Duggal and J. D. Michael. Method for making an OLED device. US Patent 7 198 533, assigned to General Electric Company (Niskayuna, NY), April 3, 2007. 43. N. N. Barashkov, T. V. Sakhno, N. N. Alekseev, T. M. Murav’eva, L. M. Bulgakova, and L. A. Gorbunov. “Spectral and luminescence properties of copolymers of poly(methy methacrylate) and poly(allyl carbonate) with fragments of anthracene and diphenyloxadiazole.” Zh. Prikl. Spektros., 53:386– 391, 1990. 44. X. Jiang, R. A. Register, K. A. Killeen, M. E. Thompson, F. Pschenitzka, and J. C. Sturm. “Statistical copolymers with side-chain hole and electron transport groups for single-layer electroluminescent device applications.” Chem. Mater., 12(9):2542 – 2549, 2000. 45. J. Bettenhausen, M. Greczmiel, M. Jandke, and P. Strohriegl. “Oxadiazoles and phenylquinoxalines as electron transport materials.” Synth. Met., 91(1-3): 223–228, December 1997. 46. K. Inoue. “Functional dendrimers, hyperbranched and star polymers.” Prog. Polym. Sci., 25(4):453–571, May 2000. 47. H. Yasujima, M. Murakami, and S. Yoshimura. “Electrical properties of pyrolytic polyoxadiazole.” Synth. Met., 18(1-3):527–530, February 1987. 48. M. Murakami, K. Watanabe, and S. Yoshimura. Production of graphite fiber. JP Patent 63 256 721, assigned to Japan Res Dev Corp; Matsushita Electric Ind Co. Ltd, October 24, 1988.
11 Poly(naphthalates) The industrial synthesis methods of polyesters trace back to Carothers. Poly(ethylene terephthalate) was discovered by Whinfield and Dickson in the 1940s.1–3 At the same time, Poly(ethylene naphthalate) was described in the literature.4 The industrial production started soon afterwards by ICI and DuPont.
11.1 MONOMERS 11.1.1 Naphthalenedicarboxylic acid Crude 2,6-naphthalene dicarboxylic acid (2,6-NDA) can be prepared by oxidizing 2,6-dialkylnaphthalenes in the liquid phase with molecular oxygen in the presence of a transition metal catalyst and an oxidation promoter. Typically, such catalysts include mixtures of cobalt and manganese promoted with bromine as an oxidation promoter. The 2,6-NDA prepared by the process, contains impurities, such as trimellitic acid (TMLA), and aldehydes. Typical amounts of impurities are shown in Table 11.1. TMLA is produced by the oxidation of one of the rings of the 2,6-dimethylnaphthalene molecule. 2-Formyl-6-naphthoic acid results from the incomplete oxidation of one of the methyl groups of the 2,6-dimethylnaphthalene molecule.5 When bromine is used as an oxidation promoter, the bromination of the naphthalene ring occurs during the oxidation reaction and results in the formation of bromonaphthalenedicarboxylic acid. The loss of one methyl 347
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High Performance Polymers Table 11.1: Impurities in Crude Naphthalenedicarboxylic Acid6 Compound
Amount/[ppm]
Cobalt Manganese Trimellitic acid 2-Formyl-6-naphthoic acid
140 676 130 5,590
or carboxylic acid substituent during the oxidation reaction results in the formation of 2-naphthoic acid. If the crude 2,6-NDA is used as a starting material for preparing poly(ethylene naphthalate) (PEN), the resulting PEN is occasionally colored. Mold staining may take place in the molding process to decrease transparency of the molded products, resulting in a lower product quality. In order to obtain high quality PEN, the crude 2,6-NDA needs to be purified before it is used as a starting material for preparing PEN.6 2,6-NDA can be purified by esterification with methanol and allow to fractionally crystallize the dimethyl-2,6-naphthalene dicarboxylate (NDC) and monomethyl-2,6-naphthalene dicarboxylate. Another purification method consists in dissolving the crude material acid in water in a supercritical or subcritical state, and recrystallizing. A two-stage condensation process is simpler, in which an oligomer is produced that is purified before the second condensation to the high molecular product.6 The esterified partial oxidation products formed during the esterification process have significantly lower boiling points than the esters of the dicarboxylic acid. Thus, without any intervening chemical treatment, such as hydrogenation or treatment with sulfite, crude aryl dicarboxylic acids can be purified by esterification followed by distillation. The purified dicarboxylic acid esters can then be subjected to direct polyesterification to produce the polyester resin.5 The impurities distilled from the esterification effluent can be recycled to the oxidation reactor where they act as oxidation promoters, thereby optionally allowing for a bromine-free oxidation process for substituted aryl hydrocarbons.
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11.2 POLYMERIZATION AND FABRICATION Conventional and novel processes for the polymerization of polyesters as well as the properties of this class of polymers have been reviewed by Pang.2 Conventional methods of polycondensation run via the transesterification of the dimethyl ester with diols, or via direct esterification of the diacids with the diols. More recently developed routes try to remove the cyclic oligomers that are formed during polycondensation. Additionally, there is a route, which converts the cyclic oligomers to high-molecularweight linear polymers by ring-opening polymerization methods.
11.2.1 Poly(ethylene naphthalate) PEN was first described in 1948.4 Commercially, PEN is prepared by a two-step reaction consisting of:7 1. Esterification between 2,6-naphthalene dicarboxylic acid and glycol to obtain a low-molecular-weight esterified compound, and 2. Polycondensation reaction of the esterified compound to obtain a higher molecular weight PEN product. Either NDC or the acid itself can be used. Using the dimethyl ester, zinc acetate or manganese acetate as a catalyst at a reaction temperature of 180–260°C is needed to produce (β -hydroxyethyl)naphthalate, or its low-molecular-weight prepolymer. When the acid is used, no catalyst is needed for the first step of esterification. After the esterification, the low-molecular-weight esters can be readily polycondensed in the presence of a polymerization catalyst, such as antimony trioxide at a reaction temperature of 280–300°C at a reduced pressure of less than 1 torr to produce a high-molecular-weight polymer.7 Since PEN has naphthalene rings in the molecular structure and a higher melt viscosity than poly(ethylene terephthalate) (PET), it requires a higher polymerization temperature in comparison to PET. Therefore, PEN is more subject to discoloring by impurities and oxidation than PET. Several variations of the process of manufacturing PEN have been suggested. On an industrial scale, if the plants are nearby, the process for producing 2,6-NDA can be coupled with the process for producing PEN. This eliminates some intermediate steps, such as drying 2,6-NDA and handling solid 2,6-NDA.8
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11.2.1.1
Esterification
Completing the esterification reaction takes a prolonged period of time. The esterification reaction may be accompanied by the formation of reaction products, which, for example, deteriorate the appearance of obtained PEN molding. Therefore, accelerating the esterification reaction enables not only shortening the production time for PEN but also reducing the formation of reaction products, which deteriorate the quality of the final product, so that its technological value is magnificent. The reaction of 2,6-NDA with ethylene glycol leads to the formation of byproducts such as etherified oligomers carrying the diethylene glycol unit. These moieties deteriorate the quality of product, for example, the appearance of obtained PEN. In order to reduce the diethylene units in the condensate, the condensation can be conducted in a two-stage process. In the first step, 2,6-NDA is condensed with ethylene glycol in the presence of water in an autoclave under a nitrogen pressure of 10 kp cm−2 at 250°C for 2 h. Then the autoclave is cooled and the liquid mixture of crystallized product is recovered. The crystallized product consists of NDA, the monoester 2-carboxyl-6-hydroxyethoxycarbonylnaphthalene, the diester 2,6-bis-(hydroxyethoxycarbonyl)naphthalene and some oligomers. If in the first step of condensation the autoclave is pressurized not only with nitrogen, but also partially with hydrogen, then impurities in the NDA are hydrogenated.6 The hydrogenated impurities are soluble in an ethylene glycol aqueous solution from which the oligo esters can be recrystallized. Still another method is to allow the aldehydes, which are contained in the crude 2,6-NDA, to react with a sulfite to give aldehyde adducts, and then dissolving the aldehyde adducts in the alcohol aqueous solution.6 An alternative procedure has been proposed that continuously feeds a slurry with the reactants, but are made up of methanol instead of water to accelerate the esterification step.7 The process is possible even at atmospheric pressure. 11.2.1.2
Polycondensation
The recovered crystallized product from the intermediate purification autoclave process9 is further esterified as usual with ethylene glycol at 260–290°C, where the emerging water is distilled off.
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Side reactions are the formation of dehydration of the pendent 2-hydroxyethyl ester to result in pendent vinyl ester groups. These can further react under the ejection of acetaldehyde. Suitable catalysts are germanium compounds, such as germanium oxide, zinc acetate, manganese acetate,9 or a combination of antimony trioxide and trimethyl phosphate.7 However, there are environmental and public health concerns about the degree of metal loading and the use of heavy metal, including antimony-based catalysts in the manufacture of food-grade polymeric packaging materials.10 11.2.1.3
Crystallization
PET and PEN, produced by melt-phase polymerization, are almost completely amorphous in nature. PEN can crystallize either from the glassy state or from the melt state. An α -crystal modification of PEN is obtained when PEN crystallizes from the glassy state, whereas a β -crystal modification is obtained when PEN crystallizes from the melt state at a higher temperature. However, the α -crystal modification can also be obtained when PEN is crystallized from a low-temperature melt.11 The addition of a nucleating agent, sodium benzoate (SB), affects the crystal modification and melting behavior of PEN when PEN/SB is crystallized at a higher temperature, but not at a lower temperature. A mixture of α -crystals and β -crystals of PEN is obtained. An overlapped dual melting peak is observed in differential scanning calorimetry (DSC) curves when PEN is crystallized at a higher temperature in the presence of SB, instead of a single crystal form and a single melting peak for the crystallization of pure PEN.12 Amorphous polyester polymers are usually converted from the amorphous state to the crystalline state prior to solid state polymerization to raise their sticking temperature. This is performed in order to keep pellets or chips of the polyester prepolymer from sticking together as a solid mass when later handled at an elevated temperature.13 When an amorphous polyester is heated from an ambient temperature to above its glass transition temperature Tg , it will become soft and sticky before it starts to crystallize. The sticking temperature of an amorphous polyester is usually about 20°C above its Tg . The crystallization rate of the polyester will not be fast enough for technical operations until its temperature is further raised to about 30°C above its sticking temperature. To achieve the maximum crystallization rate, the temperature of the polyester
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must be raised even higher. PET has a Tg of 74°C and a sticking temperature of about 95°C. The crystallization rate of PET is low until the temperature is raised to above 125°C, In practice, PET is usually crystallized at temperatures between 150–190°C. PEN has a Tg of about 120°C and a crystalline melting point Tm of 270°C. It exhibits a crystallization peak between 180°C and 220°C. Its sticking temperature is about 140–150°C, when it is in the amorphous state. The recommended crystallization temperature range for PEN is 180–220°C. In the crystallization process, the polyester undergoes a sticky stage. This takes place in the period between the time the polyester temperature exceeds the sticking temperature and the time the polyester becomes well crystallized. Therefore, most commercial crystallizers for continuous crystallization of polyesters must provide vigorous agitation to prevent agglomeration or lumping of the polyester pellets. Two types of continuous crystallizers have been widely used, namely, agitated vessels and fluidized beds. However, the prepolymer may adsorb moisture from the atmosphere at ambient conditions during pelletizing and other operations. The moisture or water content of the prepolymer constitutes the major volatile component, which must be accounted for during the heat-up of the pellets during crystallization. When PEN pellets are exposed to the required crystallization conditions, the pellets undergo a sudden and rapid expansion as they are heated to near the crystallization temperature. This results in a puffed up skin of most of the pellets, which become very sticky. Within seconds, the pellets agglomerate tightly into big lumps, not withstanding vigorous agitation. To avoid the puffed up skin, PEN pellets may be crystallized at a pressure at as high, or higher than, the vapor pressure of the volatile components contained in PEN pellets. Thereby the deformation of the PEN pellets during crystallization is avoided.13 Supercritical Carbon Dioxide Treatment. The crystallization process can be influenced by the treatment with supercritical carbon dioxide.14 A higher-order structure with fine crystallites is obtained at in a temperature range, of 110–170°C. After the treatment, the glass transition temperature decreases by more than 50°C and the PEN films are crystallized. The large
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353
8 7
t1/2 [min]
6 5 4 3 2 1
PEN PEN+3% Ceraflour 933
0 160
180
200
220
240
T oC Figure 11.1: Half Times of Crystallization Rate viz. Crystallization Temperature of PET and PEN with Additive15
decrease in Tg is attributed to the sorption of the CO2 molecules into the PEN film. An increase in the CO2 pressure increases the amount of absorbed CO2 and reduces the Tg , which can promote crystallization. The crystallite size decreases with decreasing treatment temperature. It is suggested that the CO2 treatment promotes the creation of nuclei in the amorphous state at low temperatures, followed by the formation of fine crystallites. Additives. Certain additives, such as a low molar mass poly(ethylene), Ceraflour 991™, a low molar mass poly(amide), Ceraflour 993™, and poly(1,4-butylene sebacate) substantially accelerate the crystallization of PEN at both low and high temperatures.15, 16 The crystallization rate was further investigated by the measurement of t1/2 in DSC experiments. The t1/2 represents the overall crystallization rate and is determined by the rates of nucleation and linear growth. A plot of the half-life times against the crystallization temperature is shown in Figure 11.1. The nucleation rate of primary nuclei is controlled by the free en-
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thalpy of crystallization of a nucleus of critical size (ΔG ) and the free energy of activation (ΔGη ), which governs the diffusion of polymer segments across the phase boundary. In the high-temperature region, ΔG dominates controlling the nucleation rate, whereas ΔGη governs the nucleation rate in the low-temperature region for cold crystallization. At low temperatures, the action is attributed to an improvement of the molecular motion of PEN. Namely, the glass transition temperature decreases by the addition of the additives. At high temperatures, it is supposed that a phase separation occurs between additive and PEN. The separated droplets of the additives could serve as heterogeneous nuclei to initiate primary nucleation. Avrami Equation. The kinetics of crystallization is often described by the Avrami equation.17 1 − φ = exp(−kt n ). φ k t n
(11.1)
Volume fraction of crystalline material Kinetic constant Time Exponent of the Avrami equation
Eq. 11.1 can be rearranged into ln ln
1 = ln k + n lnt. (1 − φ )
During the isothermal crystallization of PEN, a relatively high crystallinity is achieved. The rate can be described by the Avrami equation with the exponent n = 2.5. The activation energy for isothermal crystallization is determined to be 250 kJ mol−1 .18 11.2.1.4
Solid State Polymerization
After the PEN prepolymer has been crystallized, it can be dried and solid state polymerized in a batch or continuous process.13 Solid stating or solid state polymerization is a technique to increase the molecular weight of polyesters. Higher molecular weight polyesters are commonly produced from lower molecular weight polyesters of the same composition by solid state polymerization.
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Solid state polymerization of prepolymers is generally considered advantageous in that the handling of high-molecular-weight ultra-high viscosity molten polymers is eliminated during the polymerization phase. Thermal degradation is also essentially avoided during solid state polymerization of the prepolymers. In melt polymerizations, the major part of the reaction is transesterification due to the fact that thermal pyrolysis nullifies much of the esterification of carboxyl end groups. A much larger percentage of the reaction in a solid state polymerization is esterification.19 The esterification involves residual carboxyl groups and hydroxyl groups. Another type of reaction is the addition of residual hydroxyl groups to vinyl ester groups. During prolonged solid state heating, the polymer chains aggregate into supermolecular structures known as spherulites, which are in some ways similar to grain structures in metals. Spherulites grow radially from a point of nucleation until other spherulites are encountered. Big spherulites cause brittleness and reduction in tensile strengths of polymers. The size of the spherulites can be controlled by the number of nuclei present with more crystalline nuclei resulting in more but smaller spherulites. Short heat times also result in smaller spherulites. The absence of large spherulites results in stronger, less brittle polymers.20 Suitable solid state polymerization temperatures can range from a temperature just above the threshold temperature of the polymerization reaction up to a temperature within a few degrees of the sticking temperature of the PEN prepolymer. In the solid state polymerization of crystalline PEN, the temperature employed ranges from 240°C to about 265°C. As the solid state polymerization of PEN prepolymer proceeds, its sticking temperature increases. Thus, the solid state polymerization temperature can be gradually increased in the course of the process.13 The solid state polymerization is conducted in the presence of a stream of an inert gas or under a vacuum. The reactor will be designed in such a way that the inert gas will flow homogeneously through the polyester prepolymer. The intrinsic viscosity of the prepolymer measured in 60:40 phenol:tetrachloroethane will typically have 0.2 dl g−1 and will rise to about 0.8 dl g−1 . The residence time is from 6 to 24 h. The use of a foamed PEN prepolymer, combined with a devolatilization step prior to solid state polymerization, provides a particularly fast and productive solid state polymerization process for a PEN polymer. Us-
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ing a foamed PEN prepolymer allows a lower solid state polymerization temperature.21 A rapid high-temperature solid state polymerization of crystalline thermoplastic polymers may be done under conditions of mechanically induced surface stress and friction applied to polymer particles at the incipient melt point temperatures.20 This technique demands a quenching of the polymers to temperatures below the glass transition temperatures by a direct contact evaporative cooling with cryogenic liquids. Rapid high-temperature solid state polymerization provides the preparation of high-molecular-weight polymers exhibiting high intrinsic viscosities and superior mechanical properties. The method substantially eliminates chemical degradation of the polymers. The immediate direct quenching prevents the growth of large spherulites, which cause polymer brittleness and degrade mechanical properties of the solid state polymerized polymers. The method has been exemplified with poly(amide)s (PA)s and poly(butylene terephthalate) (PBT). The rate of solid state polymerization for materials that do not contain antimony catalysts or germanium catalysts from the previous steps of preparation may be increased by adding a catalytic amount of zinc p-toluene sulfonate.10 Transesterification. In 1:1 blends of PET and PEN, for transesterification levels higher than 23%, the blends tend to transform into a one-phase system and the cold crystallization of PET is strongly inhibited due to the significant reduction of the PET segment length. For lower levels of transesterification the blends are phase separated.22 The transesterification reaction has been studied for 1:1 blends of poly(pentylene terephthalate) (PPT) with PEN. Blends of PEN and PPT are initially immiscible. On heating or annealing at temperatures of 300°C for a prolonged time, the original two phases merge into one single phase composed of two polyesters and some minor fractions of copolyesters. The gain of miscibility is not to transesterification as can be confirmed by nuclear magnetic-resonance spectroscopy (NMR) experiments. X-ray analysis shows that the blend completely loses its ability to crystallize only when heated at 300°C for a time of 60 min or longer, indicating formation of fully random copolyesters.23 The kinetics of transesterification reaction in PET/PEN blends was
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357
studied by the model compounds ethylene dibenzoate (BEB) and ethylene dinaphthoate (NEN). The exchange reaction between BEB and NEN was followed by 1 H-NMR spectroscopy.24 The overall transesterification reaction found to follow a second-order law. The reversibility of the transesterification was confirmed by heating a mixed sequence of 1-benzoate 2-naphthoate ethylene (BEN). Both forward reaction of the equimolar amounts of the reagents and reverse reaction come to equilibrium at the same molar ratio of the reactants and reaction products of roughly 0.25:0.50:0.25 for BEB, BEN, and NEN, respectively. Transesterification reactions can be promoted by electron beam irradiation.25 The transesterification reaction can be written formally as −NEN− + −TET ←→ −NET− + −TEN−
(11.2)
Here E is an ethylene glycol unit, T is a terephthalate unit and N is a naphthalate unit. If the initial mole fractions of NEN and TET units are a and b, with a + b = 1 and the mole fraction x of the copolymer NET, then, assuming a second-order reversible reaction the kinetic law: dx = k1 (a − x)(b − x) − k2 x2 dt
(11.3)
emerges. In equilibrium dx/dt = 0 and x = xe . Setting k = k1 = k2 , and inserting in Eq. 11.3 a simpler form can be obtained, dx = k(xe − x). (11.4) dt Setting the transesterification ratio r = x/a and integrating Eq. 11.4 the following relation can be obtained: ln
b = kt. b−r
(11.5)
The extent of transesterification can be obtained by NMR techniques. Thus, the kinetic constant k = k1 = k2 can be obtained by a plot of − ln[(b − r)/r] against the reaction time t. A linear relationship is obtained for different blend compositions, indicating that the assumed mechanism is valid.26 The rate constants obtained for different initial mol fractions of NDA at 300°C are exemplified in Table 11.2.
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High Performance Polymers Table 11.2: Rate Constants for Transesterification at 300°C26 mol-% NDA
k/[min−1 ]
3.2 13.6 31.2 56.3 78.2
0.08 0.07 0.05 0.08 0.11
At 290°C, the rate constant is 0.050 min−1 and at 310°C, the rate constant is 0.141 min−1 . From these data, an energy of activation of 140.4 kJ mol−1 is calculated. Degree of Polymerization. By monitoring the intrinsic viscosity as a function of time at various solid state temperatures, an empirical relation for the change of the molecular weight has been established,27 i.e. √ Mn = Mn,0 + k t. Mn Mn,0 k t
(11.6)
Number average of molecular weight at time t Number average of molecular weight at initial time Kinetic constant Time
The Arrhenius parameters can be obtained when conducting the experiments at different temperatures. Thus, for PEN the relation √ 27922.83 4 6 ×k t Mn = 1.33 × 10 + 1.78 × 10 × exp − RT are obtained and for PET28 √ 22800 ×k t Mn = 1.99 × 10 + 624 × 10 × exp − RT
4
10
is obtained.
11.2.2 Copolymers In copolymers of poly(ethylene terephthalate-co-naphthalate)s with low amounts of naphthalate, a melting point depression is observed, while the
Poly(naphthalates)
359
glass transition temperatures are higher than that of PET. The crystallization rates of the copolymers decrease with increasing comonomer content. The tensile properties of the copolymers with 3–4% of naphthalate are significantly improved compared to PET. Thus, the properties of PET can be improved with the use of small amounts of naphthalate, with no significant increase of cost.29 Random poly(ethylene-co-butylene 2,6-naphthalate) copolymers are crystallizing over the entire range of composition30 and they exhibit a eutectic point. The copolymers form either exclusively PEN-type crystals or poly(butylene naphthalate) (PBN)-type crystals. A transition from the PEN-type crystal to the PBN-type crystal occurs approximately at a ratio of 1:1 of the alcohol components in the copolymer.31 This phenomenon is addressed as isodimorphic co-crystallization. Poly(1,4-cyclohexylenedimethylene terephthalate-co-1,4-cyclohexylenedimethylene 2,6-naphthalate) (P(CT-co-CN)) copolymers behave similarly with respect to crystallization behavior than poly(ethylene-co-butylene 2,6-naphthalate) copolymers.32 Poly(ethylene 2,6-naphthalate-co-1,4cyclohexylenedimethylene 2,6-naphthalate) (P(EN-co-CN)) copolymers do not crystallize in the middle of copolymer composition, whereas poly(butylene 2,6-naphthalate-co-1,4-cyclohexylenedimethylene 2,6-naphthalate) (P(BN-co-CN)) and poly(hexamethylene 2,6-naphthalate-co-1,4-cyclohexylenedimethylene 2,6-naphthalate) (P(HN-co-CN)) copolymers exhibit clear melting and crystallization peaks. This behavior indicates that both P(BN-co-CN) and P(HN-co-CN) copolymers exhibit a co-crystallization behavior. P(BN-co-CN) copolymers exhibit a eutectic melting and isodimorphic co-crystallization. In contrast, the melting temperature of P(HN-co-CN) copolymer increases continuously with increasing CN content without showing a eutectic melting temperature. This indicates that P(HN-co-CN) copolymer shows isomorphic co-crystallization.33, 34 Crystallization modifiers like isophthalic acid (IPA) and 1,4-cyclohexanedimethanol (CHDM) are often copolymerized into PET and PEN polyesters to form copolyesters that have better processing properties. We will classify terephthalate and naphthalate polyesters as poly(arylate)s. The preferred synthesis route for these copolymers is transesterification. There are two steps in the preparation of poly(ethylene-1,4-cyclohexanedimethylene arylate) (PECA). The first is the formation of 2,6-bis-(hydroxyethyl)arylate (BHEA)
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High Performance Polymers
and bis-(hydroxymethylcyclohexane)-arylate (BHCA), respectively, from the transesterification of 2,6-dimethyl naphthalate or dimethyl terephthalate (DMT) with ethylene glycol or CHDM. The second step is formation of PECA from polycondensation of the BHEA and BHCA mixture at elevated temperatures and reduced pressure.35 Modest levels of IPA slow down crystallization and raise the oxygen-barrier properties. Higher levels of IPA break up crystallinity and lead to amorphous copolyesters with good barrier properties. On the other hand, these copolyesters, show poor impact and other mechanical properties. However, modest levels of CHDM slow down crystallization and decrease oxygen-barrier properties. Higher levels of CHDM are well known to form families of amorphous copolyesters. These materials are widely used in commerce in a multitude of applications including heavy gauge sheet, signage, medical packages, etc. The copolyesters have excellent impact resistance and other mechanical properties, but have lower oxygen-barrier properties than IPA modified copolyesters and lower oxygen-barrier properties than PET.36 In copolymers composed of PEN and poly(ethylene-2,7-phenanthrate) (PEP) the glass transition temperatures (Tg ) increased with increasing PEP component.37 11.2.2.1
Aromatic Dihydroxyethoxy Compounds
Copolyesters of 2,6-naphthalates and aromatic dihydroxyethoxy derivatives, c.f., Table 11.3, have better solubilities than PEN or PBN in aprotic solvents. The thermal properties and the moisture absorption property of the copolyesters are comparative or superior to those of PEN or PBN.38 Due to improved solubility, the copolyesters have useful applications, such as paints, varnishes, and structural adhesives. Several comonomers listed in Table 11.3 have been synthesized, built, and used as comonomers. The dihydroxyethoxy compounds were prepared reacting e.g. 1 mol of bisphenol A with 2.1 mol ethylene carbonate in the presence of potassium iodide at 80–120°C. Yields of product of more than 70% were usually obtained. The copolyesters were synthesized using the products mentioned in Table 11.3 and 2,6-bis-(hydroxyethyl) naphthalate, or 2,6-bis-(hydroxybutyl) naphthalate, respectively, by a melt condensation technique, distilling off ethylene glycol.38 Zinc acetate and antimony trioxide were used as
Poly(naphthalates)
361
Table 11.3: Comonomers for Naphthalate Polyesters38 Compound Bis-(2-hydroxyethyl)-bisphenol A Bis-(2-hydroxyethyl)-hydroquinone Bis-(2-hydroxyethyl)-biphenol Bis-(4-(2-hydroxyethoxy)benzene)-ether Bis-(4-(2-hydroxyethoxy)benzene)-sulfone Bis-(2-hydroxyethyl)-bisphenol H Bis-(4-(2-hydroxyethoxy)benzene)-fluorene
mp. °C 110–111 105–106 210–211 119–121 180–181 109–110 126–128
catalysts. The copolyesters were extensively characterized with respect to intrinsic viscosities, solubilities, thermal properties, and, moisture absorption. Details can be found in the literature.38 11.2.2.2
Poly(ether ester) elastomers
Block poly(ether ester) (PEE) elastomers are synthesized with DMT and NDC. Alcohol components are 1,4-butanediol, and poly(tetramethylene ether) glycol of a molecular weight of 1000. Poly(tetramethylene ether glycol terephthalate) units are functional as the soft segment in the block structures. The introduction of the 2,6-NDA group into the hard segment, consisting of terephthalate, 2,6-naphthalene dicarboxylate, and 1,4-butanediol units, improves the UV resistance of the resulting PEE.39 11.2.2.3
Terpolymers
Semi-crystalline polyesters containing terephthalate, isophthalate, and naphthalate moieties exhibit excellent gas barrier properties. A high density level is achieved by a combination of strain-induced crystallization and thermal crystallization. Ethylene glycol, 2,6-NDA, terephthalic acid (TPA), and IPA are condensed using 10% aqueous tetramethylammonium hydroxide solution. Antimony trioxide and cobalt acetate are used as catalysts. The reaction is conducted in an inert atmosphere initially under pressure, up to 260°C, then the pressure is reduced in steps and the temperature is increased up to 274–288°C. The polymer was extruded into strands by the use of a melt pump on the bottom of the reactor. The strands were cooled in a water bath and
362
High Performance Polymers
chopped into amorphous pellets. The pellets had a 0.60 dl g−1 inherent viscosity in 60/40 phenol/tetrachloroethane at 30°C. After pelletization, the molecular weight of the polymer can be increased by solid state polymerization. A density of 1.455 g cm−3 can be achieved by a combination of straininduced crystallization and thermal crystallization. In strain-induced crystallization the polyester material is stretched at a suitable rate and temperature to achieve crystallization within the polyester. With polyester compositions dealt with here, typical temperatures are about 80°C–140°C; typical stretch rates are about 300 to about 1500%/s. The stretch ratio suitable is about 8 to about 24. For fiber stretching or orientation, the stretch ratio of about 2 to about 8 is suitable. Molded articles, in particular containers, such as bottles and jars, made from the polyester resin copolymers can be manufactured by using melt molding methods. Containers and bottles can be manufactured without the necessity of adding crystallization accelerators or crystallization retardants. The materials have low levels of acetaldehyde.40
11.2.3 Blends 11.2.3.1
Blends of PET and PEN
Adding PEN to a PET polymer increases the thermal performance. However, in a wide range of composition, the blend is substantially amorphous, which means the material cannot be crystallized. In a stretch blow molded article, crystallization is needed because it provides the necessary levels of orientation, and barrier properties, and controls the material distribution. There is also a problem with incompatible phases rendering the article opaque. PET/PEN blends can be subjected to a solid state polymerization to increase the intrinsic viscosity or to reduce the acetaldehyde generation. In the course of the solid state polymerization, the extent of transesterification is increased. A low level of transesterification of the PEN/PET preform causes poor transparency, while a high level of transesterification prevents strain-induced crystallization and poor mechanical properties. Transesterification can be measured by NMR. There, the signal intensities of the ethylene protons associated with 2,6-naphthalene dicarboxylate/ethylene glycol terephthalate sequences, compared to the signals
Poly(naphthalates)
363
of a completely random copolymer made with from 2,6-NDA, TPA, and ethylene glycol are compared. Since the molecular weight increases during heat treatment, both condensation and the transesterification should be taken into account.41 The addition of a phosphite stabilizer Bis-(2,4-di-tert-butylphenyl)pentaerythritol diphosphite (Ultranox 626™) reduces the extent of transesterification during solid state polymerization. Triphenyl phosphite was expected to inhibit transesterification, but this proposition could not be confirmed experimentally.42 In contrast, the addition of 2,2 -bis-(l,3-oxazoline) (BOZ) to blends of PET and PEN can significantly accelerate the transesterification between PET and PEN at 275°C.43 The activation energy of the transesterification reaction for a PET/PEN reactive blend with BOZ of 94.0 kJ mol−1 is lower than that without BOZ of 168.9 kJ mol−1 .44 It is possible to control both the rate of change of intrinsic viscosity (IV) and the rate of transesterification of a blend of PET and PEN during solid state polymerization. The method comprises providing PEN with a first IV and a PET with a second IV. The PEN and PET are reacted in the presence of an ethylene glycol compound in an amount sufficient to achieve a desired final IV and final level of transesterification in the copolymerized PEN/PET product.45 Due to the improved thermal resistance, the materials can be used for hot fill containers. 11.2.3.2
Blends of PBT or Poly(amide) and PEN
PBT/PEN blends exhibit an enhancement in the mechanical properties. Blends of PA 6.6 and PEN become more brittle than the constituent homopolymers. No chemical reaction occurs in both PA 66/PEN and PBT/PEN blends under melt processing. However, the domain size in PBT/PEN blends is at least on an order of magnitude smaller than that for PA 66/ PEN blends.46, 47 The glass transition temperatures of PBT/PEN blends are shown in Table 11.4. The system has two glass transition temperatures. 11.2.3.3
Blends of HDPE and PEN
HDPE is a well-known and successful bottle forming material for use in the simple one-step extrusion blow molding process by which the largest proportion of plastic bottles and similar containers are manufactured. HDPE also possesses a number of good properties required in finished bottles,
364
High Performance Polymers Table 11.4: Glass Transition Temperatures of PBT/PEN Blends47 % PEN a
0
30
40
50
60
70
100
Tg /°C (PBT) 59.2 Tg /°C (PEN) a percent by weight
65.5 117.3
65.2 117.4
65.7 119.2
70.0 123.1
76.3 123.2
132.6
such as hot fill capability, chemical resistance, and impact on the H2 O barrier. The main drawbacks of HDPE bottles are poor gas barrier, high UV transmission, and a lack of optical transparency. In contrast, PEN meets the requirements, but more complex forming techniques are needed. PEN bottles cannot be produced by simple extrusion blow molding. PEN requires the use of the more complex two-step injection stretch blow molding process. In this process, a preform is first injection molded and then stretch blow molded to produce the final bottle. This is the same process used to produce bottles of the related polyester, PET. As mentioned, a suitable technology consists in the fabrication of multilayer bottles. However, such multi-wall bottles have to be produced by the more complex and expensive co-extrusion blow molding technology. Using blends of PEN and HDPE can enhance the objected properties of HDPE bottles, using a simple one-step extrusion blow molding process. Problematic is the incompatibility of HDPE and PEN. This can be circumvented by using an ethylene/methyl acrylate copolymer as a compatibilizer.48 The blend is useful in extrusion blow molding to make articles such as bottles.
11.2.3.4
Blends of Poly(trimethylene terephthalate) and PEN
Blends of poly(trimethylene terephthalate) (PTT) and PEN are miscible in the amorphous state over a wide range of compositions.49 This is evidenced by a single, composition-dependent glass transition temperature (Tg ). The variation of the Tg with composition can be predicted by the Gordon-Taylor equation,50 with the fitting parameter being 0.57. Tg =
w1 Tg,1 + kw2 Tg,2 w1 + kw2
(11.7)
Poly(naphthalates) w1 w2 Tg,1 Tg,2
365
Weight fraction of component 1 Weight fraction of component 2 Glass transition temperature of component 1 Glass transition temperature of component 2
Another widely used equation originates from Fox,51 which reads as: 1 w1 w2 = + . Tg Tg,1 Tg,2
(11.8)
The cold crystallization peak temperature decreases linearly with increasing PTT content form 180°C for pure PEN to 65°C for pure PTT. 11.2.3.5
Blends of Poly(styrene) and PEN
Poly(styrene) (PS) is not compatible with PEN. Styrene/glycidyl methacrylate (SG) copolymers react with the terminal groups of PEN during melt blending. This results in the formation of SG-g-PEN copolymers in the blend. These copolymers tend to reside along the interface to PS and thus function as effective compatibilizers in blends of PS and PEN.52 The compatibilized blends exhibit higher viscosity, finer phase domain, and improved mechanical properties. In blends compatibilized with the SG copolymer having a high content of glycidyl methacrylate, highly grafted copolymers are produced. The length of the styrene segment in these grafted copolymers is too short to penetrate deep enough into the PS phase to form effective entanglements, resulting in lower compatibilization efficiency in blends of PS and PEN. 11.2.3.6
Blends of Poly(carbonate) and PEN
Blends of bisphenol A poly(carbonate) (PC) and PEN can be obtained without the addition of a catalyst in a batch mixer at 290°C. All the blends at various compositions prepared exhibited two phases and have good mechanical properties. In the formation of a copolymer in the mesophase, it is postulated that it effectively compatibilizes the system. The formation of a block copolymer is considered to be due to transesterification reactions between PEN and PC. This was verified by extraction experiments and analysis of the soluble and insoluble fractions.53
366
High Performance Polymers Table 11.5: Properties of the Co-polyester Composition Property Intrinsic viscosity Acetaldehyde content Permeability (CO2 ) Haze number Transesterification
11.2.3.7
Value 0.835 11 9.1 9.8 9.2
dl g−1 ppm cm3 m−2 d−1 % mol-%
Copolymers
In order to improve the heat resistance and the gas barrier properties of the PET polyester resins, the usage of blends of PET with PEN has been proposed. Blends of PET and PEN generate acetaldehyde when melt kneaded at elevated temperatures to improve the compatibility. This causes problems, such as change of taste of the contents filled in the container, and the lowering of transparency. Polyester compositions made from ethylene glycol and a mixture of TPA as a major ingredient, and 2,6-NDA have been proposed. These suffer from insufficient gas barrier properties and generate acetaldehyde. A polyester composition having better gas barrier properties with less acetaldehyde ejection consists in a blend of prepolymers of PET and PEN, or a copolymer. This blend is subjected to the solid state polymerization process.54 The PEN copolymer is prepared from 2,6-dimethyl naphthalate, dimethyl isophthalate, ethylene glycol, and poly(tetramethylene glycol), with an average molecular weight of 1,000 Dalton. A dry blend of 90% of a prepolymer of the PET, and 10% of the prepolymer of the PEN is then melt kneaded at a molding temperature of 295°C by means of a single-screw extruder at a residence time of 120 s. The strand with a diameter of 2.5 mm is cut into cylindrical chips. The prepolymer blend exhibits crystallizing at a temperature of 150°C. The chips are heated at 170°C for 2 h with nitrogen for crystallization. Then they are heated up to 210°C for 16 hours to perform a solid state polymerization. The properties of the oriented film produced from the polyester composition are shown in Table 11.5.
Poly(naphthalates)
367
11.2.4 Poly(1,3-propylene 2,6-naphthalate) 11.2.4.1
Condensation
Poly(1,3-propylene 2,6-naphthalate) (3GN) is synthesized from dimethyl2,6-naphthalene dicarboxylate and 1,3-propanediol.55 The ingredients are reacted under atmospheric pressure under nitrogen in the presence of Tyzor™ titanium tetraisopropoxide catalyst. The vessel is heated to 240°C over a period of about 330 min. At 188°C, methanol started to evolve. In the final stage, the pressure in the reaction vessel is reduced and the temperature is increased to about 280°C within 90 min and polymerization was allowed to proceed an additional 30 min. The polymer obtained has a translucent white color with a melting point of 201–203°C, a crystallization temperature of 166°C, and a glass transition temperature of 79°C. The inherent viscosity of the polymer was 0.56 dl g−1 , with a number average molecular weight Mn of 22,000 Dalton and a weight average molecular weight Mw of 36,000 Dalton. Physical blends of 3GN polymer compositions with other polymers have been described.55 11.2.4.2
Stretching
3GN has a unique combination of properties that provides a number of advantages over PEN when used in multilayers with PET.56 3GN has a lower melting point of 181–213°C than PEN (264–267°C) and PET (250–256°C), but its glass transition temperature of 79°C for unstretched films and 94°C for fully oriented films is similar to that for PET (70°C for unstretched films and about 80°C for fully oriented films, respectively). The preferred orientation temperature for 3GN and 3GN-rich copolymers and blends is 90–135°C. The preferred orientation temperature for PET is 90–115°C, thus allowing 3GN to be biaxially oriented under optimum thermal stretching conditions for PET. On the other hand, PEN has a significantly higher glass transition temperature of 113–125°C for unstretched films, and 140°C for fully oriented films and requires higher orientation temperatures, of about 120–150°C. In PEN/PET multilayer laminates, this property results in uneven thickness, opaqueness, and poor strength for the oriented PET layer due to a reduction in strain hardening during the stretching of PET at the higher temperatures.
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High Performance Polymers
In addition to the advantage of the lower Tg , 3GN also has a slow crystallization rate, which is slower than the crystallization rate of PEN. This results in the 3GN layer being more amorphous and translates into improved transparency of the 3GN layer in blow molded and extruded articles and may also account for the improved adhesion with PET in comparison to multilayer structures of PET and PEN. Moreover, 3GN has excellent barrier properties and is therefore useful as a barrier layer in PET films and containers. Unoriented Films. Unoriented multilayer films can be obtained by coextrusion of layers of 3GN and PET. Alternatively, multilayer films can be formed in a continuous lamination process using heat or optionally adhesive layers to bond the separate layers. Press lamination can also be used to form multilayer films by pressing layers of individual films at elevated temperature and pressure. Prior to film formation, the polymers are generally dried by heating to a temperature of 5°C below the crystallization temperature of the polymer under vacuum or inert atmosphere. The films can be rapidly cooled after extrusion to inhibit crystallization.56 Unoriented multilayer films are useful in a number of end-uses, including thermoforming processes, forming shaped articles, such as bottles, or they can be stretched to form oriented flat films. For example, a film having a thickness of 10 mil is suitable for thermoforming into cups having a wall thickness of 1 mil. Trays having a thickness of 10 mil can be obtained from a 100 mil film. Oriented Films. Oriented multilayer films are prepared by stretching heated unoriented films.56 The film can be stretched in the direction coincident with the machine direction, or the direction perpendicular to the machine direction to obtain an uniaxially oriented film. A biaxially oriented film is stretched in both the machine direction and perpendicular to the machine direction. Biaxial stretching may be done sequentially by drawing first in the machine direction, followed by stretching in the transverse direction. Alternately, the stretching in two directions can take place simultaneously. Prior to stretching, the films are preheated to the stretching temperature of about 90–115°C. Preferably, the films are biaxially stretched to about 2 to 4 times the original length of the unstretched film in each direction to provide oriented films having good barrier and physical properties.
Poly(naphthalates)
369
In the tubular-film process, the co-extrudate consisting of layers of 3GN and PET is extruded through a narrow die to form a tube. Pressurized air of controlled temperature is blown into the tube, which is then inflated to a larger diameter bubble.57 Biaxial orientation is induced in the film while it is being stretched in the machine and transverse directions. Alternatively, in the tenting frame process, the multilayer film is heated to an optimum orientation temperature and stretched in the tenting frame. During stretching, strain orientation and crystallization occurs, which results in improved physical properties. The stretched films are heat-set at a temperature above the crystallization temperature of the polymers. During heat-setting, the oriented film is heated and annealed while the film is dimensionally constrained. This stabilizes the structure of the polymers in the multilayer film by increasing the crystallinity, which reduces shrinkage. Heat-setting temperatures are in the range of 167–180°C for about 2-5 minutes. Heat-set oriented films have excellent clarity, with a light transmission of greater than 80%. The oriented films exhibit excellent adhesion between the 3GN and PET layers, eliminating the need for an adhesive tie layer. The oriented films are useful in end-uses requiring good oxygen-barrier resistance, such as food packaging applications. 11.2.4.3
Poly(1,3-propylene terephthalate/2,6-naphthalate)
Poly(1,3-propylene terephthalate/2,6-naphthalate) copolyesters are synthesized from poly(propylene 2,6-naphthalate) and poly(1,3-propylene terephthalate) by transesterification at 260°C in an inert atmosphere with 50 ppm of Ti(OBu)4 . All the polymers show a good thermal stability. The main effect of copolymerization is a lower crystallinity and a decrease of Tm with respect to homopolymers.58 The Fox equation, Eq. 11.8 describes the Tg -composition data well.
11.3 PROPERTIES 11.3.1 Mechanical Properties In general, PEN is superior to PET. The continuous service temperature of PEN is 160°C, in contrast to 105°C for PET.59 Comparative mechanical properties of PEN and PET films are summarized in Table 11.6.
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High Performance Polymers
Table 11.6: Mechanical Properties of Biaxially Stretched PEN and PET Films59 50 μ Films
PENa
PETa
PENb
PETb
Young’s Modulus, MPa Tensile Strength, MPa Stress at 5% Elongation, MPa Elongation at Break, % Thermal Shrinkage at 150°C, % a Machine direction (MD) b Transverse direction (TD)
5000 265 80 80 0.6
3800 190 140 140 1.3
5350 150 70 70 0.4
4200 110 100 100 1.3
11.3.2 Thermal Properties The glass transition temperature of PEN is 122°C, which is ca. 40°C higher than that of PET. Its melting temperature is 267°C, whereas PET has a melting temperature of 254°C.29
11.3.3 Electrical Properties The electrical properties of PEN have been investigated. PEN shows electroluminescence.60 During electric conduction, degradation occurs. Ageing experiments utilizing corona discharges revealed that the electric conductivity is greatly affected.
11.3.4 Optical Properties The photo-degradation of PEN consists in the formation of oligomers. Photo-oxidation is responsible for the formation of acidic end groups as major products.61 In the case of PBN,62 a mechanism for the yellowing has been proposed. The mechanism is shown in Figure 11.2. An intense photo-yellowing resulting from the conversion of the naphthalate units into more conjugated structures, accompanied by the formation of gel, is observed. The photochemical reactions are restricted rather to the surface of the polymer within a layer around 10 μ m.
11.3.5 Gas Permeability Organic polymers have a comparatively high gas permeability. In Table 11.7, the permeability of some polymers is compared. The gas transport
Poly(naphthalates)
371
O C O CH2 CH2 CH2 CH2
O C O CH2 CH2 CH2 CH2
Figure 11.2: Mechanism of Yellowing
properties in PEN are dependent on the degree of crystallinity.63 A decrease of the permeability is observed at thermally crystallized samples. No change of the sorption properties of the amorphous phase is observed by thermal treatment. A low gas permeability measured at the biaxially stretched films is related to both a change of the free volume sizes distribution and a tortuosity effect. The barrier properties of biaxially stretched films are kept even after annealing the film at 250°C. In a series of experiments, the TPA in PET has been modified with other aromatic dicarboxylic acids, such as IPA, o-phthalic acid, and certain isomers of naphthalene dicarboxylic acid, i.e., 2,6-NDA, 1,5-NDA, and 1,8-NDA.64 The oxygen permeability and other properties of the amorphous copolymers of PET modified with amounts of 10% of another carboxylic acid unit are shown in Table 11.8. Inspection of Table 11.8 reveals that among the naphthalene dicarboxylic acid isomers, only 2,6-NDA is really effective in reducing the permeability. Most effective is o-phthalic acid, however, there is a depression of the glass transition temperature. By annealing, the degree of crystallization increases up to ca. 30% and the barrier properties are markedly increased.
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High Performance Polymers
Table 11.7: Oxygen Permeability of Some Polymers65 Pa
Polymer Poly(ethylene), low density Poly(ethylene), high density Poly(styrene) Poly(vinyl chloride) Poly(acrylonitrile) Thermotropic liquid crystalline polymers Poly(ethylene terephthalate) + SiOx 12 nm a
2500 500 2000 5.9 5 0.4 0.15
P: permeability/1016 Std cm3 cm cm−2 s−1 Pa−1
Table 11.8: Oxygen Permeability of Modified PET64, 66 Amorphous Polymers64 Modifier % PET Isophthalic acid o-Phthalic acid 2,6-NDA 1,5-NDA 1,8-NDA
0 10.0 10.0 10.0 10.0 10.0
Crystalline polymers66 Modifier % o-Phthalic acid Isophthalic acid 2,6-NDA
10.0 10.0 10.0
ρ /[g cm−3 ]
Pa
Db
Sc
Tg /°C
1.3350 1.3369 1.3381 1.3345 1.3339 1.3344
0.469 0.398 0.364 0.390 0.464 0.470
5.6 4.7 4.7 4.3 4.9 5.2
0.098 0.098 0.089 0.105 0.108 0.104
81 80 74 89 91 89
ρ /[g cm−3 ]
Pa
Db
Sc
Ta /°Cd
1.3381 1.3792 1.3754
0.141 0.154 0.228
2.3 2.5 2.9
0.068 0.072 0.091
117 122 129
P: permeability/[Stdcm3 cm m−2 d−1 atm−1 ] D: diffusivity/ 1012 [m2 s−1 ] c S: solubility/[Stdcm3 cm−3 atm−1 ] d Annealed for 360 min. at T a a
b
Poly(naphthalates)
373
The permeability can be reduced by a physical vapor deposition (PVD) process and chemical vapor deposition (CVD). PVD processes operate under reduced pressure and include evaporation and sputtering, in the absence of chemical reactions in the gas phase and at the substrate surface. An overview of PVD processes show that CVD processes utilizes volatile precursors that are decomposed by means of heat, photons, or plasma. Plasma enhanced CVD (PECVD), is applicable for thermally sensitive substrates, such as polymers. It has become the most widely used process for the deposition of silicone coatings. Thin oxide films deposited on polymer substrates by vapor deposition techniques are used in various applications. The combination of polymer materials with functional and protective coatings offers a number of key advantages over alternative bulk materials, such as light weight, complex shape, design freedom, transparency, or tailored optical characteristics.65 Thin aluminum oxide layers are widely used as a gas barrier coating on polymeric materials. It has been found that a single layer of Alx Oy improved the moisture barrier of PET by an order, PC by two orders of magnitude, while no improvement was observed for PEN.67 PET, PEN, and PC exhibit different surface roughness and surface energy. These parameters are important factors that affect the growth of Alx Oy . It was found that a smooth substrate surface is a prerequisite to get a good moisture barrier independent of the barrier property of the polymer substrate. The surface roughness decreases from PEN via PET to PC. For this reason, the treatment with aluminum oxide is not effective for PEN.
11.3.6 Chemical Resistance For 2,6-NDA-based polyesters, the alcohol component has been varied. Ethylene glycol has been partially replaced by alcohols with aliphatic side chains, based on 1,3-propanediol (HH), such as 2,2-dimethyl-1,3-propanediol (CC), 2,2-diethyl-1,3-propanediol (C2C2) and 2-butyl-2-ethyl-1,3propanediol (C2C4).68 Copolymers were prepared by the melt condensation method using dimethyl 2,6-naphthalate. The crystallinity and the density of annealed films decrease with increasing content of comonomer and length of alkyl side chain in the comonomer. The alkaline resistance is considerably increased by the incorporation of a comonomer having an alkyl side chain. All copolymers have a have higher solubility, higher glass transition
374
High Performance Polymers Table 11.9: Chemical Resistance of PET and PEN59 Chemical Agent 1% Hydrochloric acid 1% Hydrochloric acid 10% Hydrochloric acid 10% Hydrochloric acid 1% Sodium hydroxide 1% Sodium hydroxide 10% Sodium hydroxide 10% Sodium hydroxide Ammonia gas Ammonia gas Thermal agingb Thermal agingc a b c
Weeks
PETa
2 5 2 5 2 20 1 2 2 10 2 2
72 74 4 0 76 60 0 0 15 0 0 20
PENa 85 106 69 60 97 126 70 50 93 96 80 50
% Retention of elongation at break 2 weeks at 180°C, 0% RH 2 weeks at 130°C, 100% RH
temperature, and a better thermal stability than PET. The thermal properties are comparable to PEN, and for 1,3-propanediol modified samples even better than those of PEN. Longer alkyl side chains in the alcohol decrease the thermal properties. Chemical resistance data are collected in Table 11.9. The chemical resistance is expressed at the retention of elongation at a break after chemical treatment.
11.4 SPECIAL ADDITIVES 11.4.1 Flame Retardants Polyamides, such as PA 6, PA 12 or PA 6.6, PET and PEN, can be made flame retardant by the addition of a mixture of melamine cyanurate and an organo poly(phosphonate).69 The poly(phosphonate) acts in addition as a plasticizer, improving the mechanical properties of the polymer and assisting the dispersion of the melamine cyanurate.
11.4.2 Protective Coatings Naphthalate polyester articles can be coated with polymerizable and crosslinkable compositions. The composition is made from the matrix and a
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large amount of benzotriazole derivatives. This class of compounds absorbs UV light. The cured compositions help protect the naphthalate polyester from UV exposure and other weathering effects.70 The matrix consists of acrylic resins and methacrylic resins. The compositions are photopolymerizable.
11.5 APPLICATIONS 11.5.1 Poly(ethylene naphthalate) The first commercial thermoplastic aromatic polyester was PET. More recently, significant attention has been focused towards PEN, because fibers and films made from PEN have improved strength and thermal properties relative to products made from PET.5 PEN has found many applications: Higher tensile strength and dimensional stability makes PEN film an excellent choice for the manufacture of magnetic recording tape and electronic components. Additionally, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen, and water vapor, films made from PEN are useful for manufacturing hot fill food containers. PEN can also be used to prepare high-strength fibers useful for the manufacture of tire cord.38 Polyesters made from mixtures of TPA and 2,6-NDA or NDC have also been found to have unique and desirable properties, such as resistance to gas diffusion. This property makes them suitable for manufacturing for food and beverage containers.5 PBN exhibits a very rapid crystallization rate and its Tg is difficult to detect by simply heating the quenched sample.71 Since the crystallization rate of PBN is much faster than that of PBT, its processing cycle time can be shortened. Because of its facile processability, PBN can be molded by either injection or extrusion into electrical, electronic, and machine parts. PBN exhibits excellent mechanical strength, heat resistance, dimensional stability, resistance to chemical, acid and alkaline and impermeability to gasoline and gasohol. Therefore, it is particularly suited for making into automobile parts that come in contact with fuel, oil, and combustion gas. In addition, fiber reinforced PBN for high temperature and humidity circumstances has been reported.38 Blends of PBN with poly(vinylphenol) (PVPh) are miscible over the whole range of compositions in the amorphous state.72
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In blends of PBN and poly(ether imide) (PEI), one single Tg was observed. The glass transition temperature increases monotonously with the increase of PEI content.73 The glass transition temperatures of the blends fit the Fox equation, indicating a mixing of the components.
