Journal of Chromatography Library - Volume 10
GAS CHROMATOGRAPHY OF POLYMERS
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Journal of Chromatography Library - Volume 10
GAS CHROMATOGRAPHY OF POLYMERS
JOURNAL OF CHROMATOGRAPHY LIBRARY Volume 1 Chromatography of Antibiotics by G. H. Wagman and M. J. Weinstein Volume 2 Extraction Chromatography edited by T. Braun and G. Ghersini Volume 3 Liquid Column Chromatography. A Survey of Modern Techniques and Applications edited by Z. Deyl, K. Macek and J. Janak Volume 4 Detectors in Gas Chromatography by J. SevEik Volume 5 Instrumental Liquid Chromatography. A Practical Manual on High-Performance Liquid Chromatographic Methods by N. A. Parris Volume 6 Tsotachophoresis. Theory, Instrumentation and Applications by F. M. Everaerts, 3. L. Beckers and Th. P. E. M. Verheggen Volume 7 Chemical Derivatization in Liquid Chromatography by J. F. Lawrence and R. W. Frei Volume 8 Chromatography of Steroids by E. Heftmann Volume 9 HPTLC -High Performance Thin-Layer Chromatography edited by A. Zlatkis and R. E. Kaiser Volume 10 Gas Chromatography of Polymers by V. G. Berezkin, V. R. Alishoyev and I. B. Nemirovskaya
Journal of Chromatography Library
- Volume 10
GAS CHROMATOGRAPHY OF POLYMERS
V.G. Berezkin, V.R. Alishoyev and I.B. Nemirovskaya Institute of Petrochemical Synthesis, Acurleiny of Sciences of tlie U.S.S.R., Moscow
ELSEVIER SCIENTIFIC PUBLISHING COMPANY AMSTERDAM - OXFORD - NEW YORK 1977
ELSEVIER SCIENTIFIC PUBLISHING COMPANY 335 Jan van Galenstraat P.O. Box 211,Amsterdam, The Netherlands Distributors f o r the United States and Canada: ELSEVIER/NORTH-HOLLAND INC. 52, Vanderbilt Avenue New York, N.Y. 10017
ISBN : 0-444-41514-9 Copyright 0 1977 by Elsevier Scientific Publishing Company, Amsterdam All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher, Elsevier Scientific Publishing Company, Jan van Galenstraat 335, Amsterdam
Printed in The Netherlands
Contents Foreword. by N . S. Nametkin
........................................
IX
Preface to the English edition
..........................................
XI
Introduction
......................................................
1. Basic principles of gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Examples of applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of complex mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of physicochemical quantities .......................... Preparation techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GCequipnient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample injectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Katharometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flame-ionization detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theoretical concepts of the GC separation process ........................ Eddy diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Resistance t o mass transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Qualitative and quantitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Qualitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Standard compound method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method using tabulated data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method of several phases ....................................... Computational methods and correlation ratios . . . . . . . . . . . . . . . . . . . . . . Physicochemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preliminary treatment of the sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quantitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Internal normalization method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Absolute calibration method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Internal standard method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 . Gas chromatographic methods for the analysis of monomers and solvents . . . . . . . Specific features of the GC analysis o f impurities .......................... Methods for improving the sensitivity of the determination of impurities . . . . . . . Increasing the size of the sample being analyzed ........................ Use of high-sensitivity detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of non-isothermal methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concentration methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparative elution chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xi11
1 6 6 7 7 9 10 12 12 12 13 13 20 21 21 22 22 23 23 24 24 25 .25 26 28 28 28 29 33 34 41 41 43 44 47 47
VI
CONTENTS
Chromatography without a carrier gas (high-concentration GC) . . . . . . . . . . Frontal chromatography...................................... Methods for demasking impurities against the background of the main component Utilization of selective sorbents and selective detectors . . . . . . . . . . . . . . . . . . Analytical reaction GC .......................................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . The study of polymer formation reactions ............................... Investigation of polymerization reactions ............................... Study of copolymerization reactions ................................... Study of polycondensation reactions ................................... References ....................................................... 4 . Determination of volatile compounds in polymer systems
...................
Direct analytical methods ............................................ Multi-stage methods for determining volatile components . . . . . . . . . . . . . . . . . . . Determination of volatile components in polymer solutions . . . . . . . . . . . . . . . Application of extraction methods .................................. Application of methods for separating volatile impurities from polymers . . . . Specific features of the GC analysis of solvents. monomers. plasticizers and stabilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47 48 51 51 52 52 59 59 72 76 80 85 87 93 93 100
102 103 108
5 . Study of the kinetics and mechanisms of chemical transformations of polymers at elevated temperatures ............................................ Static methods for studying chemical conversions of polymers . . . . . . . . . . . . . . . Dynamic methods for studying polymer conversion processes . . . . . . . . . . . . . . . . Periodic GC analysis of volatile products ............................. Automatic analysis of volatile products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of GC detectors for the continuous analysis of volatile products . . . . . . . . Impulse pyrolysis of polymers ..................................... Specific features of procedures for studying thermooxidative degradation . . . . . . Application of GC in studying the degradation of polymers . . . . . . . . . . . . . . . . . . References .......................................................
113 114 116 116 121 124 127 131 140 140
6 . Reaction gas chromatography of polymers ............................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
145 156
7 . Pyrolysis gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equipment and experimental procedure ................................. Identification of polymers ........................................... Determining the composition of polymer systems (mixtures and copolymers) .... Non-analytical applications of pyrolysis GC .............................. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References ........................................................
159 160 175 179 183 189 190
CONTENTS
VI 1
8. Inverse gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Identification of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of the molecular weight of oligomers ....................... Investigation of the themiodynamics of interaction of volatile compounds with polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The study of phase transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Investigation of the kinetics and equilibria of chemical reactions . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
206 209 215 221
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
225
195 196 199
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Fore word The extensive use of polynieric materials in industry, construction, space technology, the home and inany other areas is typical of the progress made in the second half of this century, and therefore books dealing with methods for the investigation o f polymer chemistry and for testing polymeric materials are of great importance. This book by V. G. Berezkin and his co-workers V. R. Alishoyev and I. B. Nertiirovskaya is concerned with the application of gas chromatography t o polymer chemistry. The following features of the book should be particularly noted. (1) The book is up t o current scientific and experiniental standards. It generalizes and systematizes a huge body of information scattered over a great number of publications (the bibliographical references in the book exceed 900). The text covers all of the basic fields of application of gas chromatography in polymer chemistry. (2) The text is clear and critically oriented, and the advantages and disadvantages o f the methods described are emphasized. This greatly facilitates the choice of the best method for a particular problem. (3) The book is meant for experimentalists, and the presentation adopted by the authors niakes it useful and valuable both to specialists working in the field of analytical gas chromatography and to researchers who lack appropriate experience. The latter group should first study Chapter 1, which expounds the fundamentals of the chromatographic method, covering sufficient ground for practical application. The book is of interest t o a wide circle of specialists engaged in polymer chemistry. This book was first published in Russian in the U.S.S.R.in 1972. It met with wide acclaim and was out of print in a very short time. I hope that this enlarged English edition will also be well received by readers throughout the world.
CorrespondingMember of the U.S.S.R. Academy of Sciences, Doctor of Chemical Sciences, Head ofA. V, Topchiev Institute of Petrochemical Synthesis, Academy of Sciences of the U.S.S.R.
NIKOLAI SLRGEYEVICH NAMETKIN
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Preface to the English edition At present, gas chromatography is the most widespread method for the analysis of organic compounds. A very extensive and important field of application of gas chromatographic methods is polymer chemistry and technology, although polymers cannot be analyzed directly by gas chromatography. This book reflects the basic principles of the application of gas chromatography in polymer chemistry: monomer and solvent analysis, the study of polymer formation processes, investigations of the disintegration of high-molecular-weight compounds and the study of polymers by pyrolytic and inverse chromatography. Chapter 1 deals with the fundamentals of the gas chromatographic method. With this information, those who have no previous knowledge of gas chromatography and have not used it can apply the methods described in this book in their practical work. Despite the wide application of gas chromatography in research and analysis in polymer science and industry, many of the existing solutions are, unfortunately, far from optimal. This is due to many reasons, one of which is the tremendous output of information, scattered over many, often inaccessible, publications. When writing the book, the authors attempted to carry out a systematic rationalization of the extensive published material. They hope that the use of the text will considerably facilitate the choice of the best method of investigation for a particular problem and thus permit valuable infonnation on substances and processes in polymer chemistry to be obtained. This English edition contains many additions reflecting progress in the application of gas chromatography in polymer chemistry since the Russian edition was published. Chapters 4-6 were written by V. G. Berezkin and I. B. Nemirovskaya, Chapter 7 by V. R. Alishoyev and V. G. Berezkin and Chapter 3 by V. G. Berezkin with the participation of Yu. B. Amerik. The other parts of the book were written by V. G. Berezkin, who also planned the arrangement and did the general editing. The authors thank A. N. Genkin for a number of valuable remarks on Chapter 8 and B. M. Kovarskaya for a useful discussion of Chapter 5. The authors are deeply grateful to Corresponding Member of the U.S.S.R. Academy of Sciences K . V. Chmutov and Correspon’ding Member N. S. Nametkin for their support and encouragement.
V. G . BEREZKIN V. R. ALISHOYEV I. B. NEMIROVSKAYA
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Introduction The success of any scientific research depends largely on the rational choice and use of experimental methods. Many modern processes in industry have become possible only as a result of the development of new, efficient methods of control. In chemistry, which is the science of the structure and transformation of substances, the central experimental problem often consists in determining the composition of complex mixtures, trace impurities that contaminate the main substance and physicochernical characteristics of substances. The achievements in chemistry and the chemical industry during the past two decades are due in many respects t o the vigorous development of gas chromatography (GC), the wide use of which has led t o revolutionary changes in methods of organic and gas analysis and in many physicochemical methods. This development is attributed t o the following features of GC. ( I ) GC is a universal method; in a single analysis, it is possible to determine the qualitative and quantitative composition of a complex mixture containing up t o several hundred volatile components. ( 2 ) High-sensitivity detectors, which are used for recording the results of chromatographic separations, permit the determination of harniful impurities present in concentrations as low as I O F - 1O-I" %. (3) GC is a rapid method that readily lends itself t o automation. (4) GC can be used successfully for determining both equilibrium distribution values and kinetic and diffusion characteristics o f the systems under investigation (volatile standards and solid or liquid stationary phases). GC is also widely applied in polymer chemistry, although under ordinary conditions high-molecular-weight compounds are not volatile, and the chromatography of polymers at high pressures is in its early days. This is due to the fact that in polymer chemistry manv important processes (for instance, polymer formation or degradation reactions) involve, as initial reactants or reactions products, low-molecular-weight substances that can be successfully analyzed by classical GC methods. GC methods are not always used in their optimal version, however, and they are applied very non-uniformly t o different problems in polymer chemistry. GC is used most widely in those fields where the forms of its application are traditional. Thus, GC is the basic method for determining impurities in monomers and solvents for polymerization and is widely applied in studyingvolatile degradation products. It is used much less for investigating the thermodynamics of the interactions of,standard volatile compounds with high-molecular-weight compounds by the method of inverse GC. Pyrolysis GC, in which the polymer system under investigation is characterized by means of the volatile pyrolysis products, is probably the only example of a method developed jointly by researchers working in the fields of GC and polymer chemistry, a method widely used for the identification of polymers, the quantitative analysis of copolymers and the determination of their structures. There is no doubt, however, that in the near future other modifications of GC will be developed specially for polymer studies. The authors hope that this book will arouse interest in its readers and will promote a wider application of GC methods in polymer chemistry. The authors have systematized and generalized the different applications of GC methods in polymer chemistry and have attempted to outline some future prospects for development of these methods in this important field.
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Chapter I
Basic principles of gas chromatography The development of gas chromatographic (CC) methods has led to revolutionary changes in analytical chemistry and also in experimental methods of physical chemistry and methods for the separation of volatile compounds, as GC has a number of important advantages over other methods [ l ] . Firstly, GC is universal and can be applied to the analysis of a very wide range of substances from hydrogen isotopes to oligomers and metals, with boiling points within the range -250-1000°C. The method enables one to obtain, in a single analysis, information not only on a single substance but also on the contents of all (or most) of the components present in a complex mixture. Secondly, the practical application of the method is simple, the equipment used being of a standard type with automatic recording of the results of an analysis. Thirdly, GC ensures a high efficiency of separation within a relatively short analysis time (1 -30 min). The development of GC is still continuing. About 1500-2000 papers are published annually in this field, and the total number of publications up to 1970 was about 20,000 [2]. A study of the published information indicates the major role of gas chromatography among methods for the analysis of organic compounds and gases. Thus, for instance, in 1974, the papers in the principal analytical journals (Analytical Chemistry, Zeitschriff fur Analytische Chemie, Analyst (London),Zhurnal Atialiticheskoi Khimii and Zauodskaya Laboratoriya) were distributed as follows: all types of chromatography, 58.8%; GC, 34.6%;spectral methods, 25.7%; electrochemical methods, 6.6%; other methods (colunietric, gravimetric and others), 8.9%. Thus, in the analytical chemistry of organic compounds and gases, the total number of papers on GC exceeds those on spectral methods, and one in three papers is based on GC. The development of CC began after James and the Nobel Laureate Martin published the first work on gas-liquid chromatography [3] . Chromatography as a general method of separation, however, was first discovered in 1902 by the Russian botanist Mikhail Seniyonovich Tsvet (1 872-1919), who proposed an adsorption chromatographic method of separation in the liquid phase and described its application to the analysis of chlorophyll in plants [4]. The basic units of the chromatograph are the chromatographic column and the detector. The column (Fig. 1 .I, C) separates the test mixture into its components and the detector (Fig. 1 . l , D) records (in the carrier gas flow) the concentrations of the separated Components. The results of the separation are recorded automatically. Fig, 1.I shows schematically the separate stages of the chromatographic separation of a three-component mixture, illustrating the positions of the chromatographic zones in the column at definite time intervals, and the relationship between the separation process and the recorded chromatogram. At the moment of injection of the test mixture, the zones of all three substances are located at the head of the column. Under the action of the flow of carrier gas, the components of the mixture begin to move along the column Keferences p. 29
:
BASIC PRINCIPLES 01;GC
2
(a)
C
i
t
t
t
i
t
1:ig. 1.1. Schematic representation of the Chromatographic separation of a three-component mixture. (a) Dynamics of chromatographic process (position of chromatographic zones in column at definite time intervals); (b) recorded chromatogram.
at different speeds, which are determined by the nature of the compounds being separated and the type of sorbent used. The first t o be eluted from the column is component 1 (sloping hatching) then component 2 (vertical hatching) and finally component 3 (horizontal hatching). Separation in GC is based on different distributions of the molecules of the components being separated between the mobile gas phase and the stationary phase. A dynamic equilibrium is established between these phases for each component of the test mixture. Under the action of the flow of carrier gas, the components of the test mixture move along the chromatographic column with different speeds. The speed of this motion depends, for each component, on its distribution constant between the gas and stationary phases. The speed of motion of a chromatographic zone is inversely proportional to the distribution constant, i.e., readily sorbable Components move along the sorbent layer more slowly that sparingly sorbable components. A quantitative description of the elution process in GC can be obtained most readily by kinetic treatment of the e!ementary processes of the motion of the molecules of the
BASIC PRINCIPLES OF C C
3
test compounds in the column. It is assumed that the following conditions are fulfilled in a chromatographic separation: (1) the molecules of the test compounds are in dynamic equilibrium between the gas and stationary phases, and this equilibrium is independent of the presence of other components in the sample; (2) the molecules of the test conipounds move along the column only in the gas phase; ( 3 ) the carrier gas velocity, the temperature and the properties of the sorbent are constant along the length of the column and across its section, and the pressure drop can be neglected. In the course of separation in the column, a definite proportion of the niolecules of a given coniponent is in the gas phase at any instant, namely nm/(n, + n m ) , where nm and n, are the numbers of molecules of the given component in the mobile and stationary phases, respectively. Consequently, if the total retention time of this type of molecule in the column is t R , the average retention time of a molecule in the gas phase is tRn,,/(n, + nm). As the molecules are moving along the column only in the gas phase, the molecules of the test compound will pass through a Column of length L , with a mean carrier velocity zl, within a time f, (n,/n, n,). Therefore, the following equation holds:
+
Using eqn. 1.1, one can determine the retention time as the average time for the passage of the molecules of the test substance from the head to the end of the column:
L n tR = Y - ( l + 2 ) Id
m
Considering that the gas hold-up is L/ii and nJnm = K,(V,/V,), where K , is the distribution constant of the substance between the liquid and gas phases and V,/V,, is the ratio of the volume of the stationary phase t o that of the mobile phase in the colurnn, we obtain, after simple rearrangements:
or
where tN is the net retention time and F is the volume velocity of the carrier gas as measured at the column temperature. Eqn. 1.4 can be used to obtain an expression for the net retention volume, V N . The retention volume is the volume of the carrier gas necessary to elute a compound from the column under given conditions:
V,=t,F=K,V, (1.5) where VN is the net retention volume, VM is the hold-up volume of the column and VR is the retention volume (in the absence of a pressure drop). Hence, under standard experimental conditions, the retention volume (adjusted in ternis of the hold-up volume of the column) is directly proportional to the distribution I/,=
V,
-
References p. 29
BASIC PRINCIPLES OF GC
4
constant of a given compound between the mobile and stationary phases and is a value characteristic for a given compound. This means that each substance, independent of its concentration in the sample, will be eluted from the column after a definite time, which is characteristic of the substance. The retention time is the same type of constant for a given compound as other widely used characteristics such as the boiling point and specific gravity. Eqn. 1.5 substantiates the use of GC in the qualitative analysis of the components of a mixture and in measuring distribution constants. Fig. 1.2 exhibits the relationship between the distribution constant and the retention values of the substances being analyzed. Fig. 1.2a shows the distribution isotherms of substances A and B, and Fig. 1.2b indicates the position of the chromatographic zones in the column a certain time after the beginning of the analysis. Substance B is sorbed better than substance A by the stationary phase (KO,,, KD,*). Therefore, most of the molecules of B will be in the stationary phase and a smaller proportion in the carrier gas flow. For substance A, we have the opposite situation. Therefore, the zone of substance A will move along the column faster than the zone of substance B. At the column outlet, the separated components of the test mixture proceed, in the carrier gas flow, t o the detector, the response of which is proportional to the concentration or flow-rate of the components of the test substance in the carrier gas. The detector readings are recorded automatically by an electronic potentiometer. The diagram obtained, which reflects the results of the chromatographic separation, is called a chromatogram. A typical chromatogram of a hydrocarbon mixture is shown in Fig. 1.3. On the basis of the chromatogram one can determine the qualitative composition of the mixture analyzed. Let us consider the basic elements of a chromatogram [5]. The baseline is the portion of the chromatogram (for instance, between peaks 1 and 2) obtained when only the carrier gas is eluted from the column. A chromatographic peak is the portion of the chromatogram corresponding to the detector signal when one or several components are eluted from the column. The retention time ( t R ) is the time elapsed from the moment the sample is injected into the column to the appearance of the peak maximum. The hold-up time (tM)is the retention time of a compound that is not sorbed by a given stationary phase. The adjusted retention time It;) is the total retention time less the hold-up time ( t i = t R - tM).The peak width 010) is the segment of the zero line obtained (a)
I
(b) A
0
a
Fig. 1.2. Distribution isotherms (a) and chromatographic separation (b) of a two-component mixture of A and B. a and c are the concentrations of the compounds analyzed in the stationary and mobile phases, respectively.
BASIC PRINCIPLES OF GC
5
\ 1 IU-..L',L 12
8
4
, 0
Tlme (min)
Fig. 1.3. Parameters of a chromatogram. Separation conditions: chromatograph, Tsvet-4, column 200 x 0.4 cni filled with 10%Apiezon K o n Chronlosorb P; temperature, 6 5 ° C . Peaks: 1 = air; 2 = cyclohexadiene; 3 = cyclohexane; 4 = rnethylcyclohexane. The designations of the parameters are explained in the text.
by interpolation of the baseline in the interval from the beginning to the end of the peak. In chromatographic practice, use is generally made of the peak width at half-height (/A, s), which is easier to determine from the chromatogram. The peak height ( h ) is the distance from the peak maximum to its base, measured in a direction parallel to the detector signal axis. The peak area (S) is the area enclosed between the line bounding the peak and its base. Qualitative analysis in CC is carried out on the basis of measurement of the retention times (or retention volumes). It is more convenient to identify unknown components by using relative, rather than absolute, retention values. The relative retention time (or References p. 29
6
BASIC PRINCIPLES OF GC
relative retention volume), r,,,,
is determined by the equation
VN , A - K D,A tR,B-tMB vN9B KO,E Relative retention values are determined exclusively by the distribution coefficients of a given compound and the compound used as the standard; they are independent of such experimental conditions as the carrier gas velocity, the amount of the stationary phase, the column length and the sample size. The literature contains a great number of experimental data on relative retention values of various compounds on different stationary phases (see, for instance, refs. 6 and 7). Peaks of unknown components of the mixture being analyzed are identified by comparing the relative retention times of the maxima of these peaks, which are determined directly from the chromatogram, with the tabulated values for known compounds. In developing the chromatographing procedure, after the problem of the chromatographic separation of the final sample components has been solved, their quantitative determination must be carried out. The size of the signal given by the detector used in GC is directly proportional to the concentration of a component in the carrier gas under constant experimental conditions: - "R.A
r~~~-
= 'R,A-
tM,A -
tlR,B
h(t) = l/Ric(t)
(1 *7)
where h is the detector signal, c(t) is the concentration at time t and Ri is a constant. Therefore the amount, i, of the component being analyzed is directly proportional to the area of its chromatographic peak:
q i= J c(t)dt = J Rih(t)dt = R, J h(t)dt = R , S
(1.8)
The content of the jth component in the mixture can be calculated by the following equation:
KS..100
p. = 1 1 ZKiS, This is the simplified scheme for the interpretation of chromatograms. In conclusion, we shall give some examples that characterize the main types of application of the method.
EXAMPLES OF APPLICATIONS Analysis of complex mixtures GC is widely used for the analysis of complex mixtures, beginning with the separation of methane molecules containing different hydrogen isotopes (Fig. 1.4a [8] ) and ending with high-boiling oligomers of organosilicon compounds (Fig. 1.4b [9] ). Usually, the chromatographic analysis lasts a few minutes, tens of minutes or more rarely, hundreds of minutes. Some mixtures, however, can be analyzed within a few seconds (Fig. 1 . 4 ~[lo]). It should be noted that GC is used not only for laboratory analyses but also for the control and regulation of engineering processes. Fig. 1.4d [ 111 shows a chromatogram
I
EXAMPLES OF APPLICATIONS
of an analysis of a sample collected from a flow of a polymerizate of a synthetic ethylene-propylene rubber (SKEP) using a Soviet-designed process chromatograph (KhP-2 16). The problems involved in the analysis of complex mixtures have been discussed in books [12, 131. Determination of physicocheinical quantities GC methods are widely used for detennining physicocheniical characteristics such as distribution coefficients, activity coefficients, heats of solution, heats of adsorption, adsorbent surface areas, coefficients of diffusion in gas and liquid phases and rate constants of heterogeneous and homogeneous reactions [ 14-17] . Fig. 1.5 [ 181 illustrates two chromatograms of the decomposition products of tert.-butyl hydroperoxide. Chromatograni (a) was obtained at the beginning of the reaction of the decomposition of the peroxide, and chrotnatogram (b) during the course of the reaction. The decomposition of the rut.-butyl hydroperoxide leads to shrinking of the peak of the hydroperoxide (5) and to the appearance of new peaks of the reaction products on the chromatograni (1-3). Chromatographic study of changes in the concentrations of the initial compounds and reaction products with time enables one t o obtain, in most instances, the infomiation necessary for describing the kinetic behaviour of the system. GC methods are also widely used for studies of adsorption phenomena and measurement of the surface areas of soIids. As an exaniple, Table I .1 [ 191 lists data that characteriLe the good agreement between surface areas measured by classical methods and b y GC. GC enables one t o obtain within a much shorter time, results that are comparable in accuracy with those of classical methods. Preparation techniques GC can be used for isolating pure components from a mixture. Automated preparative equipment is available in which sample injection, separation and collection of pre-assigned
rABLk 1 1 COMPARISON or suRr ACL AREAS or CATALYSTS AS D L T L R M I N L D BY CHROMATOGRAPHIC AND STATIC MrTHODS I’ROM ARGON ADSORPTION [ 191 Saniple
Specific surface area (mz/g) Volume niethod (on vacuiini set-up)
Chelate polynicr Molybdenum f o 11 Titanium oxide Silica gel A h niin a Aluniinosilica tc Silicoii-magnesia catalyst __ References p. 29
0.08
-.
4.2 20.3 135 336 414
Difference (%)
Chromatographic method of therriial desorption
0.085 0.01 4.2 20.3 132 332 398 -
1-6.3
0.0 0.0 -2.2 -1.2 -3.9
BASIC PRINCIPLES OF GC
8
(a)
I
I
I
300
I
320
310
I
I
330
340
Time (mtn)
2
1
k I
l
I
6
Time bet)
I
I
12
2
I
I
I
60
40
20
Time (min)
9
EQUIPMENT
1
40
-
L
1
1
~~
2o
1 1
40
20
1
J
Time i m i n )
Fig. 1.5. Chromatogram of a solution of ferf.-butyl hydroperoxide in the presence of manganese stearate (a) at the beginning of and (b) during the reaction. Conditions: column, 120 X 0.4 cm; sorbent, 30% dinonyl phthalate o n Celite-545; temperature, 50°C. Peaks: 1 = acetone; 2 = fer?.-butanol; 3 = ?err.-butyl peroxide; 4 = toluene (internal standard); 5 = ?err.-butyl hydroperoxide;6 = chlorobenzene (solvent).
fractions is carried out automatically. In order to obtain pure reactants, including monomeric solvents, methods of preparative chromatography are used on an industrial scale [20-231.
GC EQUIPMENT Fig. 1.6 [24] shows schematically a simple gas chromatograph. The carrier gas proceeds from a cylinder (1) through a reducer (2), a pressure regulator ( 3 ) and a flow stabilizer (4) to the reference cell of the detector (6), then through the sample introduction system (7) to the chromatographic column (9), which is located together with
Fig. 1.4. Examples of the application of gas chromatography. (a), Chromatogram of isotopic molecules of methane [8]. Conditions: capillary glass column (47 m X 0.22 m m I.D.), on the inner walls of which was a layer of active silica formed as a result of treatment of the glass capillary with a 10% solution of sodium hydroxide at 100°C; temperature, 77°K; carrier gas, nitrogen-helium (7:3); flow-rate, 1 ml/min. The upper chromatogram was obtained with the use of an ionization chamber as detector and the lower chromatogram with a flame-ionization detector. Peaks: 1 = I4CH,; 2 = CH,,H; 3 = CH,'H,; 4 = CH'H,; 5 = C3H,; I ' = "CH,; 2' = I3CH,; 3' = CH,IH; 4' = CH 12H1 '3 5' = CH,,H,; 6' = C'H,. (b), Chromatograni of high-boiling organosilicon compounds [ 9 ] : CH, 1,-0-SiCH,(C,H,), .Conditions: column, 100 X 0.4 cm; sorbent, (C,H,),CH,Si-fO-Si< C*HS 6.85%PFMS-6 on INZ-600 treated with dimethyldichlorosilane vapour; temperature, 354°C. Peaks: 1, n = 0; 2, n = 1; 3, n = 2; 4, n = 3. (c), Chromatogram of the rapid separation of a gas mixture [ 1 0 ) . Conditions: column, 100 X 0.4 cm; sorbent, 20% molecular sieves CaA o n Celite; temperature, 20°C. Peaks 1 = hydrogen; 2 = oxygen; 3 = nitrogen; 4 = methane; 5 = carbon monoxide. (d), Chromatograms of the analysis of a flow of the polymerizate of an ethylenepropylene copolymer n4th a KhP-216 process chromatograph. Peaks: 1 = ethylene; 2 = propylene. References p. 29
10
BASIC PRINCIPLES OF GC
----
-7
!----- -1 I
I
4I’
Fig. 1.6. Simple gas chromatograph. For explanation, see text.
the detector in a thermostat (10). The pressure at the column inlet is measured by a pressure gauge (5) and the volume velocity of the carrier gas is checked periodically by a foam meter (1 1). The sample is injected with a syringe (8) into the flow of carrier gas upstream of the column through a sample injector (7). The flow of carrier gas carries the sample to the column where its components are separated into separate zones. The separated substances (chromatographic zones) enter the detector (6), which determines the concentration (or mass flow) of the components in the carrier gas. The detector signal, which is proportional to the concentration (or mass flow), is automatically recorded by a potentiometer (12). Detailed consideration of the separate units can be found elsewhere [25,26]. Sample injectors
In laboratory practice, gaseous and liquid samples are usually injected into the chromatograph with syringes similar to medical syringes. A simple device for introducing the sample into the chromatograph with a syringe is shown in Fig. 1.7a. The syringe needle is introduced through a rubber gasket into a heated evaporator through which a continuous flow of carrier gas is passing. The sample to be injected rapidly evaporates and is transferred to the chromatographic column in the vapour state. In chromatography, gaseous samples are usually injected by means of a system with detachable tubes of known volume [27] , a diagram of which is given in Fig. 1.7b. The same idea was used in developing an automated system for injecting gaseous samples with diaphragm valves [28]. For automatic injection of samples from a liquid flow, use is
EQU IPM I:NT
11
(b)
3
14
Pig. 1.7. Sample injectors. (a), Sample injector,for introducing samples with a syringe: 1 = carrier gas inlet; 2 = rubber seal; 3 = heater; 4 = to chromatographic column. (b), Injector forgaseous samples: 1 = Ilow of gas to be analyzed; 2 = Ilow of carrier gas; 3 = chromatographic column; 4 = saniple injection loop of known volume. (c), Automatic sample injector with moving rod forliquid samples: 1 = seal; 2 = membrane seal; 3 = moving rod; 4 = injection volume; 5 = body; 6 = sample flow; 7 = flow of carrier gas.
made of sample introduction systems with a moving rod (Fig. 1 . 7 ~ )The . role of the calibrated sample injection volume is played by the channel in the rod. As the rod moves, a definite volume of the liquid sample filling the calibrated channel is transferred from the flow of sample into the flow of carrier gas, where the liquid sample is evaporated and the vapour is carried to the chioinatographic column by the flow of carrier gas. Capillary colunins require only very small samples for analysis. This problem is usually solved not by designing miniature sample introduction systems, b u t by using the flow division method. Thus, only a very small portion of the carrier gas containing part of the sample is directed t o the capillary column. References p. 29
12
BASIC PRINCIPLES OF GC
Columns In a chromatographic column, the components of a mixture are divided into separate zones. At present, two basic types of chromatographic column are used, namely packed and capillary columns [29,30]. The packed columns can be subdivided into preparative columns (diameter greater than 10 mm), analytical columns (diameter 3-6 mm) and capillary packed columns (diameter 0.5-2.0 mm). The length of the column with the packing is 0.8-10 m. Capillary columns (diameter 0.2-0.6 mm) are generally used without a packing, the inner walls being coated with a film of the liquid stationary phase. The length of capillary columns is 20-100 m. In spite of their high efficiency, capillary columns are used much more rarely than packed columns because high-efficiency capillary columns with reproducible characteristics are more difficult to prepare. The material of construction of chromatographic columns must be resistant to adsorption and catalytically inert. In most instances, columns made of stainless steel, glass, polymers and copper are used. Packed columns are filled with narrow fractions of either solid adsorbents with a developed surface or solid supports whose surface is coated with a layer of the liquid stationary phase. In order to reduce the adsorption and catalytic activity, mineral solid supports are treated with the vapour of dimethyldichlorosilane or hexamethyldisilazane, which deactivate the hydroxyl groups of the surface. Diatomite supports are used most often and for polar compounds good results are obtained by using inert polymer supports. Detailed information on solid supports is given elsewhere [31,32]. Detectors
A detector determines quantitatively the concentration (mass flow) of the test components in the carrier gas after they have been separated in the chromatographic column. The characteristics of the detector largely determine the accuracy and sensitivity of the entire analysis and the detector is therefore one of the most important units of the chromatographic installation. Hence “the history of development of gas chromatography is to some extent the history of development of the detector” [33]. Two types of detectors are widely used in GC: concentration detectors, whose readings depend on the concentration of the substance in the carrier gas, and mass (flow) detectors, whose readings are determined by the rate of feed (mass flow) of the test substance carried to the detector in the flow of carrier gas. The readings of the concentration detector depend only slightly on the flow-rate, whereas those of the flow-rate detector change sharply with the flow-rate. An example of the first type of detector is the katharometer, and of the second type the flameionization detector. We shall now consider the different types of detectors. Katharometer The principle of operatir. of the katharometer is based on the change in the electric resistance of the sensor (filament, coil) in relation to the heat conductivity of the gas leaving the GC column. The heat conductivity of the gas in the low-concentration range
THEORETICAL CONCEPTS OF THE SEPARATION PROCESS
13
depends linearly on the concentration of the eluted substances in the flow of carrier gas. Therefore, the resistance of the sensor changes linearly with the concentration of the detected substance in the flow of gas leaving the column. Use is generally made of the differential method, in which the working and reference cells of the detector are wired as a Wheatstone bridge, and a flow of pure carrier gas passes through tlie reference cell. When pure carrier gas passes through both cells, the bridge is in equilibrium. When a zone of a substance is eluted from the column, the composition of the gas mixture in the working cell changes, and so do the temperature and resistance of the sensor wire, and the potentiometer records the imbalance of the bridge, which is proportional t o the concentration of the substance in the carrier gas. The main advantage of the katharometer is its universality. The katharometer can be used for detecting permanent gases, various inorganic compounds (including such aggressive coinpounds as nitrogen dioxide, hydrogen chloride and fluoride gases, if a katharometer of special design is used) and vapours of organic compounds. In quantitative calculations, it is necessary to take into account'that the detector signal depends on the type of compounds being examined [34-391. Wide use is also made of the gas density balance [40-421, the sensitivity of which is slightly lower than that of the katharometer. The gas density balance, however, has the following advantages over the katharometer: (1 ) no preliminary calibration is necessary for qualitative analysis; (2) analysis of more aggressive gases is possible, because the vapours of the test substances do not come into contact with the sensitive elements; and (3) readily available gases are used as carrier gases.
E'larn~-ionizutb~~ detector The flame-ionization detector [43,44] is widely applied for detemiining organic coinpounds and especially impurities. The principle of operation is based on a sharp decrease in the electric resistance of a hydrogen flame when trace amounts of organic compounds, which forni ions in tlie course of oxidation, are introduced into it. These ions are collected at the electrodes, one of which is usually a burner nozzle. The very low ionization current that then arises is amplified and recorded by a potentiometer. A pure hydrogen flame usually generates a background current of the order of 10-"-10-12 A; when the test organic substances are introduced into the flame, currents of 10-12-10-7A are generated. Fig. 1.8 shows a flame-ionization detector of the type DIP-2. In recent years, selective ionization detectors [45-481 have been increasingly applied: the electron-capture detector for determining halogen-containing compounds [49], the themionic flame detector for phosphorus- and nitrogen-containing compounds [SO] and the mass spectral detector for most compounds [ S l , 521.
THEORETICAL CONCEPTS OF THE GC SEPARATION PROCESS
I n GC, when the Lones of the test compounds are nioved along the sorbent layer by the flow of carrier gas, two opposite effects occur simultaneously; the distance between the concentration maxima of the chromatographic zones of adjacent components References p. 29
BASIC PRINCIPLES O F GC
14
I
36 mrn
I
Fig. 1.8. Flame-ionization detector, type DIP-2: 1 = body; 2 = electrode/burner; 3 =diffuser for air supply; 4 = electrode/collector; 5 = upper detachable cup; 6 = air inlet orifice; 7 = inlet of eluate with hydrogen.
increases (this effect improves the separation) and so does the width of the chromatographic zones (this effect impedes the separation). The theory of GC explains the relationship between separation and the experimental parameters, and also the observed regularities in the two basic chromatographic characteristics, i.e., the retention value and peak broadening.
THEORETICAL CONCEPTS OF THE SEPARATION PROCESS
15
The retention volume (see eqn. 1.5) is directly proportional to the distribution constant of the test c o n ~ p o u n dbetween the liquid and gas phases and t o the volume of the liquid stationary phase in the column. This equation has been used for determining the distribution constant of organic con~poundsin the gas-liquid system. The values of the distribution constants are in good agreement with the corresponding values obtained by the static method [53], thus supporting the validity of eqn. 1.5, which is the basic equation of GC. It should be noted, however, that in the general case, when the distribution isotherpi is non-linear, the distribution constant in the retention volume equation is a function of the concentration. In this instance the retention volume also depends on the concentration, and this effect leads t o the formation of asyminetrical Chromatographic zones. When determining retention values in CC it is necessary to take into account the compressibility of the carrier gas [3], due to which the flow-rate, pressure and density of the carrier gas vary according t o a definite law along the length of the column. Let us determine the retention value, making an allowance for the compressibility of the carrier gas. Taking into account eqn. 1.3, we can write the following expression for the retention time: (1.10)
To calculate the integral, we shall use Boyle’s law: UP
(1.1 1)
= UOPO
and Poiseuille’s l a w (1.12) where u and p are the linear velocity and pressure of the carrier gas at some point x in the chromatographic column, uOand p o are the linear velocity and pressure, respectively, of the carrier gas at the column outlet, kpem,,is the permeability constant and 77 is the viscosity of the carrier gas. Expressing u and dw from eqns. 1.1 1 and 1.12, we can calculate the integral (1.13) where p i is the pressure at the column inlet. The value of the term kpenn./77u0p0can be found from the equation (1.14)
or kpenn. =
2L
- 77UOPO P’- P i References p. 29
(1.15)
BASIC PRINCIPLES OF GC
16
Thus (1.16) and hence (1.17) or (1.18) where j is the pressure-gradient correction factor of the gas: (1.19) tk is the adjusted retention time taking into account the pressure drop in the column. The corrected retention volume,yi, and the corrected gas hold-up volume of the column, pM, can be calculated by similar equations: = VRj
v$=
(1.20) (1.21)
VMj
V i is the retention volume that would have been measured if the carrier gas were incompressible. Because the correction for compressibility is less than unity, the corrected retention volume is less than that observed at the column outlet. Fig. 1.9 shows the dependence of the pressure-gradient correction factor on the ratio of the pressures at the inlet and outlet of the column. It can be seen that the pressuregradient correction factor greatly depends on this pressure ratio. As an illustration, let us consider the determination of the corrected volume from the experimental results. As a result of experimental measurements, the following experimental parameters were determined: retention time of the test substance, 6.0 min; retention time of helium (non-sorbable component), 0.5 min; carrier gas velocity measured at 25°C (298°K) and at an atmospheric pressure of 760 mmHg, 50 ml min; column temperature, 100°C (373°K); water vapour pressure at 25"C, 24 mmHg; gauge pressure at the column inlet, 900 mmHg. First, we shall determine the true velocity in the chromatographic column. In measuring the carrier gas volume velocity, Fo, with a foam velocity meter, one must take into consideration the correction for the water vapour pressure, p H I O ,at the measurement temperature, T,, and the correction for the gas volume due to the difference between the column temperature, T,, and that of the velocity meter, T , : (1.22) In this instance: F O = 5 O (298 =)
[l-
(g)]=60.5cm2/min
THEORETICAL CONCEPTS OF THE SEPARATION PROCESS
17
c i
1
3
3
-b L-
4
5
6
PI /Po
Fig. 1.9. Dependence of the correction for the compressibibty of the carrier gas o n the ratio of the pressure at the inlet and outlet of the column and o n the pressure drop. The pressure at the inlet is 1 atm.
Let us calculate the retention volume, V A = fRFo= 6.0 min X 60.5 ml/min = 363 ml, and the corrected retention volume, taking into account that p i / p o = 1.18, j = 0.98 and = j V R = 334 ml. Subtracting the corrected gas hold-up volume (V; = jFdM = 29.6 ml) from the corrected retention volume, we obtain the value of the net retention volume, VN = 304 ml. Absolute retention values are used mainly in determining physicochemical quantities (activity coefficient, distribution constant, etc). As noted above, for identification purposes use is made of relative retention values or functions of relative retention values. The second basic characteristic of the chromatographic process of separation of a compound is the broadening of its chromatographic zone. A sufficiently general and formal description of zone broadening was given by Martin and Synge [54] on the basis of the theory of theoretical plates. In this'theory the chromatogi iphic column is considered as a system consisting of a set of successive sections. Each section is a theoretical plate in which equilibrium of the test compound between the liquid and gas phases is established instantaneously. The chromatographic process is simulated as two operations that are repeated many times, namely (1) instantaneous transfer of the mobile phase from a given plate to the next plate in the absence of mass exchange between the phases, and (2) establishment of equilibrium of the test compound between the gas and stationary phases on each plate. For instance, let a column contain five theoretical plates (Fig. 1 .lo) [ 5 5 ] . In the initial position, each plate is filled with a gas, and the zero plate contains a sample of the test compounds A and B, one of which (A) is not sorbed in the stationary phase, while the other (B) is sorbed; the mass distribution ratio D, = 1, i.e., half of the molecules of substances B are in the mobile phase and the other half in the stationary phase. We introduce into the column a volume of pure carrier gas equal to the volume of the gas phase of the plate. The gas phase of the zero plate (together with substances A and B References p. 29
BASIC PRINCIPLES OF GC
18 ........ o c .......
I
0 0
, .
.. ..
.. ..
..
..
O Q 0.3
3
. . . . . . . . . . . .
.. .. .. ..
.... . . o o
0 0 ' 0 0
. . . . . . .
. . 0 "
I
I
I
I
I
I
I
0 0
. . . . . .
1
..
. . . . . . .
.. . " 0 0 0
.. .. .. ..
.. ..
....
..
..
0 0
0 0
0 0
..