11.5.2 Films 11.5.2.1
Magnetic Recoding Media
Biaxially oriented films of PET polyesters are frequently used in the field of magnetic recording media, such as audio tapes, video tapes, computer tapes, and floppy disks. With the tendency to reduce the size and weight of electric and electronic equipment and to improve their performance, the demands become increasingly exacting. For example, in the field of magnetic recording media, the base film must be thin enough to realize long-time recording and reductions in size and weight. In a typical linear magnetic tape storage device, if a 10% track mismatch is tolerable, a lateral deformation of less than about 5 μ m is desirable.74 At the same time, it is important to retain the stiffness of a film by improving elastic modulus. In some cases, an extremely thin base film formed from conventional PET is unsatisfactory in terms of elastic modulus. The mechanical and thermal properties of PET, PEN, and PA films have been extensively characterized.75–77 PEN is superior to PET in elastic properties, whereas it is inferior to PET in viscoelastic properties. PEN films were found to have better damping properties than those of PET films. This implies that they may have better performance in handling during the tape manufacturing. However, the PEN film has a lower tear strength and thus lower delamination resistance than an ordinary PET film. Particularly when it is stretched like a biaxially oriented film, the tear strength of an intermediate or final product in the process of molding is low in many cases. Therefore, the film is broken so frequently that a product cannot be obtained, for example, in the production process of a sequentially biaxially oriented film of PEN. Even if a product is obtained, a film, which is readily torn in a specific direction is obtained. A PEN copolymer containing minor amounts of IPA component or decalin-2,6-dicarboxylic acid component is helpful in improving the mechanical properties.78 The acid components are preferably used as methyl esters. Further, the amounts of diethylene glycol moieties formed by side
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reactions should be less than 3 mol-%. If the amount of the diethylene glycol component is larger than 3 mol-% the delamination resistance of the resulting film is increased, whereas crystallinity is lost with the result that mechanical strength is greatly reduced. To suppress the by-production of diethylene glycol during the production of the modified PEN, it is advantageous to make the time required for the esterification as short as possible. From the modified polyesters, biaxially oriented films can be produced by conventional methods. Biaxially oriented films can be obtained having Young’s moduli of 500 kg mm−2 or more, in both longitudinal and transverse directions. However, if Young’s moduli are too high, delamination resistance becomes insufficient. The desired density is 1.350 g cm−3 or more. If the density is lower than this value, orientation crystallization becomes insufficient, and mechanical strength deteriorates the material. Further, the delamination resistance becomes unsatisfactory. On the other hand, to keep the surface of the film flat, the density is desirably at 1.362 g cm−3 or less. The biaxially oriented film has an excellent anti-curling property. The anti-curling property means that a film hardly remains curled when rolled once and then unrolled. Whitening has rarely been observed at a fold when the film is folded. This property is called delamination resistance and is an important index for the evaluation of both a base film for a magnetic recording medium and a base film for a photo film. The delamination resistance is expressed as folded-line delamination whitening width or folded-line delamination whitening ratio. A folded-line delamination whitening ratio of 10% or less is desirable. The films can be as thick as 0.5 to 250 μ m. One of the features of the base film for a magnetic recording medium is that the base film has a surface roughness of 2.0 nm or less. If an adhesive layer, barrier layer or magnetic layer, is coated or deposited on the base film having the surface roughness of more than 2.0 nm, the flatness of the film is impaired; thereby, for example, deteriorating electromagnetic conversion characteristics disadvantageously. Crystallization is appropriately hindered by the copolymerization of an IPA component or a decalin-2,6-dicarboxylic acid component to reduce surface roughness caused by the growth of fine crystals by heat-setting. To make the surface flatter, there is a method in which the heat-setting temperature is reduced to prevent crystallization from forming spots, and
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a method in which the surface of a film is crystallized when a polymer extruded from a die is cooled by a casting drum. These methods are the most effective. In these methods, the temperature of the casting drum is maintained at 40–80°C and cold water is poured onto the film on the casting drum to quench it. 11.5.2.2
Photographic Films
A cellulose triacetate film has been used as a base film for a photo film. This triacetate film involves safety and environmental problems because an organic solvent is used in its production process. In addition, it has limits in mechanical strength and dimensional stability. Therefore, PET films are used partly as a substitute base film. However, when kept in the form of a roll and unrolled, PET films remain curled and this curl is difficult to remove. The usage of a PEN film for a photo film is more satisfactory with respect to mechanical strength, dimensional stability, and in anti-curling property when rolled to a small diameter. However, the PEN film is susceptible to delamination, especially when the film is rolled and perforated. The occurrence of delamination makes it difficult to use it as a base film of a photo film because a delaminated portion is whitened. A PEN copolymer containing minor amounts of IPA component or decalin-2,6-dicarboxylic acid component is helpful in improving the mechanical properties.78 It acts in the same way as for magnetic recording media described in Section 11.5.2.1. 11.5.2.3
Encapsulation for Flexible Organic Solar Cells
Conjugated polymers, such as poly(p-phenylene vinylene) (PPV) used for organic solar cells are known to be rather unstable in air. PPV-based solar cells operate only a few hours in air. An appropriate encapsulation is mandatory for this type of devices. Organic polymers as such cannot serve the necessary low oxygen transmission rates to ensure an acceptable lifetime. However, special techniques of deposition can reduce the gas transmission rates. A PEN-based ultra-high barrier material, which is entirely fabricated by PECVD has been introduced as a coating material for flexible organic
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solar cells.79 As encapsulation films, PEN substrates were preferred to PET because of the higher temperature of glass transition. 11.5.2.4
Substrates for Electronic Devices
The progress in the field of plastic electronics is based on developments in materials and in fabrication technology. A transistor fabricated from polymers exclusively, in which source and drain electrodes are printed by using flexographic printing have been introduced. In the polymer transistor device, poly(aniline), regio-regular poly(3-hexylthiophene), PVPh, and poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) are used as source and drain electrodes, active channel semiconductor material, insulator, and gate electrode, respectively. As substrates, flexible sheets of PET and PEN are used.80
11.5.3 Fibers Polyesters, such as PET are now widely used in the manufacture of fibers for textiles and other applications. While PET has many desirable properties that make it suitable for manufacturing fibers, there is a continuing need for polyester fibers that have improved properties, or properties that are different from PET, thereby opening new uses for polyester fibers. For example, PEN has found applications in high performance sailcloth materials or in industrial filtration applications. Micro fibers can be obtained from fibers using a laser thinning method.81 Polyester fibers with improved properties are prepared from blends of PET with PEN, or a blend of PET with a copolymer having terephthalate and naphthalate units.82 In addition, recycled PET can be used, thus providing a valuable use for recycled polyester materials. The polyester fibers have high shrink properties, which make them useful in fiber applications where crimp retention or high bulk is desired. Typical applications are in carpet yarns, hi-loft non-woven fabrics used as interlinings, cushioning media, and filtration media, as well as in specialty yarns for weaving and knitting. The fibers have a lower melting temperature compared to PET, which makes them useful as binder fibers in non-wovens, particularly in combination with PET homopolymer fibers. Wide-angle X-ray scattering and DSC measurements of cold drawn PEN stretched from an amorphous fiber that is spun at low speeds indicate that the strain-induced crystallization can occur at a temperature be-
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Table 11.10: Examples for Commercially Available PEN Polymers Tradename
Producer
Remarks
Hipertuf® Kaladex® Nopla® PenTec® Pentex® Skypet® PEN Teonex®
M&G Polymers DuPont Teijin Kolon Industries, Inc. Performance Fibers, Inc. Honeywell SK Chemicals DuPont Teijin
PEN for drinking bottles PEN films PEN & PET83 PEN fiber Modified PEN fiber PEN83 Biaxially stretched film
low the glass transition temperature and that α -crystals are formed. In contrast, when the same PEN is subjected to constrained annealing, its amorphous characteristics remain unchanged even though the annealing was performed at 200°C.84 The results suggest that the application of stretching stress is more important than elevated temperatures in producing α -crystals. At higher speeds, β -crystals are formed to some extent. A detailed prescription for the preparation of a PET copolymer modified with 20 mol-% naphthalate (PETN-20) is disclosed in the literature.82 Ternary blends from a thermotropic liquid crystalline polymer, PEN, and PET were prepared by melt blending and melt spinning to fibers. The mechanical properties of ternary blend fibers could be significantly improved by annealing at 180°C for 2 h. This is attributed to the development of more ordered crystallites and to the formation of more perfect crystalline structures.85 The interfacial adhesion between PEN and liquid crystalline polymer phases is enhanced when the blends are processed with dibutyltindilaurate as a reactive catalyst to promote transesterification.86
11.6 SUPPLIERS AND COMMERCIAL GRADES Examples for commercially available grades and tradenames are shown in Table 11.10. Tradenames appearing in the references are shown in Table 11.11.
11.7 SAFETY NDC poses no significant health hazards through the normal exposure routes, such as inhalation, ingestion, or contact with skin and eyes. As
Poly(naphthalates)
Table 11.11: Tradenames in References Tradename Description
Supplier
Aerosil® Degussa-Hüls Fumed Silica70 Bynel® (Series) DuPont Anhydride modified ethylene vinyl acetate resin, adhesion promoter56, 70 Cabosil™ M5 Cabot Corp. Silica70 Carbowax® (Series) Union Carbide Corp. Poly(ethyleneoxide glycol) (PEG)19 Darocure® 1173 Ciba 2-Hydroxy-2-methyl-1-phenylpropan-1-one, photoinitiator70 Dyneon® HTE Dyneon Fluoropolymer70 Ebecryl® (Series) Cytec Industries (UCB) Urethane acrylate70 Ecdel® Eastman Copolyester ether elastomer70 Elvamide® DuPont Low melting poly(amide)56 Engage™ resins DuPont Low density poly(ethylene)70 Esacure® Fratelli Lamberti S.p.a. Photoinitiators70 Highlink® (Series) Clariant GmbH Colloidal silica sols70 Irgacure® 184 Ciba 1-Hydroxycyclohexylphenylketone (photo initiator)70 Klebosol® Clariant GmbH Silica sol70 Laromer® LR 8739 BASF AG Urethane acrylate monomer70
381
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Table 11.11 (cont): Tradenames in References Tradename Description
Supplier
Ludox® (Series) Dupont Silicon colloid70 Melinar® Laserplus DuPont Poly(ethylene terephthalate) (PET), bottle grade56 Mylar® (Series) DuPont Poly(ethylene terephtalate)55 Nalco® 2327 Nalco Chemical Co. Silica hydrosol70 PCTA Durastar 1000 Eastman Chemical Co. Copolyester based on 65 mol % terephthalic acid, 35 mol % isophthalic acid and CHDM36 Perspex® CP63 Lucite International Acryl glass70 PETG 6736 Eastman Chemical Co. Copolyester based on terephthalic acid and EG and CHDM36 Photomer™ 6210 Henkel (Cognis) Urethane acrylate oligomer, rheology modifier resin70 Polymeg® Qo Chemicals, Inc. (Lyondell Chemical Co.) Poly(tetramethylene glycol)19 Rexflex® W111 Rexene Corp. Poly(olefin), flexible70 Solef® Solvay Poly(vinylidene fluoride)70 TEGO® RAD 2100 Tego Degussa Poly(siloxane), acrylic, Radically crosslinkable flow and wetting additive70 Tinuvin® 144 Ciba Geigy Bis(1,2,2,6,6-pentamethyl-4-piperidinyl) butyl(3,5-di-tert-butyl-4-hydroxybenzyl)malonate, UV absorber70 Tinuvin® 234 Ciba 2-(2-hydroxy-3,5-di-α -cumylphenyl)-2H-benzotriazole70 Tyzor® TPT DuPont Titanium tetraisopropoxide (tetraisopropyltitanate), catalyst55, 56 Uvinul® D-50 BASF AG 2,2 ,4,4 -Tetrahydroxy benzophenone, UV absorber70
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of 2004, there are no specific personal protective equipment requirements for handling or using NDC.87 Studies suggest that NDC poses minimal genotoxic risk to humans. NDC flakes are combustible and care must be taken to prevent fires or explosions. Solid NDC flakes by themselves do not present a significant hazard. NDA is may cause mild eye irritation.87
11.8 ENVIRONMENTAL IMPACT AND RECYCLING The separation of containers made from PET, PEN, and poly(vinyl chloride) can be achieved by identifying the nature of the individual containers by spectroscopic methods. For example, when PET is sorted from PEN, ultraviolet light is transmitted through the object and then into the splitter. The first stream is run through a filter of substantially 380 nm while the second stream is fed through a filter of substantially 400 nm. The filters are narrow band interference filters. The ratio of the amount of energy passing through the 380 nm and the amount of energy passing through the 400 nm filter is calculated. If the ratio approaches 1, then the object is classified as a PET bottle. In contrast, if the ratio approaches 0, the object can be classified as PEN.88 The materials are then automatically sorted from a conveyor belt. PET can be chemically recycled by alcoholysis. It is believed that a small amount of PEN in PET containers will not hinder PET recycling. Supercritical carbon dioxide acts as a plasticizer for polymers. It has been proposed to introduce supercritical carbon dioxide in the depolymerization process to facilitate the depolymerization process.89
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blow molding operation. Carl Hanse Verlag, Munich, 2 edition, 2003. 58. C. Lorenzetti, L. Finelli, N. Lotti, M. Vannini, M. Gazzano, C. Berti, and A. Munari. “Synthesis and characterization of poly(propylene terephthalate/2,6-naphthalate) random copolyesters.” Polymer, 46(12):4041– 4051, May 2005. 59. Naphthalates: General properties of naphthalate-containing polymers. Company Data Sheet N-9, BP Sales Administration and Customer Service, 150 West Warrenville Road, 605-CS 3, Naperville, Illinois 60563-8460, June 2001. 60. J. Guastavino, D. Mary, E. Krause, C. Laurent, and C. Mayoux. “On the electrical properties of poly(ethylene naphthalate 2,6-dicarboxylate) biaxially-oriented films.” Polym. Int., 46(1):72–76, May 1998. 61. J. Scheirs and J.-L. Gardette. “Photo-oxidation and photolysis of poly(ethylene naphthalate).” Polym. Degrad. Stabil., 56(3):339–350, June 1997. 62. J. Scheirs and J.-L. Gardette. “Photo-oxidation of poly(butylene naphthalate).” Polym. Degrad. Stabil., 56(3):351–356, June 1997. 63. L. Hardy, E. Espuche, G. Seytre, and I. Stevenson. “Gas transport properties of poly(ethylene-2,6-naphtalene dicarboxylate) films: Influence of crystallinity and orientation.” J. Appl. Polym. Sci., 89(7):1849–1857, August 2003. 64. A. Polyakova, D. M. Connor, D. M. Collard, D. A. Schiraldi, A. Hiltner, and E. Baer. “Oxygen-barrier properties of polyethylene terephthalate modified with a small amount of aromatic comonomer.” J. Polym. Sci., Part B: Polym. Phys., 39(16):1900–1910, August 2001. 65. Y. Leterrier. “Durability of nanosized oxygen-barrier coatings on polymers.” Prog. Mater Sci., 48(1):1–55, 2003. 66. A. Polyakova, E. V. Stepanov, D. Sekelik, D. A. Schiraldi, A. Hiltner, and E. Baer. “Effect of crystallization on oxygen-barrier properties of copolyesters based on ethylene terephthalate.” J. Polym. Sci., Part B: Polym. Phys., 39 (16):1911–1919, August 2001. 67. H. Low and Y. Xu. “Moisture barrier of alxoy coating on poly(ethylene terephthalate), poly(ethylene naphthalate) and poly(carbonate) substrates.” Appl. Surf. Sci., 250(1-4):135–145, August 2005. 68. Y. M. Sun and H. H. Liu. “Novel copolyesters containing naphthalene structure III. Copolyesters prepared from 2,6-dimethyl naphthalate, ethylene glycol, and 2,2-dialkyl-1,3-propanediols.” J. Appl. Polym. Sci., 81(11):2754– 2763, September 2001. 69. L. H. Martin. Flame-retardant molded component. US Patent 6 828 365, assigned to T&N Technology Limited (Cawston, GB), December 7, 2004. 70. S. J. McMan, S. A. Johnson, C. L. Jones, E. W. Nelson, and E. S. Goenner. UV resistant naphthalate polyester articles. US Patent 7 153 588, assigned to 3M Innovative Properties Company (St. Paul, MN), December 26, 2006.
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71. M.-Y. Ju and F.-C. Chang. “Multiple melting behavior of poly(butylene-2,6naphthalate).” Polymer, 42(11):5037–5045, May 2001. 72. J. Y. Lee and J. Y. Han. “Miscibility in binary blends of poly(vinyl phenol) and poly(n-alkylene 2,6-naphthalates).” Macromol. Res., 12(1):94–99, February 2004. 73. C. H. Lin and C. S. Wang. “Miscibility of poly(etherimide) and poly(butylene naphthalate) blends.” Polym. Bull., 46(2-3):191–196, April 2001. 74. B. Bhushan. Mechanics and Reliability of Flexible Magnetic Media. Springer-Verlag, 2nd edition, 2000. 75. B. Bhushan, T. J. Ma, and T. Higashioji. “Tensile and dynamic mechanical properties of improved ultrathin polymeric films.” J. Appl. Polym. Sci., 83 (10):2225–2244, March 2002. 76. T. J. Ma and B. Bhushan. “Dynamic mechanical and thermal analyses of magnetic particle and metal evaporated tapes and their individual layers.” J. Appl. Polym. Sci., 89(2):548–567, July 2003. 77. T. J. Ma and B. Bhushan. “Mechanical, hygroscopic, and thermal properties of ultrathin polymeric substrates for magnetic tapes.” J. Appl. Polym. Sci., 89 (11):3052–3080, September 2003. 78. R. Tsukamoto, S. Ito, M. Teramoto, S. Watanabe, K. Furuya, S. Kawai, and K. Suzuki. Biaxially oriented film. US Patent 6 124 043, assigned to Teijin Limited (Osaka, JP), September 26, 2000. 79. G. Dennler, C. Lungenschmied, H. Neugebauer, N. S. Sariciftci, M. Latreche, G. Czeremuszkin, and M. R. Wertheimer. “A new encapsulation solution for flexible organic solar cells.” Thin Solid Films, 511-512:349–353, July 2006. 80. T. Makela, S. Jussila, H. Kosonen, T. G. Backlund, H. G. O. Sandberg, and H. Stubb. “Utilizing roll-to-roll techniques for manufacturing source-drain electrodes for all-polymer transistors.” Synth. Met., 153(1-3):285–288, September 2005. 81. A. Suzuki and M. Tojyo. “Poly(ethylene-2,6-naphthalate) microfiber prepared by carbon dioxide laser-thinning method.” Eur. Polym. J., 43(7):2922– 2927, July 2007. 82. S. Sakellerides. Polyester fibers containing naphthalate units. US Patent 5 955 196, assigned to BP Amoco Corporation (Chicago, IL), September 21, 1999. 83. IDES Integrated Design Engineering Systems. The Plastics Web®. IDES Inc. 209 Grand Avenue Laramie, WY 82070 USA [electronic] http://www.ides.com/prospector/, 2006. 84. G. Wu, M. Liu, X. N. Li, and J. A. Cuculo. “Structure development and physical properties achieved in the drawing and/or annealing of PEN fibers.” J. Polym. Sci., Part B: Polym. Phys., 38(11):1424–1435, June 2000. 85. J. Y. Kim, E. S. Seo, S. H. Kim, and T. Kikutani. “Effects of annealing on structure and properties of TLCP/PEN/PET ternary blend fibers.” Macromol. Res., 11(1):62–68, February 2003.
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86. S. M. Hong, H. O. Yoo, S. S. Hwang, K. J. Ihn, and C. H. Lee. “Structures and physical properties of poly(ethylene 2,6-naphthalate)/liquid crystalline polymer blends.” Polym. J., 33(6):457–463, 2001. 87. Naphthalates: Physical properties and handling information for dimethyl 2,6-naphthalene dicarboxylate. Company Data Sheet N-2, BP Sales Administration and Customer Service, 150 West Warrenville Road, 605-CS 3, Naperville, Illinois 60563-8460, November 2004. 88. M. G. Roe and G. R. Kenny. System and method for distinguishing an item from a group of items. US Patent 5 966 217, assigned to Magnetic Separation Systems, Inc. (Nashville, TN), October 12, 1999. 89. S. A. Khan, G. W. Roberts, and J. R. Royer. Co2-assisted deploymerization, purification and recycling of step-growth polymers. US Patent 6 919 383, assigned to North Carolina State University (Raleigh, NC), July 19, 2005.
12 Poly(phthalamide)s The earliest commercially utilized poly(amide) (PA) is silk. Silk is a natural PA. It is produced by the larvae of the silkworm. Spider silk is a natural PA, composed from modest amino acids. It can be regarded as a natural high performance polymer. The first synthetic PAs were produced in the 1930s, now termed as nylons, which are pure aliphatic. On the other side of the product palette of PAs are pure aromatic PAs, which are addressed as aramids. In aramids, both the diacid and the diamine moieties are of an aromatic nature. Whereas nylons are considered as engineering polymers, aramids are classified as high performance polymers. This chapter focuses on partially aromatic PAs, which are settled in between the two extremes. From the chemical view, partially aromatic PAs can be regarded as nylons, where the aliphatic moieties are replaced to some extent by aromatic groups. Most commonly, aromatic diacid moieties are introduced, however, there are types that additionally use aromatic diamines. Partially aromatic PAs are considered as poly(phthalamide)s (PPA)s, when at least 60% of the diacid component is constituted from aromatic acids, prevalently from terephthalic acid (TPA) or isophthalic acid (IPA).1 PPAs and some types of partially aromatic PAs are considered as high performance polymers. The idea to substitute aliphatic diacids with aromatic diacids goes back to the mid 1950s.2 In the literature, PPAs are not always explicitly declared as such, but are rather treated as a group of PAs. For this reason, a literature search using poly(phthalamide) will not cover all relevant entries. 391
392
High Performance Polymers Table 12.1: Examples for PA Nomenclature Aconym
Components
PA 6, Perlon 6 PA 12 PA 66, Nylon 66 PA 6I PA 6T PA 6T/6I PA 6T/66
ε -Caprolactam Laurolactam Hexamethylenediamine, adipic acid Hexamethylenediamine, isophthalic acid Hexamethylenediamine, terephthalic acid Copoly(amide) Copoly(amide)
Table 12.2: Properties of PA and Partially Aromatic PA Fibers3 Property
PA 6T
PA 66
Melting point [°C] Density [g cm−3 ] Tensile strength [MPa] Elongation at break [%]
370 1.21 425 35
265 1.14 490 57
12.1 MONOMERS Before we discuss the monomers that are used in PPAs, it is in order to talk briefly about the nomenclature of these types of polymers. In nylons, there is common nomenclature to identify the types by a number code. The numbers refer to the chain lengths of the diamine and the diacid. In the case of aromatic compounds, for IPA, I is used and for TPA, T is used. In PPA, a similar nomenclature is used. Examples are given in Table 12.1. However, for fully aromatic PAs the nomenclature is not extended for practical use. The properties of PA and PPA are shown in Table 12.2. Monomers for PPAs are shown in Table 12.3. Diacids are shown in Figure 12.1, diamines are shown in Figure 12.2, and Lactams are shown in Figure 12.3. Common monomers for copoly(amide)s are adipic acid, TPA, hexamethylenediamine (HMD) and ε -caprolactam.4 2-Methylpentamethylenediamine and 2-ethyltetramethylenediamine are used for transparent PPA in combination with IPA.5, 6 These diamines can be prepared by hydrogenation of the corresponding dinitriles. For example, 2-methylpentamethylenediamine can be prepared by hydrogenating 2-methyleneglutaric dinitrile, which is a dimerization product of acrylonitrile. m-Xylylenediamine (MXDA) is produced by the hydrogenation of isophthalonitrile.7 It can be used to improve the barrier properties in PPAs.8
Poly(phthalamide)s
Table 12.3: Monomers for Poly(phthal amide)s9 Diacids
References 4
Terephthalic acid Isophthalic acid Adipic acid Suberic acid Azelaic acid Sebacic acid Bis(4-carboxyphenyl)phenylphosphine oxide Diamines
4 4 4 4 10
References
Hexamethylenediamine Trimethylhexamethylenediamine 2-Methylpentamethylenediamine 2-Ethyltetramethylenediamine Decamethylenediamine Neopentyldiamine 4,4 -Diaminodicyclohexylmethane 2,2-(4,4 -Diaminodicyclohexyl)propane 3,3 -Dimethyl-4,4 -diaminodicyclohexylmethane Piperazine m-Xylylenediamine Aminocarboxylic acids
ω -Aminoundecanoic acid Lactams
ε -Caprolactam Enantholactam Laurolactam End capping Agents Benzoic acid
11 12, 13 5 5 14 15 4, 16 4, 16 6, 16 4 8
References 4
References 4 4 4
References 9
393
394
High Performance Polymers
O
O C
O
C
HO
C OH
HO C O HO
Terephthalic acid
HOOC
(CH2)4
COOH
Adipic acid
HOOC
(CH2)7
Isophthalic acid
HOOC
(CH2)6
COOH
Suberic acid
COOH
Azelaic acid
HOOC
(CH2)8
COOH
Sebacic acid
Figure 12.1: Diacids Used for Poly(phthal amide)s
A process for the manufacture of MXDA containing polymers has been described.17 MXDA finds use both in amorphous types15 and crystalline types.18
12.2 POLYMERIZATION AND FABRICATION Generally, PAs are produced by the dehydrating polycondensation of a diamine monomer and a dicarboxylic acid monomer.
12.2.1 Conventional Route Conventional techniques for the polycondensation of PAs employ an aqueous solution of ingredients. Polymerization is accomplished by the gradual removal of the water from the mixture at elevated pressures by gradually increasing the temperature of the reaction medium. In this manner the majority of the water is removed and the temperature of the reaction medium exceeds the melting point of the PA. The process parameters, temperature and pressure, are chosen in such a way that in the reaction mixture phase separation does not occur. The removal of the water in the final stages of polymerization occurs by gradual reduction of the pressure and increasing the temperature.19 In the final stage of polycondensation, degradation reactions may diminish the quality of the final product.
Poly(phthalamide)s
CH3 (CH2)6
H 2N
NH2
H 2N
CH2
CH
(CH2)3
2-Methylpentamethylenediamine
Hexamethylenediamine
CH3
CH3 H 2N
CH2 H 2N
CH2
CH
NH2
(CH2)2
CH2
NH2
Neopentyldiamine
CH2
NH2
4,4′-Diaminodicyclohexylmethane H 3C
C CH3
2-Ethyltetramethylenediamine H 2N
CH2
NH2
H N
N H
Piperazine
CH3
H 2N
NH2
CH2
H 2N
NH2
3,3′-Dimethyl-4,4′-diaminodicyclohexylmethane m-Xylylenediamine CH3 H 2N
C
NH2
CH3 2,2-(4,4′-Diaminodicyclohexyl)propane
Figure 12.2: Diamines Used for Poly(phthal amide)s
N
O
ε-Caprolactam
N
O
Enantholactam
N
O
Laurolactam
Figure 12.3: Lactams Used for Poly(phthal amide)s
395
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High Performance Polymers
In particular, a technique of polymerization is as follows: A solution of hexamethylenediamine in water is fed into the reactor. To this solution, adipic acid, isophthalic acid, and terephthalic acid are added. The amine is in slight molar excess of 5–10%. 2% of benzoic acid is added as an end capping agent. During the addition of the acids the temperature increases to around 70°C, because of the neutralization reaction. Eventually, as a catalyst, zinc hypophosphite is added. The reactor is sealed, purged with nitrogen, and a slight excess pressure of nitrogen is left. Then the reactor is heated to ca. 120°C. and held at that temperature. After completion, the material is still further heated and dried in vacuum at an elevated temperature. There are techniques for semi-continuous production based on this method, using two reactors in parallel to provide continuous feed of salt solution to downstream processing by alternating between the two reactors.9 The polymer obtained by using the monomers mentioned is addressed as PA 6T/6I/6A. In comparison to pure aliphatic PAs, side reactions are more difficult to control for PPA. Side reactions of HMD consist in the formation of ammonia, and self-condensation of HMD.
12.2.2 Instant or Aerosol Process In another method of polymerization, the aerosol process, the final stage of the polycondensation is accomplished at temperatures in the range of 275–330°C under pressure. After reaching equilibrium, the pressurized mixture is passed through an orifice so that an aerosol is formed. Turbulent flow conditions are necessary, in order to prevent a deposition on the walls of the reactor. The reactor is addressed as a tubular flash reactor, its walls heated to 370–430°C. During flash, residual water evaporates quickly from the surface of the aerosol.12, 20 The reaction mixture never reaches the wall temperature, but is around 275–320°C. After passing the flash reactor, the mixture is guided into a finishing reactor, which is a twin screw extruder.
12.2.3 Batch Processes To ensure the production of PA with uniform and stable quality, it is important to maintain the preset mole balance and the preset polymerization conditions. Various methods have been employed to maintain the target values of mole balance, polymerization time, polymerization temperature,
Poly(phthalamide)s
397
polymerization pressure, etc. The mole balance is a particularly important process factor to be precisely controlled. In batch polymerization methods, diamine may escape out of the reaction system. The amount escaped may vary from batch to batch in an unforeseeable manner. Thus, the mole balance must be adjusted on the fly. Various methods to control the mole balance are available, including:21 • Neutralization titration to determine the concentration of terminal carboxyl groups and amino groups, • Real-time viscometry, and • Real-time infrared (IR) spectroscopy. In titration analysis a sample must be withdrawn from the reactor, solidified, dissolved in a specific solvent, and subjected to neutralization titration. This method is a time-consuming process. The viscosimetry relates the melt viscosity to the polymerization degree. However, the method is not directly sensitive to the mole balance. IR spectroscopy is quite suitable for non-destructive analysis and real time analysis. In addition, production methods of poly(ester)s have been reported, where the properties of the poly(ester)s are measured using a IR spectroscopy. The reaction conditions are adjusted according to the measured values. Instead of TPA, dimethyl terephthalate can be used as a monomer.5
12.2.4 Continuous Routes Processes for the continuous polymerization of PAs have been described, e.g., in a coil reactor22 or in flash reactor.19 Rapid heating and short residence times in the high-temperature stage improve the properties of the polymers. One process proceeds as19 follows: 1. An aqueous mixture of diacids and diamines is fed continuously into a pre-polymerization unit. The mixture is oligomerized up to a degree of polymerization of 4–10 under pressure at elevated temperatures. 2. The mixture is continuously pumped into a heated pipeline reactor, the flasher. The main function of the flasher is to heat up the reaction mixture under pressure. 3. The last vessel in the polymerization system is the finishing vessel where the vapor and liquid streams discharged from the flasher are separated and liquid stream is collected in a melt pool in the vessel.
398
High Performance Polymers
One advantage of this process is the minimal time that the polymeric mixture is exposed to high temperatures, which is typically less than 15 minutes by optimizing the heat transfer at the later stages of the process.
12.2.5 Interfacial Condensation The method of interfacial condensation starts with the monomers dissolved in mutual immiscible solvents. Instead of the acids, the respective acid chlorides are used which are dissolved in carbon tetrachloride. The amine component is prepared in aqueous solution. The pH of this solution is still increased by adding KOH. The KOH neutralized the HCl produced during condensation. Both solutions are mixed with a blender and the reaction is complete after a time in the range of a few minutes.15 Interfacial condensation has been used to prepare amorphous PPA materials. A variant of interfacial condensation has been exemplified with pure aromatic PAs, however, it is claimed that the process works also for partially aromatic PAs.23 The process runs continuously. In the first reaction step, diacid chlorides and diamines are reacted in an organic solvent, such as tetrahydrofuran without removing the HCl from the reaction system so that low-molecular-weight prepolymers are formed. In the second step, the organic solution of the prepolymer is contacted with an aqueous solution of an acid acceptor, such as sodium carbonate. The acid acceptor binds the HCl and effects a further condensation reaction of the prepolymer in the organic solvent. This process efficiently produces polyamides having a high degree of polymerization at a high rate of polymerization. In comparison to conventional interfacial condensation, the molecular weight of the final product can be more easily controlled.
12.2.6 Ester Recycling Route Esters obtained from the degradation of poly(ethylene terephthalate) (PET) can be used as raw materials for PPA.24 PPA can be condensed after the degradation of the polyester used. The ester groups can be converted almost completely into amide groups. After removal of the alcohols and diols released, the materials can be processed in a conventional manner. In particular, to a mixture of PET bottle recyclate and PA 6, benzoic acid is added as chain stopper. Further, disodium phosphate serves as an antioxidant. The mixture is molten under inert gas. To the melt, HMD is charged. After discharging and cooling, the ethylene glycol released
Poly(phthalamide)s
399
during the transamidation is removed in a vacuum drying oven for 4 hours at 180°C. A partially aromatic co-poly(amide) is obtained with a melting point of 255°C. and a glass transition temperature of 83°C. Several variants of the process have been described.24
12.2.7 Side Reactions In the course of polycondensation various side reactions may occur. Partly aromatic copoly(amide)s prepared by conventional processes have triamine contents greater than 0.5%. This effects a deterioration in the product quality and to problems in preparation by a continuous method. For example, dihexamethylenetriamine, formed from HMD is used in the preparation.25 Copoly(amide)s with a low triamine content have the same solution viscosity but lower melt viscosities compared with products of the same composition which have a higher triamine content. The processability and the product properties are significantly improved by a low triamine content. The dimerization of the diamine can be suppressed by using special methods of polymerization.25 Short residence times in the hightemperature stage of the polycondensation process suppress the formation of triamines.26 PAs with 2-methylpentamethylenediamine are difficult to prepare by conventional melt condensation methods. Namely, 2-methylpentamethylenediamine can be easily cyclized to methylpiperidine under the liberation of ammonia. Thus one potential amide group is lost and the compound becomes monofunctional and acts as chain stopper.6
12.2.8 Blends and Copolymers Poly(hexamethylenediamine terephthalic acid) exhibits a melting point of 360°C. It cannot be melt processed as such without appreciable decomposition.27 For this reason, copolymers are fabricated, in which the aromatic compound is partially substituted by aliphatic compounds. 12.2.8.1
Blends with Poly(propylene)
Poly(propylene) (PP) resins are improved in mechanical properties and in rigidity without loss in thermal processability when blended with PPA resins together with a minor amount of compatibilizer.28 Blends of PP and functionalized PP with PPA in a weight ratio of 1:3–3:1 generally exhibit
400
High Performance Polymers
excellent impact properties.29 Similarly, filled compositions, including glass-filled PPs are improved when blended in the same way. In general, the two polymer phases are incompatible. For this reason, a compatibilizer must be added to the blends in order to achieve good dispersing and mutual anchoring of the two polymer phases. Suitable compatibilizers are carboxylated or anhydride grafted poly(olefin)s. Such compatibilizers are generally commercially available, e.g., Polybond®, Exxelor®, Hostamont®, Admer®, Orevac®, Epolene®, and Hostaprime®.16 12.2.8.2
Blends with Poly(arylene ether)
The combination of poly(arylene ether) (PAE) resins with PAs into compatibilized blends results in improved overall properties, such as chemical resistance and high strength. The properties of these blends can be further enhanced by the addition of various additives, such as impact modifiers, flame retardants, light stabilizers, processing stabilizers, heat stabilizers, antioxidants and fillers.30 Such blends are attractive for a variety of end-use articles in the automotive market, especially for under hood and various exterior components. Some applications, for example connectors, have very thin wall sections and therefore require resins that have very low viscosities in order to completely fill the molding tools. The melt flow can be enhanced by mixing with a dendritic polyester resin. The inclusion of only 0.5% of a dendritic polyester resin can increase the melt flow rate by as much as 100%.30 Compatibilizing agent for blends of PAE and partially aromatic PA resins are citric acid, fumaric acid, or maleic anhydride.31 Compatibilization is achieved in a twin screw extruder by melt mixing. 12.2.8.3
Poly(amide)–Poly(amide) Blends
PAs using only crystalline PAs as a polymeric matrix exhibit insufficient weld strength due to their crystallinity when subjected to welding. The weld strength with respect to various welding methods can be improved by adding small amounts of amorphous partially aromatic PA to crystalline PA types.32, 33 The properties of the blends can be still further improved by adding organically modified montmorillonite to form layered silicate nanocomposites.34, 35
Poly(phthalamide)s
401
12.2.9 Fabrication Techniques Fabrication of PPAs into molded articles, extruded profile goods, and laminates requires processing the resin at temperatures very near the resin decomposition temperature, together with severe shear stress during molding or extrusion operations.36 Melt spinning for producing fiber and yarn subject the resin to severe stress through the application of high shear at high temperatures.
12.2.9.1
Reactive Melt Processing
Reactive Melt Polymerization. The liquid composite molding (LCM) method is established for the fabrication of thermosets. In contrast, reactive thermoplastic LCM processes are developed to only a few engineering polymers. The technique was applied to low-melting PA 6T/6I oligomers.37 On heating, the material melts at around 135°C. Shear thinning is observed at a viscosity of ca. 102 Pa s. The viscosity still decreases on further heating. However, at 180°C a high-temperature crystallization occurs. In the range of 220–290°C, polymerization takes place. The results indicate that LCM should be a viable processing technology.
Reactive Melt Blending. High molecular PA types or poly(ester amide) block copolymers can be prepared by melt blending techniques.38 As chain extenders, bislactams are used, such as terephthaloyl bislaurocaprolactam, isophthaloyl bislaurocaprolactam or the corresponding biscaprolactams. These compounds have the drawback of a relatively low reaction rate, thus requiring long residence times in the extruder. As a result, undesired side reactions, may occur, e.g., discoloration. It has been found that carbonyl bislactam is much more reactive than the other bislactam compounds mentioned above.38 Carbonyl bislactam can be obtained through the reaction of the lactam with phosgene. Preferably, the carbonyl bislactam is added to the melted polyamide or polyester product stream in the polymerization process as it leaves the polymerization reactor.
402
High Performance Polymers Table 12.4: Properties of Amodel® AT-1002 HS a39
Property Density Mold Shrink, Linear-Flow Mold Shrink, Linear-Trans Water Absorption 24 hrs Tensile Modulus Tensile Strength Tensile Stress at Break b Tensile Elongation @Yld Tensile Elongation Brk 11 Flexural Modulus Flexural Strength Shear Strength Notched Izod Impact Notched Izod Impact Strength b Unnotched Izod Impact Unnotched Izod Impact Strength b DTUL 66psi - Annealed Surface Resistivity Volume Resistivity Dielectric Strength Dielectric Constant a Solvay Advanced Polymers b 23°C
Value 1.13 0.020 0.021 0.5 2760 83.4 68.3 5.0 2210 103 64.1 128 12.6 801 177 163 8.0E+13 1.2E+16 16.1 3.3
Unit
Standard
g cm−3
ISO 1183 ASTM D955 ASTM D955 ASTM D570 ASTM D638 ASTM D638
cm/cm cm/cm % MPa MPa MPa % MPa MPa MPa J m−1 kJ m−2 J m−1 kJ m−2 °C Ω Ω cm kV mm−1
ASTM D638 ASTM D638 ASTM D790 ASTM D790 ASTM D732 ASTM D256 ISO 180 ASTM D256 ISO 180 ASTM D648 ASTM D257 ASTM D257 ASTM D149 ASTM D150
12.3 PROPERTIES Partially aromatic poly(amide)s and copoly(amide)s have been developed for use in high-temperature applications. Crystalline and semi-crystalline copoly(amide)s comprising at least about 40 mol-% partially aliphatic terephthalic units are known for their particularly good thermal properties and performance in demanding environments.36 Such PPAs have relatively high melting points, about 290°C. On the other hand, the degradation temperatures of such materials do not greatly exceed their melting points. Therefore, the requirements for melt processing are more rigorous and complex than those for lower melting PAs such as PA 66, which melts at 260–265°C.9 Properties of a neat PPA type are given in Table 12.4.
Poly(phthalamide)s
403
12.3.1 Mechanical Properties PPAs are typically low in ductility as reflected by generally low values for elongation at break and the lack of significant extensibility in tensile testing.29 The ductility and rigidity can be improved by blending with poly(olefine)s.
12.3.2 Thermal Properties The thermal stability of PAs was investigated in a series of papers by Lánská. The method of preparation influences the thermal stability of PAs.40 The side reactions are dependent on temperature. Thus, the polymers are containing different concentrations of structures which accelerate or retard the thermo-oxidation process. However, at polymerization temperatures below 250°C, the reaction time dos not influence the subsequent oxidation of the polymer. The oxidative thermal stability is markedly affected by the concentration of both carboxylic and basic end groups.41 Carboxylic acid groups initiate the homolytic decomposition of 6-hydroperoxy-6-hexanelactam. Thus, alkoxyl and hydroxyl radicals are formed as intermediates. Eventually, the monoamide of adipic acid is formed. PAs with carboxylic end groups are oxidized in a similar way. The rate of oxidization increases with the content of carboxylic groups.42 In the course of thermo-oxidative degradation of PAs stabilized with antioxidants, such as phenols and secondary aromatic amines and hindered nitroxy radicals chemiluminescence is observed.43 This effect is attributed to redox inhibition reactions.
12.3.3 Chemical Properties In addition, PPA shows a good chemical resistance. In particular, the resistance to decalcifying agents such as amidosulfonic acid and to hot oils, battery acid, and brake fluid has been pointed out.44
12.4 SPECIAL ADDITIVES For types used for molding, customary additives are stabilizers and antioxidants, flameproofing agents, agents against thermal decomposition and decomposition by ultraviolet light, lubricants, mold release agents, colorants,
404
High Performance Polymers Table 12.5: Additives for Molding Types26 Additive
Usage
Sterically hindered phenols Copper halides Resorcinols Salicylates Benzotriazoles Benzophenones Stearyl alcohol Stearic acid derivates N-alkylstearamides Rubbers Glass fibers Wollastonite Poly(dibromostyrene)
Antioxidant Antioxidant UV stabilizer UV stabilizer UV stabilizer UV stabilizer Mold release agent Mold release agent Mold release agent Impact modifier Filler & Reinforcement Filler & Reinforcement Flame retardant45
dyes and pigments and plasticizers.26 Some additives are summarized in Table 12.5.
12.4.1 Fillers When glass fibers are compounded in polyamides in high content, warping of the molded product can become a problem. Wollastonite exhibits better properties in this aspect.46 Wollastonite is a white mineral that consists essentially of calcium metasilicate. It is commonly used as an inorganic filler material of thermoplastic polymers for molding. The wollastonite fibers are treated with silane surface treatments by using γ -aminopropyltriethoxysilane or γ -glycidylpropylmethoxysilane.
12.4.2 Antioxidants PPAs must be processed at higher temperatures than aliphatic PAs. The processing temperature may be close to the degradation temperature. For this reason, stabilization is indispensable for molding operations of such materials.36, 47 During extended molding operations deposits are built up over time and form solid, intractable residues. When hot gases and decomposition byproducts cannot escape, adiabatic compression in the final feed zone may occur. Thus the mold temperature still further raises. The resin may become carbonized and darkened. Eventually, the molding op-
Poly(phthalamide)s
405
Table 12.6: Antioxidants for Poly(phthalamide) Antioxidant
References
Phenolic antioxidant Pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] N,N -hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamide)
46 46
Phosphorous auxiliary antioxidant 2-[[2,4,8,10-Tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2] dioxaphosphepin-6-yl]oxy]-N,N-bis[2-[[2,4,8,10-tetrakis (1,1-dimethylethyl)dibenzo[d,f][1,3, 2] dioxaphosphebin6-yl]oxy]-ethyl]ethanamine (IRGAFOS® 12)
46
Sulfur auxiliary antioxidant 2,2-Thiodiethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] Tetrakis[methylene-3-(dodecylthio)propionato]methane
46 46
eration must be halted and the engine must be cleaned. In the worst case, because of the formation of intractable residues, a costly replating or other refinishing of the mold cavity may be mandatory. Since PPAs contain both aliphatic and aromatic moieties, the basic knowledge on pure aliphatic PAs is expected to be sound also for partially aromatic PAs. Antioxidants are summarized in Table 12.6. The structure of IRGAFOS® 12 is shown in Figure 12.4. For aliphatic PAs, the short-term thermal stability needed for most processing can be realized by incorporating a hindered phenolic antioxidant such as di-tert-butyl cresol.36 Primary antioxidants based on secondary aromatic amines are efficient stabilizers of the oxidation reactions of PA6. The efficiency can be further increased by a combination with a phosphite antioxidant. In contrast, primary antioxidants based on phenol are ineffective in the oxidation of PA6.48 Obviously, in a PA melt, phenols undergo reactions and the reaction products are unable to react with the amide peroxy radical. On the other hand, antioxidants of the hindered amine light stabilizer type interrupt the oxidation reaction of PA6 after an induction period. During this induction period, they are oxidized to the N-oxyl form, which is essentially reactive
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High Performance Polymers
O
O P O CH2 O
P O O CH2 CH2 CH2
N CH2 CH2 O P O O
Figure 12.4: IRGAFOS® 12
Poly(phthalamide)s
407
Table 12.7: Thermal Stability of Phthalamide-Caprolactam Copolymers36
ppm Cu
End groups COOH NH2 μ eq g−1 μ eq g−1
250 56 196 34 246 18 231 2 121 54 113 20 127 22 110 0 2 46 1 40 IV: Inherent viscosity
46 62 71 100 39 60 67 98 60 65
IV dl g−1
Initial tensile strength k psi
0.89 0.89 0.92 0.84 0.93 0.94 0.92 0.90 0.89 0.89
30.8 30.9 30.7 29.6 31.7 32.4 30.0 31.2 29.5 28.9
h to 50% tensile loss 220°C 210°C 960 1120 1340 1570 840 935 1175 1330 685 735
1125 1635 1565 2090 1075 1265 1480 1620 800 935
with the amide peroxy radical.48 Dispersions of solid cuprous phthalate and potassium iodide have been used at levels of 60 ppm Cu to stabilize PA 66 and copolymers containing HMD and IPA moieties. However, copper compounds are decarboxylating agents for aromatic acids. Adding such stabilizers in order to inhibit the oxidation of the aliphatic portion of the polyamide may promote the thermal decomposition of the aromatic portion of the polymer. The thermal stability of the aromatic part can be improved by controlling the balance of end groups or by introducing end cap groups to reduce the number of acid and amine end groups when a copper-based thermal oxidative stabilizer is used. End capping can be achieved by the introduction of acetamide end groups. The acid end groups are conveniently reduced by using an stoichiometric excess of diamine. Improved high temperature PAs, in particular partially aromatic PAs, containing small amounts of carboxylic acid end groups are stabilized with copper iodide or copper bromide as thermal stabilizing additive. These PAs exhibit an improved thermal oxidative stability.36 The thermal stability of phthalamide-caprolactam copolymers with a different amount of end groups and copper stabilizer is shown in Table 12.7. The inherent viscosity, i.e., the molecular weight and the initial tensile strength of the samples are kept approximately equal in the series of experiments. The inherent viscosity is determined by dissolving
408
High Performance Polymers
the samples 60:40 phenol:tetrachloroethane at an elevated temperature at a concentration of 0.1 g/25 ml and measuring the efflux times in a Cannon Ubbelohde Viscometer at 30°C. Additional independent research agreed that a copper stabilizer can effectively improve the long-term thermal stability of the formulation. However, the production of mold vent-clogging byproducts is also significantly increased, leading to rapid clogging of vents.47 Actually, formulations without a copper stabilizer do not show rapid clogging. In order to suppress the formation of mold vent-clogging byproducts, high molecular, i.e., low volatile, hindered phenols and secondary aryl amines must be added beside copper stabilizers. Such formulations exhibit, in addition to thermal stability, improved injection molding characteristics.
12.4.3 Impact Modifiers The impact modification of aliphatic PAs has been known for a long time. Impact modifiers for PPA are acrylic polymers composed from of ethylene, n-butylacrylate, acrylic acid, and maleic anhydride. Other types of impact modifiers are ethylene propylene rubbers, grafted with maleic anhydride.26
12.4.4 Flame Retardants When a PA is to be used in applications requiring self-extinguishing characteristics and flame retardant properties, it is necessary to resort to the addition of a flame retardant. Flame retardant formulations, known in the field of poly(ester)s, can be widely adapted to PAs. Brominated poly(styrene) in combination with an antimony compound, such as sodium antimonate, can be used.45 The performance is improved when a minor amount of calcium oxide is added. The thermal stability of these compositions is much better in comparison to formulations with magnesium oxide or zinc oxide. Flame retardant compositions are particulary useful for electronic applications.49 Halogen compounds have the disadvantage that they release highly corrosive and highly toxic degradation products such as hydrogen chloride and hydrogen bromide in a fire. To avoid the disadvantages associated with halogen compounds, there is a tendency to use halogen free flame retardants. Halogen free flame retardants for partially aromatic PAs are poly(phosphonates).50 Transparent PAs retain their transparency. The poly(phosphonates) must be added in amounts of 8–15%.
Poly(phthalamide)s
409
Triarylphosphine oxide moieties can be incorporated directly in the backbone of the PA.10 The polar group enhances the solubility of the PAs and imparts flame retardancy.
12.5 APPLICATIONS Partially aromatic PAs are finding wide acceptance for the use in applications where elevated temperatures and severe environments may be encountered. Applications include45 connectors for electrical and electronic devices, automotive applications, coatings, cookware, lawn and garden tools, power tools, and medical devices. The thermal stability of some PPA types allows welding processes under lead-free conditions in electronic applications.51
12.5.1 Transparent Types An empirical rule has been proposed that predicts whether a co-poly(amide) is amorphous or crystalline. This rule is based on the stereochemical contributions of the constituent monomers to the overall polymer chain structure.52 It turns out that PAs with high melting points are crystalline if more than 80% of the monomer units are symmetrical. Thus dissymmetry favors amorphous polymers. Crystallinity is related to the optical properties of a polymer. In general, crystalline PAs are not transparent. If a transparent PA is desired, the PA must be amorphous rather than crystalline. Crystallinity can be determined by observing a melting point in the polymer. The amorphous character is indicated by the lack of a melting point. Amorphous PPA based on IPA and HMD have a low dimensional stability at elevated temperatures. To overcome this drawback, the partial replacement of HMD with an isomer mixture of 2,2,4-trimethylhexamethylenediamine and 2,4,4-trimethylhexamethylenediamine, respectively, has been proposed. PAs composed from adipic acid, TPA and 4,4-dimethyl1,7-heptanediamine are transparent.15 In addition, 2-methylpentamethylenediamine, 2-ethyltetramethylenediamine,5 and 3,3 -dimethyl-4,4 -diaminodicyclohexylmethane6 are diamines for transparent PAs. Also, the combination of MXDA and IPA results in transparent types. Glass transition temperatures of some amorphous partially aromatic PAs are shown in Table 12.8. Increasing the proportion of neopentyldi-
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High Performance Polymers
Table 12.8: Glass Transition Temperatures of Some Amorphous Partially Aromatic PAs15 Composition TPA/IPA (40/60)-HMD40/60 TPA/IPA (40/60)-NPDA/HMD (50/50) IPA NPDA TPA-NPDA TPA/IPA (60/40)-NPDA 2,6-Naphthalenedicarboxylic acid-NPDA
Tg /[°C] 127 147 184 188 190 213
amine (NPDA) in the polymer composition increases the glass transition temperature of the polymer. If only the symmetric diamine HMD is used, the glass transition temperature remains low.
12.5.2 Compositions for Welding When thick-walled hollow molded articles and molded articles with a shape having thick-walled sections and thin-walled sections are produced, the molded article is produced in two parts. These parts are eventually connected by welding. Alternatively, the two parts are placed in another mold so that they face each other at the parting lines. Then, molten resin is injected about the periphery of the opposed faces to connect the parts. This technique is addressed as insert molding, or overlap molding.33 Other welding methods include:32 1. 2. 3. 4. 5. 6. 7. 8.
Injection welding, Vibration welding Ultrasonic welding, Spin welding, Hot plate welding, Heat ray welding, Laser welding, and High-frequency induction heating welding.
Pure crystalline PAs show an unsatisfactory weld strength. Thus, these materials cannot be used for parts requiring a high weld strength. In order to improve the weld strength, the compositions should contain crystalline and amorphous partially aromatic PAs.32, 33
Poly(phthalamide)s
411
The weld strength with respect to various welding methods is improved by adding small amounts in the range of 3% of amorphous partially aromatic PA to crystalline PA types.32 The resin composition may be formed by blending respective resin pellets, and melt mixing it in the stage of obtaining a final product or, alternatively, by previously melt mixing in a uniaxial or biaxial extruder and then subject it to molding. The resin composition may be used in extrusion molding, blow molding, or in injection molding. For example, 97% PA 6 and 3% of PA 6I/6T are uniformly mixed in advance, and kneaded to prepare pellets of a PA resin composition. Before further use and examination of their properties, the pellets are dried for 24 h under reduced pressure at 110°C.32
12.5.3 Electroplated Articles Motor vehicles include a substantial number of chrome-plated parts for both decorative and functional purposes. The overall appearance of the vehicle is significantly enhanced by these highly reflective chrome surfaces. However, trim elements also serve a functional purpose in that they help to absorb impact when the vehicle is involved in a collision and when the vehicle contacts flying gravel, road debris, roadway abutments or the like. Accordingly, plated metal on a trim element preferably must withstand impact without chipping, cracking, or delaminating. Conventionally, automotive trim elements have been manufactured from metals. From economic aspects, plateable plastics are an interesting alternative, because they reduce the vehicle weight and have a much greater design flexibility than metals. When parts are formed from plastic materials, a significant cost savings can be realized in comparison to metal parts. A wide variety of plated plastics are known, such as acrylonitrilebutadiene-styrene (ABS) and poly(carbonate) (PC). ABS has been plated to provide decorative articles such as headlamp surrounds, and plumbing and marine hardware. PC has been utilized as the substrate for plated motor vehicle door handles. PPA resins are interesting for high-temperature applications. Articles formed from mineral-filled PPA resins can be metallized by electroless and electroplating techniques.53 Special formulations of PPA
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High Performance Polymers
Table 12.9: Peel Strength as a Function of Trivalent Chromium Concentration and Rinse Temperature53 Trivalent Chromium conc. /[g l−1 ]
Peel Strength /[N]
27 32 45 52
0 0 9.8–19.7 35.6–53.4
Etch Rinse Temp. /[°C]
Peel Strength /[N]
54 39 33
5.8 20.9 30.7
can be metallized by the same electroless plating techniques developed for ABS resins. Those techniques consist of: 1. 2. 3. 4. 5.