Pig. 1.10. Schematic representation of the chromatographic process based o n the theory of theoretical plates for compounds A (not retained by the stationary phase) and B (retained by the stationary phase). 0-4, successive stages of the chromatographic process. The plot depicts the distribution of concentrations of compounds A ( 0 ) and B ( 0 ) after five stages of gas phase transfer. Upper part of column, gas phase; lower part, liquid stationary phase. Q = Amount of substance analyzed; 1 = distance from the column inlet.
which are located in it) will go to the first plate, the gas phase of the first plate to the second, and so on. In the zero and first plates the substance will be distributed between the two phases in equilibrium. This process will be repeated upon injection of each new portion of carrier gas. After four elementary volumes of carrier gas have passed through the column, the substance will start to be eluted from the column. The elution curve of substance B is shown in Fig. 1.1 1 [ 121 . The asymmetry of the elution curve is due to the excessively small number of plates (N). At N > 50, the peaks are already almost symmetrical, and at N > 100 the chromatographic zones correspond to the Gauss equation. The solution of the problem leads to the following equation for the chromatographic zone based on the theory of theoretical plates: (1.23) where n is the number of theoretical plates in the column, q is the size of the sample analyzed, and V is the volume of the carrier gas that has passed through the column. From eqn. 1.23, it follows that
I
' R C
c
max.
u"u
max.
v; (2?T)f
(1.24)
In estimating the number of theoretical plates in the column, the real process is compared with the above-described ideal process of separation, which yields the same results. The characteristics of zone broadening in the theory of theoretical plates is the number of theoretical plates of the chromatographic column.
.
T H E O R r T I C A L CONCI PTS O F T H E SEPARATION PROCFSS
19
I 010
“v
rig. 1 . 1 1. Elution curve tor n column of five theorctical plates calculated o n tlie basis of the theory of theoretical plates. cy = Fraction of substance emerging from column; r i v = number of volumes of carrier gas passed through column and corresponding to one theoretical plate.
Using eqn. 1.23, one can propose a method for detemiining the number of theoretical plates. The peak width at a height c = c,,,,,/e can be calculated from the equation (1.25) Solving this equation, we obtain
n=2
(
v:: v; ve
(1.26)
-~
Considering that V g = V e = (1/2)be, where b, is th width of th chromatographic peak at a height h,,a,./e, we have (1.27) I n practice, the following equation is usually used for calculating the number of theoretical plates : (1.28) where x is the chart distance from the moment of sample injection t o the emergence of the peak maximum and)’ is the peak width at half-height. The efficiency of a chromatographic column increases with the length of the column used. Therefore, a inore invariant value is the height equivalent to a theoretical plate (HETP), o r h). h = L/n
(1.29)
The concept of the chromatographic process in the theory of theotetical plates is rather fomial; this theory does not consider the actual causes of chromatographic zone broadening. Van Deeniter er al. [56] and Klinkenberg and Sjenitzer [57] developed a velocity theory in which Lone broadening in a packed column is attributed to a number of kinetic causes. References p. 29
20
BASIC PRINCIPLES OF GC
v (rnl/rnin)
Fig. 1.12. Dependence of HETP on linear velocity of carrier gas. 1 =Contribution to HETP from molecular diffusion; 2 = contribution from mass transfer; 3 = contribution from eddy diffusion.
According to the velocity theory, the dependence of h on u is expressed by the equation h = A + (B/u) iCu
(1.30)
The first term of this equation ( A ) reflects the contribution to HETP from eddy diffusion, the second (Blu) from the molecular longitudinal diffusion and the third (Cu) from the resistance to mass transfer. Fig. 1.12 illustrates the dependence of HEPT on the carrier gas velocity and shows the contributions corresponding to each process that causes zone broadening. Further development of the theory of spreading in chromatography was carried out by Giddings [ 5 8 ] . We shall now consider in more detail the separate groups of processes that lead to zone broadening .
Eddy diffusion In any packed column, zone broadening is due, in particular, to the many possible routes, of different lengths, by which the molecules of the test substance move along the column in the flow of carrier gas, Le., to the multiplicity of channels along which the carrier gas moves in the packing. Therefore, depending on the length of the route, some molecules will reach the end of the column earlier and others later compared with the average molecule hold-up time in the column. Thus, the multi-route progress of the carrier gas through the packing layer leads to broadening of the chromatographic zone. This cause of broadening is called eddy diffusion. In Van Deemter’s equation, eddy diffusion is expressed by the term A , and A = 2hdp
(1.31)
where A is a coefficient characterizing the shape of the particles and the uniformity of their packing in the column, while dp is the average diameter of the sorbent grains. The value of A is independent of the nature of the test compound, its retention value and the nature of the carrier gas. A more rigorous description of eddy diffusion was given by Giddings [ 5 8 ] .
THEORETICAL CONCEPTS OF T H E SEPARATION PROCESS
21
Molecular diffusion
In the course of separation, there is always a concentration gradient in the gas phase of a chromatographic zone and molecular diffusion therefore always takes place in the gas phase, leading to peak broadening. The contribution from molecular diffusion is reflected in Van Deeniter’s equation by the second term (1.32) where y is the obstruction factor, which takes into account the sinuosity of the diffusion paths in the packing and Dg is the diffusion coefficient of the test substance in the carrier gas. The factor y is equal to or less than unity [59]. Molecular diffusion depends on the properties of both the test substance and the carrier gas. When the principal cause of broadening is the longitudinal molecular diffusion, it is expedient to use dense gases for reducing the zone broadening, i.e., it is preferable to use nitrogen or argon rather than hydrogen or helium as the carrier gas. Resistance to mass transfer During the motion of a chromatographic zone along the column, the front edge of the zone is predominantly characterized by the process of sorption, i.e., transfer of molecules from the gas to the stationary phase. After the maximum, the opposite phenomenon is observed, namely desorption takes place at the rear edge, i.e., transfer of molecules to the test compound from the stationary to the gas phase. Both of these processes occur rapidly, although not instantaneously. Therefore, the zone of the substance in the gas phase slightly leads the zone of the substance in the stationary phase, which also contributes to peak broadening [60]. The third term of Van Deemter’s equation reflects the contribution from this process to the HETP: (1.33) where k = KV,/Vg is the extraction coefficient, d, is the thickness of the liquid stationary phase (LSP) film and D, is the diffusion coefficient of the test compound in the LSP. The value of C depends on various factors, the most important being the thickness of the LSP fdm. The value of C increases directly proportionally to the square of this thickness. It should be noted that the value of h , although it is an important characteristic of the column, which defines the broadening of a chromatographic zone, cannot be regarded as the only value that determines the possibility of solving a particular analytical problem. In developing an analytical procedure, the problem very often reduces to the separation of at least two compounds with similar properties. For a quantitative assessment of the separation of the chromatographic zones of two compounds present in a mixture in similar concentrations, a number of separation criteria have been proposed that are a function of the difference between the retention References p. 29
22
BASIC PRINCIPLES OF GC
values and the widths of the chromatographic zones. The IUF'AC Committee [61] recommended the use of the following value as a separation criterion: R , = 2Y10, - Y*)
(1.34)
where y is the chart distance between the peak maxima of compounds 1 and 2 and y A andyB are the peak widths of compounds 1 and 2 at the base of the peaks. The value of R , varies from 0 to "0; the peaks are completely separated at R , = 1 . In the Soviet literature, the quantity K 1 is usually adopted as a separation criterion [33], where
K = 1/2R, The peak resolution is determined by the sorbent selectivity, a (a= f R , J f R , J , column efficiency, (N), and the mass distribution ratio, D, (see ref. 62):
(1.35)
the
(1.36)
(1.37)
where fi is the volume ratio of the mobile to stationary phase in the column, K is the distribution constant and a,K and N refer to the second, dower component. The development of a satisfactory separation procedure can often be reduced to the determination of the optimal conditions under which the value of the separation criterion for a pair of compounds that are difficult to separate would be the highest. The dependence of the separation coefficient on the experimental parameters is considered in detail elsewhere [33].
QUALITATIVE AND QUANTITATIVE ANALYSIS The advances made in GC were due largely to the development of efficient identification methods, the characteristic feature of which is a wide use of a combination of various physical and chemical methods for identifying the peaks in a chromatogram [48,63-651. The general scheme of application of some widely used identification methods in GC is shown in Fig. 1.13.' Qualitative analysis Qualitative analysis often includes the following stages: (1) preliminary preparation of the sample, ( 2 ) chromatographic separation with the use of chemical reactions and selective detectors, (3) isolation and physicochemical study of separate fractions, (4) GC re-examination of separation fractions. Thus, in order to determine the composition of a test mixture, bothchromatographic methods based on the measurement of the retention values and methods based on the physicochemical properties of the test components are applied. In the following sections we shall consider the principal' methods of identification.
QUALITATIVE A N D QUANTITATIVE ANALYSIS
-
r
3
-
7
4
23
5
-
J
1;ig. 1.1 3. General scheme of qualitative analysis in gas chromatography. 1 = Sample to be analyzed; 2 = preliminary preparation of sample (separation, chemical treatment, physicochemical investigation); 3 = chromatographic separation, chemical analysis; 4 = selective detectors; 5 = physicochernical study of separated fractions; 6 = gas chromatograph; 7 = re-cxamination of separate fractions.
Staridard compound method
This method is based on the introduction into the test mixture of standard substances that are assunied to be already present in the mixture. The coincidence of the retention times is usually the basis for identifying the peak of the test compound as the standard coinpound. I t should, however, be noted that this condition is not sufficient for the qualitative identification of a compound, because identical (or very similar) retention times may characterire several substances. The reliability of this method increases with the use of more efficient columns and columns with different phases, the nature of which detenniiies the sequence of emergence of the coniponents and their retention values. As many compounds are not readily available, in piactical chromatography reaction methods are used for the preparation of standard mixtures [66]. Metliod using fabiilafed data
The determination of the qualitative composition of a mixture by this method is based on a cornparison of expeiiinentally detemiined retention values of peaks with tabulated retention values for known coinpounds. Standard compounds with tabulated retention values are introduced into the test mixtuie; in order to increase the reliability of the method, several control measurements must be made for corn pounds with diffeient structures, the presence of which i n the sample being analyzed is assumed, so as to establish the identity of the chromatographic properties of the given column with that used for tabulating the data on the retention values. Tabulated retention values have been published [6, 71.
Refcrenccs p. 29
24
BASIC PRINCIPLES OF GC
Method of several phases In this method, an unknown mixture is analyzed, not on a single column, but on several columns with different phases [66]. This technique increases the reliability of the chromatographic identification of the substance and permits the type of compound being analyzed to be determined. Fig. 1.14 [67] shows the logarithmic dependence of the retention times of various compounds for two stationary phases, paraffin oil (tR,J and tricresyl phosphate (tR,2).It follows from the data presented that compounds of the same type are characterized by straight lines that do not pass through the origin. This is an important regularity, which is utilized for determining the type of test compound.
Computational methods and correlation ratios When tables of retention values lack data on some compounds;it may be useful to use correlation equations that relate the logarithm of the retention values with the properties of the test compounds [3] (for instance, the number of carbon atoms or the boiling point). In many instances, in order to determine retention values, one can use computational methods based on the additive scheme [68-701. For instance, eqn. 1.38 is valid for the retention values of alkanes:
rii
log V = Enii
(1.38)
where riiis the increment of the logarithm of the retention value corresponding to a definite combination of bonds (the structural element) and nii is the number of structural elements of type ij in the molecule of the compound. The additive scheme is based on the assumption that the molecular interaction of the test compound with the liquid stationary phase can be regarded as an interaction of a sum of definite structural elements of the mdecule, each of which is characterized by a definite contribution to the retention value. Fig. 1.15 depicts the relationship between the calculated and experimental values of the logarithms of the retention times of alkanes. The agreement is satisfactory. cc d
B
I
loo lo-
1-
0
P
/p /,$7O
-
!
A0 1
I
I
‘6
I
O8
$;,:
/ il0I, O6
,
QUALITATIVE A N D QUANTITATIVE ANALYSIS
6
3
25
0
Fig. 1.16. Separation of mixture of hexafluoroben7ene ( I ) , propyl chloride (2) and n-heptane (3) with the use of a selective flarne-emission detector. (a), Chlorine; (b), fluorine; (c), carbon.
Physicochemical methods This group of methods is based on joint utilization of chromatographic and physicochemical methods. The application of selective detectors, which record only compounds of one or several definite classes, pennits information on the nature of the test compounds t o be obtained; this information, coupled with the chromatographic data, enables one reliably t o identify the components of the text mixture. As an example Fig. 1 .I6 [71] shows three chromatograms of the same mixture (hexafluorobenzene, propyl chloride and ti-heptane, recorded with flame-emission detector. Each chromatogram was obtained by recording the intensity of a definite spectral line which was selective for chlorine-, fluorine-, or carbon-containing compounds. This technique made it possible t o establish the elemental composition of the compounds being analyzed. Methods of analytical reaction GC are also widely applied for identification purposes [64, 721. these methods use chemical reactions in a unified chromatographic scheme for analytical purposes. The deduction method [ 7 3 ] is often used for group identification. In this method, two chromatographic analyses of the initial mixture are carried out: one is an ordinary analysis without the use of chemical reactions, while the other utilizes in the chromatographic scheme a reactor containing an absorbent (reagent) that forms non-volatile compounds with certain classes of chemical substances. Therefore, the chromatogram of the second analysis shows no peaks of the reacting compounds (the chromatogram of the second analysis can be obtained from that of the first analysis by deducting the peaks of the reactants), which indicates that they belong t o compounds of a definite class. This method was first used for determining the content of unsaturated compounds in hydrocarbon mixtures. Unsaturated compounds were absorbed in the reactor by concentrated sulphuric acid deposited on silica gel.
Preliminary treatment of the sample
I n thus method, both chemical and physical techniques can be used. For instance, after a reaction of a mixture of fatty acids with bromine, compounds with one unsaturated References p. 29
26
BASIC PRINCIPLES OF GC
bond are determined by displacement of the peaks on the chromatogram, and compounds with several double bonds are determined by the disappearance of their peaks, as the introduction of bromine into the molecule of an organic compound sharply reduces its volatility. Extraction is also used as a subtraction method. For example, an efficient method for identifying alcohols is preliminary extraction with propylene glycol of components of a sample dissolved in carbon tetrachloride. Propylene glycol efficiently extracts alcohols, but not aldehydes, ketones, hydrocarbons or esters, which remain in the carbon tetrachloride solution. Acids, phenols and amines, which are also readily soluble in propylene glycol, can be removed by treatment with an alkali or acid. The above general identification methods open up wide opportunities for establishing the composition of unknown mixtures. Quantitative analysis An important stage in chromatographic analysis is the qualitative interpretation of chromatograms, as a result of which the quantitative contents of the components in the test mixture can subsequently be determined. The accuracy of the results obtained depends on a number of factors, in particular on the method of analysis selected, the characteristics of the detector used, the method of calibration and calculation and the nature of the components being analyzed [74]. In accordance with eqn.l.8, the amount of a substance in a chromatographic zone is directly proportional to the area of the chromatographic peak in the chromatogram. In this connection we shall consider methods for determining the area of a chromatographic zone, assuming that it is a Gaussian curve. (1) The area of a chromatographic zone is commonly expressed as the product of the peak height, , and the peak width at half-height:
(1.39)
A more general expression for determining the area of chromatographic peaks, which permits the calculation of the area of partially separated peaks, has been suggested [75] : = K8
hmax. P6
(1.40)
If 0 = 0.5,0.75 or 0.9, then K , = 0.941, 1.66 or 2.73, respectively. (2) The area of a chromatographic zone can be determined as the product of the peak height and the retention time [76] : (1.41) (3) The area of a chromatographic zone is proportional to the height of the chromatographic peak: (1.42)
27
QUALITATIVE AND QUANTITATIVE ANALYSIS
Therefore, the amount of a substance in a chromatographic zone is directly proportional to the following values, which are determined directly from the chromatogram: hmau.pe ; hmax,xm,, or h,,,,,. However, if in the first method the coefficient of proportionality between the amount of substance and the value being measured is largely determined b y the metrological characteristics of the detector and its opening conditions, in the second and third methods this coefficient also depends on the conditions of the chromatographic experiment. In particular, in the second method the value of the area depends o n the number of theoretical plates, which, in general. changes from one substance to another. In the third method, the coefficient of proportionality depends on the retention value and separation efficiency. It should be noted that with large samples, deviations from the above linear dependence also occur and the retention times change. These deviations must occur in all instances when the ratio of the volume of the test sample to the peak width (in volume units), as measured with a vanishingly small sample siLe, exceeds 0.4 [77]. When using ‘manual’ methods for increasing the accuracy, the peak width must be measured by means o f a measuring magnifying glass with reticule divisions of 0.1 mni [78] . In recent years, electronic integrators have been widely used in chromatography [79-811. The use of integrators considerably reduces the processing time, ensures a high accuracy and reduces the cost of processing. Table 1.2 [82] conipares different methods of chromatogram processing. I t can be seen that the use of electronic integrators improves the accuracy and reduces the processing time. The amount of a substance in a chromatographic Lone is detennined both by the characteristics of the detecting systeni and by the parameters of the chromatographic peak. Therefore, in order t o detennine the content of a component in a sample, it is not sufficient to find the area (or other parameters) of the chromatographic peak of the substance, but it also necessary t o determine the coefficient of proportionality, which depends on the type of the detecting system, the experimental conditions, the nature of the test sample, and so on. Let us consider the quantitative methods for determining the content of the coniponents in the test mixture. TABLE 1.2 COMPARISON 01 SOML CIIROMATOGKAhl PROCFSSING METIIODS ~~
Characteristic
-
.~
Manual methods Use o i planimeter
Chromatogram 45- 60 processing time (min) Reproducibility 4.06 (%,)
Electronic digital integrator
____ Area Cutting o u t calculation* and weighing
45-60
50-60
100-200
15-30
5-10
4.06
2.58
1.74
1.29
0.44
*Height multiplied by width at half-height.
References p. 29
Electromechanical integrator
Triangulation method
28
BASIC PRINCIPLES OF GC
Internal normalization method The modern version of this method in GC was suggested by Keulemans and co-workers 183,841. The calculation is made with eqn. 1.9 taking into account the correction coefficients for the separate compounds (see, for instance, ref. 82).
Absolute calibration method In this method, the dependence of the area (or another parameter) of the chromatographic peak on the absolute amount of the test substance is determined experimentally for each component, Le., one of the following dependences is determined: qi
= Ks = K h
= Khph,,,,kJ
(I .43)
The absolute calibration method is widely used in chromatography (see for instance refs. 85 and 86. It should be noted that absolute calibration must be checked periodically; the frequency of this checking must be determined empirically. Usually, in re-calibrating, one can restrict the check to a few points on the calibration graph.
Internal standard method This method was first used in GC by Ray [87]. A mixture of unknown composition, into which a known substance is specially introduced at a concentration R , is analyzed. The concentration of the standard is calculated with reference to the entire test mixture, which is taken as 100%. The content of the components (P)in the test mixture is calculated by the equation
P =A S i R f s t d . 'std.
or p =
fhih8
(1.44)
f h std. hstd.
where fi and f s t d . are the correction coefficients depending on the individual sensitivity of the detector to the component i and the standard. If R is constant, calibration graphs are obtained of percentage of impurity versus the ratio of the peak height of the component i to that of the standard. The use of relative calibration methods enables one greatly to improve the accuracy of measurements, because the effect of the experimental conditions on the analytical results is reduced as the change in the analytical parameters usually affects the retention time of the standard substance and that of the sample components equally (although there are exceptions). The corresponding equation can be obtained on the basis of eqns. 1.401.42 and 1.44: (1.45)
where f i = I / ( q - CUO) and fstd.= l/(astd.- ao).Note that the relative values in this equation are much less dependmt on the experimental conditions than are the absolute values of the height and area of the chromatographic peak. Another advantage of the method is that it is no longer necessary t o measure accurately the volume of the test
REFERENCES
29
sample except when calibration with a constant sample size is used in order to take into account the non-linearity of the detector. The method can also be used t o measure the content of individual components even when not all o f the compounds are recorded on the chromatogram. In choosing the standard compound, one must ensure that it is compatible with the sample being analyzed. In order t o increase the accuracy of the analysis, it is desirable that the substance used as the standard should be similar t o the test components in terms of the retention value and their content in the mixture being analyzed. The method automatically takes into account possible losses of the test con1 ponents during preliminary preparation of the sample [88] (for instance, during extraction, distillation, adsorption and other extraction and concentration procedures). Many of the above calculation and calibration methods are based on the assumption that the components of the test sample are not adsorbed and remain unchanged during the analysis, the detector readings are linear over a wide range of concentrations and are independent of the presence of other components in the sample and the values of the correction coefficients published in the literature are adequate. Unfortunately, many of these conditions are not fulfilled in practice. Therefore, accurate quantitative results can be obtained only by calibrating a given device with a mixture of known composition that contains all of the compounds present in the test mixture. The calibration of the device must be checked periodically. In conclusion, we wish t o emphasize the necessity of applying statistical methods [65,89,901 for estimating the accuracy of the chromatographic procedures used.
REFERENCES 1 J. Janik, Chromatogr. Rev., I 1 (1969) 203. 2 J. I. Walraven, Joint Symposium o n Accurate Methods of Anaiysis f o r Major Constituents, London, April 1570, Paper 1 1. 3 A. T. James and A. J. P. Martin,RiocAem. J., 50 (1952) 679. 4 M. S. Tsvet, Khromatograficheskii Adsorptsionnyi Analiz (Chronlatographic Adsorption Analysis), Academy Press, Moscow, 1946. 5 R. P. W. Scott (Editor), Gas Chromatography 1560, Butterworths, London, 1960. 6 J. S. Lewis, Coinpilafion of Gas Chromatographic Data, ASTM Special Technical Publication No. 343, American Society for Testing and Materials, Philadelphia, Pa., 1963. I W. 0. McReynolds, Gas Chrornatograpkic Retention Data, Preston Technical Abstracts Co., Evaston, Texas, 1966. 8 F. Bruner, C. P. Cartoni and M. Posanzini,Anal. Chem., 41 (1969) 1122. 9 G. N. Turkeltaub and B. M. Luskina, Zavod. I,ab., 35 (1969) 545. 10 V. G. Berezkin and N. S. Nikitina, Zh. Fiz. Khitn., 42 (1968) 2942. 1 1 V. L. Kepke and G. 1:. Sokolin, Zavod. Lab., 36 (1970) 1301. 12 S. Dal Nogare and R. S. Juvet, Gas- Liquid C/iromatograp1iy, Theory and Practice, Intersciencc, New York, 1962. 1 3 A. V. Kiselev and Y. I. Yashin, GasAdsorption Chromatography, Plenum Press, New York, 1969. 14 C. E. Doring, in E. Leibnitz and H. G . Struppe (Editors), Handbirch der Gas-C/iromatographie, Acadeniische Verlagsgesellscliaft, Leipzig, 1966, p. 737. 15 M. S. Vigdergauz and R. I. IzrnayIov, Prinieneniye Gazovos Khrornatografii dlya Opredeteniya Fiziko-khimicheskikh Svo-vsrv Veshchests (Application of Gas Chroniatograpliy for the Determination of Physicochcniical Properties of Substances). Nauka, Moscow, 1970.
30
BASIC PRINCIPLES OF GC
16 S. Z. Roginsky, M. I. Yanovsky and A. D. Berman, Osnovy Primeneniya Khromatografii v Katalize (Fundamentals of Application of Chromatography in Catalysis), Nauka, Moscow, 1972. 17 A. B. Kiselev and V. P. Dreving (Editors), Experimentalnyie Methody v Adsorbtsii i Molekuliarnoy Khromatografii (ExperimentalMethods in Adsorption and Molecular Chromatography), Moscow University Press, Moscow, 1973. 18 D. A. Vyakirev, N. F. Shushunova, I. I. Chuyev, M. K. Shchennikova and G. G. Kuravskaya, Neftekhimiya, 9 (1969) 861. 19 N. E. Buyanova, G. B. Gudkova and A. P. Karnaukhov, Kinet. Katal., 6 (1965) 1085. 20 K. V. Aleksyeyva, V. G. Berezkin,S. A. Volkov and E. G. Rastyannikov, Polucheniye bChistykh Veshchestv Merodom Preparativnoy Gazovoy Khromatografii (Obtaining Pure Substances by Preparative Gas Chromatography), TSNIITNeftekhimprom, Moscow, 1968. 21 A. Zlatkis and V. Pretorius (Editors), Preparative Gas Chromatography, Wiley-Interscience, New York, London, Sydney, Toronto, 1971. 22 K. I. Sakodynsky and S. A. Volkov, Preparativnaya Gazovaya Khromatografiya (Preparative Gas Chromatography), Khimiya, Moscow, 1972. 23 V. A. Averin, V. Ya. Shnol' and B. G. Distanov, Gazov. Khromatogr., No. 9 (1969) 142. 24 M. Taramasso, Gas Chromatographia, Franco Angeli Editore, Milan, 1966. 25 J. Krugers (Editor), instrumentation in Gas Chromatography, Centrex, Eindhoven, 1968. 26 B. V. Stolyarov, I. M. Savinov and A. G. Vitenbeg, in B. V. Ioffe (Editor),Bukovodstvo k Prakticheskini Rabotam PO Gazovoy Khromatografii (Manual on Practical Work in Gas Chromatography), Leningrad University Press, Leningrad, 1973, p. 284. 27 F. van de Craats, Anal. Chem., 14 (1956) 136. 28 J. Hoomeijer, A. Kwantes and F. van de Craats, in D. H. Desty (Editor), Gas Chromatography 1958, Butterworths, London, 1958, p. 288. 29 W. R. Supina, The Packed Column in Gas Chromatography, Supelco, Bellefonte, Pa, 1974. 30 L. S. Ettre, Open Tubular Columns in Gas Chromatography, Plenum Press, New York, 1965. 31 D. M. Ottenstein,J. Gas Chromatogr., 1 (1963) 1 1 . 32 V. G. Berezkin, V. P. Pakhomov and K. 1. Sakodynsky, Tverdyie Nositeli v Gazovoy Khromatografii (Solid Supports in Gas Chromatography), Khimiya, Moscow, 1975. 33 A. A. Zhukhovitskii and N. M. Turkeltaub, Gazovaya Khromatografiya (Gas Chromatography), Gostopekhizdat, Moscow, 1962. 34 L. C. Browning and J. 0. Watts, Anal. Chem., 29 (1957) 94. 35 E. G. Hoffman,Anal. Chem., 34 (1962) 1216. 36 A. E. Messner, D. D. Rosie and P. A. Argabright,Anal. Chem., 31 (1959) 230. 37 K. L. Grob, D. Mercer, T. Gribbon and H. Wells, J. Chromatogr., 3 (1960) 545. 38 H. Veening and G. D. Dupre,J. Gas Chromatogr., 4 (1966) 153. 39 L. D. Hiushaw, J. Gas Chromatogr., 4 (1966) 300. 40 A. J. P. Martin and A. T. James, Biochem. J., 63 (1956) 138. 41 A. G. Nerheim,Anal. Chem., 35 (1963) 1640. 42 A. A. Datskevich, Gazov. Khromatogr., No. 1 (1964) 79. 43 J. Harley, W. Nel and V. Pretorius, Nature (London), 181 (1958) 177. 44 J. G. McWilliain and R. A. Dewar, Nature (London), 182 (1958) 1664. 45 E. R. Adlard, Crit. Rev. Anal. Chem., 5, May (1975) 13. 46 V. V. Brazhnikov, Differentsialnyie Detektory dIya Gazovoy Khromatografii (Differential Detectors f o r Gas Chromatography), Nauka, Moscow, 1974. 47 V. A. Botin, Radioionizatsionnoye Detektirovaniye v Gazovoy Khromatografii (Radioionization Detection in Gas Chromatography), Atomizdat, Moscow, 1974. 48 L. S. Ettre and W. H. McFadden (Editors), Ancillary Techniques of Gas Chromatography, WileyInterscience, New York, London, Sydney, Toronto, 1969. 49 J. E. Lovelock and S. R. Lipsky,J. Amer. C h e m SOC., 82 (1960) 431. 50 A. Karmen,Anal Chem., 36 (1964) 1416. 51 D. Henneberg, Z. Anal. Chem., 183 (1961) 12. 52 V. L. Talroze, V. V. Reznikov and G. D. Tantsyrev, Dokl. Akad. Nauk SSSR, 159 (1964) 182. 53 J . C. Giddings and R. L. Mallik,i,:d Eng. Chem., 59 (1967) 18.
REFERENCES
31
54 A. J. P. Martin and R. L. M. Synge,Biochem. J . , 35 (1941) 1385. 55 M. Singliar, Pracrica Oomatografiei de Gaze, Trad. Dinliniba Slovaca, Buchurest, 1963. 56 J . J. van Deemter, F. .I.Zuiderweg and A. Klinkenbcrg, Cheni. Eng. S c i , S (1956) 271. 57 A. Klinkenherg and I:. Sjenitzer, Chem. Eng. Sci., 5 (1956) 258. 58 J . C. Giddings, Dynamics of Chromatography, Part 1, Marcel Dekker, New York, 1965. 59 E. Glueckauf, in D. 11. Desty (Editor), Gas Orromatography 1958, Academic I’rcss, New York, 1958, p. 33. 60 G. Schay, Theoretische Gnindlageti der Gas-Chroniatoxraphie, VEB Deutscher Verlag der Wissenschaften, Berlin, 1960. 61 IUPAC, Pure Appl. Chem., 37 (1974) 445. 62 Cs. Horvath, in 1,. Ettre and A. Zlatkis (Editors), The Practice of’ Gas Chromatography, Interscience, New York, London, 1967, p. 129. 6 3 D. A. Leathard and R. C. Shurlock, Identification Techniques in Gas Chronzatography, Wiley, London, New York, Sydney, Toronto, 1970. 64 V. ti. Berezkin, Analytical Reaction Gas Chromatography, Plenuni Press, New York, 1968. 65 R. Kaiser, Chromatoyraphie in der Gasphase, Bibliographes Institu!, Mannlieim, 1969. 66 V. G. Berezkin, L. Sojik and I. UndeovB, J. Chromatogr., 9 8 (1974) 157. 67 E. Bayer, in D. H. Desty (Editor), Gas Chromatography, Buttcrworths, London, 1958, p. 333. 68 V. G. Rerezkin, Neftekhimija, 1 (1961) 169. 69 V. G. Berezkin and V. S. Kruglikova, Neftekhiniiya, 2 (1962) 845. 70 G. Schoniburg, Advan. Chrornatogr., 6 (1968) 21 1. 71 A. J. McCormack, S. C. Tong and W. D. Cook, Anal. C%em.,3 7 (1965) 1470. 72 M. Reroza and M. N. Inscoe, in L. S. Ettre and W. H . McFadden (Editors), Ancillary Techniques of Gas Chromatography, Wiley-Interscience: New York, London, Sydney, Toronto, 1969. 73 R. L. Martin,Anal. Chem., 32 (1960) 336. 74 L. A. Kogan, Kolitscizesti~entzayaGazovaya Khromatografiya (Quan titative Gas Chroma tography), Khimiya, Moscow, 1975. 75 A. A. Zhukovitskii. B. A. Kazanskii, 0. D. Sterligov and N. M. Turkeltaub, Dokl. Akad. Nauk SSSR, 123 (1958) 1037. 76 J. Novik, Chetn. Listy, 59 (1965) 1021. 77 V. I. Kallnanovski and A. A. Zhukovitzki,J. Chromafogr., 18 (1965) 243. 78 V. G . Baranova, A. G. I’ankov and Ya. I. Turian, Ostrovy F’izikokhimicheskikh Metadov Analiza i Kontrol Proizvodstva Izoprena (Fundamentals of’f)lzysicochernicalMethods o f Analysis and Control of Isoprene ProducfionJ, NIITEKhim, Moscow, 1965. 79 J. M. Gil1,J. Chromatogr. Sci., 10 (1972) 1. 80 ti. G. Struppe (Editor), .4spects of Gas Chromatography, Deut. Akad. Wiss., Berlin, 1971, p. 144. 81 Chromatographia, 7, No. 9 (1972). 82 H. M. McNair and E. J . Bonelli, Basic Gas Chromatography, Varian Aerograph, Walnut Creek, Calif., 1967. 83 A. 1. M. Keulenians and A. Kwantes, Anal. Chim. Acfa, 13 (1955) 357. 84 A. I. M. Keulernans, A. Kwantes and G. W. Rijndcrs, Anal. Chim. Acta, 16 (1957) 29. 85 N. Brenner and L. S. Ettre,Anal. Chem., 31 (1959) 1815. 86 A. I. Dolgina, A. D. Alekseyeva and A. N. Meshcheryakova, Gazov. Khromatogr., No. 4, (1966) 120. 87 N. If. Ray, J. AppL Chetn., 4 (1954) 21. 88 11. P. Burchfield and E. E. Storrs, Biochemical Applications of Gas Chromatography, Academic Press, New York, 1962. 89 A. N. Zaydel, Elementarnyiye Otsenki Oshihok Izmerenii (Elementary Estimates o f Measurements ErrorsJ, Nauka, Leningrad, 1967. 90 K . Doerffel, Statistik in der Analytischen Chemie, Deutscher Verlag fiir Grundstoffindustrie, Leipzig, 1966.
This Page Intentionally Left Blank
Chapter 2
Gas chromatographic methods for the analysis of monomers and solvents Multi-ton polymerization processes that can be realized only with very pure starting materials (monomers, solvents, protective gases, etc.) are being employed on an everincreasing scale in the petrochemical and chemical industries. As an illustration, we can cite the requirements imposed by the industry on the purity of hydrocarbon monomers. The maximum permissible concentrations are 4 lo4% for acetylene hydrocarbons, for cyclopentadiene, 1 * 6 * lo”% for piperylene, 5 * for ethers and esters, 1* for water, 2 1 0 % for sulphur as mercaptans and 5 10% for carbon monoxide [ I ] . The maximum permissible concentration of impurities usually must not exceed 10-2-104%, depending on their reactivity, i. e., under polymerization conditions the allowed concentration is from several molecules to several tens of hundreds of molecules of an impurity per million molecules of the monomer. The content of impurities in the solvents and other auxiliary compounds used in the polymerization process must not exceed the maximum permissible concentrations for monomers. Such stringent requirements are necessary because reactions of formation and transformation of polymers are very sensitive t o the presence of small amounts of impurities in the reaction medium or in the polymer chain [2-41. Numerous instances are cited in the literature where a change in leads to an increase the concentration of impurities within the range from lo-’ to in the induction period, a decrease in the rate and extent of transformation and an (undesirable) change in the molecular weight distribution of the polymers formed. In some instances, however, the presence of impurities has a favourable effect on the course of a polymerization reaction [5-71. The effect of impurities on the kinetics and mechanism of polymerization processes is complicated and has been insufficiently investigated. Therefore, in order to obtain correct information on the regularities of polymerization reactions, it is necessary to use starting materials with a high degree of purity and to control the presence of impurities in the polymerization medium during the reaction. Unfortunately, despite the importance of this problem in polymer chemistry, the control of impurities is not always given adequate attention. The analysis of impurities in “pure” compounds and the determination of impurities in commercial products is one of the principal fields of development in contemporary analytical chemistry [8]. In accordance with the definition of Dal Nogare and Juvet [9], we shall regard the components of a test mixture as impurities if their concentration is equal to or less than lo-’%. At present, the gas chromatographic (GC) analysis of impurities is an important independent field of GC that is characterized by a number of specific features of separation, special methods and equipment for analysis and, unfortunately potential sources of errors [ 10-141. In this chapter, we shall describe the specific features of impurity analysis and experimental methods that are of interest for the determination of impurity components in monomers and solvents.
-
-
References p. 52
-
34
GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
The development of GC methods for the analysis of volatile impurities in monomers and solvents has solved most of the practical problems that might arise. The use of high-sensitivity and selective ionization detectors often make it possible to determine impurities in trace concentrations down to 10-6-10-10%. Wide opportunities for increasing the sensitivity of GC determinations of impurities arise by applying methods based on the use of the thermal parameters (temperature programming, chromathermography, etc.), which result in a considerable enrichment of impurity components in the sample, methods with preliminary concentration of impurities and methods involving analytical reaction GC. The universality of GC, the possibility of using high-sensitivity detectors for recording purposes and the rapidity of the method explain the wide use of GC for the analysis of impurities in monomers and solvents.