Etching with a chromic acid-sulfuric acid solution, Treating the etched surface with a neutralizing solution, A catalyzing solution of a noble metal salt solution, An acid accelerating solution, and An electroless plating solution.
However, in the case of PPAs poor metal adhesion is observed. Unfilled impact-modified PPA resins are very difficult to plate successfully. Various methods have been suggested for improving the adhesion between plated metal and a plastic substrate, such as pre-etch conditioning.54 However, these methods are not promising for PPA. A process has been developed for electroplating a PPA resin, modified with ethylene propylene diene monomer rubber, ethylene-propylene rubber, and styrene-butadiene rubber. As etching solution, chromic acid is used. However, it has been found that the concentration of Cr3+ is crucial for the success of the method.53 The concentration of Cr3+ is in the range of 50–55 g l−1 . Low levels of Cr3+ result in poor adhesion of the final metal plating, while high levels of Cr3+ can cause the formation of small blisters in the metal plating. The influence of the process parameters on the peel strength is shown in Table 12.9.
Poly(phthalamide)s
413
Table 12.10: Compositions of Hot-melt Adhesives55 Monomer feed Caprolactam Laurolactam Adipic acid Azelaic acid Sebacic acid Dodecane diacid Terephthalic acid HMD Piperazine 2-Methylpentanediamine
mol-% 30 40 10 10 10 — — 25 5
25 50 — — 19 — 6 20 —
27 40 3 11 10 — 9 25 —
30 50 — — — 20 — — 20
30 40 — 15 15 — — — —
40 40 20 — — — — 20 —
—
5
8
—
30
—
[dl g−1 ] a
RV 1.37 1.36 1.34 Melting point [°C] 104 100 79 a RV: Relative viscosity: 0.5% in m-cresol
1.41 113
1.38 109
1.35 121
12.5.4 Hot-melt Adhesives Hot-melt adhesives based on aliphatic copoly(amide)s are used in the textiles field. The desired melting temperatures are around 80–130°C. These properties are adjusted by the special combination of the monomers. Statistical copoly(amide)s suitable as hot-melt adhesives are synthesized through hydrolytic polycondensation of the individual monomer components. A prolongation of the open time is obtained by the use of one additional diamine in addition to HMD. If small amounts of piperazine or 2-methyl-1,5-diaminopentane are incorporated into the polymer, products with a greater longevity of their adhesive properties are be obtained. If piperazine is added, the effect even seems to be a little stronger.55 Examples for compositions of hot-melt adhesives and their corresponding melting temperatures are shown in Table 12.10. The materials are to be addressed as partially aromatic PAs, however, with a low content of aromatic units. Even with aromatic acid components, low melting compositions can be obtained. However, the proportion of aromatic diacids may not be greater than 10 mol-%. A further increase in the content of aromatic compounds results in products with a high glass transition temperature. Such products would not crystallize any more and remain completely amorphous. These substances are mostly unsuitable as hot-melt adhesives, since they display a bad resistance to washing and dry cleaning.
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Table 12.11: Examples for Commercially Available Poly(phthal amide)s Tradename
Producer
Amodel® Balpound™ Grivory™ Laramid® Zytel® HTN PPA
Solvay Advanced Polymers Shikoku Chemicals Corp. Ems-Grivory Lati SpA DuPont
12.6 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 12.11. Tradenames appearing in the references are shown in Table 12.12.
12.7 SAFETY Most of the diacids used as monomers are classified as eye, skin and respiratory irritants. The diamines are more toxic than the diacids, and moreover corrosive and may cause burns. Usually, during fabrication, there should be no contact with the monomers, not even when reactive processing of the oligomers is used. Powders from some PA types have been reported as a potential irritant. Additional information must be consulted from the respective manufacturer. In a vessel of polymerization plant for PPA, incompletely reacted portions of the feed caused a pressure build up. In the course of subsequent maintenance, the gases escaped in an explosive manner.56
12.8 ENVIRONMENTAL IMPACT AND RECYCLING Polymeric, oligomeric or monomeric esters from aromatic dicarboxylic acids of any origin, even from polyester waste materials can be used as raw materials for PPA.24 The method contributes to the solution of the recycling problem concerning poly(ester)s in general. The process is described in detail in Section 12.2.6.
Poly(phthalamide)s
Table 12.12: Tradenames in References Tradename Description
Supplier
Admer® L 2100 Mitsui Petrochemical Corp. Poly(ethylene) grafted with 0.1% maleic anhydride16 Admer® Mitsui Chemicals, Inc. Adhesive resins, maleic anhydride grafted poly(ethylene) or poly(propylene)16 Amodel® (Series) Amoco Poly(phthalamide)28, 29, 45 Amodel® 1000 Amoco Poly(phthalamide)9, 36 Amodel® A 1000 Amoco Hexamethylene terephthalamide isophthalamide adipamide terpolymer29 Amodel® AF 1113 Amoco Aromatic copolyamide 6.6/6.I/6.T 449 Amodel® AF 4133 Amoco Aromatic copolyamide 6.6/6.T 549 Amodel® X4000 Amoco Hexamethylene terephthalamide isophthalamide adipamide terpolymer 65/359 Arlene® CH 230 Mitsui PA 6.6/6.T 549 Arlene® Mitsui Poly(phthalamide)45 Arnitel® Akzo Poly(ester) elastomer16 Carbowax® (Series) Union Carbide Corp. Poly(ethyleneoxide glycol) (PEG)14 Denka® SMI Denki Ragaku Kogyo Styrene maleide imide copolymer49 Exxelor® PO 1015 Exxon Poly(propylene) grafted with 0.3% maleic anhydride16, 28 Exxelor® VA 1801 Exxon Ethylene propylene rubber grafted with 0.6% maleic anhydride9, 29, 47
415
416
High Performance Polymers
Table 12.12 (cont): Tradenames in References Tradename Description
Supplier
Exxelor® VA 1803 Exxon Ethylene propylene rubber grafted with 0.4% maleic anhydride16, 26 Grivory® HTV-4X2VO Ems 6.6/6.T49 Grivory® HTVS-3X2VO Ems 6.6/6.T 749 Hytrel® DuPont Poly(ester) elastomer16 Kraton® Shell Styrenic block copolymer9, 29, 47 Lucalen® A 3710 MX BASF AG Copolymer of LDPE and 7% acrylic acid16 Lupolen® (Series) BASF AG Poly(ethylene)16 Naugard® 445 Uniroyal Chemical Co. 4,4 di(α ,α -Dimethyl-benzyl)diphenylamine47 Novolen® 1100 BASF AG Isotactic poly(propylene)16 Novolen® 2500 HX BASF AG Propylene/ethylene block copolymer, 10% ethylene16 Novolen® 3200 HX BASF AG Propylene/ethylene block copolymer, 2.5% ethylene16 PDBS® 80 Great Lakes Chemical Corp. 49 Poly(dibromostyrene) Primacor® 1410 XT Dow Ethylene acrylic acid copolymer with 10% acrylic acid9 Pyrocheck® 68 PB Ferro Corp. Brominated poly(styrene)45, 49 Rilsan® B MNO Elf Atochem PA 1249 Selar® PA3426 DuPont PA 6 T/I15
Poly(phthalamide)s
Table 12.12 (cont): Tradenames in References Tradename Description
Supplier
Stanyl® KS 200 DSM Low molecular weight PA 4.649 Stanyl® KS 300 DSM Medium molecular weight PA 4.649 Surlyn® DuPont Ionomer resin9 Trogamid® T Dynamit Nobel PA from terephthalic acid, 2,2,4-trimethylhexamethylenediamine and 2,4,4-trimethylhexamethylenediamine15 Tuftec® (Series) Asahi Chemical Industry Styrenic block copolymer31 Udel® Polysulfone Solvay Poly(bisphenol A sulfone)54 Ultramid® (Series) BASF AG Poly(amide)25, 26 Victrex® PES (Series) Victrex PLC Poly(aryl ethersulfone)49 Zytel® DuPont Poly(amide)9
417
418
High Performance Polymers
REFERENCES 1. Standard specification for polyphthalamide (PPA) injection molding materials. ASTM Standard ASTM D5336-03, ASTM International, West Conshohocken, PA, 2003. 2. P. Schlack. Verfahren zur Herstellung hochpolymerer linearer Polyamide. DE Patent 929 151, assigned to Hoechst AG, June 20, 1955. 3. J. Preston. Polyamides, aromatic. In H. F. Mark, N. Bikales, C. G. Overberger, and G. Menges, editors, Encyclopedia of Polymer Science and Engineering, volume 11, pages 381–409. Wiley Interscience, New York, 2nd edition, 1988. 4. H. Reimann, G. Pipper, H.-P. Weiss, C. Plachetta, and E. M. Koch. Partly aromatic copolyamides of reduced triamine content. US Patent 5 298 595, assigned to BASF Aktiengesellschaft (Ludwigshafen, DE), March 29, 1994. 5. G. Bier, F. Blaschke, H. aus der Funten, and G. Schade. Transparent polyamides. US Patent 4 111 921, assigned to Dynamit Nobel Aktiengesellschaft (Troisdorf, DE), September 5, 1978. 6. G. Schade, N. Vollkommer, and H. Wemheuer. Method of preparing modified or unmodified poly-(alkylpentamethyleneterephthalamide). US Patent 4 163 101, assigned to Dynamit Nobel Aktiengesellschaft (Cologne, DE), July 31, 1979. 7. K. Amakawa and T. Shitara. Process for production of xylylenediamine and/or cyanobenzylamine. US Patent 7 119 230, assigned to Mitsubishi Gas Chemical Co., Inc. (Tokyo, JP), October 10, 2006. 8. J. D. Matlack, J. G. Villanueva, B. A. Newman, L. D. Lillwitz, M. L. Luetkens, Jr., and G. E. Schmidt. Polyamide having improved gas barrier properties from adipic acid, isophthalic acid and m-xylylene diamine. US Patent 5 175 238, assigned to Amoco Corporation (Chicago, IL), December 29, 1992. 9. R. A. Montag, G. A. Corbin, and D. W. Garrett. Polyphthalamide composition. US Patent 6 306 951, assigned to BP Corporation North America Inc. (Chicago,IL), October 23, 2001. 10. Y. Zhang, J. C. Tebby, and J. W. Wheeler. “Polyamides incorporating phosphine oxide groups: IV. Aromatic-aliphatic polymers.” Eur. Polym. J., 35(2): 209–214, February 1999. 11. M. Kosaka, Y. Muranaka, and K. Wakatsuru. Process for preparing aromatic polyamides. US Patent 6 133 406, assigned to Mitsui Chemicals, Inc. (Tokyo, JP), October 17, 2000. 12. J. A. Richardson, W. Poppe, B. A. Bolton, and E. E. Paschke. Polycondensation process with mean dispersion residence time. US Patent 4 831 108, assigned to Amoco Corporation (Chicago,IL), May 16, 1989. 13. W. Poppe, Y.-T. Chen, and E. E. Paschke. Crystalline copolyamide from terephthalic acid, isophthalic acid and C6 . US Patent 4 617 342, assigned to
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Amoco Corporation (Chicago, (IL), October 14, 1986. 14. H. Ng. Partially aromatic polyamides and a process for making them. US Patent 6 355 769, assigned to DuPont Canada, Inc. (Mississauga, CA), March 12, 2002. 15. E. E. Paschke, W. Poppe, and D. P. Sinclair. Amorphous polyamide from neopentyl diamine. US Patent 5 081 223, assigned to Amoco Corporation (Chicago, IL), January 14, 1992. 16. A. Gottschalk, H. Fisch, G. Pipper, and M. Weber. Polyamide/polyolefin blends. US Patent 5 883 186, assigned to BASF Aktiengesellschaft (Ludwigshafen, DE), March 16, 1999. 17. U. Presenz, S. Schmid, R. Hartmann, and H. R. Luck. Method for producing a polyamide compound. US Patent 6 881 477, assigned to EMS-Chemie AG (Domat/Ems, CH), April 19, 2005. 18. Y.-T. Chen. Composition comprising crystallizable polyamide from terephthalic acid, adipic acid, aliphatic diamine and m-xylylene diamine. US Patent 5 194 577, assigned to Amoco Corporation (Chicago, IL), March 16, 1993. 19. J. Willis-Papi and T. Mutel. Single-phase or multi-phase continuous polyamide polymerization processes. US Patent 6 759 505, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), July 6, 2004. 20. J. A. Richardson, W. Poppe, B. A. Bolton, and E. E. Paschke. Polycondensation process with aerosol mist of aqueous solution of reactant salts. US Patent 4 603 193, assigned to Amoco Corporation (Chicago,IL), July 29, 1986. 21. K. Tanaka, H. Kurose, T. Shida, and M. Kikuchi. Production method of polyamide. US Patent 7 138 482, assigned to Mitsubishi Gas Chemical Company, Inc. (Tokyo, JP), November 21, 2006. 22. W. Nielinger, W. Alewelt, R. Binsack, L. Bottenbruch, and H.-J. Fullmann. Process for the preparation of copolyamide from adipic acid, terephthalic acid and hexamethylene diamine. US Patent 4 762 910, assigned to Bayer Aktiengesellschaft (Leverkusen, DE), August 9, 1988. 23. A. W. Etchells, III and F. K. Mallon. Continuous process for the production of polyamides. US Patent 7 009 028, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), March 7, 2006. 24. H. Wagner, J. Ensinger, and E. Krumpschmid. Method of producing partially aromatic copolyamides from aromatic dicarboxylic acid esters. US Patent 5 895 809, assigned to EMS-Polyloy GmbH (Gross-Umstadt, DE), April 20, 1999. 25. W. Goetz, C. Plachetta, U. Wolf, G. Blinne, and H. Reimann. Toughened partly aromatic copolyamides. US Patent 5 504 146, assigned to BASF Aktiengesellschaft (Ludwigshafen, DE), April 2, 1996. 26. H. Reimann, G. Pipper, H.-P. Weiss, C. Plachetta, E. M. Koch, G. Blinne, W. Goetz, and P. Steiert. Impact modifying rubber and partly aromatic
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27.
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High Performance Polymers copolyamides. US Patent 5 252 661, assigned to BASF Aktiengesellschaft (Ludwigshafen, DE), October 12, 1993. R. G. Keske. Polyphthalamides. In J. C. Salamone, editor, Concise Polymeric Materials Encyclopaedia, pages 1264–1265. CRC Press, Boca Raton, FL, 1999. G. T. Brooks, B. L. Joss, and C. L. Myers. Polypropylene-polyphthalamide blends. US Patent 5 283 284, assigned to Amoco Corporation (Chicago, IL), February 1, 1994. G. P. Desio, R. A. Montag, and G. A. Corbin. Polyphthalamide blends. US Patent 5 436 294, assigned to Amoco Corporation (Chicago, IL), July 25, 1995. A. Adedeji. High flow compositions of compatibilized poly(arylene ether) polyamide blends. US Patent 6 794 450, assigned to General Electric Company (Pittsfield, MA), September 21, 2004. M. D. Elkovitch, J. R. Fishburn, and S.-P. Ting. Poly (arylene ether)/polyamide composition. US Patent 7 182 886, assigned to General Electric Company (Schenectady, NY), February 27, 2007. K. Nakamura and A. Miyamoto. Polyamide resin composition showing excellent weld strength. US Patent 6 541 559, assigned to UBE Industries, Ltd. (Ube), April 1, 2003. M. Nozaki and T. Kuroe. Polyamide composition for welding. US Patent Application 2004 0 087 735, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), May 6, 2004. L. Incarnato, P. Scarfato, G. M. Russo, L. Di Maio, P. Iannelli, and D. Acierno. “Preparation and characterization of new melt compounded copolyamide nanocomposites.” Polymer, 44(16):4625–4634, July 2003. L. Incarnato, P. Scarfato, L. Scatteia, and D. Acierno. “Rheological behavior of new melt compounded copolyamide nanocomposites.” Polymer, 45(10): 3487–3496, May 2004. R. G. Keske. Partially aromatic polyamides having improved thermal stability. US Patent 5 962 628, assigned to BP Amoco Corporation (Chicago, IL), October 5, 1999. N. Pini, C. Zaniboni, S. Busato, and P. Ermanni. “Perspectives for reactive molding of PPA as matrix for high-performance composite materials.” J. Thermoplast. Compos. Mater., 19:207–216, 2006. J. A. Loontjens, B. J. M. Plum, and P. M. M. Nossin. High-molecular polyamide. US Patent 6 750 316, assigned to DSM N.V. (Heerlen, NL), June 15, 2004. IDES Integrated Design Engineering Systems. The Plastics Web®. IDES Inc. 209 Grand Avenue Laramie, WY 82070 USA [electronic] http://www.ides.com/prospector/, 2006. B. Lánská and J. Šebenda. “Thermo-oxidation of lactam polymers related to the conditions of their preparation by hydrolytic polymerization.” Eur. Polym.
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J., 21(10):891–894, 1985. 41. B. Lánská and J. Šebenda. “The effect of carboxylic and basic end-groups on the thermo-oxidation of hydrolytic polymers of lactams.” Eur. Polym. J., 22 (3):199–202, 1986. 42. B. Lánská. “Thermo-oxidation of lactam-based polyamides with carboxylic end-groups: Decomposition of 6-hydroperoxy-6-hexanelactam in the presence of carboxylic acids.” Eur. Polym. J., 30(2):197–204, February 1994. 43. B. Lánská, L. Matisová-Rychlá, and J. Rychlý. “Chemiluminescence of polyamides III. Luminescence accompanying thermooxidation of lactam-based polyamides stabilized by antioxidants.” Polym. Degrad. Stabil., 72(2):249– 258, May 2001. 44. R. Hagen. “Alternatives to high-performance plastics. Special properties of partially aromatic copolyamide.” Kunststoffe, 87:622–624,626, 1997. 45. M. G. Reichmann. Flame retardant high temperature polyphthalamides having improved thermal stability. US Patent 5 773 500, assigned to Amoco Corporation (Chicago, IL), June 30, 1998. 46. M. Nozaki, R. Koshida, T. Tasaka, and T. Ushida. Aromatic polyamide compositions for molding. US Patent 6 784 279, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), August 31, 2004. 47. J. M. Hurley and B. H. Bersted. Method for reducing mold deposit formation during moldings of polyamide and composition therefor. US Patent 6 518 341, assigned to Solvay Advanced Polymers, LLC (Alpharetta, GA), February 11, 2003. 48. B. Lánská. “Stabilization of polyamides–I: The efficiency of antioxidants in polyamide 6.” Polym. Degrad. Stabil., 53(1):89–98, 1996. 49. H. G. J. Havenith, W. J. M. Sour, J. Tijssen, and R. M. Leeuwendal. High-melting polyamide composition for electronic applications. US Patent 6 441 072, assigned to DSM N.V. (Heerlen, NL), August 27, 2002. 50. W. Nielinger, H. Kauth, and H.-J. Fuellmann. Flame-resistant polyamides. DE Patent 3 613 490, assigned to Bayer AG (DE), October 29, 1987. 51. K. J. Steffner. “Lead-free soldering. High-temperature polyamides.” Kunststoffe, 95:195–198, 2005. 52. J. G. Dolden. “Structure-property relationships in amorphous polyamides.” Polymer, 17(10):875–892, October 1976. 53. R. J. Timmer. Method for electroplating elastomer-modified polyphthalamide articles. US Patent 5 928 727, assigned to Lacks Industries, Inc. (Grand Rapids, MI), July 27, 1999. 54. L. P. Donovan, E. Maguire, and L. A. Kadison. Pre-etch conditioning of polysulfone and other polymers for electroless plating. US Patent 4 125 649, assigned to Crown City Plating (El Monte, CA), November 14, 1978. 55. G. Poessnecker, J. Spindler, and E. Kinkelin. Low-melting copolyamide and their use as hot-melt adhesives. US Patent 6 590 063, assigned to EMSChemie AG (Domat/EMS, CH), July 8, 2003.
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56. N. N. Thermal decomposition incident. Investigation Report 2001-03-I-GA; NTIS number PB2002-107365, U.S. Chemical Safety and Hazard Investigation Board, National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161, June 2002.
13 Aramids Aramid is an acronym originating from aromatic poly(amide). Wholly aromatic poly(amide)s (PA)s were described in the 1960s.1–3 Aramid fibers were originally intended to replace steel belting in vehicle tires. Aramids are reviewed in the literature.4–7 In addition to wholly aromatic PAs, PAs are termed as aromatic PAs, when at least one constituting component is of an aromatic nature.8 These types of partially aromatic PAs are in between nylons and aramids. The particular advantages are easier techniques of polymerization and fabrication. So, if the properties of partially aromatic PAs are sufficient for certain particular applications they can be used instead of wholly aromatic PAs. Partially aromatic PAs are dealt with in this chapter only marginally; they are summarized in the poly(phthalamide) chapter.
13.1 MONOMERS Monomers for poly(arylamide)s are shown in Table 13.1. As for nylons, the monomers are used as a combination of diamines and diacids, or derivatives of diacids, respectively. The acid groups are activated as they are converted into acid chlorides. The most common diacid chlorides are shown in Figure 13.1. Most common diamines are shown in Figure 13.2. Wholly aromatic PAs based on 1,4-bis(4-carboxyphenoxy)naphthalene or 2,6-bis(4-carboxyphenoxy)naphthalene and aromatic diamines are readily soluble in a variety of organic solvents such as dimethylacetam423
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High Performance Polymers
Table 13.1: Monomers for Poly(arylamide)s Diamines
Remarks
1,4-Phenylenediamine 1,3-Phenylenediamine 3,4 -Diaminodiphenyl ether 4,4 -Diaminodiphenyl ether
Kevlar®, Twaron® Nomex® Technora® Films9
Diacids
Remarks
Terephthaloyl chloride Isophthaloyl chloride 2-Chloroterephthaloyl chloride 1,4-Bis(4-carboxyphenoxy)naphthalene 2,6-Bis(4-carboxyphenoxy)naphthalene 5-Amino-2-(4-aminophenoxy)-pyridine
Kevlar®, Twaron® Films9 Organically soluble10 Organically soluble10 Organically soluble11
AB2 Types
Remarks
2,3-Bis(4-aminophenyl)-quinoxaline-6-carboxylic acid 2,3-Bis(4-aminophenyloxyphenyl)-quinoxaline-6-carboxylic acid
Hyperbranched polymers12 Hyperbranched polymers12
Cl
Cl
O
O
O
O
C
C
C
C
Cl
Terephthaloyl chloride
Cl
Cl
2-Chloroterephthaloyl chloride
O
O C
C
Cl
Cl
Isophthaloyl chloride
Figure 13.1: Diacid Chlorides
Aramids
NH2
H 2N
H 2N
1,3-Phenylenediamine
425
NH2
1,4-Phenylenediamine
O
NH2
H 2N 3,4′-Diaminodiphenyl ether H 2N
O
NH2
4,4′-Diaminodiphenyl ether H2N CH2 CH2 CH2 CH2 CH2 CH2 CH2 NH2 1,6-Diaminohexane H2N (CH2)10 NH2 Decamethylene diamine
Figure 13.2: Diamines
ide and N-methyl-2-pyrrolidone (NMP).10, 13 Naphthalene based diacid monomers are shown in Figure 13.3. Transparent, tough and flexible films of these polymers can be cast from the solutions. 5-Amino-2-(4-aminophenoxy)-pyridine is another type of diamine for organically soluble wholly aromatic PAs.11 The basic structure of the PA is not very much changed. Note that 5-amino-2-(4-aminophenoxy)-pyridine is very similar to 4,4 -diaminodiphenyl ether. Obviously, it is sufficient in introduce asymmetry into the molecule to achieve solubility. The thermal properties of the resulting aramids are essentially unchanged. Polymers based on AB2 types have been reported to be used in combination with bismaleimide polymers, in order to initiate curing.12 Monomers for hyperbranched poly(arylamide)s are shown in Figure 13.4.
13.2 POLYMERIZATION AND FABRICATION Aromatic polyamides are prepared via two major general routes:12 1. Polycondensation reaction via an aromatic diacid chloride and a
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High Performance Polymers
O
O
HO C
O
O
C OH
1,4-Bis(4-carboxyphenoxy)naphthalene O C OH
O HO C
O
O 2,6-Bis(4-carboxyphenoxy)naphthalene
Figure 13.3: Naphthalene Based Diacid Monomers10
NH2
O HO C
N N NH2
2,3-Bis(4-aminophenyl)-quinoxaline-6-carboxylic acid
O HO C
O
NH2
O
NH2
N N
2,3-Bis(4-aminophenyloxyphenyl)-quinoxaline-6-carboxylic acid
Figure 13.4: Monomers for Hyperbranched Poly(arylamide)s12
Aramids
Cl NH2 +
H 2N
Cl C
C O
H
427
O
H N
N C O
C O
Figure 13.5: Synthesis of Aramids from Diamines and Dichlorides
diamine and 2. Direct polycondensation reaction of a dicarboxylic acid and a diamine. Further, other routes have been suggested that seem to be used rather rarely. For the first route, the polymerization is usually conducted at temperatures at or below 0°C under an inert atmosphere. This is necessary, because of the extreme moisture sensitivity of diacid chlorides and the highly exothermic nature of the reaction between an amine and a carboxylic acid chloride. In order to achieve a high molecular weight for the resulting polyamides, the diacid chloride monomers must be purified prior to polymerization. The dicarboxylic acid monomers used in the second route are cheaper, in comparison to the diacid chloride monomers. They are much less sensitive to moisture and relatively easy to purify via recrystallization. However, the condensation reaction proceeds badly under ambient temperature conditions. Therefore, a phosphorus based promoter such as triphenyl phosphite is used to activate the carboxylic group and the reaction is conducted at elevated temperatures.
13.2.1 Acid Chloride Route Kevlar® is synthesized by the condensation of 1,4-phenylenediamine and terephthaloyl chloride. The synthesis is shown in Figure 13.5. NMP is used as a solvent, together with CaCl2 as an ionic component.14 The process
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High Performance Polymers
is performed at 0°C. Before the usefulness of CaCl2 was discovered, a mixture of hexamethylphosphoramide and NMP had been proposed.15
13.2.2 Acid Route The preparation of amides via the free acid, diamine and catalysts, is also referred to as the Yamazaki reaction.16 Organic phosphor containing compounds, such as triphenyl phosphite are used as catalysts. In addition, in NMP/pyridine solvents, salts such as LiCl and CaCl2 are dissolved that favor polymerization. The reaction has to be conducted at elevated temperatures in order to achieve high-molecular-weight polymers. Isophthalic acid gives somewhat higher molecular weight polymers than terephthalic acid. Nevertheless, for certain types of PAs, products with only low molecular weights are obtained.17 The acid route is used rather for the preparation of specialized types of polyamides.
13.2.3 Carbon Monoxide Route As pointed out already above, aromatic polyamides are conventionally prepared by condensing aromatic diacid chlorides and aromatic diamines in polar aprotic solvents. A disadvantage of such a process is that the variety of aromatic polyamides produced is limited by the small number of commercially available diacid chlorides. Moreover, the diacid chlorides are hydrolytically sensitive. A process has been suggested that uses carbon monoxide, an aromatic dichloride and a diamine. As catalyst, a palladium complex is used, bis(triphenylphosphine)palladium(II)chloride.18 The catalyst induces the carbonylation of aryl aromatic chlorides.
13.2.4 Partially Aromatic Poly(amide)s Partially aromatic polyamides consist of aromatic dicarboxylic acid and aliphatic diamine monomer units. Such polyamides are generally characterized by high melting points, high glass transition temperatures, low moisture absorption and, unlike aliphatic polyamides such as nylon 6 and nylon 66, good dimensional stability under moist conditions. The combination of high temperature and dimensional stability render
Aramids
429
partially aromatic polyamides particularly suitable for use in electronics, engineering plastics, films and fibers.19 Partially aromatic PAs can be prepared from the acid, instead of the acid chloride in a multi step process. The first steps are conducted as a solid state polymerization with increasing temperature steps, and optionally feeding monomers after each reaction step. The final steps are proceeding as a melt condensation reaction.8 Alternatively, instead of the acids, the corresponding esters can be used as starting materials.19 Esterification of the dicarboxylic acid advantageously lowers its melting point to a temperature that allows melting of the acid while minimizing the thermal degradation. The admixture of the dicarboxylic acid component and the diamine component in the form of a melt is thereby facilitated. Further, the partially aromatic polyamide formed by these reactants likewise contains alkyl side chains. These side chains also depress the melting point of the final PA. Thus the resulting PA can be more readily processed than the corresponding polyamide that lacks such alkyl side chains.
13.2.5 Fibers Fibers are delivered in three basic forms: 1. Continuous multi filament yarn, 2. Staple fibers (cut and crimped), and 3. Pulp (short cut and fibrilated). 13.2.5.1
Spinning
Aramid fibers have been reviewed in the literature.5, 20 The conventional aromatic PAs suffer from being insoluble in organic solvents. Therefore, for Kevlar®, fiber spinning is done in concentrated sulfuric acid as a solvent. The spinning mass used is prepared by mixing sulfuric acid with the polymer at 70–100°C.21, 22 In air gap spinning, the solution of the aramid is forced through a spinneret. The face of the spinneret is in contact only with a gas, usually air. After travelling a short distance through the air, the solution in the form of a fine jet enters a coagulant.
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High Performance Polymers
The coagulant extracts the solvent from the polymer, resulting in the formation of the polymer fiber.23 The coagulant is water. The addition of small amounts of a drag reducing polymer allows a higher throughput or the production of fibers with a smaller diameter. Poly(ethylene oxide) is a suitable drag reducer. 13.2.5.2
Modification and Treatment
Fabric blends of aramid fibers and flame resistant cellulosic fibers have been described. These fabric blends are popular for use in protective garments. These blends are more comfortable than fabrics made of aramid fibers alone.24 p-Aramid fiber has a highly ordered fibrillar structure with a propensity for fibrillation attributable to the lack of lateral forces between macromolecules. As the p-aramid content of a fabric increases above 5%, the extent of fibrillation of the p-aramid fibers increases and the actual fibrillations can become more noticeable and objectionable. With the wear, abrasion and laundering that occurs as the fabric article is used over time, fabrics lose their aesthetic appeal. The appearance of the fabric can be restored by a dye treatment. Highly fibrillated areas of cloth can be placed in an aqueous bath at Basic Yellow 40. The materials is contacted with the aqueous dye solution for 30 minutes followed by a treatment of a 10% detergent solution of hexylene glycol at 60°C for 10 minutes. Finally, the materials are rinsed thoroughly with water and dried.25 Hydrophobically finished aramid fabric can be produced by a water-repellent agent. In addition, the application of an antistatic agent is advantageous.26 The water repellant agent is a mixture of fluoroacrylate polymers.
13.2.6 Aramid Paper Aramid paper pulp may consist both of the meta and the para form of aramid. Short fibers, staple fibers, pulp, and polymer particles of aramid swollen with water can be used as starting material for aramid paper. Papermaking is done in a conventional manner. Aramid paper pulp can contain both aramid floc and aramid fibers. Flocs are addressed as short fibers cut from longer aramid fibers. High porosity papers can be made using high levels of floc instead of fibers.27
Aramids
431
Alkaline treatment of the pulp results in papers with enhanced tensile strength.28 It is believed that the alkaline treatment effects hydrolysis reaction of the surface of the polymer. Thus the surface contains an increased amount of amino groups. These materials can be used in slipping clutches. Aramid pulp is widely used as a substitute for asbestos.29 The aramid paper is used as insulating paper. In this case, mica, ground quartz, glass fibers, alumina, or talc, can be incorporated to improve the insulating properties. In contrast, if alumina laminae, carbon black, or stainless steel short fibers are incorporated, electrical conductive papers are obtained. Aramid paper is also used as a reinforcing agent in honeycombs.
13.2.7 Honeycombs Aramid honeycombs find use in the fields of aerospace, transport equipment, etc. Aramid honeycomb sheets comprising a nylon-type resin, particularly a p-aramid honeycomb resin, have flame retardant, tough and other excellent properties required for a honeycomb core structure. However, the method of fabrication of aramid honeycombs is a rather complicated process. The process is described in detail in the literature.30, 31
13.2.8 Aramid Films Aromatic PA films are produced by casting a polymer solution in sulfuric acid onto an endless belt. In addition, the solution contains microparticles of silica to improve the surface properties. The casting solution is guided after intermediate heating into a coagulation bath and the film is released from the belt and post treated.32 It is possible to condense the aramide directly in an NMP solution and cast the solution. This process directly produces a transparent film from an aromatic polyamide dope without requiring the step of dissolving the aromatic polyamide in concentrated sulfuric acid. The process does not require any acid resistant equipment and is inexpensive.33 A procedure for the production of a film has been reported as follows:9 In NMP, 0,85 mol 2-chloro-p-phenylenediamine, and 0.15 mol 4,4 -diaminodiphenyl ether are dissolved. 0.985 mol 2-chloroterephthaloyl chloride is added. After 2 hours, the polymerization is complete. The mixture is then neutralized with lithium hydroxide. The polymer solution is filtered and cast onto an endless belt. The solvent is evaporated at 160°C. A
432
High Performance Polymers Table 13.2: Properties of an Aramid Fiber a 34 Property Density
Value 1.44
Unit g cm−3
Straight Test on Conditioned Yarns Tensile Modulus Elongation Break Breaking tenacity Specific Heat (25°C) Thermal Conductivity Decomposition Temperature (Air) a Kevlar™ 29, DuPont
70,500 3.6 2,900 1.42 0.04 427–482
MPa % MPa J kg−1 K−1 W m−1 K−1 °C
film with a polymer content of 45% can be continuously peeled off from the belt. Then the film was guided into a bath of NMP and water to extract the residual solvent, the inorganic salt, and impurities. The film is stretched between nip rollers in the longitudinal direction at a stretching ratio of 1.20. Applications of the films are in: • Magnetic recording media, • Acoustic diaphragms for audio speakers, and in • Electronic applications.
13.3 PROPERTIES Properties of an aramid fiber are shown in Table 13.2. Extensive tables of chemical stability are given in the literature.34 Nomex® can be chlorinated without any significant decomposition, however, Kevlar® decomposes under the same conditions of chlorination. Experiments with model compounds revealed that the p-diaminophenylene moiety is oxidized to a quinone intermediate when treated with hypochlorous acid, that is not stable. In contrast, with Nomex® such a reaction mechanism is not possible.35
13.3.1 Mechanical Properties The tensile strength of aramids are comparable to cast metals. In additions, they exhibit low creep and low water absorption. Therefore, these materials
Aramids
433
are suitable for metal replacement. In the case of high loading rates an increase in material stiffness and strength compared to the static behavior may occur. This is addressed as strain rate effect. This effect is important in dynamic finite element calculations. Actually, in taking account of this effect the material may be designed more lightly. A strain rate effect has been demonstrated for aramid paper honeycombs.36 13.3.1.1
Friction and Wear Properties
Aramid-containing composites are often used in sliding applications. The estimation and prediction of the tribological properties of such composites is a desirable skill. Despite the increasing use of polymeric composites, the knowledge of their tribological behavior is limited and lacks predictability. Studies with a pin-on-disk apparatus have been performed to elucidate the tribological behavior of aramid containing polymer composites. These studies include theoretical considerations and can serve as a directive on how to get information about tribological and wear properties in other related systems.37 13.3.1.2
Impact Behavior Properties
Impact problems are becoming increasingly important to industry, with respect to safety issues. The designer has to take into account accidental loads of the material caused by dropped objects, collisions or explosions. In particular with respect to aramid materials, ballistic protection applications are an important issue. Advantageously, the materials should have a large capacity to absorb kinetic energy. There are standard test methods for testing the impact strength under ordinary conditions.38, 39 In addition, there is an ISO Standard with respect to bullet resistance of protective clothing,40 but remarkably there is no ASTM Standard with respect to this topic. In classical ballistic tests, a projectile is shot at a stationary target which consists of the material to be tested. The residual kinetic energies are studied. A recently developed technique for lightweight materials uses the reverse ballistic impact. In contrast to the classical tests, the target is moving and the projectile is a rest.41 Nitrogen from a commercially compressed gas bottle can be used as acceleration gas. Velocities of up to
434
High Performance Polymers
400 m s−1 can be achieved, depending on the nature and the pressure of the gas. As an exemplary result of the study, is has been found that in comparison to standard shooting tests with soft bullets, woven aramid panels have a slightly better resistance than knitted panels with a comparable areal density.41
13.3.2 Thermal Properties The high fluidity of aramids at elevated temperature enables fabricating thin parts by conventional injection molding techniques. The flow properties are retained even at a glass fiber content as high as 60%.
13.3.3 Optical Properties Aramid is sensitive to UV light. The effects of simulated solar UV irradiation on the mechanical and structural properties of a poly(p-phenylene terephtalamide fiber have been studied.42 UV irradiation deteriorates the surface and defect areas of the fiber by photoinduced chain scission. Tenacity, break extension and energy to break of the filaments decrease rapidly and almost linearly. After 144 hours of irradiation, the energy to break drops below 40% to the initial value. Oxidation of the end groups occurs in air. The crystalline structure remains nearly unchanged.
13.4 SPECIAL ADDITIVES 13.4.1 Ultraviolet Stabilizers Aramid fibers have an inherently poor resistance to ultraviolet light. Thus, fabrics made from aramid fibers change in color when exposed to ultraviolet light. In addition, there is a significant loss of strength to the fabric. Ultraviolet absorbers or light screeners are often incorporated into the aramid fibers during manufacture or used to treat the aramid fibers in subsequent processing steps to improve their performance. In the normal textile dye process, dye molecules typically penetrate the fiber and become entrapped therein. Alternatively, the dye molecules may chemically bond with the fiber. However, aramid fibers are difficult to dye using conventional techniques.
Aramids
435
Thus, ultraviolet stabilization of aramid fibers is not easily accomplished by ultraviolet absorbers or light screeners in the dye bath. Actually, the normal dye process does not improve the ultraviolet stability of aramid fibers. Textile pigment printing, involves the printing of an insoluble coloring material on a textile fabric. The pigment, which has no affinity for the fibers of the fabric, is adhered to the fabric by a resin binder. It has been shown that a suitable pigment may serve as an UV stabilizer as such.43 Resin binders are acrylic copolymer binders, styrene-butadiene latex binders, or modified nitrile polymer binders. Tetrabutyl titanate was used as a sol-gel precursor of a nanosized TiO2 coating of aramid fibers. The photostability of the aramid fiber increased by this treatment.44
13.4.2 Electrically Conductive Modifier Often there is a need to drain or dissipate electrical charges off polymeric surfaces. Aramids are likely to collect electrical charges. Therefore, there is an inherent threat of sparking on discharge. Sulfonated poly(aniline) can be added to aramid in order to increase the electrical conductivity.45, 46 The electrically conductive composites prepared by these methods are not sufficiently conductive for shielding of electromagnetic interference. Still more effective in producing conductive aramids is electroless plating. A high electrically conductive fiber is obtained. The fiber is impregnated with metal complexes using supercritical carbon dioxide. The metal complexes are activated by reduction in hydrogen and then immersed into an electroless plating solution. The process has been using palladium(II)-hexafluoroacetylacetonate. The palladium complex can be activated in the absence of hydrogen.47 This reduces the risks of explosion in a technical process.
13.5 APPLICATIONS Aramid fibers are notorious for their application in bulletproof jackets, more generally addressed as ballistic resistant fabric articles.48, 49 Less exciting applications of aramids are in automotive, electrical, and electronic fields. They are also used in medical devices. Specific uses are summarized in Table 13.3.
436
High Performance Polymers Table 13.3: Fields of Uses for Aramids Usage
Examples
Friction materials Gaskets Medical applications Optical applications Papers Protective applications Reinforcing fibers Ropes and Cables Sporting equipment
Brake pads, linings, clutch facings Prosthetics, fibrous bone cement Optical fiber cables Insulating paper, friction paper Fire-fighting, cut protection, ballistics Tires, pipes Sail clothes, tennis strings
13.5.1 Friction Materials For the formulation of friction materials, both powdery and fibrous materials are mixed together. The binder consists of phenolic resins. Alternative binders include melamine resins, epoxy resins, or poly(imide) resin. Nowadays, in place of asbestos, fibrous reinforcements are used that include glass fiber, steel fiber, aramid fiber, potassium titanate fiber, etc. Since these fibrous reinforcements have their own specific properties, in practice, a mixture of them is used.50 Potassium titanate fiber is a hard inorganic fiber. It can improve the strength, the heat resistance and the wear resistance of the friction material. In addition, it can enhance the friction coefficient of the friction material through its abrasive property. Friction modifiers include inorganic friction modifiers such as alumina, silica, magnesia, zirconia, chrome oxide, or quartz and organic friction modifiers such as synthetic rubber or cashew dust. Graphite, or molybdenum disulfide serve as solid lubricants. Copper fiber increases the thermal conductivity of the formulation. Typical components in a friction material are shown in Table 13.4. The formulations are cured at high pressure and temperatures. It has been found out that potassium hexatitanate and potassium octatitanate fibers should be used together. In this way, the performance of the mixture can be increased considerably.
13.5.2 Gaskets Gaskets and other seals are required for use in many applications where adjacent surfaces are to be sealed to prevent fluid or gas leakage.51 Com-
Aramids
437
Table 13.4: Components in a Friction Material50 Component
Amount %
Phenolic Resin Cashew Dust Barium Sulfate Zirconia Graphite Copper Fiber Aramid Fiber Potassium Hexatitanate Fiber Potassium Octatitanate Fiber
10 10 25 2 8 10 5 15 15
Table 13.5: Components of a Gasket Formulation52 Component Aramid Hydrogenated NBR Calcium terephthalate Kaolin Othera Phenolic resin a
% 7.6 7.0 30.4 38 15 2
Graphite, vermiculite, aluminum trihydrate, magnesium hydroxide
posite gasket materials made by a wet laid process are comprised of a fiber component which is distributed within an elastomeric binder matrix, together with property improving solid fillers.52 The content of the fiber component is typically 3–15%. The elastomeric binder comprises about 3–15%. The rest are fillers. Aramids are used in fiber reinforced gaskets instead of asbestos fibers. Binders typically are synthetic rubbers. Typical components of a gasket formulation are shown in Table 13.5.
13.5.3 Reinforcing Materials Aramid fibers are widely used as reinforcing fibers in high performance composites. One disadvantage is the poor adhesion to the matrix materials. This arises from the lack of functional groups in the polymer.53 To overcome the lack of adhesion, the fibers are treated by so-called finish formulations, which is essentially a surface treatment.
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High Performance Polymers
Aramid tire cords have been treated by argon plasma etching and plasma polymerization of acetylene. The combination of argon plasma etching and acetylene plasma polymerization results in a greatly improved pull-out force of 91 N in comparison to 34 N with the untreated aramid tire cord. Thus, the plasma treatment improves the adhesion to rubber compounds.54 A surface modification by plasma treatment has been reported, using oxygen as the treatment gas.55 Immediately after the plasma treatment, the treated fibers exhibit lower water contact angles, higher surface oxygen and nitrogen contents, and larger interfacial shear strengths to epoxy resins. After ageing, the fibers still retain their improved properties to some extent. The effectiveness of such a finish can be tested by various methods, including:53 • Analysis of the composition of the surface with X-ray photoelectron spectroscopy, • Contact angle measurements, • Single-fiber pull-out tests, and • Composite mechanical testing.
13.5.4 Catalyst Supports Catalysts having inorganic supports are often heavy and awkward to deal with. They are generally in the form of small particles and have associated dust and fines. Such catalysts are often brittle or may become brittle in use. They may fracture or crumble causing increases in pressure drop or decreases in throughput of the chemical reaction that they are designed to facilitate. A p-aramid polymer supporting a catalyst agent has been presented for several reactions.56 The composition exhibits an improved catalytic activity in comparison to m-aramid polymer catalyst support. The actual catalyst agent is adhered on or within the support. The technique of depositing or incorporating the catalyst agent or precursor of the catalyst agent onto the supporting polymer depends on the catalyst composition. Not all catalytically active materials can tolerate the conditions required to spin p-aramid polymer or to cast p-aramid polymer films. In these cases, it is preferred that the catalyst composition be made by depositing the desired catalyst agent or the precursor of the catalyst agent on the surface of the p-aramid polymer.
Aramids
439
Examples for p-aramid supported catalysts are palladium catalysts for the dehydrohalogenation reaction and catalysts for the hydrogenation reaction of organic compounds.
13.5.5 Carbon Fiber Precursors Poly(m-phenylene isophthalamide) and other aramids have been proposed as precursor for activated carbon materials.57 These materials exhibit a very homogeneous micropore size. This property makes them usable as adsorbents, molecular sieves, catalysts or electrodes. Poly(m-phenylene isophthalamide) derived carbon fibers can be activated by carbon vapor deposition of benzene. The activated carbon fibers are suitable as molecular sieves for air separation.58 Carbon fibers can be obtained from the aramid by pyrolysis at 750–850°C. The pyrolysis may take place in Ar or CO2 . The fiber may be pre-impregnated with H3 PO4 . Steam, CO2 , and H3 PO4 serve as activators. The activation converts the amide groups in the polymer precursor into complex and heterogeneously distributed nitrogen functionalities.59
13.5.6 Cryogenic Fuel Tanks Cryogenic fuel tanks are essential components for space transportation systems. Materials for cryogenic fuel tanks must safely carry pressure, external structural loads, resist leakage, and operate over an extremely wide temperature range. Aramids exhibit a wide range in service temperature and are therefore candidates for such applications. A wide variety of skin and core materials have been tested for helium gas permeability.60 It turned out that Nomex® is superior in comparison to Kevlar®. A low level of permeability could be achieved, which meets the requirements.
13.5.7 Hyperbranched Aramids When monomers of the AB2 types are used, hyperbranched polymers will be formed on polymerization.12 The polymers can be employed to initiate bismaleimide polymerization. Another application of these polymers is to increase the toughness for thermosets such as bismaleimide polymers and epoxies.
440
High Performance Polymers
Table 13.6: Examples for Commercially Available Poly(arylamide)s Tradename
Producer
Remarks
Aramica® Armos® Heracron®
Asahi Chimvolokno JSC Kolon Industries, Inc. A. L. Hyde Co. Solvay Advanced Polymers Dupont Toray Industries DuPont Termotex Co., Mytishchi61, 62 Teijin Chemicals Teijin Chemicals Teijin Chemicals Difco Performance Fabrics, Inc. Teijin Twaron B.V.
p-Aramid film
Hydlar® Ixef® Kevlar® Mictron® Nomex® Rusar® Sulfron® Technora® Teijinconex® Thermatex® Twaron®
Reinforced aramid fiber
p-Aramid fiber p-Aramid film m-Aramid fiber
Sulfur modified aramid m-Aramid fiber Aramid and aramid blend fabrics
13.6 SUPPLIERS AND COMMERCIAL GRADES Aramides are available in a variety of grades. These include: • • • •
Glass fiber reinforced, Mineral filled Flame retardant equipped, and Impact modified types.
Fibers are delivered as pulps, which are chopped and refined fibers with a high surface area. The pulps are used as specialty additives that enhance performance by providing excellent reinforcement. Lightweight strength reinforcement can be achieved with continuous filaments. Honeycomb cores are available for aerospace industries. Spun yarn is used for protective coatings.63 Suppliers and commercial grades are shown in Table 13.6. Aramid fibers are used for uncountable other matrix polymers as reinforcing fibers. These types are not included in Table 13.6. Tradenames appearing in the references are shown in Table 13.7.
Aramids
Table 13.7: Tradenames in References Tradename Description
Supplier
Amodel® 1000 Amoco Poly(phthalamide)64 Aracon® Micro-coax, DuPont Metal coated Kevlar® fiber47 Basofil® BASF AG Melamine resin fiber43 Caprolan® Shaw Industries Nylon 648 Carbowax® (Series) Union Carbide Corp. Poly(ethyleneoxide glycol) (PEG)19 Conex® Teijin m-Aramid56 Kevlar® DuPont Aramid30, 43, 47, 48, 56 Kraton® Shell Styrenic block copolymer48 Leomin® AN Clariant GmbH Oleyl phosphonate lubricant, textile auxiliary26 Nomex® DuPont m-Aramid24, 43, 48 OLEOPHOBOL® (Series) Ciba Fluoroacrylate polymer, Oil and water repellent26 Polyox® 301 Union Carbide Corp. Poly(ethylene oxide)23 Twaron® Teijin Twaron B.V. Aramid26, 42, 43
441
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13.7 SAFETY Inhalation toxicology studies with fibrils made from p-aramid have been documented.65 Responses were detected, but the interpretation of these studies were regarded as controversial. There is some evidence for the biodegradability of p-aramid respirable-sized fiber-shaped particulates,66 which concept was supported in subsequent studies.67 Actually, material safety data sheets report the possibility of lung injury, if fiber dust is inhaled for a prolonged time. On the other hand, aramids are not considered to be dangerous by skin contact or by ingestion.
13.8 ENVIRONMENTAL IMPACT AND RECYCLING Traditionally, asbestos has been used in friction materials such as vehicle brake and clutch components because of its toughness and non-flammability. The use of asbestos has been restricted in many countries. Aramid, as an alternative material, has been increasingly used instead of asbestos. Aramid is a tough, synthetic fibrous material which is believed to be safer to health than asbestos. Methods have been developed to differentiate between asbestos and aramid.68 The method is based on rubbing a reference material against a sample of the unidentified material. The electrostatic charge produced at the rubbing location is then measured. In order to develop a beneficial application for fly ashes, automotive brake lining friction composites have been developed. These are based on phenolic resin, aramid pulp, glass fiber, potassium titanate, graphite, aluminum fiber and copper powder, in addition to fly ash.69
REFERENCES 1. H. W. Hill, Jr., L. S. Kwolek, and W. P. Morgan. Polyamides from reaction of aromatic diacid halide dissolved in cyclic nonaromatic oxygenated organic solvent and an aromatic diamine. US Patent 3 006 899, assigned to Du Pont, October 31, 1961. 2. L. S. Kwolek, W. P. Morgan, and R. W. Sorenson. Process of making wholly aromatic polyamides. US Patent 3 063 966, assigned to Du Pont, November 13, 1962. 3. H. F. Mark, S. M. Atlas, and N. Ogata. “Aromatic polyamide.” J. Polym. Sci., 61:S49–S53, 1962.