SPECIFIC FEATURES OF THE GC ANALYSIS OF IMPURITIES The separation of complex mixtures whose components are present in comparable concentrations has been studied comprehensively [9, 15-20]. Therefore, it was desirable to find the extent to which the previously developed concepts and known regularities retain their importance in impurity analysis. With this aim in view, Berezkin el al. [21] studied the effect of the concentrations of the components in a test mixture on their retention times and broadening of chromatographic zones for both light and heavy impurities. The investigation was carried out under conditions such that the absolute value of the concentration of the principal component and the size of the sample did not differ substantially from those used in ordinary analysis. It was found that the retention times of impurities remain virtually unchanged and therefore, in identifying impurity zones, one can use tables of retention values obtained for mixtures with approximately equal concentrations of the components. Of course, retention values are independent of the concentrations of the test substances only when it is possible to neglect the adsorption of the test impurities at the gas-liquid stationary phase interface and, particularly important, at the liquid stationary phase-solid support interface. Reversible adsorption of test compounds at the interfaces under the conditions of impurity analysis may affect two main characteristics of chromatographic separation. Firstly, adsorption causes additional spreading of the impurity zones, with a corresponding decrease in the sensitivity of the determination; secondly, it makes the retention volume dependent on the sample size and the concentration of the components of the mixture. On the other hand, if one uses for chromatographic separation a sorbent on which the retention is determined by adsorption of the liquid stationary phase ( U P ) on the interface as well, the retention times of the test components, in general, must depend on experimental conditions such as the concentration of the components in the test sample, the type of solid support, the method of deposition of the U P , the method of column ageing, the frequency of injection of the sample into the column and the nature of the other components, especially the main substance. The dependence of the retention values on the concentration of the components in a test mixture (ignoring their mutua! influence) must be similar to the dependence of the
srrcmc FJ A T U R ~ OF S THL ANALYSIS OF IMPURITIES
35
retention volunie on the sample size. The latter was studied in a number of investigations [22-341 with different supports, compounds and LSP‘s, and also in relation t o the sample composition and size. A particularly pronounced dependence of the retention volunie on the size (concentration) of the sample of the substance analyzed is usually observed for polar compounds in separation on non-polar phases deposited on a solid diatomite support. Thus, for instance, the retention value of ethanol on a non-polar LSP (6%)squalane on Spheroclironie-I) increases by 300% as the sample size decreases from 0.7 t o 0.1 pI [ 3 3 ] . For a net retention volume, VN, the following equation [ 3 S ] is valid in gas-liquid chroniatograpliy, provided that one takes into account the dissolution of the test compounds in the LSP,adsorption a t the gas -LSP interface and adsorption at the LSP-solid support interface:
where cX is the concentration of the substance in the gas phase, cI is the concentration of the substance in the liquid phase, c I x is the concentration of the substance on the surface of the liquid phase, cS is the concentration of the substance on the surface of the solid support, Vl is the volume of the UP in the column and Sl is the area of the solid support, coated with a layer of U P in the colunin. If the sainple adsorption and absorption isotherms are linear, then the following equation can be written for cI = li!cx, c I g = kl,cg, cs = k s c l , the relative retention volume and the retention index [36] :
(2.2)
where I is the Kovits retention index, PI is the percentage of the LSP on the solid support (the weight ofthe latter is lOO.l,) and X I , Xz and h3 are constants. Using coluinns with different contents of the liquid phase, one can detemiine the invariant values
KI/Klstd.and lo= 100 log
, which depend exclusively on the distribution
of tlie substance in the systen; UP-solid support. The determination of the indicated values was considered by BereLkin 1361. If the adsorption of test compounds at the interfaces is small conipared with adsorption in tlie LSP, then eqn. 2.3 becomes the known eqn. 1.6. As follows froni eqn. 2.2, if the contribution from adsorption to the retention volume is substantial, then in order to obtain reproducible values of the retention voluine it is necessary t o standardiLe not only the LSP, but also tlie solid support. In this instance, however, gas-liquid chromatography loses one of its main advantages, namely simplicity of preparation of the column with reproducible relative retention volumes. For this reason, investigatois naturally tend t o carry out chromatographic separations by References p. 52
GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
36
means of ‘pure’ gas-liquid chromatography, when the retention volume is determined only by the dissolution in the U P . Berezkin and Pakhomov [32] derived an equation describing the dependence of the retention volume on the sample size, assuming non-linear adsorption of the compound on the surface of the solid support. Let us consider the dependence of the retention volume on the concentration (or on the sample size, which is proportional to it), assuming that we can neglect the adsorption of the components at the interface of the LSP with the carrier gas; adsorption in the LSP phase is described by the Henry equation, while the adsorption at the UP-solid support interface is described by the Freundlich equation (0 < 1): as = acf
With the above assumptions, eqn. 2.3 can be rearranged:
where VNstd. = Klstd, V, and Xis the coefficient of proportionality in the equation c,,,. = Xq/V, (see, for instance, ref. 19). It follows from the equation obtained that the relative retention volume increases with a decrease in sample size, and increase in the surface area of the solid support and non-linearity of the adsorption isotherm, i.e., with an increase in 1 - p. The experimental data obtained by Scholtz and Brandt [22] and Berezkin and Pakhomov [32] are fairly well described by eqn. 2.5 and the values of p obtained are in good agreement with the generally accepted concepts of the nature of the adsorption of organic compounds on the surface of solids. As in impurity analysis the real concentration (or the ‘individual’ sample size) of test impurities usually differs by an order of magnitude from the corresponding values in the analysis of ordinary mixtures, even small deviations of the sorption isotherms from linearity m3y lead to perceptible changes in retention values. Therefore, in many instances one can expect the retention values to depend on the concentration of the components in the mixtures being analyzed. Indeed, it was shown experimentally [37, 381 that the identification of polar impurities according to retention times obtained with the use of mixtures in which the same compounds were present in higher concentrations may lead to errors. These features must be taken into account when identifying unknown mixture components. Therefore, it is possible to carry out qualitative identifications of compounds in chromatograms by using either literature data on retention times obtained under conditions of impurity anaIysis or ordinary tabulated retention times; in the latter instance one must first show that the retention times of the expected (assumed) polar impurities are independent of their concentration. The efficiency of a GC column also depends on the concentration of the test compounds. An experimental study on the dependence of the peak width of test compounds on their concentration in the sample was carried out by Berezkin et al. [21] , and the relationship between the peak width and the retention time has also been investigated [34,39]. The presence of the main component in the mixture does not affect the efficiency of separation of impurities, which can be completely separated from the zone
SPECIFIC FEATURES OF THE ANALYSIS OF IMPURITIES
31
of the main substance. These results disagree with the widely accepted notion that in impurity analysis (all things being equal) a much better separation will be achieved if the impurity is eluted ahead of the main substance. One of the specific features of impurity analysis is that the zone of the main substance is usually eluted from the column with a very wide peak, with a greatly broad tail that may overlap (mask) the zones of impurities or impede their determination. Broadening of the zone of the main component in the low concentration range (Le., at concentrations that are a factor of 10-3-10-6 of the maximal concentration has been studied [40]. In Fig. 2.1 the shapes of toluene peaks obtained on injection of equal samples (2 pl) into the column are compared; in recording the chroniatograms, different detector sensitivity scales were used. It can be seen that as the detection sensitivity changes, the peak width and shape change abruptly. In effect, by recording the main peak in the same sensitivity range as for the impurities, we are investigating its shape ‘under a microscope’, the role of which is played by the sensitive detector. The broadening of a chromatographic zone in the low concentration range near the baseline is evidently due t o the same causes as the broadening of zones in ordinary chromatographic analysis, but their ratio is different in this instance. It is known [9, 191 that the broadening of chromatographic zones is usually due t o diffusion and kinetic factors. Longitudinal diffusion broadening of a chromatographic band is caused by molecular and eddy diffusion. These effects lead t o symmetrical broadening of the narrow initial sample peak. In considering this group of causes of peak broadening, it must be noted that while the initial widths of the zones of impurities and of the main substance are the same, the concentration at the maximum of the zone of the main component is 10-5-108times that of the impurities. Hence, in accordance with Fick’s first law, the zone of the main component must be broadened t o a much greater extent than the impurity zones.
x
2
P lJ
i a,
1
-
~-
~
8
~~~~
Time
Fig. 2.1. Change in shape of peak of toluene (sample size 2 PI) with different detection sensitivities: 1 = scale 10” A; 2 = 10- A; 3 = 10” A; 4 = 10- A. References p. 52
38
GC METHODS FOR THE ANALYSIS O F MONOMERS AND SOLVENTS
A quantitative estimation of the dependence of the broadening of a chromatographic zone of the main component on the carrier gas velocity and other experimental parameters was carried out by Berezkin and Tatarinskii [40]. The resistance to mass transfer and longitudinal diffusion are the principal causes of broadening of the peak of the main component in the low concentration range. The important role of the kinetic factors in the formation of the wide band of the main substance was also confirmed by the linear dependence of its width on the particle diameter of the solvent and the decrease in zone width after modification of the support with a small amount of a polar substance (1% of triethanolamine) in chromatographing polar ethanol. Note that the increase in longitudinal diffusion may explain the increase in the width of the chromatographic peak, but it cannot explain its symmetry (see Fig. 2.1). The asymmetry of a chromatographic zone is often due to the non-linearity of the distribution isotherm. This effect may indeed occur, especially in the chromatography of polar main components on LSPs deposited on adsorption-active supports. Thus, if the adsorption isothenn of the main component from the U P on a solid support has a convex shape (for instance, for the Langmuir isotherm), we can write for the retention volume [35] :
where c, = zpcJ( 1 + pc,) and z and p are the parameters of the Langmuir equation. It follows from eqn. 2.6 that the retention volume increases with decreasing concentration of the test component, ie., the chromatographic zone of the main substance must have an extended rear edge. An asymmetrical peak of the main component, however, is also observed with a linear diffusion isotherm. Thus, for instance, the toluene peak (see Fig. 2.1) is clearly asymmetrical, although the retention times of the peak maximum of toluene coincide when chromatographing different-sized samples with the use of different sensitivities (from 10” to A), i.e., over a wide concentration range, indicating the linearity of the distribution isotherm. As the zone of the test substance moves along the column under the influence of the flow of gas carrier in the tail part of the chromatographic zone, the concentration of the substance in the gas phase is reduced with respect to the equilibrium concentration in the stationary phase [41], which leads to progressive broadening of the rear edge of the zone of the main component. Asymmetrical broadening of the peak of the main substance may be caused by various kinetic factors, such as slow kinetics of desorption of the substance from the sorbent grains or slow kinetics of desorption of the substance from stagnant zones. A theoretical explanation of the asymmetry of chromatographic zones due to kinetic effects in preparative separations was given by Giddings [42], and experimental verification was carried out in Takizawa et al. [43]. In selecting the optimal conditions for the separation of impurities and the main component, it is important to estimate the separation quantitatively in relation to the experimental parameters. The separation of the main component and the impurity has been studied [19,44, 451, and it was shown that with an increase in the ratio of the concentration on the main component to the impurity concentration at the zone maximum, the number of
srt:cmc FLATURLS OF THE ANALYSIS OF IMPURITIES
39
theoretical plates of the chromatographic column necessary t o obtain a desired separation greatly increases. Thus, for instance, from the results obtained by Genkin [45] it follows that if tlie difference between the retention times of two components is equal to three standard deviations, one can detect 0.6%'of impurity in the chrornatogram, and when the difference between the retention times increase to four standard deviations, lo-% of impurity. Note that these investigations were carried out with the assumption that the shape of the peak of the main component corresponds t o the Gaussian curve. In this connection, it is advisable t o use for separation high-efficiency columns made of inert materials (glass, stainless steel, etc.). The analysis of impurities on classical and packed capillary columns has been considered [46-481. In order to characterize the separation of the chroniatographic zones of the main component and impurity, several workers [40, 49-52] proposed special, senii-empirical criteria and established their relationship with the experimental parameters by using the concepts of the theory of theoretical plates [53]. hi the course of a chroinatographic separation, the surface of the adsorbent in gas adsorption chromatography, or the surface of the solid support in gas-liquid chromatography, may also exhibit irreversible adsorption of the compounds being separated, or catalytic transformations of the components of the test mixture and the LSP [9, 54-62]. These phenomena may lead t o serious errors in interpreting the analytical results, especially in qualitative impurity analysis. Thus, for instance, if the degree o f conversion of the test compounds in the course of a chromatographic separation is negligible, being a factor of only 104-10-6, then in ordinary analysis, when the components are contained in the mixture in comparable concentrations, the course of such reactions will not affect the results obtained. In the analysis of impurities, however, even such a small conversion of the main coinponent may result in the formation of ghost peaks corresponding t o the conversion products. Therefore, special attention must be paid t o the catalytic activity of the sorbents with respect to the main coniponent. Interfering peaks may also be caused by desorption from the surface of the equipnient or the solid support of compounds that are relatively strongly adsorbed froin previous samples during the analysis of more polar substances (displacement effect) 163-661, and the release of volatile substances from tlie rubber seals used in the sample injectors of gas chromatographs [67-691 and from other parts of the chromatographic equipnient [70] . Irreversible adsorption of the components of test mixtures on the surface of the solid support in the course of a GC separation may also cause large errors. For the analysis of mixtures with comparable concentrations of the components, this phenomenon was studied by Kusy [ % I , although it has been noted earlier [ 5 9 , 6 0 ] . Direct proof that adsorption of test compounds on the support occurred under the conditions of a chromatographic method was obtained by tracer studies [61], and the effect of adsorption on tlie surface of a solid support 1621 on the quantitative results in impurity analysis was studied by Berezkin et al. [7 11. The best results can be obtained by using solid supports modified by small additions of polar non-volatile or volatile compounds [72]. It is also necessary t o check the correctness of the procedures developed for the analysis of impurities by analyzing standard mixtures the composition of which is known and is similar t o that of the samples being analyzed, because in analyzing impurities the separation of all of the components is necessary but does not ensure the correctness of the results. References p. S Z
40
GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
In recent years, some use has been made of the method of reducing the adsorption effects based on saturation of the carrier gas with the vapour of volatile (usually polar) substances. The choice of the carrier gas depends on the particular problem and primarily on the composition of the impurities to be determined. Thus, for instance, the use of water vapour [72-741, formic acid [63], ammonia [64] and other substances has been described. In some instances it is advisable to use, as the stationary phase, the vapour of a substance that is not recorded by high-sensitivity detectors (for instance, water or ammonia vapour when using a flame-ionization detector, or hexane vapour when using an electron-capture detector). A survey of the use of vapour mobile phases in elution GC has been published by Rudenko et al. [7S] . TABLE 2.1 SOME PROCESSES THAT IMPEDE THE ANALYSIS OF IMPURITIES BY GC Process
Analytical consequences
Recommended techniques
Reversible adsorption at the liquid stationary phase-solid support interface, and also on the surface of the solid support not covered by liquid phase, and on the surface of chromatographic equipment.
Non-reproducibility of retention time; difficulties in identification. Spreading and asymmetricity of chromatographic zones, reduced accuracy and sensitivity of determination.
Modification of solid supports. Inert materials in chromatographic equipment. High-efficiency columns, equipment ‘training’.
Irreversible adsorption of components on the surface of the solid support and on the surface of chromatographic equipment.
Complete or partial loss of impurity components; erroneous, low results.
Modification of the surface of the solid support and the surface of equipment. Equipment ‘training’.
Displacement of impurities in the course of analysis.
False peaks; erroneous, high results.
Inert carrier gases and materials in chromatographic equipment. Use of equipment for analysis of impurities of the same type. Pure carrier gases and thermally stable liquid stationary phases.
Chemical conversions of main component and impurities.
Loss of some components. False peaks.
Inert sorbents and materials in chromatographic equipment.
Spreading of zone of main Masking of impurity zones by component in low concentration main component zone. range.
High-efficiency columns. Methods of reaction chromatography and selective detectors. Intermediate isolation of main component fraction with masked impurities for the purpose of concentration.
IMPROVING THIS SENSITIVITY OF T H E DETERMINATION OF IMPURITIES
41
In analyzing aggressive polar compounds, when during the analysis they may be adsorbed on the support or the equipment, or a direct reaction with components of the liquid stationary phase may occur, a useful method is 'training' of the column, i.e., the introduction of the test compound into the chromatograph at regular intervals for a considerable length of time [76]. Adsorption phenomena should also be taken into account when preparing standard mixtures for calibration of the chromatograph. In this connection. we wish t o point out the expediency of using dynamic methods, among which the most reliable are, in our opinion, the diffusion [77-85] and electrochemical [86, 871 methods. A survey ofniethods for the preparation of reference vapour-gas mixtures in the ultramicro concentration range has been published b y Popov and Pechennikova [88]. Table 2.1 lists some processes that impede the GC analysis of impurities.These features must be taken into account in developing effective procedures for the analysis of impurities. In developing procedures for the analysis of impurities in practical systems, one usually has t o overcome specific difficulties associated with insufficient detection sensitivity and poor resolution of the impurity and the main component. In order t o solve these problems in GC, a number of methods have been developed, the application of which permits the sensitivity of the determination of impurities to be increased and their separation from the main substance t o be improved.
METHODS FOR IMPROVING THE SENSITIVITY OF THE DETERMINATION OF IMPURITIES
In order to improve the sensitivity of the methods for GC separation of impurity components, i.e., t o reduce the threshold concentration o f the impurities, which can still be reliably determined, the following methods have been developed and applied: (1) increasing the size of the sample being analyzed; (2) utilization of high-sensitivity detectors; (3) non-isothernial chromatographic methods; (4) concentration techniques. We shall now consider these methods in more detail. Increasing the size of the sample being analyzed Under the conditions of elution analysis, continuous broadening of the chromatographic zones takes place in the column, and this decreases the concentration at the maximum of the impurity zone. This fact limits the sensitivity of the GC determination of the test compounds, as the impurity component can be recorded by the concentration detector only when the concentration (or flow) of the substance at the zone maximum exceeds the minimal determinable value for the detector used in a given device. In accordance with the theory of theoretical plates [53], the maximal concentration in a zone increases with an increase in the number of theoretical plates, n(4), and the sample size, 4 , and with a decrease in the retention volume, V, :
References p. 5 2
42
GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
It follows from this equation that one of the simplest methods for improving the sensitivity of a procedure is to increase the size of the test sample. When large samples are used for analysis, however, the width of the chromatographic zones increase, thereby affecting the efficiency of separation. This effect limits the application of the method of large samples to impurity analysis. In practice, when developing a satisfactory procedure, the task reduces to the selection of,the optimal sample size, which is a ‘compromise’ with respect to two factors that change in opposite directions with an increase in the test sample, namely sensitivity and com ponen t separation. Theoretical consideration of the shape of the chromatographic peak in relation to the sample size has been carried out [20, 89-91]. Ingenious calculation methods, which are of great practical and theoretical interest for the selection of the optimal experimental conditions, were developed by Kalmanovskii and Zhukhovitskii [92]. The change in column efficiency with increasing sample volume has also been studied [44,45,93-951 . If the concentration dependence of the test sample has a rectangular shape (the ‘piston rod‘ method), then, with an increase in the size of the sample (i. e. , of the width of the ‘rod’, the width of the chromatographic peak will also increase and so will its maximal concentration. As demonstrated [92], the maximal concentration of the substance in the chromatographic zone increases approximately linearly with sample size up to samples for which the ratio of the sample width to the peak width with a vanishingly small sample is 0.8. For samples of this size, the relative peak width at half-height increases by only 15%compared with the peak width with a vanishingly small sample, i.e., this unfavourable effect on separation can often be neglected. The concentration at the peak maximum in this instance is 65%of that in the sample. The analysis of impurities in monomers with the use of large samples and a chromatograph with a katharometer has been described [93-1001. With large samples, the broadening of the initial chromatographic zone is incomplete; only the edges of the zone broaden, and the elution curves have a step-like shape. This version of chromatography was proposed independently by Zhukhovitskii and Turkeltaub [IOl-1031 and Reilley et al. [lo41 and was termed stepwise chromatography [101-1031. Analysis under the conditions of stepwise chromatography has a number of advantages over the ordinary version: (1) a higher signal stability; (2) directly proportional dependence of the height of the zone (step) on the concentration of the substance in the test sample, which simplifies the calculations and the quantitative interpretation of chromatograms; (3) errors caused by small irreversible adsorption of the test substance on the sorbent layer are partially eliminated; and (4) the sensitivity slightly increases (the concentration of the substance in the chromatographic zone is equal to that in the initial sample). A drawback of stepwise chromatography compared with the ordinary variant of elution chromatography is a lower efficiency associated with the greater width of the step-like zones. Therefore, stepwise chromatography can conveniently be used for the analysis of impurities in systems whose final components can be separated very well. Stepwise chromatography can be applied to monomer analysis; for instance, Palamarchuk [ 1051 used it for determining trimethylchlorosilane and methyltrichlorosilane impurities in dimethyldichlorosilane. A stepwise method for determining water as an impurity in the butane-butylene fraction has also been described [106].
IMPROVING THL SI NSITIVITY OE THI DLTLKMINATION or IMPURITICS
43
In order t o reduce the broadening of an initial zone during sample injection and to decrease tlie size of the sampling volume, the use in stepwise chromatography has been suggested [lo71 of a therniostated sampling volume filled with a sorbent, which is blowed with the test mixture t o saturation. The use of this method for sample injection enables one to reduce the spread when introducing large samples and also t o carry out relative concentration of heavy impurities. If the sample size is not limited, it may be useful in impurity analysis to apply the method of vacancy chromatography, which was developed by Zhukhovitskii and co-workers [ 108-1 101 and also Reilley et d.[ 1041. In vacancy chromatography, the test mixture, and not the carrier gas. passes through the column. If, for instance, we introduce a certain volume of pure carrier gas into the flow of this mixture upstream of the chromatographic column, vacancies (regions of reduced concentration) are formed that move along the column at different speeds characteristic of the impurity components. Zhukhovitskii and Turkeltaub [ 1031 pointed out the following advantages of vacancy chromatography: (1) the test mixture is passed continuously and there is n o need t o use a carrier gas; (2) the sampling process is simplified; (3) the total concentration of the mixture components is measured continuously; and (4) the concentration being detennined is not measured instantaneously, but is averaged over a certain period of time. An interesting version of vacancy chromatography, which may also find application in impurity analysis, is differential chromatography [ 1 101 . In differential chromatography, the test mixture is continuously passed through the column and a mixture of pre-assigned composition is sampled periodically. The peaks on the chromatograni characteriLe the deviation of tlie current concentrations from the pre-assigned concentrations. If the concentrations of the components in the test mixture do not differ from those of the corresponding conipo~indsin the pre-assigned mixture, no vacancies (or peaks) are formed. Vacancy chromatography is currently used in the analysis of impurities in pure gases [Ill].
Use of high-sensitivity detectors An impurity can be recorded only if the following conditions are met: for a concentration detector L'max.'>
Clim.
and for a continuous flow detector
c,,x.F
jiim.
(2.9)
I n analydng impurities, the most important characteristic of the detector is the sensitivity threshold, although for practical use of a detector other characteristics are also important. When using chromatographs with ordinary katharonieters, it is possible t o determine [ 112-1 141. If special conditions impurities at concentrations of about (thermostating, flow stability, etc.) are observed, a high-sensitivity katharometer makes and when the it possible t o determine impurities at concentrations as low as katharonieter is equipped with themiisters, even 5 * 10% [ 115, 1161 . The sensitivity of the katharometer can be increased by one or two orders of magnitude compared with References p. 5 2
44
GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
the detector with filament sensitive elements by using fim-type sensitive elements (for instance, a thin layer of platinum plated on a quartz filament) [ 117, 1181 . The sensitivity of the katharometer can be increased 10-fold by using an a.c. power supply with subsequent strengthening of the signal and phase detection, also by using a reference signal [119]. In determining gaseous compounds, the sensitivity of the katharometer can also be improved by reducing the temperature of the detector block (walls) [ 1201 . For organic compounds, the sensitivity of detection with a katharometer can be increased by first converting the test compounds into carbon dioxide [121] or hydrogen [ 1221. It has been shown [123] that the last-mentioned method permits the sensitivity of the katharometer to be increased by a factor of 20-30. The katharometer is a simple, reliable and universal detector, but its application in impurity analysis is limited by its relatively low sensitivity. The katharometer is usually employed in determining impurities of inorganic gases in procedures involving a concentration stage. Ionization detectors, the sensitivity of which is 102-107 times that of the katharometer, are used much more frequently in methods for determining impurities. In order to determine organic impurities in monomers, use is generally made of the flame-ionization detector. This method was instrumental, for instance, in developing the procedure for the analysis of impurities in ethylene [67, 124-1261, propylene [127, 1281 and styrene [129, 1301. The flame-ionization detector is also used in industrial analysis for determining impurities in inonomers and inert gases [ 13, 1 31, 1321 . Improvement in detection sensitivity is one of the main aims in the development of chromatographic equipment. The importance of this problem in chromatography was emphasized by Martin [133]. Detailed surveys on chromatographic detectors have been made by Brazhnikov [134] and Adlard [ 1351. The advances achieved in recent years in the field of impurity analysis due to the use of high-sensitivity detectors with batch-produced devices are demonstrated in Fig. 2.2 [136]. It can be seen that the widely used ionization detectors permit the determination of impurities present in monomers and solvents at very small concentrations. Note that the application of concentration methods usually makes it possible to increase the sensitivity, i.e., to reduce the concentration being determined for any detector used a further 10-1000-fold. Concentration can be achieved either by employing special methods or by applying non-isothermal methods. Application of non-isothermal methods Compared with separation under isothermal conditions, the use of the thermal effects in impurity analysis enables one to increase, in the course of a chromatographic separation, the concentration of the test substance at the maximum of a chromatographic zone and hence to improve the sensitivity of determination. Also temperature programming makes it possible to reduce the analysis time and to analyze impurities that differ widely in their boiling points. The widest use in chromatographic practice is made of the temperature programming method [I371 in which the temperature is increased along the entire length of the
IMPROVING THE SENSITIVITY OF THE DETERMINATION OF IMPURITIES
45
U
C
cn
0’
6
10-8
c (%I
Fig. 2.2. Determination of impurities with high-sensitivity chrolnatographic detectors. 1 = Katharometer; 2 = flame-ionization detector; 3 = electron-capture detector; 4 = thermionic detector; 5 = helium detector; C, concentration (being determined) of component in sample.
column during the separation of a mixture. Harris and Habgood [138] noted that when the principal task is the separation of two closely spaced peaks, the best separation can probably be obtained under an isothermal regime, but for widely differing substances tern perature programming niay improve the degree of separation. The temperatureprogramming method is particularly useful when the difference in the boiling points of the sample components exceeds 50- 100°C, and in some instances (for example, when using selective sorbents and in rapid analysis) with a smaller temperature difference. Zhukhovitskii and Turkeltaub [ 139, 1401 were among the first investigators t o point out the possibility of improving the sensitivity of impurity analysis in GC by using temperature programming. Questions of the enrichment of both light and heavy impurities in temperature prograniming were specially considered by Datskevich ef al. [141], who found that in GC with temperature programming the concentration of the test compounds at the zone maximum on the initial curve niay considerably exceed (25- t o 30-fold) the concentration in the initial sample. References p. 5 2
46
GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
An important contribution to chromatographic practice was made by the work of Janlk on analysis with temperature programming [ 142- 1441 . The use of time-programmed column heating permits the time required for the analysis of impurities t o be reduced and impurities that boil over a wide temperature range to be analyzed, on a single column [128, 145-1471. A disadvantage of the thermal methods is that in some instances a reduced separation efficiency compared with the isothemial regime is obtained. Berezkin and Tatarinskii [ 1481 proposed a version of temperature programming (the ‘thermal shock’ method) that over comes this disadvantage. Zhukhovitskii and co-workers [ 149-1 531 proposed and developed a new version of GC utilizing thermal effects, namely chromatography, the application of which is particularly promising for the analysis of impurities. l o achieve a separation, chromatography uses a temperature field moving along the column; the change in the column temperature along the length of the column does not occur instantaneously, as in temperature-programmed chromatography, but over a certain period. In stationary chromatography, the direction of the movement of the carrier gas and the motion of the oven coincide; the temperature gradient in the oven has a negative value, that is, the temperature increases in the direction of movement of the oven. Molecules that ‘outrun’ the chromatographic zone for any reason enter the region of the cold sorbent, where the speed of their motion along the column decreases. Molecules that on the contrary, ‘lag behind’ the chromatographic zone enter a zone that contains hotter sorbent compared with the sorbent in the centre of the zone. The speed of molecular motion along the column in the hot zone is higher than the average speed of the molecules of a given component, and therefore the ‘lagging’ molecules quickly ‘overtake’ their chromatographic zone. Compared with chromatography with temperature programming stationary chromathermography makes it possible to obtain: (1) symmetrical peaks, even with a non-linear sorption isotherm, which does not, in chromathermography, lead to zone asymmetry, as in this instance the rear (usually broadened) edge of the chromatographic zone is at a higher temperature than its front edge; and (2) a considerable enrichment of concentration of the impurity components. In order to obtain narrow zones, it is advisable to use columns of small diameter and a fine-grained sorbent. A considerable contribution t o the development of stationary chromatography for impurity analysis was made by Kaiser. He demonstrated the possibility of the determination, in’gases, of heavy impurities at concentrations of lo-’’% when using very large samples [154-1581 . A modification of this method for determining heavy impurities in solvents and volatile liquids was proposed by Berezkin and Starostina [ 1591 ; In chromatographic practice, despite its above advantages in impurity analysis, chromathermography is used much more rarely than chromatography with temperature programming; in our opinion, this is associated with the need for more sophisticated equipment. Berezkin and co-workers [160, 1611 and later Fatscher and Vergnand [162] proposed a new version of chromathermography in which separation is effected simultaneously along the entire length of the column under a negative temperature gradient. This version of chromathermography consists in using, for separation, a constant temperature gradient dong the column together with temperature programming.
IMPROVING THE SI NSITIVITY OF THE DCTrRMINATlON O € IMPURITIES
41
The variation of temperature with time under a negative temperature gradient along the colunm is equivalent to the motion of a thennal field under conditions of chromathemiography [ 161 ] . This method perniits chr(~iiiatliennograp1iicseparation with the use of a column of any length and shape. The method was checked experimentally for the detemiination of inipurities in toluene; the concentrations at the peak maximum hicrease 10- t o 15-fold conipared with the isothennal method. An ingenious pulse-thernial method of gas analysis was developed by Dantsig [ 1631. One of the advantages o f the method tein perature prograniming is the (usually) weak dependence of the chroniatographic separation efficiency on the size of the sample being analyzed [164, 1651. The use of different versions of temperature programming considerably increases the sensitivity of the detemiination of impurities and the role and importance of nonisothemial methods in such analyses will increase in the future. Concentration methods If the conventional methods of chromatographic separation are not adequate (for instance, because of the low detector sensitivity or poor separation of the impurity and the main coinponent), special methods of sample preparation can be used (in particular, pre-concentration arid separation of the impurity from the main component). Concentration is also used when non-chromatographic niethods are employed for identification of impurities (special optical methods, mass spectrometry, nuclear magnetic resonance spectroscopy, etc.). Therefore, impurity concentration is often a necessary stage in the analytical investigation of nionomers and solvents.
Prepura t iw d u t ion c I i ro ma tography Preparative elution chromatography is widely used for concentration in impurity analysis [9, 19, 166-1701 owing to high separation efficiency, the possibility of separating comparatively large samples, repeated separation cycles and complete automation of the separation process. One of the principal sources of errors in quantitative analysis when using preparative elution GC for concentration is incomplete trapping of the separated fractions. In order to trap the fractions isolated on a preparative column, cold traps are used, the design of which depends on the size of the preparative column, the properties of the impurities isolated and the methods used for their subsequent analysis [171-1771. Roggus and Adains [ 1781 were among the first researchers who used comparative elution chromatography for the analysis of impurities. The applications of the method to impurity analysis are described elsewhere [ 179- 1851 . CIi 1’0rnatography
w i t h i i t a carrier gas (high-concen tru tion GC)
The main drawback of concentration methods based on elution chromatography is the low concentration of the impurities eluted from the chromatographic column and the need for a special operation to trap impurity components from the flow of carrier gas. References p. 5 2
48
GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
The methods of chromatography without a carrier gas (which Zhukhovitskii et al. [186] also call high-concentration chromatography) permit one to obtain, under isothermal conditions, a concentrated zone of both light and heavy impurities at the column outlet. In chromatography without a carrier gas, the procedure (the absence of a carrier gas, large samples, etc.) and the separation mechansim differ from those adopted in elution chromatography. In chromatography without a carrier gas, the zones are not separated from each other by a layer of carrier gas but adjoin each other. The different velocities of the gas flow in the zones formed and the different adsorbabilities of the components being separated are the cause of the formation of zones with clearly defined boundaries, whose broadening is limited. In contrast to isothermal elution chromatography, chromatography without a carrier gas permits enrichment such that virtually pure components are obtained at the column outlet, and the difference in the flow-rates in the chromatographic zones leads to a sharp decrease in edge broadening [186] . A concentrated impurity zone can be directed straight to the analytical column for detailed separation into individual components, or to some other analytical device for their identification. Important contributions to the development of some versions of chromatography without a carrier gas have been made by Dubinin and co-workers [187,188], Schay [41], Zhukhovitskii and co-workers [186, 189,1901, Claesson [191], Altshuller and co-workers [192, 1931 and Guiochon and Jacob [194]. A survey of new versions of chromatography without a carrier gas has been published by Sazonov [ 1951. Below we consider the principal versions of chromatography without a carrier gas that have been or may be used for impurity concentration. Frontal chromatography As a general method of concentration, frontal chromatography was developed by Mirzayanov et al. [ 1961 ;individual applications of frontal chromatography for concentration were described previously [197, 1981. Frontal concentration of poorly adsorbed impurities can be effected in two ways: (1) by filling the column before the experiment with an inert gas that is less sorbable than the impurity [196, 1991 ; or (2) in the absence of an inert gas filler, when a vacuum is set up in the column [200]. The first method is simpler as regards the equipment, while the second is useful when concentrating very light gases if it is difficult to select a filler gas that meets the condition impurity distribution constant > filler gas distribution constant. In the simplest system for the analysis of poorly adsorbed impurities, the concentration and separation columns are connected in tandem, i.e.. they are actually combined in a single column [196,201]. The method of frontal concentration has been used successfully for analyzing poorly adsorbed impurities in ethylene [196,201,202] and propylene [203], and also for determining the content of light (C4-C,) hydrocarbons in dimethylformide and acetonitrile [204]. Some time ago, a Soviet design office developed a laboratory-type gas chromatograph ('Luch'), which is also based on frontal-adsorption enrichment of light impurities in a pre-evacuated column [205].
IMPROVING THE SENSITIVITY OF T H E DETERMINATION OF IMPURITIES
49
The chromatograph can be used, in particular, for determining helium, neon and hydrogen in micro-impurities in atmospheric air, hydrogen in argon, etc. The minimal determinable concentration of light gas impurities is 10-5-10-6% (the volume of the sample analyzed being up t o 1000 ml). For concentrating heavy impurities, wide use is made of the frontal method coupled with subsequent thermal desorption [206-2131. In this method, a definite amount (up to tens of litres or more) o f the sample being analyzed is passed through a trap (sometimes a cold trap) filled with sorbent. It should be noted that the use of high temperatures for the desorption of impurities from the concentration column may lead t o various undesirable side reactions, especially when determining unstable compounds. Therefore, in many instances it is better t o use the method of displacement (washing out) of impurities with a suitable solvent [ 2 1 4 , 2 1 5 ] . All the above methods for adsorbing impurities in a concentration trap are based on complete absorption of heavy impurities by the trap packing from the entire volume of the gas being analyzed. A radically different method was proposed by Novik et al. [216], in which the gas sample is passed through a small concentration column (with the corresponding stationary phase) that is at the tem perature of the surrounding medium, until the test impurities begin t o emerge, i.e., the concentration of the impurities in the sorbent is in equilibrium with that in the initial mixture along the entire length of the column. This method has the following advantages: (1) there is no need for accurate measurement of the volume of the gas passed through the trap, it being sufficient t o determine its excess and to know the precise temperature of the concentrator; (2) the method permits a selective increase in the sensitivity of impurity determination or elimination of components that interfere with determination by selecting the appropriate packing (for instance, the use of a non-polar liquid phase can eliminate the effect of water vapour, which is sometimes the cause of difficulties in sorption methods of concentration); and (3) there is a possibility of ‘smoothing out’ the amount of individual components in the trap, as the concentration effect usually increases proportionally with the increase in molecular weight. A disadvantage of the method is the need to maintain a constant temperature during sorption (concentration). A technique for the application of this method in analyzing monomers (ethylene and propylene) was successfully developed by Lulova and co-workers [2 17-2 191 . When analyzing heavy impurities, Zhukhovitskii and Turkeltaub developed a modification of the frontal method, the thermodynamic method [ 1 9 , 2 2 0 , 2 2 1 ] , which has been widely applied by both Soviet and other workers [ 154-1 58, 222-2241. In the thermodynamic method, a heated oven with a negative temperature gradient is displaced periodically along the column while the mixture is fed continuously into the column. The heavy components (impurities) adsorbed at the head of the column start t o move along the column in the oven zone under the effect of the heat field of the test mixture and are periodically eluted from the column. The role of the carrier gas in this instance is played by the main component of the test mixture. In order t o increase the working length of the layer, it is advisable t o use a circulation diagram [225] and a moving sorbent [226]. For the purpose of concentrating impurities, use has been made successfully (in addition t o the frontal method) of other modifications of chromatography without a carrier gas: References p. 52
v1
TABLE 2.2
0
METHODS OF ANALYTICAL REACTION GC FOR THE ANALYSIS OF IMPURITIES Reacting compound
Main substance
Impurity
Carrier gas
Change in characteristics of test compounds as a result of chemical reactions
Change in characteristics of detection of test compounds as a result of chemical reactions
Retention time increases
Detection sensitivity increases
Retention time decreases
Detection of impurities against background of main substance forming non-detectable compound [2611
Separation of impurities and main component forming low-volatility compound with reagent [249] Frontal-chemical concentration [251-2531 Impurity concentration by using chemical absorbents forming low-volatility compound with impurity [254,255]
Detection sensitivity decreases
4:
3a
Conversion of nondetectable Separation of inipurity and compounds into compounds main component by recordable by high-sensitivity converting impurities into volatile compound with the detectors, by means o f use of: (a) b u b h h g liquid (a) single-stage conversion reactor [256] or (b) tubular [258] or (b) two-stage reactor [257] conversion of compounds that do not contain carbon [259] Non-selective concentration of zones of impurities and main substance as a result of chemical binding of part of carrier gas [260]
n
z>
z U m
Z
DEMASKINC IMPURITIES
51
thermal displacement [227-2301, displacement [231-2331 and elution-thermal displacement chromatography [234-2361. Wide use has also been made of non-chromatographic methods of concentration, which are combined with subsequent GC analysis. Of course, in order t o solve a practical problem by the optimal concentration method, any selective separation method can be used, such as distillation [237], recrystallization [237], extraction [238], liquid column chromatography [239, 2401 and thin-layer chromatography [241] . The phase equilibrium method is particularly promising as a method of concentration for subsequent detemiination of trace impurities by gas chromatography [242]. A detailed survey of the method has been published by Vitenberg et al. [243] .
METHODS FOR DEMASKING IMPURITIES AGAINST THE BACKGROUND OF THE MAIN COMPONENT When analyzing impurities in specially pure substances, apart from the usual problems of separation of all impurity components of a complex mixture, which often have similar properties, other problems arise that are associated with the masking of the zones of the impurity components by the broad zone of the main substance. Utilization of selective sorbents and selective detectors
As the final result of a chromatographic analysis is determined by characteristics of both the sorbent and the detector, in the analysis of impurities the use of selective sorbents and detectors is of special importance. To determine the test impurities that are masked by the zone of the main component, one can use a selective sorbent on which the retention times of the impurity and the main components differ widely. Another solution t o this problem consists in the use of a selective detector whose sensitivity t o the impurity component is much higher than that t o the main substance. In this version, it is also possible t o detemiine the content of the impurity component, even if the test impurity is eluted in the zone of the main substance. The sorbent selectivity (a) for determining impurities must meet the condition
a > 1 +%
(2.10)
tm
where (Y = r i / r , , ri and tm are the retention times of the impurity and the main component, respectively; and p m is the width of the zone of the main component (the region of the chromatographic zone in which impurities cannot be determined). For a preliminary choice of a selective LSP it is advisable to use published tables of relative retention times. In gas-liquid chromatography, selective stationary sorbents of various types are known (complexing phases, liquid crystals, volatile solvents, etc.), which have been surveyed in detail [15, 16, 18, 2441. The selectivity of solid sorbents has been comprehensively discussed by Kiselev and Yashin [62]. As the column efficiency with respect t o the main component increases, i e . , when p, decreases, the requirements imposed on the selectivity o f the sorbent are relaxed. Therefore, particular care must be taken t o reduce the zone width of the main component. References p. 52
52
GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
In recent years, selective detectors have found wide application in GC; they show high sensitivity only to certain groups of compounds. Comprehensive surveys of selective detectors has been published by KrejEi and Dressler [245] and Adlard [246]. The mass spectrometer is the most selective detector. The use of combined chromatography-mass spectrometry for the analysis of impurities is a particularly promising method [247-2481. Analytical reaction GC
In recent years, a new aspect of GC has been developed, namely analytical reaction GC, which has been applied successfully in the analysis of impurities. It is interesting to note that even in the first investigations on analytical reaction GC, problems of determining impurities masked by the main component [249] and of improving detection sensitivity [250] were solved. The combination of the chromatographic and chemico-analytical methods resulted in a new method, the potential of which is much greater than those of either of the two original methods. The combined methods of analytical reaction gas chromatography that have been developed for analyzing impurities, assuming different reactivities of the impurity and main components, are listed in Table 2.2. Methods of analytical reaction chromatography are currently widely used in analytical practice [262] . REFERENCES 1 L. S. Kofman and V. S. Vinogradova, Izv. Akad. Nauk SSSR, Ser. Khim, (1965) 375. 2 V. A. Kargin, Osnovnyie Problemy Khimii Polimerov (Principal Problems of Polymer Chemistry), Plenarnyi Doklad na VIII Mendeleyevskom Syezde PO Obshchey i Prikladnoy Khimii, Academy Press, Moscow, 1958. 3 S. E. Bresler and B. L. Erusalimsky, Fizika i Khimya Makromolekul (Macromolecule Physics and Chemistry), Nauka, Moscow, Leningrad, 1965. 4 Kh. S . Bagdasarian, Teoriya Radikal'noy Polimerizatsii (Radical Polymerization Theory), Nauka, Moscow, 1966. 5 E. B. Lyudvig, A. R. Gantmakher and S. S. Medvedev, Dokl. Akad. Nauk SSSR, 156 (1964) 1163. 6 R. H. Biddulph, P. H. Plesch and P. P. Rutherford, J. Chem. Soc., (1965) 275. 7 K. Ueno, K. Hayashi and S. Okamura, Polymer, 7 (1966) 431. 8 P. Auger, Current Trends in Scientific Research, UNESCO, Geneva, 1963. 9 S. Dal Nogare and R. S. Juvet, Gas-Liquid Chromatography, Interscience, New York, London, 1962. 10 V. Svojanovsk?, M. KrejEi, K. Tesa'fik and J. Janak, Chromatogr. Rev., 8 (1966) 90. 11 V. G. Berezkin and V. S. Tatarinskii, Gas Chromatographic Analysis of Trace Impurities, Consultants ' Bureau, New York, London, 1973. 12 R. Kaiser, in K. V. Chmutov and K. I. Sakodynsky (Editors), Uspekhi Khromatografii, Nauka, Moscow, 1972, p. 193. 1 3 E. V. Vagin, in K. V. Chmutov and K. I. Sakodynsky (Editors), Uspekhi Khromatografii, Nauka, Moscow, 1972, p. 262. 14 V. G. Baranova, A. G. Pankov and N. K. Loginova, Metody Analiza v Proizvodstve Monomerov dlya Sinteticheskikh Kauchukov (Methods of Analysis in Production of Monomers for Synthetic Rubbers), Khimiya, Moscow, 1975. 15 E. Leibnitz and H. G. Struppe (Editors), Handbuch der Gas-Chromatographie, Akademische Verlagsgesellschaft, Leipzig, 1966. 16 R . Kaiser, Chromatographie in der Gasphase, Bibliographisches Institut, Mannheim, 1969. 17 J. C. Giddings, Dynamics of Chromatography, Part I, Marcel Dekker, New York, 1965. 18 W. R. Supina, The Packed Column in Gas Chromatography, Supelco, Bellefonte, Pa., 1974.
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GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
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GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS
223 H. Pauschmann, Z. Anal. Chem., 236 (1968) 159. 224 H. Oster, Siemens-Z., 42 (1968) 703. 225 A. A. Zhukhovitskii and N. M. Turkeltaub, Gazovaya Khromatografiya, Trudy I Vsesoyuznoy Konferentsii, Academy Press, Moscow, 1960, p. 107. 226 V. R. Alishoyev, V. G. Berezkin and V. P. Pakhomov, Izv. Akad. Nauk SSSR, Ser. Khim, (1967) 686. 227 R. Henjes, 0 1 Kohle, 14 (1938) 1079. 228 N. Turner, Nut. Pet. News, 35 (1943) 234. 229 M. I. Yanovsky, Candidate's Thesis, Institute of Physical Chemistry, Academy of Sciences of t h e USSR, Moscow, 1947. 230 0. V. Altshuler, 0. M. Vonigradova, V. R. Linde, S. 2. Roginsky, Yu. N. Chirkov and M. 1. Yanovsky, Gazovaya Khromatografiya, Trudy I1 Vsesoyuznoy Konferentsii, Nauka, Moscow, 1964, p. 198. 231 E. Gliieckauf and G. P.Kitt, Vapour Phase Chromatography Symposium, Butterworths, London, 1957, p. 422. 232 0. V. Altshuler, 0. M. Vinogradova, 0 . 2 . Roginsky and Yu. N. Chirkov, Dokl. Akad. Nauk SSSR, 152 (1963) 892. 233 0. V. Altshuler, 0. M. Vinogradova, S. Z. Roginsky and Yu. N. Chirkov, in A. A. Zhukhovitskii (Editor), Gazovaya Khrornatografiya, Trudy 111 Vsesoyuznoy Konferen tsii, Izdaniye Dzerzhinskogo Filiala OKBA, Dzerzhinsk, 1966, p. 317. 234 V. G. Berezkin and E. G. Rastyannikov, Neftekhimiya, 6 (1966) 487. 235 V. G. Berezkin and E. G. Rastyannikov, Izv. Akad. Nauk SSSR, Ser. Khim. (1969) 749. 236 V. S. Mirzayanov and V. L. Dyuzhev-Mal'tsev, Zh. Anal. Khim., 29 (1974) 185. 237 V. G. Arakelian, L. S. Sarycheva, N. G. Dzhurinskaya, Z. N. Rasskazova, Z. F. Azarian, E. A. Rozina and V. P. Evdakov, Gazov. Khromatogr., No. 10 (1969) 96. 238 S. D. Nogare and L. W. Safranski, J. Chem. Educ., 35 (1958) 14. 239 J. J. Kirkland (Editor), Modern Practice of Liquid Chromatography, Wiley-Interscience, New York, London, Sydney, Toronto, 1971. 240 V. R. Alishoyev, V. G. Berezkin and V. S. Tatarinskii, Zavod. Lab., 34 (1968) 148. 241 1. Klesnient and A. Kasberg,Izv. Akad. Nauk Est. SSR Khim., Geol., 17 (1968) 258. 242 B. V. Ioffe, A. G. Vitenberg and V. N. Borisov, Zh. Anal. Khim., 27 (1972) 1811. 243 A. G. Vitenberg, B. V. loffe and V. W. Borisov, Chromatographia, 7 (1974) 610. 244 B. L. Karger, Anal. Chem., 39 (1967) 24A. 245 M. KrejEi and M. Dressler, Chromatogr. Rev., 13 (1970) 1. 246 E. R. Adlard, Crit. Rev. Anal. Chem., 5 (1975) 13. 247 R. Schubert, Anal. Chem., 44 (1972) 2084. 248 T. A. Gough and K. S. Webb, J. Chromatogr., 64 (1972) 201. 249 N. H. Ray, Analyst (London), 80 (1955) 853 and 957. 250 G. E. Green, Nature (London), 180 (1957) 295. 251 V. G. Berezkin and 0. L. Gorshunov, Izv. Akad. Nauk SSSR, Ser Khim., (1965) 2069. 252 V. S. Mirzayanov, V. G. Berezkin and A. Datskevich, Author's Certificate. 171,660 (1964); Byull. Izobr., No. 11 (1965). 253 V. S. Mirzayanov and V. G. Berezkin, Metody Analiza i Kontrolya Y Khimicheskoy Promyshlennosti, No. 1, NIlTEKhim, Moscow, 1966, p. 28. 254 V. G. Berezkin and 0. L. Gorshunov, Zh. Anal. Khim., 21 (1966) 1487. 255 V. G. Berezkin, 0. L. Gorshunov and 2. P. Markovich, Zavod. Lab., 29 (1967) 1067. 256 V. G. Berezkin, A. E. Mysak and L. S. Polack, Neftekhimiya, 4 (1964) 156. 257 E. Bayer, Angew. Chem., 69 (1957) 732. 258 U. Schwenk, H. Hachenberg and M. Fordcrreuther, Brennst.-Chem, 42 (1961) 295. 259 V. G. Berezkin, A. E. Mysak and L. S . Polak, Izv. Akad. Nauk SSSR,Ser. Khim., (1964) 1871. 260 K. A. Golbert, 0. L. Gorshunov and V. G. Berezkin, Zavod. Lab., 29 (1967) 799. 261 V. S. Mirzayanov, V. G. Berezkin and V. A. Nikol'sky, Zh. Anal. Khim., 21 (1966) 1239. 262 E. E. Kugucheva and A. V. Alekseyeva, Usp. Khim., 42 (1973) 2247.
Chapter 3
The study of polymer formation reactions In reactions for the formation of polymers, volatile substances are generally used as starting materials, and in some reactions (for instance, polycondensation) volatile products are released [ 11 . As a rule, the presence of a polymer in the reaction mixture does not hinder the use of gas chromatography (GC). Methods for determining volatile products in polymer systems have been developed in great detail (see Chapter 4). Therefore, GC methods can be used directly for determining the reaction kinetics from the changes in the concentrations of the monorners consumed or the products formed. In connection with the application of GC methods for studying the kinetics of the reactions of formation or transformation of macromolecules in relation t o the type of reaction, it was found expedient t o consider the application of GC to the study of polymerization (copolymerization) and polycondensation reactions and some chemical transformations of macromolecules.
INVESTIGATION OF POLYMERIZATION REACTIONS In the kinetic scheme of radical polymeri7ation, it is possible, in most instances, to single out three stages (see, for instance, ref. 2): (1) R-R+2R', ( 2 ) R,' M+R,+ ' and (3) R,' + R , '+R,+ 1. Reaction (1) represents initiation (formation-of radicals from initiator molecules), reaction (2) is the growth of the polymer chain and reaction (3) is chain te 1-11ina 1 t ion. For the simple scheme, the total rate of radical polymerization is described by the equation
+
where I is the rate o f initiation (for chemical initiation, I = 2fidis.c,where f i s the initiation efficiency, kdis, is the disintegration rate of the initiator and c is the initiator concentration), k , , is the velocity constant of the growth of the polymer chain, kter,is the chain termination constant and [MI is the monomer concentration. It follows from eqn. 3.1 that the rate of polymerization depends on the rate of the initiation reaction, the ratio of the constants of the rates of the reactions of growth (2) and rupture (3) and the monomer concentration. If we know the rate of the initiation reaction, we can determine, from eqn. 3.1, the ratio kgr./kter,l/':
This ratio is usually determined by studying the kinetic characteristics of the polymerization rate in the steady state. In order to determine the absolute values of the constants kter.and k , , , it is necessary t o determine the value of one of these constants References p. 80
60
POLYMER FORMATION REACTIONS
or another ratio between them. Detailed surveys of the principal methods for determining the kinetic parameters of the polymerization reaction have been published [2-51. The general principles of the kinetics and equilibrium of chemical reactions in solutions were considered by Moelwyn-Hughes [ 6 ] . Experimental methods are usually based on the determination of the time dependence of the polymer concentration in the reaction medium. The kinetic parameters of a reaction, however, can be determined, as follows from eqn. 3.1 also by measuring the decrease in the concentration of the monomer as it is consumed in the polymerization reaction. In order to determine the monomer concentration in the reaction medium, it is useful to apply GC [7-lo]. The use of GC has certain advantages. In Table 3.1 GC is compared with dilatometry, which is evidently the most widely used method for determining the kinetic parameters of polymerization reactions. The only advantage of dilatometry compared with chromatography in our opinion, is, that with concentrated solutions its application in simple cases enables one to determine the degree of conversion of the monomer with greater accuracy, especially at low degrees of conversion. As regards the other characteristics compared in Table 3.1, however, it is preferable t o use GC. The field of application of GC is wider; it can be used for determining the kinetic parameters in highly dilute solutions. Investigations of fast reactions in such solutions is of special interest [ l l ] . The information obtained when applying GC is also much more extensive. In this method, one can study, in addition to the main polymerization reaction, side reactions that occur in the reaction medium (for instance, isomerization). There is also the possibility of recording the polymer concentration continuously (particularly at very low concentrations) by direct use of transport-disintegration detectors for liquid chromatography. In these detectors, the following operations are effected continuously and automatically: (1) removal of the solvent from the eluate, as GC detectors are equally sensitive to solvents and test substances; and (2) transfer of the disintegration products of the non-volatile substance (polymer) to the gas phase. In order t o obtain optimal results, the eluate emerging from the chromatographic column is usually applied to the moving eluate support, which is a transporter made in the form of metallic helix or ribbon. In its further mechanical motion, the eluate support passes successively through two heated zones; in the first, the solvent is removed from the eluate with the aid of the gas flow, and in the second, the eluate support, together with the test substance, is heated in an atmosphere of inert gas to the temperature of pyrolysis of the substance being analyzed. The volatile pyrolysis products are blown by the flow of the inert carrier gas into the GC detector, where they are detected. The detector signal is proportional t o the amount of the substance being detected. In order t o investigate the polymerization kinetics, the chromatographic column must be replaced by a reactor, the reaction mixture being applied continuously to the transporter. Surveys on this type of detectors have been published [12-151. If pure initial monomers are not readily available, it is better to use GC for the study of the kinetics, as it requires less material than dilatometry. The sample volume required for a single GC ana!ysis is about 1 p1 and the amount of monomer sufficient for a series of measurements thtrefore does not exceed 0.1 ml.
POLYMERIZATION REACTIONS
61
TABLI., 3.1 COMPARISON OF GC AND DILATOMETRIC MITHODS FOK DFTERMINING TIlE KINETIC PARAMETERS OT: POLYMl+,RIZATIONRLACTIONS --
Characteristic
Dilatometric method
GC method
Principle
Determination of dccrease in volunie of reaction mixture as a result of formation of a polymer characterized by higher density than that of initial monomer Degree of conversion with concentrated solutions can usually be determined to within 0.1% Study of polymerization reactions using pure monomers and concentrated solutions; sensitivity of the method reduces with decreasing monomer concentration i n reaction mixture Sensitivity of method increases with amount (volume) or reacting monomer in reaction mixture; necessary monomer volume is 10-50 nil Equipment is siniplc b u t nonstandard; method is rather time consuming as any change in polymerization conditions necessitates re-calibrat ion of equipment
GC detcrniination of monomer concentration in samples periodically collected from reactor
Accuracy
Field of application
Volume of reaction mixture
Equipment and expcrimental proccdurc
Dcgrec of conversion can be determined to within 0.5-5 % (relative) Study of polymerization reactions in concentrated and highly dilute solutions (up to 0.001-0.0001% initial concentrations of niononiers in reaction medium) Minimal amount (volume) of monomer necessary for one experiment is small, and when using dilute solutions is 0.01-0.005 a Analytical cquipmcnt (chromatograph) is complcw, b u t standard. calibration o f chromatograph is independent of polymerization conditions
i n 1 if the kinetics. The amount of monomer can be further reduced t o 5-10 * are studied in dilute solutions. Hence the GC method of control permits the use of seminucro- and microscale methods for studying polymerization kinetics. In studying polymerization kinetics over a wide temperature range, as well as in investigating changes of other parameters (solvent, catalyst, etc.), dilatoinetry is more laborious than GC because a change in polymerization conditions may lead t o a change in the type of polymer obtained (atactic, syndiotactic, etc.), each type having a different density, and also to a change i n the rate of side reactions, the products of which may differ in density from that of the initial monomer. Therefore, in experimental investigations of this nature, changes in the polynierization conditions usually necessitate the t iine -con s um ing re-calib ration of the dil at ome te r. From a comparison of the dilatometric and GC methods of studying polymerization kinetics, one can conclude that GC is the preferred method in most instances. 111 fact, GC is being used more and inore often in investigations o f the kinetics and mechanisms of polymerization reactions [ 16-27] .
Refercnccs p. 80
62
POLYMER FORMATION REACTIONS
An important contribution to the development of GC methods for investigating polymerization reactions was made by Guyot and co-workers [16, 17, 19-22]. Guyot was one of the first workers to realize the potential of this method for determining the kinetic parameters of the polymer formation reaction. He applied GC in his studies of the polymerization of butadiene [16, 17, 191, propylene [17, 19, 201 and vinyl chloride [21]. At first, Guyot et al. [17] proposed a method for controlling the degree of conversion of a monomer based on the analysis of samples collected from the gas phase in an autoclave over the mixture being polymerized. Later [16], in studying faster reactions with the aim of eliminating possible errors due to the deviation of the composition of the gas phase from the equilibrium composition, the sample of the reaction mixture was collected from the liquid phase. The latter method was used in studying the stereospecific polymerization of butadiene, which is in the gaseous state under ordinary conditions. The reaction was carried out in an autoclave at a low pressure in benzene solution [16]. In order to determine the degree of conversion, liquid samples (0.2-0.5 ml) were collected periodically from the reactor and evaporated in a pre-evacuated vessel (500 ml). An aliquot of the sample from this vessel was analyzed chromatographically. For the internal standard [7, 181 use was made of n-butane, which was previously introduced into the reaction mixture. In another, simpler method [24-261, the reaction is usually carried out in a small themostated reactor in a protective atmosphere of an inert gas, the pressure of which is slightly higher than atmospheric. The degree of polymerization is determined on the basis of the GC analysis of samples of the reaction mixture. Samples are collected periodically from the reactor by means of a syringe with a long needle, which is introduced into the reactor through a cap made of self-sealing rubber similar to that used in the sample introduction system of a gas chromatograph. The collected sample is quickly transferred to a test-tube with a.reagent that immediately terminates the polymerization reaction. In order to avoid the need for accurate measurement of the volumes of the reagent and reaction mixture, use is made of the internal standard method, the internal standard method being previously introduced into the reaction mixture at a known concentration. The internal standard is a compound that does not affect the polymerization reaction, is stable (chemically inert), is soluble in the reaction mixture and can be well separated from all of the volatile components of the reaction mixture by chromatography. The liquid sample collected from the reactor, together with the polymerization inhibitor introduced into it, is analyzed by GC on standard equipment. In order to separate the non-volatile polymer from the volatile components of the reaction mixture, a special cartridge (a short column) with an inert packing is introduced between the column and the sample introduction system, or a special sample introduction system is filled with an inert, friable material (for instance, a support). The non-volatile polymer accumulates on the inert packing, which is replaced periodically. The degrees of conversion in the polymerization reaction is determined by calculating the change in the concentration of the volatile reaction components from the chromatograms of the reaction mixture.
POLYMERIZATION REACTIONS
63
All of the above operations can be automated, and to determine the polymerization kinetics a special device can be developed on the basis of the industrial automatic chromatograph . When investigating processes that occur at moderate rates, GC is usually applied for determining the content of the volatile components in ‘frozen’ reaction mixtures. Unfortunately, direct injection of liquid reaction mixtures into the chromatograph is often impossible because of the presence of substances that interfere with the analysis or because the reaction is resumed upon evaporation of the sample. Therefore, samples are usually subjected t o a preliminary treatment, which is often time consuning and is a potential source of errors. To obviate these difficulties in the application of GC for investigating many liquidphase reactions, Levitin [28] attempted to use an analysis of the vapour phase, because in vapours the reaction usually ceases automatically, often owing to the non-volatility of one of the reagents or the catalyst. Also, many types of reaction do not proceed in the gas phase or proceed much more slowly than in solution. Below we consider the conditions for studying liquid-phase reactions by vapour analysis, the possible errors of the method and its field of application. The suitability of the method for kinetic investigations has been verified for the reduction of butyl bromide and benzyl chloride with sodium borohydride in dimethylformamide. Let us assume that the liquid-phase reaction passes through a non-flow-through system at a constant temperature. If the composition of the solution changes slowly enough, so that an interphase equilibrium is established, it is possible, in principle, to determine the concentrations of the volatile components by using the results of vapour analysis. In the general case, for real multiconiponent solutions, this problem is very complicated. In kinetic investigations it is often simplified because use is usually made of solutions that are diluted with respect to the components being analyzed, and the measurements are carried out at the lowest possible degree of conversion, so that the composition of the solution changes only insignificantly. Also, when using vapour analysis, it is important to have an inert standard substance in the solution. In this instance, the use of an internal standard not only eliminates the need for knowledge of the sample size, but also enables one to eliminate or at least greatly reduce the dependence of the results of the analysis on the change in the concentration of other components, and also on surface phenomena (increase in partial pressures because of the formation of drops of intensive mixing, which is necessary for the rapid attainment of solution-vapour equilibrium). Therefore, the reacting solution can be regarded as pseudo-binary relative to the component being analyzed. In order t o calculate the concentrations, it is necessary to carry out a calibration, i.e., t o determine from the experimental points the following dependence, at constant temperature:
for cStd,= constant, where i is the ratio between the signals of the test components and the standard, and c and cStd,are their concentrations. Calibration naturally hdS to be effected by using non-reacting solutions, i.e., in the absence of one of the reagents or the catalyst. The resulting inaccuracy can be removed, at least partly, by adding, during calibration, a substance similar to the substance that is lacking in such an amount References p. 80
64
POLYMER FORMATION REACTIONS
that the principal thermodynamic characteristics of the reacting solution could be reproduced (for instance, the ionic strength). The calibration and the calculation of the analytical results are simplified if IHenry’s law is still obeyed accurately enough in the adopted concentration range of the component being analyzed and, in addition, the signal is proportional t o the amount of substance in the saniple. Under these conditions, the relative signal for the substance in the vapour is proportional to its concentration in solution. It is obvious that the lag of the vapour from the changes that occur in the solution must lead t o systematic errors in kinetic measurements. The measured product concentrations will be lower than the true values, while those of the initial substances will be greater. It thus follows that the method always produces slightly low values of the kinetic const ants. For the quantitative determination of the errors, it is necessary t o consider the kinetics of interphase mass transfer. In kinetic experiments, when the major proportion of the component being analyzed must be in solution, its partial pressure is insignificant compared with the total pressure if the latter is at atmospheric or higher pressures. Under these conditions, the resistance t o mass transfer is determined by the transport of the substance from the interface, where phase equilibrium is achieved, to the gas mass [29]. The change in the partial pressure of the component resulting from convective diffusion can be described approximately by the equation dp/dt = K ( P ~ p ) . The equilibrium partial pressure, p e , depends only on the processes in the liquid phase, which contains most o f the molecules of the component. The effect of the nature of the migrating substance on the rate of mass transfer is determined by the diffusion coefficient of the vapour of this compound in the given gas, and the dependence of the mass transfer coefficient, K , on the diffusion coefficient, D , is lower the more intensive is the mixing. For a turbulent regime, it is taken that K D ’ / 3 [29]. For a wide range of organic substances, the diffusion coefficients in vapours differ not more than two-fold [ 3 0 ] .This fact (taking into account the equation) makes it possible to approximately equate the mass transfer coefficients of different components, the error not exceeding 30%. In the range of applicability of Henry’s law, the systematic error in concentration determinations due t o the non-equilibrium composition of the vapour is equal t o the deviation of the partial pressure from the equilibrium value. The use of the equation permits this error t o be expressed through values determined experimentally:
The equation makes it possible, in doubtful instances, t o check whether the use of the method has led t o considerable errors. The sensitivity of the device restricts the application of the method to the determination of high-boiling components of the reaction mixture in accordance with the inequality n-. < PV/RT, where n-. is the minimal determinable amount of a given substance in the sample and V is its volume. In order to evaluate the limits of applicability of the method, Levitin [28] adopted the following values in this inequality: I/ = 1 ml, T = 300°K and nmin.= lO-’and 10-’Omole for katharometers and ionization detectors,
POLYMERIZATION REACTIONS
65
respectively. In this instance it is necessary that the vapour pressure of the test component over the solution should not exceed 2 and 2 * 10-3 mmHg in analysis with a katharometer and an ionization detector, respectively. This condition, although very tentative, shows that when using high-sensitivity ionization detectors, the method is suitable for determining the concentrations of a wide range o f substances. The suitability of the method for kinetic investigations has been checked for the reduction of certain halogen alkyls with sodium borohydride in diniethyl formamide. It is shown that when using standard reaction devices, the method can be used for studying processes with a semiconversion of over 10 min. As an example of the application of GC t o the study o f kinetics, let us consider in more detail the investigation of the kinetics of the polymerization of vinylcyclohexane conducted by Kleiner andco-workers [25, 261. The specific feature of this reaction that complicates its investigation is that the polymerization reaction is accompanied by some side reactions. The concentration of the components of the reaction mixture was determined on a 50 m X 0.2 mm I.D. capillary column with squalane as the liquid stationary phase at 80°C. The carrier gas was nitrogen, the hydrogen velocity was 40 ml/min and the air velocity 600 ml/niin. The sample size was 8 pg. The analysis was conducted on a Chrom-2 chromatograph. The kinetics of polymerization and other transformations of vinylcyclohexane in the presence of various complex catalysts were determined on the basis o f the changes in the concentration of the monomer and other by-products in samples collected periodically from the reaction mixture. The collected sample was immediately spiked with isopropanol in order to ‘freeze’ the reaction. To simplify the quantitative calculations, tz-octane was introduced into the reaction mixture as the internal standard. Fig. 3.1 shows chromatograms from the analysis of a reaction mixture at (a) 80°C and (b) 30°C. The analysis at 30°C was carried out with the aim of separating the isomers o f ethylcyclohexane. It follows from the results that a number o f side reactions take place in the reaction medium in addition t o polymerization, and they yield I-, 3and 4ethylcyclohexane, ethylidenecycloliexane and etliylcyclohexane. The concentration of the polyvinylcyclohexane formed was calculated on the basis of the results of the chromatographic analysis using the material balance equations. The calibration plot is given in Fig. 3 . 2 . The relative error is about 1%. Some of the results obtained are given in Fig. 3.3. It can be seen that the application of GC permits the kinetics not only of the polymerization reaction, but also of the side reactions, to be determined. Ilyina et al. [24] used GC for determining the sequence of the polymerization reaction of propylene sulphide with respect to the monomer in order to study the dependence of the reaction rate constant on temperature and the dependence o f the product yield on the catalyst concentration (isobutylaluminium chloride). The method permitted the optimal conditions for carrying out the reaction t o be determined. The GC determination o f volatile components in the reaction mixture thus makes it possible t o study the kinetics of polymerization and of side reactions and t o find the optimum conditions for the final reaction. We believe that GC methods and equipment can also be used for the direct determination of the reaction kinetics from the polymer yield if the yield of one component References p. 80
POLYMER FORMATION REACTIONS
1
+
~~~
~
I
10
I , , . .
<
72
)
FQ
.
I
#
’
hl
&
-,k3
:g, 1.5
’
137 ’
I
llrne (rn,”)
Pig. 3.1. Chromatograms of the reaction mixture formed o n polymerization of vinylcyclohexane. (a), Analysis temperature 80°C. Peaks: 1 = isopropanol (reaction inhibitor); 2 = n-heptane (solvent); 3 = n-octane (internal standard); 4 = vinylcyclohexane; 5 = ethylcyclohexane; 6 = ethylidenecyclohexane. (b), Analysis temperature 30°C. Peaks: 1 = n-heptane; 2 = vinylcyclohexane; 3 = ethylcyclohexane; 4 = 1-ethylcyclohexane; 5 = 3-ethylcyclohexane and 4-ethylcyclohexane; 6 = ethylidenecyclohexane. The dotted line (peak 7) is a mixture of isomers of vinylcyclohexane with an internal double bond, eluted from the column as a combined zone a t 80°C (the retention time at this temperature is 13.6 min).
_1
20 lh/lOhO
Pig. 3.2. Calibration graphs for determining the content of vinylcyclohexane ( l ) , ethylcycloh’exane (2) and isomers of vinylcyclohexane with an internal double bond (3) in reaction mixtures by the internal standard (n-octane) method. G/G, is the ratio of the concentration of the test component to that of the internal standard and lh/l,h, is the ratio of the product of the retention time and the peak height of the test component to the corresponding values for the internal standard. The retention time and peak height are determined directly from the chromatogram and are usually expressed in millimetres.
or the total yield of the products of the thermal degradation of the polymers formed is measured quantitatively. In order to carry out the degradation of polymers, one can use either the pyrolytic chromatography technique [31] or the recently developed systems for detection in liquid chromatography [ 12, 321 based on continuous collection
POLYMERIZATION REACTIONS
10
t
t
Trme (rnBnj
Fig. 3.3. Kinetic curves for tlie conversion of vinyicyclohexane in the presence of various catalysts:
+ TiCI,; (b), TiCI,; (c), A1(C,HJ3; (d), Al(iso-C,H,),. Curves: 1 = polyvinylcyclohexane; 2 = ethylcycloliexane; 3 = vinylcyclohexane; 4 = isomers with internal double bond.
(a), Al(iso-C,H,),CI
of the sample on a moving wire and subsequent evaporation of the volatile compounds arid degradation of the non-volatile compounds, the volatile products of which are recorded with a high-sensitivity GC detector. With this method, it will be possible t o determine very low degrees of conversion with sufficient accuracy. It should be noted that GC can undoubtedly be used for determining other characteristics of polymerization reactions. In particular, it can be applied for studying the kinetics of the initiation reaction, investigating the structures of composite catalysts, determining changes in the catalyst concentration during polymerization and measuring elementary polymerization constants with the use of known kinetic methods (method of inhibition, method of a revolving sector, etc.) based on the deterinination of the degree of conversion under different reaction conditions. Guillot and Guyot [ 3 3 ] also used GC for studying the constants of the chain transfer reaction. The previously used methods based on ainperometric titration, utilization of tracer atonis or determination of the average molecular mass of the polymer fornied are very specific or insufficiently accurate. In order to determine the chain transfer constants, Guillot and Guyot [ 3 3 ] used the results of the GC analysis of a reaction mixture into which an inert internal standard had been injected. This procedure made it possible t o obtain, in one experiment, the dependences of the conversion of monomer and the consumption of the chain transfer agent on time. This method was used for determining the chain transfer constants in the free radical polymerization of styrene in the presence of rz-butylmercaptane. The reaction was carried out in a reactor at a low nitrogen pressure and 60°C. The reaction started upon addition of 0.8 g of azobisisobutyronitrile to tlie reaction mixture (240 g of toluene, 93.8 g of styrene and 0.222 g of n-butylniercaptan). The collected samples were cooled at 0°C and an aliquot was analyzed on a chromatograph with a flame-ionization detector. The separation was conducted at 110°C on a column (500 X 0.2 cni) filled with Chromosorb W impregnated with 1076 silicone elastomer SE-30. Fig. 3.4 shows the kinetic curves for the consumption of styrene (1) and n-butylinercaptan (2). It can be seen that 17-butylniercaptan reacts almost completely, the degree of monomer conversion being 15%. The transfer constant, k A ,was determined References p. 80
68
POLYMER FORMATION REACTIONS
'O4A
o,o/02
';,.,u
I
10
I
1
20
Time ( h )
Fig. 3.4. Kinetic curves for the consumption of styrene (1) and n-butylmercaptan(2). (Y = degree of conversion.
on the basis of the equation
where [A] is the concentration of n-butylmercaptan and [MI is the concentration of monomer. The value obtained, kA = 25 2 , agrees fairly well with the value determined by the Walling method. An important field of application of GC in research and industry is the determination of the concentration of complex organometallic catalysts in the reaction medium, and also the determination of their composition and structure. The methods used are generally based on the decomposition of the catalysts and on the quantitative determination of the reaction products or of the unreacted reagent [34] . GC can be used successfully for solving these problems, because the reagents used for analytical purposes and the products formed are usually volatile (hydrogen, hydrocarbons, alcohols, iodoalkenes, etc.). In addition it is also possible to carry out the direct GC determination of certain volatile organometallic compounds and metal chlorides. Analytical methods for determining the composition of complex catalysts have been described [35, 361 . It should be noted that the field of application of GC in the analytical chemistry of complex organometallic catalysts can be greatly extended. Examples of the utilization of G.C for the investigation of complexing, the structure of complexes and polymerization kinetics can be found in the literature [37-401. GC has been used for studying certain features of the radiation of polymerization monomers, and has also been used successfully for characterizing side reactions. The formation of volatile by-products in radiation polymerization can be caused either by degradation of the monomer or solvent, or of the polymer formed. One of the main difficulties in studying side reactions is that they have small yields of radiochemical products. The use of GC devices, however, made it possible to determine the radiochemical yields of by-products in the polymerization of isoprene, which are 0.10 for hydrogen, 2 lo4 for methane and 1 10-3molecule~per 100 eV for ethylene [41]. The application of GC to the investigation of side reactions in radiation polymerization has also been described [42,43]. The use of GC for studying the mechanism of radiation
*
-
-
POLYMERIZATION REACTIONS
69
telomerizatioii of acetaldehyde and isobutylene was described by Iizuka et al. [44]. It is desirable to use GC methods more widely for studying the kinetic regularities of radiocheinical polymerization processes, both homogeneous and heterogeneous. The application of GC for studying polymerization reactions of olefins on solid catalysts has been described [ 4 5 , 4 6 ] . GC can also be used directly for determining volatile polymerization products; this aspect is of interest both for the study of the kinetics and mechanism of polymerization and for characterizing the final products obtained in the course of oligomerization. Using GC, Karlinszky er al. [47] investigated the composition of the products formed on oligornerization of propylene in the presence of silicotungstic acid. For identification of dimeric products,they studied the infrared spectra of fractions that had previously been isolated by preparative GC. Preparative separation was carried out under the following conditions: column, 600 X 3.5 cm, 25% diethyl phthalate on refractory brick; temperature, 40°C; nitrogen velocity, 40 I/h. The use of the combination of preparative GC and infrared spectroscopy permitted the identification of 1 1 isomers, which constituted 7570 of the dimeric fraction. GC was also used for determining the total yield of propylene triniers and tetramers in this reaction. Fig. 3.5 shows the chrornatogram of the dimeric fraction produced in the oligomerization of propylene, obtained with the use of a capillary column at the Institute of Instrumental Analytical Chernistry of the Czechoslovak Academy of Sciences. This chromatogram illustrates the great posssibilities of GC in studying the composition of complex mixtures. GC has also been used for determining the composition of oligomers (dimers, trimers and tetramers) of isoprene [48] in relation to the conditions of their production. In order t o establish the nature of the addition of the molecules, the dimer obtained was hydrogenated and then the products formed were studied by GC. Chromatographic separation was carried out on a sorbent containing 10%polyphenyl ether on Celite-545 at 75°C on a 9-m column. Both 2,6-dimetliyloctane ('head-to-tail' dimer) and 2,7dimethyloctane ('tail-to-tail' dimer) were found in the hydrogenated products. The analysis of the isoprene dimerization products was also described by Sakodynsky [49]. The GC separation of styrene oligomers has been discussed by a number of workers [50-53]. A characteristic feature of the methods used was the joint utilization of GC and other physicochemical (infrared and mass spectroscopy) methods. Fig. 3.6 shows the chromatograni of the separation of styrene oligomers obtained by cationic polymerization. The separation was carried out on a 300 X 0.4 cm column filled with 570silicone SE-30 on Chromosorb W, with temperature programming from 100 to 270°C at lO"C/min. A more detailed investigation of the composition of styrene oligorners with the use of mass spectroscopic methods and capillary chromatography was conducted by Stein and Mosthaf [53]. For polymerization products of relatively low molecular weight, it is possible, on the basis of GC data, to obtain the molecular-weight distribution of the polymers formed, which is of interest as a characteristic of the commercial product. Such procedures were developed for poly(ethy1ene glycols) and products of the addition of ethylene oxide t o alcohols. Thus, for instance, Miva [54] developed a procedure for the analysis of poly(ethy1ene glycol) with 11 monomer units in the polymer chain. The analysis was References p. 80
POLYMER FORMATION REACTIONS
70
5
I 22
Time (min)
Fig. 3.5. Chromatogram of dimeric products of the oligomerization of propylene obtained with the aid of a capillary column. Peaks: 1 = propane; 2 = propylene; 3 = isobutane; 4 = isobutylene; 7 = 3-methylbutene-l ; 8 = 2-methylbutane; 10 = 2-methylbutene-l ; 11 = 2-methylbutene-2; 13 = 3,3-dimethylbutene-1; 1 4 = 2,2-dimethylbutane; 15 = 4-methylpentene-1 or 3-methylpentene-1; 16 = 2,3-dimethylbutane; 17 = 2,3-dimethylbutene-1; 18 = 4-methyl-tmns-pentene-2; 19 = 2-methylpentane; 20 = 4-methyl-trans-pentene-2;21 = 3-methylpentane; 22 = 2-methylpentene-l ; 24 = 2-ethylbutene-1 ; 25 = 2-methylpentene-2; 26 = 3-methyl-trans-pentene-2;27 = 3-methyl-cis-pentene-2; 28 = 2,edimethylpentane; 29 = 2,3-dimethylbutene-2; 5, 6, 9, 12, 23, 30-32, not identified.
I 0 I
100
I
5
1
10 Time (rnin)
I
150
I
15 I
200 T ("C)
250
Fig. 3.6. Chromatogram of styrene oligomers. Peaks: 1 = benzene; 2 = styrene; 3 = cyclic distyrene; 4 = linear distyrene; 5 = tristyrene; 6 = tetrastyrene.
carried out with the use of glass columns (275 X 0.6 cm) filled with a sorbent containing 10%Apiezon L on acid-washed and silanized Chromosorb W (60-80 mesh). The separation temperature was 250°C and the velocity of the carrier gas (helium) was 40 ml/min. Blocking of absorption-active polar groups permits the range of the poly(ethy1ene glycols) analyzed to be extended. Calzolari et al. [55] analyzed the more volatile
71
POLYMERIZATION REACTIONS
derivatives of poly(ethy1ene glycols) (methyl, phenyl, trimethylsilyl, etc.). The separation was cartied out on a column ( 2 2 X 0.3 cm) filled with 5% silicone SE-30 on silanized Chromosorb W (60-80 mesh). The temperature of the sample introduction system was 375-400°C and the detector temperature was 375°C. During the separation, the column temperature was increased from 100 to 350°C at the rate of 7-8"C/min. A flameionization detector was employed, and hexaethylene glycol was used as the internal standard. Fig. 3.7 illustrates chromatograms of the commercial poly(ethy1ene glycols) PEG-300, PEG400 and PEG-600 in the form of their diphenyl derivatives, C6H50(CH2CH20),C6Hj. In a later paper [56], when discussing quantitative analysis with the aim of determining the molecular-weight distribution in commercial products, Calzolari et al. showed that because of the losses that occur during the preparation of the derivatives in the chromatographic analysis, the molecular-weight distribution, as determined by this method, may differ from the true value. The most accurate results were obtained in analyzing poly(ethy1ene glycols) in the form of their C6H50(CH2CHzO),C6Hs and C6H5S(CHzCHzO),CH2CH2SC6Hjderivatives. In this instance it is possible to analyze by GC poly(ethy1ene glycols) with a molecular weight of 700, i e . , t o determine the molecular-weight distribution of PEG-300 and PEG-400. The accuracy of determination of the molecular-weight distribution of poly(ethy1ene glycol) derivatives was considered by Calzolari et aZ. [57] . Tlie application of GC for separating the products of the addition o f ethylene oxide to dodecanol-1 containing up to 15 units of ethylene glycol in the form of their trimethylsilyl derivatives was considered by Tornquist [58] . A GC method for determining the molecular-weight distribution of previously acetylated poly(ethy1ene glycol) and poly(propy1ene glycol) and glycerine adducts with propylene oxide was described by Gnauck and Fijolka [59]. (a)
t
I
T ("C)
% a 0
P kj
:
w
n
T ("C)
Fig. 3.7. Chromatograms of commercial poly(ethy1ene glycols): (a) PEG-300; (b) PEG-400; (c) PEG-600. The elution temperature is indicated o n the abscissa. References p. 80
POLYMER FORMATION REACTIONS
12
The use of GC for studying the lunetics and mechanism of the telomerization of ethylene with carbon tetrachloride has been described [27,60]. GC can be used for analyzing C3-CI7 tetrachloroalkanes [27] . GC can be used directly for analyzing polymeric products containing up to 10-20 monomer units. This range can be extended to higher molecular weights by using the mobile phase above its critical point [61, 621 and also by using it in the vapour phase [63]. GC can also be used for determining the degree of polymer cross-linking [64]. With the aid of a microtome, 3 X 5 X 0.02 mm samples are cut off, extracted with methanol, dried in a vacuum and weighed. The samples are subsequently placed in xylene, then removed, the liquid solvent is removed from the surface of the entire sample with the aid of a filter paper, and the sample is placed in a vessel containing 1 ml of chloroform and 1 mg of cumene (internal standard). After 30 min, the content of xylene in the chloroform is determined by GC. From the results, it is possible to calculate the degree of swelling of the sample. The method enables one to determine the change in the degree of cross-linking across the width of the product.
STUDY OF COPOLYMERIZATIONREACTIONS Gas chromatography can also be used for studying copolymerization reactions, as during the formation of a copolymer the monomer concentrations change continuously. In the copolymerization of two monomers, M I and M2, their molecules are consumed in the following four elementary reactions:
(1) R; + MI
k
R;
where R; and R; are the growth radicals with the last monomer units, M I and Mz, respectively, and kll, k12.kzl and kz2are the rate constants of the addition of the monomers to the radical. The rates of consumption of each of the monomers, MI and M2, in the copolymerization reaction are expressed by the following equation [65] :
d[M1l - k l l [R;] [MI] dt
+ k21[R;1 [MI]
(3.4)
The steady-state conditions within the limits of the polymerization increment can be written as
k12[R;I [Mzl =k21[R;l [MI]
(3.6)
73
COPOLYMERIZATION REACTIONS
Combining eqns. 3.4-3.6, we obtain the general equation of the copolymerization reaction relating the composition of the copolymer formed with that of the initial monomer mixture:
d[MiI dr
-
_ _ .ri [Mil
mi - [Mil m2
[M21
+ [MzI [MI] + [M2l r2
(3.7)
where the quantities r 1 = k l l / k 1 2and r2 = k22/kZ1 are called the copolymerization constants, and m l and m2 are the molal fractions of the corresponding units in the copolymer. A more cumbersome expression is obtained for the overall copolymerization rate:
-d([Mll dt
+ [Mz])=
(r:[M11’+2[M11 [MzI + r - ? [ M 2 1 2 ) ( y ~ 5 / 6 ~ ) {r: [MI]’ + 2(@r1~262/6 1) [MI] [MzI + ( ~ 2 6 2 / 61)’
(3.8)
v i is the initiation rate and ktll, kt22 and k,,, are the rate constants of the bimolecular chain termination on interaction of the corresponding radicals. There are several methods for calculating the copolymerization constants. The most common methods, however, are the differential and integral methods of Mayo and Lewis [65]. For calculations by these methods, it is necessary to determine experimentally the dependence of m l / m 2on [ M I ] [M2]. In the application of the differential method, the degree of conversion in copolymerization must not exceed 5-10%. In this instance, it can be assumed that a compositionally homogeneous copolymer is formed, while the composition of the initial mixture changes only insignificantly. By rearranging eqn. 3.7, one can express the value of r2 as
Substituting in eqn. 3.9 the experimentally determined pairs of values of rnl/rnz and [ M I ] / [ M 2 ] , one can obtain a family of curves in the coordinates r2, r , , whose common point of intersection corresponds to the values r2 and r I . If the degree of conversion in copolymerization exceeds lo%, one uses the integral method for determining the copolymerization constants. Integrating eqn. 3.9 between [ M , ] 0 / [ M z l o a n d [Ml]c/[M2]c and introducing the parameter p = (1 - r l ) (1 - r z ) , we obtain the following equation:
(3.10)
where [ M I ] and [M2] ,,are the cotiionomer concentrations in the initial mixture and [MI] and [M,] are the concentrations of the unreacted comonomers on completion of copolyrneriza tion. References p. 80
14
POLYMER.FORMATION REACTIONS
Using the experimental values of [MI] o, [M2] o, [MI]. and [M2Ic, and also a set of arbitrary values of p (Fig. 3.8), one can determine the straight line rz = f(rl). In the second experimental cycle, we find a new equation of the straight line r2 = f'(rl) by substituting in eqn. 3.10 the new values [MI] '0, [MJ '0, [MI]', and [Mz] ',, and the same set of values of p . The point of intersection of the straight lines rZ = f(rl) and rz = f'(rl) corresponds to the desired values of r l and r2. In practice, when determining the copolymerization constants, one usually determines the dependence ml/mz = f ( [Ml/[Mz]). However, the determination of the composition of the copolymer is often a highly time-consuming and inaccurate stage in finding the constants of r l and r2. Moreover, in this instance such an important characteristic as the composition of the monomer mixture at any instant corresponding to a definite degree of conversion is not known. Also, a knowledge of the composition of the monomer mixture at any stage of copolymerization permits the accurate calculation of the compositional non-homogeneity of the copolymers formed. In this connection, it would be expected that the control of the composition of the monomer mixture during copolymerization by means of GC must be extremely efficient not only for determining the copolymerization constants, but also for studying the kinetics of joint polymerization. It should be noted that such a widely used method for studying the kinetic regularities of polymerization as dilatometry is unsuitable for investigating the kinetics of copolymerization reactions, as the composition (density) of the copolymer formed changes continuously. Originally, GC was used by Jones [66] and Harwood et al. [67] for determining the composition of an unreacted monomer mixture. They determined the copolymerization constants in the system styrene (1)-methyl methacrylate (2); it was found that rl = 0.51 ? 0.02 and rz = 0.46 0.02. Polymerization was carried out to different degrees of conversion in a toluene solution. To terminate the process abruptly, an inhibitor (hydroquinone) was introduced into the system. The calculation was made by the integral method. _+
PP
Fig. 3.8. Graphical method of Mayo and Lewis, withp = 0.25,0.5, 1 , 2, -2, -1 and -0.5.
75
COPOLYMERIZATION REACTIONS
Makarov et al. [68] also used GC to determine the copolymerization constants of the same pair of monomers. As the polymer does not appear on the chromatograni in GC, quantitative interpretation of the results is conveniently made with the aid of a 'marker', for which purpose the solvent was used. Because the weight concentration of the solvent in the reaction mixture does not change with time, the weight concentration of any monomer in the mixture can be determined from the equation
cM = kic,SM/Ss
(3.1 1)
where cM (%, w/w), is the monomer concentration, c, (%, w/w), is the solvent concentration, S, and S, are the monomer and solvent peak areas, respectively, on the chromatogram and k , is the relative calibration coefficient of the ith component with respect to the marker substance. In determining r , and r2 by this method, one must choose a solvent whose boiling point is 60-80°C higher than the copolymerization temperature, otherwise the error in the concentration value will be large. The results of chromatogram processing for the mold ratio of styrene t o methyl methacrylate of 1 : 1 are presented in Fig. 3.9a as the dependence of the molal concentration of the monomers on the reaction time: (&/Mi)/ C(c',/M, + c,/M,), where Miand M, are the molecular weights of the monomer and of the solvent, respectively. The copolymerization was carried out t o a high degree of conversion (84%).The copolymerization constants were calculated by both the differential and the integral equation of the copolymer composition. The method permits the copolymerization constants to be calculated from the data of a single polymerization experiment (for a single monomer ratio). The copolymerization constants determined were r I = 0.52 (styrene) and r2 = 0.44 (methyl methacrylate) and were in good agreement with the data published in the literature. Using this method, Mashlyakovsky ef al. [69] determined the copolymerization constants of the fluoroanhydride of 2-methylbutadiene-l,3-phosphonic acid (1) with styrene (2) to be r l = 1.15 and r2 = 0.54. The application of other calculation methods [65] leads to practically the same values of r , and rz. Copolymerization in a two monomer-one radical system has been investigated [70].
36
?2
100
144
lA0 3 0 -
Ik
54
.
d0
i
126
162
Time ( m i n )
Fig. 3.9. Variation of monomer concentration with time in the copolymerization of styrene (1) with methyl methacrylate (2) at 60°C. (a), Number of moles of monomer in mixture; (b), molal ratio 1: I ; 01 = tan p; k = ( a , / a 2 ) / ( Q , / Q=2 1.04. )
References p. 80
POLYMER FORMATION REACTIONS
16
When using GC for determining the copolymerization constants of over 30 pairs of monomers, the following interesting fact was discovered [7 11 . With equimolecular mixtures of the initial monomers, the kinetic curves of the variation of monomer concentration with time are fairly well approximated by the linear dependences cM = f(t) at the initial stage of the process (Fig. 3.9). The slope of such a curve (a= tan 0) is equal to the empirical constant of the process of consumption of each monomer: a1 = k l l + k21 and a2= kz2 k I 2 .Also, for most of the pairs of monomers investigated, the following ratio is valid:
+
where the constant k is close to unity. These dependences permit the rapid and accurate determination of the copolymerization ability of the monomers, which can be characterized by the values of k l , a1and a2,and calculation of the values of Q2provided that Ql is known; the quantity Q characterizes the reactivity of a monomer in terms of conjugation energy. Mano and De Almeida [72] proposed a simple equation, which makes it possible to calculate the constants r1and rz directly from the retention time. The results obtained are in good agreement with the experimental data. The application of GC to the determination of the composition of ternary copolymers of acrylonitrile, vinyl acetate and a-methylstyrene, based on the conversion during the copolymerization reaction, was described by Lupu [73] , and the ternary copolymerization of acrylic acid, butyl acrylate and methyl methacrylate was studied by GC with sample collection by Balashov et al. [74] .