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4. H. H. Yang. Kevlar Aramid Fiber. John Wiley & Sons, Chichester, 1993. 5. S. S. Kathiervelu. “Aramid fibers.” Synthetic Fibres, 32:12–15, 2003. 6. Y. Imai. “Recent advances in synthesis of high-temperature aromatic polymers.” React. Funct. Polym., 30(1-3):3–15, June 1996. 7. S. Reboullai. Aramids. In J. W. S. Hearle, editor, High-performance fibres, Woodhead Publishing Limited series on fibres, pages 23–61. CRC Press, Boca Raton, 2004. 8. M. Kosaka, Y. Muranaka, and K. Wakatsuru. Process for preparing aromatic polyamides. US Patent 6 133 406, assigned to Mitsui Chemicals, Inc. (Tokyo, JP), October 17, 2000. 9. A. Tsukuda, T. Ieki, and Y. Nakajima. Highly size-stabilized polymer film and magnetic recording medium using the film. US Patent 6 797 381, assigned to Toray Industries, Inc. (Tokyo, JP), September 28, 2004. 10. G.-S. Liou, S.-H. Hsiao, and J.-C. Yang. Organic soluble wholly aromatic polyamides and preparation of the same. US Patent 5 856 572, assigned to Industrial Technology Research Institute (Hsinchu, TW), January 5, 1999. 11. I. In and S. Y. Kim. “Soluble wholly aromatic polyamides containing unsymmetrical pyridyl ether linkages.” Polymer, 47(2):547–552, January 2006. 12. J.-B. Baek and L.-S. Tan. Quinoxaline-containing AB2 monomers for hyperbranched aromatic polyamides. US Patent 6 552 195, assigned to The United States of America as represented by the Secretary of the Air Force (Washington, DC), April 22, 2003. 13. G.-S. Liou and Y.-T. Chern. “Synthesis and properties of new polyarylates from 1,4-bis(4-carboxyphenoxy)naphthyl or 2,6-bis(4-carboxyphenoxy)naphthyl and various bisphenols.” J. Polym. Sci., Part A: Polym. Chem., 37:645–652, 1999. 14. L. Vollbracht and T. J. Veerman. Process for the preparation of poly-p-phenyleneterephthalamide. US Patent 4 308 374, assigned to Akzo N.V. (Arnhem, NL), December 29, 1981. 15. H. Blades. High strength polyamide fibers and films. US Patent 3 869 429, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), March 4, 1975. 16. N. Yamazaki and F. Higashi. New condensation polymerizations by means of phosphorus compounds. In A. Berlin, editor, Polymerization Processes, volume 38 of Adv. Polym. Sci., pages 1–25. Springer, Berlin, 1981. 17. G. Keil, K. Heinrich, and P. Klein. Wholly aromatic polyamide. US Patent 4 987 216, assigned to Hoechst Aktiengesellschaft (Frankfurt am Main, DE), January 22, 1991. 18. R. J. Perry. Preparation of aromatic polyamides from carbon monoxide, a diamine and an aromatic chloride. US Patent 5 693 746, assigned to Eastman Chemical Company (Kingsport, TN), December 2, 1997. 19. H. Ng. Partially aromatic polyamides and a process for making them. US Patent 6 355 769, assigned to DuPont Canada, Inc. (Mississauga, CA), March
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12, 2002. 20. V. Gabara, J. D. Hartzler, K.-S. Lee, D. J. Rodini, and H. H. Yang. Aramid fibers. In M. Lewin, editor, Handbook of Fiber Chemistry, volume 16 of International Fiber Science and Technology Series, chapter 13, pages 975– 1029. CRC Taylor & Francis, 3rd edition, 2007. 21. D. R. Owens. Process of wet spinning polyamides and prevention of gel formation. US Patent 3 154 610, assigned to Celanese Corp., October 27, 1964. 22. H. T. Lammers. Process for the manufacture of fibres from poly-p-phenylene terephthalamide. US Patent 4 320 081, assigned to Akzo N.V. (Arnhem, NL), March 16, 1982. 23. S. D. Ittel and H. Shih. Air gap spinning process for aramids. US Patent 5 393 477, assigned to E. I. Du Pont de Nemours and Company (Wilmington, DE), February 28, 1995. 24. F. Lapierre. Fabric blends of aramid fibers and flame resistant cellulosic fibers. US Patent 6 576 025, assigned to Difco Performance Fabrics, Inc. (North Charleston, SC), June 10, 2003. 25. P. D. Yarborough and H. M. Ghorashi. Process for restoring the natural appearance of para-aramid clothing. US Patent 6 669 741, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), December 30, 2003. 26. C. K. Böttger, R. Hartert, K. R. Stolze, J. Jager, H. van de Ven, and P. G. Akker. Method for producing a hydrophobically finished aramid fabric and use thereof. US Patent 7 132 131, assigned to Teijin Twaron GmbH (Wuppertal, DE), November 7, 2006. 27. L. J. Hesler and S. C. Park. Aramid papers containing aramid paper pulp. US Patent 5 026 456, assigned to E. I. Du Pont de Nemours and Company (Wilmington, DE), June 25, 1991. 28. H. Werlich, W. Zwilling, and U. Wecker. Process for preparing p-aramide paper, p-aramide pulp, and the use of the paper and the pulp. EP Patent 1 277 880, assigned to Teijin Twaron Gmbh (DE), January 22, 2003. 29. M. Iwama and T. Takahashi. Process for preparing para-aromatic polyamide paper. US Patent 6 942 757, assigned to Teijin Twaron B.V. (Arnhem, NL), September 13, 2005. 30. P.-Y. Lin. High shear modulus aramid honeycomb. US Patent 5 137 768, assigned to E. I. Du Pont de Nemours and Company (Wilmington, DE), August 11, 1992. 31. K. Nomoto. Aramid honeycombs and a method for producing the same. US Patent 6 544 622, assigned to Showa Aircraft Industry Co., Ltd. (Tokyo, JP), April 8, 2003. 32. T. Yamada and M. Nakatani. Aromatic polyamide film, method for producing the same, and magnetic recording medium and solar cell using the same. US Patent 5 853 907, assigned to Asahi Kasei Kogyo Kabushiki Kaisha (Osaka, JP), December 29, 1998.
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33. K. Akiyoshi, K. Iwasaki, M. Niwano, and Y. Ohbe. Process for producing aromatic polyamide film. US Patent 5 659 007, assigned to Sumitomo Chemical Company, Limited (Osaka, JP), August 19, 1997. 34. Anonymous. Kevlar® aramid fiber. Technical Guide H-77848 4/00, DuPont Advanced Fibers Systems, Richmond, VA, 2001. [electronic] http://www2.dupont.com/Kevlar/en_US/tech_info/index.html. 35. A. Akdag, H. B. Kocer, S. D. Worley, R. M. Broughton, T. R. Webb, and T. H. Bray. “Why does kevlar decompose, while nomex does not, when treated with aqueous chlorine solutions?.” J. Phys. Chem. B, 111:5581–5586, 2007. 36. S. Heimbs, S. Schmeer, P. Middendorf, and M. Maier. “Strain rate effects in phenolic composites and phenolic-impregnated honeycomb structures.” Compos. Sci. Tech., 67(13):2827–2837, October 2007. 37. T. Ø. Larsen, T. L. Andersen, B. Thorning, A. Horsewell, and M. E. Vigild. “Comparison of friction and wear for an epoxy resin reinforced by a glass or a carbon/aramid hybrid weave.” Wear, 262(7-8):1013–1020, March 2007. 38. Test methods for determining the izod pendulum impact resistance of plastics. ASTM Standard ASTM D256-06a, ASTM International, West Conshohocken, PA, 2007. 39. Plastics - determination of Charpy impact properties - part 1: Non-instrumented impact test. ISO Standard 179, International Organization for Standardization, Geneva, Switzerland, 2005. 40. Protective clothing - body armour - part 2: Bullet resistance; requirements and test methods. ISO Standard ISO/FDIS 14876-2, International Organization for Standardization, Geneva, Switzerland, 2002. 41. M. Hockauf, L. Meyer, F. Pursche, and O. Diestel. “Dynamic perforation and force measurement for lightweight materials by reverse ballistic impact.” Composites Part A, 38(3):849–857, March 2007. 42. H. Zhang, J. Zhang, J. Chen, X. Hao, S. Wang, X. Feng, and Y. Guo. “Effects of solar uv irradiation on the tensile properties and structure of PPTA fiber.” Polym. Degrad. Stabil., 91(11):2761–2767, November 2006. 43. G. M. Kent, K. L. Johnson, D. R. Gadoury, and R. L. Mumford. Ultraviolet stability of aramid and aramid-blend fabrics by pigment dyeing or printing. US Patent 6 451 070, assigned to BASF Corporation (Mount Olive, NJ), September 17, 2002. 44. Y. Xing and X. Ding. “Uv photo-stabilization of tetrabutyl titanate for aramid fibers via sol-gel surface modification.” J. Appl. Polym. Sci., 103:3113–3119, 2007. 45. C.-H. Hsu. Electrically conductive fibers. WO Patent 9 722 740, assigned to Du Pont (US), June 26, 1997. 46. J. D. Hartzler. Electrically-conductive para-aramid pulp. US Patent 6 436 236, assigned to E. I. du Pont de Nemours & Company (Wilmington, DE), August 20, 2002.
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47. X. Zhao, K. Hirogaki, I. Tabata, S. Okubayashi, and T. Hori. “A new method of producing conductive aramid fibers using supercritical carbon dioxide.” Surf. Coat. Tech., 201(3-4):628–636, October 2006. 48. D. C. Prevorsek, G. A. Harpell, and D. Wertz. Ballistic resistant fabric articles. US Patent 5 677 029, assigned to AlliedSignal Inc. (Morristown, NJ), October 14, 1997. 49. Chitrangad. Aramid ballistic structure. US Patent 6 030 683, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), February 29, 2000. 50. A. Hikichi, M. Haruta, and K. Horiguchi. Friction material. US Patent 6 670 408, assigned to Akebono Brake Industry Co., Ltd. (Tokyo, JP), December 30, 2003. 51. J. H. Bickford, editor. Gaskets and Gasketed Joints. Marcel Dekker, New York, 1998. 52. R. P. Foster. Gasket material. US Patent 5 240 766, assigned to Hollingsworth & Vose Company (East Walpole, MA), August 31, 1993. 53. P. J. de Lange, P. G. Akker, E. Mader, S.-L. Gao, W. Prasithphol, and R. J. Young. “Controlled interfacial adhesion of twaron(r) aramid fibres in composites by the finish formulation.” Compos. Sci. Tech., 67(10):2027–2035, August 2007. 54. H. M. Kang, T. H. Yoon, and W. J. Van Ooij. “Enhanced adhesion of aramid tire cords via argon plasma etching and acetylene plasma polymerization.” J. Adhes. Sci. Tech., 20:1155–1169, 2006. 55. Y. Ren, C. Wang, and Y. Qiu. “Influence of aramid fiber moisture regain during atmospheric plasma treatment on aging of treatment effects on surface wettability and bonding strength to epoxy.” Appl. Surf. Sci., 253(23):9283– 9289, September 2007. 56. T. A. Koch, V. Gabara, E. W. Tokarsky, and J. J. McEvoy. Aramid polymer catalyst supports. US Patent 6 159 895, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), December 12, 2000. 57. S. Villar-Rodil, F. Suárez-García, J. I. Paredes, A. Martínez-Alonso, and J. M. D. Tascón. “Activated carbon materials of uniform porosity from polyaramid fibers.” Chem. Mater., 17(24):5893–5908, 2005. 58. S. Villar-Rodil, A. Martinez-Alonso, and J. M. D. Tascon. “Carbon molecular sieves for air separation from nomex aramid fibers.” J. Colloid Interface Sci., 254(2):414–416, October 2002. 59. J. Boudou, P. Parent, F. Suarez-Garcia, S. Villar-Rodil, A. Martinez-Alonso, and J. Tascon. “Nitrogen in aramid-based activated carbon fibers by tpd, xps and xanes.” Carbon, 44(12):2452–2462, October 2006. 60. M. Bubacz, A. Beyle, D. Hui, and C. C. Ibeh. “Helium permeability of coated aramid papers.” Composites Part B, 39(1):50–56, January 2008. 61. I. V. Tikhonov. “New organic materials with improved consumer properties and articles made from them.” Fibre Chemistry, 30(5):312–317, September 1998.
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62. I. V. Slugin, G. B. Sklyarova, A. I. Kashirin, and L. V. Tkacheva. “Rusar para-aramid fibers for composite materials for construction applications.” Fibre Chemistry, 38:25–26, 2006. 63. DuPont™ Kevlar®. [electronic] http://www2.dupont.com/Kevlar/en_US/products/index.html, 2007. 64. R. G. Keske. Partially aromatic polyamides having improved thermal stability. US Patent 5 962 628, assigned to BP Amoco Corporation (Chicago, IL), October 5, 1999. 65. D. B. Warheit. “A review of inhalation toxicology studies with para-aramid fibrils.” Ann. Occupat. Hyg., 39(5):691–697, October 1995. 66. D. B. Warheit, K. L. Reed, K. E. Pinkerton, and T. R. Webb. “Biodegradability of inhaled p-aramid respirable fiber-shaped particulates (RFP): Mechanisms of RFP shortening and evidence of reversibility of pulmonary lesions.” Toxicol. Lett., 127(1-3):259–267, February 2002. 67. D. B. Warheit, K. L. Reed, J. D. Stonehuerner, A. J. Ghio, and T. R. Webb. “Biodegradability of para-aramid respirable-sized fiber-shaped particulates (RFP) in human lung cells.” Toxicol. Sci., 89:296–303, 2006. 68. G. L. Hearn, J. A. Amner, and P. J. Walker. Method and apparatus to identify asbestos and aramid. US Patent 6 144 208, assigned to Ford Global Technologies, Inc. (Dearborn, MI), November 7, 2000. 69. S. Mohanty and Y. Chugh. “Development of fly ash-based automotive brake lining.” Tribol. Int., 40(7):1217–1224, July 2007.
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14 Poly(amide imide)s Poly(amide imide)s (PAI)s have been used in the late 1960s as wire coating materials.1–3 Soon afterwards this class of polymers were considered for aerospace applications.4 Varieties with urethane units and ester units have been discussed. Now various grades, suitable for extrusion and injection molding are available. Thermosetting types are also on the market. PAIs are in between poly(amide)s (PA)s and poly(imide) (PI) in their properties.
14.1 MONOMERS Common monomers are summarized in Table 14.1. Diacids used for Poly(amide imide)s are shown in Figure 14.1, diamines and diisocyanates are shown in Figure 14.2, and other compounds are shown in Figure 14.3. The only diacid anhydride of practical importance is trimellitic acid
O HOOC O O Trimellitic acid anhydride
Figure 14.1: Diacids Used for Poly(amide imide)s
449
450
High Performance Polymers
H 2N H 2N
NH2
NH2
O
1,3-Phenylene diamine
Diaminodiphenyl ether
NH2
OCN
CH2
NCO
H 3C
CH2 NH2
H 3C
Diisocyanatodiphenyl methane
CH3
Isophorone diamine
H 2N
NH2 O CH CH C CH CH
1,5-Bis(3-aminophenyl)-1,4-pentadien-3-one CH3 H 2N
O
O
NH2
H 3C 2,2′-Dimethyl-4,4′-bis (4-aminophenoxy) biphenyl H 3C
CH3 CH3
H 2N
O
C
O
NH2
CH3 H 3C
CH3
3,3′,5,5′-Tetramethyl-2,2-bis[4-(4-amino-phenoxy)phenyl]propane
Figure 14.2: Diamines and Diisocyanates Used for Poly(amide imide)s
Poly(amide imide)s
H 3C
O
H 2N N COOH
COOH
O 3-Aminobenzoic acid
4 -(α-Methylnadimido)-benzoic acid O N
COOH
HOOC O 2-(4-Carboxyphenyl)-1,3-dioxoisoindoline-5-carboxylic acid O
O
HOOC
COOH N O
O
N O
Oxy-bis(N-(4-phenylene)-trimellitic imide)
Figure 14.3: Amino and Imino Acids Used for Poly(amide imide)s
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High Performance Polymers Table 14.1: Monomers for Poly(amide imide)s Diacids and Anhydrides
References 5, 6
Trimellitic acid anhydride Diamines
References 6, 7
Diaminodiphenyl ether Diisocyanatodiphenyl methane (MDI) m-Phenylenediamine 2,2 -Dimethyl-4,4 -bis(4-aminophenoxy)biphenyl 1,5-Bis(3-aminophenyl)-1,4-pentadien-3-one Isophorone diamine Amino and Imino acids
5 6 8 9 10
References
3-Aminobenzoic acid 4-(α -Methylnadimido)-benzoic acid Oxy-bis(N-(4-phenylene)-trimellitic imide) 2-(4-Carboxyphenyl)-1,3-dioxoisoindoline-5-carboxylic acid
11 5 6 9
anhydride. Trimellitic acid is produced by the oxidation of pseudocumene.12 4-(α -Methylnadimido)-benzoic acid is prepared from 4-aminobenzoic acid and α -methylnadic anhydride.5 Oxy-bis(N-(4-phenylene)-trimellitic imide) is the product of condensation of trimellitic acid anhydride and 4,4 -diaminodiphenyl ether.6 Analogous bisimides can be prepared from trimellitic acid anhydride and MDI, 4,4 -diaminodiphenylmethane, or isophorone diamine. Diaminodiphenyl ether is a synonym for 4,4 -oxydianiline (ODA). These compounds are prepared by refluxing alkali aminophenates with chloronitrobenzene in dimethylformamide.13 3,4 -Diaminodiphenyl ether is prepared by dehydrogenating 3-amino-2-cyclohexene-1-one.14 2,2 -Dimethyl-4,4 -bis(4-aminophenoxy)biphenyl has a non-coplanar disubstituted biphenylene moiety and flexible aryl units. The incorporation of the disubstituted biphenylene in a polymer chain will not change the rod-like structure of the polymer backbone but reduces the interchain interactions. The tendency to crystallize and the transition temperatures are lowered and the solubility is enhanced.8 The diamine is synthesized by coupling 2,2 -dimethylbiphenyl-4,4 diol with p-chloronitrobenzene. Subsequently, the nitro groups are reduced. For similar reasons, 3,3 ,5,5 -tetramethyl-2,2-bis(4-(4-amino-phenoxy)phenyl)propane has been introduced as a diamine monomer.15 Poly-
Poly(amide imide)s
453
mers that are soluble in N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAC), N,N-dimethylformamide, m-cresol, pyridine, and tetrahydrofuran have been prepared. The solubility in polar aprotic solvents can be enhanced by the introduction of ether and alkyl flexible groups.16 Oligoether spacers have been synthesized by the condensation of trimellitic acid anhydride with 1,8-diamino-3,6-dioxaoctane.
14.2 POLYMERIZATION AND FABRICATION PAIs can be prepared by the condensation of a monoanhydride of a tricarboxylic acid and a primary diamine. Further PAI can be prepared by the reaction of dicarboxylic acid chlorides and diamines. Here, either the diamines or diacid halides already contain imide linkages, or mutatis mutandis, amide linkages. Classical PAI are usually obtained by reacting equimolar amounts of trimellitic acid halide anhydride and a diamine.17
14.2.1 Isocyanate Route The isocyanate route comprises the condensation of aromatic diisocyanate with aromatic tricarboxylic acid anhydride to give PAI without a poly(amic acid) as an intermediate.18 The isocyanate route may encounter problems as gelation could occur during the reaction. Further, it is difficult to get linear high-molecular-weight polymers due to the formation of byproducts.6 In the precipitation polymerization with aromatic diisocyanates problems in safety in working environment and cost emerge, since the reaction is carried out using poisonous nitro compounds or expensive sulfolane type solvents.19 PAI resins prepared by this method have poor melt flowability, melt processability, mechanical properties, and heat resistance.6 When oligomeric isocyanate compounds are used, PAI types with soft segments can be fabricated. These types can be used in pervaporation membranes, in order to tailor the properties.20
14.2.2 Acid Chloride Route The acid chloride method comprises the condensation of aromatic tricarboxylic acid chloride with aromatic diamine. This method is subdivided into:6
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High Performance Polymers 1. Low temperature homogeneous solution polymerization method, and 2. Low temperature precipitating polymerization method.
A typical example of the low temperature homogeneous solution polymerization method is the polymerization reaction at room temperature in a nonaqueous polar solvent such as DMAC.7 The low temperature precipitating polymerization method comprises the polymerization reaction in an organic solvent which is sparingly soluble in water, such as methyl ethyl ketone, and in an aqueous solvent by using triethylamine as an acid acceptor. This reaction is a kind of interfacial polymerization method.6
14.2.3 Direct Polymerization Route The direct polymerization method comprises the direct polymerization of aromatic diamine with the aromatic tricarboxylic acid anhydride in the presence of a dehydration catalyst. Dehydration catalysts include triphenyl phosphite, triphenyl phosphate, and tri-n-butyl phosphite.11 The reaction water is removed by heating to above 200°C in a nitrogen stream. The procedure can be performed in two stages, using different catalysts in each stage.19 Alternatively, the azeotropic condensation technique is used to remove the water. In the modified direct polymerization route, diaminodiphenyl ether, m-phenylenediamine (MPD) and trimellitic acid anhydride in a ratio of total 1 mol of diamino compound and 2 mol of anhydride compound are condensed in NMP using xylene to remove the water formed. The resulting diimidedicarboxylic acid is treated with thionyl chloride to form the acid chloride on the fly and the PAI is created by adding a mixture of diaminodiphenyl ether and MPD.6 PAI with a definite head-to-tail backbone can be prepared by condensation of trimellitic acid anhydride with 4-amino-4-nitrodiphenyl ether. Then the nitro group is reduced to give a monomer having amine and acid functional groups. This monomer is subjected to direct polymerization.21 The procedure is useful for the preparation of dissymmetric polymers. Further compounds with amino groups and nitro groups are 3-nitroaniline, 2-methyl-5-nitroaniline, and, 3-nitromesidine, i.e, 2,4,6-trimethyl-3-nitroaniline.
Poly(amide imide)s
O
O
O
C
C
N
O
N
N
H
455
O O
H
O
O H O
NH2
N
N C N
N NH2
Figure 14.4: Monomers for Photoactive PAI22
14.2.4 Microwave Polymerization Monomers for photoactive PAI types are shown in Figure 14.4. The photoactive diamine with the naphthalenic side group is prepared by the reaction of 1-naphthaldehyde with sulfuryl chloride, followed by the condensation with 2,4,6-triamino-1,3,5-triazine.22 In an analogous study, 4,4 -diaminodiphenyl ether was used as diamine and the phenanthrene unit was used instead of the naphthalene unit.23 It was attempted to imidize the monomers shown in Figure 14.4 by conventional methods. The reaction does not proceed even by applying long reaction times, high temperatures and azeotropic condensation. However, the application of microwave radiation produces polymers with quantitative yield and high inherent viscosity within a short time. The choice of an appropriate solvent is essential. The coupling with microwave irradiation increases with the dielectric constant. The use of a small amount of a polar solvent, which is heated when irradiated in a microwave oven, acts as: 1. Primary absorber, and 2. Solvent. It was found that for the condensation of anhydrides with amines, o-cresol is a particularly good additive.24, 25
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14.2.5 End Capped Poly(amide imide) The thermal integrity and solvent resistance can be greatly enhanced by capping the amide imide backbones with monomers that can be crosslinked.26 If an excess of the diamine is used, the free amino end groups can be end capped, e.g., with nadic anhydride or maleic anhydride. These materials can be crosslinked via the reactive vinyl group by a thermosetting reaction. Synthesis methods of such end capped materials have been developed in which diisocyanates and aromatic anhydride acids are used.5 It has been proposed that a wide variety of oligomeric materials with different functional groups can be blended to tailor the properties of the cured products17, 26
14.2.6 Unsaturated Poly(amide imide) An unsaturated PAI has been prepared from 1,5-bis(3-aminophenyl)-1,4pentadien-3-one and 2-(4-carboxyphenyl)-1,3-dioxoisoindoline-5-carboxylic acid.9 Thionyl chloride serves to activate the acid group as intermediate acid chloride. As cocatalyst, lithium chloride is used. The reaction runs at 0–5°C. The polymers exhibit a glass transition temperature of 220°C. Crosslinking of the polymer can be achieved with dibenzoylperoxide. Then the transition temperature raises up to 235°C.
14.2.7 Blends Blends of a PAI and poly(aryl ether ketone) exhibit improved solvent resistance and hydrolytic stability.27 Blends of sulfonated poly(ether ether ketone) and PAI have been tested as membrane materials for direct methanol fuel cells.28, 29 Miscible blends can be obtained. Blends of poly(urethane)s (PU)s and PAI, as the minor component have been reported for membrane applications.30 The resulting membranes are immiscible. Phase separation occurs when the amount of PU decreases.
14.2.8 Foams Foaming of PAI will result in heat resistant foams. There are such foams made from other types of polymers. Advantages and drawbacks of typical
Poly(amide imide)s
457
Table 14.2: Advantages and Drawbacks of Heat Resistant Foams10 Polymer
Advantage
Disadvantage
Poly(isocyanurate)
Good mechanical property, flame retardancy and morphological stability, cheap Excellent heat resistance
Poor heat resistance
Poly(benzimidazole) Poly(imide)
Poly(amide imide)
Excellent heat resistance, flame retardant, good electrical properties low permeability Toughness, good mechanical strength
Poor oxidation resistance Expensive
high heat resistant foams are compared in Table 14.2. PI resin foams are regarded as one of the most applicable materials by virtue of their excellent heat stability and flame retardancy. Researches are targeted to improve the physical properties and to simplify the process of preparation. On the other hand, PAI resins exhibit a better heat resistance than poly(ether imide) resins. In addition, they can be more easily processed in the melt. 14.2.8.1
Continuous Physical Foaming
PAI of cellular structure has long been known and observed in various stages of polymer preparation and processing.31 Actually, in a variety of situations, foaming is not desired or intentionally produced. For example, it has been known for several years that PAI can foam when purged from the nozzle of the injection barrel of an injection molding machine. Thus, a family of publications emerged that teach how to prevent foaming.32, 33 Accidentally produced foams exhibit an irregular, open-cell structure. Interconnected, randomly sized and shaped voids often vary in size, shape and distribution from one area of a foam to another. On the other hand, PAI foams can be produced intentionally. For example, satisfactory foam structure and properties can be obtained by heating a commercial Torlon® PAI powder in a closed mold to above glass transition temperatures for a time sufficient to soften and fuse the particles.31 Then, absorbed volatiles expand the polymer and fill the mold. The residence times range from several minutes to several hours. After heating,
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High Performance Polymers
the mold is cooled to below the glass transition temperature. The expanded cellular mass solidifies and a foamed article conforming to the mold cavity is obtained. This technique is sometimes referred to as confined free rise foaming or confined free expansion foaming. It is employed for the fabrication of shaped articles of simple geometry such as panels, sheets and blocks. Foamed articles can be also obtained by extrusion techniques. 14.2.8.2
Chemical Foaming
Prepolymer Foaming. Special prepolymers of PAI that allow simple foaming techniques have been developed. These types are characterized by a low melt viscosity. The use of isophorone diamine in PAI contributes to the decrease of melt viscosity since the packing density is decreased due to three methyl substituents.10 Foams are prepared similar to PI resins by heating a prepolymer of poly(amide amic acid). The water during the reaction will cause foaming. In detail, the process consists of:10 1. Pulverizing the poly(amide amic acid) resin prepolymer, into the size of 400μ m 2. Laminating the obtained poly(amide amic acid) powder in mold uniformly 3. Compressing, and 4. Foaming under a pressure of 300 kp cm−2 at 330°C. Prepolymers are prepared from a mixture of isophorone diamine (30 mol-%) and ODA (70 mol-%) in DMAC and reacted with stoichiometric amounts of trimellitic acid anhydride chloride at 0–10°C. Triethylamine is eventually added as an acid scavenger. Acetic anhydride and pyridine can be added as reaction catalysts for imidization of the poly(amide amic acid) if desired. The glass transition temperature of the PAIs is 270–320°C. The density of the foams is 0.1–0.5 g cm−3 . Table 14.3 shows the properties of the final materials dependent on the degree of imidization. Isocyanate Foaming. Alternatively, PAI foams can be prepared by the reaction of multifunctional isocyanates with multifunctional imido carboxylic acids.34, 35 The reaction is catalyzed by tertiary amines. Best results are obtained with polymeric diisocyanates from the MDI type. Surfactants
Poly(amide imide)s
459
Table 14.3: Properties of Foams10 Imidization Time/[h] Degree/[%] 0 12 24
Foam Density/[g cm−3 ] Properties
24.3 63.6 79.6
0.15 0.38 —
Flexible foam Rigid foam No foam
Table 14.4: Properties of Torlon® 4301 Property
Value
Density 1.46 Water Absorption 24 hrs 0.28 Tensile Modulus 6830 Tensile Strength 163 Tensile Elongation Brk 7 Flexural Modulus (23°C) 6890 Flexural Strength (23°C) 215 Compressive Modulus 5310 Compressive Strength 166 Notched Izod Impact 64.1 Unnotched Izod Impact 406 Thermal Conductivity 0.53 Surface Resistivity 8.0E+17 Volume Resistivity 8.0E+15 a Solvay Advanced Polymers
a36
Unit
Standard
g cm−3 % MPa MPa % MPa MPa MPa MPa J m−1 J m−1 W m−1 K−1 Ω Ω cm
ASTM D792 ASTM D570 ASTM D638 ASTM D638 ASTM D638 ASTM D790 ASTM D790 ASTM D695 ASTM D695 ASTM D256 ASTM D256 ASTM C177 ASTM D257 ASTM D257
can be added to improve the uniformity of the foam and to reduce voids and other imperfections.34
14.3 PROPERTIES Most PAIs are injection-moldable, amorphous, thermoplastic materials. They absorb water when subjected to humid environments or immersed in water. Properties of a general purpose PAI type are given in Table 14.4.
14.3.1 Mechanical Properties PAIs exhibit high strength at high temperatures up to 260°C. They exhibit excellent resistance to creep and wear under these conditions. Therefore, these materials are used for rotating and sliding components in automotive
460
High Performance Polymers
and industrial applications, such as bearings and bushings, seal rings, wear pads and piston rings .37 External lubrication is not needed.
14.4 APPLICATIONS Thermoplastic PAI resins find use in many applications, such as adhesives, coatings, filled and unfilled molding compositions, fibers, films, composites, and laminates. In particular they are used in aircraft, automotive electric and electronic applications, as coatings, seals, and in oil and gas processing equipment.
14.4.1 Membranes Membrane applications include those for direct methanol fuel cells, gas separation, pervaporation and chemical catalysis. 14.4.1.1
Gas Separation Membranes
PAI exhibit interesting properties for membrane separation applications. Both permeability and selectivity may be enhanced by the incorporation of bulky pendent groups. Such groups make the molecular structure rigid and keep voids. In comparison to PIs, PAI membranes can be more easily fabricated. Fluorinated aromatic PAI can be used for highly selective membranes. Some are summarized in Table 14.5. These compounds are condensed with various amines to give PAI types.38 These include the monomers are shown in Figure 14.5 and in Figure 14.2. The gas separation performance can be tailored by variations in the polymer structure. A series of PAI have been condensed using trimellitic acid and also 3-amino-4-methylbenzoic acid and various diamines. It was found that the permeability or helium increases with increasing fluorine content. In addition, nonlinear moieties contribute to the permeability.39 In PAI with oligo(tetrafluoroethene) segments, the substitution patterns of the phenyl group in the diamine moieties do not notably influence the diffusivity.40 14.4.1.2
Catalytic Membranes
Catalysts are used for the hydrogenation of edible oils such as sunflower oil. Traditionally, silica gel supported nickel catalysts in slurry reactors
Poly(amide imide)s
461
Table 14.5: Monomers for Membranes Amino and Imino acids
References 38
2,2-Bis[N-(4-carboxyphenyl)-phthalimidyl]hexafluoropropane Diamines
References
4,4 -Bis(4-aminophenoxy)biphenyl 2,2 -Dimethyl-4,4 -bis(4-aminophenoxy)biphenyl 3,3 ,5,5 -Tetramethyl-bis[4-(4-aminophenoxy)phenyl]sulfone 2,4,6-Trimethyl-1,3-phenylenediamine 3,3 -Dimethoxybenzidine 3,3 ,5,5 -Tetramethylbenzidine 3,3 -Dimethyl-l,1 -binaphthalene Other Monomers
38 38 38 39 39 39 39
References 39
3-Amino-4-methylbenzoic acid
O O C
CF3
O O
C
N
N
CF3
HO O
C OH
O
2,2-Bis[N-(4-carboxyphenyl)-phthalimidyl]hexafluoropropane CH3
H 3C O H 2N
O
S
O
NH2
O H 3C
CH3
3,3′,5,5′-Tetramethyl-bis[4-(4-aminophenoxy)phenyl]sulfone
Figure 14.5: Monomers Used in Gas Separation Membrane Applications38
462
High Performance Polymers
O
R
H
N R
N
OH O
O
H
O
H R
N R
N O
O
Figure 14.6: Cyclization of the Amic Acid Form to the Imide Form
have been used. Nickel is now substituted by palladium or platinum in order to reduce the formation of trans isomers in the course of isomerization. Since the catalysts have to be reduced after hydrogenation, reuse is desirable, in particular in the case of noble metal catalysts. The catalytic membranes based on PAI and other high temperature stable materials have been developed.41 Membranes based on PAI and aluminum oxide are dense and exhibit a low permeability. However, when poly(ethylene oxide propylene oxide) is added and the casting temperature is kept below 9°C, a dramatic increase of the permeability of nitrogen, oil and water is observed. The membrane can be made catalytically active by adding a previously prepared supported catalyst instead of a plain filler to the casting solution. An alternative method is to impregnate the membrane with a PdCl2 or H2 PtCl6 solution. In this case, the metal is fixed to the membrane either by chemical reduction using borohydride or by calcination of the membrane at temperatures of 175–200°C. Using the latter method, the catalyst is less sensitive to be washed out during the hydrogenation reaction. In contrast to nickel catalysts, where temperatures of 170–200°C are necessary for hydrogenation, the membranes work satisfactorily at a temperature of 100°C and hydrogen pressures of 4 bar.
14.4.2 Coatings and Adhesives There are PAI-types that are used as coatings.42 The types with thermosetting properties are used. The polymer is delivered in the unimidized or amic acid form. Upon heating, the polymer will undergo cyclization to the imide form. The cyclization reaction is shown in Figure 14.6. The imidization reaction occurs in the temperature range of 90–150°C. Solvents for the
Poly(amide imide)s
463
amic acid form include DMAC, dimethyl sulfoxide, dimethylformamide, and NMP. NMP is preferred, because it has a low odor and a relatively low level of toxicity. The solvents are relatively expensive. Therefore, cheaper materials, which are solvents only in a limited range of concentration are used. These materials are addressed as diluents. Diluents can only be used within their solubility limits. Diluents include aromatic hydrocarbons, ethyl acetate, acetone, cyclohexanone, and acetanilide.42
14.4.2.1
Wire Enamels
Originally, an enamel was understood as a vitreous material applied to metal or to porcelain. However, the meaning has been extended to coatings and is commonly used by experts and in standards.43 The use of PAI in wire enamels is well known.44 Conventional products are PAI wire enamels which consist of e.g., trimellitic acid anhydride and MDI. As a solvent, NMP is used which is in some cases extended with a hydrocarbon. NMP is responsible for the high level of NO emissions from coating plants coupled with a waste air incinerator. Furthermore, NMP responds poorly to additives, e.g., to enhance the levelling of wire enamels. For these reasons, attempts have been made to substitute NMP by other solvents. It was suggested to use cresol instead of NMP, however, other types of PAI must be used because of the lack of solubility in cresol.45 PAI types that are soluble in cresol can be produced by the reaction of trimellitic acid anhydride with cresol to give the cresyl ester. This ester is reacted with MDI and then with 4,4 -diaminodiphenylmethane. PAI is rendered soluble in cresol by the modification with ε -caprolactam. A disadvantage is that this type is no longer purely aromatic and the thermal properties are adversely affected. It was found that aromatic imide and amide forming components with a functionality of more than two yield products that are soluble in cresol.45 Additives for wire enamels are phenolic resins, melamine resins, fluorinated compounds, or, benzyl alcohol. Crosslinking catalysts are zinc octoate, cadmium octoate, or titanates, such as tetrabutyl titanate. Extenders are xylene, toluene, ethylbenzene, or cumene, and commercial available similar compounds.
464
High Performance Polymers
14.4.2.2
Craze Resistant Coatings
Formations with enhanced resistance to craze formation have been described.46 Increasing the molecular weight has little influence on suppressing varnish-induced crazing in PAI films. However, craze resistance is associated with curing and crosslinking. The ratio of imide to amide and the surface tension of the film coatings influence the tendency to craze formation. Further, a dispersed second phase minimizes the residual stress and reduces detrimental craze formation. A series of formulations have been described.46 The components of the PAI are trimellitic acid anhydride, MDI, adipic acid, isophthalic acid, dicarboxyl terminated poly(acrylonitrile-co-butadiene). The solvent is NMP. A slurry of poly(tetrafluoroethylene) is added to the polymer.
14.4.2.3
Adhesive Coatings
Formulations suitable for high strength, high-temperature adhesives based on PAI, have been developed. Excellent bond strengths are observed with stainless steel, aluminum and titanium alloys, and PI films.42 Siloxane-modified PAI resin compositions have been developed for the production of interlaminar adhesive films for wiring boards. The composition strongly adheres to the PI base layer and copper foil.47, 48
14.4.3 Fibers PAI fibers have been proposed for the use in high-temperature bag filters for exhaust gas facilities. Conventionally, aramid fibers have been used as heat resistant fibers for bag filters. PAI fibers are more easier produced than aramid fibers. For example, a polymer prepared from trimellitic acid anhydride and MDI in NMP can be extruded from the solution through a one-hole nozzle. This is followed by a dry spinning apparatus equipped with a furnace having a length of 1.5 m. Fibers are obtained at 270°C at 220 m min−1 . The undrawn fibers are thoroughly dried in vacuo to a residual solvent content of less than 1%. Then the fibers are passed through a heating zone in a nitrogen atmosphere, whereby the fibers are at a drawing ratio of 5.49
Poly(amide imide)s
465
14.4.4 Optical Applications 14.4.4.1
Optical Waveguide Materials
Attempts have been made to use PAI types as optical waveguide materials for light transfer in the near infrared wavelength range. The wavelength range of light for optical communications has been shifted from 800 nm to 1550 nm, which corresponds to the near infrared wavelength range. Conventional polymers absorb light of 1000–1700 nm i.e., in the near infrared wavelength range. Absorption of light in the near infrared wavelength range by organic polymers is caused by overtones of harmonics due to stretching and deformation vibrations of carbon-hydrogen bonds in alkyl, phenyl and other similar functional groups. Conventional polymers are not suitable as optical waveguide materials in the near infrared wavelength range because of a large optical loss. This is also true for PAI types, which exhibit, however, other attractive properties for the application in optical devices. In order to reduce the optical loss, the light absorption wavelength region of a polymer must be shifted from the near infrared wavelength range to a longer or shorter wavelength region. Substitution of the hydrogen atoms by fluorine atoms has been suggested. However, if hydrogen is substituted by fluorine, the refractive index of the polymer is lowered. Another drawback is that the surface tension is lowered, which in general causes poor adhesion. PAI with a higher refractive index than conventional fluorinated PI consist of chlorinated monomers. When using such a PAI type as a material for the core of an optical fiber, the selection range on the material for cladding becomes more wide. In addition, the coating performance and the adhesiveness to a substrate are improved compared to a conventional PI. Imide containing monomers based on 3,5,6-trichloro-4-chloroformyl phthalic acid anhydride have been proposed for optical applications, since the refractive index is sufficiently high.50 The monomers are obtained by the condensation of 3,5,6-trichloro4-chloroformyl phthalic acid anhydride, c.f. Figure 14.7, with various diamines to form imide containing monomers. In addition to the monomers shown in Figure 14.7, several other diamines have been exemplified.50 After formation of the imide monomer by reaction of the phthalic end groups, the PAI is formed by the reaction of the chloroformyl groups with an additional diamine. Essentially, the same diamines are utilized for the imidization reaction.
466
High Performance Polymers
Cl
O
Cl O Cl
C O
O
Cl
3,5,6-Trichloro-4-chloroformyl phthalic acid anhydride H 2N
NH2 H 2N
1,3-Diaminobenzene
NH2 4,4′-Diaminobiphenyl
F H 2N
F
NH2
F
F
F
H 2N F
NH2
F F
F 1,3-Diaminotetrafluorobenzene
F
F
F
4,4′-Diaminooctafluorobiphenyl CCl3
H 2N
C
NH2
CCl3 2,2-Bis(4-aminophenyl)hexachlorpropane F H 2N
F
F CCl3
F
C F
CCl3 F F
NH2 F
2,2-Bis(4-aminotetrafluorophenyl)hexachloropropane
Figure 14.7: Monomers for Optical Applications50
Poly(amide imide)s
467
Table 14.6: Diamines with Azo Groups Diamine
References
2,4-Diamino-4 -fluoroazobenzene 2,4-Diamino-4 -methylazobenzene 2,4-Diamino-4 -trifluoromethoxyazobenzene 2,4-Diamino-4 -nitroazobenzene 2,4-Diamino-4 -(4-nitrophenyl-diazenyl)azobenzene 4-(4 -Nitrophenyl-diazenyl) phenyl-1,3-diamine
O2N
NH2
O2N
51 51 51 51 52 52
N N Cl
H 2N NH2
H 2N O2N
N N
NH2
Figure 14.8: Synthesis of 4-(4 -Nitrophenyl-diazenyl) phenyl-1,3-diamine
14.4.4.2
Photochromic Materials
Polymers containing photochromic groups are of interest in optical applications. For example, photochromic lenses darken on exposure to UV light. Photochromic polymers can be obtained by the introduction of azobenzene groups. These groups undergo cis-trans-isomerizations on exposure to light. Diamines with azo groups are shown in Figure 14.6. The infrared band of the N=N linkage overlaps with that of the C=C vibration of the benzene ring at around 1600 cm−1 . The PAIs exhibit high glass transition temperatures and high thermal stability. Lateral alkyl substituents increase the light-induced orientation effect.51 Azo based chromophores are also used as side chains in PAI for nonlinear optical applications. The synthesis of such compounds is shown in Figure 14.8.
468
High Performance Polymers
14.4.4.3
Electrochromic Materials
Electrochromism is the reversible change in optical properties of a material caused by redox reactions.53, 54 The redox reactions can be initiated when the material is placed on the surface of an electrode. When the electrochromic material is capable of showing several colors, it is addressed as polyelectrochromic. Changes in color may occur when a chromophore is forced to change its absorption spectrum by the application of electric potential. Thereby the absorption may change from the UV region into the visible region. Electrochromic materials are used to control the flow of light. These applications are summarized as smart windows or mirrors. Applications are in anti-glare car rear-view mirrors,55 smart sunglasses, and in devices for optical information and storage. Several classes of organic electrochromic materials are known. The triphenylamine unit can be used to be impart electrochromism into PAI resins. In particular, N,N-bis(4-aminophenyl)-N ,N -diphenyl-1,4-phenylenediamine56, 57 and 4,4 -diamino-4 -methoxytriphenylamine can be condensed with bis(trimellitimide)s to get electrochromic PAI types.58 Triphenylamine forms a radical cation on anodic oxidation which dimerizes into tetraphenylbenzidine. The redox potential can be tuned by the substituting the aromatic ring. The redox behavior of the PAI can be characterized by cyclic voltametry. Films are cast on an indium tin oxide-coated glass substrate as a working electrode in dry acetonitrile. The electrochromism is examined by an optically transparent thin-layer electrode coupled with a UV-vis spectroscopy.
14.5 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 14.7 Tradenames appearing in the references are shown in Table 14.8.
14.6 SAFETY Material safety data sheets for PAI are available from the manufacturer. The usual precautions when handling hot polymers are recommended. Irritations are caused due to mechanical effects. Overheating in the course of processing generates fumes of unknown toxicity. There is essentially
Poly(amide imide)s
469
Table 14.7: Examples for Commercially Available Poly(amide imide)s Tradename
Producer
AI Polymer Alphamide® Pyropel® Sintimid™ Torlon® Vylomax® Pebax
Mitsubishi Gas Company Quadrant Engineering Plastic Products Albany International Ensinger Solvay Advanced Polymers Toyobo Atofina
Table 14.8: Tradenames in References Tradename Description
Supplier
Desmodur® (Series) Bayer AG Oligomers based on 4,4 -diphenylmethane diisocyanate45 Expandex® 150 Olin Chemicals Calcium salt of 5-phenyltetrazole, blowing agent31 Galwick® PMI 41 Wetting fluid Isonate® Dow Isocyanate based formulation for foams45 Lupranat® (Series) BASF Isocyanate based formulations45 Pluronic® (Series) BASF AG Ethylene oxide/propylene oxide block copolymer, defoamers41 Porewick® PMI Wetting fluid41 Radel® A Solvay Poly(ether sulfone)41 Solventnaphtha™ Aromatic Hydrocarbon solvent45 Solvesso® Exxon Higher aromatic solvent mixtures45 Torlon® (Series) Solvay (Amoco) Poly(amide imide)6, 31 Vylomax® Toyobo Poly(amide imide)41
470
High Performance Polymers
no literature on the pyrolysis of PAIs. However, it may be expected that the mechanism of degradation will run both as observed in PAs and PIs. Of course, the monomers used for PAIs are more hazardous than the polymers. For example, ODA may cause cancer and heritable genetic damage. It is toxic by inhalation, in contact with skin, and if swallowed. It is further toxic to aquatic organisms, as it may cause long-term adverse effects in the aquatic environment.
REFERENCES 1. Wire coating. NL Patent 6 611 895, assigned to du Pont de Nemours, E. I., and Co., February 27, 1967. 2. Polyamide-imide resins. GB Patent 1 119 791, assigned to Sumitomo Electric Industries, Ltd., July 10, 1968. 3. E. G. Redman and J. S. Skinner. Poly(amide imide) electrical insulation. FR Patent 1 501 198, assigned to Mobil Oil Corp., November 10, 1967. 4. R. E. Mauri. Organic materials for structural applications. NASA Spec. Publ. SP-3051, USA., 1969. 5. Y. Camberlin and P. Michaud. Linear aromatic poly(amideimide)s having latent maleimide endgroups. US Patent 5 086 154, assigned to Rhone-Poulenc Chimie (Courbevoie, FR), February 4, 1992. 6. K.-Y. Choi, D.-H. Suh, M.-H. Yi, Y.-T. Hong, and M.-Y. Jin. Process for preparing polyamideimide resins by direct polymerization. US Patent 5 955 568, assigned to Korea Research Institute of Chemical Technology (Daejeon, KR), September 21, 1999. 7. J. R. Stephens. Preparation of film forming polymer from carbocyclic aromatic diamine and acyl halide of trimellitic acid anhydride. US Patent 3 920 612, assigned to Standard Oil Company (Chicago, IL), November 18, 1975. 8. D.-J. Liaw and B.-Y. Liaw. 2,2’-dimethyl-4,4’-bis (4-aminophenoxy) biphenyl, and polymers prepared therefrom by polycondensation. US Patent 5 844 065, assigned to National Science Council (TW), December 1, 1998. 9. S. Maiti and A. Ray. “Unsaturated polyamideimide from 1,5-bis(3-aminophenyl)-1,4-pentadien-3-one.” Makromol. Chem., Rapid Comm., 2:649–653, 1981. 10. K.-Y. Choi, M.-H. Yi, M.-Y. Jin, and Y.-T. Hong. Polyamideamic acid resin prepolymers, high heat resistant polyamideimide foams prepared therefrom, and processes for preparing them. US Patent 5 824 766, assigned to Korea Research Institute of Chemical Technology (Daejeon, KR), October 20, 1998.
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11. M. Minami and M. Taniguchi. Method of preparing a soluble high molecular weight aromatic polyamide imide composition. US Patent 3 860 559, assigned to Toray Industries, Inc. (Tokyo, JA), January 14, 1975. 12. S. Nagao, I. Kuzuhara, and H. Ogawa. Process for producing trimellitic acid. US Patent 6 835 852, assigned to Mitsubishi Gas Chemical Company, Inc. (Tokyo, JP), December 28, 2004. 13. P. H. Merrell and M. F. Ellis. Preparation of diaminodiphenyl ethers. US Patent 4 539 428, assigned to Mallinckrodt, Inc. (St. Louis, MO), September 3, 1985. 14. S. E. Jacobson. Method for the manufacture of 3,4’-diaminodiphenyl ether. US Patent 5 434 308, assigned to E. I. Du Pont de Nemours and Company (Wilmington, DE), July 18, 1995. 15. D.-J. Liaw and B.-Y. Liaw. Preparation of soluble polyamide, polyimide and poly(amide-imide). US Patent 6 087 470, assigned to National Science Council of Republic of China (Taipei, TW), July 11, 2000. 16. S. Mehdipour-Ataei and F. Zigheimat. “Soluble poly(amide imide)s containing oligoether spacers.” Eur. Polym. J., 43(3):1020–1026, March 2007. 17. H. R. Lubowitz and C. H. Sheppard. Oligomers with multiple chemically functional end caps. US Patent 5 969 079, assigned to The Boeing Company (Seattle, WA), October 19, 1999. 18. M. Nakano and T. Koyama. Novel polyimidamide resin. US Patent 3 541 038, assigned to Hitachi Chemical Co. Ltd, November 17, 1970. 19. T. Sakata, K. Hattori, and Y. Mukoyama. Process for the production of high molecular weight polyamide-imide resin. US Patent 5 047 499, assigned to Hitachi Chemical Co., Ltd. (Tokyo, JP), September 10, 1991. 20. A. Jonquières, R. Clément, and P. Lochon. “New film-forming poly(urethane-amide-imide) block copolymers: influence of soft block on membrane properties for the purification of a fuel octane enhancer by pervaporation.” Eur. Polym. J., 41(4):783–795, April 2005. 21. K.-Y. Choi, J. H. Lee, Y.-T. Hong, M. Y. Jin, K.-S. Choi, and H.-J. Park. Polyamide-imide having head-to-tail backbone. US Patent 6 433 184, assigned to Korea Research Institute of Chemical Technology (Daejeon, KR), August 13, 2002. 22. S. Khoee, F. Sadeghi, and S. Zamani. “Preparation, characterization and fluorimetric studies of novel photoactive poly(amide-imide) from 1-naphthaldehyde and 2,6-diaminopyridine by microwave-irradiation.” J. Photochem. Photobiol., A, 189(1):30–38, June 2007. 23. S. Khoee and S. Zamani. “Synthesis, characterization and fluorimetric studies of novel photoactive poly(amide-imide) from anthracene 9-carboxaldehyde and 4,4’-diaminodiphenyl ether by microwave irradiation.” Eur. Polym. J., 43(5):2096–2110, May 2007. 24. S. E. Mallakpour, A.-R. Hajipour, and S. Khoee. “Polymerization of 4,4 -(hexafluoroisopropylidene)-N,N -bis(phthaloyl-L-leucine) diacid chlor-
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Poly(amide imide)s
38.
39.
40.
41.
42.
43. 44. 45.
46. 47.
48.
49.
50.
473
[electronic] http://www.solvayadvancedpolymers.com/static/wma/pdf/2/5/2/ TBTorlon4435.pdf. S.-H. Huang, C.-C. Hu, K.-R. Lee, D.-J. Liaw, and J.-Y. Lai. “Gas separation properties of aromatic poly(amide-imide) membranes.” Eur. Polym. J., 42(1): 140–148, January 2006. D. Fritsch and N. Avella. “Synthesis and properties of highly gas permeable poly(amide-imide)s.” Macromol. Chem. Phys., 197(2):701–714, February 1996. Z. K. Xu, M. Bohning, J. Springer, N. Steinhauser, and R. Mulhaupt. “Gas transport properties of highly fluorinated polyamideimides.” Polymer, 38(3): 581–588, February 1997. D. Fritsch and G. Bengtson. “Development of catalytically reactive porous membranes for the selective hydrogenation of sunflower oil.” Catal. Today, 118(1-2):121–127, October 2006. Anonymous. Torlon® AI-10 coatings. Technical Bulletin T-49977, Solvay Advanced Polymers, Alpharetta, Georgia, 2006. [electronic] http://www.solvayadvancedpolymers.com/static/wma/pdf/9/5/9/ 1/Torlon_AI10_Coatings_TB.pdf. Standard test methods for magnet-wire enamels. ASTM Standard ASTM D3288-03, ASTM International, West Conshohocken, PA, 2003. N. J. George. Polyamide-imide resins. US Patent 3 554 984, assigned to The P. D. George Co. (St. Louis, MO), January 12, 1971. S. König, K. W. Lienert, and G. Schmidt. Polyamide-imide resin solution and the use thereof for producing wire enamels. US Patent 7 122 244, assigned to Altana Electrical Insulation GmbH (Wesel, DE), October 17, 2006. J. J. Xu. Polyamideimide composition. US Patent 6 914 093, assigned to Phelps Dodge Industries, Inc. (Ft. Wayne, IN), July 5, 2005. K. Takeuchi, T. Saito, and K. Nanaumi. Siloxane-modified polyamideimide resin composition, adhesive film, adhesive sheet and semiconductor device. US Patent 6 252 010, assigned to Hitachi Chemical Company, Ltd. (Tokyo, JP), June 26, 2001. K. Takeuchi, T. Saito, and K. Nanaumi. Adhesive film formed of a siloxanemodified polyamideimide resin composition, adhesive sheet and semiconductor device. US Patent 6 475 629, assigned to Hitachi Chemical Company, Ltd. (Tokyo, JP), November 5, 2002. C. Inukai, T. Kurita, and K. Uno. Polyamide-imide fibers for a bag filter. US Patent 5 681 656, assigned to Toyo Boseki Kabushiki Kaisha (Osaka, JP), October 28, 1997. D.-H. Suh, E.-Y. Chung, and T.-H. Rhee. Polyamideimide for optical communications and method for preparing the same. US Patent 6 028 159, assigned to SamSung Electronics Co., Ltd. (Suwon, KR) Korea Research Institute of Chemical Technology (Daejeon, KR), February 22, 2000.
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51. E. Schab-Balcerzak, B. Sapich, and J. Stumpe. “Photoinduced optical anisotropy in new poly(amide imide)s with azobenzene units.” Polymer, 46(1):49– 59, January 2005. 52. H.-L. Chang, H.-L. Lin, Y.-C. Wang, S. A. Dai, W.-C. Su, and R.-J. Jeng. “Thermally stable NLO poly(amide-imide)s via sequential self-repetitive reaction.” Polymer, 48(7):2046–2055, March 2007. 53. W. Dautremont-Smith. “Transition metal oxide electrochromic materials and displays: A review - 1. oxides with cathodic coloration.” Displays, 3(1):3– 22, January 1982. 54. P. M. S. Monk, R. J. Mortimer, and D. R. Rosseinsky. Electrochromism and Electrochromic Devices. Cambridge University Press, Cambridge, 2007. 55. A. R. Watson and M. A. Bryson. Rearview mirror assemblies incorporating hands-free telephone components. US Patent 7 266 204, assigned to Gentex Corporation (Zeeland, MI), September 4, 2007. 56. S.-H. Cheng, S.-H. Hsiao, T.-H. Su, and G.-S. Liou. “Novel electrochromic aromatic poly(amine-amide-imide)s with pendent triphenylamine structures.” Polymer, 46(16):5939–5948, July 2005. 57. G.-S. Liou, S.-H. Hsiao, and Y.-K. Fang. “Electrochromic properties of novel strictly alternating poly(amine-amide-imide)s with electroactive triphenylamine moieties.” Eur. Polym. J., 42(7):1533–1540, July 2006. 58. C.-W. Chang and G.-S. Liou. “Stably anodic green electrochromic aromatic poly(amine-amide-imide)s: Synthesis and electrochromic properties.” Org. Electron., 8(6):662–672, December 2007.