STUDY OF POLYCONDENSATION REACTIONS In polycondensation reactions, together with the formation of polymeric compounds, low-molecular-weight products are also formed. In general, a polycondensation reaction with interaction of two different compounds can be represented in the following form [75] : x(a-A-a)
+ x(b-B-b)
-+
a(-AB),
b
+ (2.x
--
1)ab
or, if both functional groups are included in the same molecule, the,reaction scheme can be written as x(a-A-b)+
a(-A),b
+ (x - 1)ab
Depending on the nature of the functional groups of the initial substances, polycondensation may be based on different processes (esterification, amidation, alkylation, sulphidation, dehydrogenation, oxidation, etc.) [76] . As distinct from polymerization reactions, in polycondensation reactions some of the end products, as well as the initial compounds, are volatile low-molecular-weight compounds. Therefore, GC method can be used directly in studying the kinetics of polycondensation reactions by measuring the volatile products. This technique permits the investigation of the kinetics of polycondensation reactions with high accuracy and at low degrees of conversion, in contrast to the kinetics of polymerization reactions.
POLYCONDENSATION REACTIONS
77
GC has been used successfully for studying the kinetics of the main and side reactions in the course of polycondensation. The potentialities of GC in studying the formation of polymers by the method of polyrecoinbination (a particular case of polycondensation) were demonstrated by Sosin et al. [77] for tlie complex reaction of polymer formation on treating diphenylmethane with tert.-butyl peroxide: (CbHj)2CHZ + R'
-+
(CbH5)2CH'
+ RH
2(C6H5)2CHS + (c6Hj)zCH-CH(C6 (C~HS)~CH-CH(C~HS)~ + R'
+
(R = (CH3)co' or CH;)
H5)z
( C ~ H ~ ) ~ C H ( C ~ H+ S )(~CC~'H S ) Z C H [ ( C ~ H ~n)H~ C I
The application of GC has made it possible t o study the mechanism of the side reactions of decomposition of tert.-butyl peroxide, and t o explain the appearance in the reaction medium of such products as di-terf.-butyl ether and isobutylene oxide, and, in the gaseous phase, ethane, ethylene, propane, propylene and isobutane. A detailed GC study of the products permitted the quantitative assessment of all tlie routes by which tert.-butyl peroxide was consumed. The use of GC has also made it possible to resolve several kinetic aspects of the reaction, and, in particular, t o determine the activation energy of the detachment of the hydrogen atom from diphenylniethane by the method of competing reactions. An interesting series of investigations into the kinetics of tlie curing of various resins lids been carried out by Japanese workers [78-81]. GC was used for determining volatile reaction products and infrared spectroscopy for studying the resins. With the aid of GC they studied the kinetics and mechanism of tlie curing of carbamide-formaldehyde resins [78] , dialkyl ethers of diniethylalcarbaniide [79] , niethylolmelaniine [SO] and saligenin [81 ] . An important characteristic of the polycondensation reaction is its reversibility. GC can be used for assessing tlie reversibility of the polycondensation reaction. Velichkova and co-workers [82, 831 studied the equilibrium of tlie reaction R1 *
CO * OR2 + HCI + R1 * CO * C1 + R20H
by GC for poly [9,9-bis(4-oxyphenyI) fluorotereplitlialate] and the model compounds 9,9-fluorenyl bis(2-oxyphenyl benzoate) and phenyl benzoate. The reaction mixture was analyzed on columns with PEG 2000 and Apiezon L deposited on Chromosorb W. The sensitivity of the clirotnatographic methods used was sufficient f o r recording the phenol and benzol chloride formed upon 1% conversion of tlie model compounds. It was shown that under tlie conditions of polyarylate synthesis at 220°C in a ditolylrnethane solution in a flow of dry hydrogen chloride, the reaction of their formation is irreversible, as n o products of the above reactions were found in the reaction mixture. For a quantitative determination of the equilibrium constant, the reaction was carried out in sealed ampoules and the degree of conversion was determined by GC. The equilibrium constant of tlie at 220"C, ie., this reaction for phenyl benzoate is 2.35 lo4 at 40°C and 4.48 reaction can be neglected in the formation of polyarylates. The application of GC to the quality control of starting materials and in investigations into the effect of its purity on the quality of the resulting polymer was studied by
-
References p. 80
I8
POLYMER FORMATION REACTIONS
Strakhov et al., [84]. It was found that 1,4-butanediol, used as a chain elongator in the synthesis of polyurethane thermoplastics, contains various impurities. Monofunctional impurities (n-butanol and butyric acid monoester) may cause chain termination and the formation of a product with a reduced molecular weight. The application of CC for assessing side exchange reactions that take place during the synthesis of polyarylates by high-temperature polycondensation from the chloroanhydrides of dicarbonic acids and bisphenols was described by Vinogradova et al. [85]. As the synthesis of polyarylates with different structures but with the same molecular characteristics (molecular weight, polydispersity) is difficult, an investigation of the effect of the chemical structure of the bisphenol component of the polyarylates on exchange reactions was carried out on model compounds. Re-esterification of the dibenzoates of substituted dioxydiphenylmethanes was effected with phenol or n-chlorobenzoic acid. The degree of conversion was determined by CC, with the aid of calibration plots, from the amount of phenol reacted or the amount of benzoic acid formed. This method was used for determining the re-esterification rate constants for bisphenols with various structures. The GC control of the re-esterification and aminolysis stage in polyurethane synthesis was applied by Melaerts [86]. If the products formed on polycondensation differ substantially in volatility from the initial compounds, GC detectors can be used for the direct recording of the release of these products. The inert gas flow, passing through the reactor, purges the reaction mixture of the volatile products; then the carrier gas flow passes through the column with a sorbent (or a reagent) to drive off the vapour of the initial components, and enters a detector, which continuously records the changes in the concentration of the reaction product. If the product concentration is below the sensitivity threshold of the detector used, which can occur with very slow reactions, reactions in dilute solutions or when running a reaction with trace amounts of the initial substances, it is expedient to introduce a concentration step (for instance, a cold trap) into the chromatographic scheme. In this instance, by periodically feeding products to the input of the chromatographic column, one can analyze a number of volatile products instead of a single product and thus considerably improve the sensitivity of the method. For automatic recording of the products formed, it is desirable to use the method and device developed for studying the kinetics of polymer degradation [87]. This method is described in more detail in Chapter 5. It should be noted that if the reaction proceeds relatively rapidly and the release of products from the reaction mixture occurs slowly, it is expedient to run the reaction in thin liquid or solid layers of a reaction mixture, deposited on an inert porous support. The deposition technique is similar to that for the deposition liquid stationary phases on a solid support in gas-liquid chromatography. In this instance the product equilibrium between the gas and condensed phases will be established sufficiently quickly. An ingenious method for stu.dying the kinetics of the reactions that occur in polymerization and accompanied by the release of volatile products has been proposed by Franck [88]. When poly(viny1 alcohol) [or its mixture with poly(acry1ic acid)] is heated, a cross-linking reaction occurs that is accompanied by the release of water vapour. Rates of reactions can be studied by recording the release of water vapour. The reaction was run in a special tubular reactor, which was filled with poly(viny1 alcohol) film. The reactor was placed in an air thermostat and was introduced into the equipment in place
POLYCONDENSATION REACTLONS
79
of the chromatographic column. The reactor was heated at a pre-assigned constant rate in the chromatograph thermostat t o a definite temperature, after which the reaction proceeded under isothermal conditions. During the reaction, a flow of carrier gas was passed through the reactor at a constant velocity. The water released on cross-linking of the poly(viny1 alcoliol) was determined with the aid of the detector on the basis of heat conductivity. A typical graph of the variation in water concentration in the carrier gas flow is shown in Fig. 3.10. The reaction was studied at temperatures in the range 1 50&3OO0C. The use of GC for studying the kinetics of the cyclodehydration reaction in the synthesis of thermostable polymers has been described [8Y],a comprehensive study having been made of the kinetics of the reaction of solid-phase imidization of polyaniido acids. The above examples indicate the adequate universality of GC, which permits it t o be applied t o a wide range of reactions. It should be noted that gas chromatography is now used not only in scientific research, but also for control of polymer production in industry [YO--Y2]. I n conclusion, we shall indicate some possibilities for the investigation of polycondensation reactions and reactions in polymers by inverse chromatography and the pulse method. In distinction t o the conventional version of dilution chromatography, in inverse chromatography the unknown substance (that being analyzed) acts as the liquid stationary phase, on which a mixture of known standard compounds of different classes is separated. The tnain characteristics of the stationary pliases studied in inverse GC are the retention values of the components of the standard mixture. The method o f inverse GC is discussed in detail i n Chapter 8. If the reaction mixture of non-volatile components is used as the liquid stationary phase, the variation in its composition with time, which is accompanied by variations in the concentration of the functional groups, can be controlled periodically by changing the elution characteristics of the non-reacting compounds. Although inverse GC iias so far been applied niainly to non-polymeric compounds, in our opinion this method can also be used+ccessfully for studying polymer formation reactions, especially reactions that are accompanied by a change in the functional groups. In the pulse methods for studying the kinetics of chemical transformations, a pulse of the volatile compound is fed into the carrier gas flow at the inlet of the reactor column, I
-1
30
50
70
T ("C)
2
8
14
Tlme (min)
Fig. 3.10. Concentration curve for water released in a carrier gas flow on cross-linking of poly(viny1 alcohol). Initial temperature, 30°C; rate of temperature increase to 9O"C, 100"C/min. 1 , Nonisothermal conditions; 2, isothermal conditions. References p. 80
80
POLYMER FORMATION REACTIONS
while the second reaction component (the non-volatile compound) is used as the liquid stationary phase. The reaction proceeding in the reactor column is assessed by the change in the amount (concentration) of the volatile reaction component. The use of pulse methods for studying the kinetics of liquid-phase reactions was described by Berezkin [93]. Their utilization in studying the kinetics of polycondensation reactions seems to hold much promise. As an illustration, we shall consider a study of the kinetic characteristics of the condensation of maleic anhydride with polyisobutylene [94]. The study of a bimolecular reaction, when two initial reacting components differ widely in volatility, is the most convenient example of the application of the pulse chromatographic method. In studying the kinetics of the addition of maleic anhydride to polyisobutylene, Mysak et al. [94] used a chromatographic column connected in tandem with a reactor column. The chromatographic column served to separate the mixture of maleic anhydride and the standard and was a 1 m X 4 mm I.D.stainless-steel tube filled with Chromosorb W which was acid-washed, treated with hexamethyldisilazane and impregnated with 10% silicone elastomer SE-30. The condensation reaction was run in the reactor column, which was a 5 m X 4 mm I.D.stainless-steel tube filled with Chromosorb W which was washed with an acid, treated with hexamethyldisilazane and impregnated with polyisobutylene of molecular weight 700 in the amount of 20% of the weight of Chromosorb W. When a sample of the mixture of maleic anhydride with the standard (dimethyl ester of succinic acid) was introduced, the components were separated on the column with the silicone elastomer SE-30, and they entered the reactor column separately, where the maleic anhydride reacted with the polyisobutylene. The course of the reaction was judged by the difference in the ratio of maleic anhydride to the standard in the initial mixture and after leaving the reactor column. The contact time was calculated as the difference between the total retention time of the maleic anhydride (necessary for passing the column with the silicone elastomer and the reactor column) and the retention time when passing only through the separation column. The concentration of the polyisobutylene was 1.25 mole/l, while that of the maleic anhydride was 6 * lo-’ mole/l, Le., about 2 * was consumed during one experiment. Hence, under the conditions of a pulsed chromatographic regime, the condensation reaction can be regarded as an irreversible reaction of first order with respect to the maleic anhydride. The linear dependence of the logarithm of the reaction rate constant on the inverse of the absolute temperature was used for the graphical determination of the activation energy, which is equal to 12 k 0.5 kcal/mole in the temperature range 176-205OC.
REFERENCES 1 T. Teiji, Reakeii Poluchenya Sinteticheskih Polymerov (Reactions to obtain Synthetic Polymers), A. P. Sergeev (Editor of Russian edition), Khimia, Moscow, 1963. 2 Kh. S. Bagdasarian, Teoriya Radikal’noi Polimerizatsii (Radical Polymerization Theory), Nauka, Moscow, 1966. 3 G. P. Gladyshev and K. M. Gibov, Polimerizatsiya pri Clubokikh Stepenyakh Prevrashcheniya i Metody ee Issledovaniya (Polymerization with Deep Degrees of Conversion and Methods for its Investigation), Nauka Kazakhskoy SSR, Alma-Ata, 1968.
REFERENCES
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POLYMER FORMATION REACTIONS
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REFERENCES
83
86 W. Melaerts, Anti. Sci, Text. Belg., (1969) 16. 87 V. R. Alishoev. V. G. Berezkin, I. B. Neniirovskaya and B. M. Kovarskaya, Mukromol. Chem., 143 (1971) 207. 8 8 A. Franck, Makromol. Chetn., 96 (1966) 258. 89 I. B. Nemirovskaya, V. G. Berezkin and B. M. Kovarskaya, Vvsokornol. Soedin., Ser. A , 15 (1973) 1168. 90 J. Fischer, in E. Leibnitz and H. G. Struppe (Editors), Hatrdhuch der Gas-Chromatographie, Gecst & Portig, Leipzig, 1966, p. 597. 91 M. 1. Afanasiev, A. A. Datskevich, S. G. Ivanova and E. 1. Chiirkin, Neftepererah. Neftekhim. (Moscow), No. 6 (1 970) 46. 9 2 V. L. Kepke and G. F. Sokolin, Zavod. L u ~ . 36 , (1 970) 1301. 93 V. G. Berezkin, Usp. Khim., 37 (1 968) 1348. 94 A. E. Mysak, Yu. A. Kanchenko, V. G. Berezkin and N. P. Mysak, Kitzet. Kutal., 16 (1975) 257.
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Chapter 4
Determination of volatile compounds in polymer systems The determination of volatile compounds in polymers is important, as the properties of polymer materials and the fields of their application often depend strongly on their contents of various ingredients (initial monomers, plasticizers, antioxidants, degradation products, etc.). Similar problems arise in physicochemical investigations, for instance in studying the solubility of volatile organic compounds in polymer materials, determination o f gas solubility, analysis of volatile degradation products, determination of polymerization kinetics at high degrees of conversion, study of ageing of plastics and determination of the purity and properties of plasticizers. Thus, the determination of volatile compounds in polymers is one of the most common analytical problems in polymer chemistry. To determine the contents of monomers, plasticizers and other compounds in polymers, various methods have been used together with gas chromatography (GC), e.g., chemical [ 1, 21 and polarographic [3] methods and infrared and nuclear magnetic resonance spectroscopy [4, 51. Tlie results of the determination of the contents of light components obtained by different methods (for instance, by GC and polarography [ 6 ] , are in good agreement. I n practice, however, GC is the most popular method, because it is faster, more convenient and relatively simple. Owing to its high sensitivity, GC is considered to be a universal method for solving problems of this type. In determining volatile compounds contained in a polymer, the first stage is the isolation (separation) of the volatile compounds from the polymer, and the second is the GC analysis of the separated volatile compounds. Sometimes, for instance in analyzing volatile compounds in liquid polymers, these two stages can be combined. GC methods proper for determining substances contained in polymers (monomers, plasticizers, etc.) have been developed more comprehensively. In most instances, when the chemical nature of the substances in the polymers is known, a skilled worker can, in principle, assess beforehand the possibility of solving the problem posed, establish its complexity and also outline rational ways for developing an appropriate procedure. Methods for the isolation (separation) of volatile compounds from polymer compounds have been developed to a much lesser extent. The incompleteness of separation of the volatile substances from the polymer, and also contamination of the separated fraction with products formed during separation (for instance, with products of thermal degradation if the separation process takes place at elevated temperatures), are frequent sources of errors in determining the compositions of volatile substances in non-volatile polymers. Therefore, in developing an appropriate procedure, one must pay special attention to the first, non-chromatographic, stage of the analysis. It is expedient, in working out procedures for determining volatile compounds in non-volatile samples, t o carry out control tests, using for the analyses samples of polymers with a known content of the impurities being analyzed. Two types of method for determining volatile compounds in polymers have been described in the recent literature: (1) direct chromatography, in which sample is References p. 108
86
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
introduced, without any preliminary preparation, into the chromatograph, which is usually equipped with an appropriate device, and (2) two-stage methods, in which GC determination is preceded by preliminary treatment of the polymer sample with the aim of separating the volatile products (for instance, by extraction or chromatographic separation in the liquid phase) or preparing the sample for subsequent GC separation (for instance, dissolution). Methods in which vapour-liquid phase equilibrium [7] is used for separating the volatile components from the polymers deserve special mention among the two-stage methods because of their rapidity, simplicity and universality. Thus, for instance, Rohrschneider [8] developed a method for the separation and quantitative determination of styrene and other aromatic hydrocarbon impurities in polystyrene by automatic GC analysis of the gas phase over the solution of the polystyrene under investigation. A polystyrene sample (1 80-200 mg) is placed in a 13-ml bottle, 2 ml of dimethylformamide containing about 3 mg of butylbenzene (internal standard) are added, the bottle is sealed with a rubber cap and held at 70°C for at least 2 h, after which the gas phase is subjected to chromatographic analysis. It should be noted that in some instances it is also expedient to use the liquid-liquid phase equilibrium for this purpose. The application of phase equilibrium or partition chromatography also permits the qualitative identification of the volatile products simultaneously with their quantitative analysis [9]. The direct analytical methods are based on rapid separation of the volatile compounds from the polymers at elevated temperatures. They are less laborious, but the incompleteness of the separation and degradation is a potential source of errors. Two-stage methods are more universal and more reliable. In these methods, the concentration process often takes place simultaneously with separation at the stage of preliminary preparation of the sample, and this increases the sensitivity of the determination of volatile compounds in polymers. In most of the GC methods for determining volatile compounds in polymers, the standard sample injector port of the chromatograph has to be changed. Mlejnek [lo] proposed the following classification of methods for determining volatile products in polymers based on differences in the method of sample injection: (1) Application of a special syringe for injecting a solid polymer sample into the stationary sample chamber of the chromatograph [ 11-20] . (2) The use, in a stationary sample chamber, of various detachable inserts permitting rapid and convenient removal of non-volatile products from the sample chamber [ 18-27] . (3) Special, usually automatic, 'two-chamber devices for introducing a known sample of the test material (in a boat or a capillary) into the heated zone of the evaporator. The method cannot be used for samples with a high content of volatile compounds [28-381. (4) The introduction of a sealed ampoule with the test substance into the evaporator, where destruction of the ampoule (mechanically or by melting) and evaporation takes place. The method is universal [39-481. (5) Sundry methods [49-561. The method of determining volatile substances in polymers can also be used for identification of polymers. Thus, it has been shown [57] that chromatograms of volatile substances isolated from natural rubber subjected to various treatments (blowing out
DIRECT ANALYTICAL METHODS
87
volatile substances in a vapour and air flow a t an elevated temperature) are characteristic of each type of rubber and can be used for their identification. As polymer samples that have been subjected t o the effects of different chemical agents and physical factors (temperature, radiation, etc.) contain complex mixtures of various volatile compounds, they can be identified most efficiently by combined GC-mass spectrometry (MS). The potentialities of this method in polymer chemistry were assessed by Bennett and Paul [58]. The method can be used with advantage in solving the following problems: (1) analysis of impurities in monomers, plasticizers, etc.; ( 2 ) determination of the unreacted monomer in a polymer; (3) evaluation of thermostability of a polymer by analyzing the disintegration products; (4) investigation into the molecular-weight distribution of poly(ethy1ene glycols) and other oligomers b y analyzing their volatile derivatives; (5) investigation into copolymerization kinetics by analyzing the unreacted monomers; (6) investigation of volatile products of polymer reactions; and (7) investigation of polymers by studying the volatile products of their pyrolysis, thermal disintegration. etc. The application of GC-MS for determining the volatile products released upon heating from a methyl arsenite-siloxane polymer has been described [59]. I n conclusion, it should be noted that the importance of the methods for determining volatile products released from polymers is increasing owing t o the increasing importance of polymeric materials in industry and in the home.
DIRECT ANALYTICAL METHODS In direct methods of determination, the test sample of the polymer is introduced into a zone at elevated tempetature, where the volatile compounds diffuse from the polymer into the gas pliase. The temperature of separation of light compounds must be sufficiently high for the test substances t o be transferred to the gas plzase, as an increase in temperature increases the rate of diffusion of the test compounds and reduces their distribution constant in the gas-polymer system. The temperature increase, however, cannot be too great; it is usually linuted by the thermal stability of the polymer and the volatile compounds contained in the sample. However, if during degradation the polymer forms volatile products that differ substantially from the thermostable compounds being analyzed, the heating temperature can be above the degradation temperature of the polymer. As the diffusion coefficients of organic compounds in liquid and solids differ by several orders of magnitude, the isolation of volatile substances from non-viscous liquid polymers and polymer solutions proceeds much more rapidly. Therefore, the methods and equipment for determining volatile cornpounds in polymers differ, depending on their physical state. The deterinination of volatile products in liquid polymers (or their solutions) is simpler than the determination of volatile compounds in solutions of solid polymets. A procedure for the analysis of solvents in non-viscous varnishes by direct introduction of the sample into the standard sample chamber of the chromatograph on t o a layer of glass-wool has been described [60]. In analyzing solutions of solid polymers, one must bear in mind the possibility of 'trapping' of the volatile products by the film of the solid polymer formed on evaporation of References p. 108
88
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
the volatile solvent. Therefore, in analyzing systems of this type, one should perhaps use a binary solvent consisting of a heavy component, which is non-volatile under the experimental conditions, and a volatile solvent. When analyzing liquid polymers or their solutions, the sample is injected with a syringe into the standard sample chamber of the chromatograph, which is heated to a high temperature. In the high-temperature zone, the light components of the sample rapidly evaporate, while the liquid polymer slowly moves to the chromatographic column under the effect of gravity and the gas flow. Therefore, in procedures of this type, one usually installs, before the chromatographic column, a preliminary section, which consists of a short column filled with a granulated material characterized by very low retention with respect to the test compounds [for instance, glass beads, quartz glass or an inert support (such as Chromosorb G) impregnated with 0.1-0.5% of liquid stationary phase] . The preliminary column protects the chromatographic column from contamination, retaining polymers and high-boiling compounds. In many instances, it is convenient to combine the functions of the evaporator and the preliminary column by filling the evaporator volume with an inert packing. The deposition of the sample on to a layer of inert material promotes the distribution of the polymer as a thin layer on the packing, and hence promotes rapid diffusion of the volatile compounds from the polymer, because the time necessary for the diffusion of the compound from the film is proportional to the square of its width. However, this solution to the problem is not the best for all evaporator designs. For analyzing volatile compounds in polymer mixtures, it will probably be necessary to develop special units that combine the functions of the evaporator and preliminary column. A disadvantage of the known methods for determining volatile compounds in polymer systems is the need for periodic cleaning of the evaporator and the preliminary column in order to remove the remaining polymer and other volatile compounds. When a large number of samples have to be analyzed, and during prolonged continuous operation, it may be advantageous to use a periodically moving layer of the inert material located between the sample injector point and the chromatographic column. The relevants device, which was developed for the analysis of heavy impurities in the gas flow, was described by Alishoyev et al. [61]. Some other methods have been described in which the removal of non-analyzed non-volatile compounds (residue) was carried out periodically after each experiment [48, 62,631 or continuously [64]. The method for determining water and solvents in viscous polyorganosiloxanes was developed by Luskina et al. [65]. In order to increase the sensitivity of the determination of water, reaction of water with calcium carbide was used. The sampling device consists of a four-way tap, the sample injector proper, in which the water and solvents are isolated from the polymer sample at an elevated temperature, and a metal tube (reactor) filled with pieces of calcium carbide 2-3 mm across. The sample injector is a metal block, the upper part of which has a socket for a cup with the sample and the lower part is fitted with a heater. The block is covered with a bolted metal lid. With the aid of the four-way tap, the flow of carrier gas can either be fed into the sample injector in order to displace the water vapour and the solvents or, by-passing the injector, directly into the chromatographic colunin. This system can be used as an attachment to a laboratory chromatograph.
DIRECT ANALYTICAL METHODS
89
When carrying out the analysis, the cup (0.5-1 .O g) was placed in the sample injector and the system was swept out with a flow of carrier gas (nitrogen) for several minutes. Then the four-way tap was turned so that the carrier gas by-passed the sample injector, which was heated to 150°C for 15 min. The water evaporated from the sample was displaced by the carrier gas from the sample injector into the tube containing the calcium carbide, where a reaction proceeded rapidly at 20°C with the release of acetylene. The acetylene passed through a chromatographic column (100 X 0.4 cm) filled with 15% of tricresyl phosphate on INZ-600. Determination of solvents was carried out in the same apparatus, without the use of a reactor containing calcium carbide, but the chromatographic column was thermostated at 150°C. The lowest determinable concentration when using a chromatograph with a katharometer was 0.005% for water and 0.10% for solvents. A rapid method for determining water in stained facing materials based on polyethylene was proposed by Keister and Harrington [66]. A weighed sample (ca. 200 mg) is placed in a platinum boat, which is introduced into the cold zone of a tubular reactor with an oven (Fig. 4.1). After sealing of the device and stabilization of the flow of helium, the boat with the sample is moved, with the aid of magnet M1, into the oven zone heated to 300°C. The sample quickly melts and the water enters the flow of carrier gas and is transferred to a column (240 X 0.47 cm) filled with 20% Carbowax 1540 on Haloport F. The separation takes place at 100°C and at a helium flow-rate of 60 ml/min. The method can be used for analyzing samples with a humidity of about 0.02%. The water content is calculated on the basis of a calibration plot. The relative standard deviation is 2.3%. The method permits one determination to be carried out in 10 min and can be used for the analysis of materials based on polyethylene, polyesters and cellulose. Other methods for determining small amounts of water have also been described [67-691. Hudy [62] developed a simple and accurate method and equipment for determining solvents in varnishes and resins without previous separation of the volatile compounds from the polymer and pigment. The test varnish sample is placed in part of a capillary (0.05-mm bore) made of a low-melting material (polyethylene or an indium alloy), which is sealed off and loaded in a nickel boat (length 50 mm). The boat containing the capillary is introduced into a tubular reactor with an oven (heated zone 110 X 13 mm), sealed off and swept out with a flow of carrier gas (40 ml/min). Then, with the aid of a magnet, the boat is moved into the hot zone (for analysis of solvents in varnishes a temperature of 180-200°C is sufficient), where the capillary melts and the volatile
3
4
5
Fig. 4.1.Diagram of tubular reactor: 1 = carrier gas inlet; 2 = j o i n t for sample injection; 3 = magnet M 1 ; 4 = boat with sample; 5 = oven; 6 = magnet M2.
References p. 108
90
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
components of the sample enter the chromatographic column for separation. The column (360 X 0.5 cm) was filled with 20% Apiezon N or diethylene glycol succinate on Chromosorb W (60-80 mesh). Chromatographic separation was carried out at 100°C. The non-volatile residues remained in the boat and were removed from the oven on completion of analysis. The internal standard method was used for quantitative calculations [48] . Table 4.1 lists the relative retention times of the varnish solvents usually used (with respect to n-butanol). . Baybayeva er al. [70] described the application of GC to the analysis of dyes and varnishes. An ingenious method for determining the total content of low-molecular-weight compounds in plasticizers, whose presence in polymers is usually blamed for the noxious smell of polymer products, has been described [64]. The plasticizer sample (ca. 0.2 ml) was introduced through the sample introduction system into an empty column (400 X 0.25 cm) and distributed over its walls as a thin layer, as a result of which favourable conditions were created for the isolation of volatile compounds. The volatile compounds passed with the flow of carrier gas through a column whose walls were coated with a layer of plasticizers remaining from previous determinations, and entered an argon TABLE 4.1 RELATIVE RETENTION TIMES OF SOME VARNISH SOLVENTS [62] Temperature, 100'C. Solvent
Acetone Methyl ethyl ketone Methyl isobutyl ketone Methanol Ethanol Isopropanol n-Butanol (standard) Amyl alcohol Hexanol Methyl acetate Ethyl acetate Isopropyl acetate n-Propyl acetate Amyl acetate Isobutyl acetate n-Butyl acetate Hexyl acetate Toluene Ethylbenzene n-Xylene o-Xylene nHexane n-Heptane Isooctane
Stationary phase
-
Apiezon N
Diethylene glycol succinate
0.4 1 0.69 1.61 0.33 0.36 0.46 1.00 1.88 2.13 0.41 0.68 0.86 1.15 1.70 1.89 2.29 3.80 2.75 5.25 5.90
0.33 0.42 0.63 0.37 0.42 0.34 1.oo 1.62 1.03 0.34 0.39 0.23 0.52 0.59 0.56 0.76 0.17 0.73 0.95 1.00 1.29 0.1 3 0.14 0.13
1.0
0.86 1.51 1.52
DIRECT ANALYTICAL METHODS
91
ionization detector. The sensitivity of the method is about and the analysis time is 20 min. The method can also be used for automatic control. It is of interest for determining volatile components in non-viscous polymer solutions and in oligomers. I n the Design Office for Automation in Petroleum Processing and Petrochemistry, Kepke and Sokolin [71, 721 developed an industrial gas chromatograph specially designed for the analysis of volatile monomers in polymer systems. In this device, the removal of the non-volatile solvent is achieved by washing the sample introduction system with a liquid solvent. The chromatograph was used successfully for determining the content of ethylene and propylene monomers in the initial charge and in the polymerizate in the production of synthetic rubber and for determining divinyl in the charge in the production of BR rubber. The isolation of light components from solid polymer samples is difficult. To reduce the desorption time, it is always desirable to use the finest possible polymer powder for analysis. As the time of desorption (evaporation) of volatile components from the polymer and the width of the initial zone are usually sufficiently large, no subsequent efficient chromatographic separation is possible. Therefore, the determination of light components in polymer systems by direct introduction of the polymer sample into the heated evaporator usually cannot be carried out. To overcome these difficulties, desorption of volatile substances is conducted for a definite period of time, either in a small enclosed volume (tlus volume is then included in the flow of carrier gas upstream of the chromatographic column) or in a continuous-flow system with a trap placed between the desorption chamber and the chromatographic column. On completion of desorption, the trap is heated rapidly, as a result of which the products released from the polymer enter the chromatographic column from the trap pulse-wise. An ingenious version of the first method was suggested by Wandel and Tengler [73]. A special device (Fig. 4.2) located upstream of the chromatographic column has a heated chamber through which the carrier gas passes. The chamber contains a glass ampoule with the test polymer sample. After desorption for a period of time at a given temperature (e.g. 5 min, ZSO'C), the ampoule is broken and the volatile products are transferred by the flow of carrier gas t o the chromatographic column for subsequent analysis. Although the above technique was developed specially for determining plasticizers [73], it can also be used for determining trace amounts of solvents, monomers, etc., in polymers. I
I
Fig. 4.2. Device for GC analysis of volatile compounds in polymers. 1 = Tube for carrier gas; 2 = rod with piston for breaking glass ampoule with sample; 3 = lid; 4 = body; 5 = ampoule; 6 =joint; 7 = chromatographic column. References p. 108
92
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
Reichle and Tengler [74] described a similar procedure for the determination of acrylonitrile in polyacrylonitrile. A 0.2-0.3-g amount of powdered polyacrylonitrile in a sealed glass ampoule was introduced into a chamber heater to 9 5 O C . After 10 min, the ampoule was broken with a special piston. The acrylonitrile vapour in the flow of carrier gas entered the chromatographic column (300 cm) with p,$-oxydipropionitrile for separation, and the results were recorded with a flame-ionization detector. The method permits the content of a monomer in polyacrylonitrile down to 0.001% to be determined. It should be noted that the method of automatic sample injection described in the literature [75] and also the method of sample introduction in sealed capillaries made of Wood’s alloy [49], which readily lends itself to automation, can be applied to the analysis of volatile compounds in polymers. A simple method for analyzing plasticizers in polymers with the use of an ordinary pyrolytic cell of the filament type has been developed by Zulaica and Guiochon [51]. The polymer samples (ca. 0.005 g) were placed in a pyrolytic cell and then pyrolyzed at about 625OC for 10 sec. Poly(viny1 chloride)-based polymer materials containing dibutyl phthalate, dibutyl sebacate, tri-n-butyl phosphate and bis(2-ethylhexyl) adipate as plasticizers and also mixtures of these plasticizers were analyzed. In order to obtain quantitative results, it is advisable to use the absolute calibration method, because it takes into account losses of plasticizers. The possible errors of this method are associated with its first stage, i.e., heating of the test polymer sample to a high temperature. During this heating, the polymer may be decomposed to light products, for instance to the corresponding monomer. Therefore, this method cannot be widely used for determining the content of a monomer in a polymer, but can be applied successfully only for analyzing thermostable impurities of plasticizers, solvents and other compounds. Slight decomposition of volatile test components, and also other sources of losses, can be taken into account during calibration. Tubular-type pyrolytic cells [51] can also be used for determining the content of both inhibitors and lighter products (for instance, volatile solvents and monomers). In thls type of procedure the boat containing the polymer sample is quickly introduced (for the necessary time interval) into the hot zone of the tubular reactor heated to the pre-assigned temperature. if the test components pass into the gas phase within 10-20 see, the heating is effected in the flow of carrier gas and the equipment can be used without modification. If, however, the isolation of the volatile components at the selected temperature requires a long time, then in order to correlate it with the subsequent GC analysis one must either shut off the flow of carrier gas to the pyrolytic chamber for the required period of time and carry out the transfer of the volatile components from the polymer to the gas under static conditions, or introduce, between the analyzer and the chromatographic column, a trap for removing the volatile components from the flow of carrier gas. After the trap temperature increases, the volatile impurities enter the chromatographic column as a narrow zone in the flow of carrier gas for separation. The second method seems to be the simpler. The use of traps for removing heavy impurities from the flow of gas has been described [ 3 7 , 7 6 , 7 7 ] . In a particular instance, especially in the analysis of relatively high-boiling compounds (‘heavy’ solvents, plasticizers, inhibitors, etc.), we can recommend the use of a chromatographic column at low temperatures as a trap. The subsequent chromatographic analysis is carried out at
MULTI-STAGE METHODS
93
elevated temperatures. This method of concentration was used successfully for the analysis of impurities in air [ 7 8 ] , of solvents and water in nitrocellulose and also of water and phenol in phenolic resins [37]. MULTI-STAGE METHODS FOR DETERMlNING VOLATILE COMPONENTS When determining monomers, plasticizers, solvents and other volatile conipounds in polymers, extensive use is made of multi-stage methods, which include one or several operations of preliminary preparation of the sample for subsequent GC analysis. Tlie methods usually applied for separating volatile impurities from polymers include extraction, dissolution with subsequent deposition of the polymer and thermal desorption in the flow of carrier gas. It is also undoubtedly expedient to use other effective, primarily chromatographic, methods for separating the volatile components, such as gel permeation, thin-layer and column chromatography. As the multi-stage methods are very laborious and complex, they should be used only when simpler methods fail (for instance, because of the thermal instability of the polymer), or for single determinations, when the special development of a simple method is not justified. The only exceptions are methods in which the preliminary stage is not the separation process, but the dilution of the test polynier solution or the dissolution of a solid polymer. This simple technique permits the complex task of determining volatile components in a solid polymer or in viscous solutions t o be reduced to a simpler task, i.e., deterininatioii of volatile components in polymer solutions. Naturally, the concentration of the test conipounds in the sample will decrease, but high-sensitivity GC ionization detectors can determine impurities in a sample at concentrations of 10-4-10-5%. Therefore, this method of analysis is also sufficiently simple and sensitive. Determination of volatile components in polymer solutions When preparing solutions of polymer systems for subsequent GC analysis, it is advisable to bear in mind the following recommendations. (1) The liquid stationary phase must be sufficiently selective that the solvent can be well separated from the test components. The solvent zone must not mask the impurities being determined. (2) For quantitative assessment, it is best to add to the polymer solution a known amount of the substance that is used as the internal standard. (3) It is expedient t o use dilute polymer solutions for analysis. This procedure will reduce the errors due to the possible ‘trapping’ of the test components of the polymers and increase the operating time of the chromatograph between periodic cleaning of the sample introduction system or replacement of the preliminary column. This method of analysis has been widely used for determining volatile compounds in polystyrene [79-841, acrylates and niethacrylates [ 7 4 , 8 5 ] , phenol- formaldehyde resins [ 8 6 ] and some other types of polymers [87] and oligomers [88--90]. As an exaniple, Tdbk 4.2 lists the conditions for determining residual monomers in styrenebased polymers; the corresponding chroniatogranis are given in Fig. 4.3. It follows from these data that the method is sufficiently sensitive and rapid. References p. 108
TABLE 4.2
\o
P
DETERMINATION OF MONOMER IMPURITIES IN STYRENE-BASED POLYMERS [ 801 Polymer analyzed
Sample preparation
Conditions of chromatographic analysis Column length X I.D. (cm)
Determination of acrylonitrile 1 g was dissolved in 10 ml in acrylonitrile-butadieneof dimethylformamide; styrene (ABS) copolymer internal standard n-octane
250 X 0.6
+ 35 X
Retermination of styrene and acrylonitrile in polystyrene and styreneacrylonitrile copolymer
1.25 g was dissolved in 15-20 ml of dimethylformamide
200 X 0.32
Determination of monomers and ethylbenzene in ABS copolymer
0.5 g was dissolved in 1.0 ml of a 0.5% solution of toluene in dimethylformamide; internal standard toluene
100 x 0.3
+ 100 x
0.6
0.3
Notes
Sorbent
100 mymin N,, 100°C, threshold concentration 0.008%. 20% Carbowax o n Chromosorb W + 10% Apiezon L on Chromosorb
Concentration of 0.00 1% cannot be determined because of asymmetry of acrylonitrile peak
N,, 140"C, threshold concentration 0.001%. 15%Carbowax 1500 on Chromosorb W
Analysis should be carried out at low hydrogen flow-rates
< 0
z r r
M
n
30 ml/min N,, 120°C, threshold concentration 0.001%. 20% Tween 81 o n Chromosorb W + 10%Resoflex 446 on Chromosorb
0
5 C
F
2 r
<
E;a Cn
4
MULTI-STAGE METHODS (b)
n4
1
Time
95
I
I:ig. 4.3. Determination of nionoiner impurities in styrene-based polymers. (a), Chromatogram of standard solution of acrylonitrile in n-octane. Peaks: 1 = internal standard; 2 = acrylonitrile. Total analysis time, 5 inin. (b), Determination of acrylonitrile impurity in acrylonitrile-butadiene-styrene (ABS) copolynier.Peaks: 1 = acrylonitrile; 2 = toluene; 3, 4 = not identified. Total analysis time, 7 inin. (c), Determination of volatile impurities in ABS copolymer. Peaks: 1 = butadiene; 2 = acrylonitrile; 3 = toluene; 4 = ethylbenzene; 5 = styrene; 6 = solvcnt. Total analysis time, 6 inin. Analysis conditions are indicated in Table 4.2.
The procedure for analyzing volatile impurities in polystyrene has also been worked out in detail by Crompton and co-workers [81, 821. The polystyrene was dissolved in propylene oxide and the solution was introduced into the evaporator, which contained a glass tube filled with glass-wool. A column (450 X 0.6 cm) filled with 10%Carbowax 15-20 M on Celite (60-72 mesh) at 80°C was used for separating the volatile compounds. The separated components were recorded with a flame-ionization detector. Table 4.3 187-1 gives the relative retention times of the impurities (with respect t o styrene), the content of which was calculated by the internal standard (n-undecane) method. Highsensitivity methods for determining the monomer and other volatile impurities in styrene-based polymer systems have been described [83] . In all procedures of this type, the important stage is the assessment of the correctness of the method and the instrument calibration. As an illustration, we shall describe the procedure for the preparation of a standard solution of toluene in a polymer [ 8 3 ] .The polymer was dissolved in chloroform, re-precipitated with methanol and dried in a vacuum drier at 60°C for 24 h. Then 0.5 g of the polymer obtained was dissolved in 5 ml of redistilled diniethylformamide, washed with 1 ml of a 0.5% solution of toluene in dimethylforniamide, and an aqueous standard monomer solution was added that g of monomer, corresponding to 0.01 -1 .O% of the monomer contained 5 10-’-5 concentration in the polymer. A calibration graph was plotted of the ratio of the monomer and toluene peak heights versus monomer concentration in the polymer on the basis of chromatographic analyses of the monomer solution in the polymer.