15 Poly(imide)s In 1951, Flory reported the condensation reaction of diacid chlorides, e.g. with potassium salts of imides, e.g. the condensation of sebacyl chloride with potassium phthalimide.1 In this way, N-acyl diimides are formed. Flory pointed out the possibility of forming polymers, when components with higher functionality are used. Poly(imide)s (PI)s ∗ from pyromellitic acid were reported in 1955 by Edwards and Maxwell at DuPont.2 The diamines used were of aliphatic nature. Later, in addition, aromatic diamines were used.3 The two major types of PIs are: • The thermoset type and • The thermoplastic type. Poly(ether imide)s (PEI)s are a particular class of PI which combine the high-temperature characteristics of PI but still have sufficient melt processability to be easily formed by conventional molding techniques such as compression molding, gas assist molding, profile extrusion, thermoforming and injection molding.4 To the thermoset type belong bismaleimides and bisnadimides as well as oligomeric end capped imides. End capping occurs with reactive phenylethyl groups. These types are used for reactive injection molding and related techniques. The chemistry of formation of the imide moiety is quite similar for both the thermoset type and the thermoplastic type. There are several monographs on PIs.5–9 Bismaleimides are a separate subclass ∗ Inconsequently,
mostly written as polyimides
475
476
High Performance Polymers
of PIs that are dealt with here only marginally since the focus is directed rather to thermoplastic PIs.
15.1 MONOMERS The synthesis of monomers suitable for PIs and the formation of the respective polymers has been reviewed thoroughly.10 Monomers are collected in Table 15.1. Dianhydride compounds used for PIs are shown in Figure 15.1. Dianhydride compounds are classified into: 1. Benzene dianhydrides, 2. Bridged dianhydrides, and 3. Bis(ether anhydride)s. In all important dianhydride compounds, the anhydride groups are attached to aromatic moieties. Pyromellitic dianhydride (PMDA) is prepared by the oxidation of durene, which is 1,2,4,5-tetramethylbenzene. The synthesis is completely analogous to the synthesis of phthalic anhydride. The starting compound for biphenyltetracarboxylic dianhydride is dimethyl phthalate. Dimethyl phthalate is dimerized in the presence of palladium catalysts, the ester groups are hydrolyzed and eventually dehydrated.11 Biphenyltetracarboxylic dianhydride can also be synthesized by the direct dimerization of phthalic anhydride.12 Phthalic anhydride is heated in the presence of palladium acetate to 280°C. A mixture of biphenyl-3,3 ,4,4 -tetracarboxylic dianhydride and biphenyl-2,3,3 ,4 -tetracarboxylic dianhydride is obtained in a total yield of ca. 20%. 3,3 ,4,4 -Oxydiphthalic anhydride (4,4 -ODPA) can be prepared from chlorophthalic anhydride in o-dichlorobenzene and potassium carbonate. Hexaethylguanidinium chloride is used as a phase transfer catalyst.13 A synonym for 4,4 -bis(3,4-dicarboxyl phenoxyphenyl)-isopropylidene dianhydride is Bisphenol A dianhydride. The synthesis of a wide variety of other dianhydride compounds is detailed in the literature.10 Modifiers for PI are shown in Figure 15.2. 4-(Phenylethynyl)-1,2-benzenedicarboxylic acid anhydride (PEPA) is used to introduce functional groups PIs, in order to make them heat curable. Citraconic anhydride (CA) acts in the same way. In addition, these modifiers act as end capping agents.
Poly(imide)s
477
Table 15.1: Monomers for Poly(imide)s14 Dianhydrides Pyromellitic dianhydride (PMDA) 3,3 ,4,4 -Biphenyl dianhydride (4,4 -BPDA) 3,3 ,4,4 -Benzophenone dianhydride (4,4 -BTDA) 3,3 ,4,4 -Oxydiphthalic anhydride (4,4 -ODPA) 1,4-Bis(3,4-dicarboxyl-phenoxy)benzene dianhydride (4,4 -HQDPA) 1,3-Bis(2,3-dicarboxyl-phenoxy)benzene dianhydride (3,3 -HQDPA) 4,4 -Bis(3,4-dicarboxyl phenoxyphenyl)-isopropylidene dianhydride (4,4 -BPADA) 4,4 -(2,2,2-Trifluoro-1-pentafluorophenylethylidene) diphthalic dianhydride (3FDA) Diamines 4,4 -Oxydianiline (ODA) m-Phenylenediamine (MPD) p-Phenylenediamine (PPD) m-Toluenediamine (TDA) 1,4-Bis(4-aminophenoxy)benzene (1,4,4-APB) 3,3 -(m-Phenylenebis(oxy))dianiline (APB) 4,4 -Diamino-3,3 -dimethyldiphenylmethane (DMMDA) 2,2 -Bis(4-(4-aminophenoxy)phenyl)propane (BAPP) 1,4-Cyclohexanediamine 2,2 -Bis[4-(4-amino-phenoxy) phenyl] hexafluoroisopropylidene (4-BDAF) 6-Amino-1-(4 -aminophenyl)-1,3,3-trimethylindane (DAPI) Modifiers Maleic anhydride (MA) Citraconic anhydride (CA) Nadic anhydride (NA) 4-(Phenylethynyl)-1,2-benzenedicarboxylic acid anhydride (PEPA)
478
High Performance Polymers
O
O
O
O
O
O
O O
O
O O
O
Pyromellitic dianhydride 3,3′,4,4′-Biphenyl dianhydride O
O
O
O
O O
O
O
O
O
O
O O
O
3,3′,4,4′-Benzophenone dianhydride 3,3′,4,4′-Oxydiphthalic anhydride O
O O
O
O
O
O
O
1,4-Bis(3,4-dicarboxyl-phenoxy)benzene dianhydride O
O
O
O
O
O
O
O
1,3-Bis(2,3-dicarboxyl-phenoxy)benzene dianhydride O O O
O
CH3 O
C CH3
O
O O
4,4′-Bis(3,4-dicarboxyl phenoxyphenyl)-isopropylidene dianhydride
Figure 15.1: Dianhydride Compounds Used for Poly(imide)s
Poly(imide)s
479
O O O 4-(Phenylethynyl)-1,2-benzenedicarboxylic acid anhydride H 3C
O O O
Citraconic anhydride
O O O Nadic anhydride
Figure 15.2: Modifiers for Poly(imide)s
Diamine compounds used for PIs are shown in Figure 15.3. 4,4 -Diamino-3,3 -dimethyldiphenylmethane can be synthesized from the condensation of o-toluidine with formaldehyde. 6-Amino-1-(4 -aminophenyl)-1,3,3-trimethylindane (DAPI) is used in curable PIs. Poly(amic acid) and PI with DAPI in the backbone and end capped with allylnadic moieties are soluble in polar organic solvents in very high concentrations.15 The resulting PIs show excellent toughness. PIs based on 4,4 -ODPA and bis[4-(p-aminophenoxy)phenoxy]dimethylsilane have been reported to exhibit good solubility and film-forming capability.16
15.2 POLYMERIZATION AND FABRICATION There are several routes to synthesize PIs. Most commonly, PIs are synthesized from dianhydride compounds and diamine compounds. There are also commercial processes that use dianhydride compounds and isocyanate compounds to get PIs.
15.2.1 Conventional Route The synthesis of PIs runs in two steps. In the first step, the poly(amic acid) is formed from dianhydride and diamine. N-Methyl-2-pyrrolidone (NMP) is used as a solvent for the two stage process.
480
High Performance Polymers
H 2N
NH2
O
NH2
H 2N
m-Phenylene diamine
4,4′-Oxydianiline CH3
H 3C H 2N
NH2
CH2
NH2
H 2N
4,4′-Diamino-3,3′-dimethyldiphenylmethane p-Phenylene diamine H 2N
O
O
NH2
1,4-Bis(4-aminophenoxy)benzene CH3 H 2N
O
O
C CH3
2,2′-Bis(4-(4-aminophenoxy)phenyl)propane NH2
CH3 H 2N CH3 H 3C
6-Amino-1-(4′-aminophenyl)-1,3,3,-trimethylindane
Figure 15.3: Diamine Compounds
NH2
Poly(imide)s
481
In the second step, the poly(amic acid) is dehydrated to the PI. In course of imidization of the poly(amic acid), water is produced. The dehydration may be performed: 1. By heating up to 250°C. This process is termed as thermal dehydration. 2. By azeotropic dehydration. An azeotropic solvent, such as toluene or xylene is used to remove water. 3. By chemical dehydration. In the presence of acetic anhydride, the imidization already proceeds at 100°C. When highly basic diamines, are used, e.g., alicyclic diamines are used as a diamine monomer, a salt is produced by the neutralization reaction before forming a poly(amic acid) at the point that the alicyclic diamines are mixed with acid dianhydrides and precipitated. Since this salt is in a stable state, it is neither possible to form a poly(amic acid) by polymerization reaction nor to imidize poly(amic acid) by the reaction with a hydrating reagent.17 When water is chemically removed, a basic catalyst, such as triethylamine, pyridine, β -picoline, or isoquinoline, and acetic anhydride is added. Water produced in the imidization is chemically removed by the reaction with acid anhydride. However, this method, requires a process for purification of a reaction mixture to remove the residue, such as tertiary amines and acetic anhydrides, out of the system. Some PIs are not processable. When the final dehydration step makes the materials no longer processable, most of the intermediate can still be cast into films from the cast solutions.
15.2.2 Isocyanate Route A one-step route involves the reaction of an acid dianhydride and a diisocyanate. This method usually requires heating at 250°C. Under ordinary pressure to complete imidization because of the low reactivity of the acid dianhydride towards the diisocyanate.17 Diisocyanate and acid dianhydride are mixed in equimolar amounts, often previously dissolved in a suitable solvent. 3,3 ,4,4 -Benzophenone dianhydride (4,4 -BTDA) is mixed at room temperature with in diphenylmethanediisocyanate using N,N-dimethylformamide (DMF) as a solvent and subsequently heated in vacuum up to 210°C. A PI with a molecular
482
High Performance Polymers Table 15.2: Poly(imide)s Produced by the Aqueous Route18 Dianhydride
Diamine
Type
4,4 -BTDA
TDA MPD 3,4 -Oxydianiline + APB
P84 Ultem® 1000 PETI-5
4,4 -BPADA 4,4 -BPDA + PEPA
weight of 50,000 Dalton and a glass transition temperature of 250°C is obtained. To reduce the reaction time between diisocyanates and dianhydrides, catalysts such as alkali metal methoxides,19 or alkali metal fluoride can be optionally combined with a quaternary onium salt.20 However, the addition of catalysts are not advised in the productions of yarns that should exhibit good characteristics.21 Certain fibers and non-wovens are made commercially by the isocyanate route, such as P84.22 For this family, 4,4 -BTDA and PMDA are used as dianhydride components, and toluene diisocyanate and diisocyanatodiphenyl methane are used as diisocyanate components.23
15.2.3 Aqueous Route Water can be used as a solvent for the synthesis of PI. The synthesis route has been demonstrated for the production of commercial PIs.18 In contrast to the conventional route, the aqueous route runs via carboxylic salt and amine salt precursors. In the first step of the synthesis, the dianhydride is refluxed in water in order to completely hydrolyze it. After cooling down to room temperature, the aqueous solution of the dicarboxylic acid is neutralized with the respective diamine and a salt is formed. The salt precipitates out from the aqueous solution. The precipitate is dried and heated up to 180–220°C for dehydration to form the final PI product.18, 24 Products that have been produced by the aqueous route are shown in Table 15.2. The infrared (IR) spectra of commercially available Ultem® 1000 and the type synthesized by the aqueous route are virtually indistinguishable.18 Water offers environmental and economic benefits. However, there emerges one disadvantage of this method, as the process is complete in one step. The aqueous poly(imide) synthesis route results in products that
Poly(imide)s
O
483
O X
N Ar N
+ -O Ar
O-
X O
O
O
O O Ar
N Ar N O O
O
Figure 15.4: Nucleophilic Displacement Polymerization
are not processable, if the resulting PI is insoluble and infusible.25 Alternatively, fluorinated monomers can be used to get processable and hightemperature stable products. Or else, low-molecular-weight intermediate thermoset products could be produced.
15.2.4 Nucleophilic Displacement Polymerization The nucleophilic displacement polymerization starts with the monomeric bisimides, that are halogen substituted at the aromatic ring. The bisimides are chain extended at the aromatic ring using bisphenols. The process is shown schematically in Figure 15.4.
15.2.5 Transimidization The transimidization reaction to get PIs goes back to the early days of PI fabrication. However, rather low-molecular-weight polymers were obtained initially. The transimidization reaction is shown in Figure 15.5. The concept of transimidization has been refined.26 The route comprises the reaction of 4-halotetrahydrophthalic anhydride with an activating primary amine, such as 2-pyridylamine to yield 4-halotetrahydrophthalimide. The activating group supports the subsequent aromatization. The aromatization is highly affected by the proper choice of the catalyst. Acti-
484
High Performance Polymers
O
O NH + H2N Ar
HN O
NH2
O
O N O
O N Ar
+ 2 NH3
O
Figure 15.5: Transimidization Reaction
vated carbon treated with copper compounds gives yields in the region of 90%. The 4-halophthalimide is coupled by the reaction with the disodium salt of a bisphenol to yield a bisimide. The bisimide may then be directly treated with a diamine to yield poly(etherimide)s.
15.2.6 Chemical Vapor Deposition Chemical vapor deposition (CVD) of polymers is an elegant technique to place coatings on various substrates without using solvents.27 However, the equipment needed is more complicated in comparison to conventional techniques. CVD is used in various branches of industry for corrosion resistant and protective coatings. Films of poly(amic acid) and PI can be deposited by CVD. The dianhydride and diamine monomers are co-evaporated. The process is completely dry in nature. The rate of vaporization must be controlled to provide proper stoichiometric amounts of material to be deposited.28 PIs obtained by CVD exhibits better tensile properties and lower gas permeability in comparison to samples obtained from solution casting. This is attributed to crosslinking reactions in the PI obtained by CVD.29 Details of a recent experimental setup have been discussed in the literature.30
Poly(imide)s
485
N +
H 2N
NH2 O
O
N
O O
O
O
O
O
O
O
O N
O O
+
H 2N
O NH2
Figure 15.6: Reaction of Hindered Biphenols with a Diamine31
15.2.7 Hindered Biphenols It is possible to synthesize the corresponding biphenol dianhydrides from strongly hindered biphenols. The reaction with diamines, c.f. Figure 15.6, results in high-molecular-weight PEIs. The PEIs thus obtained are soluble in a variety of organic solvents and exhibit high glass transition temperatures.31, 32
15.2.8 Poly(isoimide)s The isoimide moiety and the isomerization reaction is shown in Figure 15.7. Poly(isoimide)s can be converted from poly(amic acid)s with car-
486
High Performance Polymers
N O O
N O O
O N O
O N O
Figure 15.7: Isomerization of the Isoimide to the Imide
bodiimides or trifluoroacetic anyhdride.33 Poly(isoimide)s received interest as intermediates for PIs, because the ring closure of the poly(amic acid) runs faster with carbodiimides, e.g., dicyclohexylcarbodiimide, in comparison to acetic anhydride.34 However, the use of a carbodiimide yields the isoimide moiety exclusively, whereas acetic anhydride yields the ordinary imide. However, by the treatment with suitable isomerizing agents, such as 1-hydroxybenzotriazole, or 3-hydroxy-1,2,3-benzotriazin-4-one, the isoimide can be converted to the imide. This technique is suitable for the synthesis of liquid thermosetting maleimide resins. Poly(isoimide)s are more soluble and exhibit a lower melt viscosity than comparable PIs. For this reason, they are used intentionally as an alternative class of PIs. For example, poly(isomaleimide) can be chain extended with a poly(nucleophilic) monomer, such as a poly(thiol), a poly(ol), or a poly(amine).35
15.2.9 Functionalized Poly(imide) 15.2.9.1
PETI Types
When a monofunctional component is added to the monomeric mixture, the molecular weight is lowered. PEPA is a compound that bears one anhydride group and one phenylethynyl group. The anhydride group of PEPA reacts with the amide group and acts as a chain stopper during condensation. In a later stage, the pendent phenylethynyl groups are ready for thermal curing. In the same way, nadic anhydride can be used as a reactive modifier.36 Materials built up according to this concept exhibit low melt viscosities. They are stable for several hours at 210–275°C. The thermal curing of the phenylethynyl group does not occur to any appreciable extent at tem-
Poly(imide)s
487
Table 15.3: Melting Points of PETIs14 PETI Type 3,4 -Bis[4-(phenylethynyl)phthalimido]diphenylether
PEPA-3,4 -ODA N,N -[2,2-(4-phenoxyphenyl) hexafluoroisopropylidene]bis-(4-phenylethynylphthalimide) PEPA-4-BDAF N,N -(4,4 -diphenyleneethylene)bis(4-phenylethynylphthalimide) PEPA-4,4 MDA N,N -(1,4-phenylene)bis(4-phenylethynylphthalimide PEPA-p-phenylenediamine N,N -(1,3-phenylene)bis(4-phenylethynylphthalimide PEPA-m-phenylenediamine (MPD) N-[4-(3-phenoxy)-4 -phenylethynylbenzophenone]4 -phenylethynylphthalimide PEPA-APDE
mp. [°C] 297 260 285 296 248 167
peratures below 300°C. Thermal cure takes place from 300–350°C. The phenylethynyl groups react to provide a crosslinked resin system.37 Phenylethynyl terminated imide (PETI) monomers can be used as reactive diluents for in melt processable PETI oligomers.14 Melting points of such monomers are shown in Table 15.3. The viscosities of the PETI oligomers can be reduced by blending them with the PETI monomers. The oligomers have molecular weights in the range of 2,000–5,000 Dalton. In PIs based on the dianhydrides 4,4 -BTDA and 3,3 ,4,4 -biphenyl dianhydride (4,4 -BPDA), those that are terminated with 4-(1-phenylethynyl)1,8-naphthalic anhydride (PENA) show superior properties in comparison to PEPA.38 Oligomers derived from PENA can be cured at lower temperatures and the corresponding cured polymers exhibit a better hydrolytic stability than those of PEPA. 15.2.9.2
Citraconic Anhydride Types
Other types of low viscosity resins suitable for processing by resin transfer molding and resin infusion are based on 4,4 -BPDA, 2,2 -bis(4-(4-aminophenoxy)phenyl)propane and CA.39 1 mole of 4,4 -BPDA is used; the other compounds are fed in amounts of 2 mole. Thus, a compound of the structure CA−BAPP−BPDA−BAPP−CA is formed. The oligomer is end capped with CA moieties. The PI resins exhibit melting at temperatures of 150–175°C. The melt viscosities at 200°C
488
High Performance Polymers
are less than about 2,000 cP. Curing of the end capped units is achieved at 330–350°C. Carbon fabric reinforced composite can be prepared from the materials. 15.2.9.3
Isocyanate Types
Aromatic PIs with anhydride end groups can be chain extended by reacting them with aromatic polyisocyanates.40
15.2.10 Bis(maleimide)s The most common thermosetting PI resins are bis(maleimide) resins. A wide variety of these resins is known. Commercially available bismaleimide thermoset compositions are well known for their high modulus, and excellent resistance to thermal degradation. On the other hand, these thermoset compositions are also well known for their brittleness.41 The utility of the bismaleimide class of thermosets can be improved by less brittle formulations that retain the desirable thermal and elastic properties. An improvement in the performance of maleimide thermosets can be achieved through the incorporation of an imide-extended maleimide compounds. These maleimide compounds are readily prepared by the condensation of appropriate anhydrides with appropriate diamines to give amine terminated compounds. These compounds are then condensed with excess maleic anhydride to yield imide-extended maleimide compounds.
15.2.11 Poly(imide sulfones) The desirable properties of PIs and poly(sulfone)s can be combined into a single resin, such as in a poly(etherimide sulfone).4 These resins have low levels of residual volatile species and low levels of reactive groups. Thus articles may be prepared from these resins, which are essentially free of voids, bubbles, splay, silver streaks or other imperfections. Monomers that introduce the sulfone group into the polymer are shown in Table 15.4. Poly(etherimide sulfone)s with a residual volatile species concentration of less than about 500 ppm can be prepared. The resins have a good heat resistance and a good melt processability.
Poly(imide)s
489
Table 15.4: Monomers with Sulfone Groups4 Dianhydrides 4,4 -Bis(3,4-dicarboxyphenoxy)diphenyl sulfone dianhydride 4,4 -Bis(2,3-dicarboxyphenoxy)diphenyl sulfone dianhydride Diamines Diaminodiphenyl sulfone Bis(aminophenoxy phenyl)-sulfone
15.3 PROPERTIES PI is widely used as a protective material or an insulation material in the electronic field due to its good properties, e.g., high mechanical strength, high thermal resistance, and solvent resistance. Selected physical properties of a thermoplastic PEI are shown in Table 15.5. Ultem® is obtained from bisphenol A dianhydride and MPD. It has a glass transition temperature of 217°C. It should be emphasized that there are a lot of different PI resins with deviating properties. Thus the data given in Table 15.5 are not plainly representative for PIs. PEI are in frequent use as insulating materials for integrated circuit boards and printed wiring boards because they are excellent in heat resistance, electrical insulating properties, and mechanical properties. The electrical properties can still be improved by blending with poly(phenylene ether), which has a low relative permittivity, but otherwise poor in mechanical strength, moldability and heat resistance. Alternatively, PEI types with low relative permittivity and low dielectric dissipation factor have been developed42 . Another approach is to provide porous PI materials.43 The introduction of trifluoromethyl side groups to the backbone enhances the optical properties.44 PIs are sensitive to hydrolysis.45 Several PI types are miscible among one another.46, 47 Miscible blends are often of importance to facilitate the fabrication into final articles. Table 15.6 shows common miscible blends of PIs with other polymers. The solubility of PIs can be increased by the introduction of flexible moieties in the backbone.
15.4 SPECIAL ADDITIVES PEI resins are known for high heat distortion temperatures and glass transi-
490
High Performance Polymers
48
Table 15.5: Properties of Ultem® 1000 Property Density Melt Mass-Flow Rate (MFR) a Water Absorption 24 hrs Water Absorption Equil (23°C) Tensile Modulus Tensile Strength Yield Tensile Elongation Yld Tensile Elongation Brk Flexural Modulus b Flexural Strength Yield b Poisson’s Ratio Taber Abrasion Resistance c Notched Izod Impact (23 °C) Unnotched Izod Impact (23 °C) Reverse Notch Izod Impact d Gardner Impact (23 °C) Rockwell Hardness (M-Scale) DTUL 66psi - Unannealed e DTUL 264psi - Unannealed e Vicat Softening Point Thermal Conductivity Volume Resistivity a
Value 1.27 9.0 0.25 1.3 3590 110 7 60 3520 165 0.36 10.0 53.4 1330 1300 36.6 109 210 201 219 0.22 1.0E+17
Unit
Standard
g cm−3 g/10 min % % MPa MPa % % MPa MPa
ASTM D792 ASTM D1238 ASTM D570 ASTM D570 ASTM D638 ASTM D638 ASTM D638 ASTM D638 ASTM D790 ASTM D790 ASTM E132 ASTM D1044 ASTM D256 ASTM D256 ASTM D256 ASTM D3029 ASTM D785 ASTM D648 ASTM D648 ASTM D1525 ASTM C177 ASTM D257
mg J m−1 J m−1 J m−1 J °C °C °C W m−1 K−1 Ω cm
(337°C/6.6 kg) b (100 mm Span) c (1000 Cycles) d (3.20 mm) e (6.40 mm)
Table 15.6: Miscible or Partially Miscible Blends of PIs Poly(imide)
Other Polymer
References
Matrimid® 5218 Matrimid® Matrimid® 5218 YS-30 (ODA+4,4 -ODPA) XU-218 (Ciba)
Torlon® 4000T Polybenzimidazole Poly(ethersulfone) Poly(ether ether ketone) PEI
49 50 51 52 53
Poly(imide)s
491
tion temperatures that make their use as coatings, molded articles, composites, and the like, very attractive where high temperature resistance is desired. Due to their high glass transition temperature and high melt viscosity, however, PEIs are difficult to process into finished products. Molding, extruding, spraying, and the like must be performed at high temperatures to plasticize the PEI resin. Two properties that limit the use of PEI compositions, particularly in injection molding applications, are mold release and melt flow.54 High melt flow is essential for achieving fast molding cycles and molding of complex parts. Mold release agents have been developed, i.e., stearic acid. This additive also improves the flow properties. Carbon fibers have been used to make blends of PEI and poly(ethylene terephthalate) electrically conductive. Such compositions display good dimensional stability at elevated temperatures especially when heated rapidly using electromagnetic radiation, which renders them useful in articles and operations where rapid assembly is important.55
15.5 APPLICATIONS 15.5.1 Foams PI foams are employed:56 • In joining metals to metals or metals to composite structures, • As structural foam, having increased structural stiffness without large weight increases, and • As low density insulation for thermal and acoustic applications. Originally three general routes for the production of poly(imide) foams have been known:57 1. A monomer mixture composed of an ester of benzophenone tetracarboxylic acid and a polyamine, with a content of 9% volatiles is heated to a critical temperature at which foaming occurs contemporaneously with polymerization of the tetracarboxylic and polyamine components until the PI foam is formed.58 2. A mixture of diamines is added to an alcoholic solution of the half ester of benzophenone tetracarboxylic acid and reacted to form a heavy syrup. This syrup is heated to form a PI precursor resin. Eventually, the precursor is pulverized into a powder which is
492
High Performance Polymers spread over aluminum foil on an aluminum plate and heated up to 315°C.57, 59 3. Microwave radiation can be used for converting the PI precursor into a cellular structure which is then normally subjected to final curing in a thermal oven.
Certain methods for making PI foams utilize solutions of diamines and dianhydrides or dianhydride derivatives in a low-molecular-weight alkyl alcohol solvent. The precursor solutions or powders are then processed into foams through the expulsion of water and alcohol during a thermal imidization process. Some processes require the application of microwave radiation to initiate the foaming process. Foams in a wide range of densities, especially very low densities, along with the desired combination of mechanical properties and flame resistance can be prepared by foaming an isocyanate solution. Two separate solutions are prepared. One solution contains the dianhydride, catalysts for foaming the isocyanate and other ingredients, such as blowing agents and flame retardants. The other solution contains the isocyanate. A preferred solvent is DMF. When the solutions are combined, foaming starts. The PI foam is formed by heating the foam in a microwave oven.56 Bright yellow foams are obtained. 15.5.1.1
Aerogels
Aerogels are solid materials that consist of a highly porous network of micro-sized and meso-sized pores. The pores of an aerogel can frequently account for over 90% of the volume when the density of the aerogel is about 0.05 g cm−3 . In general, aerogels are prepared by a supercritical drying technique, i.e., a sol-gel process, to remove the solvent from a gel. The process is conducted in a way that no solvent evaporation can occur. Thus no contraction occurs. An aerogel is prepared in 3 steps:60 1. Dissolution of the solute in a solvent, 2. Formation of the sol, and formation of the gel, 3. Solvent removal without causing pore collapse. PI aerogels are produced by synthesizing a PI highly diluted in a solvent. Then the solvent is replaced by supercritical carbon dioxide. When
Poly(imide)s
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the solvent is not miscible with supercritical carbon dioxide, it can be replaced by another solvent that is miscible with supercritical carbon dioxide. Postcuring before or after replacement with supercritical carbon dioxide can be done. The PI aerogels are suitable for thermal insulation materials with service temperatures ranging from cryogenic temperatures up to 500°C. From PI aerogels, carbon aerogels can be prepared, as the PI aerogel is pyrolyzed.60
15.5.2 Membrane Technology PIs are attractive membrane materials for gas separation because of their good gas separation and physical properties. Many attempts have been made to modify the chemical structure of PIs in order to construct both highly permeable and permselective membrane materials. Blends of PIs have been demonstrated to exhibit improved performance in gas separation applications.23 PIs may suffer from severe ageing and performance decay due to densification or plasticization. A basic study on the plasticization with PI based on 4,4 -hexafluoroisopropylidene diphthalic dianhydride and 2,3,5,6tetramethyl-1,4-phenylenediamine has been presented, using CO2 , N2 , and O2 as permeants.61 Whereas the permeability of N2 , and O2 increases with increasing temperature, the permeability of CO2 decreases with increasing temperature. This effect is attributed to the fact that the gas solubility decreases with increasing temperature, as frequently observed with glassy polymers. The permeability varies with the pressure or more strictly the fugacity of the gas. A convex curve shape with a minimum is characteristic for a plasticizing effect. The minimum is referred to as the plasticization pressure. Crosslinking of the PI provides membranes with antiplasticization properties and good chemical resistance. Crosslinking can be effected by several methods, including:62 • UV light induced photochemical crosslinking reactions in benzophenone containing PIs, or • The formation of interpenetrating networks using polymer blends, and subsequent thermal treatment at elevated temperatures.
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High Performance Polymers
15.5.2.1
Crosslinked Membranes
Dendrimers based on diaminobutane serve as crosslinking agents for fluorinated PI types and are active at room temperature.62, 63 PI types that are built from 2,4,6-trimethyl-1,3-phenylenediamine and 3,3 ,4,4 -benzophenone tetracarboxylic dianhydride are photochemically crosslinkable due to the benzophenone incorporated. It is essential that pendent alkyl groups are attached to the aromatic structure.64 However, it has been objected that photochemical crosslinking is not a practical method for fabricating gas separation membranes cost-effectively.65 15.5.2.2
Blends
Glassy polymeric materials are often plasticized when used in gas membranes due to sorption. This can be overcome by annealing or crosslinking, however, this method does not influence the selectivity of the membrane, instead the permeability is decreased. Another method to stabilize the plasticization is to use polymer blends, as demonstrated with Matrimid®5218 and a copoly(imide) P84. The material is stabilized against carbon dioxide plasticization and the selectivity for a mixture of carbon dioxide and methane is improved.66 Hollow fiber membranes composed of blends of PIs with enhanced resistance towards hydrocarbons have been developed.67 15.5.2.3
Mixed Matrix Membranes
In contrast to ordinary membranes, mixed matrix membranes are composed of an organic polymer and therein embedded inorganic particles such as zeolites, carbon molecular sieves, or nanoparticles. Mixed matrix membranes are believed to achieve higher performance than conventional polymeric membranes. In addition, the poor mechanical properties of inorganic membranes can be improved by embedding them in a flexible polymeric matrix.68 Mixed matrix membranes with zeolite as organic particles and polymeric matrices such as poly(vinyl acetate) (PVAc), Ultem®, and Matrimid® have been prepared. The selectivity of nitrogen to oxygen nearly reaches a factor of 2 over conventional pure polymeric membranes. PI is superior to PVAc in mixed matrix membranes. This is attributed to a higher intrinsic selectivity of the polymer itself. Aminosilanes can be used as cou-
Poly(imide)s
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Table 15.7: Gas Permeability and Separation Factors for Carbon Membranes Obtained at 1173 K in Vacuum69 Gas He CO2 O2 N2 CH4
Permabilty mol m−2 s−1 Pa−1 7.26 0.79 0.26 0.13 0.006
Mixture He/N2 CO2 /N2 O2 /N2 CO2 /CH4
Separation Factor 558.27 60.87 19.69 138.53
pling agents. The hydroxyl group is reactive to the zeolite surface and the amino group may react with the imide group.68 15.5.2.4
Carbon Membranes
Carbon membranes can be prepared by pyrolysis of Kapton® at different temperatures. The pyrolysis occurs both in vacuum and in nitrogen flow. Carbon membranes prepared at 1273 K in vacuum exhibit the highest micropore volume. In contrast, membranes prepared at 1073 K in vacuum exhibit the highest energy of adsorption.69 The gas permeability and separation factors for the carbon membranes are shown in Table 15.7. The selectivity obtained is strongly dependent on the method of pyrolysis, either in vacuum or in nitrogen flow and at the temperature of pyrolysis. In general, the selectivity is much better when the pyrolysis is done in vacuum. In contrast, the membranes prepared in nitrogen flow exhibit a higher permeability than the membranes prepared in vacuum at the same temperature. 15.5.2.5
Methane Enrichment in Biogas
Membranes for the separation of biogas have been reported.70 Raw biogas contains 55–65% CH4 , 30–45% CO2 , further H2 O and H2 S. Utilizing PI membranes, the CH4 content can be enriched to greater than 90%. 15.5.2.6
Pervaporation Membranes
The separation of azeotropic mixtures is difficult using conventional processes such as distillation. In contrast, pervaporation is a promising technique because of its simplicity and low energy consumption. Pervaporation
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High Performance Polymers
uses asymmetric membranes to separate liquid mixtures, the dense and selective surface of the membrane being in contact with the liquid, while the opposite side is evacuated or flushed by a carrier gas. A systematic study on the physical properties and pervaporation performances of various types of PI membranes is available.71 In most types of membranes investigated, the separation factors of mixtures tend to increase with the operating temperature. A correlation between flux and interchain distance (d-spacing) is demonstrated. In contrast, there is no clear relationship between the flux and the fractional free volume. PI materials have been used for the dehydration of water/alcohol mixtures by the pervaporation method.72 P84 co-PI hollow fibers73 and zeolite filled P84 co-PI membranes74 have been used for the pervaporation dehydration of isopropanol. In addition, P84 co-PI based dual-layer hollow fiber membranes serve for the dehydration of tetrafluoropropanol.75
15.5.2.7
Ion Exchange Membranes
Crosslinked sulfonated PI types have been developed for the use as cation exchange membranes. The sulfonated PIs have excellent proton conductivity and a low-cost of preparation. These membranes can be used as polymer electrolyte membranes in hydrogen or a direct methanol fuel cell for electric vehicles and portable electric power sources.76 The synthesis of amines with sulfonic acid groups, 4,4 -bis(4-aminophenoxy)benzophenone-3,3 -disulfonic acid and 4,4 -bis(4-aminophenylthio)benzophenone-3,3-disulfonic acid starts from 4,4 -dichlorobenzophenone by sulfonation with sulfuric acid. In the second step, 4-aminophenol is coupled in the presence of anhydrous potassium carbonate.77 When 3,5-diaminobenzoic acid (DBA) is used as a diamine, PIs with pendent carboxyl groups are formed.78 These carboxyl groups are crosslinkable with aliphatic linear diols. The performance of the membrane is strongly dependent on the chain length of the crosslinking diols. Chain length shorter than four carbon atoms result in a compact structure of the material, whereas longer spacers exhibit higher water uptake and proton conductivity but lower methanol permeability as observed in a non-crosslinked material. Membranes may be reinforced with a porous poly(tetrafluoroethylene) (PTFE) material. In particular, the porous PTFE is impregnated with
Poly(imide)s
497
sulfonated PI in a solution of dimethyl sulfoxide. The PTFE increases the hydrolysis stability of the PI.79 Care must taken for the stability under long-term application conditions. Membranes with phthalic moieties are not satisfactorily stable under fuel cell conditions.80 The stability can be significantly improved by naphthalenic monomers instead of phthalic monomers. However, such types are difficult to fabricate and finalize. In general, PIs are sensitive to hydrolysis. The sensitivity is enhanced by the introduction of sulfonic acid groups. The vulnerability to hydrolysis arises from a thermal activated oxidation process.81 IR studies of the damaged membranes show a significant chemical degradation which is attributed to hydrolysis of the imide moieties. Thus the main chains are broken. In addition to the main chain degradation, the surface or the membrane which is exposed to the anodic compartment seems to undergo an additional oxidative process.80 In addition to IR investigations, the degradation can be traced by viscosity measurements.82
15.5.3 Sensor Technology Patterned copper tracks can be deposited on a flexible PI film. The devices can be used for electronic applications. With a conventional desktop ink-jet printer an aqueous palladium(II) solution is sprayed onto a surface-treated PI film. The palladium(II) is then reduced by treatment with sodium borohydride (NaBH4 ). Finally, a copper layer is deposited by electroless copper plating.83 The surface treatment of the PI, in order to accept the palladium(II) consists either of oxidation with alkaline KMNO4 at 80°C for 60 min or simply with NaOH at room temperature for 72 h. The latter treatment gives better results. Structures with dimensions down to 100 μ m with good copper adhesion can be built. In a similar way, gold pattern layers have been put on PI substrates by micro contact printing.84 In micro contact printing, a polymeric stamp that is wetted with a potassium hydroxide solution is pressed onto the PI substrate. The alkaline treated regions of the substrate become hydrophilic and are prone to hold a palladium(II) solution. In the same way as described above, the adhered palladium ions are reduced by NaBH4 and further electroplated. Flexible PI electrode microelectrode arrays have been developed for electric potential probing for in vivo applications.85 Implantable micro-
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High Performance Polymers
electrodes can be used to record neuronal action potentials or local field potentials from within the brain. The PI arrays consist of alternating layers of PI and platinum. The devices are fabricated by reactive ion etching. Nanocomposite materials constituted by Ag nanoparticles embedded in thin films of fluorinated PI can be used as optical sensors for organic vapors.86 In comparison to ordinary PI, fluorinated PI exhibit an improved optical transparency in the visible range. The PIs consist of 4,4 -hexafluoroisopropylidene diphthalic dianhydride and tetramethyl-p-phenylenediamine, or 4,4 -BPDA and 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane. In order to enhance the porosity of the films, small amounts of a blowing agent, azodicarbonamide (ADC) is added to the PI in solution before casting. ADC can be selectively removed by a thermal treatment. The interaction of the analyte with film effects a change of the plasmon absorption peak of the nanoparticles caused by a change of the average refractive index of the surrounding medium in the visible to near UV region.
15.5.4 Polymer Matrix Electrolytes Electrolytes used in lithium batteries can be liquid or polymer-based electrolytes. Lithium batteries including liquid electrolytes have been on the market for several years. Lithium ion rechargeable batteries having liquid electrolytes are mass produced for applications such as notebook computers, camcorders and cellular telephones.87 Lithium batteries based on liquid electrolytes technology have some drawbacks. The liquid electrolyte generally requires hermetic sealing, which may reduce the energy density. In addition, for safety reasons, lithium ion rechargeable batteries and lithium-metal primary batteries having liquid electrolytes are designed to vent automatically when certain abuse conditions exist, such as a substantial increase in internal pressure which can be caused by internal or external overheating. If the cell is not vented under extreme pressure, it can explode because the liquid electrolyte used in liquid Li cells is extremely flammable. An alternative to lithium batteries with liquid electrolytes are those with solid polymer electrolytes. Solid polymer electrodes are generally gel type electrolytes which trap solvent and salt in pores of the polymer to provide a medium for ionic conduction. Typical polymer electrolytes are shown in Table 15.8. The polymer electrolyte generally functions as a separator, being in-
Poly(imide)s
499
Table 15.8: Polymers for Electrolytes87 Polymer Poly(ethylene oxide) Poly(acrylonitrile) Poly(methyl methacrylate) Poly(vinylidene fluoride) Poly(imide)
terposed between the cathode and anode films of the battery. Because its electrolyte is generally a non-volatile material which does not generally leak under normal operating conditions, a lithium battery having a polymer electrolyte is intrinsically safer than a lithium battery having a liquid electrolyte. Polymer electrolytes eliminate the need for venting and package pressure control which are generally required for operation of lithium batteries having liquid electrolytes. Special PI types for use as polymer electrolytes have been synthes87 ized. It has been found that the average dielectric strength equivalence of the polymer increases with the number of imide rings present per unit weight. Higher equivalent dielectric strength is believed to generally lead to an improved salt interaction, which can improve the ionic conductivity of the polymer matrix electrolyte. The maximum ionic conductivity generally occurs when the system of PI, salt and solvent, forms a completely homogeneous, clear matrix. Any phase separation is expected to reduce the ionic conductivity values.
15.5.5 Films and Coatings for Electronic Applications PI films that are transparent and colorless find applications as liquid crystal display materials, optical communication materials, waveguide materials, or solar battery protection films.88 During fabrication, PI films are likely to turn yellow or brown due to thermal degradation resulting from severe thermal history. In order to fabricate high transparent films, special techniques must be employed. Including fluor atoms into the structure to make the materials more heat resistant. Another method consists of the following steps in film fabrication:88 1. Mixing and polymerizing the diamine constituent and the acid di-
500
High Performance Polymers anhydride constituent in a solvent so as to obtain a polyamic acid solution, 2. Heating the polyamic acid solution under reduced pressure to obtain a PI resin, 3. Dissolving the PI resin in a solvent, 4. Casting and drying.
Metallized PI films are used in electronic applications, e.g. for flexible printed circuit boards. Conventional techniques for the fabrication use adhesive bonding to a copper foil. However, the demand for high density packaging of electronic apparatuses requires a further reduction in the thickness of these substrates. It is possible to sputter metal particles into the surface of the PI film at a thickness of 20 nm, which forms the intermediate layer for subsequent formation of a conductive layer of copper or a copper alloy.89, 90 PI resins find use as coverlays. A coverlay is generally used to protect printed circuits during subsequent processing, primarily solder operations, or from environmental effects during use.91 The PIs can be used in combination with epoxies. In addition, additives, such as phosphorous-based flame retardants or adhesion promoters, may be used in the formulations. PI is used to form an alignment film for liquid crystalline displays (LCD)s, which can provide a uniform and stable alignment effect to the liquid crystal molecules.92
15.5.6 Photosensitive Compositions Photosensitive PI resin compositions are used for insulating films in electronic applications, such as semiconductor and LCD devices. In optical devices, increased transparency is a desirable property. Basically, it is differentiated between: • Negative type resins, wherein the light exposed portion is crosslinked and made insoluble by light, and • Positive type photosensitive resin wherein the light exposed portion becomes soluble. For PIs, both types are available. It has been pointed out that positive types are advantageous over negative types.93 With positive types, pinholes are unlikely to be formed. With negative types, if dust adheres, that portion is not exposed and therefore is etched out, thus forming a hole.
Poly(imide)s 15.5.6.1
501
Negative Types
PI as such shows negative type photosensitivity when irradiated with an actinic beam.94 Polymerizable vinyl groups can be introduced into PI precursors by treating dianhydrides with vinyl containing alcohols, such as 2-hydroxyethyl methacrylate (HEMA), converting into the acyl chloride and condensing with diamines, e.g., DBA.95 The reaction scheme is shown in Figure 15.8. Actually, a siloxane containing precursor polymer exhibits improved adhesive properties to the substrate. To the mixture in a suitable solvent, the photoinitiator and the sensitizer are added, e.g., Michler’s ketone and 4-diethylaminoethyl benzoate. As polymerizable comonomers, higher diacrylate esters may be added. The composition is spin cast onto a substrate and dried. After irradiation, the irradiated regions become crosslinked and insoluble. The coating is developed in aqueous tetramethylammonium hydroxide solution. Eventually, the film is converted into PI by heating up to 400°C. In this step the photosensitive group is released as the precursor is converted into the PI. According to this principle, other types of PI precursors based on PMDA, 4,4 -oxydianiline and HEMA have been prepared and characterized.96 The photoinitiator can be directly introduced into the polymeric backbone. 4,4-bis[(4-amino)thiophenyl] benzophenone has amino groups and is photosensitive at the same time.97 15.5.6.2
Positive Types
A positive type photosensitive PI resin composition can be made up from a solvent-soluble PI, a poly(amic acid) compound, and by an o-quinonediazide compound.98 The latter compound releases acid groups when irradiated by UV light. The method of etching is based on the principle that although the PI resin itself is not photosensitive, the photosensitive substance becomes alkali-soluble upon exposure and it is dissolved in an alkali together with the PI resin.99 Solvent-soluble PI types are needed for this application. For such PI resins special components, are reported, which are summarized in Table 15.9. Diamines are shown in Figure 15.9 and dianyhdrides are shown in Figure 15.10. Such solvent-soluble PIs are used in photosensitive compositions. They are dissolved in a suitable solvent, e.g., ethyl lactate and
502
High Performance Polymers
O
O
O
O
O
O + O
HOR = HO CH2 CH2 O C CH CH2 O
O
HO RO
OH OR
O
O SOCl2 O
O
Cl RO
Cl OR
O
O H2NR1NH2 O
O
HN RO
NHR1 OR
O
O
Figure 15.8: PI Precursors with Pendent Vinyl Groups95
Poly(imide)s
Table 15.9: Monomers for Soluble PIs100 Dianhydrides Cyclobutanetetracarboxylic dianhydride 4,4 -Hexafluoroisopropylidene diphthalic dianhydride 3,3 ,4,4 -Biphenyl dianhydride Diamines 2,2 -Bis(3-amino-4-toluyl)hexafluoropropane 3,5-Diaminobenzoic acid 2,2 -Bis(trifluoromethyl)benzidine Bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane Bis[4-(3-aminophenoxy)phenyl]sulfone
CF3 H 2N
NH2 F3 C
2,2′-Bis(trifluoromethyl)benzidine H 2N
NH2 CF3 CH3
C
H 3C
CF3 2,2′-Bis(3-amino-4-toluyl)hexafluoropropane O O
O
S O
H 2N
NH2 Bis[4-(3-aminophenoxy)phenyl]sulfone CH3
H 2N
CH2
CH2
CH2
Si CH3
CH3 O Si
CH2
CH2
CH2
CH3
Bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane
Figure 15.9: Diamines for Soluble PIs
NH2
503
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High Performance Polymers
O
O
O
O
O
O
Cyclobutanetetracarboxylic dianhydride O O
O
O O
O CH3
5-(2,5-Dioxotetrahydrofuryl)-3-methyl-3-cyclohexene1,2-dicarboxylic dianhydride O O O
O CF3 C
O
O CF3 4,4′-Hexafluoroisopropylidene diphthalic dianhydride
Figure 15.10: Dianyhdrides for Soluble PIs
Poly(imide)s
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Table 15.10: Types of Photo Acid Generators101 Compound type Diazonaphthoquinones Nitrobenzylsulfonates Triaryl sulfonium salts (Φ3 S+ SbF− 6) Diphenyliodonium salts Polymer types
Table 15.11: Compounds for Reactive End Capping101 Compound Nadic anhydride Itaconic anhydride 2,3-Dimethylmaleic anhydride Ethynylaniline
NMP and admixed with the poly(amic acid). The photosensitive compound is added to this mixture. The compound generates an acid by irradiation with light. These compounds are also addressed as photo acid generators. A photosensitive compound is prepared by the condensation of 4,4 -[1-[4-[1-(4-hydroxyphenyl)-1-methylethyl]phenyl]ethylidene] with 1,2-naphthoquinone-2-diazido-5-sulfonyl chloride. Other types of photo acid generators are summarized in Table 15.10. The final composition is coated onto the substrate by spin-coating. After drying, the material can be exposed to UV light through a mask. Subsequent etching is done with an aqueous alkaline solution.100 A varied method uses only poly(amic acid ester)s. These compounds are the precursors for PIs. After etching, the substrate is heated and the PI is formed on the fly.93 The precursors can be modified with reactive end capping groups.101 These groups are converted into reactive groups that effect additional crosslinking during the final conversion step. End capping groups are shown in Table 15.11.
15.6 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 15.12. Further details concerning the chemical constitution of commercial available PIs and special properties can be found in the book of Bessonov [5, chapter 4].
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Table 15.12: Examples for Commercially Available Poly(imide) Polymers Tradename
Producer
Remarks
Ableloc® Apical® Aurum® Avimid® Cirlex® Extem® Imidex® Kapton® Kerimid® Matrimid® Meldin® Neoflex® Onlymide® OP-PEI. . . GF PRL PEI-G. . . P84 Polycoustic® Pyralin® Pyre® Pyrocoat® Solimide® Tecapei® Tempalux® HI Toray® Ultem® Unitem Upilex® Upimol® Vespel®
Indopco Kaneka Mitsui Chemicals DuPont, Cytec DuPont General Electric Westlake Plastics DuPont Ciba Huntsman (Ciba) St.-Gobain Performance Plastics Mitsui Chemicals Kolon Industries, Inc. Oxford Polymers Polymer Resources Ltd Lenzing Johns Manville International, Inc. DuPont I.S.T. Corp. Furukawa Electric Inspec Foams, Inc. Ensinger Westlake Plastics Toray Industries, Inc. General Electric Nytef Plastics Ube Industries Ube Industries DuPont
Film Film, TS TP
TP: Thermoplastic type GF: Glass fiber reinforced type TS: Thermosetting type
Film TP Film TS
TS Laminate TP, GF TP, GF PI+PTFE Foam Adhesive Wire coating Coating Foam PEI PEI Film Coating TP Film Powder TS
Poly(imide)s
507
Tradenames appearing in the references are shown in Table 15.13.
15.7 SAFETY There are plenty of material safety data sheets that should be consulted before handling PIs. In the case of thermosetting types, the uncured products are regarded as hazardous wastes. On proper handling, the materials are not considered harmful. For solution types, potential hazards may be caused from volatile solvents. The vapors generated by fluorocarbon containing formulations on heating may cause fever (polymer fume fever) on inhalation. Some grades of PEI are not recommended for use in medical applications which require biocompatibility. Microparticles made from PEIs are used as polymeric supports in extracorporal blood detoxification.102 Certain copolymers, poly(anhydride-co-imide)s have been proposed for controlled macromolecule delivery. These copolymers are well tolerated in acute toxicity studies in rats and therefore show promise as biomaterials.103
15.8 ENVIRONMENTAL IMPACT AND RECYCLING PIs serve as catalysts for transition metal complexes. These metal complexes are suitable for the epoxidation of olefins. The heterogeneous PIsupported transition metal complex catalysts provide superior catalytic activity, selectivity and stability in the epoxidation of higher olefin. Because of the heterogeneous nature, the catalysts can be easily separated from the reaction product, which eases recycling of the catalysts.104
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High Performance Polymers
Table 15.13: Tradenames in References Tradename Description
Supplier
Antiblaze (Series) Flame retardant56 Arcol®
Rhodia Inc.
Poly(propylene oxide)35 Avimid® Poly(imide)25 Capcure® 3-800 Thiol end-capped polymer35 Carbowax® (Series) Poly(ethyleneoxide glycol) (PEG)35 Cariflex® Triblock copolymer55 Conductex® Carbon black55 Crystar® 5005 Poly(ethylene terephthalate)54 DABCO® 8154 Cyclic tertiary amine catalyst56 DABCO® K-15
Bayer AG, ARCO Chemical Co. DuPont Henkel Union Carbide Corp. Shell Columbian Chemical Corp. DuPont Air Products and Chemicals, Inc. Air Products and Chemicals, Inc.
Metallic based catalyst56 Doverphos® S9228 Dover Chemical Corp. Bis(2,4-dicumylphenyl pentaerythritol) diphosphite54 Epikote® (Series) Yuka Shell Co. Bisphenol A/F epoxies91 Epon® (Series) Resolution Performance Products LLC. Corp. (Shell) Diglycidyl ethers of bisphenol A35 Ethacure® 100 Albemarle Corp. Diethyltoluene diamine35 Eupergit® C250L Röhm Epoxy-activated poly(methacrylamide)102
Poly(imide)s
509
Table 15.13 (cont): Tradenames in References Tradename Description
Supplier
Exolit® OP 1311 Clariant GmbH Mixture of aluminum salts of diethylphosphinate and melamine polyphosphate91 H-2™ Shell Mixture of ethylenediamine and methyl isobutyl ketone ketimine (curing agent)35 Hycar® (Series) Lubrizol Advanced Materials, Inc., (B.F. Goodrich Co.) Amine-terminated butadiene-acrylonitrile35 Irganox® 1010 Ciba Geigy Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), phenolic antioxidant54 Jeffamine® (Series) Huntsman Petrochemical Corp. Amine capped polyalkoxylene glycol35 Kapton® DuPont-Toray Co., Ltd. Poly(imide)25, 91 Kraton® Shell Styrenic block copolymer55 Lenzing® P84 Lenzing AG Benzophenone tetracarboxylic dianhydride-MDI-2,4-TDI copolymer, poly(imide)18, 23, 62, 65, 67 Lindol® XP Plus Azko Nobel Chemicals, Supresta Tricresyl phosphate91 Loxiol® G40 Cognis Fatty acid isoalcohol ester54 LP-2™ Morton International, Inc. Poly(sulfide)35 Matrimid® Ciba Geigy Poly(imide)65, 67, 68 Melapur® 200 DSM Melamine poly(phosphate) (flame retardant)91 Niax® A-33 O Si Specialities, Inc. 56 Amine catalyst Niax® L-620 General Electric,O Si Specialities, Inc. Silicone emulsifiyer, for flexible poly(urethane) foam56 Niax® L-6900 O Si Specialities, Inc. Surfactant56
510
High Performance Polymers
Table 15.13 (cont): Tradenames in References Tradename Description
Supplier
Noryl® PPE PS Blend32 PACM™ 20
General Electric
Bis(4-aminocyclohexyl)methane35 PAPI® (Series) Isocyanate56 Polycat® (Series)
Air Products and Chemicals, Inc. Huntsman Polyurethanes Air Products and Chemicals, Inc.
Amine based catalysts56 PolyTHF® CD BASF AG THF copolymers35 Primene® MD Rohm & Haas 1,8-Diamino-p-menthane35 Primene® Rohm & Haas Primary aliphatic amines with highly branched alkyl chains35 Rubinate® (Series) Huntsman Polyurethanes Isocyanate56 Sepharose® CL GE Healthcare Crosslinked poly(saccharide)102 Surlyn® DuPont Ionomer resin55 Sylgard® 184 Dow Silicone elastomer65 Teflon® Dupont Tetrafluoro polymer47, 64, 91 Terathane® DuPont Poly(tetramethyleneoxide glycol) (PTMEG)35 Teric® (Series) ICI Poly(ethylene glycol) mono(nonylphenyl) ether24 Thermid® National Starch and Chemical Co. PETI type Poly(imide)37 Tone® (Series) Union Carbide Corp. Polyols35
Poly(imide)s
Table 15.13 (cont): Tradenames in References Tradename Description
Supplier
Ultem® (Series) Poly(imide), thermoplastic18, 24, 54, 55, 68 Unilink® (Series)
General Electric
Aromatic secondary diamines35 Upilex® Poly(imide)25 Versalink® (Series)
Dorf Ketal Chemicals (UOP, Inc.) Ube Industries, Ltd.
Air Products and Chemicals, Inc. Amine terminated poly-THF and PPO for PU resins35 Vircol® 82 Rhodia Inc. Flame retardant56
511
512
High Performance Polymers
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57. J. R. Barringer, H. E. Broemmelsiek, C. W. Lanier, and R. Lee. Polyimide foam products and methods. US Patent 5 096 932, assigned to Ethyl Corporation (Richmond, VA), March 17, 1992. 58. E. Lavin and I. Serlin. Process for the preparation of a polyimide foam. US Patent 3 554 939, assigned to Monsanto Co., January 12, 1971. 59. J. Gagliani. Fire resistant resilient foams. Report NASA-CR-147496, Res. Dep.,Solar, San Diego, CA, 1976. 60. W. Rhine, J. Wang, and R. Begag. Polyimide aerogels, carbon aerogels, and metal carbide aerogels and methods of making same. US Patent 7 074 880, assigned to Aspen Aerogels, Inc. (Marlborough, MA), July 11, 2006. 61. X. Duthie, S. Kentish, C. Powell, K. Nagai, G. Qiao, and G. Stevens. “Operating temperature effects on the plasticization of polyimide gas separation membranes.” J. Membr. Sci., 294(1-2):40–49, May 2007. 62. T.-S. N. Chung, M. L. Chng, and L. Shao. Polyimide membranes. US Patent 7 169 885, assigned to National University of Singapore (Singapore, SG), January 30, 2007. 63. L. Shao, T.-S. Chung, S. H. Goh, and K. P. Pramoda. “Transport properties of cross-linked polyimide membranes induced by different generations of diaminobutane (DAB) dendrimers.” J. Membr. Sci., 238(1-2):153–163, July 2004. 64. R. A. Hayes. Polyimide gas separation membranes. US Patent 4 717 393, assigned to E. I. Du Pont de Nemours and Company (Wilmington, DE), January 5, 1988. 65. J. W. Simmons and O. M. Ekiner. Polyimide blends for gas separation membranes. US Patent 7 018 445, assigned to L’Air Liquide, Societe Anonyme a Directoire et Conseil de Surveillance Pour l’Etude et l’Exploitation des Procedes Georges Claude (Paris, FR) N/A (, March 28, 2006. 66. A. Bos, I. Punt, H. Strathmann, and M. Wessling. “Suppression of gas separation membrane plasticization by homogeneous polymer blending.” AIChE Journal, 47:1088–1093, 2001. 67. J. W. Simmons, S. Kulkarni, and O. M. Ekiner. Method for separating hydrocarbon-containing gas mixtures using hydrocarbon-resistant membranes. US Patent 7 025 804, assigned to L’Air Liquide, Societe Anonyme A Directoire et Conseil De Surveillance Pour L’Etude et L’Exploitation Des Procedes Georges Claude (Paris, FR) N/A (, April 11, 2006. 68. T.-S. Chung, L. Y. Jiang, Y. Li, and S. Kulprathipanja. “Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation.” Prog. Polym. Sci., 32(4):483–507, April 2007. 69. A. C. Lua and J. Su. “Effects of carbonisation on pore evolution and gas permeation properties of carbon membranes from kapton(r) polyimide.” Carbon, 44(14):2964–2972, November 2006.
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70. M. Harasimowicz, P. Orluk, G. Zakrzewska-Trznadel, and A. Chmielewski. “Application of polyimide membranes for biogas purification and enrichment.” J. Hazard. Mater., 144(3):698–702, June 2007. 71. Y. Xu, C. Chen, and J. Li. “Experimental study on physical properties and pervaporation performances of polyimide membranes.” Chem. Eng. Sci., 62 (9):2466–2473, May 2007. 72. X. Qiao, T.-S. Chung, and K. Pramoda. “Fabrication and characterization of BTDA-TDI/MDI (P84) co-polyimide membranes for the pervaporation dehydration of isopropanol.” J. Membr. Sci., 264(1-2):176–189, November 2005. 73. R. Liu, X. Qiao, and T.-S. Chung. “The development of high performance P84 co-polyimide hollow fibers for pervaporation dehydration of isopropanol.” Chem. Eng. Sci., 60(23):6674–6686, December 2005. 74. X. Qiao, T.-S. Chung, and R. Rajagopalan. “Zeolite filled P84 co-polyimide membranes for dehydration of isopropanol through pervaporation process.” Chem. Eng. Sci., 61(20):6816–6825, October 2006. 75. K. Y. Wang, T.-S. Chung, and R. Rajagopalan. “Dehydration of tetrafluoropropanol (TFP) by pervaporation via novel PBI/BTDA-TDI/MDI co-polyimide (P84) dual-layer hollow fiber membranes.” J. Membr. Sci., 287(1): 60–66, January 2007. 76. Y.-M. Lee, H.-B. Park, and C.-H. Lee. Crosslinked sulfonated polyimide films. US Patent 7 157 548, assigned to Hanyang Hak Won Co., Ltd. (KR), January 2, 2007. 77. F. Zhai, X. Guo, J. Fang, and H. Xu. “Synthesis and properties of novel sulfonated polyimide membranes for direct methanol fuel cell application.” J. Membr. Sci., 296(1-2):102–109, June 2007. 78. H. B. Park, C. H. Lee, J. Y. Sohn, Y. M. Lee, B. D. Freeman, and H. J. Kim. “Effect of crosslinked chain length in sulfonated polyimide membranes on water sorption, proton conduction, and methanol permeation properties.” J. Membr. Sci., 285(1-2):432–443, November 2006. 79. L. Wang, B. Yi, H. Zhang, Y. Liu, D. Xing, Z.-G. Shao, and Y. Cai. “Sulfonated polyimide/PTFE reinforced membrane for PEMFCs.” J. Power Sources, 167(1):47–52, May 2007. 80. G. Meyer, G. Gebel, L. Gonon, P. Capron, D. Marscaq, C. Marestin, and R. Mercier. “Degradation of sulfonated polyimide membranes in fuel cell conditions.” J. Power Sources, 157:293–301, June 2006. 81. C. Perrot, G. Meyer, L. Gonon, and G. Gebel. “Aging mechanisms of proton exchange membrane used in fuel cell applications.” Fuel Cells (Weinheim, Germany), 6:10–15, 2006. 82. H.-J. Kim, M. H. Litt, E.-M. Shin, and S. Y. Nam. “Hydrolytic stability of sulfonic acid-containing polyimides for fuel cell membranes.” Macromol. Res., 12:545–552, 2004.
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83. S. Busato, A. Belloli, and P. Ermanni. “Inkjet printing of palladium catalyst patterns on polyimide film for electroless copper plating.” Sens. Actuators, B, 123(2):840–846, May 2007. 84. S. Yoon, D. Kim, S. Park, Y. Lee, H. Chae, S. Jung, and J.-D. Nam. “Direct metallization of gold patterns on polyimide substrate by microcontact printing and selective surface modification.” Microelectron. Eng., 85(1):136– 142, January 2008. 85. K. C. Cheung, P. Renaud, H. Tanila, and K. Djupsund. “Flexible polyimide microelectrode array for in vivo recordings and current source density analysis.” Biosens. Bioelectron., 22(8):1783–1790, March 2007. 86. A. Quaranta, S. Carturan, M. Bonafini, G. Maggioni, M. Tonezzer, G. Mattei, C. de Julian Fernandez, G. Della Mea, and P. Mazzoldi. “Optical sensing to organic vapors of fluorinated polyimide nanocomposites containing silver nanoclusters.” Sens. Actuators, B, 118(1-2):418–424, October 2006. 87. C. G. Wensley, A. Vallee, D. Brouillette, and S. Gustafson. Polyimide matrix electrolyte. US Patent 7 198 870, assigned to Solicore, Inc. (Lakeland, FL) Avestor Limited Partnership (Boucherville, Quebec, CA), April 3, 2007. 88. M. Nishinaka and T. Itoh. Polyimide film and process for producing the same. US Patent 7 247 367, assigned to Kaneka Corporation (Osaka, JP), July 24, 2007. 89. S. Katsuki and H. Mii. Process for preparing metal-coated aromatic polyimide film. US Patent 7 232 610, assigned to UBE Industries, Ltd. (Yamaguchi, JP), June 19, 2007. 90. M. Aida. Metallized polyimide film and manufacturing method therefor. US Patent 7 241 490, assigned to Mitsubishi Shindoh Co., Ltd. (Tokyo, JP), July 10, 2007. 91. T. E. Dueber, M. W. West, B. C. Auman, and R. V. Kasowski. Polyimide based adhesive compositions useful in flexible circuit applications, and compositions and methods relating thereto. US Patent 7 220 490, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), May 22, 2007. 92. C.-D. Lee, H.-M. Liu, K.-H. Shen, and J.-M. Chen. Vertical alignment polyimide and vertical alignment film compositions for lcd. US Patent 7 132 137, assigned to Industrial Technology Research Institute (Hsinchu, TW), November 7, 2006. 93. T. Yamanaka, K. Okada, and K. Takagahara. Polyimide precursor, manufacturing method thereof, and resin composition using polyimide precursor. US Patent 7 189 488, assigned to Kaneka Corporation (Osaka, JP), March 13, 2007. 94. H. Itatani, S. Matsumoto, T. Itatani, T. Sakamoto, S. Gorwadkar, and M. Komuro. Method for forming polyimide pattern using photosensitive polyimide and composition for use therein. US Patent 6 777 159, assigned to PI
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95.
96.
97.
98.
99.
100.
101.
102.
103.
104.
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R&D Co., Ltd. (Kanagawa, JP) The National Institute of Advanced Industrial Science and Technology (Tokyo, JP), August 17, 2004. H. Kikkawa, F. Kataoka, I. Takemoto, J. Tanaka, K. Isoda, S. Uchimura, M. Kaji, and M. Sugiura. Photosensitive polyimide precursor and its use for pattern formation. US Patent 6 319 656, assigned to Hitachi Chemical Company, Ltd. (Tokyo, JP), November 20, 2001. L. T. T. Nguyen, H. N. Nguyen, and T. H. T. La. “Synthesis and characterization of a photosensitive polyimide precursor and its photocuring behavior for lithography applications.” Opt. Mater., 29(6):610–618, February 2007. X. Jiang, H. Li, H. Wang, Z. Shi, and J. Yin. “A novel negative photoinitiator-free photosensitive polyimide.” Polymer, 47(9):2942–2945, April 2006. K. Ishii, T. Nakayama, T. Nihira, and H. Fukuro. Positive type photosensitive polyimide resin composition. US Patent 6 677 099, assigned to Nissan Chemical Industries, Ltd. (Tokyo, JP), January 13, 2004. M. Yukawa, T. Abe, and N. Kohtoh. Positive photosensitive polyimide resin composition comprising an o-quinone diazide as a photosensitive compound. US Patent 5 288 588, assigned to Nissan Chemical Industries Ltd. (Tokyo, JP), February 22, 1994. T. Nakayama, M. Kato, and T. Nihira. Positive photosensitive polyimide resin composition. US Patent 7 026 080, assigned to Nissan Chemical Industries, Ltd. (Tokyo, JP), April 11, 2006. M. S. Jung, S. K. Jung, Y. Y. Park, B. S. Moon, and B. K. Kim. Positive-type photosensitive polyimide precursor and composition comprising the same. US Patent 6 600 006, assigned to Samsung Electronics Co., Ltd. (Kyungki-do, KR), July 29, 2003. W. Albrecht, K. Lutzow, T. Weigel, T. Groth, M. Schossig, and A. Lendlein. “Development of highly porous microparticles from poly(ether imide) prepared by a spraying/coagulation process.” J. Membr. Sci., 273(1-2):106– 115, March 2006. J. Hanes, M. Chiba, and R. Langer. “Degradation of porous poly(anhydride-co-imide) microspheres and implications for controlled macromolecule delivery.” Biomaterials, 19:163–172, 1998. S. K. Ihm, C. G. Oh, J. H. Ahn, J. C. Kim, and D. C. Sherrington. Polyimide-supported transition metal complex catalyst and process for preparing epoxy compounds using the same. US Patent 6 063 943, assigned to Korea Advanced Institute of Science and Technology (KR), May 16, 2000.
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16 Liquid Crystal Polymers Liquid crystals were discovered by the botanist Friedich Reinitzer∗ ,1, 2 although the first observation of liquid crystals can be likely ascribed to George-Luis LeClerc.3 After their discovery, liquid crystals were considered for a long time as a curiosity. Engaged with the constitution of cholesterol, for cholesteryl benzoate Reinitzer reported two phase transitions in the course of melting. Melting starts with the formation of a cloudy liquid, which transforms on further heating into a clear liquid. This type of liquid crystal is addressed as thermotropic. In addition, there are lyotropic liquid crystals that change their behavior as both a function of concentration in a solvent and of temperature. Liquid crystals originate from the phenomenon that the crystal structure is partly maintained in the liquid phase above the melting point. In particular, they exhibit a long range orientational order, but not a positional order. The various types of liquid crystal phases, also called mesophases can be classified according to their type of ordering into: 1. 2. 3. 4. 5.
Nematic phases, Smectic phases, Cholesteric phases, Chiral phases, and Discotic phases.
∗ Friedich Reinitzer, born Feb. 25, 1857 in Prague, now Czech Republic, died Feb. 16, 1927 in Graz, Austria
521
522
High Performance Polymers
a
b COO N N
O COO C C
COOCO CNH
CONH
c (CH2)n
(CH2O)n
O
Figure 16.1: Units for Liquid Crystalline Polymers. a: Stiff Units, b: Linking Units, c: Flexible Spacer Units
Various subclasses of liquid crystals have been described.3, 4 From the molecular view, liquid crystals are built up of rod-like stiff moieties in the molecule. This is also true for liquid crystal polymers (LCP)s. Discotic liquid crystalline molecules are disc-shaped molecules.5 Examples are Hexaalkanoyloxy benzenes, hexaalkoxy triphenylenes, bis-(4n-decylbenzoyl)methanato copper (II), hexa-n-alkanoates of truxene and octa substituted phthalocyanines.6 Besides the type of the liquid crystal, LCP can be classified into main chain LCP and side chain LCP. Both main chain LCPs.7–12 and side chain LCP13, 14 have been reviewed in the literature.
16.1 MONOMERS Typically monomers for LCPs consist of stiff units connected with linking units. These units are summarized in Figure 16.1. Typical stiff units include phenyl, biphenyl, and naphthoic units. The linking units include ester, ether, amide, and other units. Thus LCPs do not belong to an unique class of polymers in the common sense. Flexible spacer units are made up from aliphatic chains or polyether chains. Common monomers for LCPs are shown in Table 16.1 and Figure 16.2. Polymers exclusively based on the monomers of either terephthalic
Liquid Crystal Polymers
HOOC HOOC
COOH Isophthalic acid
COOH
Terephthalic acid OH
HO
COOH
HOOC
4-Hydroxybenzoic acid
2-Hydroxy-6-naphthoic acid CH3 CH3
COOH
CH3 HO
C C COOH
HO
4-Hydroxycinnamic acid
Lithocholic acid OH
COOH HO
HOOC 2,6-Naphthalenedicarboxylic acid HO
OH
2,6-Dihydroxynaphthalene
HO
Hydroquinone
OH 4,4′-Biphenol
HO
OH
4-Aminophenol
Figure 16.2: Monomers Used for Liquid Crystal Polymers
523
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High Performance Polymers Table 16.1: Monomers for Liquid Crystal Polymers Basic Monomers
References
Terephthalic acid Isophthalic acid 2,6-Naphthalenedicarboxylic acid 4-Hydroxybenzoic acid 2-Hydroxy-6-naphthoic acid 4-Hydroxycinnamic acid Lithocholic acid Hydroquinone tert-Butylhydroquinone 4,4 -Biphenol 2,6-Dihydroxynaphthalene 4-Aminophenol
15 16 15 17 7 18 18 7 19 15 15
Functionalized Monomers Benzoic acid, 4-[4-[(3-ethyl-3-oxetanyl)methoxy]butoxy]-, 1,4-phenylene ester Benzoic acid, 4-[(3-ethyl-3-oxetanyl)methoxy]-, 4-[[4-(octyloxy)phenoxy]carbonyl]phenyl ester
20 20
acid (TPA), 4-hydroxybenzoic acid (HBA) or hydroquinone (HQ) are intractable because of their crystallinity. Therefore, copolymers in which the benzene moieties are partly replaced by naphthalene or by biphenyl moieties are in fabricated.7 Amide groups yield LCP with high glass transition temperatures. This behavior results from the fact that amide groups are capable of forming hydrogen bonds that fix the chains. The replacement of HQ by 4,4 -biphenol (BP) reduces the melting temperature. In the same way, the use of comonomers reduces the melting temperature, because the polymeric backbone is made up of moieties of different size which disturbs intermolecular aggregation. The introduction of naphthoic groups introduces kinks in the backbone which leads to a loss of symmetry. In contrast, the introduction of linear groups that can easily rotate, such as are present in TPA and HQ increases the melting point considerably. 4-Hydroxycinnamic acid and lithocholic acid are used in biocompatible materials.18 Polymers composed from these compounds can be used for medical parts such as cell incubation dishes, surgical suture threads,
Liquid Crystal Polymers
R
COO OCO
R=
525
R
O(CH2)5OOCHC CH2
N C
COO
O(CH2)4OOCHC CH2
Figure 16.3: Photopolymerizable Liquid Crystalline Monomers21
bone fixation screws, and artificial blood vessels.
16.1.1 Acetyletion Often the monomers listed in Table 16.1 are not used as such, but in acetylated form.17, 22 4-Acetoxybenzoic acid can be prepared from HBA by neutralizing with sodium hydroxide, cooling with ice and treatment with acetic anhydride. After adding concentrated hydrochloric acid, a slurry is obtained from which the raw product can be isolated. The raw product is recrystallized from methyl isobutyl ketone. The preparation of 1,4-Diacetoxybenzene needs refluxing of HQ with acetic anhydride. Several other acetylated monomers have been prepared in this way.17
16.1.2 Functionalized Monomers Monomers with reactive groups such as acrylic and cyclic ether units are used in the fabrication of liquid crystalline displays (LCD)s. A monomer with acrylic units is shown in Figures 16.4 and 16.4. A discotic photopolymerizable liquid crystal monomer is shown in Figure 16.4. The starting compound for discotic monomers is hexahydroxytriphenylene. Suitable compounds bearing acrylic units are attached to the hydroxy groups.23 In general, discotic liquid crystal compounds exhibit a large refractive index of birefringence. This means that the material is highly anisotropic. A series of other examples are given in the literature.24 The method of preparation of the optical compensation film has been described in detail.24
526
High Performance Polymers
R
R
R
R R
R=
OCO
R
O(CH2)4OOCHC CH2
Figure 16.4: Discotic Monomer24
Monomers with oxetane units, c.f. Figure 16.5, can be prepared in three steps by the reaction of:20 1. Dibromoalkane with a hydroxy oxetane compound, 2. Reaction with 4-hydroxybenzoic acid, and 3. Esterification of the carboxyl groups with HQ. The synthesis is sketched in Figure 16.6. The first two steps are a Williamson ether synthesis and the final step is an esterification.
16.2 POLYMERIZATION AND FABRICATION 16.2.1 Copoly(ester)s Instead of using diols and hydroxycarboxylic acids as monomers for condensation, the diacetate of the aromatic diols and the monoacetate of the aromatic hydroxycarboxylic acid can be used.19 Then the process of polycondensation proceeds as transesterification. The diacetate of the aromatic diols is used in slight stoichiometric excess in order to compensate losses in the course of condensation. For example, a mixture of the monomers is heated with continuous stirring in the polymerization vessel from 130°C to 270°C during 75 min at atmospheric pressure under nitrogen flow. Then the mixture is heated up to 325°C within 195 min. At this stage, approximately 85% of the stoichiometric amount of the acetic acid is distilled off. Eventually, the temperature
Liquid Crystal Polymers
527
O O O (CH2)7
C
O C
O
O
(CH2)4 O
CH3
CH2 CH2
CH3
O Benzoic acid, 4-[(3-ethyl-3-oxetanyl)methoxy]-, 4-[[4-(octyloxy)phenoxy]carbonyl]phenyl ester O O (CH2)4
C O
O
O
O (CH2)4 O
CH2 CH2 O
O
C
CH2 CH3
CH2 O
Benzoic acid, 4-[4-[(3-ethyl-3-oxetanyl)methoxy]butoxy]-, 1,4-phenylene ester
Figure 16.5: Liquid Crystalline Monomers with Oxetane Units20
CH3
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High Performance Polymers
OH CH2 CH2
CH3 + Br
CH2 CH2 CH2 CH2 Br
O O CH2 CH2 CH2 CH2 Br CH2
O
CH2
CH3
HO
C OH
O
O
O CH2 CH2 CH2 CH2 O
C OH
CH2 CH2
CH3
HO
OH
O O O
C
(CH2)4
C O
O
O
O (CH2)4 O CH2
CH2 CH2 O
O
CH3
CH2 O
Figure 16.6: Synthesis of Oxetane Functionalized Monomers
CH3
Liquid Crystal Polymers
529
is raised to 330°C and the pressure is reduced from atmospheric pressure to less than 15 mm Hg.19 A process of polymerization has been described that starts with an acid chloride, and a hydroxy acid in xylene as a solvent. After reaction, the hydrogen chloride is neutralized. In the next step, additional diacid and aromatic diol is added, together with acetic anhydride. Thus the acetylated products are created on the fly in the polymerization vessel. Finally, the actual transesterification polycondensation is performed.25
16.2.2 Poly(ester amide)s The addition of 4-aminophenol to compositions used commonly for poly(ester)s allows the production of poly(ester amide)s. For example, a mixture of 2,6-naphthalene dicarboxylic acid, TPA, BP, HBA, and 4-aminophenol is condensed in the presence of acid anhydride.15 Such materials exhibit good soldering resistance so that they can be utilized as an electric or electronic material such as electric connectors, sockets for integrated circuits, etc.
16.3 PROPERTIES LCPs are much more expensive than ordinary engineering polymers. For this reason, there is a tendency to use LCPs as a minor component in polymeric formulations. Some commercial available LCPs, including Xydar® and Zenite® have been extensively characterized by infrared spectroscopy, differential scanning calorimetry, polarized light microscopy, thermogravimetry, and elemental analysis.26 Some selected properties of a neat LCP are shown in Table 16.2.
16.3.1 Mechanical Properties In polymeric composites, in the course of mold processing LCPs are oriented in the direction of flow. Thereby fibrils are formed. These fibrils reinforce the matrix polymer. This type of composite is addressed as an in situ composite.27
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High Performance Polymers Table 16.2: Properties of Vectra® A950 a
Property
Value
Density Water Absorption Equil (23°C, 50% RH) Tensile Modulus Tensile stress Brk Tensile strain Brk Flexural modulus Flexural strength Vicat softening point Melting temperature a
1.40 0.03 10.6 182 3.4 9.1 158 128 280
28
Unit
Standard
g cm−3
ISO 1183 ISO 62 ISO 527 ISO 527 ISO 527 ISO 178 ISO 178 ISO 306 ISO 11357
% GPa MPa % GPa MPa °C °C
Ticona, unreinforced grade for extrusion
Table 16.3: Dependence of Glass Transition Temperature and Melting Point on Composition Mol-% 4-ABA a
Tg /[°C]
Tm /[°C]
0.35 110 152 0.50 120 152 0.65 112 184 0.75 114 306 0.85 — 334 a 4-ABA: 4-acetoxybenzoic acid, Comonomer: 3-benzoyl-4-acetoxybenzoic acid
16.3.2 Thermal Properties The incorporation of rigid moieties into the main chain increases the melting temperature. However, a melting temperature causes a poor melt processability. The melting temperature can be reduced by structural modification, such as the introduction of:7 • Kinks or bends, • Bulky side groups, or • Flexible spacers. The changes of glass transition temperature and melting point with composition of co(polyester)s prepared from 3-benzoyl-4-acetoxybenzoic acid and 4-acetoxybenzoic acid are shown in Table 16.3.
Liquid Crystal Polymers
531
The glass transition temperature does not change significantly with composition. However, the melting point increases with the amount of 4-acetoxybenzoic acid, which is less bulky than 3-benzoyl-4-acetoxybenzoic acid.17 Aromatic polyesters are highly heat resistant due to their high crystallinity.
16.3.3 Electrical and Optical Properties The orientation of liquid crystals can be influenced by an electric field. This property makes LCPs attractive for LCDs. Some liquid crystals undergo a spontaneous electric polarization even in the absence of an external electric field. This phenomenon is referred to as a ferroelectricity.
16.4 APPLICATIONS LCPs have found a number of applications, including:7, 14 • • • • • • • • •
High stiffness and high strength fibers, Precision molded components, Barrier films, Solid polyelectrolytes, Melt processing additives, Reversible information storage devices, Endoscopic surgical instruments, Sporting goods, and Electro-optical displays.
16.4.1 In Situ Composites In situ composites27 are composites composed of a thermoplastic polymer and a LCP. The concept is applicable to various classes of polymers. Recent studies and polymers are that form in situ composites are summarized in Table 16.4. During fabrication, the LCP forms fine fibrils that reinforce the thermoplastic polymer matrix. The mechanism of reinforcement resemble those of glass fibers. However, when light weight is a key issue, in situ composites are superior in comparison to glass fibers.7
532
High Performance Polymers Table 16.4: Matrix Polymers for In Situ Composites Polymer
Reference
Ethylene acrylic elastomer Fluorocarbon elastomers Poly(amide) Poly(carbonate) Poly(ester) Poly(ethylene terephthalate) Poly(ethylene naphthalate) Poly(ether ether ketone) Poly(ethylene) Poly(propylene) Poly(sulfone) Styrene-(ethylene butylene)-styrene triblock copolymer
29 30 31, 32 33–36 37 38, 39 40 41 42 43–45 27 46–50
In addition, LCPs display viscosities that are considerably lower than other polymers. For this reason, the processability of LCP reinforced composites is better. Of course the price of LCP is much higher than the price of glass fibers. The particular orientation of the LCP in the course of flow effects a shear thinning at low shear rates. High shear forces favor the formation of fibrils. In aromatic poly(ester)/LCP blends, the viscosity ratio is more responsive in controlling the morphology.40 Since most of the LCPs are immiscible with conventional polymers, the mechanical properties are less than those predicted in theory. This results from poor interfacial adhesion. Actually, this problem can be overcome by proper compatibilization. The same compatibilizers, which are common in other fields, can be used. The compatibilizers help to improve the dispersion of the fibers and increase the fiber aspect ratio. Compatibilizers are summarized in Table 16.5. Phase diagrams for compatibilized systems can be constructed in order to predict the conditions under which in situ fibrillation occurs.51 The conditions of processing largely influence the morphology of the two phases. A morphology is ideally where the LCP is expanded into fibrils, however, a spherical droplet morphology may be obtained. Another issue is the high melting temperature of LCPs. Thus, in combination with conventional matrix resins, degradation of the matrix resin may be
Liquid Crystal Polymers
533
Table 16.5: Compatibilizers for LCP Blends Matrix
Compound
References
Poly(propylene) Poly(propylene) Poly(propylene) Poly(ether imide) Poly(amide)
Maleic anhydride grafted PP Thermoplastic elastomers Poly(styrene-ethylene butylene-styrene) Poly(epoxide) Maleic anhydride/styrene copolymer
52 53 53 54 55
observed. The formation of fibrils in poly(carbonate) (PC)/LCP blends have been shown to be enhanced by the addition of glass beads.35, 56 Nano silica acts in the same way. For example, in PC/LCP blends the addition of nano silica results in a reduction of the viscosity.36 The reduction of viscosity is correlated with the fibrillation of the LCP which is promoted by nano silica. In blends of LCP and poly(sulfone) the addition of ca. 5% of nano silica effects the formation of long and perfectly orientated fibrils in the capillary flow.57 The nano silica forms a network in the matrix that increases the elasticity. This effect is responsible for the improvement of the formation of fibrils. In unsaturated poly(ester) resins, the addition of LCP improves the adhesion to glass fibers.37 In addition, the mechanical properties are enhanced. In elastomer matrix polymers, the addition of LCP causes an enhancement of thermal stability50
16.4.2 Optical Data Storage Nematic and smectic side chain LCP are suitable as optical data storage media. A particular technique is the heat-mode recording technique.58 By locally heating, an optical scattering center is generated. However, heatmode recording has the disadvantages of slow response time and low resolution.14 In addition, liquid crystalline or amorphous polymer films containing the azobenzene group which is sensitive to light can be used for optical data storage. Azobenzenes undergo isomerization processes under the action of light.59 Azobenzenes attached to polymers exhibit a directed orientation if they are exposed to polarized light of a suitable wavelength. Exposure to linearly polarized light leads to orientation of these groups perpendicularly
534
High Performance Polymers
to the direction of polarization. A light-induced double refraction in the polymer may occur. The idea to use this effect for reversible optical data storage goes back to Todorov60, 61 and Ringsdorf,62 which is now addressed as photomode recording. The advantage of photo-mode recording over heat-mode recording lies in superior resolution, fast writing speed and the possibility of multiplex recording.14 Thus, when the film of such materials is irradiated with linearly polarized or unpolarized light, optical information can be written, erased, or rewritten on the polymer film. Namely, the irradiation causes an optically induced birefringence.63 The information written in this way can be probed by measuring the optical properties of the material. In this way, the information is retrieved. On a molecular base, the azobenzene converts from the trans state into the cis state. By this isomerization reaction, the previously unoriented azobenzene groups are aligned perpendicular to the plane of polarization of the incident light. The alignment results in a high birefringence of the irradiated areas. Azo groups can be introduced into polymers by monomers based on acrylate or methacrylate units to which side chains that contain azo groups are attached. The synthesis of such a monomer is shown in Figure 16.7. In addition, mesogenic units are introduced in the polymer. The mesogenes are bonded in the same way as the azo dyes. They need not necessarily absorb the actinic light since they act as a passive molecular group. Their task is to intensify the light-inducible double refraction and stabilize it after the action of the light.59 In order to improve the solubility of the polymer, other moieties may be incorporated: 1. As monomer units randomly integrated into the main chains. 2. As a side group at the bonding site between the azobenzene and spacer. 3. As a terminal group at the free end of the azo dye. For example, the solubility may be improved by dye molecules containing pendent hydroxyethyl groups or by the incorporation of dimethylacrylamide in the polymer. The incorporation of dimethylacrylamide improves also the reversibility of the lighting dynamics. In order to produce storage devices, polymers from the monomers are
Liquid Crystal Polymers
535
OH O HO
O
+
N H
O
N H
Br
OH O
N
O HO
N N+
N N
N
N
O
N
N
N N
N
O HO
Figure 16.7: Synthesis of Azobenzene Containing Methacrylate59
536
High Performance Polymers
produced and polymer films are produced by spin-coating from a solution. Films with a thickness of typically 200 nm are prepared. In the writing process, the samples are irradiated from the polymer side with polarized laser light incident perpendicular thereto (writing process). An argon ion laser at a wavelength of 514 nm serves as a light source with an intensity of 100 mW cm−2 In this way, trans-cis-trans-isomerization cycles are induced in the azobenzene side group molecules of the polymer, leading to a net orientation of the side groups away from the polarization direction of the laser. These reactions can be followed by the double refraction Δn in the plane of the polymer film. The behavior of the induced double refraction at a wavelength of 633 nm is determined experimentally with a helium-neon laser of mW cm−2 , the so-called reading laser. Another type of azobenzene containing monomers are malonic es63 ters. The azo dye, Disperse Red 1, is fixed as the ester functionality. Poly(malonic ester)s are then prepared by the reaction of the active hydrogens in the malonic ester with α , ω -alkanes, or aromatic compounds, such as dibromoxylylenes in presence of sodium hydride.
16.4.3 Stationary Phases Conventional stationary phases separate according to their polarity and to their mutual interactions to the analyte. In contrast, LCPs have the additional technical feature that they separate by the molecular shape of the analyte. The majority of investigations focuses on gas chromatography (GC). Low molecular liquid crystal materials may suffer from high volatility. For this reason, separations that need high temperatures are achieved more successfully by LCPs. Liquid crystal materials including LCPs can be characterized by inverse gas chromatography64 Commonly, polymers with a poly(siloxane) backbone and pendant side chain liquid crystalline groups are used. The side chain liquid crystalline groups are organic complexes with zinc, nickel, or crown ethers.65 This type of polymers is addressed as mesomorphic poly(siloxane). Side chains based on 4-biphenyl-4-allyloxybenzoate exhibit a special separation performance for racemic compounds.66 Certain liquid crystalline side chain polymers based on acrylate have been used as stationary phases for both liquid chromatography (LC) and
Liquid Crystal Polymers
537
GC.67, 68 The preparation of LCPs suitable for the separation of polychlorinated dibenzodioxins, dibenzofurans, and other polychlorinated aromatics have been described.69 In addition, an extensive compilation of stationary phases for GC composed from LCPs with examples of applications has been presented in the literature.65 Less common than in GC is the application of LCP as a stationary phase in LC.70 Two basic methods to prepare liquid crystalline stationary phases are available, namely: 1. Bonding low-molecular-weight liquid crystalline molecules to silica gel particles, or 2. Coating or bonding of LCPs onto silica. Of course, the first method results likewise in polymers as the second method, however, in the first method hybrid materials containing inorganic and organic materials are produced. An acid chloride functionalized liquid crystalline material was tried to bond to the silanol group in silica. The direct reaction does not seem to be successful; also, a spacer with a substituted dichlorodimethyl silane is not successful. The reaction scheme is shown in Figure 16.8. However, a separation performance is observed rather by deposition than by bonding of the liquid crystals.71 The liquid crystalline materials used in this study was based on cholesteric moieties. It is suspected that the reaction failed because of the large size of the cholesteryl group. In another study, dimethylchlorosilane was added to the allyl group of 4-methoxyphenyl-4-allyloxy benzoate. This intermediate could be bonded to silica.72 Several other routes to fix liquid crystalline moieties on silica have been reviewed.70 The major advantage for the usage of LCPs as stationary phases for LC applications is that coating of the polymer on the silica gel is a simple process. However, also comb-shaped polymers prepared by octadecylacrylate and 3-mercaptopropyltrimethoxysilane as chain transfer agent can be immobilized on silica gel by bonding. It was shown that the telomer behaves as nematic material in the range of 42–47°C. The separation of geometrical isomers could be achieved.73
16.4.4 Liquid Crystal Displays The history of LCDs has been presented by Mosley74 and Castellano.75 The development of LCD compositions requires an expert knowledge that
538
High Performance Polymers
CH3 Si
OH + Cl
Si
CH3
O
CH3 O
CH3 Si O Si
OH
CH3 Cl O LC CH3 Si O Si CH3
O O O LC O
Figure 16.8: Fixing Liquid Crystalline Moieties on Silica70, 71
is far beyond organic chemistry, which is presented elsewhere.76 We will discuss here only the basic principles of LCDs and review LCPs that are used in this field. A schematic sketch of a LCD is shown in Figure 16.9. In the simplest case a LCD is built up from a: 1. 2. 3. 4. 5. 6.
Vertical polarizer, Indium tin oxide (ITO) electrode array deposited on glass, Liquid crystal matrix, ITO electrode array deposited on glass, Horizontal polarizer, Mirror or light source.
The electrode surfaces that are in contact with the liquid crystal matrix are treated by a layer in order to align the liquid crystal molecules in a particular direction. This alignment layer consists of a polymer that is brushed in a certain direction. There are several principles in how to do the alignment. In a twisted nematic device, the surface alignment directions at the two electrodes are perpendicular to each other. For this reason, the
Liquid Crystal Polymers
Pola
rize
r
ITO
539
Liqu ITO Pola Mirr or o id c elec rize r lam ryst r trod trod a es es lline p mat eria l
elec
Figure 16.9: Sketch of a Liquid Crystal Display
molecules align in a helical mode throughout the cell in the absence of an electrical field. For a twisted nematic device, this is the transparent mode. When an electric field is applied, the liquid crystals align parallel to the electric field. Thus a change of the angle of rotation of the light occurs which changes the transparency of the device. When the last element is a mirror, the LCD is of the reflective type; when a light source is mounted, the LCD is a backlight type. There are additional components in an LCD, such as color filters, reflective retardation elements, etc. In a vertically aligned LCD the situation is reverse. The long axes of the liquid crystal molecules align themselves vertically to the substrates in a state where no electric field is formed between the electrodes. Accordingly, using polarizing plates, light is completely blocked when there is no electric field. Since brightness of the dark mode is extremely low, a higher contrast ratio can be realized than obtained in a twisted nematic device.77 The polymer of an alignment layer may be a poly(imide), which is spin-coated from solution. After drying, a film with a thickness of 0.1 μ is formed at 200°C. This film is subsequently rubbed in one direction so that it functions as the alignment layer. Basic studies soon revealed that the viscosities of most nematic polymers are too high so that they are not suitable candidates in fast switching devices. However, these materials may be useful in display related auxiliary components, such as polarizers, retardation films and polymeric dispersing matrices.14
540
High Performance Polymers
16.4.4.1
Polymer Fixation
The liquid crystalline material in the LCD is not necessarily a LCP. However, in order to fix the liquid crystalline material conventional polymers are used for embedding and LCP are also used. Side chain LCPs combine the properties of liquid crystals and polymers with flexible main chains. Since the liquid crystal moieties are fixed on the backbone of the main chains of the polymer, this type of liquid crystals may not flow away, as it may be the case in monomeric liquid crystals. Common liquid crystal materials can consist of:78 • Polymer network liquid crystals, or • Polymer dispersed liquid crystals. The polymer network liquid crystal is made by the polymerization phase separation method. A solution is made of the liquid crystal (70–90%) and a polymerizable monomer or oligomer, which is placed into the cell. Polymerization is initiated by UV light. In the course of polymerization, a phase separation between the liquid crystal and a polymer may occur. Thus the polymer forms a network pattern in the liquid crystal.78 Monomers may consist of a mixture of 2-hydroxyethyl methacrylate, phenoxyethylacrylate, poly(ethylene glycol diacrylate), and poly(tetramethylene glycol).79 Liquid crystals of the cyanobiphenyl type, cyanophenylcyclohexane type, and cyanohexylcyclohexane type exhibit a high responsive to an electric field. In a polymer fixation system, a liquid crystal composite in which a polymerizable component is mixed in a liquid crystal, is sealed between the substrates. The liquid crystal molecules are tilted by applying a voltage between the substrates and polymerization is started. Thus, a liquid crystal layer in which the molecules are tilted in a predetermined tilt direction is obtained, and the tilting direction of the liquid crystal molecule can be fixed. Materials that are capable of polymerization by heat or by UV are selected as monomers.80 A liquid crystal monoacrylate in amounts of 2.5% is added to a liquid crystal. After the liquid crystal material is injected between substrates, the monomers are cured by irradiating the liquid crystal layer with ultraviolet rays while a voltage of 5 V is applied to the liquid crystal layer. By this procedure, it is possible to form polymers aligned in the tilt orientation of
Liquid Crystal Polymers
541
the liquid crystal molecules. In other words, the liquid crystal alignment at the time of low voltage application can be fixed. Photopolymerizable liquid crystalline monomers are mostly based on acrylate groups that are introduced in the liquid crystal unit. Examples are shown in Figures 16.3 and 16.4. In addition, liquid crystalline monomers with pendent oxetane units, c.f. Figure 16.5 can be polymerized by ringopening.20 Compositions are spin-coated and polymerization is initiated by UV light. 16.4.4.2
Viewing Angle Dependency
Conventional LCDs may suffer from a viewing angle dependency. This means that the display quality is lower when the display is viewed slantingly in comparison when it is viewed from a normal position.21 Usually a wide viewing angle is desirable. The viewing angle can be widened in the hybrid aligned nematic mode liquid crystal display or in the optically compensated bend mode liquid crystal display.81 In the latter type, the liquid crystal molecules are aligned substantially parallel to the electrodes in the outer regions of the cell, then are increasingly slanted until reaching the center where the liquid crystal molecules are substantially perpendicular to the two electrodes. A wide viewing angle is achieved as a result. To obtain such a bent alignment of the liquid crystal molecules, a horizontal orientation agent that is oriented in the same direction is used and a high voltage is initially applied. Also, liquid crystal molecules move in the same orientation when operating, realizing a wide viewing angle as well as fast response times. The viewing angle can be widened if an additional layer is introduced between the liquid crystal cell and the polarizer. Photopolymerizable cholesteric moieties, modified with spaces and acrylic units, can be used to build up such retardation layers.21 Examples of such monomers are shown in Figure 16.3.
16.4.5 Electrically Conductive Compositions Injection-moldable electrically conductive composition based on LCP are useful in a wide variety of applications, including:82 • Electrochemical devices, • Battery current collectors,
542
High Performance Polymers Table 16.6: Materials for Electrically Conductive Compositions82 Component
Remark
Zenite® HX8000 LCP PPI-1204-Ni60 Graphite fiber a PPI-1208-Ni60 Graphite fiber a PPI-1204-NiCu40 Graphite fiber b Thermocarb® CF300 Graphite Powder a Ni coated, resin impregnated b Ni−Cu coated, resin impregnated
Manufacturer DuPont Composite Materials Composite Materials Composite Materials Conoco
• High efficiency radio frequency interference shielding, and • Fuel cell applications. Electrically conductive compositions are fabricated by blending an injection-moldable LCP and nickel-coated graphite fibers that are impregnated with a non-liquid-crystalline thermoplastic binder resin. The blending is done below the melting point of the LCP. The mixture is processed in an injection molding machine. A balance among conductivity, processability, and structural properties is desirable.82 Materials for electrically conductive compositions are summarized in Table 16.6.
16.5 SUPPLIERS AND COMMERCIAL GRADES Suppliers and commercial grades are shown in Table 16.7. Monomers and structural units used in commercial available LCPs have been summarized in the literature.7, 8 Vectra® types are random copoly(ester)s based on HBA and 2-hydroxy-6-naphthoic acid or copoly(ester amide)s based on 4-aminophenol and TPA.7 Xydar® types are fully aromatic poly(ester)s based on HBA, BP, and TPA.7 Neat poly(ethylene terephthalate), is not a LCP. However, by the introduction of HBA it becomes a LCP. The ethylene groups act as flexible spacers that reduce the nematic transition temperature. Commercial available types of this kind are X7G and Rodrun types. Tradenames appearing in the references are shown in Table 16.8.
16.6 ENVIRONMENTAL IMPACT AND RECYCLING Since in situ composites contain engineering polymers, recycling is a relevant issue. The properties of recycled PC/LCP blends have been investi-
Liquid Crystal Polymers
543
Table 16.7: Examples for Commercially Available Liquid Crystal Polymers Tradename
Producer
Ekonol® Laxtar™ Novaccurate® Octa® Rodrun . . . Siveras™ Sumikasuper® Titan® Vecstar® Vectra® A Vectran® Xydar® Zenite® RTP Compounds 34. . . X7G
Sumitomo Chemical Lati Mitsubishi Engineering Plastics Dainippon Ink & Chemicals Unitika Toray Industries Sumitomo Chemical Eastman Kuraray Co., Ltd. Celanese Ticona, Polyplastics Co., Ltd. Ticona Solvay Advanced Polymers LLC DuPont RTP Company Eastman
Table 16.8: Tradenames in References Tradename Description
Supplier
Ekono® E-101
Sumitomo Chemical Co., Ltd.
Poly(p-hydroxybenzoic acid)25 Irgacure® 907 Ciba 2-Methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone (photo initiator)21 Optomer® AL1254 Japan Synthetic Rubber Co. Poly(imide)21 Rodrun® Unitika Liquid crystalline polymer33 Thermocarb® Conoco Graphite fiber82 Zenite® DuPont Liquid crystalline polymer82
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High Performance Polymers
gated. Test samples were prepared by blending PC and LCP and injection molding to obtain a virgin sample. This sample was crushed followed by injection molding to get a single-stage recycled sample. Crushing and injection molding was repeated to get multi stage recycled samples.33 Up to 4-stages of recycled samples were prepared. The virgin sample and the recycled samples have been characterized by mechanical and rheological properties. In the course of recycling, Young’s modulus remains nearly constant. The tensile strength decreases by ca. 20%. in contrast, the melt flow rate increases significantly with the number of recycling stages. Another route of recycling aims to separate the LCP from the matrix polymer.83 In the case of a poly(propylene) (PP) matrix, reactive extrusion was used to reduce the molecular weight of the PP. The reduction of the molecular weight facilitates subsequent phase separation in mineral oil. By this method, ca. 70% of the LCP can be reclaimed with a purity of more than 96%. It was found that virgin LCP can be partially substituted by reclaimed LCP in order to fabricate PP/LCP blends; the mechanical properties are not altered.
REFERENCES 1. F. Reinitzer. “Beiträge zur Kenntniss des Cholesterins.” Monatsh. Chem., 9 (1):421–441, December 1888. 2. T. J. Sluckin, D. A. Dunmur, H. Stegemeyer, and Editors, editors. Crystals that Flow: Classic Papers from the History of Liquid Crystals. The Liquid Crystals Book Series. Taylor & Francis, Inc., New York, 2004. 3. P. Palffy-Muhoray. “Orientationally ordered soft matter: The diverse world of liquid crystals.” Liq. Cryst. Comm., pages 1–16, August 2007. [electronic] http://www.e-lc.org/docs/2007_08_26_01_36_22. 4. P. Palffy-Muhoray. “The diverse world of liquid crystals.” Physics Today, 60 (9):54–60, September 2007. 5. S. Chandrasekhar and G. S. Ranganath. “Discotic liquid crystals.” Rep. Prog. Phys., 53(1):57–84, 1990. 6. S. Chandrasekhar, S. K. Prasad, G. G. Nair, D. S. S. Rao, S. Kumar, and M. Manickam. Liquid crystal display device. US Patent 6 558 759, assigned to Centre for Liquid Crystal Research (ID), May 6, 2003. 7. S. C. Tjong. “Structure, morphology, mechanical and thermal characteristics of the in situ composites based on liquid crystalline polymers and thermoplastics.” Mater. Sci. Eng., R, 41(1-2):1–60, September 2003. 8. A. A. Collyer, editor. Liquid Crystalline Polymers: From Structures to Applications, volume 1 of Polymer Liquid Crystal Series. Elsevier, London, 1992.
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37. X.-B. Yu, C. Wei, and F.-A. Zhang. “Studies on the mechanical and dynamic mechanical properties of thermotropic liquid crystalline polymer/unsaturated polyester/glass fiber in situ hybrid composites.” Polym. Adv. Tech., 17:534– 539, 2006. 38. L. Pan and B. Liang. “A comparative study of in-situ composite fibers reinforced with different rigid liquid crystalline polymers.” J. Appl. Polym. Sci., 70:1035–1045, 1998. 39. W. Grasser, H.-W. Schmidt, and R. Giesa. “Fibers spun from poly(ethylene terephthalate) blended with a thermotropic liquid crystalline copolyester with non-coplanar biphenylene units.” Polymer, 42(21):8517–8527, October 2001. 40. J. Y. Kim and S. H. Kim. “In situ fibril formation of thermotropic liquid crystal polymer in polyesters blends.” J. Polym. Sci., Part B: Polym. Phys., 43:3600–3610, 2005. 41. J. He, Y. Wang, and H. Zhang. “In situ hybrid composites of thermoplastic poly(ether ether ketone), poly(ether sulfone) and polycarbonate.” Compos. Sci. Tech., 60:1919–1930, 2000. 42. S. Saengsuwan, S. Bualek-Limcharoen, G. R. Mitchell, and R. H. Olley. “Thermotropic liquid crystalline polymer (Rodrun LC5000)/polypropylene in situ composite films: Rheology, morphology, molecular orientation and tensile properties.” Polymer, 44:3407–3415, 2003. 43. B. Wanno, J. Samran, and S. Bualek-Limcharoen. “Effect of melt viscosity of polypropylene on fibrillation of thermotropic liquid crystalline polymer in in situ composite film.” Rheol. Acta, 39:311–319, 2000. 44. S. Saengsuwan, G. R. Mitchell, and S. Bualek-Limcharoen. “Determination of orientation parameters in drawn films of thermotropic liquid crystalline polymer/polypropylene blends using WAXS.” Polymer, 44:5951–5959, 2003. 45. P. Sukananta and S. Bualek-Limcharoen. “In situ modulus enhancement of polypropylene monofilament through blending with a liquid-crystalline copolyester.” J. Appl. Polym. Sci., 90:1337–1346, 2003. 46. S. Saikrasun, S. Bualek-Limcharoen, S. Kohjiya, and K. Urayama. “Thermotropic liquid-crystalline copolyester/thermoplastic elastomer in situ composites. I. Rheology, morphology, and mechanical properties of extruded strands.” J. Appl. Polym. Sci., 89:2676–2685, 2003. 47. S. Saikrasun, S. Bualek-Limcharoen, S. Kohjiya, and K. Urayama. “Thermotropic liquid-crystalline copolyester (Rodrun LC3000)/thermoplastic elastomer (SEBS) in situ composites: II. Mechanical properties and morphology of monofilaments in comparison with extruded strands.” J. Appl. Polym. Sci., 90:518–524, 2003. 48. S. Saikrasun, S. Bualek-Limcharoen, S. Kohjiya, and K. Urayama. “Anisotropic mechanical properties of thermoplastic elastomers in situ reinforced
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Liquid Crystal Polymers
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High Performance Polymers
76. S.-T. Wu and D.-K. Yang. Fundamentals of Liquid Crystal Devices. Wiley Series in Display Technology. Wiley, Chichester, 1st edition, 2006. 77. J.-H. Mun and J.-K. Song. Liquid crystal display having wide viewing angle. US Patent 7 209 204, assigned to Samsung Electronics Co., Ltd. (Gyeonggido, KR), April 24, 2007. 78. K. Yamagishi, K. Kurematsu, O. Koyama, and T. Nakazawa. Matrix substrate, liquid crystal display device using it, and method for producing the matrix substrate. US Patent 6 927 829, assigned to Canon Kabushiki Kaisha (Tokyo, JP), August 9, 2005. 79. T. Asada. Liquid crystal display device and its production method. US Patent 6 924 873, assigned to Tadahiro Asada (Kyoto, JP), August 2, 2005. 80. K. Hanaoka. Liquid crystal display and method of manufacturing the same. US Patent 7 113 241, assigned to Sharp Kabushiki Kaisha (Osaka, JP), September 26, 2006. 81. J.-J. Lyu, C.-H. Lee, and H.-S. Chang. Liquid crystal display with a wide viewing angle using a compensation film. US Patent 7 265 802, assigned to Samsung Electronics Co., Ltd. (KR), September 4, 2007. 82. M. K. Bisaria, P. Andrin, M. Abdou, and Y. Cai. Injection moldable conductive aromatic thermoplastic liquid crystalline polymeric compositions. US Patent 6 379 795, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), April 30, 2002. 83. M. C. Collier and D. G. Baird. “Separation of a thermotropic liquid crystalline polymer from polypropylene composites.” Polym. Compos., 20:423–435, 1999.