-
References p. 108
-
96
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
TABLE 4.3 RELATIVE RETENTION TIMES OF IMPURITIES (WITHRESPECT TO STYRENE) IN POLYSTYRENE [ 821 ~
Compound
Relative retention time
,Benzene Toluene n-Undecane Ethylbenzene rn-Xylene p-Xylene Cymene o-Xylene n-Propylbenzene rn-Ethyltoluene p-Ethyltoluene Isobutylbenzene
0.17 0.29 0.40 0.47 0.50 0.50 0.62 0.66 0.77 0.84 0.84 0.92
Compound
Relative reten tion time
tert. -Butylbenzene
0.92 1.oo 1.00 1.05 1.35 1.44 1.44 1.60 1.86 1.86 1.86
sec. -Butylbenzene
Styrene o-Ethyltoluene rn-Diethylbenzene p-Diethylbenzene n-Butylbenzene a-Methylstyrene o-Methylstyrene rn-Methylstyrene p-Methylstyrene
A rapid method for determining the styrene content in polystyrene was devised by Graebel [84].A batch of sample was dissolved in dimethylformamide and, after centrifuging off the impurities and adding an internal standard (cyclohexyl formate), the solution obtained was analyzed on a column with diethylene glycol succinate. Investigations along these lines include the determination of foaming agents (for instance, pentane) in polystyrene foam [91]. The procedure for determining the unreacted monomer in polyacrylonitrile, based on analysis of the polymer solution in dimethylformamide (internal standard acetonitrile), was described by Podmore [75]. A method for the direct determination of caprolactam in nylon-6 was described by Zilio-Grandi et al. [92], whose results permit the analysis time to be reduced from 5 to 2.5 h [93]. The proposed method is based on dissolution of the polymer sample in 85% formic acid containing quinoline, which is used as internal standard for quantitative calculation. The solution obtained (1 111) was analyzed directly by GC with flame-ionization detection. The separation was carried out at 200°C on a column (80 X 0.4 cm) filled with 10% Carbowax 20M on Chromosorb W treated with dimethyldichlorosilane. The column was replaced after 400 analyses. The caprolactam content was determined within the concentration range from 0.1 to' 10%. The dissolution method can be applied successfully for determining not only monomers? but also heavier compounds such as antioxidants and plasticizers. For determination of inhibitors of Ionol(2,6-di-ferf.-butyl-p-cresol) and Tinuvin P [2-(2'-hydroxy-5'-methylphenyl)benzotriazole] , the polymer sample was dissolved in carbon disulphide or methylene chloride, and a sample of the solution obtained was separated on a column (30 X 0.3 cm) filled with 25% poly(ethy1ene glycol)-pentaerythritol adipate and 2% orthophosphoric acid on Chromosorb W [94]. The separation on Ionol was carried out at 135OC, and that of Tinuvin P at 220°C. An efficient GC method for determining the antioxidants 2(3)-terf. -butyl-4-hydroxyanisole (BHA), 1,2-dihydroxy-6-ethoxy-2,2,4-trimethylquinoline (ethoxyn) and 3,5-di-ferf. -butyl-4-hydroxytoluene (BHT) when present simul-
MULTI-STAGE METHODS
91
taneously in a solution was elaborated by Choy et al. 1951 . A sample containing the antioxidant was separated at 188°C on a column (1 52 X 0.3 cm) filled with 10%silicone SE-30 on refractory brick (60-80 mesh). GC methods are also widely used in analyzing emulsions (latexes) etc. [85, 96-1001. In this instance, it is also desirable t o carry out dissolution or elution as a preliminary operation prior t o analysis. The use of more dilute systems reduces the risk of 'trapping' of light compounds by the polymer, reduces the associated analytical errors and enables one, by decreasing the viscosity of the system, to use the conventional syringe technique for injection of the sample into the chromatograph. In the work of Shapras and Claver [85] , a three-component latex containing 44% of solid residue was diluted 1 : 10 with water and 50 p1 of the solution were poured into the sample introduction system at 140°C. The separation was carried out at 100°C on a column (1 80 X 0.6 c m j filled with 10% stearamidopropyldimethyl-0-hydroxyethylammonium nitrate on Chromosorb W. Acrylonitrile, styrene and hexyl acrylate were determined. The sensitivity threshold of the method is ca. 5 lo4%,. A detailed analysis of the determination of unreacted monomers in polymer emulsions was carried out by Fossich [98]. He developed a procedure for analyzing the emulsion of the vinyl acetate copolymers. About 1 g of well shaken emulsion is nuxed wit.11 10 nil of methanol and 1 nil of solution of the internal standard. The emulsion obtained is stable for 24 11. A 3-pl sample is taken for analysis. Columns (1 80 X 0.3 cm) filled with 10% Carbowax 1500 on Chromosorb W (40-60 mesh) are used. The conditions are given in Table 4.4. The sensitivity threshold of the method is 0.01 5% for vinyl acetate, 0.01 % for tz-butyl acrylate, and 0.02% for 2-ethylhexyl acrylate. Fossicli [98] also proposed an efficient design of a sample chamber for analyzing polymer systems (Fig. 4.4). A glass inert with a glass-wool packing on which the non-volatile polymer remains is placed in the sample introduction system. This design facilitates rapid cleaning of the sample introduction system by replacing the inserts. Glass is one of the most convenient inert materials for making inserts. An analysis of various latex systems was discussed by Wilkinson et al. [99] . As the selection of the optimal solvent is an important problem in analysis, Table 4.5 lists suitable solvents for some latexes. The GC analysis of latexes was described by Panora er al. [loo] . During the production of polyester, it is necessary to determine rapidly and accurately the residual monomers, solvents and other volatile compounds and the application of GC for this purpose has been described [86, 101-1031.
-
TABLE 4.4
GC ANALYSIS OF MONOMERS I N POLY(V1NYL ACLTATE) EMULSIONS 1981 Conditions
Test monomer Vinyl acetate
Temperature
50°C
n-Butyl acrylate
2-Ethylhcxyl acrylate
SO"(', increased atter
50"C, increased after 20 min to 105°C 2% (w/w) Octanol-2 in methanol
6 min to 85°C Internal standard
References p. 108
0.4% n-Octane in methanol
2% (w/w) Cunicne in methanol
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
98
Fig. 4.4. Sample introduction system for polymer analysis. 1 = Syringe; 2 = serum cap; 3 = syringe needle; 4 = glass liner; 5 = glass-wool plug. TABLE 4.5 SOLVENTS FOR SOME TYPES OF LATEXES [ 991 Type of latex
Solvent
Vinyl chloride-vinylidene chloride Vinyl acetate-vinyl propionate Butadiene- styrene*
Cyclohexanone Cyclohexanone Cyclohexanone 2,2-Dimethoxypropane 2,2-Dimethoxypropane 2,2-Dimethoxypropane-acetic anhydride Acetic anhydride2,2-dimethoxypropane Acetic anhydride Acetic anhydride
Styrene- 2-ethylhexyl acrylate Styrene- 2-ethylhexyl acrylate-acrylonitrile* Ethyl acrylate-butyl acrylate-styrene* Ethyl acrylate- methyl methacrylate Acrylonitrile-butyl acrylate .-
-
*It is believed I991 that some correction experiments are necessary.
In many instances, in order to improve the separation of light components, it is advisable t o use as the solvent a higher-boiling compound compared with the compounds being analyzed. An example of such a procedure is the determination of residual amounts of ethylene oxide, propylene oxide, methanol and tetrahydrofuran in polyesters by Mokeyeva and Tsarfin 11031. Chlorobenzene was used to dissolve the polyester. A chlorobenzene solution of the polyester was injected into a pre-column (30 X 0.3 cm) on whose packing the polyester was retained, while the low-boiling components were separated from the chlorohenzene and entered the chromatographic column (200 X 0.4 cm) with the carrier gas for separation. Both columns were filled with 20%Ethofat on Chromosorb W (80-100 mesh). The chronatographic set-up is shown schematically in Fig. 4.5.
M U LTI-STAG E METHODS
4g5
99
1’
3’
Fig. 4.5. Chromatographic set-up for the analysis of polymer solutions. (a), Tandem connection of pre-column and main column; (b), parallel connection. 1 = Switching device; 2 = pre-column; 3 = chromatographic column; 4 = sample introduction system; 5 = detector; 6 = switching control head.
The columns are connected in tandem (Fig. 4 . h ) when tlie sample is introduced before the separation of tlie light components and chlorobenzene. After 7 inin, the arrangement is changed; the clilorobenzene remains in the pre-column, while the test components are separated in the main chroniatographic column (position b). After the rearrangement (position b), the chlorobenzene is rapidly eluted within 1 5 min) t o the atmosphere from the short pre-column, while the separation of the light components on the main column is completed within 10 Inin. The pre-column packing, in which polyester is accumulated, is replaced after each 80-100 analyses. The quantitative determination of impurities in polyesters was carried out by the absolute calibration method. The method is applicable to the analysis of contents of ethylene oxide exceeding 5 lo-’%, propylene oxide 0.0170, methanol 0.05% and tetrahydrofuran 0.7% (with a polyester batch of 1 g). When a polymer is thermally unstable, the polymer solution obtained is not analyzed immediately, but after the precipitation (isolation) of the polymer. In this instance the mother liquor containing no non-volatile polymer is used for GC analysis. This method simplifies the concluding stage of the determination, Le., GC analysis, and allows for the use of standard procedures and equipment. Also, by evaporating the mother liquor or applying some other methods of concentration (for instance, extraction), one can increase the concentration of the volatile compounds being analyzed. It should be noted remembered, however, that there may be losses of the test components during re-precipitation of the polymer and evaporation of the mother liquor. As an illustration, we shall now consider the method for determining free phenol and free formaldehyde in phenol-formaldehyde resins [ 8 6 ]. A batch of an alkaline solution of a resin (10 g) is mixed with an internal standard (n-butanol for formaldehyde and p-cresol for phenol), then it is diluted with water (1:2) and divided into two parts. One part is acidified, with vigorous stirring, with hydrochloric or sulphuric acid. The resin precipitate is filtered out and the filtrate is analyzed for formaldehyde at 130°C on a column (488 X 0.6 cni) filled with 1% octaacetate saccharose on Teflon-6. As part of tlie phenol is trapped by the resin precipitate during precipitation, the extraction method was used in the determination of phenol. A 10-nil volume of dietliyl ether was added to the second part of the dilute solution and, with vigorous stirring, the
-
References p. 108
100
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
polymer solution was neutralized with the acid. A sample of the ether layer was chromatographed at 130°C on a column (366 X 0.6 cm) filled with 10%silicone SF-96 on Fluoropak, thus determining the phenol content. The analysis time did not exceed 15 min. The accuracy of determination with the use of a katharometer was k0.05% for phenol and +0.06% for formaldehyde. The procedure developed by Stevens and Percival [86] was used successfully for the analysis of free fural in phenol-fural resins and of acrolein in phenol-acrolein resins [104]. Jones [lo51 showed that better separation of formaldehyde, methanol and water is achieved on columns filled with Phasepak P and Q, Porapak R and Q, or on specially treated diatomite supports (Universal A and B). Determination of the contents of residual monomers in some polystyrene plastics was developed by Kleshcheva et ul. [ 1061 . A batch of polymer (cu. 15 g) was dissolved in 10 ml of chloroform with heating for 1 h on a water-bath equipped with an inverted cooler. After cooling, the polymer was precipitated by adding 7-10 ml of methanol. The mother liquor was analyzed at 130°C on a column (250 X 0.6 cm) filled with 25% poly(ethy1ene glycol adipate) on INZ-600, using isopropylbenzene as the internal standard. Esposito and Swann [lo71 determined the content of monomeric styrene (a crosslinking agent) in polyester resin. A batch of 1-2 g of polyester resin was mixed with 0.3 g of methyl isobutyl ketone (internal standard) and dissolved in 1 ml of methyl ethyl ketone. The polymer was precipitated with 4 ml of pentane, and then centrifuged. The mother liquor was analyzed at 85°C on a column (180 X 0.63 cm) filled with 20% Triton X-305 on Chromosorb W. In many instances, when determining the content of volatile impurities in a polymer, chemical methods of decomposition of the polymer can be used with advantage. One such method was applied when determining the content of diethylene glycol in poly(ethy1ene terephthalate) [1081.Poly(ethy1ene terephthalate) was decomposed by treatment with 85% hydrazine at 1 15°C. Under these conditions, fine filaments decompose within 1-2 min, finely ground granulate (diameter less than 0.8 mm) within 0.5-1 .O h and unground granulate (6 X 4 X 2 mm) within 10 h. The diethylene glycol content in the hydrazinolysis products is determined by gas chromatography with the use of benzyl alcohol or tetraethylene glycol dimethyl ester as internal standard. The method is simple and rapid. Application of extraction methods Extraction methods are very efficient for separating volatile impurities from polymers and their application is especially expedient in investigating solid insoluble polymers and polymers of low thermal stability. At present, extraction is used most frequently for determining plasticizers, inhibitors and other high-boiling components of polymer systems [73]. This and other problems of the application of GC in polymer chemistry were considered by Stevens [109]. The efficiency of extraction is largely determined by the selection of the most suitable solvent, which must dissolve efficiently the volatile component being determined and must not dissolve the polymer (a low swelling ability of the polymer promotes
MULTI-STAGE METHODS
101
extraction). The sample for extraction is prepared in the form of small pieces. The completeness of the extraction of a component is best verified by control tests. One should bear in mind that the polymer products may also be dissolved. Extraction is carried out with a Soxhlet apparatus; the procedure for the extraction of plasticizers was described by Robertson and Rowbey [I 101. A 4-g amount of polymer is extracted in a Soxhlet apparatus with 100 ml of dietliyl ether. After the solvent has been distilled off, its residue is removed by heating, first on a water-bath (1 h) and then in a drying cabinet. When extracting with diethyl ether, plasticizers with a molecular weight below 1000 are usually removed [ I 111. In order to extract plasticizers, the same sample is further extracted with an azeotropic mixture of carbon tetrachloride and methanol. Such extraction procedures for poly(viny1 chloride) and cellulose ethers were considered in a monograph [73]. As an example we shall describe the procedure for the extracting plasticizers from poly(viny1 chloride). About 10 g of poly(viny1 chloride) are ground to particles of diameter not exceeding 0.7 mm, weighed with an accuracy of 0.1 mg and placed in a paper cartridge (7.5 X 3 cm), which is inserted in a Soxhlet extractor. The flask (250 nil) of the Soxhlet apparatus is filled with 120 ml of diethyl ether and heated until the ether boils. The solvent vapour is condensed in the inverted cooler and flows down into the extractor, where the cartridge containing poly(viny1 chloride) is located. The intensity of heating must be such that the solvent (extracting agent) is replaced every 3-4 min. After 6-8 h of extraction, the plasticizers go into solution quantitatively. The solution containing the extracted plasticizers is evaporated at a decreased pressure to a volume of 50 ml. The concentrated solution is filtered off, and the entire solvent is evaporated by heating on a water-bath. The quantitative content of the plasticizers is determined by weighing after final drying of the residue in a vacuum cabinet at 60-70°C for 2 h. Note that in the GC determination of the plasticizer content in the initial polymer, it is possible to analyze the ether solution; the stages of weighing and evaporation* of the solvent are unnecessary. The extraction method has been used in determining the content of ‘heavy’ monomers, for instance caprolactani, in nylon-6. Aqueous extraction was carried out in a Soxhlet apparatus for 5 h [112]. The extract obtained was separated by GC at 195°C on a column (180 X 0.3 cm) filled with 10%silicone SE-30 on Diatoport (60-80mesh). For quantitative calculations, use was made of the internal standard method [bis-2-(2 methoxy)ethyl ether] . Extraction methods have also been used for extracting stabilizers from polymers. Schroder et al. [I 131 described the use of extraction for isolating antioxidants and thermostabilizers from formaldehyde polymers. Antioxidants were extracted from the polymers with chloroform, while thermostabilizers were extracted from the chloroforminsoluble part with methanol. A similar method for determining antioxidants was described by Schwecke and Nelson [114] . Extraction of phenyl salicylate and resorcinol nionobenzoate from plastics and subsequent determination by GC and thin-layer chromatography has been described [ 1 151 . Rose [ I 161 suggested that up to 0.0270 of residual styrene monomer in polystyrene can be determined by using the extraction method. Polystyrene (1 g) was heated for 30 min in 50 ml of diethyl ether, whereupon the extract was removed and the operation *Evaporation may be useful for concentrating the plasticizer solution. References p. 108
102
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
repeated in the pure solvent. Decalin (2 ml) was added to the combined extract and the ether was distilled off. The concentrated extract was analyzed at 125°C on a column (100 X 0.01 cm) filled with 10% Apiezon L o n Celite (100-120 mesh). The combined application of GC and thin-layer chromatography, infrared spectroscopy and qualitative reactions has been described [ 1 171 . A disadvantage of the extraction method is the long extraction time but this can be reduced by an increasing masstransport rate of the process. A method for determining stabilizers in rubbers with the use of GC, chemical methods and MS was described by Gotti [ 1181 . A series of investigations for determining the content of various ingredients in polymers was published by Styskin and co-workers [119-1241. Application of methods for separating volatile impurities from polymers Apart from extraction and precipitation from solution, other methods for separating volatile components of polymer systems from polymer compounds have been described. Although these methods have not found wide application, they will undoubtedly be useful in solving certain particular problems. Solvent extraction from varnishes and glues can be achieved successfully by lowtemperature vacuum distillation. The distilled solvent is analyzed directly by GC [ 1251 . A method of this type has been developed by Hoover [ 1261 . A 1-ml volume of test dye is introduced into a heated glass tube filled with glass beads. The flow of nitrogen passing through the tube carries the vapour of the volatile substances to a cold trap, in which the solvent vapour is then trapped. A sample of a condensed mixture of solvents is collected with a microsyringe and analyzed on a gas chromatograph. The distillation method has also been used for separating the unreacted monomer from the polymer part of an aqueous emulsion [127] . A sample of aqueous emulsion of a copolymer of ethyl acrylate and styrene (50 ml) was diluted with 125 nil of water, then exactly 3 ml of toluene, 20 ml of hydroquinone and a small amount of a defoamer were added and the mixture was distilled; about 3 ml of organic layer were collected in the receiver. Part of this layer (0.5 ml) was dried with anhydrous sodium sulphate and then separate samples (0.5 pl) were analyzed by GC, the contents of ethyl acrylate, toluene and styrene being determined. An interesting method for the chromatographic separation, in the liquid phase, of volatile plasticizers (didecyl phthalate, dinonyl phthalate, tricresyl phosphate, etc.) from castor oil, which cannot be determined by GC, was described by Rau et al. 11281. A test sample of a solution of plasticizers for imitation leather is deposited on a column containing silica gel, from which the plasticizers are eluted with a mixture of carbon tetrachloride and diisopropyl ether (85: 15) and then analyzed on a gas Chromatograph. The castor oil is eluted from the column with diisopropyl ether. The determination of alkylphenols [ 1291 by combining thin-layer and gas-liquid chromatography can also be used for determining alkylphenols in polymers. The application of combined methods based on combining liquid (column and thinlayer) chromatography, which is used for separating volatile compounds from the polymer and dividing them into groups, with GC seems to hold much promise in the analysis of polymer systems.
SPECIFIC FEATURES
103
SPECIFIC FEATURES OF THE GC ANALYSIS OF SOLVENTS, MONOMERS, PLASTICIZERS AND STABILIZERS The GC analysis of monomers and solvents has been thoroughly developed. Data on the retention values of these compounds on various liquid stationary phases are given in many monographs [ 7 8 , 130-1351 and reference books [136, 1371. The literature contains niuch less data on higher-boiling components of polymer,systems such as TABLE 4.6 CONDITlONS FOR T I E CC ANALYSIS 01: PLASTICIZERS [ 7 3 ] ~~
~~
No.
Liquid stationary phase*
Column (I.D. = 4 mm) Material
Length (m)
Glass Glass Glass Glass Glass Glass Glass Glass Glass Glass
3 3 1
6 7 8 9 10
15% Ultramol 111 30% Polyglycol P-200 15% Ultraniol 111 15% Ultramol 111 10% Resoflex LAC-2R-446 1S% Ultramol Ill 15% Ultramol 111 10% Reoplex 400 lo%,Reoplex 400 10% Reoplex 400
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
10% Polyglycol P-2000 10% brnpol 1040 20% Polyglycol P-2000 10% Resoflex LAC-2R-446 10% Resoflex LAC-2R-446 10% Reoplex 400 15% Ultramol 111 10% Resoflex LAC-2R-446 10% Resoflex LAC-2R-446 15% Ultramol 111 10% Resoflex LAC-2R-446 5% Sihcone rubber GT, SC-30 10% Resoflex LAC-2R-446 10%Resoflex LAC-2R-446 15% Ultraniol 111 15% Ultrainol I11 1S%, Ultrarnol 111 15% Ultramol 111 10% Rc\oilex LAC-2R-446 1.55, Ultrarnol 111
Glass Glass Glass Glass Copper Glass Glass Glass Steel Glass Glass Steel Copper Copper Glass Glass Glass Glass Steel
1 2 3 4 5
*Support Kieselguhr (0.2-0.3 mm).
References p. 108
Glass
1
1 1
1 1 2 2 0.3 2 2 2 0.3 1 1 1 0.5 1 0.5 1.2 0.5 0.5 1 1 1 0.5 0.5 I
Column temperature ("C or "C/min) 70 70 80 160 160 160 100-200 90 140 80-160 (1.25) 130 160 160 170 236 160 240 2 20 230 240 2 30 140-350 (15) 190 230 190 2 30 220 2 30 230 210
Velocity of helium carrier gas (ml/min) 40 54 176 244 160 200 115 40 62 42 60 82 116 90 90 40 196 160 48 208 116 100 75 120 150 170 208 190 52 160
104
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
TABLE 4.7 RELATIVE RETENTION VALUES OF PLASTICIZERS AND SOME SOLVENTS The conditions refer to the numbers given in the f i s t column in Table 4.6 [ 731. Compound
Conditions .~
2
1
Methanol Methyl ethanoate Methyl propionate Methyl butyrate Methyl 2-ethylbutyrate
1.0 1.0* 2.0 3.78 7.98
4.79 1.0* 1.50 2.31 3.84
~
3, Methyl 2-ethylcaproate Methyl caprylate Methyl pelargonate Methyl benzoate Methyl caprate Mathyl laurate
Methyl adipate Methyl laurate Methyl myristate Methyl azelate Methyl phthalate Methyl sebacate Methyl palmitate Methyl citrate Methyl stearate Methyl oleate Methyl linoleate Methyl linolenate Methyl ricinoleate
Glyceryl t riacetate Glyceryl diacetate Glyceryl monoacetate
0.46 1.00* 1.92 2.46 3.86 13.70 4
7
0.86 1.0* 2.43 3.29 4.36 5.13 5.78 7.06 14.13 14.13 16.42 20.63 36.55
0.91 1.0* 1.43 1.57 1.74 1.78 1.83 1.97 2.24 2.24 2.31 2.40 2.81
14
13
1.00
1.65 1.96 25
Dimethyl phthalate Diethyl phthalate Dibutyl phthalate Dirnethoxyethyl phthalate Di-2-ethylhexyl phthalate Benzyl butylphthalate Diisononyl phthalate Dime thylcyclohexyl phthalate Dibenzyl phthalate
0.73 1.00* 3.11
1.00 1.21 1.43
26
0.40 1.0* 1.60 2.52 2.96 2.96 14.40
23 0.80 1.00" 2.95
24
1.00 3.50 3.50
4.78 7.50
SPECIFIC FEATURES
105
TABLE 4.7 (cotrtCiuedJ ._~.
Compound __
.
.
~-
Conditions 11
Ethylene glycol Butanediol Butanediol-l,3 Butanediol-l,4 Diethylene glycol Glycerin Triethylene glycol
1.0* 1.00* 1.91 3.72 4.18 11.63 17.80 19
Benzyl butyladipate Benzyl (ethylhexyl) adipate
0.07, 1.00*, 4.13 0.29, 1.00, 4.13 28
Tributyl phosphate Tri-2-ethylhexyl phosphate Trichloroethyl phosphate Di(~~Iicnyl-2-ethylliexyl) phosphate Diphenyl cresylphosphate
29
0.13 0.12 1.00* 1.00* 1.00 1.82 5.46 1.00 Several peaks 12
Phenol o-(l'resol m-and pCresol Xylene
1.00* 1.15 1.54 2.52 17
Dibutyl adipatc Di-2-ethylhexyl adipate Diisononyl adipate Dinonyl adipate Dibutyl azelate Di-2-ethylbutyl azelate Dihexyl azelate Di-2-ethylhexyl azelate
0.16 1.00* 1.15, 1.35, 1.72 1.35, 1.78, 2.31 0.41 1.oo* 1.43 2.56 21
Ethyl o-acetylcitratc Triethyl citrate Butyl o-acetylcitrate Butyl citrate 2-Et1;ylhexyl o-acetylcitrate Dimethyl sebacate Diethyl sebacate Dibutyl sebacate Di-2-ethylhexyl sebacate Dibenzyl sebacate
18
1.oo 1.00*
3.29 3.57 23.80 1 .oo 1.00* 3.29 17.28
27 1.00 1.00* 5.28 5.28
0.19 1.00* 1.31 1.31, 1.76, 2.33 0.47 1.00* 1.39 2.34 22
1.00 1.00* 1.42 1.56 2.29 0.84 1.00*
1.39 2.06 2.30
20
0.75 1.00* 3.33 18.83
(Continued on p . 106) References p. 108
106
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
TABLE 4.1 (continued) Compound
Conditions 5
Dimethyl adipate Dimethyl azelate Dimethyl sebacate Dimethyl phthalate Triniethyl citrate
6
0.35 1.00* 1.42 1.70 2.95
0.33 1.00* 2.04 1.73 2.82
15 Phenyl salicylate Resorcinol monobenzoate
1.oo* 12.00 16
Methyl benzoate Methyl salicylate Phenol
0.59 1.00*
1.59 30
Dibutyl adipate Di-2-ethylhexyl adipate
1.oo * 6.35 8
Methanol Ethanol Propanol n-Butanol Ethylene glycol monomethyl ether Ethylene glycol monoethyl ether Ethylene glycol monopropyl ether n-Hexanol Ethylene glycol monobutyl ether Methylcyclo hexanol n-1-leptanol 2-Ethylhexanol Isononanol n-Octanol Ethylene glycol lsodecanol n-Decanol Benzyl alcohol *Internal standard.
1.oo 1.oo 2.2s 2.75 3.75 4.63 7.00 8.50 11.62 15.23 15.23 18.75 21.25
9
0.81 1.00* 1.50 1.72 2.05 2.66 4.61
10 0.79 1.00* 1.57 2.50 3.29 3.86 5.28 6.00 7.50 7.78 7.78 9.43 10.00 11.22 13.62 14.95 17.29 20.10
SPECIFIC FI ATURES
107
plasticizers and inhibitors. The CC determination of volatile compounds present in polymers is often complicated by the high polarity of the test compounds. Therefore, in the practical application of GC, the following difficulties must be borne in mind: (1) deviations of the experimental retention values from the published data; and (2) understated values of concentrations of the test compounds. These effects are due to the adsorption of the sorbents of the test compounds at the interfaces of the liquid stationary phase with the solid support and the gas phase. Reversible adsorption at these interfaces leads t o uncontrollable fluctuations of the retention values, irreversible adsorption on the surface of the solid support and a decrease in the detectable concentration of the polar component or even to its complete loss. The nature of these factors, which complicate the analysis, was considered by Berezkin and Tatarinskii [77]. The analysis of high-boiling compounds has a number of difficulties, which have been considered in detail [ 138, 1391 .
Time (min)
Fig. 4.6. Examples of the G€ analysis of plasticizers. (a), Chromatogram of a mixture of methyl esters of mono-, di- and triciirboxylic acids. Conditions: glass column, 100 X 0.4 cm; sorbent, Ultrainol 111 o n Kieselguhr; temperature increase from 100 t o 200°C at 4"C/niin. Peaks: 1 = adipic acid; 2 = lauric acid; 3 = azelaic acid; 4 = niyristic acid; S = phthalic acid; 6 = sebacic acid; 7 = palmitic acid, citric acid; 9 = stearic acid; 10 = oleic acid; 11 = linoleic acid; 12 = linolenic acid; 1 3 = ricinoleic acid. (b),Chromatogram of a mixture of alcohols. Conditions: glass column, 200 X 0.4 cm; sorbent, 10% Reoplex 400 on Kieselguhr; temperature increase from 80 t o 100°C at 1.25"C/min. Peaks: 1 = niethanol: 2 = ethanol; 3 = propanol; 4 = n-butanol; 5 = ethylene glycol monomethyl ether; 6 = ethylene glycol monoethyl ether; 7 = ethylene glycol nlonopropyl ether; 8 = n-hexanol; 9 = ethylene glycol monobutyl ether; 10 = nietliylcyclohexanol; 11 = n-heptanol; 12 = 2-ethylhexanol; 1 3 = isononanol; 1 4 = n-octanol; 15 = ethylene glycol; 16 = isodecanol; 17 = n-decanol; 18 = benzyl alcohol. (c), Chromatogram of a mixture of esters of sebacic acid. Conditions: stainlesssteel column, 120 X 0.4 cm, S% SE-30 on Chromosorb; temperature increase from 140 to 350°C at 15"C/min. Peaks: 1 = dimethyl sebacate; 2 = diethyl sebacate; 3 = dibutyl sebacate; 4 = di-2-ethylhexyl sebacate; 5 = dibenzyl sebacate. References p. 108
108
VOLATILE COMPOUNDS IN POLYMER SYSTEMS
Note that in reproducing a known procedure or in developing a new procedure, it is necessary to carry out control analyses on samples of a polymer with a known content of the impurities being determined. A detailed discussion of GC methods for determining plasticizers has been published [73] Fig. 4.6 shows chromatograms for several plasticizers, and Tables 4.6 and 4.7 [73] list data on the chromatographic conditions and the retention values of plasticizers under isothermal conditions, respectively. In conclusion, it should be noted that the methods for analyzing volatile components in polymer systems have been developed thoroughly, and GC can be regarded as the principal method for analyzing solvents, monomers and plasticizers in polymers. The main task in this field, as we see it, is the selection of optimal methods and the development of standard packing procedures for GC analysis. The methods developed for analyzing solvents, plasticizers and other compounds is also of interest in determining volatile degradation products in polymers.
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80 D. Simpson, Brit. Plast., 41 (1968) 78. 81 T. R. Crompton, L. W. Myers and D. Blair, Brit. Plast., 38 (1965) 740. 82 T. R. Crompton and L. W. Myers, Eur. Polym. J., 4 (1968) 355. 83 P. Shapras and G . C. Claver, Anal. Chem., 36 (1964) 2282. 84 W. Craebel, Z. Lebensm- Unters.-Forsch., 130 (1966) 180. 85 P. Shapras and G. C. Claver, Anal. Chem., 34 (1962) 433. 86 M. P. Stevens and D. F. Percival, Anal. Chem., 36 (1964) 1023. 87 V. N. Paukov, L. B. Kotlyarsky and M. I. Band, Lakokras. Muter. Ikh. Primen., No. 4 (1973) 50. 88 A. A. Karpishin, Plast. Massy, No. 3 (1974) 78. 89 V. 1. Lushchik, V. R. Zlobina and T. A. Ermolayeva, Lakokras. Mater. Ikh. Primen., No. 6 (1973) 49. 90 B. B. Shiryhayev and E. B. Kozhukhov, Zavod. Lab., 38 (1972) 1303. 91 R. F. Heide, Brit. Plast.. 38 (1965) 366. 92 F. Zilio-Grandi, G. M. Sassu and P. Callegaro, Anal. Chem., 41 (1969) 1847. 93 G. C . Angemach and A. C. Moody, Anal. Chem., 39 (1967) 1005. 94 C. B. Roberts and J. D. Swank, Anal. Chem., 36 (1964) 271. 95 T. K. Choy, J. J. Quatrone, Jr. and N. J . Alicino, J. Chromatogr., 12 (1963) 171. 96 J. Brodsky, Kunststoffe, 51 (1961) 20. 97 F. M. Nelsen, F. T. Eggertsen and J. J. Holst, Anal. Chem., 33 (1961) 1150. 98 G. N. Fossich, J. Oil Colour Chem. Ass., 49 (1966) 477. 99 L. B. Wilkinson, C. W. Norman and J. P. Buettner, Anal. Chem., 36 (1964) 1759. 100 R. V. Panova, R. A. Belova and T. S . Lavrushina, Tsent. Nauch.-Issled. Inst. Inf: Tech.-Econ. Issled., l(1969) 21. 101 L. P. Gratsianskaya, L. N. Lishtvinova and M. I. Goryayeva, Izv. Akad. Nauk Kaz. SSR, Ser. Khirn Nauk, (1965) 86. 102 L. P. Blaushard, F. Singh and M. D. Baijal, Can. J. Chem., 44 (1966) 2676. 103 R. N. Mokeyeva and Ya. A. Tsarfin, Plast. Massy, No. 9 (1968) 60. 104 M. P. Stevens, Anal. Chem, 37 (1965) 167. 105 K. Jones, J. Gas Chromatogr., 5 (1967) 432. 106 M. S. Kleshcheva, V. A. Balandina, V. T. Usaclieva and L. B. Korolyova, Vysokomol. Soedin., Ser. A , 11 (1969) 2595. 107 G. C. Esposito and M. H. Swann,J. Gas Chromatogr., 3 (1965) 192. 108 13. D. Dinse and E. TuEek, Faserforsch. Textiltech., 21, No. 5 (1970) 205. 109 M. 1’. Stevens, Characterisation and Analysis of Polymers by Gas Chromatography, Marcel Dekker, New York, London, 1969. 110 M. W. Robertson and R. M. Rowbey, Brit. Plast., 33 (1960) 26. 111 A. D. Clarke and E. Barill, Brit. Plast., 31 (1958) 16. 112 G. S. Ongemach, V. A. Dorman-Smith and W. E. Beier, Anat. Chern., 38 (1966) 123. 113 E. Schroder, E. Hagen and M. Helmstedt, PIaste Kautsch., 14 (1967) 560. 114 W. M. Schwecke and J. H. Nelson, J. Agr. Food Chem., 12 (1964) 86. 115 M. Wandel and H. Tengler, Fette, Seifen, Anstrichm., 66 (1964) 815. 116 R. L. Rose, Plastics, 30 (1965) 65. 117 M. Gillio-Tosand A. Vimercati, Kunststoffe, 56 (1966) 409. 118 J. Ligotti, Mater. Plast. Elast., 39 (1973) 889. 119 I. K. Sharpanova, E. P. Taraday, E. L. Styskin and V. N. Provorov, Kauch. Rezina, No. 10 (1971) 57. 120 Ya. A. Gurvich, E. L. Styskin and A. A. Grinberg, Kauch. Rezina, No. 1 (1 971) 53. 121 Ya. A. Gurvich, S. G. Kumok and E. L. Styskin, Khim. Prom., (1971) 556. 1 2 2 E. L. Styskin, Kauch. Rezinu, No. 2 (1973) 49. 123 E. L. Styskin,Zavod. Lab., 39 (1973) 27. 124 Ya. A. Gurvich, 0. F. Starikova and E. L. Styskin, Khim. Prom., (1973) 9. 125 J . Haslam and A. R. Jeffs, Analyst (London), 83 (1966) 455. 126 W. S. Hoover, F & M Technical Paper, No. 24, published by F & M Scientific Division of Hewlett-Packard; cited in A. H. Latimer, SPEJ., 23, No. 7 (1967) 50. 127 0. Tweet and W. K. Miller, Anal. Chem., 35 (1963) 852. 128 J. H. Rau, G. Balback and H. Jzsse, Milliund Textilber., 45 (1964) 539.
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129 V. A. Zakupra, V. S. Dobrov, L. Ya. Grona and A. P. Lizogub, Khitn. Tekhnol. Topliv. Masel, No. 5 (1 970) 56. 130 A. A. Zliukhovitskii and N. M. Turkeltaub, Cazovaya Khromatografiya fGas Chromatography), Gostoptekhizdat, Moscow, 1962. 131 S. Dal Nogare and R. S. Juvet, Gas-Liquid Chromatography, Interscience, New York, 1962. 132 A. B. Littlewood, Gas Chromatography, Academic Press, New York, 1962. 1 3 3 11. P. Burchiield and E. E. Storrs, Biochemical Applications o f Gas Chromatography, Academic Press, New York, 1962. 134 R. Kaiser, Chromatographie in der Gas Phase, Bibliographisches Institut, Mannheim, 1969. 135 K. A. Gol'bert and M. S. Vigdergauz, Kurs Gazovoy Khromatografii (A Course in Gas Chromatography), Khimiya, Moscow, 1974. 136 J . S. Lewis, Compilation of Gas Chromatographic Data, ASTM Special Technical Publication No. 343, American Society for Testing and Materials, Philadelphia, Pa., 1963. 137 W. 0. McReynolds, Gas Chromatographic Retention Data, Preston Technical Abstracts, Evanston, Texas, 1966. 138 B. M. Luskina and G. N. Turkeltaub, Gazo-Zhidkostnaya Khromatografiya Vysokokipyashchikh Soyedinenii (Gas- Liquid Chromatography of High-boiling Compounds), NIITEKhim, Moscow, 1970. 139 A. I:. Slilyakhov, in A. A. Zhukhovitskii (Editor), Khromatografiya, VINITI, Moscow, 1974, p. 81.
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Chapter 5
Study of the kinetics and mechanisms of chemical transformations of polymers at elevated temperatures During the processing and use of polymeric materials, especially at elevated temperatures, various chemical processes occur that are accompanied by the release of low-molecularweight products and the formation of cross-linked structures. A change in the structure of the initial polymer leads in most instances to a deterioration in its properties. As the detailed theory of the chemical transformations of polymers under the influence of high temperature, oxygen, radiation, etc., is just being developed, detailed experimental studies into the kinetics and mechanisms of transformations in various systems and the experimental assessment of the stability of polymers and the role of various inhibiting additives are of great importance. No less important is the development of methods for the technical assessment of polymers under industrial conditions and the determination of harmful volatile products formed during the production and use of polymers, especially in enclosed premises. As a result of some particular properties of polymeric compounds, investigations into the kinetics of chemical transformations in polymer systems are more complicated than studies of transformations of low-molecular-weight compounds. In studying reactions with the participation of high-molecular-weight compounds, one should remember that they are affected by characteristics of polymer systems such as the average molecular weight, the molecular-weight distribution, the structure of the terminal groups, the presence of branching and the presence of ‘weak’ units in the molecule. The role of impurities is also important. Even before the start of an experimental investigation into chemical reactions of polymers, it is essential that the initial compounds be characterized as fully as possible, as different batches of the product and even different test samples may have different compositions. Thus, for instance, the composition of .he surface layer of a product being synthesized may differ widely from that of the internal layers, especially if the synthesis is carried out in the presence of oxygen or, for instance, the composition of the initial components changes during the synthesis. A changesin composition is also possible during the grinding of samples, which is often effected in order to prepare polymers for investigation. The size and geometry of the sample also play an important role in the degradation kinetics. These factors determine diffusion processes, which strongly affect the process kinetics, including weight losses and secondary reactions due to interaction of the decomposition products with each other or with the polymer. The low heat conductivity of polymer systems may give rise to temperature gradients in the polymer. All of these features of the kinetic behaviour of polymer systems must be taken into account when carrying out experimental investigations into the kinetics of chemical transformations in polymers. At elevated temperatures, reactions of polymeric compounds are often accompanied by the formation of volatile compounds or absorption of the gaseous active reagent (for instance, oxygen), and therefore methods based on recording the changes in the weight References p. 140
114 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES
of the polymer sample or the change in the pressure in a closed system have usually been applied for assessing these processes. These methods, however, yield only limited information on the reactions of polymers. Therefore, in recent years, together with the above methods, much progress in studying the decomposition of polymers has been made by developing methods that permit not only the total yield of volatile products, but also the kinetics of the isolation of the separate volatile components to be determined. I n the past two decades, gas chromatographic methods in particular have been widely used in determining stable volatile products formed during the transformations of polymers. Davidson et al. were the first to use GC for analyzing polymer decomposition products [ l ] . GC is a simple and effective method for studying the rate of release of separate volatile products (in relation to time and temperature), and yields extensive information for quantitative investigations into the kinetics and mechanisms of various degradation reactions. GC can be used for studying transformations not only of the polymer itself, but also of the polymer-inhibitor system. It is extremely convenient in industry for the assessment of the behaviour of a polymer during its decomposition and also for comparative investigations. GC can be used in studies of the degradation of polymers under static or dynamic conditions. Static conditions imply running the reaction in an enclosed volume, where the polymer and the decomposition products are in constant contact; under dynamic conditions, there is no constant contact between the polymer and degradation products. In the most common version of dynamic conditions, the polymer sample is located in a flow of gas that removes volatile products continuously from the reaction zone. In such investigations, a gas chromatograph may form part of the apparatus for studying the kinetics of chemical transformations of polymers and can be used independently; in the latter instance the test products have to be transported (mostly in a trap) from the kinetic chromatographic installation.
STATIC METHODS FOR STUDYING CHEMICAL CONVERSIONS OF POLYMERS In their studies of the pyrolysis of polymers, Feneberg and Weigel [2] used an ampouk-type reactor containing a test polymer sample of known weight. After heating the ampoule and completion of the pyrolysis of the polymer, the decomposition products were analyzed by GC. The static method has been widely applied in kinetic investigations. Tatarenko and Pudov [3,4] used the static ampoule method for degradation investigations, with subsequent chromatographic analysis of the products formed for the purpose of investigating the reactions of macro-radicals formed on decomposition of poly(propy1ene hydroperoxide), and for studying the mechanism of formation of low-molecular-weight hydrocarbons. The hydroperoxide was obtained by oxidation of the polymer at 130°C and oxygen pressure of 400 mtnHg. The hydroperoxide concentration varied from 0.39 lo4 to 4.0 lo4 mole per gram of polymer, depending on the oxidation time. A sample of oxidized polypropylene with a known content of hydroperoxide was heated in a previously evacuated enclosed system at 130°C for 80 min, up to complete decomposition of the hydroperoxide, after which the volatile products formed as a result of the decomposition were analyzed chromatographically.