Index TRADENAMES Ableloc® Poly(imide), 506 Accuguard™ Poly(phenylene ether), 164 Accutech™ Poly(phenylene ether), 164 Aciplex® Perfluorosulfonic acid membrane, 271 Acnor™ Poly(phenylene ether), 164 Acronal® 4F Poly(n-butylacrylate), 322 Admer® Adhesive resins, maleic anhydride grafted poly(ethylene) or poly(propylene), 415 Admer® L 2100 Poly(ethylene) grafted with 0.1% maleic anhydride, 415 Aerosil® Fumed Silica, 381 AI Polymer Poly(amide imide), 469 Albis PPS Poly(phenylene sulfide), 198 Alftalat® AN 739 Polyester, 322 Alphamide® Poly(amide imide), 469
551
552
High Performance Polymers
Amodel® (Series) Poly(phthalamide), 414, 415 Amodel® 1000 Poly(phthalamide), 415, 441 Amodel® A 1000 Hexamethylene terephthalamide isophthalamide adipamide terpolymer, 415 Amodel® AF 1113 Aromatic copolyamide 6.6/6.I/6.T 4, 415 Amodel® AF 4133 Aromatic copolyamide 6.6/6.T 5, 415 Amodel® X4000 Hexamethylene terephthalamide isophthalamide adipamide terpolymer 65/35, 415 Antiblaze (Series) Flame retardant, 508 Apical® Poly(imide), 342, 506 Aqua-Cleen® Ethoxylated mercaptan, surfactant, 200 Aracon® Metal coated Kevlar® fiber, 441 Aramica® Aramid, 440 Arcol® Poly(propylene oxide), 508 Arlene® Poly(phthalamide), 415 Arlene® CH 230 PA 6.6/6.T 5, 415 Armos® Aramid, 440 Arnitel® Poly(ester) elastomer, 415 Ashlene® Poly(phenylene ether), 164 Aurum® Poly(imide), 506 Avaspire™ Poly(ether ether ketone), 228 Avimid® Poly(imide), 506, 508 Balpound™ Poly(phthalamide), 414
Index Basofil® Melamine resin fiber, 441 Baytron® P Complex of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonic acid), 125 Black Pearls® Carbon black, 165 Blendex™ Poly(2,6-dimethylphenylene ether), 165 Boltorn® (Series) Dendritic poly(ester)s, 165 Bondfast® Epoxy functional poly(olefin), 200 Buna® AP 437 EPDM, 165 Bynel® (Series) Anhydride modified ethylene vinyl acetate resin, adhesion promoter, 381 Cabelec® Conductive carbon black masterbatch in PA 6, 165 Cabosil™ M5 Silica, 381 Calprene® Styrene-(ethylene-butylene)-styrene triblock copolymer, 165 Capcure® 3-800 Thiol end-capped polymer, 508 Caprolan® Nylon 6, 441 Carbowax® (Series) Poly(ethyleneoxide glycol) (PEG), 381, 415, 441, 508 Cariflex® Triblock copolymer, 165, 508 Celstran® PPS Poly(phenylene sulfide), 198 Cirlex® Poly(imide), 506 Comshield® PPS Poly(phenylene sulfide), 198 Conductex® Carbon black, 165, 508 Conex® m-Aramid, 441 CoorsTek Neat PES Poly(arylene ether sulfone), 270
553
554
High Performance Polymers
CP-45X Developer, 322 Crylcoat® 2392 Polyester, 322 Crystar® 5005 Poly(ethylene terephthalate), 508 DABCO® 8154 Cyclic tertiary amine catalyst, 508 DABCO® K-15 Metallic based catalyst, 508 Darocure® 1173 2-Hydroxy-2-methyl-1-phenylpropan-1-one, photoinitiator, 381 DC® -704 Silicone oil, 229 DC® -710 Silicone oil, 229 Denka® SMI Styrene maleide imide copolymer, 415 DER® 332 Bisphenol A diglycidyl ether based epoxy resin, 296 Desmodur® (Series) Oligomers based on 4,4 -diphenylmethane diisocyanate, 469 Desmodur® W Bis-(4-isocyanatocyclohexyl) methane (H12 MDI), 322 Desmophen® 690 Branched lacquer polyester with OH groups, 322 Diaion® (Series) Sulfonic acid type ion exchange resin modified with 2-mercaptoethylamine, 271, 296 Disflamoll® DPK Diphenylcresyl phosphate, 165 DOVERPHOS® S9228 Bis(2,4-dicumylphenyl pentaerythritol) diphosphite, 508 Dowex® (Series) Anion and cation exchangers, 296 DYLARK® Copolymers of styrene with maleic anhydride, 165, 271 Dyneon® HTE Fluoropolymer, 381 Ebecryl® (Series) Urethane acrylate, 381 Ecdel® Copolyester ether elastomer, 381
Index
555
Edgetek® -PK Poly(ether ether ketone), 228 Edgetek™ Poly(arylene ether sulfone), 270 Edgetek™ PPS Poly(phenylene sulfide), 198 Ekonol® Poly(p-oxybenzoate), 200, 543 Ekono® E-101 Poly(p-hydroxybenzoic acid), 543 Elvamide® Low melting poly(amide), 381 Emi-X* PPS Poly(phenylene sulfide), 198 Engage™ resins Low density poly(ethylene), 381 Ensinger PEEK Poly(ether ether ketone), 228 Epikote® (Series) Bisphenol A/F epoxies, 508 Epispire Poly(arylene ether sulfone), 270 Epon® (Series) Diglycidyl ethers of bisphenol A, 165, 508 ERL™ Alicyclic epoxides, 165 Esacure® Photoinitiators, 381 Ethacure® 100 Diethyltoluene diamine, 508 Eulexin® Nonsteroidal antiandrogen, 84 Eupergit® C250L Epoxy-activated poly(methacrylamide), 508 Exolit® OP 1311 Mixture of aluminum salts of diethylphosphinate and melamine polyphosphate, 509 Expandex® 150 Calcium salt of 5-phenyltetrazole, blowing agent, 469 Extem® Poly(imide), 506 Exxelor® PO 1015 Poly(propylene) grafted with 0.3% maleic anhydride, 415
556
High Performance Polymers
Exxelor® VA 1801 Ethylene propylene rubber grafted with 0.6% maleic anhydride, 415 Exxelor® VA 1803 Ethylene propylene rubber grafted with 0.4% maleic anhydride, 416 Flemion® Fluoropolymer ion-exchange membrane, 229, 271 Fluorinert® Fluorinated oil, 229 Fortafil® Carbon fiber, 165 Forton® (Series) Poly(phenylene sulfide), 198, 200 Freon® 113 1,1,2-Trichloro-1,2,2-trifluoroethane, 229 Gafone™ Poly(arylene ether sulfone), 270 Galwick® Wetting fluid, 469 Gatone™ PEEK, 228, 229 Poly(ether ether ketone), 228 Geloy® resin ASA copolymer, 165 Glycolube® (Series) Fatty esters, flow promotor, mold release agent, 200 Gore-Select® Microporous expanded PTFE membrane (ePTFE), ion conductive membrane, 229, 271 Grafil® fibers Carbon fiber, 165 Grafoil® Flexible graphite, 229 Grivory® HTV-4X2VO 6.6/6.T Poly(phthal amide), 414, 416 Grivory® HTVS-3X2VO 6.6/6.T 7 Poly(phthal amide), 416 H-2™ Mixture of ethylenediamine and methyl isobutyl ketone ketimine (curing agent), 509 Hakkol FWA-SF Triazinylaminostilbene fluorescent brightening agent, 322 HB® -40 Hydrogenated terphenyl, 229
Index
557
HERACRON® Aramid, 440 HiFill® PPS Poly(phenylene sulfide), 198 Highlink® (Series) Colloidal silica sols, 381 Hiloy® PPS Poly(phenylene sulfide), 198 Hipertuf® Poly(ethylene naphthalate) for drinking bottles, 380 Hycar® (Series) Amine-terminated butadiene-acrylonitrile, 509 Hydlar® Aramid, 440 Hytrel® Poly(ester) elastomer, 416 Hytrin® Cardiovascular preparation, 84 Igetabond® Epoxy functional poly(olefin), 200 Imidex® Poly(imide), 506 Irgacure® 184 1-Hydroxycyclohexylphenylketone (photo initiator), 381 Irgacure® 907 2-Methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone (photo initiator), 543 Irganox® 1010 Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), phenolic antioxidant, 166, 509 Irganox® 1076 Octadecyl-3-(3 ,5 -di-tert-butyl-4 -hydroxyphenyl) propionate, 166 Irganox® 1098 N,N -hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide), 166 Isonate® Isocyanate based formulation for foams, 469 Ixef® Aramid, 440 Jeffamine® (Series) Amine capped polyalkoxylene glycol, 509 Kaladex® Poly(ethylene naphthalate) films, 380
558
High Performance Polymers
Kapton® Poly(imide), 166, 200, 342, 506, 509 Kerimid® Poly(imide), 506 Ketaspire™ Poly(ether ether ketone), 228 Ketjenblack® Conductive carbon black, 166 Ketron® PEEK Poly(ether ether ketone), 228 Kevlar® Aramid, 166, 229, 440, 441 Klebosol® Silica sol, 381 Konduit* PPS Poly(phenylene sulfide), 198 Kraton® Styrenic block copolymer, 166, 200, 416, 441, 509 Krytox® Fluorinated oil, 229 Kynar® Poly(vinylidene fluoride), 229 Laramid® Poly(phthalamide), 414 Laromer® LR 8739 Urethane acrylate monomer, 381 Larpeek Poly(ether ether ketone), 228 Laxtar™ Liquid crystalline polymer, 543 Lenzing® P84 Benzophenone tetracarboxylic dianhydride-MDI-2,4-TDI copolymer, poly(imide), 229, 506, 509 Leomin® AN Oleyl phosphonate lubricant, textile auxiliary, 441 Lexan® Poly(carbonate), 271 Lindol® XP Plus Tricresyl phosphate, 509 Lotader® Epoxy functional poly(olefin), Adhesive, 200 Loxiol® G40 Fatty acid isoalcohol ester, 509
Index LP-2™ Poly(sulfide), 509 Lubri-Tech™ PPS Poly(phenylene sulfide), 198 Lubriblend® PPS Poly(phenylene sulfide), 198 Lubricomp* PPS Poly(phenylene sulfide), 198 Lubrilon® PPS Poly(phenylene sulfide), 198 Lucalen® A 3710 MX Copolymer of LDPE and 7% acrylic acid, 416 Ludox® (series) Silicon colloid, 382 Lupersol® 256 2,5-Dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, 322 Lupolen® (Series) Poly(ethylene), 416 Lupranat® (Series) Isocyanate based formulations, 469 Luran® SAN copolymer, 166 Luvican® Poly(vinyl carbazole), 52 Luxprint® 7144 Carbon conductor ink, 125 Luxprint® 7145L Silver conductor ink, 125 Luxprint® 7151 Electroluminescent phosphor paste, 125 Luxprint® 7153E Barium titanate paste, 125 Lynite® Poly(ethylene terephthalate), 296 Makrolon® Poly(carbonate), 271 Mark 2112 Tris(2,4-di-tert-butyl phenyl) phosphite, 229 Matrimid® Poly(imide), 229, 506, 509 Melapur® 200 Melamine poly(phosphate) (flame retardant), 509
559
560
High Performance Polymers
Meldin® Poly(imide), 506 Melinar® Laserplus Poly(ethylene terephthalate) (PET), bottle grade, 382 Merlon® Poly(carbonate), 271 Mictron® Aramid, 440 Mindel® Poly(arylene ether sulfone), 270 Mobiltherm® (Series) Heat transfer oil, 230 Multiposit® XP-9500 Thermoset epoxy resin, 342 Mylar® (Series) Poly(ethylene terephtalate), 342, 382 Nafion® Sulfonated PTFE, for membrane applications, 230, 271 Nafion® 1100 EW Sulfonated PTFE, Nafion membrane, of equivalent weight (EW) of 1100, 230 Nalco® 2327 Silica hydrosol, 382 Naugard® 445 4,4 di(α ,α -Dimethyl-benzyl)diphenylamine, 416 Neoflex® Poly(imide), 506 Neosepta® Perfluorinated ion exchange membranes, 271 Niax® A-33 Amine catalyst, 509 Niax® L-620 Silicone emulsifiyer, for flexible poly(urethane) foam, 509 Niax® L-6900 Surfactant, 509 Nirez® 2150/7042 Terpene phenol flow modifier, 166 Nomex® m-Aramid, 440, 441 Nopla® Poly(ethylene naphthalate)-Poly(ethylene terephthalate), 380 Noryl* PPS+PPE Poly(phenylene sulfide), 198
Index Norylux™ PPO Poly(phenylene ether), 164 Noryl® PPE PS Blend, 164, 166, 510 Novaccurate® Liquid crystalline polymer, 543 Novolen® 1100 Isotactic poly(propylene), 416 Novolen® 2500 HX Propylene/ethylene block copolymer, 10% ethylene, 416 Novolen® 3200 HX Propylene/ethylene block copolymer, 2.5% ethylene, 416 Octa® Chemicals, 543 OLEOPHOBOL® (Series) Fluoroacrylate polymer, Oil and water repellent, 441 Onlymide® Poly(imide), 506 OP-PEI. . . GF Poly(imide), 506 Optomer® AL1254 Poly(imide), 543 Oxalon® Poly(oxadiazole) fibers, 342 PACM™ 20 Bis(4-aminocyclohexyl)methane, 510 PAPI® (Series) Isocyanate, 510 Paraloid® Acrylate rubber, impact modifier, 271 Parylene C Chlorinated Parylene type, 83 Parylene D Dichlorinated Parylene type, 83 Parylene HT Fluorinated Parylene type, 83 Parylene N Standard Parylene polymer, 83 PCTA Durastar 1000 Copolyester based on 65 mol % terephthalic acid, 35 mol % isophthalic acid and CHDM, 382 PDBS® 80 Poly(dibromostyrene), 416
561
562
High Performance Polymers
Pebax Poly(amide imide), 469 PEEK-OPTIMA® Granular Poly(ether ether ketone), 228 PenTec® Poly(ethylene naphthalate) fiber, 380 Pentex® Modified Poly(ethylene naphthalate) fiber, 380 PEN™ Poly(arylene ether nitrile), 296 Perspex® CP63 Acryl glass, 382 PETG 6736 Copolyester based on terephthalic acid and EG and CHDM, 382 Photomer™ 6210 Urethane acrylate oligomer, rheology modifier resin, 382 Pluronic® (Series) Ethylene oxide/propylene oxide block copolymer, defoamers, 469 PMC EP PX1000 Poly(phenylene ether), 164 Polectron® Poly(vinyl carbazole), 52 Policarb™ Poly(vinyl carbazole), 52 Polycat® (Series) Amine based catalysts, 510 Polycoustic® Poly(imide), 506 Polymeg® Poly(tetramethylene glycol), 382 Polymist® (Series) Poly(tetrafluoroethylene) lubricant powders, 271 Polyox® 301 Poly(ethylene oxide), 441 PolyTHF® CD THF copolymers, 510 Porewick® Wetting fluid, 469 Primacor® 1410 XT Ethylene acrylic acid copolymer with 10% acrylic acid, 416 Primef® PPS Poly(phenylene sulfide), 198
Index Primene® Primary aliphatic amines with highly branched alkyl chains, 510 Primene® MD 1,8-Diamino-p-menthane, 510 Primospire Benzoyl-substituted Parylene type, 83 PRL PEI-G. . . Poly(imide), 506 PRL PPX Poly(phenylene ether), 164 Proscar® Medicinal preparation for treatment of the prostate gland, 84 Pyralin® Poly(imide), 506 Pyre® ML Pyromellitic dianhydride/4,4 -oxydianiline poly(imide), 506 Pyrocheck® 68 PB Brominated poly(styrene), 416 Pyrocoat® Poly(imide), 506 Pyropel® Poly(amide imide), 469 QR Resin QR-4000 Poly(phenylene ether), 164 Radel® A Poly(ether sulfone), 270, 271, 296, 469 Radel® R Poly(biphenyl sulfone), 230, 271 ReoPro® Glycoprotein IIb/IIIa inhibitor, 84 Rexflex® W111 Poly(olefin), flexible, 382 Rilsan® B MNO PA 12, 416 Rodrun® Liquid crystalline polymer, 543 RTP Compounds ESD Poly(arylene ether sulfone), 270 RTP PPS (Series) Poly(phenylene sulfide), 198 Rubinate® (Series) Isocyanate, 510
563
564
High Performance Polymers
Rusar® Aramid, 440 Ryton® (Series) Poly(phenylene sulfide), 198, 200, 296 Sandostab® 4020 Pentaerythritol tetrakis(3-laurylthiopropionate), 166 Sandostab® -P-EPQ Tetrakis(2,4-di-tert-butyl phenyl)-4,4,-biphenylene diphosphonite, 230 Santowax® R Mixed terphenyls, 230 Sapron™ S SMA copolymer, 166 Schulatec® PPS Poly(phenylene sulfide), 198 Selar® PA3426 PA 6 T/I Poly(amide), 416 Sepharose® CL Crosslinked poly(saccharide), 510 Septon® Hydrogenated styrenic block copolymer, 166, 200 Shieldex® C 303 Ca ion-exchanged silica, anticorrosion pigment, 322 Silicone KF351A Poly(dimethyl siloxane) surfactant, 322 Siltem® STM 1500 Poly(ether imide), 230 Sintimid™ Poly(amide imide), 469 Siveras™ Liquid crystalline polymer, 543 Skypet® PEN Poly(ethylene naphthalate), 380 Sniamid® ASN 32 Poly(amide), 166 Solef® Poly(vinylidene fluoride), 382 Solimide® Poly(imide), 506 Solprene® Styrenic block copolymer, 166, 200 Solventnaphtha™ Aromatic Hydrocarbon solvent, 469
Index Solvesso® Higher aromatic solvent mixtures, 322, 469 Stanyl® KS 200 Low molecular weight PA 4.6, 417 Stanyl® KS 300 Medium molecular weight PA 4.6, 417 Stat-Kon* PPS Poly(phenylene sulfide), 198 Statiblend® PPS Poly(phenylene sulfide), 198 Stilan® Poly(etherketone), 230 Styvex Poly(phenylene ether), 164 Sulfan® B Sulfur trioxide, 230 Sulfron® Aramid, 440 Sumikaexcel® Poly(arylene ether sulfone), 270 Sumikasuper® Liquid crystalline polymer, 543 Sumiploy® Poly(arylene ether sulfone), 270 Supec® Poly(phenylene sulfide), 198, 200 Surlyn® Ionomer resin, 166, 417, 510 Sylgard® 184 Silicone elastomer, 510 T-4 Poly(phenylene sulfide), 200 Taronyl Poly(phenylene ether), 164 Taxol® Antiproliferative preparation, 84 Tecapei® Poly(imide), 506 Technora® Aramid, 440 Tedur® Poly(phenylene sulfide), 198, 200
565
566
High Performance Polymers
Teflon® Tetrafluoro polymer, 510 Teflon® AF 1600 Amorphous copolymers of perfluoro-2,2-dimethyl-1,3-dioxole with tetrafluoroethylene, 230 Teflon® AF 2400 Amorphous copolymers of perfluoro-2,2-dimethyl-1,3-dioxole with tetrafluoroethylene, 230 Tegoglide™ 410 Poly(siloxane) surfactant, 125 Tegowet™ Poly(siloxane)-poly(ester) copolymer surfactant, 125 TEGO® RAD 2100 Poly(siloxane), acrylic, Radically crosslinkable flow and wetting additive, 382 Teijinconex® Aramid, 440 Tempalux® HI Poly(imide), 506 Tenax® Carbon fiber, 167 Teonex® Biaxially Poly(ethylene naphthalate) film, 380 Terathane® Poly(tetramethyleneoxide glycol) (PTMEG), 510 Teric® (Series) Poly(ethylene glycol) mono(nonylphenyl) ether, 510 Therma-Tech™ PPS Poly(phenylene sulfide), 198 Thermatex® Aramid, 440 Thermid® PETI type Poly(imide), 510 Therminol® 66 Partially hydrogenated terphenyls, 230 Therminol® 75 Mixed terphenyls and quaterphenyls, 230 Thermocarb® Graphite fiber, 543 Thermocomp Poly(phenylene ether), 164 Thermocomp* PPS Poly(phenylene sulfide), 198
Index
567
Thermocomp® Poly(arylene ether sulfone), 270 Thermotuf* PPS Poly(phenylene sulfide), 198 Tinuvin® 144 Bis(1,2,2,6,6-pentamethyl-4-piperidinyl) butyl(3,5-di-tert-butyl-4-hydroxybenzyl)malonate, UV absorber, 382 Tinuvin® 234 2-(2-hydroxy-3,5-di-α -cumylphenyl)-2H-benzotriazole, 382 Tinuvin® 326 2-(2 -Hydroxy-3 -tert-butyl-5 -methylphenyl)-5-chlorobenzotriazole, UV absorber, 322 Tinuvin® P, 2-(2 -Hydroxy-5 -methylphenyl)benzotriazole, UV absorber, 322 Titan® Liquid crystalline polymer, 543 Tone® (Series) Polyols, 510 Torayca® Carbon fiber, 167 Toray® Poly(imide), 506 Toreca™ Carbon fiber, 296 Torelina® Poly(phenylene sulfide), 198 Torlon® (Series) Poly(amide imide), 230, 469 Trogamid® T PA from terephthalic acid, 2,2,4-trimethylhexamethylenediamine and 2,4,4trimethylhexamethylenediamine, 417 Tronox® R-KB-2 Alumina silica treated, rutile titanium dioxide, pigment, 322 Tuftec® (Series) Styrenic block copolymer, 417 Twaron® Aramid, 440, 441 Tyneloy® Poly(phenylene ether), 164 Tyzor® TPT Titanium tetraisopropoxide (tetraisopropyltitanate), catalyst, 382 Ucarsol® Amine mixture, 230
568
High Performance Polymers
Udel® Polysulfone Poly(bisphenol A sulfone), 230, 270, 271, 296, 417 Ultem® (Series) Poly(imide), thermoplastic, 231, 271, 296, 506, 511 Ultem® 6050 Poly(ether imidesulfone), 231, 296, 342 Ultramid® (Series) Poly(amide), 167, 417 Ultrapek® Poly(arylene ether ketone), 296 Ultrapek® KR 4176 4,4 -Diphenoxybenzophenone-terephthaloyl chloride copolymer, 231 Unilink® (Series) Aromatic secondary diamines, 511 Unitem Poly(imide), 506 Upilex® Poly(imide), 342, 506, 511 Upimol® Poly(imide), 506 Uralac® P 1460 Polyester polyol, 322 UTTAP SF 50030 GF Liquid crystalline polymer, 270 Uvinul® D-50 2,2 ,4,4 -Tetrahydroxy benzophenone, UV absorber, 382 Valox® 315 Poly(butylene terephthalate), 167 Vecstar® Liquid crystalline polymer, 543 Vector® Styrenic block copolymer, 167, 201 Vectran® Liquid crystalline polymer, 543 Vectra® (Series) Liquid Crystal Polymer, composed from mainly 4-hydroxybenzoic acid or 6-hydroxy-2-naphthoic acid, further, depending on type: p-acetaminophenol, terephthalic acid, and biphenol, 201, 543 Versalink® (Series) Amine terminated poly-THF and PPO for PU resins, 511 Verton* PPS Poly(phenylene sulfide), 198
Index Vespel® Poly(imide), thermosetting, 231, 506 Vestakeep® Poly(ether ether ketone), 228 Vestamid® Poly(amide), 167 Vestenamer® 8012 Poly(octenylene), 167 Vestoran Poly(phenylene ether), 164 Victrex® 381G Poly(etheretherketone), cable coating, 231, 296 Victrex® PEEK (Series) Poly(etheretherketone), 228, 296 Victrex® PEEK 450 Poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene), poly(etheretherketone), 231 Victrex® PEK Poly(oxy-1,4-phenylenecarbonyl-1,4-phenylene), 231 Victrex® PES (Series) Poly(aryl ethersulfone), 296, 417 Vircol® 82 Flame retardant, 511 Vulcan® XC72 Carbon black, 167 Vylomax® Poly(amide imide), 469 Westlake PES Poly(arylene ether sulfone), 270 Xtel® PPS Poly(phenylene sulfide), 198 Xydar® Liquid crystalline polymer, 543 Xyron® Poly(phenylene ether), 164 Xyron® PPS+PPE Poly(phenylene sulfide), 198 Zenite® Liquid crystalline polymer, 543 Zoltek® HT Carbon fiber, 167 Zonyl® 7950 Fluorinated surfactant, 125
569
570
High Performance Polymers
Zonyl® FSO 100 Ethoxylated nonionic fluorosurfactant, 125 Zytel® Poly(amide), 167, 417
ACRONYMS α -TPT 2,5-Bis-(2-thienyl-1-cyanovinyl)-1-(2 -ethylhexyloxy)-4-methoxybenzene, 32 β -TPT 2,5-Bis-(2-thienyl-2-cyanovinyl)-1-(2 -ethylhexyloxy)-4-methoxybenzene, 32 2,6-NDA 2,6-Naphthalenedicarboxylic acid, 347 3GN Poly(1,3-propylene 2,6-naphthalate), 367 4,4 -BPDA 3,3 ,4,4 -Biphenyl dianhydride, 487 4,4 -BTDA 3,3 ,4,4 -Benzophenone dianhydride, 481 4,4 -ODPA 3,3 ,4,4 -Oxydiphthalic anhydride, 476 7-DCST 2-(4-Azepan-1-yl-benzylidene)-malononitrile, 40 AA Acrylic acid, 254 AAG 2-Acryamido glycolic acid, 260 ABS Acrylonitrile-butadiene-styrene, 411 ADC Azodicarbonamide, 156, 498 ADMET Acyclic diene metathesis, 94 ADMVN 2,2 -Azobis-(2,4-dimethylvaleronitrile), 8 AF-50 N,N-Diphenyl-7-(2-(4-pyridinyl)-ethenyl)-9,9-di-n-decyl-9H-fluorene-2amine, 45 AFM Atomic force microscopy, 52 AIBN 2,2 -Azobisisobutyronitrile, 3, 305
Index
571
Alq3 Tris-(8-hydroxyquinoline)-aluminum, 32 ASE Amplified spontaneous emission, 51 BEB Ethylene dibenzoate, 357 BEN 1-Benzoate 2-naphthoate ethylene, 357 BHCA Bis-(hydroxymethylcyclohexane)-arylate, 360 BHEA 2,6-Bis-(hydroxyethyl)arylate, 359 BisCzPro 1,3-Biscarbazolyl propane, 43 BOZ 2,2 -Bis-(l,3-oxazoline), 363 BP 4,4 -Biphenol, 239, 524 BPD 2-tert-Butylphenyl-5-biphenyl-1,3,4-oxadiazole, 34 BTDA-DATA Poly(3,3 ,4,4 -benzophenone tetracarboxylic dianhydride-3,5-diamino-1,2,4triazole), 317 C12O-PPP Poly(2-dodecyl-p-phenylene), 36 CA Citraconic anhydride, 476 CBTA Benzotriazole, 317 CHDM 1,4-Cyclohexanedimethanol, 359 CHO Cyclohexene oxide, 8 CPDHFPV Poly(9,9 -dihexylfluorene-2,7-divinylene-m-phenylene vinylene-stat-p-phenylene vinylene), 30 CTA Chain transfer agent, 15 CVD Chemical vapor deposition, 72, 93, 373, 484 DAPI 6-Amino-1-(4 -aminophenyl)-1,3,3-trimethylindane, 479
572
High Performance Polymers
DBA 3,5-Diaminobenzoic acid, 496 DCM 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylamino-styryl)-4H-pyran, 110 DMAC Dimethylacetamide, 453 DMF N,N-Dimethylformamide, 93, 189, 305, 481 DMNPAA 2,5-Dimethyl-4-(p-nitrophenylazo)anisole, 39 DMSO Dimethyl sulfoxide, 70 DMT Dimethyl terephthalate, 360 DNA Deoxyribonucleic acid, 52, 266 DR-1 Disperse Red 1, 43 DSC Differential scanning calorimetry, 351 DyC-82 Dysprosium fulleride, 24 E3VC N-Ethyl-3-vinylcarbazole, 15 ECZ N-Ethylcarbazole, 41 EL Electroluminescence, 25, 94 ENB Ethylidene norbornene, 316 EPDM Ethylene propylene diene monomer, 316 ESIPT Intramolecular proton-transfer, 36 GC Gas chromatography, 536 GF Glass fiber, 185 HALS Hindered amine light stabilizer, 191 HBA 4-Hydroxybenzoic acid, 524
Index HEMA 2-Hydroxyethyl methacrylate, 260, 305, 501 HIPS High impact poly(styrene), 154 HMD Hexamethylenediamine, 392 HPA Heteropolyacid, 159, 294 HQ Hydroquinone, 286, 524 IOL Intraocular lenses, 314 IPA Isophthalic acid, 359, 391 IR Infrared, 37, 153, 397, 482 ITO Indium tin oxide, 25, 108, 538 IV Intrinsic viscosity, 143, 194, 212, 363 LC Liquid chromatography, 536 LCD Liquid crystalline display, 500, 525 LCM Liquid composite molding, 401 LCP Liquid crystal polymer, 186, 522 LED Light-emitting diode, 14 MA Methacrylic acid, 260 MDI Diisocyanatodiphenyl methane, 452 MDMO-PPV Poly(2-methoxy-5-(3 ,7 -dimethyloctyloxy)-1,4-phenylene vinylene), 115 MEH-PPV Poly(2-methoxy-5-(2 -ethylhexyloxy)-1,4-phenylene vinylene), 30, 91 MPD m-Phenylenediamine, 454, 487 MTBE Methyl-tert-butyl ether, 160, 260
573
574
High Performance Polymers
MXDA m-Xylylenediamine, 392 NDC Dimethyl-2,6-naphthalene dicarboxylate, 348 NEN Ethylene dinaphthoate, 357 NLO Nonlinear optical, 3 NMP N-Methyl-2-pyrrolidone, 177, 211, 244, 290, 425, 453, 479 NMR Nuclear magnetic-resonance spectroscopy, 356 NOM Natural organic matter, 260 NPDA Neopentyldiamine, 410 NVK N-Vinylcarbazole, 1 NVP N-Vinyl-2-pyrrolidone, 259 ODA 4,4 -Oxydianiline, 452 ODCA 2,5-Bis-(4-carboxyphenyl)-1,3,4-oxadiazole, 329 ODPA-APB-8-AA Poly(4,4 -oxydiphthalic anhydride-1,3-aminophenoxybenzene-8-azaadenine), 317 OXD Oxadiazole, 31 P3O Poly(2,6-diphenyl-1,4-phenylene oxide), 152 PA Poly(amide), 148, 184, 252, 356, 391, 423, 449 PAE Poly(arylene ether), 194, 400 PAES Poly(arylene ether sulfone), 183, 237 PAI Poly(amide imide), 449 PANI Poly(aniline), 29, 187 PAS Poly(arylene sulfide), 179
Index PBD 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, 11, 340 PBN Poly(butylene naphthalate), 359 PBT Poly(butylene terephthalate), 185, 356 PC Poly(carbonate), 365, 411, 533 PCB Printed circuit board, 151, 194 PECA Poly(ethylene-1,4-cyclohexanedimethylene arylate), 359 PECVD Plasma enhanced CVD, 373 PEDOT Poly(3,4-ethylenedioxythiophene), 109 PEE Poly(ether ester), 361 PEEK Poly(ether ether ketone), 209, 263, 294 PEG Poly(ethylene glycol), 9 PEI Poly(ether imide), 214, 376, 475 PEK Pol(yether ketone), 209, 287 PEN Poly(arylene ether nitrile), 283 Poly(ethylene naphthalate), 118, 348 PENA 4-(1-Phenylethynyl)1,8-naphthalic anhydride, 487 PEP Poly(ethylene-2,7-phenanthrate), 360 PEPA 4-(Phenylethynyl)-1,2-benzenedicarboxylic acid anhydride, 476 PEPC Poly(N-epoxypropyl)carbazole, 13 PES Poly(ethersulfone), 239 PET Poly(ethylene terephthalate), 332, 349, 398 PETI Phenylethynyl terminated imide, 487
575
576
High Performance Polymers
PHDP Poly(1-hexyl-3,4-dimethyl-3,5-pyrrolylene), 22 PI Poly(imide), 214, 317, 449, 475 PL Photoluminescence, 25, 105 PMDA Pyromellitic dianhydride, 476 PMMA Poly(methyl methacrylate), 32, 100 PNV Poly(1,5-naphthylene vinylene), 98 POD-DPE Poly(4,4 -diphenyl ether-1,3,4-oxadiazole), 334 PODA Poly(1,3,4-oxadiazole), 329 POF Poly(9,9-dioctylfluorene), 110 PP Poly(propylene), 226, 399, 544 PPA Poly(phosphoric acid), 334 Poly(phthalamide), 391 PPE Poly(phenylene ether), 139 PPESK Poly(phthalazinone ether sulfone ketone), 258 PPS Poly(phenylene sulfide), 175, 293 PPSA Poly(1,4-phenylene sulfide-1,4-phenyleneamine), 187 PPSAA Poly(phenylene sulfide-phenyleneamine-phenyleneamine), 189 PPSO Poly(p-phenylene sulfoxide), 192 PPT Poly(pentylene terephthalate), 356 PPV Poly(p-phenylene vinylene), 31, 89, 308, 334, 378 PPX Poly(p-xylylene), 69 PPY Poly(pyrrole), 12
Index PS Poly(styrene), 15, 150, 365 PSI Poly(arylene ether sulfide), 183 PT Poly(1,2,4-triazole), 301 PTFE Poly(tetrafluoroethylene), 182, 215, 250, 496 PTK Poly(arylene thioether ketone), 251 PTT Poly(trimethylene terephthalate), 364 PU Poly(urethane), 315, 456 PVA Poly(vinyl alcohol), 118 PVAc Poly(vinyl acetate), 494 PVD Physical vapor deposition, 373 PVK Poly(N-vinylcarbazole), 1, 340 PVP Poly(N-vinyl-2-pyrrolidone), 265 PVPh Poly(vinylphenol), 375 RAFT Reversible addition-fragmentation chain transfer, 15 ROMP Ring opening metathesis polymerization, 94 SB Sodium benzoate, 351 SG Styrene/glycidyl methacrylate, 365 SPAENK Sulfonated poly(arylene ether nitrile ketone), 295 SPPEKN Sulfonated poly(phthalazinone ether ketone nitrile), 295 TAZ 3-(4-Biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole, 306 TBBPL 3,3 ,5,5 -Tetra-tert-butyl biphenol, 240
577
578
High Performance Polymers
TEP Triethyl phosphate, 265 TFPX α , α , α α -Tetrafluoro-p-xylylene, 70 THF Tetrahydrofuran, 10, 72, 98, 183 TIP Thermally induced phase inversion, 223 TMDQ Tetramethyldiphenyl quinone, 143 TMLA Trimellitic acid, 347 TNF 2,4,7-Trinitro-9-fluorenone, 1 TNFDM (2,4,7-Trinitrofluorene-9-ylidene)-malononitrile, 41 TPA Terephthalic acid, 361, 391, 524 TPD N,N -Bis-(3-methylphenyl)-N,N -diphenylbenzidine, 11 UF Ultrafiltration, 259
CHEMICALS p-Acetaminophenol, 543 Acetic anhydride, 250, 458, 481, 486, 525, 529 Acetonitrile, 15, 287, 468 4-Acetoxybenzoic acid, 525, 530, 531 2-Acetoxy-5-vinylphenyl-benzotriazole, 309 Acetylacetone, 33 Acetylene, 2, 301, 303, 438 Acrolein, 318 2-Acryamido glycolic acid, 260 Acrylamide, 254, 312 2-Acrylamido-2-methylpropanesulfonate, 312 2-Acrylamido-2-methyl-1-propane sulfonic acid, 260 Acrylic acid, 2, 24, 254, 260, 408 Acrylonitrile-butadiene-styrene, 412 Acryloyl chloride, 312, 314 5-Acryloyloxyethoxycarbonylmethyl-7-hydroxy-1,2,4-triazolo[1.5-a]pyrimidine, 311 Adipic acid, 392, 393, 396, 464
Index
579
N-Alkyloxadiazolium hydrosulfate, 339 2-Allyl-6-methylphenol, 139, 151 γ -Alumina, 193 Aluminum chloride, 248 Aluminum oxide, 12, 220, 373, 462 Amidosulfonic acid, 403 5-Amino-2-(4-aminophenoxy)-pyridine, 424, 425 6-Amino-1-(4 -aminophenyl)-1,3,3-trimethylindane, 477, 479 3-Aminobenzoic acid, 452 4-Aminobenzoic acid, 215, 452 3-Amino-2-cyclohexene-1-one, 452 3-Amino-4-methylbenzoic acid, 460, 461 4-Amino-4-nitrodiphenyl ether, 454 Amino[2.2]paracyclophane, 70 4-Aminophenol, 496, 524, 529, 542 p-Aminophenol, 287 3-(3-Aminophenyl)-5-[3 -(4-aminophenoxy)phenyl]-1,2,4-triazole, 307 3-(3-Aminophenyl)-5-[3 -(4-aminophenylsulfonyl)phenyl]-1,2,4-triazole, 307 γ -Aminopropyltriethoxysilane, 154, 404 3-Amino-1,2,4-triazole, 302, 317–320 5-Aminotriazole, 318 ω -Aminoundecanoic acid, 393 5-Amino-1-vinyltetrazole, 319 Amino-p-xylylene, 82 Ammonium carbonate, 156 Ammonium metavanadate, 141 Ammonium nitrate, 320 Aniline hydrochloride, 303 Anthracene, 13, 21 Antimony pentachloride, 187, 188 Antimony trioxide, 155, 349, 351, 360, 361 Arsenic pentafluoride, 106, 107 8-Azaadenine, 318 Azelaic acid, 393 2-(4-Azepan-1-yl-benzylidene)-malononitrile, 40 4-Azido-tetrafluorobenzoic acid, 215 1,1 -Azobis-(1-acetoxy-1-phenylethane), 11 4,4 -Azobis-(4-cyanopentanoic acid), 8 2,2 -Azobis-(2-cyanopropanol), 8 2,2 -Azobis-(2,4-dimethylvaleronitrile), 8, 11 2,2 -Azobis-2,4-dimethylvaleronitrile, 312 2,2 -Azobisisobutyronitrile, 3, 8, 9, 11, 13, 23, 305 Azodicarbonamide, 498
580
High Performance Polymers
Benzene arsonium fluoroborate, 309 Benzene-1,4-bis-(phenylene vinylene), 103 1,4-Benzenedicarboximidic acid dihydrazide, 330, 332 Benzene iodonium fluoroborate, 309 4-Benzenesulonylphenyl phenyl ether, 213 1-Benzoate 2-naphthoate ethylene, 357 Benzoic acid, 393, 396, 398 Benzonitrile, 263, 286, 318 Benzophenone, 223, 312 3,3 ,4,4 -Benzophenone dianhydride, 477, 481 3,3 ,4,4 -Benzophenone tetracarboxylic dianhydride, 494 1-Benzothiazol-3-phenyl-pyrazoline, 32 Benzotriazole, 317 5-(2H-Benzotriazole-2-yl)-2,2 ,4,4 -tetrahydroxybenzophenone, 312 5-(2H-Benzotriazole-2-yl)-2,2 ,4-trihydroxy-4 -acryloxybenzophenone, 313 3-Benzoyl-4-acetoxybenzoic acid, 530, 531 Benzoyl chloride, 146, 148 Benzyl alcohol, 463 2-Benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1, 11 Benzyl-1-pyrrolecarbodithioate, 15 4,4 -Biphenol, 524 4-Biphenyl-4-allyloxybenzoate, 536 4-Biphenylcarboxylic acid, 223 3,3 ,4,4 -Biphenyl dianhydride, 477, 487, 498, 503 4,4 -Biphenylene, 243 Biphenyl-2,3,3 ,4 -tetracarboxylic dianhydride, 476 Biphenyl-3,3 ,4,4 -tetracarboxylic dianhydride, 476 Biphenyltetracarboxylic dianhydride, 476 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, 11, 26, 31, 33, 106, 107, 110 3-(4-Biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole, 306 Bis-(allyl ether) tetrabromobisphenol A, 155 1,4-Bis(4-aminophenoxy)benzene, 477 4,4 -Bis(4-aminophenoxy)benzophenone-3,3 -disulfonic acid, 496 4,4 -Bis(4-aminophenoxy)biphenyl, 461 4,4 -Bis-(p-aminophenoxy)diphenyl-1,3,4-thiadiazole, 307 Bis[4-(p-aminophenoxy)phenoxy]dimethylsilane, 479 2,2 -Bis[4-(4-amino-phenoxy) phenyl] hexafluoroisopropylidene, 477 1,3-Bis[5 -[3 -(p-aminophenoxy)-phenyl]-oxadiazol-2-yl]benzene, 307 2,2 -Bis(4-(4-aminophenoxy)phenyl)propane, 477, 487 Bis(aminophenoxy phenyl)-sulfone, 489 Bis[4-(3-aminophenoxy)phenyl]sulfone, 503 N,N-Bis(4-aminophenyl)-N ,N -diphenyl-1,4-phenylenediamine, 468
Index
581
2,3-Bis(4-aminophenyloxyphenyl)-quinoxaline-6-carboxylic acid, 424 1,5-Bis(3-aminophenyl)-1,4-pentadien-3-one, 452, 456 1,1-Bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane, 498 2,3-Bis(4-aminophenyl)-quinoxaline-6-carboxylic acid, 424 Bis-(aminophenyl)-sulfone, 241 4,4 -Bis(4-aminophenylthio)benzophenone-3,3 -disulfonic acid, 496 Bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, 503 4,4-Bis[(4-amino)thiophenyl] benzophenone, 501 2,2 -Bis(3-amino-4-toluyl)hexafluoropropane, 503 1,4-Bis(bromodifluoromethyl)benzene, 72 3,5-Bis-(tert-butyl)-phenol, 306 1,3-Bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-phenylene, 33 1,3-Biscarbazolyl propane, 43 4,4 -Bis-((3-carboxyphenoxy)(p-benzoyl))-phenyl sulfone, 252 1,4-Bis(4-carboxyphenoxy)naphthalene, 423, 424 2,6-Bis(4-carboxyphenoxy)naphthalene, 423, 424 2,5-Bis-(4-carboxyphenyl)-1,3,4-oxadiazole, 330 Bis(4-carboxyphenyl)phenylphosphine oxide, 393 2,2-Bis[N-(4-carboxyphenyl)-phthalimidyl]hexafluoropropane, 461 1,4-Bis-(chloromethyl)-2-methoxy-5-(2 -ethylhexyloxy)benzene, 98 2,5-Bis-(chloromethyl)-1,3,4-oxadiazole, 337, 338 Bis-(4-chlorophenyl)-sulfone, 241, 242, 291 Bis-(2,4-di-tert-butylphenyl)-pentaerythritol diphosphite, 363 1,3-Bis(2,3-dicarboxyl-phenoxy)benzene dianhydride, 477 1,4-Bis(3,4-dicarboxyl-phenoxy)benzene dianhydride, 477 4,4 -Bis(3,4-dicarboxyl phenoxyphenyl)-isopropylidene dianhydride, 476, 477 4,4 -Bis(2,3-dicarboxyphenoxy)diphenyl sulfone dianhydride, 489 4,4 -Bis(3,4-dicarboxyphenoxy)diphenyl sulfone dianhydride, 489 1,4-Bis-(dichloromethyl)-benzene, 89, 91, 93, 94 Bis-((4,6-difluorophenyl)-pyridinato-N,C-2 )(picolinato)Ir(III), 33, 36 1,5-Bis-(4-(4 -fluorobenzoyl)-phenoxy)-naphthalene, 210 Bis-(4-fluorophenyl)-sulfide, 176, 187 Bis-(4-fluorophenyl)sulfone, 245, 247 2,6-Bis-(hydroxybutyl) naphthalate, 360 4,4 -Bis-(4-hydroxy-3,5-dimethylphenyl)pentanoic acid, 144 Bis-(4-(2-hydroxyethoxy)benzene)-ether, 361 Bis-(4-(2-hydroxyethoxy)benzene)-fluorene, 361 Bis-(4-(2-hydroxyethoxy)benzene)-sulfone, 361 2,6-Bis-(hydroxyethoxycarbonyl)naphthalene, 350 2,6-Bis-(hydroxyethyl)arylate, 359, 360 Bis-(2-hydroxyethyl)-biphenol, 361 Bis-(2-hydroxyethyl)-bisphenol A, 361 Bis-(2-hydroxyethyl)-bisphenol H, 361
582
High Performance Polymers
Bis-(2-hydroxyethyl)-hydroquinone, 361 2,6-Bis-(hydroxyethyl) naphthalate, 360 Bis-(hydroxymethylcyclohexane)-arylate, 360 1,1-Bis-(4-hydroxyphenyl)-ethyl-1-phenyl-2,3,5,6-tetrafluoro-4-vinylphenyl ether, 267 9,9-Bis-(4-hydroxyphenyl)-fluorene, 284, 287 2,2-Bis-(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane, 284, 293 Bis-(4-hydroxyphenyl)-hydrazide, 303 Bis-(4-hydroxyphenyl)-methane, 243 1,4-Bis-(4-hydroxyphenyl)-phenylene dihydrazide, 303 1,1-Bis-(4-hydroxyphenyl)-2-phenyl ethane, 243 2,2-Bis-(4-hydroxyphenyl)-propane, 243 Bis-(4-hydroxyphenyl)-sulfone, 240, 241, 247 Bis-(4-hydroxyphenyl) sulfone, 241 N,N -Bis-(3-methylphenyl)-N,N -diphenylbenzidine, 11, 33 1,4-Bis-(2-methylstyryl)-benzene, 103 (1,4-Bis-(1,3,4-oxadiazole)-2,5-di(2-ethylhexyloxy)phenylene)-5,5 -diyl, 30 2,2 -Bis-(l,3-oxazoline), 363 Bis-(pentafluorophenyl)-sulfide, 176, 183 Bis-(pentafluorophenyl)-sulfone, 183 4,4 -Bisphenol S, 241 Bisphenol A, 148, 155, 187, 210, 212, 239, 240, 284, 291, 360 Bisphenol A dianhydride, 476, 489 1,4-Bis(phenoxymethyl)benzene, 70, 73 1,4-Bis[(phenylmethoxy)methyl]benzene, 70, 73 2,5-Bis-(2-thienyl-1-cyanovinyl)-1-(2 -ethylhexyloxy)-4-methoxybenzene, 32 2,5-Bis-(2-thienyl-2-cyanovinyl)-1-(2 -ethylhexyloxy)-4-methoxybenzene, 32 1,4-Bis(trifluoromethyl)benzene, 70, 74 2,2 -Bis(trifluoromethyl)benzidine, 503 Boron trifluoride, 248 Bromoanil, 18 p-Bromobenzaldehyde, 141 4-Bromo-4 ,4 -dihydroxytriphenylmethane, 139, 141, 145 4-Bromodiphenyl ether, 223 1-Bromonaphthalene, 223 Bromonaphthalenedicarboxylic acid, 347 N-Bromo succinimide, 74, 319 1,4-Butanediol, 361 4-Butoxy-3-propyl-1-(4 -nitrophenylazo)benzene, 39, 42 Butyl acrylate, 184 n-Butylacrylate, 408 tert-Butyl alcohol, 8, 161 N-Butyl-N -(4-azidophenyl)thiourea, 215
Index
583
Butyl benzyl phthalate, 45 4-tert-Butylcatechol, 240 2-Butyl-2-ethyl-1,3-propanediol, 373 tert-Butylhydroquinone, 212, 524 n-Butyllithium, 17, 18, 162 n-Butyl methacrylate, 309 tert-Butyl oxide, 75 4-tert-Butylphenol, 244 2-tert-Butylphenyl-5-biphenyl-1,3,4-oxadiazole, 34, 35 2-(4-tert-Butylphenyl)-5-4-[(4-vinylphenyl)methoxy]phenyl-1,3,4-oxadiazole, 24, 341 Cadmium octoate, 463 Cadmium selenide, 49 Calcium metasilicate, 404 Calcium oxide, 408 Calcium terephthalate, 437 ε -Caprolactam, 316, 392, 393, 463 2-(Carbazol-9-yl)ethyl methacrylate, 32, 40 6-(Carbazol-9-yl)hexyl methacrylate, 40 1-(3-Carbomethoxyacryloyl)-5-amino-1,2,4-triazole, 321 2-Carboxyl-6-hydroxyethoxycarbonylnaphthalene, 350 4-Carboxyl[2.2]paracyclophane, 70 2-(4-Carboxyphenyl)-1,3-dioxoisoindoline-5-carboxylic acid, 452, 456 Cellulose triacetate, 378 Chloroanil, 18 Chlorobenzene, 23, 241, 242, 291 4-Chlorobenzenesulfinate, 245 Chlorobenzenesulfonic acid, 242 4-Chlorobenzenesulfonyl chloride, 241 3-Chloro-2,6-difluorobenzonitrile, 286 2-Chloroethyl ether, 14 2-Chloroethyl vinyl ether, 13 2-Chloro-6-fluorobenzonitrile, 286 1-Chloronaphthalene, 192, 223 p-Chloronitrobenzene, 452 p-Chlorophenol, 293 2-Chloro-p-phenylenediamine, 431 Chlorophenylsulfonyl phenoxide, 245 Chlorophthalic anhydride, 476 Chlorophyll, 49 Chlorosulfonic acid, 148, 161, 162, 249, 250, 265, 338 2-Chloroterephthaloyl chloride, 212, 424, 431 Chlorotrimethylsilane, 162
584
High Performance Polymers
Chloro-p-xylylene, 74 Cholesteryl benzoate, 521 Chrome oxide, 436 Citraconic anhydride, 146, 147, 476, 477 Citric acid, 156, 400 Cobalt acetate, 361 Copper bromide, 407 Copper 4-bromobenzenethiolate, 176, 181 Copper chloride, 145 Copper iodide, 407 Coronene, 13 Coumarin, 107, 340 m-Cresol, 453 o-Cresol, 141, 455 Cumene, 463 p-Cyanobenzoyl chloride, 288 5-Cyanoisophthaloyl chloride, 288 1,4-Cyclohexanediamine, 477 1,4-Cyclohexanedimethanol, 359, 360 Cyclohexanone, 142 Cyclohexene oxide, 8 Decafluorodiphenyl ketone, 267 Decamethylenediamine, 393 Deoxyribonucleic acid, 52, 266 1,4-Diacetoxybenzene, 525 β , β -Diacetyl-4-methoxystyrene, 39 p-Diaminobenzene, 252 3,5-Diaminobenzoic acid, 496, 503 4,4 -Diaminodicyclohexylmethane, 393 2,2-(4,4 -Diaminodicyclohexyl)propane, 393 4,4 -Diamino-3,3 -dimethyldiphenylmethane, 477, 479 1,8-Diamino-3,6-dioxaoctane, 453 3,4 -Diaminodiphenyl ether, 424, 452 4,4 -Diaminodiphenyl ether, 452, 455 4,4 -Diaminodiphenyl ether, 424, 425, 431 4,4 -Diaminodiphenylmethane, 452, 463 2,4-Diamino-4 -fluoroazobenzene, 467 4,4 -Diamino-4 -methoxytriphenylamine, 468 2,4-Diamino-4 -methylazobenzene, 467 2,4-Diamino-4 -nitroazobenzene, 467 2,4-Diamino-4 -(4-nitrophenyl-diazenyl)azobenzene, 467 2,6-Diaminopyridine, 317 3,5-Diamino-1,2,4-triazole, 317
Index 2,4-Diamino-4 -trifluoromethoxyazobenzene, 467 Dibenzoylperoxide, 456 4,4 -Dibromobiphenyl, 176, 182 1,2-Dibromoethene, 93 α , α -Dibromo-α , α , α , α -tetrafluoro-p-xylene, 74 α , α -Dibromo-p-xylene, 89, 93, 110 2,6-Di-tert-butyl phenol, 240, 286 N,N-Di(4-tert-butylphenyl)-4-(2-pyridyl) phenylamine, 33 Dibutyltindilaurate, 380 N,N -Di(carbazol-3-yl)-N,N -diphenyl-1,4-phenylenediamine, 5 1,2-Dichlorobenzene, 9 o-Dichlorobenzene, 143, 476 p-Dichlorobenzene, 175, 177 2,6-Dichlorobenzonitrile, 284, 287, 295 4,4 -Dichlorobenzophenone, 182, 252, 496 4,4 -Dichlorodiphenyl sulfone, 210, 239, 240, 243, 244, 252, 263 1,2-Dichloroethane, 143, 265, 288 3,5-Dichloro-4-(4-methoxyphenyl)-4H-1,2,4-triazole, 306, 307 1,9-Dichloro[2.2]paracyclophane, 93 Dichloro-p-xylylene, 76 Dicumyl peroxide, 155 2-Dicyanomethylene-3-cyano-2,5-dihydrofuran, 39 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylamino-styryl)-4H-pyran, 107, 110 4-(Dicyanovinyl-N,N-diethylaniline), 43 9,9 -Didodecylfluorene-2,7-diyl, 30 1,4-(N,N -Diethylamino)anthraquinone, 110 Diethylaminodicyanostyrene, 41 4-Diethylaminoethyl benzoate, 501 Diethylene glycol, 256, 260 Diethylfumarate, 13 2,2-Diethyl-1,3-propanediol, 373 2,6-Difluorobenzonitrile, 284, 286, 290, 295 4,4 -Difluorobenzophenone, 212 4,4 -Difluorodiphenyl ketone, 210, 211 4,4 -Difluorodiphenyl sulfone, 240, 243–246, 248 2,7-Difluoro-9,10-dithiaanthracene, 190 3,5-Difluoro-4 -hydroxydiphenyl sulfone, 253 3,5-Difluorophenylmagnesium bromide, 253 2,7-Difluorothianthrene, 190 4,4 -Diflurobiphenyl, 182 Dihexamethylenetriamine, 399 9,9-Dihexylfluorene-2,7-divinylene-m-phenylene vinylene, 51
585
586
High Performance Polymers