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From the analytical data, the authors concluded that the decomposition of the hydroperoxide results in the rupture of tlie molecular chains of the polypropylene with subsequent depolymerization, and the rate of rupture of the polymer chain is close t o that of peroxide decomposition. Cotter et al. [5] used gas-liquid chromatography and mass spectrometry for studying the thermal degradation of poly(p1ienylene 1,3,4oxidiazole) and similar compounds. The pyrolysis of the substances was carried out in evacuated glass ampoules. The volatile products were evaporated from the ampoule directly into the mass spectrometer chamber or dissolved in acetone and then analyzed by gas-liquid chromatography. The analysis of the volatile products enabled well substantiated mechanism for the decomposition of the polymer to be proposed. In studying decomposition under static conditions, one can also use standard equipment for pyrolysis GC. Szkkely et al. [6] modified a pyrolytic micro-reactoi for carrying out pyrolysis under static conditions [ 7 , 8 ] . A filament-type pyrolytic block with an electric heater and therrtiostated external walls, made of stainless steel, was provided with two additional gas inlets so that the reactor could be fdled with any gas, or another gas could be added t o that already in the cell. This design permits the reaction t o be run in an aggiessive medium. The reactor could be completely isolated with thc aid of taps, and a by-pass line was used t o maintain the velocity of the carrier gas flow through the chromatographic column at a constant rate when the cell was isolated. When studying the thermal degradation of polydimethylsilylene [6] in an enclosed micro-reactor, the walls of the reactor must have a temperature of at least 250°C in order to prevent condensation of the decomposition products. In this instance, the flow of carrier gas carries all of the volatile components t o the chromatographic column on completion of the experiment. The experimental procedure was as follows. A micro-reactor containing a batch of polymer was heated t o 250"C, then the tap was opened and the decomposition products were fed t o the chromatographic coluiiin by means of the flow of carrier gas for analysis. The reactor was then isolated again and tlie same sample was heated at 350°C foi 1 min (the cup containing the sample was heated). The products formed were analyzed under the same conditions. The process was repeated at four different temperatures from 250 to 6OO0C, arid thus four pyrograms of products were obtained for each sample. The combined use of the results of pyrolysis carried out under both static and dynamic conditions and the application of a two-channel system for detection [9, 101 enabled the carbon t o silicon ratio for the pyrolysis products t o be determined and a polymer degradation niechanism t o be proposed [6]. Stuart and Smith [ l 11 developed a version of the chromatographic procedure for investigating the decomposition of epoxy resins. In carrying out decomposition, they used a glass pyrolytic block [ 121 with a heated filament, which was completely enclosed in a thermostat so as to prevent condensation of the products. The system included a fast-acting tap, which turned the flow of carrier gas on and off. To carry out the pyrolysis of the polymer under static conditions, the flow of carrier gas was shut off from the cell, the sample was pyrolyzed and then the carrier gas was passed through the cell and carried the volatile products t o the chromatographic column for separation and subsequent detection. Keenan and Smith [13] used this procedure for investigating the thermal degradation of a number of epoxides. The batches of the polymer samples References p. 140
116 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES
used in the investigation weighed only 2-4 mg. They had previously studied the dependence of the variation in filament temperature on time at intervals of 100°C for temperatures from 350 to 750°C. The pre-assigned temperature was established 5 sec after the heater was turned on. The total pyrolysis time at each temperature was 15 sec. Despite the simplicity of the procedure, the static method cannot be widely used for studying the decomposition of non-volatile compounds. Prolonged contact of the decomposition products with the polymer at an elevated temperature (as well as under the influence of other physical factors, such as radioactive radiation) may lead to additional (secondary) reactions of the volatile products with the polymer or with each other. This feature of the static method must be borne in mind when using it for studying polymer degradation processes. I n all instances when it has not been specifically established that the products formed are stable under the experimental conditions used (ie., that they do not react with the polymer or with each other), it is preferable to use the dynamic system, in which the time of contact of the decomposition products with the polymer is considerably reduced. Investigations are usually carried out in the velocity range where the results are independent of the velocity of the carrier gas.
DYNAMIC METHODS FOR STUDYING POLYMER CONVERSION PROCESSES The dynamic method is most widely used in kinetic investigations. Reduction in the time of contact of the volatile products with the polymer and their removal from the reaction zone can be achieved by continuous freezing out of the decomposition products, thermal degradation in a continuous-flow system with a carrier gas or the application of the pulse method of degradation. From the point of view of the procedure it is advisable to consider the following modifications of the methods: (1 ) periodic GC analysis of volatile products in studying polymer degradation; (2) automatic GC analysis of volatile products in studying polymer degradation; (3) investigation of polymer degradation with the use of GC detectors for continuous analysis of the volatile products released; and (4) impulse polymer pyrolysis with subsequent analysis of the degradation products. Periodic GC analysis of volatile products At present, the most common technique used in studying degradation processes is a modification of the dynamic method in which the polymer decomposition products are removed from the reaction (hot) zone and trapped in cold traps, which are periodically heated in order to desorb the decomposition products, which are subsequently subjected t o GC analysis. The application of this method has been discussed extensively by the literature describing GC investigations of polymer decomposition [14-251. As all of these methods are similar from the point of view of the procedure, we shall consider only some of them. Thus, the above method was used to measure the rates of formation of various volatile products of the decomposition of hydroperoxides [ 151 . The
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decomposition of hydroperoxides obtained by oxidation of polypropylene was carried out on a circular installation in a flow of carrier gas so that the volatile decomposition products were carried away from the reaction vessel by the flow of helium circulating in the system and frozen out in traps cooled with liquid nitrogen. The sample was injected into the chromatographic column with the aid of the device depicted in Fig. 5.la. When tap 1 is in the position indicated in the figure, the carrier gas enters the column, by-passing the U-shaped capillary trap. To effect periodic analysis, the mixture of products from trap 3 is transferred t o capillary 5, then tap 2 is turned t o position 2' and, after tap 1 has been switched t o position l', the products from capillary trap 5 are carried to the chromatographic column by the flow of carrier gas. Capillary 5 is treated with hot water. In the course of work, various inert carriers and stationary phases were tested. To reduce the analysis time, white sand (particle size ca. 0.1 mm) was successfully used as the carrier. With this inert support, various liquid stationary phases were used for the separation of the volatile products of polypropylene oxidation: triethylene glycol, silicone oil, diffusion oil and P,P'-oxydipropionitrile. The best results were obtained with 0,pl-oxydipropionitrile as the stationary phase, and it was used in all subsequent analyses. Fig. 5.2 shows a chroniatogram of the volatile decomposition products of peroxides in isotactic polypropylene. The analysis time was 24-25 min, the helium velocity being 30 ml/min. Gromov ef al. [ 161 proposed an installation for studying high-temperature uninhibited and inhibited oxidation of polymers which provides for periodic GC analysis of decomposition products after their freezing out in cold coil traps. Complete freezing out (retention) of decomposition products can be achieved by using the procedure usually applied for trapping short-lived radicals with subsequent electron paramagnetic resonance analysis. In this instance, a 'finger' cooled with liquid nitrogen is placed inside a silica reactor near the surface of the polymer being heated.
To
Fig. 5.1. System for (a) introducing samples of volatile decomposition products into the chromatograph and (b) carrying out pyrolysis and collecting samples of pyrolysis products for chromatographic analysis. (a), 1, 1' = Four-way tap; 2, 2' = three-way tap; 3 = trap with products; 4 = two-way tap; 5 = capillary. (b), 1' = Ampoule with sample of polymer pyrolyzed; 2' = oven; 3" = trap with liquid nitrogen; 4" = taps. References p. 140
118 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES
Time (rnin)
Fig. 5.2. Chromatogram of volatile decomposition products of peroxides in isotactic polypropylene. Separation conditions: column length, 2.5 m;stationary phase, 0.25%~(w/w)1p,p’-oxydipropionitrileon sand; temperature, 50°C; carrier gas (helium) velocity, 30 ml/min. Peaks: 1 = hydrogen, carbon monoxide and carbon dioxide; 2 = acetaldehyde; 4 = acetone; 6 = water; 3,s = not identified.
In the silica vessel, the decomposition products formed on heating of the polymer to the destruction temperature are quickly condensed on the cooled surface of the ‘finger’, thereby reducing secondary transformations and losses of degradation products. When degradation is completed, the silica gel can be connected to the chromatograph and, after replacing the liquid nitrogen with hot water, analysis of the unfrozen volatile products can be carried out. The silica vessel serves simultaneously as a reactor and a trap in the dynamic version. A number of workers have proposed various methods for the most convenient and efficient collection of samples. Krasnov et al. [17] , in analyzing the pyrolysis products of aromatic polyamides by gas-liquid chromatography, used for collection of decomposition products a trap (Fig. S.lb) in which the liquid products were collected in a narrow branch tube upon shaking. This technique permits the gaseous and liquid products to be analyzed separately. The gaseous products were collected for analysis from the trap with a syringe (through a vacuum tube at the end of the trap). The liquid products were collected with a microsyringe after the narrow branch tube had been cut off. Bondarenko [26] used a Topler pump for concentrating the gaseous products of polymer destruction. Davis and Golden [ 191 proposed installations for trapping (concentrating) polymer decomposition products (Fig. 5.3). Using these installations in combination with subsequent GC analysis, David and Golden [ 191 investigated the kinetics and mechanism of the thermal decomposition of a number of polycarbonates. The advantage of the above methods is their simplicity, but the fact that the process consists of two stages (destruction and chromatographic analysis are carried out independently) complicates the experimental study of the degradation process. Therefore, a logical sequence with these methods was the development of techniques and devices in which the decomposition and analysis were combined in a single scheme.
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119
305 crn
6
,--a
Fig. 5.3. Devices for conducting polymer pyrolysis and concentrating the volatile products obtained. (a), 1 = Salt bath; 2 = cooler; 3 = tube joint. (b), 1 = Salt bath; 2 = reaction vessel; 3 = electric heater; 4 = trap filled with mixture of acetone and dry-ice; 5 = Pirani gauge; 6 = rotary pump; 7 = gas tank; 8 = diffusion pump; 9 = ejector; 10 = Topler pump.
In studying degradation processes by conducting periodic GC analyses of volatile products, of great importance are techniques that provide for complete trapping of the decomposition products and methods for injecting these products into the GC column during subsequent analysis. For analysis to be carried out under optimal conditions, the broadening of the zones of the components during injection of the sample into the chromatograph (Le., the initial width of the zones) must be minimal. To achieve this, one can use several methods. In the first method a vapour sample is collected periodically from the flow of carrier gas containing the decomposition products and is introduced into the gas chromatograph for subsequent analysis. This method is simple, but its application is limited by the difficulties involved in collecting a sample that would contain not only light, but also high-boiling compounds, the complex design of the sampling tap, which operates a t high temperatures, and the low concentration of the products formed. Therefore, in practical work, use is generally made of a second method, in which the products References p. 140
120 CHEMICAL TRANSFORMATIONS O F POLYMERS AT ELEVATED TEMPERATURES
concentrated from the flow of carrier gas over a definite time period are analyzed gas chromatographically. T h s method greatly increases the volume of the sample being analyzed and simplifies the experimental procedure. To illustrate such a combination, we shall discuss an investigation into the mechanism of thermal degradation of polyesters [27]. This investigation involved a method combining slow pyrolysis with freezing out of the decomposition products and subsequent GC analysis. Sample degradation was carried out in an oven consisting of a silica tube which was placed in a stainless-steel tube connected at both ends to the gas network of the installation in such a way that the silica tube prevented contact of the steel tube with the atmosphere and with the reaction products. The stainless-steel tube was located in a thick insert in order to reduce temperature variations. The insert was heated with a metal resistor and the temperature was controlled by a proportional regulator. The sample was placed in a silica trough connected to a chromel-alumel thermocouple; it was heated to the pre-assigned temperature within about 45 sec. During a 5-h experiment at 300"C, the maximum temperature fluctuation was 0.12"C. Batches of about 10 mg were used, the layer thickness being 100 pm. The results obtained under these conditions are reproducible and the spread in the yield of the volatile decomposition products with different batches is less than 2%. The sample thickness is comparable with that used in Madorsky's work [28]. The flow of carrier gas carried away the pyrolysis products from the reactions zone immediately after their formation. Several traps for concentrating these products were installed at the reactor outlet and the volatile products were fed to the chromatograph from the trap at regular intervals for analysis. The heaviest degradation products were condensed on the walls of a stainless-steel coil placed at the outlet of the pyrolytic tube, which was heated to 150°C. These compounds were low-molecular-weight products of polymerization of some fragments formed on thermal destruction of the initial polymer. Preliminary trapping of these compounds precluded their degradation, which might have occurred on subsequent desorption of all products from the main trap at 300°C during injection into the chromatograph. This also prevented the harmful effect of waxes as stationary phase during the condensation at any cold point in the system or accumulation at the inlet of the chromatography column. From the pyrolytic tube, the carrier gas, together with the decomposition products, entered two tandenpconnected 0.6-ml copper traps. The first, empty trap was immersed in a Dewar flask at -75°C (dry-ice trichloroethylene served as the coolant), and was used for concentrating heavy products. The second trap was filled with molecular sieve 5A and immersed in liquid nitrogen. This trap was designed to accumulate light products. In the work of Ferre-Rins and Guiochon [27] the main products of degradation were cyclopentanone (which was condensed in the first trap) and carbon dioxide, ethylene, ethane and oxygen (in the second trap). In this scheme, the difference between the average heights of the peak obtained on direct injection into the chromatograph and that obtained on introduction from the trap did not exceed 1.676, this being within the usual reproducibility of such experiments. Hence the extraction of products from the traps during desorption was 100%.The system of traps helped to prevent losses of products during transfer to the chromatograph. To introduce the degradation products from the traps into the gas chromatograph, the traps were heated to 300°C at the rate of 15"C/min. Each trap was connected to 3
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separate chromatograph specially designed for the analysis of products condensed in the given trap. To analyze the products from the first trap, use was made of a chromatograph with programming of the column temperature and a flame-ionization detector. Separation was carried out on a column (700 X 0.4 cm) filled with 4% Carbowax 20M on Chroinosorb. The analysis of light gases from the second trap was conducted at room temperature on a chromatograph with a katharonieter. Separation was conducted on a column (200 X 0.4 cm) filled with silica gel. The heavy products that had concentrated on the coil at 150°C were collected and weighed. The method enabled qualitative and quantitative analyses of the decomposition products t o be carried out and a possible mechanism of thermal decomposition was suggested . Automatic analysis of volatile products The combination of a reactor for polymer degradation, traps for accumulating the volatile products formed and a chromatographic installation for their analysis increased the reliability and accuracy of measurement, but the overall method remained laborious as it required constant attention. Therefore, efforts were made t o develop an automatic device for the investigation of polymer degradation. An example of the automatic analysis of volatile decomposition products is the work of Alishoev et al. [29] , who developed a method for studying the stability of polymers based on condensation and periodic automatic quantitative analysis of the degradation products formed within a definite period, which can be measured over a wide range; the processes of degradation, concentration of the volatile products formed, their desorption, and analysis are combined in one device. The method permits the determination of the kinetics of formation of the separate degradation products, their sum, and also (by re-calculation) the kinetics of the variation in weight or piessure, depending on specific modifications of the device. The general method is as follows. The carrier gas, which is simultaneously the medium in which degradation occurs, passes t o the reference cell of the katharometer and then into the degradation zone. The volatile decomposition products formed are led, together with the carrier gas, to the column filled with an appropriate adsorbent. The decomposition products are adsorbed at room temperature, and then periodically desorbed by heating (for instance during a circular motion of the oven along the column, which is made as an open ring) and separated on the chromatographic column (Fig. 5.4a). The design of the device can be modified, depending on its purpose and the charscter of the products formed; for instance, it is often expedient t o use reactors placed both before the concentrator column and after it or after the separation column, thereby making it possible to analyze the products formed for their contents of carbon, hydrogen nitrogen, oxygen, etc. [ 3 0 ] .In the therrnooxidative version, oxygen is added to the carrier gas. The method can be used for studies of radiative and photo-degradation. The method has been checked by determining the thermal decomposition products of acetylated polyformaldehyde. Formaldehyde, formed on heat treatment of polyformaldehyde, polymeri;res very readily on the cold parts of the installations (at temperatures below 80°C),which usually makes it difficult t o study the stability of the References p. 140
122 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES
a
7
6
Fig. 5.4. (a) Device for GC determination of polymer decomposition products. 1 = Gas cylinder; 2 = reduction valve; 3 = fine control valve; 4 = detector; 5 = flow meter; 6 = concentration column (TDU-1); 7 = desiccator; 8, 9 = stove with electric water; 10 = quartz tube. (b) Chromatogram of products of thermal decomposition of polyformaldehyde.
polymer, Therefore, the modification with oxidation of carbon-containing products was used. For concentration and periodic automatic desorption of the carbon dioxide, a horizontal thermodynamic installation (TDU-1) developed by Zhukhovitskii and Turkeltaub [31] was used. The TDU-1 is a chromatographic column, made in the shape of a split ring, around which a U-shaped electric oven rotates continuously, moving in the direction of the carrier gas. The sorbent consists of 30% triethanolamine on INZ-600 brick (fractions 0.25-0.5 mm). The oven rotation cycle is one revolution per 4.7 min (the rotation speed can be changed). For the test, the silica boat (connected to an iron pivot) containing ca. 10 rng of polymer sample was placed in the room-temperature zone of the silica tube; the air was swept out of the system with helium and the boat was placed in the pyrolysis zone with a magnet. The decomposition temperature was 210 f 0.5"C. The time taken for the boat to be heated to the decomposition temperature was 1-2 min. The flow-rate of the carrier gas was 30 ml/min. The carbon-containing decomposition products formed were oxidized with copper(l1) oxide to carbon dioxide and water. Water was absorbed in a desiccator, and the carbon dioxide formed during one oven cycle was concentrated and periodically desorbed. The desorption temperature was 1O O O C . In the decomposition of polyformaldehyde, the chromatogram represented a series of decreasing peaks (Fig. 5.4b) and the envelope curve directly reflects the variation of the decomposition rate with time. The method permits a series of comparative determinations to be conducted and the decomposition products formed to be analyzed in the studies of the processes of polymer decomposition and stabilization. Although only one modification, with the determination of the sum of all of the carbon-containing decomposition products, has been implemented so far, it is possible, by using the appropriate sorbents in the concentrator, to analyze both separate light and heavy and all degradation products, and to study the kinetics of the release of each of the products by using as concentrators, for instance, traps with freezing out, which provide for the possibility of rapid heating.
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It is necessary only that the analysis time should not exceed the concentration time. In addition, as has already been mentioned, the method is convenient for the selective determination of the separate decomposition products or for improving the detector sensitivity (when ionization detectors are used). Judging by the sensitivity of the modification tested, the method permits the determination of 0.002 tng of carbon dioxide. If necessary, the sensitivity can be increased in various ways, for instance, by increasing the desorption rate or sample size or by using more sensitive detectors. Thus, the possibility is created for investigating the kinetics of degradation with very small degrees of conversion. The method is recommended for studying slow processes. The installation does not require any special equipment. Using the described method for studying the stability of various substances, Lee er al. [32] obtained comparative kinetic data on the thermal stability of poly(ethy1ene glycols) (PEGs) of different molecular weight and of polyesters based o n them. Fig. 5 . 5 depicts the kinetic curves of the release of gaseous carbon-containing decomposition products of different PEGs; the process was conducted in a flow of helium at 250°C. It can be seen that with an increase in molecular weight, the thermal stability of the polymers increases, owing to the effect of the number of terminal hydroxyl groups on the stability of the PEG. Lee era!. concluded that the decomposition of the polymer starts from the ends of the molecule. Replacement of the terminal hydroxyl groups by the residue of the maleic acid considerably reduces the thermal stability of the polyesters. A combined method for studying the thermooxidative and thermal degradation of polymers was proposed by Neniirovskaya et al. [ 3 3 ]. As has already been noted, the conventional methods for investigating polymer compounds at elevated temperatures under the influence of oxygen based on recording the change in the weight of the polymer sample or the cliange in pressure in a closed system are not adequate for characterizing the process, as they indicate the overall result due t o the simultaneous occurrence of the processes of absorption of oxygen (increase in weight, decrease in pressure) and release of volatile products (decrease in weight, increase in pressure). For a more detailed description of the thermooxidation process, it is advisable t o use GC, which permits the volatile products to be investigated and can be used effectively for studying the rate of their release with time at various temperatures.
_ _ _ _ _ _ _ - 1----0
20
40
Time ( r n i n )
rig. 5.5. Variation in weight of PEG (solid lines) and diesters based o n the corresponding PEG and maleic acid (broken lines) at 250 "C. 1 = PEG-7000; 2 = PEG-3000; 3 = PEG-1550; 4 = PEG-1000. p = degree of decomposition (5%). References p. 140
124 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES
The GC investigation of the thermooxidative degradation of polymers can, in principle, be conducted by one of the methods described in the preceding chapter. It is best to use a scheme that permits the rapid removal of the oxygen from the flow of carrier gas, as some data have indicated the possibility of oxidative disintegration of liquid stationary phases in a chromatographic column if the carrier gas contains oxygen as an impurity. The oxygen can be removed from the flow of carrier gas both when studying disintegration under static conditions and when using the dynamic method with freezing out (concentration) of the decomposition products in a trap. The disadvantage of these methods is that they involve two stages. The single-stage method for studying the thermooxidative stability of polymers consists in the periodic GC analysis of the volatile products formed upon thermooxidation of the polymer in the reactor (in which the gas medium is replaced periodically) attached to the sampling tap of the chromatograph in place of the sampling loop. This method permits processes to be studied at low degrees of conversion, as the sample size is not limited. In some instances, when the regime is specially selected, because of the small consumption of oxygen its concentration in the system varies insignificantly, which simplifies the kinetic interpretation of the results. The proposed chromatographic method can be regarded as a ‘quasi-flow-through’ method. Its application enables one to record simultaneously the consumption of oxygen and the formation of volatile disintegration products. The method is simple and suitable for investigating the behaviour of polymers in any gaseous medium. In thermooxidation, the set-up permits experiments to be carried out over a wide range of oxygen pressure, which is changed by varying the nitrogen to oxygen ratio in the system. The method has been verified for the therniooxidative degradation of poly(dodecy1amide) (PA-I 2) (molecular weight 20,000). The results obtained and the procedural details are given elsewhere [ 3 3 ] . Use of GC detectors for the continuous analysis of volatile products
Of considerable interest is a modification of the dynamic method with a constant flow of the carrier gas through the reactor, in wluch the polymer degradation products formed are continuously recorded by GC detector. This mcthod is perticularly convenient when studying the destruction of a polymer with a poor product spectrum and for group recording of products when using a selective detector. It is advisable to combine the analysis of volatile decomposition products in the flow of carrier gas by means of GC methods and equipment with differential thermal analysis. This combination was first proposed by Konenko et al. [34] for studying heterogeneous systems. In this method, which involves programmed heating or cooling of the object under investigation and the standard, the temperature of the sample and the standard, the difference between their temperatures (which is proportional to the thermal effects of the physical or chemical process under study) and the overall change in the concentration or content of the spent vapours after contact with the sample (for instance, with a katharometer or a flame-ionization detector) are recorded continuously and simultaneously in any controlled atmosphere in the gas flow; also, the detailed composition of the products is periodically studied by GC as the experiment proceeds.
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Eggertsen and Stross [ 3 5 ] suggested that the thermal stability of polymers can be determined during heating on a hot platinum coil similar t o that used in pyrolytic GC cells. The coil temperature is controlled with a thermocouple. The rate of release of the volatile products is determined with the aid of a flame-ionization detector, the signal being registered by a recording simultaneously with the coil temperature. Determination of degradatioii can be carried out both with a continuous increase in temperature and at a constant temperature. When needed, pyrolysis products can be identified with the aid of gas-liquid chromatography. Fig. 5.6 shows a thermogram obtained during the thermal degradation of polystyrene. When using this method for the comparative determination of polymer stability, one usually compares the temperatures (as determined from the thermogram) at which the sample decomposition rates are 0.1, 1 and 5%/inin. These decomposition rates correspond to recording of 0.1, 1 and 5%+ respectively, of the entire peak area per minute of the thermogram. The temperatures corresponding to these rates of release of volatile products are denoted by To.,, TI and Tj in Fig. 5.6. In the device described, a high-sensitivity detector was used, which permitted the products t o be recorded with a decomposition rate of 0.0 1 %/inin, thereby determining To,oi.The results obtained from this thermogram are presented in Table 5.1 t o illustrate the method for the calculation of decomposition rates and the determination of the corresponding temperature.
1'0
30
20
10
40
Time (min)
k3g. 5.6. l'hernial degradation of polystyrene (1) and sample temperature ( 2 ) as a function of time. Degradation conditions: sample, 0.2 mg; heating rate, 6"C/niin. Total peak area at maximum sensitivity, 554 cm2 ; chart strip speed, 0.5 cm/min.
T A B L E 5.1 CHARACTERISTIC TEMPERATURES FOR THERMAL DEGRADATION OF POLYSTYRENE Decomposition rate (%/min)
Peak area (in.'/min)
Distance from zero line (in.)
Characteristic temperature ("C)
0.11 1.1 10.9
3 1 3 (To.,,,) 335 ( T o . , ) 363 ( T , ) 347 ( T J
__-___
~
0.01 0.1 1 5
0.022 0.22 2.18 10.9 ~
References p. 140
54.5
~~
~.
~
126 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES
It should be noted that the method is especially convenient for investigating thermal stability under isothermal conditions, because the temperature equilibrium can be acheved very rapidly. In the decomposition of polystyrene (0.2-mg sample) at 365"C, the decomposition rate gradually increases and reaches a maximum in 8 min. The decomposition rate corresponding to this moment is 5% of the total area of the recorded peak per minute. These results correlate fairly well with those obtained when the sample was heated at a constant rate, a polystyrene decomposition rate of 5%/niin being reached at 370°C. The method has high sensitivity, particularly at low decomposition rates, it provides the possibility of rapid and controllable temperature variation, the equipment is simple in design and the necessary sample size is small (0.02-0.6 mg). Efficient temperature control is achieved by replacing the platinum coil with a differential calorimeter [36]. A slight change in the design of the calorimeter (1) (Fig. 5.7) makes it possible to supply decomposition products through the four-way tap ( 2 ) , either directly and continuously to the detector, or, through an intermediate GC column, for separation and subsequent recording by the detector (3). This system has certain advantages over thermogravimetric analysis, which is often used in investigations of the thermal degradation of polymers: (1) The high sensitivity of the detector enables one to use very small samples (the sample size decreases by several orders of magnitude); this reduces the temperature gradient in the sample and makes it possible to determine products formed in small amounts, (2) One can carry out differential thermal analysis simultaneously with the analysis of volatile products. (3) If necessary, one can identify the pyrolysis products formed at different stages of the process by GC. When using this method, one should bear in mind that many polymers form lowvolatility products, which may condense in the connecting tubes, thus causing systematic errors. It should be pointed out that one can use the chromatograph with a programmed temperature increase as a combination device in investigating the kinetics of polymer transformations at elevated temperatures. Thus, Franck applied this method to study the kinetics of the release of volatile products during programmed isothermal curing and degradation of poly(acry1ic acid) [37] and the kinetics of the release of water during the
Fig. 5.7. Schematic diagram of apparatus for studies of thermal stability of polymers. 1 = Sample (in differential calorimeter); 2 = tap that can be turned either for measuring the total amount of volatile products or for separating and identifying all volatile products at definite moments: 3 = flameionization detector; 4 = GC column.
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cross-linking of poly(viny1 alcohol) and of a mixture of poly(viny1 alcohol) with poly(acry1ic acid) [38]. In all instances the polymer sample was heated at a constant rate until the conversion process began, after which the temperature was kept constant until the process terminated; then the temperature was increased again until a new conversion process began, and so on. A chromatographic column in which a sample of the polymer under investigation was placed was used as the reactor. The reaction was carried out in a flow of helium. With the aid of a flow divider, a small part of the gas flow before the katharometer was directed into a flame-ionization detector, which recorded only organic components. The simultaneous use of two detectors made it possible t o estimate the ratio between the organic and inorganic products. During decomposition of poly(acry1ic acid) in an inert atmosphere, one can distinguish three successive stages with maximum rates of release of volatile products a t 160, 310 and 400°C. The utilization of selective detectors is an interesting line of investigation into degradation processes, especially when studying the decomposition products of heterochain polymers. Note that the sensitivity of the detector can be changed by using different reactors [30] with absorbents that selectively retain compounds of particular classes. The use of selective detectors in combination with other methods of analysis is a promising technique in studying polymer disintegration processes. The combination of thermal analysis, chromatography and mass spectrometry has been particularly widely applied [39]. A combination of some of these methods has been used successfully for investigation the disintegration of phosphorylated poly(viny1 alcohol) [40] , polyacrylic resin and nylon-6 [41] , hydrocarbon copolymers [42] , irradiated polyethylene [43], poly(acry1ic acid) and poly (methacrylic acid) [44] . Impulse pyrolysis of polymers Originally, pyrolysis GC [30] was proposed as a method for investigating the structure and analysis of high-molecular-weight compounds [ 12, 451 , but it is now also used as a method for studying degradation processes. In pyrolysis GC, the two processes (pyrolysis of the polymer and subsequent GC analysis of the products formed) are combined in a single device. Pyrolytic cells can be classified into two types: in the first type, pyrolysis occurs on a heated filament, coil, boat, etc. [ 1 2 , 4 6 ] , while in the second type the sample is introduced into an electric oven already heated t o a pre-assigned temperature [47]. Either of these two types of cell can be used for assessing the thermal stability of a polymer and determining the kinetic characteristics by measuring the yield of volatile degradation products at a given temperature and with a definite heating time. The procedure used t o deposit the sample on the filament [48], the sample size [49], etc., are of great importance for obtaining reproducible results in methods using rapid heating of the pyrolyzed sample. The use of a ribbon-type filament [50] improves the reproducibility of the method, provided that the sample is deposited over the working area from a solution. The temperature gradient on a ribbon filament was studied by observing (through a microscope) the melting of samples of a standard compound placed at different sites on the ribbon. The temperature change on the working site is less than 4OC at a pyrolysis temperature of 400°C. References p. 140
128 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES
In kinetic investigations, not only the position of the sample is important, but also the effect of the sample size on the observed decomposition rate. Thus, for instance, Knight [51] investigated the influence of the batch sample size on the thermal degradation of polystyrene in pulse pyrolysis at 555-700°C. The pyrolysis was carried out with samples of 1-3 mg at 700°C and 1-5 mg at 550-700°C. The styrene yield in the first instance (large samples) was 75 f 5%, while in the second instance it increased with temperature from 63 f 4 to 101 k 6%. This effect is attributed to the fact that secondary reactions take place in large samples and at lower temperatures, with the resulting formation of dimers, trimers and tetramers. Hence a change in the sample size of the polymer greatly affects the yield of products. When a polymer is soluble, it is deposited on a filament from the solution in order to obtain a uniform thin film. As trace amounts of the solvent may remain in the sample, one must ensure that the solvent has been completely removed or that its traces cannot affect the decomposition rate. To prove that this is so, it will suffice to show that (a) a decrease in the amount of solvent causes no change in the rate of degradation of the polymer, or (b) the use of different solvents does not affect the decomposition of the polymer. Using the equipment described by Barlow et al. [ 121 , it is possible to investigate the dependence of the decomposition rate on the film thickness. The results of an investigation into the degradation of poly(methy1 methacrylate) (PMMA) [52] with heating on a filament coil (Fig. 5.8) showed that the specific rate of decomposition (i.e., the amount of volatile products released per second per unit weight of the sample) depends strongly on the film thickness. This effect may be due to the following causes: (1) diffusion of the volatile substances into the gas phase is impeded; ( 2 ) a different time is required for heating the separate portions of the sample to the pre-assigned temperature; (3) a temperature gradient is formed in the test sample. The role of each of the above factors depends on the pyrolysis temperature. Thus, until it is shown that the rate of decomposition is independent of the sample size, the observed rates cannot be identified with the true rates of the chemical processes at given temperatures. The use of high-sensitivity chromatographic equipment permits very small samples to be used, the film thickness of which no longer affects the rate of decomposition.
, , -0
L ,-oy-
1
2 d.10‘
3
(crn)
Fig. 5.8. Dependence of the rate of decomposition of poly(methy1 methacrylate) on film thickness of the polymer. Temperature: 1 = 425°C; 2 = 375°C; 3 = 350°C. P = degree of decomposition within 10 sec; d = film thickness.
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Using capillary colunlns and argon detectors, it is possible to investigate the decomposition of samples weighing as little as 1Ow8gand to determine products released in amounts of lo-'' (ref. 50). When working with samples of thickness less than 200 A, the specific rate of degradation of PMMA [52] and polystyrene [48] is independent of the film thickness. It has been shown that the decomposition process is described by a first-order reaction equation only for samples of thickness less than 750 A. Note that for insoluble samples it is difficult to determine the effect of the sample size on the rate of decomposition because the grinding of the sample may cause premature decomposition. The velocity of the carrier gas is also an important factor. It was shown [50] that the degree of degradation of a polymer is independent of the velocity of the carrier gas if the latter is less than 0.5 ml/min. As the velocity increases from 2.6 to 4.8 ml/tnin, the degree of conversion of PMMA decreases by 33%in some instances. This effect is due to the influence on the filament temperature of the high-velocity flow, which cools the sample and filament. Therefore, the filament must be calibrated at a futed velocity of the carrier gas. A rational combination of a pyrolytic cell with a gas chromatograph for the analysis of volatile degradation products in a single device is possible, provided that the polymer pyrolysis time (and hence the width of the initial sample zone) is sufficiently small (not more than 15-20 sec). Therefore, one must provide sample heating conditions under which the pyrolysis temperature is attained very rapidly. When using a heating coil, these conditions cannot be achieved simply by switching the electric current on and off, as a certain time is needed for the filament and the sample to reach the pre-assigned temperature. The initial heating region can be substantially reduced by changing the intensity of the heating current during the experiment. Thus, if a high current is passed through the coil at the beginning of the process and then the current is reduced, the pre-assigned temperature can be reached within 0.5-1.0 sec. The optimal heating conditions are selected experimentally in each particular instance. The temperature reached by the filament under working conditions is determined with an optical pyrometer or a thermocouple, or by observing the melting temperatures of crystals of simple substances with known melting points. The use of a tubular oven as the pyrolyzer makes it possible to conduct the experiment under more standard temperature conditions. This procedure, however, has the following disadvantages; (1) the pyrolysis time is not controlled and the decomposition reaction proceeds until complete conversion of the sample is achieved; (2) the pyrolysis products are in contact with the walls of the reactor, which are at a high temperature, and this creates favourable conditions for secondary reactions. Note that the time of initial heating to the pre-assigned temperature does not necessarily have to be very small; it depends on the mass of the boat and the sample and on their geometry. As the heating rate has a very strong effect on the qualitative and quantitative composition of the products at elevated pyrolysis temperature, some workers [ 541 thought it advisable to use pyrolysis with introduction heating of a wire made of ferromagnetic material up to the Curie point for kinetic investigations i46]. The method is promising, because the desired temperature is reached within tens of milliseconds. It should be noted that, as in other methods (for instance, the use of a heated coil), the results depend substantially on the position of the test sample on the filament [54]. It follows from the results obtained [54] that reproducible results can be obtained only over a limited range of constant temperatures. References p. 140
130 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES
The method of induction heating up to the Curie point has been used in studying the thermal degradation of modified PMMA in the range 310-450°C [55]. Methods of pyrolysis GC have been applied successfully not only for both the qualitative and quantitative characterization of polymer degradation. An example of the application of pyrolysis GC to studies of the kinetics of the degradation of polymers is the determination of the kinetics of the thermal decomposition of PMMA [ 5 6 ] . Fig. 5.9 shows a first-order plot of the time dependence of the yield of volatile products for the decomposition of a fractionated sample of PMMA (average molecular weight 580,000) at 370°C. The dependence is linear up to 30% conversion, and the apparent rate constant (kapp.) can be determined in this portion. The point of intersection of the kinetic line with the abscissa at the value 2.0 f 0.3 sec is due to the heating time of the sample and filament. Therefore, the determination of the rate constant from the point of conversion within 10 sec may result in an understated value of the rate constant. Using their own procedure [ 5 0 ] ,Lehrle and co-workers studied in detail the degradation of polyacrylonitrile and polymethacrylonitrile and the kinetics of the decomposition of non-fractionated PMMA [57], and investigated the dependence of the rate of pyrolysis on the molecular weight for fractionated and non-fractionated polymer samples at 300-500°C [ 5 6 , 581. They noted [ 5 6 ] that modification of PMMA changes the pyrolysis mechanism. When pyrolysis was carried out on a ribbon filament 2 m long [58] it was possible to obtain sufficient residue for subsequent analysis by gel chromatography. Pyrolysis GC is an effective method for estimating the thermal stability of polymers and for the qualitative characterization of degradation [ 571 . Fig. 5.10 illustrates pyrograms of some polymers, obtained at different decomposition temperatures. Each short horizontal line represents a chromatogram of the decomposition products a t a definite temperature. The polymer sample under study was deposited on a filament coil and heated for 10 sec; first at 150°C. Then the sample was heated again, the temperature being increased by 100°C; during each subsequent heating, the residue after the preceding decomposition was used as the sample. In each experiment, the chromatogram of the decomposition products was recorded. A comparison of the pyrograms obtained makes it possible to estimate the relative thermal stabilities of the
Time ( s e c )
Fig. 5.9. Dependence of the yield of volatile products of the pyrolysis of poly(methy1 methacrylate) (fraction with average mo1ecu:ar weight 580,000; temperature 370°C) on time plotted as a first-order equation. The broken line indicates the error of the determination of the average rates from a single point within 10 sec.
PROCEDURES FOR STUDYING THERMOOXIDATIVE DEGRADATION
131
samples and changes in the composition of the degradation products. This method yields more extensive information and is faster than the widely used method for determining degradation from the weight loss. In some instances this method permits the qualitative issessment of the degradation mechanism. Thus, for instance, polymers 1 and 2 are h o s t completely destroyed at low and medium temperatures with the formation of monomers, evidently by a chain mechanism, while the decomposition products of polymer 3 contain, together with the main product (monomer), small amounts of other compounds (methanol, ethanol, methyl methacrylate), indicating that additional side reactions take place. The monomer is also the main product in the decomposition of polystyrene (4) and, at much higher temperatures, of polytetrafluoroethylene (5). Chromatograms of the volatile decomposition products of polymers 6 and 7 illustrate the different behaviour of poly(viny1 chloride) and poly(viny1 acetate), for which, at medium temperatures, the main products (hydrochloric and acetic acid, respectively) are the result of detachment of the side groups, and the polyacetate formed from poly(viny1 acetate) remains as a stable residue and decomposes only at much higher temperatures, mainly with the formation of acetylene. The behaviour of copolymers and mixtures during decomposition is shown in pyrograms 8-12, from which it follows, in particular, that the polymer obtained in the polymerization of methyl methacrylate with vinyl chloride has a much lower stability. Unfortunately, rapid pyrolysis methods are unsuitable for studying the thermal degradation of most of the polymers at low temperatures.
SPECIFIC FEATURES OF PROCEDURES FOR STUDYING THERMOOXIDATNE DEGRADATION The thermooxidative degradation of polymers can, in principle, be studied by one of the above-described methods. It is better to work with the scheme that permits rapid removal of oxygen from the flow of carrier gas, because there are indications [50] that liquid stationary phases used in the chroinatographic column can be subjected to oxidative degradation if oxygen is present as an impurity in the carrier gas [59, 601. Removal of oxygen from the flow of carrier gas can be achieved either by studying decomposition under static conditions or by using the dynamic method involving freezing out (concentration) of the decomposition products in the trap [61]. An ingenious chromatographic method for characterizing the oxidative degradation of polymers has been suggested by Scholz et a/. [62] . The test polymer was deposited on an inert solid support and placed in a short pre-column situated between the sample introduction system and the chromatographic separation column. The pre-column containing the test polymer can be thermostated at 100-600°C. The polymer was oxidized in contact with oxygen, which was introduced pulse-wise into the flow of inert carrier gas. The volatile oxidation products formed were then fed into the chromatographic column for separation. The range of volatile products of oxidative degradation is characteristic of the polymer concerned. Fig. 5.1 1 shows chromatograms obtained for Carbowax 20M and Ucon Polar. The pre-column was therniostated at 203 and 202.5"C, respectively. References p. 140
1
Fig. 5.1 0. Chromatograms of products of polymer degradation at different temperatures. A polymer sample was deposited on a spiral filament at 150°C for 10 sec. The operation was repeated, the temperature being increased by 100°C. in each instance the residue after the preceding heating was used as the sample. Polymers: 1 = poly(methy1 methacrylate); 2 = poly(ethyl methacrylate); 3 = poly(methy1 acrylate); 4 = polystyrene; 5 = polytetrafluoroethylene; 6 = poly(viny1 chloride); 7 = poly(viny1 acetate); 8 = methyl methacrylate-styrene copolymer (1: 1); 9 = methyl methacrylate-styrene mixture (1: 1); 10 = methyl methacrylate-vinyl chloride bulk copolymer (1 :4);11 = methyl methacrylate-vinyl chloride static copolymer (3:7); 12 = methyl methacrylate-vinyl chloride mixture (1 :4).
PROCEDURES FOR STUDYING THERMOOXIDATIVE DEGRADATION
133
The velocity of the carrier gas can vary over a wide range. At very low velocities, however, asymmetric peaks are obtained, whereas excessively high velocities cause a sharp reduction in the oxygen hold-up time in the pre-column. The temperature of the chroniatographic column depends on the liquid stationary phase used, with an allowance for its oxidation. After the oxygen has been removed from the system, it is advisable to use programmed heating. From Fig. 5.12, it follows that the method can supply qualitative characteristics of the thermooxidative stability of polymers. In order to obtain reproducible results, it is necessary to use the same sorbent for separating the oxidation products. In this instance, when the oxidation products are identified, or for any sample under study, the ‘weak unit’ in the polymer chain can be investigated. As a result, it is possible to regulate the number of ‘weak bonds’ subject to oxidation in the given polymer. The method can also be used for comparative estimation of polymer stability to oxidation at elevated temperatures. Fig. 5.1 2 depicts the chromatograms of oxidation products for a poly(pheny1 ether) containing six rings, Convalex 10 and 0s-124. The last two polymers contain five rings and differ in the ratio of the isomers. All of the pre-columns were filled with 15% poly(pheny1 ether) on Chromosorb P (35-80 mesh). It follows from the results that the six-ring poly(pheny1 ether) forms the least number of oxidation products, while 0s-124 [five-ring poly(pheny1 ether)] forms many more, even at lower temperatures (250°C). Thus, the results show that the six-ring ether is more stable to oxidation than the five-ring ether (0s-124). The method can be used for preliminary estimation of the efficiency of stabiliz,ers. With the aid of the method described, it is also possible to study other reactions of polymers, such as hydrolysis. During the GC analysis of products of thermooxidative degradation, one often has to determine small amounts of a component in the presence of considerable amounts of oxygen. For instance, during the thermooxidation of films of impact strengthened polystyrene [63] of thickness 30 pni 0.07-g sample with a comparatively large volume of the system (200 ml) 0.03-2.00/0 concentrations of carbon monoxide and dioxide in oxygen are formed. The quantitative analysis of such a mixture involves certain difficulties. The direct GC analysis of a mixture of oxygen, carbon monoxide and carbon dioxide on a column (100 X 0.4 cm) filled with carbon SKT at 60°C results in complete separation of oxygen and carbon monoxide only when a small sample (0.05 ml) is introduced. The analysis of large samples (8-10 ml) is impossible because the oxygen peak is superimposed upon the small peak of carbon monoxide. Improven~entin the separation of these two components by increasing the column length or decreasing the temperature resulted in considerable broadening of the carbon dioxide peak, thereby decreasing the sensitivity of the method. The use of a sorbent more selective for oxygen and carbon monoxide (molecular sieves) leads to sorption of carbon dioxide which is virtually irreversible under the given conditions. Therefore, a method was developed for analyzing small concentrations of carbon monoxide and dioxide in oxygen with their separation from oxygen on an adsorption column at -70°C and subsequent desorption at 200-250°C [63]. The concentration column (30 X 0.3 cm) and the chromatographic column (100 X 0.4 cm) are fdled with carbon SKT (fraction 0.25-0.5 mm) previously dried at 200°C. The separation of the components on the column was carried out at 1 lO‘C, which speeded up the analysis cycle and ensured that clear symmetrical carbon dioxide peaks References p. 140
134 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES
U
(b)
, !JI
+
Tlme
Fig. 5.11. Chromatogram of oxidation products of polymers: Carbowax 20M and Ucon Polar. (a), 15% Carbowax 20M on Chromosorb P; (b), 15% Ucon Polar on Chromosorb P.