o-Dihydroxybenzene, 181 2,5-Dihydroxybenzoic acid, 245 4,4 -Dihydroxybenzophenone, 210 4,4-Dihydroxydiethoxydiphenyl sulfone, 256 2,4-Dihydroxydiphenyl sulfone, 241 4,4 -Dihydroxydiphenyl sulfone, 240, 241, 243, 284, 286 2,6-Dihydroxynaphthalene, 524 2,7-Dihydroxynaphthalene, 249, 284, 290 2-(2,4-Dihydroxyphenyl)-2H-benzotriazole, 313 1,6-Di(3-hydroxyphenyl)-1,6-diazaspiro[4.4]nonane-2,7-dione, 284 1,6-Di(4-hydroxyphenyl)-1,6-diazaspiro[4.4]nonane-2,7-dione, 287 2-(2,4-Dihydroxyphenyl)-1,3-2H-dibenzotriazole, 313 p-Diiodobenzene, 252 p,p -Diiododiphenyl sulfone, 252 Diisocyanatodiphenyl methane, 452, 482 Diisopropylamine, 316 Diisopropyl azodicarboxylate, 145 m-Diisopropylbenzene, 286 3,3 -Dimethoxybenzidine, 461 4-Di(2-methoxyethyl) aminobenzylidene malononitrile, 42 2,2-Dimethoxy-2-phenylacetophenone, 11 Dimethylacetamide, 225, 453, 454 Dimethylacetonitrile, 196 Dimethylacrylamide, 534 4-Dimethylaminopyridine, 147 3,3 -Dimethyl-l,1 -binaphthalene, 461 2,2 -Dimethyl-4,4 -bis(4-aminophenoxy)biphenyl, 452, 461 4-Dimethylbutylamine, 147 N,N-Dimethylcarbamyl chloride, 242 Dimethylchlorosilane, 537 3,3 -Dimethyl-4,4 -diaminodicyclohexylmethane, 393, 409 2,5-Dimethyl-2,5-di(tert-butylperoxy)-3-butane), 152 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline , 34 N,N-Dimethylformamide, 10, 162, 242, 453, 492 4,4-Dimethyl-1,7-heptanediamine, 409 2,6-Dimethyl-4-heptanone oxime, 316 1,3-Dimethylimidazolidinone, 290 Dimethyl isophthalate, 366 2,3-Dimethylmaleic anhydride, 505 2,6-Dimethyl naphthalate, 360, 366 Dimethyl-2,6-naphthalene dicarboxylate, 348, 361 2,5-Dimethyl-4-(p-nitrophenylazo)anisole, 39, 41, 42 2,5-Dimethyl-4-(4 -nitrophenylazo)phenyl benzyl ether, 43
Index
587
2,5-Dimethyl-4-(4 -nitrophenylazo)phenyl octyl ether, 43 2,4-Dimethyl-3-pentanone oxime, 316 2,6-Dimethylphenol, 141, 143, 151, 152, 163, 194 Dimethyl phthalate, 476 2,2-Dimethyl-1,3-propanediol, 373 2-(5,5-Dimethyl-3-styryl-cyclohex-2-enylidene)-malononitrile, 45 Dimethyl sulfoxide, 70, 72, 145, 286, 497 Dimethyl terephthalate, 337, 361, 397 3,5-Dimethyl-1,2,4-triazole, 301, 302, 316 3,6-Dimethyl-9-vinylcarbazole, 11 3,6-Dinitro-9-vinylcarbazole, 21 9,9-Dioctylfluorene, 50, 335 1,6-Dioxaspiro[4.4]nonane-2,7-dione, 287 1,4-Diphenoxybenzene, 210, 213 2,6-Diphenoxybenzonitrile, 288 4,4 -Diphenoxybenzophenone, 249 N,N -(4,4 -Diphenyleneethylene)bis(4-phenylethynylphthalimide), 487 4 4 -Diphenyl ether dicarboxylic acid, 330, 332 9,10-Diphenylethynylanthracene, 13 N,N-Diphenylformamide, 223 Diphenylguanidine, 143, 144 Diphenyl methyl phosphate, 223 2,6-Diphenylphenol, 139, 141, 142 N,N -Diphenyl-1,4-phenylenediamine, 7 2,2-Diphenyl-1-picryl-hydrazyl, 14 N,N-Diphenyl-7-(2-(4-pyridinyl)-ethenyl)-9,9-di-n-decyl-9H-fluorene-2-amine, 45 4,4 -Diphenyl sulfone, 243 3,6-Diphenyl-vinylcarbazole, 21 4,7-Diselenophen-2 -yl-2,1,3-benzoselenadiazole, 49 4,7-Diselenophen-2 -yl-2,1,3-benzothiadiazole, 49 Disperse Red 1, 43 3,3 -Disulfonate-4,4 -dichlorodiphenyl sulfone, 295 4,7-Di-2-thienyl-2,1,3-benzothiadiazole, 50 Dodeca-fluoro[2.2]paracyclophane, 72 Durene, 476 Dysprosium fulleride, 24 Enantholactam, 393 Epibromohydrin, 146, 151 Epichlorohydrin, 146, 151 9-(2,3-Epoxypropyl)carbazole, 2, 13 Ethyl acetate, 10, 120, 160, 463 Ethyl-2-bromo-2-methylpropionate, 39
588
High Performance Polymers
N-Ethylcarbazole, 40, 41, 43 9-Ethyl-3-carbazolecarboxaldehyde, 17 N-Ethylcarbazole-3,6-dicarboxylic acid, 334 Ethylene carbonate, 360 Ethylene chlorohydrin, 312 Ethylenediamine tetraacetic acid, 143 Ethylene dibenzoate, 357 Ethylene dinaphthoate, 357 Ethylene glycol, 360 Ethylene glycol mono methyl ether, 145, 260 Ethylene propylene diene monomer, 316 1-(2-Ethylhexyloxy)-2,5-dimethyl-4-(4-nitrophenylazo)benzene, 39, 42 1-(2 -Ethylhexyloxy)-2,5-dimethyl-4-(4 -nitrophenylazo)benzene, 44 Ethyl mercaptan, 240 Ethyl methacrylate, 309 4-Ethyl[2.2]paracylophane, 70, 78 2-Ethyltetramethylenediamine, 393, 409 N-Ethyl-3-vinylcarbazole, 15 Ethynylaniline, 505 3-Ethynylphenol, 183 9-Fluorenone, 287 4-Fluorobenzenesulfinate, 240, 245 2-Fluorobenzonitrile, 284, 290 4-Fluoro-4 -hydroxydiphenyl sulfone, 240, 245 5-[(4-Fluorophenyl)sulfonyl]-2-fluorobenzoic acid, 240, 245 (Fluorophenyl)(trifluorophenyl) sulfone, 240, 245 2-Formyl-6-naphthoic acid, 347, 348 Fumaric acid, 2, 24, 146, 147, 150, 400 Fumaroyl chloride, 305 Germanium oxide, 351 Glycidyl acrylate, 312 N-Glycidylcarbazole, 13 Glycidyl methacrylate, 314, 365 γ -Glycidylpropylmethoxysilane, 404 Glycidyl tosylate, 151 Heteropolyacid, 294 1-Hexadecylamine, 42 2-Hexadecyloxy-5-methoxybenzene-1,4bis-(4-dimethylaminophenylene vinylene), 103 1,5-Hexadiene, 12 4,4 -(Hexafluoroisopropylidene)diphenol, 267, 284, 295 4,4 -(Hexafluoroisopropylidene)-diphenyl, 267 4,4 -Hexafluoroisopropylidene diphthalic dianhydride, 493, 498, 503
Index Hexahydroxytriphenylene, 525 N,N -Hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamide), 405 Hexamethylenediamine, 392, 393, 407 Hexamethylphosphoramide, 72, 428 1,6-Hexane diisocyanate, 316 m-Hexaphenyl ether, 288 4-(4-(Hexyloxy)phenyl)-3,5-diphenyl-4H-1,2,4-triazole, 308 High impact poly(styrene), 154 R-Hirudin, 82 Hydrazine, 155, 337 Hydrazine sulfate, 332, 337 Hydrogen fluoride, 248, 267 Hydrogen sulfide, 78, 178, 179, 191 6-Hydroperoxy-6-hexanelactam, 403 Hydroquinone, 210–212, 243, 524 2-(2-Hydroxy-7-acryloyloxynaphthyl)-2H-benzotriazole, 313 2-[2-Hydroxy-4-alkoxy-(2-oxypropyl methacrylate)phenyl]2H-4-methoxybenzotriazole, 314 Hydroxyapatite, 220 p-Hydroxy benzaldehyde, 141 4-Hydroxybenzoic acid, 524, 526 3-Hydroxy-1,2,3-benzotriazin-4-one, 486 1-Hydroxybenzotriazole, 486 6-Hydroxy[1,3-bis-(4-hydroxyphenylsulfonyl]benzene, 241 7-Hydroxy-5-carboxymethyl-1,2,4-triazolo[1,5-a]pyrimidine, 311 4-Hydroxycinnamic acid, 524 1-Hydroxycyclohexyl-phenyl-ketone, 51 2-Hydroxyethyl acrylate, 311, 312 2-Hydroxyethyl methacrylate, 260, 305, 306, 315, 501, 540 (β -Hydroxyethyl)naphthalate, 349 N-Hydroxymethylmethacrylamide, 314 2-Hydroxy-6-naphthoic acid, 524 6-Hydroxy-2-naphthoic acid, 186 4 -Hydroxy phenyl-4-hydroxybenzoate, 210, 212 4,4 -[1-[4-[1-(4-Hydroxyphenyl)-1-methylethyl]phenyl]ethylidene], 505 2-(4-Hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole, 13, 14 4-(4-Hydroxyphenyl)-1(2H)-phthalazinone, 295 9-(3-Hydroxypropyl)carbazole, 41 N-Hydroxypyridine-2-thione, 254 Hypochlorous acid, 432 2-Imidazolidinone, 70 Indium tin oxide, 25, 29, 34, 108 Isophorone diamine, 452, 458
589
590
High Performance Polymers
Isophthalic acid, 337, 359, 371, 391, 392, 407, 428, 524 Isophthalonitrile, 392 Isophthaloyl bislaurocaprolactam, 401 Isophthaloyl chloride, 212, 330, 332 4,4 -Isopropylidene diphenyl, 267 Laurolactam, 392, 393, 413 Liquid crystal polymer, 522 Lithium hydroxide, 431 Lithium naphthalene, 17 Lithocholic acid, 524 Magnesia, 436 Magnesium acetate tetrahydrate, 151 Maleic anhydride, 24, 146, 147, 150, 151, 185, 400, 408, 456, 477, 488, 533 Malonic acid diethyl ester, 316 Manganese acetate, 349, 351 Melamine cyanurate, 374 2-Mercaptoethanol, 259, 260 3-Mercaptopropionic acid, 287 3-Mercaptopropyltrimethoxysilane, 537 Methacrylamide, 312 Methacrylic acid, 2, 260 Methacrylic acid 6-[3-(2-cyano-2-(4-nitrophenyl)-vinyl)-carbazol-9-yl]hexyl ester, 2–4 Methacrylic acid 6-[3-(diphenyl-hydrazonomethyl)-carbazol-9-yl]hexyl ester, 2–4 Methacrylic acid 6-[3-[2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester, 3, 4 Methacrylic anhydride, 146, 147 N-Methacryloxypropyl carbazole, 40, 41 N-Methacryloxypropyl-3-(p-nitrophenyl)azo carbazole, 40 Methacryloyl chloride, 41, 312, 314 Methanesulfonic acid, 189, 192, 196, 222, 287, 338 4-(5-Methoxy-2H-benzotriazole-2-yl)resorcinol, 314 2-Methoxy-5-(3 ,7 -dimethyloctyloxy)benzene-1,4-diacetonitrile, 124 p-(Methoxymethyl)benzyl chloride, 94 1-Methoxynaphthalene, 223 p-Methoxyphenol, 99 4-Methoxyphenyl-4-allyloxy benzoate, 537 Methoxyphenylisocyanate, 306 4-Methoxyphenylsulfonyl chloride, 253 Methyl-(4-anilino-phenyl) sulfide, 187, 188 N-(α -Methylbenzyloxy)-2,2,6,6-tetramethylpiperidine, 41 p-Methylbenzyltrimethylammonium hydroxide, 70 2-Methyl-1-butanol, 79
Index Methyl-tert-butyl ether, 160, 260 Methyl chloromethyl ether, 148, 161 2-Methyl-1,5-diaminopentane, 413 Methylene chloride, 17, 182, 265, 291 2-Methyleneglutaric dinitrile, 392 Methyl ethyl ketone, 260, 454 Methyl isobutyl ketone, 525 Methyl mercaptan, 240 Methyl methacrylate, 29, 254 α -Methylnadic anhydride, 452 4-(α -Methylnadimido)-benzoic acid, 452 2-Methyl-5-nitroaniline, 454 2-Methylpentamethylenediamine, 392, 393, 399, 409 N-Methyl-4-picolinium hexafluorophosphate, 157 Methylpiperidine, 399 4-Methyl-pyrazolo[3.4-b]quinoline, 37 N-Methyl-2-pyrrolidone, 177, 211, 244, 290, 334, 427, 453, 479 N-(4-Methylsulfinyl)phenylene-N -phenyl-1,4-phenylenediamine, 189 4-Methylsulfoxy-diphenylamine, 196 Methyltri-n-octylammonium chloride, 143 Molybdenum disulfide, 436 Monoethanolamine, 79, 224 Monomethyl-2,6-naphthalene dicarboxylate, 348 Montmorillonite, 12 Morpholine, 250 Nadic anhydride, 456, 477, 486, 505 p-Nadimidochlorobenzene, 176, 183 1-Naphthaldehyde, 455 2,6-Naphthalenedicarboxylic acid, 347, 349, 350, 361, 366, 371, 375, 524 1,5-Naphthalenediol, 330, 337 Naphthalocyanine, 37 2-Naphthoic acid, 348 1,2-Naphthoquinone-2-diazido-5-sulfonyl chloride, 505 2-α -Naphthyl-5-(4-vinylphenyl)-1,3,4-oxadiazole, 11 Neopentyldiamine, 393, 410 3-Nitroaniline, 454 Nitrogen dioxide, 78 3-Nitromesidine, 454 4-(4 -Nitrophenyl-diazenyl) phenyl-1,3-diamine, 467 N-(4-Nitrophenyl)-1-prolinol, 46 Octadecylacrylate, 537 1,1,2,2,9,9,10,10-Octafluoro[2.2]paracyclophane, 70, 73 Octafluoro[2.2]paracyclophane, 72
591
592
High Performance Polymers
2,2,3,3,4,4,5,5-Octafluoropropyl methacrylate, 306 1,3,4-Oxadiazole, 30 Oxalic acid, 218, 220 Oxaloyl chloride, 187 4-Oxoheptanedioic acid, 287 4,4 -Oxybis-(benzenesulfonylhydrazide), 156 Oxy-bis(N-(4-phenylene)-trimellitic imide), 452 3,4 -Oxydianiline, 482 4,4 -Oxydianiline, 317, 452, 477, 501 2,2 -(Oxydi-4,1-phenylene)bis[5-(4-fluorophenyl)-1,3,4-oxadiazole], 330, 337 3,3 ,4,4 -Oxydiphthalic anhydride, 476, 477, 479 N-Oxypyridine-2-thione, 254 Ozone, 78, 182, 197 Palladium acetate, 476 Palladium(II)-hexafluoroacetylacetonate, 435 Papain, 161 Pentachlorobenzonitrile, 284, 286, 291 Pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 405 Pentafluorobenzonitrile, 284, 286 Pentafluorophenyl sulfone, 267 Perchloric acid, 189 Perfluoroalkyl vinyl ether copolymer resin, 227 Perfluorooctyl methacrylate, 74 Perlenetetracarboxylic-bis-benzimidazole, 49 Perylene, 13, 49 Phenol, 141, 241, 257, 286 Phenolphthalein, 258, 263, 284, 286 Phenolsulfonic acid, 241 p-Phenoxybenzenesulfonyl chloride, 248 4-Phenoxybenzophenone, 213 p-Phenoxybenzoyl chloride, 210, 213, 248 Phenoxyethylacrylate, 540 4-(4-Phenoxyphenoxy)benzophenone, 213 p-Phenoxyphenoxybenzoyl chloride, 210, 213 N-[4-(3-Phenoxy)-4 -phenylethynylbenzophenone]4 -phenylethynylphthalimide, 487 N,N -[2,2-(4-Phenoxyphenyl) hexafluoroisopropylidene]bis-(4-phenylethynylphthalimide), 487 4-Phenoxyphenyl sulfone, 249 4-Phenoxy-2,3,5,6-tetrafluorobenzonitrile, 284, 293 Phenyl benzoate, 223 1-Phenyldecane, 223 N-Phenyl-4,5-dichlorophthalimide, 189
Index
593
3,3 -(m-Phenylenebis(oxy))dianiline, 477 N,N -(1,3-Phenylene)bis(4-phenylethynylphthalimide, 487 N,N -(1,4-Phenylene)bis(4-phenylethynylphthalimide, 487 1,3-Phenylenediamine, 424 1,4-Phenylenediamine, 424, 427 m-Phenylenediamine, 454, 477 p-Phenylenediamine, 477 4,4 -(p-Phenylenedi-1,2-ethenediyl)-diphenol, 100, 104 4,4 -(m-Phenylenedioxy)-bis-(benzenesulfonyl chloride), 253 p-Phenylene oxadiazole, 339 p-Phenylene-5,5 -tetrazole, 330, 332 p-Phenylene vinylene), 51 4-(Phenylethynyl)-1,2-benzenedicarboxylic acid anhydride, 476, 477 4-(1-Phenylethynyl)1,8-naphthalic anhydride, 487 4-(Phenylethynyl)phenol, 183 Phenylhydroquinone, 212 N-Phenylmaleimide, 150 3-Phenyl-7-methacryloyloxyethoxy-1-methyl-1H-pyrazolo[3,4-b]-quinoline, 32 3-(5-Phenylpentyl)-4-methylbenzyl chloride, 70 p-Phenylphenol, 139, 284 2-Phenyl-5-4-[(4-vinylphenyl)methoxy]phenyl-1,3,4-oxadiazole, 24, 341 2-Phenyl-5-4-[(4-vinylphenyl)methoxy]phenyl-1,3,4-oxadiazole, 2 Phosphomolybdic acid, 163, 226 Phosphorus pentoxide, 248 o-Phthalic acid, 371 Phthalic anhydride, 287, 476 Phthalocyanine, 37 β -Picoline, 481 Poly(acenaphthylene), 21 Poly(acetylene), 103 Poly(acrylic acid), 9 Poly(acrylonitrile), 162, 499 Poly(amide), 148, 185, 220, 306, 356, 376, 391 Poly(amide imide), 449 Poly(5-amino-1-vinyltetrazole), 319 Poly(amino-p-xylylene), 83 Poly(aniline), 29, 30, 187, 379, 435 Poly(arylene ether nitrile), 283 Poly(arylene ether sulfide), 183 Poly(arylene ether sulfone), 183 Poly(arylene sulfide), 179, 192 Poly(arylene thioether ketone), 175, 251 Poly(benzimidazole), 457
594
High Performance Polymers
Polybenzimidazole, 318, 490 Poly(3,3 ,4,4 -benzophenone tetracarboxylic dianhydride-3,5-diamino-1,2,4-triazole), 317 Poly[3,6-bis-(3,7-dimethyloctyloxy)-9,9-spirobifluorenyl-2,7-vinylene], 116 Poly(bis-1,2,4-triazole), 305 Poly(1,4-butylene sebacate), 353 Poly(butylene terephthalate), 185, 356, 375 Poly-γ -carbazolylethylglutamate, 18 Poly[2-(carbazol-9-yl)-1,4-phenylene vinylene], 32 Poly(carbonate), 164, 365, 373 Poly(chloro-p-xylylene), 83 Poly(cyclohexylenedimethanol terephthalate), 185 Poly(dibromostyrene), 404 Poly(dichloro-p-xylylene), 83 Poly(9,9-dihexylfluorene), 89 Poly(9,9 -dihexylfluorene-2,7-divinylene-m-phenylene vinylene-stat-p-phenylene vinylene), 30, 31, 51 Poly(2,5-dimethoxy-1,4-phenylene vinylene), 95 Poly(2-(N,N-dimethylamino) phenylene vinylene), 100 Poly(2-dimethyloctylsilyl)-phenylene vinylene, 99 Poly(9,9-dioctylfluorene), 31, 110 Poly(9,9-dioctylfluorene-co-fluorenone), 30 Poly(4,4 -diphenyl ether-1,3,4-oxadiazole), 334 Poly(2,6-diphenyl-1,4-phenylene oxide), 152 Poly(2,6-diphenyl-1-4-phenylene oxide), 141 Poly(dithiathianthrene), 189 Poly(2-dodecyl-p-phenylene), 36 Poly(N-epoxypropyl)carbazole, 13 Poly(ether ether ketone), 209 Poly(ether imide), 154, 214, 264, 376 Pol(yether ketone), 213 Poly(ether nitrile), 227 Poly(ethersulfone), 209, 264 Poly(ethylene-1,4-cyclohexanedimethylene arylate), 359 Poly(3,4-ethylenedioxythiophene), 109 Poly(ethylene glycol), 9 Poly(ethylene glycol diacrylate), 540 Poly(ethylene naphthalate), 29, 347, 349 Poly(ethylene oxide), 430, 499 Poly(ethylene terephthalate), 185, 256, 347, 351, 398 Poly(N-ethyl-3-vinylcarbazole), 14 Poly(furfuryl alcohol), 162 Poly(1-hexyl-3,4-dimethyl-3,5-pyrrolylene), 22
Index
595
Poly(3-hexylthiophene), 379 Poly(imide), 162, 189, 214, 317, 499 Poly(2-methoxy-5-(3 ,7 -dimethyloctyloxy)-1,4-phenylene vinylene), 117 Poly(2-methoxy-5-(2 -ethylhexyloxy)-1,4-phenylene vinylene), 31, 50, 91, 117 Poly(methyl-bis-(3-methoxyphenyl)-(4-propylphenyl)amine)siloxane, 42 Poly(methyl methacrylate), 32, 41, 100, 341, 499 Poly(2-methyl-5-vinyl)tetrazole, 319 Poly(1,5-naphthylene vinylene), 98 Poly(1,3,4-oxadiazole), 329 Poly(1,3,4-oxadiazole-2,5-diyl-1,2-vinylene), 337 Poly(4,4 -oxydiphthalic anhydride-1,3-aminophenoxybenzene-8-azaadenine), 317 Poly(oxyethylene), 164 Poly(oxymethylene), 164 Poly(pentylene terephthalate), 356 Poly(phenazasiline), 52 Poly(2(3)-(4-phenylbutyl)-1,4-phenyleneethylene), 84 Poly(p-phenylene), 30, 103 Poly[1,4-phenylene-1,2-di(4-benzyloxyphenyl)vinylene], 42 Poly(phenylene ether), 139 Poly(1,4-phenylene ether sulfone), 250, 256 Poly(p-phenylene ethylene), 69 Poly(1,4-phenylene ethynylene), 105 Poly(p-phenylene ethynylene), 89 Poly(m-phenylene isophthalamide), 439 Poly(p-phenylene methylene), 69 Poly(1,4-phenylene-methylsulfonium-1,4-phenyleneamine)methylsulfonate, 196 Poly(p-phenylene-1,3,4-oxadiazole), 341 Poly(phenylene oxide), 139 Poly(m-phenylene sulfide), 189 Poly(phenylene sulfide), 175, 293 Poly(1,4-phenylene sulfide-1,4-phenyleneamine), 187 Poly(phenylene sulfide-phenyleneamine-phenyleneamine), 189 Poly(p-phenylene sulfoxide), 192 Poly(p-phenylene terephthalamide), 339 Poly(p-phenylene vinylene), 31, 89, 308 Poly(phosphoric acid), 334 Poly(phthalamide), 392 Poly(propylene), 154, 226, 399, 533 Poly(1,3-propylene 2,6-naphthalate), 367 Poly(1,3-propylene terephthalate), 369 Poly(pyridopyrazine vinylene), 50 Poly(p-pyridyl-vinylene), 89
596
High Performance Polymers
Poly(pyrrole), 12 Poly(styrene), 33, 150, 365 Poly(tetrafluoroethylene), 182, 195, 215, 464, 496 Poly(tetramethylene ether) glycol, 361 Poly(tetramethylene ether glycol terephthalate), 361 Poly(tetramethylene glycol), 366, 540 Poly(p-thienyl vinylene), 89 Poly(1,2,4-triazole), 303 Poly(trimethylene terephthalate), 364 Poly(urethane), 456 Poly[1-(4-vinylbenzoyl)-5-amino-1,2,4-triazole], 321 Poly[3-(4-vinylbenzoyl)-5-amino-1,2,4-triazole], 321 Poly(vinyl butyral), 52 Poly(N-vinylcarbazole), 1 Poly(vinyl chloride), 164 Poly(vinylidene fluoride), 264, 499 Poly(1-vinylnaphthalene), 21 Poly(3-vinyl-1,2,5-oxadiazole), 319 Poly(vinyl phenanthrene), 18 Poly(vinylphenol), 375, 379 Poly(vinyl pyrene), 18 Poly(N-vinyl-2-pyrrolidone), 9 Poly(1-vinyltetrazole), 319 Poly(5-vinyltetrazole), 319 Poly(3-vinyl-1,2,4-triazole), 319 Poly(vinyltriazole), 319 Poly(p-xylene), 103 Poly(p-xylylene), 69, 93 Potassium tert-butoxide, 99 Potassium fluoride, 70, 72, 267, 286 Potassium hexatitanate, 436 Potassium iodide, 360, 407 Potassium octatitanate, 436 Potassium phthalimide, 475 1,3-Propanediol, 367, 373, 374 Propylene glycol monobutyl ether, 319 Pyrazoloquinoline, 37 4-(1-Pyrenyl)butyl vinyl ether, 13 Pyridine, 317 2-Pyridylamine, 483 Pyromellitic acid, 475 Pyromellitic dianhydride, 317, 476, 477, 501 Quinacridone, 37
Index 8-(Quinolinolate)-aluminum, 37 p-Quinone, 181 Resorcinol, 284, 286, 290 Resorcinol diphosphate, 155 Rubrene, 37 3-(N-Salicyloyl)amino-1,2,4-triazole, 317 Sebacic acid, 393 Sebacyl chloride, 475 Silicone nitride, 81 Silicotungstic acid, 226 Sodium acetate, 178, 182 Sodium antimonate, 408 Sodium azide, 303 Sodium benzoate, 351 Sodium bicarbonate, 156 Sodium borohydride, 214, 497 Sodium chlorate, 258 Sodium hydride, 536 Sodium hydrogen sulfide, 182 Sodium oxalate, 193 Sodium saccharinate, 180 Sodium sulfide, 175, 177, 189 Stearic acid, 220, 491 Stearyl alcohol, 404 Styrene acrylonitrile copolymer, 149 Styrene buadiene styrene block copolymer, 150 Styrene/glycidyl methacrylate, 365 Suberic acid, 393 Succinamic acid, 215 o-Sulfobenzoic acid, 180 Sulfonated poly(ether ether ketone), 224, 456 3,3 -Sulfonyl bis-(6-fluorobenzene sulfonic acid) disodium salt, 247 3,3 -Sulfonyl bis-(6-hydroxybenzene sulfonic acid) disodium salt, 247 Sulfonyl chloride, 242, 253 Sulfur tetrafluoride, 74 Sulfur trioxide, 105, 265 Sulfuryl chloride, 249, 455 Tannic acid, 164 Terephthalaldehyde, 74 Terephthalic acid, 317, 337, 366, 375, 391, 409, 428, 524 Terephthaloyl bislaurocaprolactam, 401 Terephthaloyl chloride, 210, 213, 424, 427 Tetrabenzyl perylene-3,4,9,10-tetracarboxylate, 114, 117
597
598
High Performance Polymers
Tetrabromocyclooctane, 155 Tetrabromovinylcyclohexene, 155 Tetrabutylammonium bromide, 98, 314 Tetrabutylammonium perchlorate, 17 Tetra-n-butylammonium tetrafluoroborate, 107, 110, 112 3,3 ,5,5 -Tetra-tert-butyl biphenol, 240 Tetrabutylphosphonium bromide, 337 Tetrabutyl titanate, 435, 463 1,1,2,2-Tetrachloroethane, 11 Tetrachloroisophthalodinitrile, 291 Tetrachloromethane, 143 Tetrachlorophthalodinitrile, 291 Tetracyanoethylene, 14, 18 Tetracyanoquinodimethane, 18 Tetraethyl lead, 160 Tetrafluoropropanol, 496 2,2,3,3-Tetrafluoropropyl methacrylate, 306 Tetrafluoro-p-xylene, 74 α , α , α α -Tetrafluoro-p-xylylene, 70 Tetrahydrofuran, 10, 17, 23, 72, 183, 398, 453 Tetrahydrothiophene, 91, 93 Tetramethylammonium hydroxide, 361, 501 1,2,4,5-Tetramethylbenzene, 476 3,3 ,5,5 -Tetramethylbenzidine, 461 3,3 ,5,5 -Tetramethyl-2,2-bis(4-(4-amino-phenoxy)phenyl)propane, 452 3,3 ,5,5 -Tetramethyl-bis[4-(4-aminophenoxy)phenyl]sulfone, 461 Tetramethyldiphenyl quinone, 143, 164 2,3,5,6-Tetramethyl-1,4-phenylenediamine, 493 Tetramethyl-p-phenylenediamine, 498 2,2,6,6-Tetramethylpiperidine-N-oxyl, 98 2,4,5,7-Tetranitro-9-fluorenone, 18 2,4,5,7-Tetranitroxanthone, 18 Tetraphenylbenzidine, 468 5,6,11,12-Tetraphenylnaphthacene, 37 (4,4 -Tetrazolyl-4 -methyl)triphenylamine, 329, 330 2,2-Thiodiethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 405 Thioglycol acid, 240 Thionyl chloride, 182, 187, 242, 254, 454, 456 Titanium tetraisopropoxide, 259, 367 m-Toluenediamine, 477 Toluene diisocyanate, 8, 482 p-Toluenesulfonic acid, 240, 286 o-Toluidine, 479
Index Tolyltriazole, 315 2,4,6-Triamino-1,3,5-triazine, 455 1,2,4-Triazole, 301, 302 Tri-n-butyl phosphite, 454 1,2,4-Trichlorobenzene, 180 1,3,5-Trichlorobenzene, 176, 180 2,3,6-Trichlorobenzonitrile, 286 3,5,6-Trichloro-4-chloroformyl phthalic acid, 465 3,5,6-Trichloro-4-chloroformyl phthalic acid anhydride, 465 Trichloroethylene, 143, 158 Tricresyl phosphate, 45, 218 Triethanol amine, 79 Triethylamine, 309, 454, 458, 481 Triethyl phosphate, 265 Trifluoroacetic anyhdride, 486 Trifluoromethanesulfonic acid, 72, 189, 223, 242, 248 (4-(4 -Trifluoromethyl)phenoxyphenyl)hydroquinone, 210 4,4 -(Trifluoromethylphenylisopropylidene)diphenol, 183 4,4 -(2,2,2-Trifluoro-1-pentafluorophenylethylidene) diphthalic dianhydride, 477 3,4,5-Trifluorophenylsulfonyl benzene, 253 Triiron dodecacarbonyl, 72 Trimellitic acid, 347 Trimellitic acid anhydride, 317, 452–454, 463, 464 Trimethylamine, 262 Trimethylchlorosilane, 148 2,2,4-Trimethylhexamethylenediamine, 409 2,4,4-Trimethylhexamethylenediamine, 409 2,4,6-Trimethyl-3-nitroaniline, 454 2,3,6-Trimethylphenol, 139, 194 2,4,6-Trimethylphenol, 139, 141, 144 2,4,6-Trimethyl-1,3-phenylenediamine, 461, 494 Trimethyl phosphate, 351 Trimethylsilyltributyltin, 72 1,3,7-Trinitrodibenzothiophene-5,5-dioxide, 18 (2,4,7-Trinitrofluorene-9-ylidene)-malononitrile, 44 2,4,7-Trinitro-9-fluorenone, 1, 5, 24, 41, 45 2,4,8-Trinitrothioxanthone, 18 Trioctylamine, 94 Trioxane, 8 Triphenylamine, 35 1,2,3-Triphenylbenzene, 223 Triphenylmethane, 223
599
600
High Performance Polymers
Triphenylmethanol, 223 Triphenyl phosphate, 218, 454 Triphenylphosphine, 145, 318, 319 Triphenylphosphine oxide, 263 Triphenyl phosphite, 252, 363, 427, 428, 454 Tris-(8-hydroxyquinoline)-aluminum, 32, 34 Tris(1-phenylisoquinoline) iridium, 33 Tris(3,4,5-trifluorophenyl)phosphine oxide, 253 1,1,1-Tri-(p-tosyloxymethyl)-propane, 100 Truxene, 522 Tungstophosphoric acid, 225 Vinylbenzoate, 32 1-(4-Vinylbenzoyl)-5-amino-1,2,4-triazole, 320 4-Vinylbiphenyl, 70, 74 N-Vinylcarbazole, 1, 2 N-Vinyl formamide, 260 9-(2-Vinyloxyethyl)carbazole, 13 9-(4-Vinylphenyl)anthracene, 32 N-Vinylphthalimide, 2, 21, 23 N-Vinyl-2-pyrrolidone, 2, 259, 260 1-Vinyl-1,2,4-triazole, 301, 302, 305 4-Vinyltriphenylamine, 41 Wollastonite, 404 p-Xylene, 69, 70, 73 2,6-Xylenol, 141 p-Xylylene diacetate, 70 m-Xylylenediamine, 392 p-Xylylene dipropionate, 70 Zinc acetate, 349, 351, 360 Zinc chloride, 181, 315 Zinc oxide, 141, 193, 218, 315, 408 Zinc phthalocyanine, 49 Zinc p-toluene sulfonate, 356 Zirconia, 436
Index
601
GENERAL INDEX Acoustic diaphragms, 432 Acrylic resins, 375 Adhesion copper, 497 finish formulation, 437 improvement, 533 interfacial, 252, 380 plasma treatment, 438 promotion, 294, 317, 368 reduction, 193, 465, 532 Adhesives, 157 high-temperature, 317 structural, 360 Adhesiveshot-melt, 413 Admittance spectroscopy, 105 Aerogels, 492 Agglomeration, 220, 352 Air bags, 319 Analysis biomaterials, 123 electrochemical, 30 organic vapors, 120 stationary phases, 536 Anchor groups, 72, 82 Anti-curling, 377 Anti-fouling properties, 259 Anti-glare mirrors, 468 Anticorrosion layers, 196 Antioxidants disodium phosphate, 398 for PPA, 405 hindered phenols, 404, 405 hindered radicals, 403 Antisense oligonucleotides, 266 Aramid paper pulp, 430 Argon plasma etching, 438 Arrhenius equation, 11, 358 Artificial blood vessels, 525 Asbestos substitutes, 431, 436, 437
Audio tapes, 376 Avrami Equation, 354 Base pairing, 266 Batteries lithium, 9, 498 rechargeable, 221, 226 solar, 499 Biaxial stretching, 367–369, 371, 376, 377 Binder fibers, 379 resins, 294, 435–437, 542 Biocompatibility, 264, 507, 524 Birefringence, 267, 525, 534 Bis-(amidrazones), 332 Bislactams, 401 Blends abrasion-resistant, 195 compatible, 157, 214, 365, 533 fillers, 400 glass transition temperatures, 364 immiscible, 250, 363 membrane applications, 456 miscible, 149, 364, 375, 490 oligomeric, 248 solder friendly, 194 ternary, 380 with PPE, 150, 162 Blocking agents, 315, 316 Blow molding, 362, 363, 411 Blowing agents, 155, 156, 498 Bone cement, 436 fixation screws, 525 substitutes, 221 Bottles, 362, 363 multilayer, 364 recycling, 383, 398 Bulletproof jackets, 435
602
High Performance Polymers
Bushings, 220, 460 Butyl rubber, 316 Capacitance transient spectroscopy, 105 Capping agents, 147, 183, 213, 214, 244, 254, 396, 456, 476, 505 Carbodiimides, 486 Carbon black, 340, 431 Carbonate method, 243 Cashew dust, 436 Casting, 5, 23, 152, 222, 263, 284, 293, 378, 431, 484 Catalysts copper, 144 crosslinking, 463 dealkylation, 141 nickel, 462 palladium, 476 photo, 157 supported, 161, 438, 439, 460 water soluble, 144 Ziegler-Natta, 8, 319 Cell incubation dishes, 524 Chain extenders, 291, 401 Chain scission, 191, 215, 259, 434 Chain stoppers, 284, 292, 398, 399, 486 Chemical vapor deposition, 73, 93, 373, 484 Chiral phases, 521 Chromatic aberration, 51 Chromophores acceptor, 21 azo-type, 38, 467 donor, 21 dual-use, 45 photorefractive, 42 sensitizer, 42 Cladding materials, 267 Co-electrolysis, 95 Coal-tar distillation, 2 Coatings p-xylylene, 73
Coatings (cont.) adhesive, 464 craze resistant, 464 gas barrier, 373 imide, 462 nonadhesive, 220 organic solar cells, 378 photosensitive, 501 powder, 315, 316 silicone, 373 solder powders, 78 waveguide, 78 wire, 293, 317, 449 Coherence gates, 44 Cold crystallization, 354, 356 Compatibility blood, 264 improvement, 146, 150, 366 improvment, 145 lack, 146, 364 Compatibilizers, 146, 150, 184, 186, 364, 365, 400, 533 Computer tapes, 376 Condensation azeotropic, 243, 454, 455 interfacial, 398 melt, 360, 429 reactive molding, 290 two-stage, 348, 350 Conductor tracks, 196 Copolymers alternating, 110, 335 aromatic, 187 block, 15, 99, 120, 212, 224 conjugated, 50 fibers, 339 functionalized, 82, 308 graft, 365 photoconductive, 23 random, 26, 95, 244, 337 telechelic monomers, 247 transesterification, 359 triblock, 13, 99, 532
Index Corrosion inhibitors, 193 triazole based, 318 Coverlays, 500 Creep resistance, 195, 226, 432, 459 Crosslinking agent, 180 blocked isocyanates, 316 capping agents, 456 diols, 496 Friedel-Crafts, 288 melt blending, 185 oxidative, 16, 179 phenylethynyl group, 487 photo, 111, 215, 259, 494 rubber, 316 thermal, 152, 183, 191, 224 Cryogenic fuel tanks, 439 Crystallites, 352, 353, 380 Crystallization, 28 additives, 43 annealing, 371 cold, 356, 365 high-temperature, 401 isodimorphic, 359 isothermal, 354 kinetics, 354 modifiers, 359 poly(ethylene naphthalate), 351 rate, 251, 291, 351, 353, 359 retardants, 362 strain-induced, 361, 362, 379 temperature, 181 thermal, 361 Cyclic voltametry, 30, 78, 468 Cyclization, 191, 248, 290, 332, 462 intramolecular, 189 Cycloaddition, 301 Cyclodehydration, 305, 332 Cyclophanes, 69, 70 Debutylation, 240, 286 Decalcifying agents, 403 Decarboxylation, 245 Decolorants, 192
603
Deep-drawability, 316 Defects band, 30, 44 conjugation, 91, 105 crystallographic, 105 oxidative, 105 structural, 213, 337 Degradation stabilizers, 315 Dehalogenation, 251 Delamination resistance, 376, 377 Dental plaque, 258, 264 Dielectric properties, 1, 74, 77, 123, 151, 154, 191, 227, 257, 455, 489 Diffraction grating, 51 Dimensional stability, 153, 294, 375, 409, 428 Dioxins, 537 Dip-coating, 91 Direct methanol fuel cells, 163, 262, 456, 460 Discoloration, 192, 349, 401 Discotic liquid crystals, 521, 526 Doctor blade techniques, 47 Donor acceptor, 21, 22, 26, 27, 51, 114, 338 Dopants, 31, 37, 106, 107 Double refraction, 534, 536 Drag reducers, 430 Drug release, 83, 265 Dye doping, 106, 107 Dye molecules, 107, 434, 534 Electrets, 154 Electric connectors, 529 Electrically conductive composites, 435, 541, 542 Electrochromism, 468 Electrodeposition, 318 Electroless plating, 412, 435 Electroluminescence, 10, 25, 89, 107, 112, 308, 336, 370 Electrolytes, 162, 301, 498, 499 Electromagnetic-shielding layers, 196 Electromers, 25
604
High Performance Polymers
Electrophotography, 1, 19, 22, 197 Electroplating, 411 Electroplexes, 26 Embrittlement, 215 Encapsulation, 118, 378, 379 End capping, 183, 214, 254, 407, 475, 505 Energy transfer Dexter, 35 Förster, 35, 51, 110 Enthalpy of crystallization, 354 Esterification, 151, 160, 311, 348, 349, 356, 358, 362, 377, 429, 526 Etching, 412, 438, 498, 505 Eutectic melting temperature, 359 Exciplexes, 25, 36 Excitons, 25, 47, 48 Exfoliation, 12, 111 Extrusion blow molding, 363 Fabric blends, 430 Femtosecond pulses, 44 Ferroelectricity, 531 Fibers Aramid, 423, 429, 432 carbon, 154, 439, 491 cellulosic, 430 copper, 436 electrically conductive, 435 filter materials, 293 glass, 8, 154, 404, 533 graft polymerization onto, 8 graphite, 341, 542 hollow, 162, 260, 494 nano, 118 optical, 80, 436, 465 partially aromatic PA, 392, 464 plasma treatment, 438 poly(ethylene naphthalate), 375 poly(ethylene terephthalate), 379 reinforcement, 151, 154, 199, 293 spinning, 152, 193, 339, 380, 401, 429 Titanate, 437
Fibers (cont.) Wollastonite, 404 Fibrillation, 430, 529, 531–533 Fillers, 186, 220, 222, 250, 404 Filters optical, 244, 539 particle, 80, 293, 464 Flame retardants, 151, 155, 250, 256, 374, 404, 408 Flexographic printing, 379 Floppy disks, 376 Fluorescence microscopy, 52, 118 Fluorescent dyes, 34, 106 Fly ashes, 442 Foaming chemical, 458 confined free expansion, 458 confined free rise, 458 continuous physical, 457 microwave, 492 Foams, 156 heat resistant, 456 isocyanate, 458 poly(imide), 491 Food applications, 199, 222, 305, 351, 369, 375 Friction composites, 442 Friction materials, 436, 442 Friction modifiers, 436 Friedel-Crafts catalysts, 161, 213, 288 chain extension, 106 polymerization, 248, 288 reaction, 241 Fuel cells, 162, 224, 225, 249, 258, 261, 263, 340, 456, 460, 496 Fullerenes, 17, 43, 49, 115 Functional dyes, 110 Functionalization, 43, 82, 145, 148, 214, 314 Functionalized monomers, 72, 524, 525, 528
Index Gas barriers, 118, 360, 361, 364, 368, 378 Gas permeability, 29, 370, 439, 495 Gas separation, 158, 159, 162, 291, 460, 461, 493, 494 Gas-generating polymers, 319 Gaskets, 226, 227, 436 Gelation, 70, 453 Gene expression, 266 Gilch reaction, 75, 97, 337 Gorham process, 78 Graphite precursors, 105, 341 Herbicide containing polymers, 320 Heterojunction, 114 Hexaethylguanidinium chloride, 476 Hi-loft fabrics, 379 High Tg materials, 262, 287, 338, 428, 467, 485, 524 High flow compositions, 150 Hofmann elimination, 70 Hole blocking, 34, 306, 308 Hole-transporting material, 18–20, 28, 34, 42, 105, 110, 308, 340 Holograms, 44 Homojunction, 113 Honeycombs, 197, 431, 433 Hydrogels, 314 Hydrogenation, 348, 392, 439, 460, 462 Impact modifiers, 154, 184, 250, 404, 408 Impedance spectroscopy, 122, 123 Impedimetric biosensors, 123 Implantates, 83 Incinerators, 293 Inherent viscosity, 213, 362, 367, 407, 455 Initiators anionic, 18, 99 cationic, 5, 12 condensation, 245, 246 photo, 11, 13, 501 radical, 3, 8, 305, 319
605
Initiators (cont.) Ziegler-Natta, 12 Injection molding, 214, 220, 226, 294, 408, 411, 434, 449, 475, 542, 544 Ink-jet printing, 29, 109, 111, 497 Insert molding, 410 Insulating paper, 431 Insulin, 82 Intercalation, 12, 111 Interdigitated electrodes, 120, 123 Intrinsic viscosity, 146, 194, 249, 334, 355, 356, 358, 361–363 Inverse gas chromatography, 536 In situ composites, 531, 542 Ion etching, 498 Isocyanates bifunctional, 8 blocked, 315, 316 foaming, 458, 492 oligomeric, 453 poly(amide imide)s, 453 poly(imide)s, 481 Kinks, 524, 530 Knoevenagel polycondensation, 97 Laminates, 151, 367, 401, 460 Laser light, 11, 536 Laser thinning, 379 LCD projectors, 193 Lenses, 314, 315, 467 Light actinic, 534 Light stabilizers, 191, 312, 400 Lithiation, 162, 254 Lubricants, 193, 215, 436 Luminophores, 31 Lyotropy, 521 Macrocycles, 248, 272 Magnetic recoding media, 376 Medical devices, 82, 409, 435 Melt blending, 157, 185, 186, 365, 380, 401 Melt stabilizers, 218
606
High Performance Polymers
Membranes asymmetric, 258 bipolar, 161 carbon, 261, 495 catalytic, 460 composite, 163, 226 crosslinked, 494 dense, 158, 225 electrolyte, 162 enzyme-functionalized, 161 filled, 161 fuel cell, 224, 261, 262, 340, 460, 496 gas separation, 158, 224, 291, 460, 494 hollow fiber, 260, 494, 496 mixed matrix, 494 molecular sieve, 162 pervaporation, 160, 453 porous, 158, 161, 222 reinforced, 496 steam sterilizable, 264 sulfonated, 161 ultrafiltration, 161, 258 Memory devices, 33, 41, 226 Mesogenic units, 534 Mesomeric effect, 5 Mesophases, 329, 365, 521 Messenger RNA, 266 Metal complexes, 32, 36, 435, 507 Metathesis, 94, 96 Methacrylic resins, 375 Michler’s ketone, 501 Microfiltration, 258 Micromachining, 80, 81 Microparticles, 227, 431, 507 Mold release agents, 404, 491 Mold staining, 348 Molecular glasses, 5 Multifunctional isocyanates, 458 monomers, 3, 150 UV absorbers, 315
Multilayer films, 120, 368 Multiplex recording, 534 Nanocomposites, 11, 111, 400, 498 Nanoparticles anatase, 259 core-shell, 17, 42 gold, 119 silica, 161 silver, 498 Nanotubes carbon, 9 poly(p-phenylene vinylene), 119 Networks crystal, 117 interpenetrating, 24, 49, 493 Non-wovens, 379, 482 Nucleation, 353 Nucleic acids, 266 Nucleotide mimetics, 266 Open-circuit voltage, 50, 51, 116, 117 Optical data storage, 38, 41, 45, 107, 468, 533 Optical nonlinearity, 3, 43 Orientation temperature, 367 Overlap molding, 410 Oxidation promoters, 347 Oxidative coupling, 139, 143 Oxidative dimerization, 16, 240, 286 Oxidative stabilizers, 405, 407 Oxide films, 373 Papermaking, 430 Permanent magnets, 294 Pervaporation, 160, 453, 495 Phase transfer catalysts, 72, 98, 337, 476 Photo acid generators, 505 Photochemical reactions, 45, 46, 49, 254, 309, 370, 493 Photoconductivity, 3, 22 Photocuring, 157, 309 Photographic films, 378 Photoinitiation, 11 Photolithography, 29
Index Photorefractivity, 37, 39, 40 formulations, 39 liquid crystals, 44 Photostability, 191, 435 Photovoltaic devices, 47, 113 Pinholes, 28, 78, 500 Piston rings, 460 Plasma coating, 78 Plasma treatment, 215, 438 Plasticization, 493, 494 Plasticizers, 43, 45, 374, 383 Poly(isocyanate)s aliphatic, 316 blocked, 315, 316 Polycondensation aerosol process, 396 chain-growth, 244 cross-coupling, 93 dehydrating, 394 Knoevenagel, 97 living, 242 melt, 256 nucleophilic substitution, 244 transesterification, 349, 529 Polydispersity, 76, 98, 99, 243, 244 Polyelectrochromic materials, 468 Polymer fume fever, 507 Polymerization actinically activated, 309 anionic, 17, 98 batch, 397 cationic, 12 charge transfer, 14 continuous, 397 CVD, 82 dispersion, 17 electro, 318 electrochemical, 9, 15 electrophilic, 249 free radical, 3, 315 Friedel-Crafts, 248, 288 graft, 8 Grignard metathesis, 95
607
Polymerization (cont.) heterogeneous, 8 interfacial, 454 living radical, 41 melt-phase, 351 microwave, 455 nucleophilic displacement, 483 nucleophilic substitution, 187 oxidative, 141, 163 oxidative matrix, 9 phase separation method, 540 photo, 11 plasma, 29, 319, 438 precipitation, 453 radical, 10 RAFT, 14 reactive melt, 401 ring-opening, 248, 272, 349 ROMP, 94 solid state, 351, 354 suspension, 149 Yamamoto, 337 Ziegler-Natta, 12 Polymers biocidal, 320 comb-shaped, 537 dendritic, 150, 400 hyperbranched, 50, 100, 145, 253, 439 ladder-type, 189 photochromic, 45, 467 photorefractive, 41, 43, 44, 46 Porcelain, 463 Powder coatings, 151 Printed circuit boards, 81, 500 Promoters, 70 adhesion, 317, 500 flow, 150 oxidation, 347, 348 reaction, 143, 427 sequence, 266 Protective garments, 430 Proteolytic enzymes, 161
608
High Performance Polymers
Pyrolysis, 69, 73, 93, 162, 191, 256, 355, 439, 495 Pyrotechnic composition, 319 Reaction spinning, 339 Reactive diluents, 487 Recording heat-mode, 533 holograms, 44 magnetic tape, 375 magnetic tapes, 376 photo-mode, 534 Redistribution reaction, 145, 146 Redox potential, 468 Reflective layers, 78, 80 Reflow characteristics, 78 Reflow soldering, 226 Rheology modifiers, 145 Rod-like structure, 452, 522 Scavenger acid, 458 radical, 98 Seals, 220, 436, 460 Self-propagating thermolysis, 319 Semiconductors, 89, 114, 338 Sensitizers, 13, 21, 22, 24, 501 Sensors gas, 120 impedimetric, 123 organic vapors, 498 pH sensitive, 119 pressure, 340 thermal, 16 Shear thinning, 401, 532 Silicon wafer, 80 Sintering, 227 Sliding applications, 433 Slurry coating, 199 Smart windows, 468 Soft segments, 361, 453 Solar cells, 47, 49, 113, 118, 378 Soldering resistance, 529 Spacers, 453, 542 Spherulites, 251, 355, 356
Spin casting, 501 Spin-coating, 5, 29, 91, 115, 505, 536 Spinning air gap, 429 dry, 464 electro, 118 melt, 193, 380, 401 reaction, 339 wet, 339, 429 Spirodilactams, 287 Spraying electrostatic, 118, 199 flame, 227 Sputtering, 29, 109, 373 Staple fibers, 429, 430 Statistical copolymer, 24, 51, 341 Stents, 82 Sterilization radiation, 264 steam, 250, 264 Sticking temperature, 351, 355 Stokes shift, 34 Stoving lacquers, 316 Stress-cracking, 155, 250 Stretch blow molding, 362, 364 Sulfurization, 251 Sunflower Oil, 460 Sunglasses, 468 Sunlight, 48, 191 Supercritical carbon dioxide, 9, 352, 383, 435, 492 Surface modification, 72, 82, 148, 154, 214, 258, 438 Surface roughness, 78, 159, 373, 377 Surface-mount technology, 194 Suzuki coupling, 93, 335 Telechelic monomers, 247 Tenting frame process, 369 Terpolymers, 184, 361 Thermal crosslinking, 111 Thermal degradation, 103, 153, 164, 191, 308, 499
Index Thermosets, 151, 176, 183, 254, 401, 439, 488 Thermosetting resins, 139, 151, 317, 449, 486, 488 Thrombin inhibitors, 82 Tire cords, 339, 375, 438 Transamidation, 399 Transimidization, 483 Transistors, 107, 379 Trim elements, 411 Twin screw extruder, 396, 400 Twisted nematic device, 538 Ullmann reaction, 5, 141, 145 Ultrafiltration, 161, 258 Ultrapure water, 222 Ultraviolet absorbers, 309, 312, 434 Ultraviolet stabilizers, 312, 313, 434 Unsaturated poly(ester) resins, 533 UV absorbers, 312 inhibition of photocuring, 309 monofunctional, 315
609
UV absorbers (cont.) polymeric, 314 polymerizable, 312, 313 Vacuum deposition, 5, 29, 82, 109 Vertically aligned LCD, 539 Video tapes, 376 Vitrification, 5 Waveguides, 76, 78, 107, 183, 267, 465 Welding, 400, 409, 410 Williamson synthesis, 306, 526 Wire coating, 212 Wire enamels, 463 Wittig reaction, 17, 97, 308 Work function, 25, 30, 108, 119 Yamamoto coupling, 337 Yamazaki reaction, 428 Yarns, 339, 379, 401, 429, 440, 482 Ziegler-Natta catalysts, 8, 319 polymerization, 12
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