-L
,
(b)
c
0
5z c
1 1
/
6
1
1
12
I
0
l
l
6
1
1
12
1
,
6
1
1
1
12
Time (min)
Fig. 5.12. Chromatograms of oxidation products of poly(pheny1 ethers). (a), Six-ring poly(pheny1 ether); (b), Convalex 10; (c), 0s-124.
were obtained. The flow-rate of the carrier gas (helium) was 50 ml/min and the analysis time was 12-1 5 min. In order to determine the completeness of desorption of the components, the areas of the peaks were compared on introduction of the carbon dioxide sample into the concentrator and on injection of an identical sample directly into the chromatographic column. The difference in areas was 5-7%, which indicated the absence of any losses of the components sorbed. The quantitative calculation of the components was carried out by the absolute calibration method. GC is also used as a method for investigating other types of degradation of polymer systems (for instance photooxidative [64] and mechanical [65] ).
PROCEDURES FOR STUDYING THERMOOXIDATIVE DEGRADATION
135
TABLE 5.2 APPLICATION O F GC IN STUDIES OF POLYMER DEGRADATION PE = Polyethylene; PMMA = poly(methy1 methacrylate); PP = polypropylene; PS = polystyrene. ~ _ _ ~
Polymer investigated
Description of investigation
Reference
Phenol condensation polymers (epoxides)
Investigation of thermal stability and thermal degradation mechanism
66
Polycarbonates
Study of kinetics of formation of pyrolysis products
61,68
Study of kinetics of thermooxidative degradation; analysis of decomposition products
69
Poly(2,6-dimethylphenylenone),
Investigation of thermal degradation; poly( 2-methyl-6-isopropylphenyleneoxide) determination of the composition of volatile products
I0
PS (mol. wt. 180,000)
Study of kinetics of thermal degradation; analysis of decomposition products
71
PE, PP atactic and isotactic, copolymers of PI< and PP
Investigations of mechanism of thermooxidative degradation; analysis of oxidation products
14
Poly formaldeh yde
16 Study of mechanism of thermooxidative degradation; analysis of oxidation products
Polycaprolactam
Investigation of mechanism of thermooxidative degradation; analysis of decomposition products
72
PMMA
Investigation of kinetics and mechanism of transformations in thermal degradation
56
Aromatic polyamides based on iso- and terephthalic acids and m, p - and o-phenylenedianiinc
Study of radiation and radiation-oxidative stability; analysis of radiolysis products
13
Poly(ethy1ene terephthalate)
Study of thermal and therniooxidative degradation; analysis of decomposition products
2s
PMMA and its copolymers
Study of thermal degradation; analysis of decomposition products
14
Epoxy resins
Study of mechanism of initiation of thermal degradation; analysis of decomposition products
15
PMMA. modified
Investigation of kinetics and mechanism of thermal degradation
55
Phenol- formaldehyde resin (type t:FI(-30)
Study of kinetics of thermal degradation; analysis of decomposition products
18
Poly(viny1 chloride)
76 Study of mechanism of thermal degradation and toxicity of decomposition products; analysis of volatile products (Continued on p. 136)
References p. 140
136 CHEMICAL TRANSFORMATIONS O F POLYMERS AT ELEVATED TEMPERATURES
TABLE 5.2 (continued) Polymer investigated
Description of investigation
Reference
Epoxy polymers: poly[ di(Zg1ycideoxynaphthyl-1-methy1)succinaniidel, poly [ di(2-glycideoxy-I-methyl)sebacinamide ]
Investigation of mechanism of thermal degradation; analysis of decomposition products
77
Polysulp hone
Study of thermal stability; analysis of decomposition products
78
Styrene- sulphone copolymer
Study of mechanisni of thermal degradation; analysis of decomposition products
79
PS
Study of mechanism of radiolysis; analysis 80 of radiolysis products
Phenol- fornialdeliyde resins and poly(pheny1ene ester) polymers at different stages of curing
81 Study of mechanism of thermooxidative degradation; analysis of oxidation products
PS and its mixturcs with poly-or-methylstyrene
Study of thermal degradation; analysis of volatile decomposition products
82
Partly hydrated terphenyls and quarterphenyls
Study of radiation stability; analysis of radiolysis products
83
Poly(oxymethy1ene diacetate)
Investigation of reactions proceeding in radiolysis products
84
Kesotropin complex; product of interaction of hexamethylenetetramine with resorcinol
Study of mechanism of modifying effect of resotropin in thermal degradation
85
Poly(styrene sulphone)
Study of thermal degradation; analysis of decomposition products
86
PP
Study of thermal degradation; analysis of volatile decomposition products
87
Copolymers of unsaturated polyesters with styrene
Study of effect of network structure of polymer on its thermal degradation; analysis of decomposition products
88
w,w'-liexametliyldimethylsiloxane
Investigation of thermal degradation products
89
Poly(viny1 chloride)
Study of kinetics of thermal decomposition; analysis of decomposition products
90
Polycarbonate
Study of intermediate oxidation products
24
Polyarylates
Study of decomposition mechanism; analysis of volatile products
22
Poly-p-xylylene
Study of Ineclianism of thermal degradation; analysis of decomposition products
91
PROCEDURES FOR STUDYING THERMOOXIDATIVE DEGRADATION
137
TABLE 5 . 2 (continued) -
Polymer investigated
Description of investigation
Reference
Poly(propy1ene hydroperoxide)
Investigation of process of formation and diffusion of low-nlolecular-weight radicals into gas phase o n decomposition; product analysis
3
PE
Study of mechanism of thermal degradation; analysis of decomposition products
92
Polyacrylonitrile
Study of thermal degradation; analysis of decomposition products
21
Polyfpropylene hydroperoxide)
Investigation of thermal degradation; analysis of low-molecular-weight decomposition products
Poly diinet hylsiloxane
Study of mechanism of thermal degradation; analysis of decomposition products
20
Polyoxyethylene and polyacetaldehyde
Investigation of mechanism of thermal degradation; analysis of decomposition products
93
Unsaturated polyesters
Investigation of thermooxidative degradation; analysis of oxidation products
94
Polyoxymethylene
Study of mechanism of thermal degradation; analysis of decomposition products
95
Poly(oxyethy1ene glycol)
Investigation of light radiolysis products
96
Poly(viny1idene fluoride) and copolymer of vinylidene fluoride with hexafluoropropylene
Investigation of thermal degradation; 97 ‘analysis of gaseous decomposition products
Poly(oxypropy1ene glycols)
Study of mechanism of thermooxidative degradation; analysis of decomposition products
98
Siloxane polymers with silylphenylcne groups
Investigation of pyrolysis mechanism; analysis of decomposition products
99
Study of thermal degradation by pyrolysis
44
PS
4
Gc PE, PP, polybutene, poly-4-methylpentene, bulk and static copolymers of PE and PP
Investigation of pyrolysis mechanism
100
Polyester resins
Study of thermal degradation
101
Poly-tert. -butyl(acrykdte)
Investigation of kinetics of formation of photolysis products
102
High-pressure poly-3,3,3-trifluoropropene Study of kinetics of thermolysis from data 1 0 3 obtained; analysis of decomposition products
(Continued on p. 138)
138 CHEMICAL TRANSFORMATIONS OF POLYMERS AT ELEVATED TEMPERATURES TABLE 5.2 (continued) Polymer investigated
Description of investigation
Reference
Polyiodophenylacetylene
Study of character of processes occurring in thermolysis; analysis of volatile decomposition products
104
Poly-e-caprylamide
Investigation of mechanism of radiolysis and radiation oxidation; analysis of radiolysis and oxidation products
105
Polycarbona tes
Investigation of changes in polymer structure during pyrolysis; analysis of gaseous pyrolysis products
106
Poly(m-phenybiieisoplithalainide), poly@-pheny leneisophthalamidc), poly(m-phen y lenetetraphtliamide)
Study of composition of volatile and liquid products of thermal degradation
17
Polyoxydiazole and its model compounds
Investigation of mechanism of thermal degradation; analysis of decomposition products
5
Polysulphides
Study of process of photolytic degradation; 1 0 7 analysis of volatile products
&,&-Dimethyl-0-propiolactone
Study of kinetics of formation of volatile decomposition products
23
Polymethylene
Study of products of thermal degradation
108
Shock-resistant PS
Analysis of small concentrations of decomposition products
PP
Study of kinetics of accumulation of monomer in polymer pyrolysis
Polydimethylsilylene
Study of mechanism of thermal decomposition
Polydimet hylsilylenc
Study of composition of thermal degradation products
Poly(acry1ic acid)
Study of kinetics of programmed isothermal curing and degradat'ion of polymer
Silicone polymers
lnvestigation of thermal processes 111 (curing, decomposition, depolynierization) by pyrolysis GC
PE
lnvestigation of products of thermal 112 degradation and decomposition mechanism
PS
Study of products of thermal degradation and decomposition mechanism
113
Copolymer of acrylonitrile and styrene
Study of products of thermal degradation and decomposition mechanism
114
Copolymer of styrene and acrylonitrik
Study of products of thermal degradation and decomposition mechanism
115
63 109 6
110
37
PROCEDURES FOR STUDYING THERMOOXIDATIVE DEGRADATION
139
TABLE 5 . 2 (contiriued) Poly iner investigated
Description of investigation
Epoxy resins
Investigation of thermal stability and mechanism of thermal degradation
PS, PMMA, poly(viny1 c l ~ o r i d e ) , poly( viiiy 1 acetate)
Study of mcchanisin of thermal degradation
116
PP
Study of effect of oxidation products o n polymer pliotostability; product analysis
117
Polydinietliylsiloxaiies
Investigation of pyrolysis mechanism; analysis of decomposition products
118
Phenolfornialdehyde resins
Study of products of thermal and themrooxidative destruction
1 I9
Copolymer of vinyl acetate with other esters
Study of decomposition niechanisin
120
Poly-in-aminost y rene
Analysis of decomposition products for thermostability studies
121
Cellulose
Study of effects of fire-proofing treatment on range of dcconiposition products
122
PMMA
Utilization of pyrolysis gas chroniatopraphy 50 data for determination of reaction niechanisni
Polyacrylonitrile
Quantitative study of degradation (inechanisni and kinetics)
51
PMMA (fractionated)
Study of thermal degradation mechanism
58
Copolymers of ethylene and propylene
Study of pyrolysis niechanisin
123
Poly-ecaprolact am
Determination of radiation- chemical yield of radiolysis and radiative oxidation products
124
PP, poIy(propy1ene hydroperoxide)
Investigation ot' kinetics of accumulation of products of oxidative decomposition in presence of antioxidant and pure initial polynicr
15
Polyesters: poly(ethy1ene glycol adipate), poly( 1,4-butanediol adipa t c ) , poly( 1,8-octanediol adipatc)
Investigations into degradation kinetics and mechanism
27
Polyoxyethylene, PP, I T
Investigation of kinetics of formation of products in pyrolysis
125
PS, polyisobutylcne, polybutadiene, PE,
Determination of rate of release of volatile decomposition products
35
Study of therniooxidative stability; analysis of oxidation products
62
PP, copolymer of acrylonitrile, butadiene and styrene W O N , Carbowax 20M, PS, poly(pheny1 ethers)
Reference 11
(Continued 011 p.140) References p. 140
140 CHEMICAL TRANSFORMATIONS O F POLYMERS AT ELEVATED TEMPERATURES
TABLE 5.2 (continued) Polymer investigated
Description of investigation
Reference
Poly(etliy1 acrylate)
Investigation of oxidative degradation; analysis of decomposition products
126
Poly(viny1 alcohol) and its mixture with poly(acry1ic acid)
Investigation of kinetics of reaction of programmed isothermal curing of polymer
83
Polycarbonates
Study of mechanism of thermal decomposition; analysis of decomposition products
19
APPLICATION OF GC IN STUDYING THE DEGRADATION OF POLYMERS The field of application of GC in the study of chemical transformations of polymers under the influence of various physical and chemical factors is extremely wide, because most of the known polymers form volatile products during decomposition. GC has been used successfully for studying the kinetics and mechanisms of degradation processes, for determining the stability of polymers, for estimating the efficiency of various inhibitors, etc. Table 5.2 summarizes investigations in which GC has been used for studying degradation processes, and demonstrates the great opportunities for using GC in this field of polymer chemistry. It should be noted, however, that in practice the advantages of GC are unfortunately far from completely utilized.
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Chapter 6
Reaction gas chromatography of polymers In recent years, some work has shown that it is possible, in principle, t o analyze polymers by gas chromatography (GC) with the use of carrier gases at high pressures [ l , 21. The prospects for the wide analytical utilization of this method are still uncertain, however, and it is possible that the use of the rapidly developing technique of liquid chromatograpliy for the analysis of polymer systems will remain the simplest and most efficient method of investigation of high-molecular-weight compounds [3-51 . In recent years, the methods of functional group analysis have undergone great changes because of the wide application of GC [6-81. The combined use of chemical and chromatographic methods has opened up a number of new possibilities; among these, of special significance are the following points: (1) increasing the resolution of the methods of functional group analysis, transforming then1 from methods of group identification into methods of identification of individual compounds, permitting the solution of problems that were previously insoluble; (2) increasing the reliability of the method, in particular the possibility of isolating the end product of the reaction froin the side products; (3) reducing the analysis time; and (4) decreasing the amount of substance necessary for carrying out the functional group analysis. The methods of functional group analysis of non-polymeric organic compounds have been thoroughly developed, and the existing methods have been surveyed in some excellent books [6, 9, 101. The application of these methods t o polymers, however, is possible only after preliminary evaluation, because the solubility of polymers is usually lower than that of monomeric coinpounds, which affects the kinetics of their reactions. If a polymer is insoluble, the existing methods can be applied only for determining the surface functional groups. Also, the reactivity of the functional groups in polymers differs froin that in non-polymeric compounds. As polymers are non-volatile under the usual conditions used in GC, this technique is used for analyzing the volatile products of the degradation of high-molecular-weight compounds, rather than the compounds themselves. In functional group analysis, GC is nua ally applied in combination with chemical methods in which either reactions occur with the formation of volatile products, or one of the initial reagents is a volatile compound. Accordingly, one can distinguish several different methods in the functional group analysis of polymers by cheiiiical reactions combined with GC. Firstly, we should mention reaction methods that lead to the formation of volatile reaction products. Thus, the determination of alkoxyl groups [l 1 1 by boiling the test compound in hydriodic acid using the Zeisel and Fanto method [12] leads t o the formation of the corresponding volatile alkyl iodides, whose chromatographic separation has been described [ 131. In polysiloxanes, the determination of alkoxyl groups can also be achieved by the GC analysis of the alcohols formed on alkaline fusion of the polymer [ 141 . Secondly, for analytical purposes one can use reactions of addition of a volatile reagent t o the polymer with GC determination of the content of the residue of this References p. 156
146
REACTION CC OF POLYMERS
reagent in the reaction mixture upon completion of the reaction. Thus, for instance, this principle served as the basis for determining the content of primary and secondary amino groups in polyethyleneimino by using the reaction of cyanoethylation [ 151 . Thirdly, the content of the separate groups in a polymer can be determined after chemical cleavage of the test polymer into separate volatile compounds. For instance, in carrying out an analysis of a copolymer of ethylene oxide and propylene 1,2-oxide [16] , the copolymer was deconiposed with a 45% solution of hydrogen bromide in glacial acetic acid for 3 h at 150°C. The reaction products contained a mixture of 1,2-dibromoethane and 1,2-dibroniopropane, which were extracted with carbon disulphide and analyzed by GC. In many instances, the realization of methods such as the above may require a series of successive chemical transformations. According to Hellman and Wall [ 171 , chemical methods are often more accurate than physical methods for the investigation of polymers. Certain limitations of chemical methods should also be mentioned [ 161 . With an increase in the molecular weight of the test polymer, the accuracy of determination of the terminal functional groups decreases. Trace impurities may result in qualitative and quantitative errors. Therefore, chemical methods of analysis of terminal groups are mainly applicable to polymers with a molecular weight from 20,000 to 30,000. Also, the reactivity of the functional groups in the polymer changes as a result of the reaction, with a change in the nature of neighbouring groups. Note that reactions in polymers often do not proceed to completion. Fourthly, the determination of the functional g r o u p present in a polymer in small amounts has a number of specific features [ 171 . The general scheme for the investigation of polymeric compounds by reaction GC can be represented as follows. The non-volatile compound to be analyzed, subjected to the action of chemical reagents (acid, alkali, oxygen, etc.) and also physical effects (high temperature, various types of radiation), yields volatile products the nature and amount of which are closely related to the structure and composition of the system being analyzed. Therefore, GC data on the composition of the volatile products of polytner degradation make it possible to characterize more or less fully the composition and structure of the initial polymer. This chapter considers chemical reaction-functional group GC based on specific reactions of chemical reagents with the polymer (which usually proceed with definite functional groups of the polymer), and GC of the volatile products of radiative and photochemical degradation. Pyrolysis GC, which is based on the thermal decomposition of the polymer, is of interest in its own right and is therefore covered in a separate chapter (see Chapter 7). As the determination of functional groups is simplified when the reaction product is a gas we gve opposite a table tor the gasometric determination of organic functional groups (Table 6.1) [9]. The work of Franc and Mike3 [18] provides a brilliant example of the use of splitting reactions, resulting in the formation of gaseous products, with the aim of identifying various nitrogen-containing functional groups. Let us consider the application of reaction GC to the analysis of polyesters. The methods for analyzing linear and branched polyesters and alkyd resins include complete decomposition of the polymer into the initial components by saponification, aminolysis or alcoholysis, conversion of the polar monomeric compounds obtained into derivatives,
REACTION GC OF POLYMERS
147
TABLE 6.1 GASOMbTRIC DETERMINATION OF ORGANIC rUNCTIONAL GRO'LJPS ~
Functional group
Principle of determination
Compound analyzed
Azo Alkene
Iieating with phenylhydrazine Catalytic hydrogenation and determination of absorbed hydrogen Reduction Formation of alkyl iodide, oxidation and reaction of iodic acid with hydrazine Reaction with Grignard reagent Nitrosation Oxidation
Nitrogen Hydrogen
Alkyl nitrate Alkoxyl Aniide and imide Ainine Hydrazine and hydrazide Hydroxyl Diazo Carboxyl Carbonyl
N-Nitro N-Nitroso Semicarbazide Ester
Sulpharnide Quinone Phenol
Reaction with lithium aluminium hydride Heating in the presence of catalyst Decarboxylation with Grignard reagent Reaction with Grignard reagent and decomposition of excess of reagent Reduction with sodium borohydride and determination of excess of reagent Reaction with sulphuric acid and mercury Reduction Reaction with sulphuric acid and mercury Oxidation Reaction with Grignard reagent and decomposition of excess of reagent Reduction with lithium aluminium hydride and decomposition of excess of reagent Oxidation with nitric acid Reaction with phenylhydrazine Reaction with lithium aluminium hydride Reaction with Grignard reagent
Nitrogen oxides Nitrogen Methane Nitrogen Nitrogen Hydrogen Nitrogen Carbon monoxide Methane Methane Hydrogen Nitrogen oxides Nitrogen Nitric oxide Nitrogen Methane Hydrogen Nitrogen dioxide Nitrogen Hydrogen Methane
which are more suitable for GC analysis, and GC analysis proper of the products obtained. Thus, in practice, carboxylic acids are analyzed in the form of the corresponding esters and polyols in the form of the corresponding acetates. As an example, we shall consider the analysis of alkyd resins based on phthalic anhydride and various polyols [ 191 . The resin sample was placed in a flask (1 25 ml), 6 nil of butylaniine was added and the mixture was boiled with an inverted cooler for 1 h. On cooling, 25 nil of acetic anhydride were added and the mixture was boiled for 90 min. Upon cooling, 35 ml of water were added t o the reaction mixture containing acetates of polyhydroxy alcohols and the mixture was boiled for 5-10 min, then cooled, transferred into a separating funnel and extracted with chloroform. The combined extract was washed with water until the acidic reaction disappeared, then filtered through filter-paper moistened with chloroform and collected into a flask (250 nil). Most of the solvent was evaporated in a jet of air over a water-bath, the remainder (5-10 ml) being used for chromatographic analysis. The polyol acetates obtained were determined by References p. 156
REACTION GC OF POLYMERS
148
gas chromatography with a programmed temperature increase from 50 to 225°C at 7.9'C/min on a column (122 X 0.6 cm) filled with 10% Carbowax 20M on Chromosorb W (60-80 mesh). Fig. 6 . l a shows a chromatogram of a mixture of all of the polyols studied on Carbowax 20M. The reaction products of excess of butylamine and acetic anhydride emerge from the column between 14 and 16 min in the form of two small separated peaks. Fig. 6.lb shows a chromatogram of the conversion products of one of the samples of an alkyd resin. Each of the alkyd resins analyzed, based on phthalic anhydride, forms a derivative of o-phthalic acid as reaction product. This product, however, does not interfere in the determination of alcohols. Other isomers of phthalic acid were not separated under these conditions. The method enables one to determine alcohols contained in the mixture at concentrations as low as 1%, but the sensitivity of the determination decreases with increasing boiling temperature. Esposito [20] extended the range of alcohols determined and neopentyl glycol, 1,4butanediol, dipropylene glycol and triethylene glycol were analyzed. Benzylamine was used for aminolysis because n-butylamine masks the peaks of the substances being analyzed. Later, polyvinylformylpropionitrile was proposed as a stationary phase for the separation of polyols [21] as it is stable for a long period at 275°C. Wittendorfer [22] described the application of this method to the analysis of alkyd resins based on trimethylolpropane. The above methods make it possible to determine any polyol contained in an alkyd resin whose acetate can be analyzed by GC. The identification and quantitative analysis of carboxylic acids (components of alkyd and polyester resins) are carried out by GC of the methyl esters. The methyl esters are less polar than the parent acids and are more suitable for GC determination. The
L
L
1.-
1-1
I
L , - -U
Fig. 6.1. (a) Chromatogram of polyol acetates on Carbowax 20M. Peaks: 1 = propylene glycol acetate; 2 = ethylene glycol acetate; 3 = reaction products; 4 = diethylene glycol acetate; 5 = glycerol acetate; 6 = trimethylol acetate; 7 = trimethylolpropane acetate; 8 = pentaerythritol acetate; 9 = mannitol acetate; 10 = sorbitol acetate. (b) Chromatogram of conversion products of an alkyd resin. Peaks: 1 = chloroform; 2 = propylene glycol acetate; 3 = reaction products; 4 = glycerol acetate; 5 = o-phthalic acid.
REACTION GC OF POLYMERS
149
rapid methods developed for the esterification of polymer saponification products ensure the rapid preparation of samples for analysis. The procedure for the preparation of samples [23] for analysis is fairly simple. A resin solution containing about 0.3 g of non-volatile substance was placed in a flask (125 ml), then 15 nil of 0.5 Nlithium methoxide were added; the mixture was heated with an inverted cooler on a water-bath to complete dissolution, and then boiled for a further 2 min. A 5-ml volume of 6 N sulphuric acid was added to the mixture, which was then transferred into a separating funnel. Subsequently, 50 ml of water and 35 ml of methylene chloride were added. The methylene chloride layer was separated, washed with water to a neutral reaction, evaporated to dryness and the residue analyzed by GC. The chromatographic analysis of methyl esters of carboxylic acids was carried out independently with the use of two columns: the first contained silicone grease on Chromosorb W (60-80 mesh), and the second a polar phase (one half was filled with 20% Carbowax 20M on Chromosorb W and the other half with 20% diethylene glycol succinate on Chromosorb W). The separation was carried out with a programmed temperature increase from 75 to 250°C for the column containing the silicone grease and from 125 to 225°C for that containing Carbowax 20M, each at the rate of 4"C/min. The method can be used for investigating resins based on various dicarboxylic acids. It is also possible to determine monocarboxylic acids, which are often present in alkyd and polyester resins. Esposito and Swann 1231 investigated o-phthalic, isophthalic, fumaric, maleic, itaconic, succinic, adipic, azelaic, sehacic, diglycolic, pelargonic and benzoic acids and also lauric, myristic, palmitic, stearic, oleic, linoleic and linolenic acids. Fig. 6.2 illustrates the separation of the methyl esters of some carboxylic acids on columns containing Carbowax 20M and silicone grease. On a polar column containing Carbowax 20M, a more distinct separation is achieved, but isophthalic and o-phthalic acids are eluted as a single peak, while maleic, fumaric, lauric and adipic acids are separated incompletely. On the column containing silicone grease, isophthalic and o-phthalic acids are separated, but some other compounds are eluted from the column as a single peak. The above procedure was used successfully in the analysis of alkyd and polyester resins. The presence of modifying additives such as phenol, rosin, nitrocellulose and melamine- and urea-formaldehyde resins does not affect the results. Tetrachlorophthalic and chlorendic acids cannot be determined by this method, but they do not affect the identification of the other acids. A slightly different version of the determination of the acids contained in alkyd resins was suggested by Gerasimora et al. [24]. A sample of alkyd resin (0.05-0.10 g) is placed in a flask with an inverted cooler, then a 100%excess of a 1%solution of sodium in absolute methanol is added and the mixture is heated for 1 h on a water-bath. A saturated solution of sodium chloride is added and the methyl esters of the acids are extracted with diethyl ether and analyzed by GC at 182°C or 240°C on columns (3 m X 4 mm; 2 m X 4 mm) filled with 20% poly(ethy1ene glycol adipate) on Chromosorb W or 20% Apiezon L on Chromosorb W. Haken [25] proposed a GC method for the analysis of benzoic acid and its p-tert.butyl homologue in the form of the corresponding methyl esters. These compounds are used as chain-length regulators in modified alkyd and linear resins. According to the proposed method [25] , a resin solution is saponified with an alcoholic solution of an References p. 156
REACTION GC OF POLYMERS
150
% 0
a
m L c L
0
c
lJ
aJ
c
c n I
0
10
20
30
0
10
20
30
,
40
Fig. 6.2. Chromatograms of methyl esters of carboxylic acids on columns containing Carbowax 20M (a) and silicone grease (b). (a) Peaks of methyl esters of acids: 1 = pelargonic; 2 = succinic; 3 = benzoic; 4 = maleic or fumaric; 5 = lauric; 6 = adipic; 7 = itaconic; 8 = diglycolic; 9 = myristic; 10 = triacetic; 11 = azelaic; 12 = palmitic; 1 3 = sebacic; 14 = o- or iso-phthalic; 15 = stearic; 16 = oleic; 17 = linoleic; 18 = linolenic. (b) Peaks of methyl esters of acids: 1 = succinic; 2 = benzoic; 3 = diglycolic, maleic and fumaric; 4 = pelargonic, itaconic and adipic; 5 = triacetic; 6 = o-phthalic; 7 = isophthalic; 8 = lauric and azelaic; 9 = sebacic; 10 = myristic; 11 = palmitic; 12 = oleic, linoleic and linolenic; 13 = stearic.
alkali, then the potassium salts of the benzoic and fatty acids are esterified with diazomethane. The separated methyl esters of the acids were analyzed on a column containing 10%butanediol succinate on Celite (80- 100 mesh); the column temperature was increased from 130 to 200°C at 5"C/min. The GC separation of methyl esters of fatty acids has also been described by other workers [26-291. Jankowski and Garner [30] proposed a method for converting esters of fatty acids into their methyl derivatives by re-esterification. The procedure for the re-esterification of esters of carboxylic acids present in plastisizers and polymers was as follows. A sample (0.2-0.5 g) was placed in a flask (250 ml) containing iodine, then 25 ml of 1 N acetic acid, 25 ml of a saturated aqueous solution of sodium chloride and 10 ml of a standard solution of diphenyl ether (internal standard) in benzene were added. The mixture was transferred into a separating funnel and the benzene layer was collected, evaporated to 10 ml and analyzed on the chromatograph. The chromatographic analysis was carried out at 195OC on a column (250 X 0.6 cm) filled with 5% Bentone-34 and 15% Carbowax 20M on Chromosorb 8' (60-80 mesh). Table 6.2 [30] compares the results obtained for some polymeric and monomeric substances when using the methods of re-esterification and saponification. It can be seen that the results are in good agreement, but the former method is preferable because of its simplicity, specificity and rapidity. Free adipic, palmitic and terephthalic acids do not form methyl esters by the above-described method. Monomeric and dimeric acids in alkyds were determined after hydrolysis by converting them into methyl esters (Paylor et al. [31] used a methanolic solution of boron trifluoride). By using a short column (30.5 X 0.3 cm) packed with 5% Dow Corning Hi-Vac silicone grease on Chromosorb W (60-80 mesh), with programmed temperature increase from 170 to 330°C at 15"C/min, and then heating for 5 min at 330°C it was possible to elute all of the monomeric acids in a single peak and the dimeric in another single peak.
REACTION GC OF POLYMERS
151
TABLE 6.2 COMPARISON OF TWO METHODS 1;ORTHE ANALY SIS OF MONOESTERSAND POLYESTERS [ 30) Initial (test) compound
Poly(et1iylene terephthalate) (commercial fibre) Polyester fibre
Dietliylene glycol adipate Diphenyl phthalate Dibutyl terephthalate Dibutyl sebacate Dibenzyl succinate Dibenzyl phthalate Sorbitol monolaurate
~
Acid whose methyl ester is determined
Content of acid (%)
Terephthalic
81.0
82.0
Isophthalic Terephthalic Adipic Adipic Phtlialic Terephthalic Sebacic Succinic Phthalic Myristic Lauric Capric Caprylic
10.0 59.1 40.5 66.6 52.2 58.0 60.6 35.5 48.0 11.7 30.1 3.4 3.0
10.0 60.0 41.0 61.6 52.2 58.4 62.9 38.2 47.8 11.5 31.9 3.3 2.8
~
.
~
Re-esterification Saponification
.
Other investigations [32] showed that the analysis of a large number of mono- and dibasic acids is more efficient if n-propyl esters are chroniatographed instead of methyl esters. In this method, the esters were obtained by interaction with n-propanol and boron trifluoride, and separation was carried out on a column packed with 3% poly(ethylene glycol) or 3% silicone oil on Celite. The method for the industrial analysis for unsaturated polyesters is different from the preceding methods in that dibasic acids (methyl esters) and glycols are separated on a single column. The resin is first separated from the solvent (styrene) by re-precipitation from light petroleum [33] and the solid resin is re-esterified with sodium methoxide in methanol for 18 h. The resulting solution of glycols and methyl esters is analyzed directly by GC on a column (336 X 0.6 cm) packed with 20% GE silicone SF-96 on Fluoropak 80 at 1 10, 150 and 18OoC or with a programmed temperature increase from 110 to 180°C at 8"lmin. The GC of polyesters used in the production of polyurethanes has been developed [34]. The method permits the identification of polyesters in a mixture and the determination of the content of oxyethylene and oxypropylene groups. Polyesters are treated with a mixture of acetic anhydride and p-toluenesulphonic acid and the acetylation products are determined by GC. The results obtained by GC and nuclear magnetic resonance spectrometry are in good agreement. A method for the quantitative determination of hydroxyl groups in polymers has been proposed [ 3 5 ] ,based on the interaction of these groups with methylmagnesium iodide and subsequent quantitative determination of the methane released. In order t o separate the methane from the benzene or diethyl ether, which are used as solvents, a column (0.51nX 4 mm) packed with 10%N-methylpyrrolidone on alumina at 351°C was used, while for separating the nitrogen and methane a column (1 .O in X 6 mm) packed with molecular sieve 5 A was used. References p. 156
152
REACTION CC OF POLYMERS
The applications of reaction GC also include the analysis of mononuclear hydroxymethylphenols in the form of acetates in uncured resols [36] . The resol analysis was described in detail by Higginbottom et al. [37]. First, water was removed from the resol, then the hydroxyl groups of the polymer were converted quantitatively into acetates with the aid of acetic anhydride in the presence of pyridine. Removal of water was accompanied by partial loss of free phenol and formaldehyde, but. it completely prevented polymerization. Formaldehyde was present in the dehydrated resol in the form of a polyforrnal, HO * C6H4 CH2 * (OCH2), * OH, which, on acetylation, was stabilized in the form of an acetate, HO C 6 h CH2 (OCH2), OCOCH3, and produced several peaks on the chromatogram. Therefore, formaldehyde was removed in the form of a compound with pyridine bisulphide. The separation of hydroxymethylphenol acetates was carried out on a column (760 X 0.3 cig) filled with silicone SE-30 with a programmed temperature increase from 100 to 300°C at 5-6"C/min. The compounds formed were eluted from the column in the order of increasing molecular weight or, with polymers, of increasing boiling temperature. Benzyl acetate was used as internal standard. In determining 2,4,6-trihydroxymethylplienoltriacetate, it is better to use bis(4-acetoxy-3,5-dimethylplienyl)methaneas internal standard. Phenol derivatives can be chromatographed in the form of trimethylsilyl ethers on a column filled with DC-550 or di-n-butyl tetrachlorophthalate [38], and also in the form of trifluoroacetates [39] on a column filled with silicon oil plus tri-o-phenyl phenylphosphate (3: 1). An ingenious method for determining alkoxy groups in phenol esters by the hydrogenation method has been developed by Klesment and Kasberg [40]. The reaction chromatography of polymers, in which low-molecular-weight products are first formed by the action of the appropriate reagents and are then analyzed by GC, is an efficient method for investigating high-molecular-weight compounds with reactive bonds in the main polymer chain. As an example, we shall describe several chromatographic procedures in which hydrolysis is used in the analysis of the composition of various polymers. In the synthesis of linear polyesters, such as poly(ethy1ene terephthalate), a number of side reactions take place, for instance conversion of ethylene glycol to diethylene glycol. The inclusion of diethylene glycol in the polymer chain has an adverse effect on the properties of the polymer. In the GC determination of the amount of diethylene glycol in poly(ethy1ene terephthalate), the best results were obtained with the use of an inert column (200 X 0.6 cm) filled with 5% Carbowax 20M (30-60 mesh) on Haloport F at 170°C [41]. In this procedure, the polyester sample was saponified with sodium hydroxide for 30 min. These conditions did not ensure complete saponification, but as the saponification kinetics of ethylene glycol and diethylene glycol ether groups are almost identical, the proportions of the contents of the two glycols in the product and hence the ratio of the peaks of the substances analyzed are constant. Upon completion of the saponification reaction, the mixture was acidified with phthalic acid and methanol was added. The concentration of diethylene glycol was then determined from a previwsly plotted calibration graph. Later, a method was woiked out in which a metal column was used [42].The polymer was hydrolyzed under pressure f w 4 h at 230"C, then hydrolyzate was analyzed directly
-
- -
6
REACTION GC OF POLYMERS
153
with the use of benzyl alcohol as internal standard on a column (300 X 0.3 cm) filled with 10%Carbowax 20M on a diatomite support at 180°C. Despite the fact that during hydrolysis at 240°C the polymer decomposes to some extent, Janssen et al. [43] hydrolyzed the polymer sample at 250°C for 16 h, then tetraethylene glycol dimethyl ester (internal standard) was introduced and the mixture was separated on Carbowax a t 290°C. A simpler version for determining diethylene glycol in poly(ethy1ene terephthalate) involving decomposition of the sample with 85% hydrazine at 115"C, has been described [44], The analysis of polyamides by GC [45] was carried out by hydrolyzing the test samples in 6 N hydrochloric acid at 130°C for 24-48 h. The results showed the presence of dicarboxylic acids, hydrochlorides of w-amino acids and diamines in the hydrolyzate. In order to carry out the GC analysis, the active hydrogen atom in the amine and carboxyl groups was repIaced with a trimethylsilyl group by treatment with N,O-bis(trimethylsilylacetamide), thus making it possible to identify the compounds of the above three classes on a single column. The reaction of the dry hydrolysis products with the reagent was carried out under the dry nitrogen in a sealed tube for 2 h at 80°C. The dicarboxylic acids were also analyzed in the form of their methyl esters, for which purpose the hydrolyzate was treated with methanol plus p-toluenesulphonic acid. The high-volatility amines were analyzed in the free state; in order to do this, their hydrochlorides were treated with potassium hydroxide in methanol. The method for determining the composition of copolyamides [46] also includes depolynierization by hydrolysis. A polymer sample was hydrolyzed in 6 N hydrochloric acid. One portion of the hydrolyzate, containing amine hydrochlorides and free acids, was evaporated, esterified with a niethanolic solution of boron trifluoride and extracted with diethyl ether. The ether extract of the methyl esters was chromatographed on a column (200 X 0.6 cm) filled with 5% diethylene glycol adipate on Chromosorb W treated with an acid (60-80 mesh). Azelaic acid, which was used as the internal standard, was added prior to esterification. A second portion of the hydrolyzate was neutralized with sodium hydroxide in order to isolate the amines and convert dibasic acids into sodium salts. The free amines were extracted with n-butanol and chromatographed on a column filled with 10%Apeizon L deposited on glass beads. Both columns were heated with a programmed temperature increase from 100 to 220°C at S"C/min. The relative error of the determination was k 5%. The quantitative and qualitative determination of alkoxyl groups in acrylic copolymers can be achieved by taking advantage of the cleavage of the alkoxyl groups by a mixture of potassium iodide and orthophosphoric acid upon heating for 2 h to 200°C [47] . The alkyl iodides released are absorbed by n-hexane on cooling the trap to -75°C; they then are analyzed by GC. For polyacrylates, a method was devised in which the alcohols formed as a result of saponification of the polymer were analyzed on a colurnn (122 X 0.6 cni) filled with 23% Oronit NI-W on firebrick (42-60 mesh) [48] . The polymer sample was dissolved in tetrahydrofuran and saponified with an alcoholic solution of potassium hydroxide at 160°C for 4 h, then the contents were neutralized and chromatographed. This method was used to analyze decyl, lauryl and 2-ethylhexyl acrylates.
References p. 156
154
REACTION G C OF POLYMERS
Earlier work [49] dealt with hydrolysis of copolymers of trioxan and the glycolformyl of orthophosphoric acid with subsequent separation of the products on dodecyl phthalate or a silicone stationary phase at 150°C. Interaction with orthophosphoric acid was also used for determining the weight ratio of oxypropylene and oxyethylene groups in poly(propy1ene oxide) and poly(ethy1ene oxide) [ S O ] . The polymer sample reacted with the orthophosphoric acid in an inert atmosphere at 350°C at the chromatograph inlet, forming propionaldehyde and acetyldehyde, respectively. The percentage of oxyalkylene groups was proportional to the amount of the aldehyde formed. Hydrolysis and GC have been used for analyzing copolymers of cis- and trans-cyclohexanediol-1,2 [51 1 , diisopropyl esters of polyoxymethylene [52] ,cyano-containing polymers [53] and also other polyesters [54] and alkyd resins [55]. It should be noted that in many instances the method permits the determination not only of the initial monomeric compounds, but also of the distribution of the monomers in the copolymer. Thus, for instance, hydrolysis and combined GC nuclear magnetic resonance analysis have been used for determining the distribution of isomers in propylene oxide-maleic anhydride block copolymers [56] and propylene oxidecitraconic anhydride block copolymers [56, 571 prepared on different catalysts. Glycol esters were separated on a column filled with mercury(I1) chloride and Carbowax 20M on Chromosorb CL (60-80 mesh). A similar method was used for establishing the distribution of monomers in polysulphite esters obtained by copolymerization of propylene oxide and sulphur dioxide on different catalysts [58]. In analytical practice, use is also made of polymer cleavage (as a result of a reaction with hydrogen bromide) with subsequent analysis of the bromide formed. For determining the composition of copolymers of methyl methacrylate, ethyl acrylate and butyl maleinate [59, 601, the combined uses of Zeisel’s method and GC has been suggested. At first [59], the method was intended for determining the composition of copolymers of methyl methacrylate with ethyl acrylate that contained less than 10%of the latter. The alkoxyl groups of the copolymer were converted into the corresponding alkyl iodides with the aid of hydriodic acid in a phenol solution, then the iodides were isolated and chromatographed on a column containing dinonyl phthalate on Celite at 150°C. Methylene chloride and ethylene chloride were used as internal standards for methyl and ethyl iodides, respectively. Later [60], in order to determine butyl maleate, which is also present in the copolymer, the analysis was carried out on a column (300 X 0.6 cni) filled with di-2-ethylhexylsebacate on firebrick at 70°C. This method was also used successfully for analyzing the weight ratio of oxyethylene and oxypropylene groups in alkylene oxide polymers [61, 621 . A polymer sample (20 mg) was converted into the dibromide by interaction with excess of a 45-50% mixture of acetic and hydrobroniic acids at 150°C for 2 h [61]. The bromides formed were dissolved in carbon disulphide and analyzed on a glass column (90 X 0.4 cm) filled with 30% silicone E-301 on Celite (30-60 mesh) at 65°C. Using the ozonation reactiw with subsequent GC analysis of aldehydes and acids, one can determine the micro-structure of rubbers [63] .
REACTION GC OF POLYMERS
155
On interaction with BF30(C2H5)2,methylsiloxane form fluorosilanes, the GC analysis of wluch yields the ratios of the groups (CH3)3Si, (CH3)*Si and CH3Si in the polymers [64] . The disadvantage of this method is that the time required t o complete the reaction at 70°C is 96 h. The fluorosilanes formed were cooled with a mixture of dry-ice and acetone, then dissolved in diethyl ether and analyzed on a capillary column (4600 X 0.05 cm) filled with silicone SE-30, and a column (122 X 0.6 cm) filled with 16.6%squalane on Teflon-6 at 35°C. In this reaction, the ethylsiloxane groups are converted into C2H5SiF5, which is eluted simultaneously with (CH&SiF*. Therefore, in practice the method is restricted t o the investigation of methylsiloxane polymers. Vinyl groups of silicone rubbers, after interaction with orthophosphoric acid (in order t o convert the vinyl groups into ethylene) [65] , are also determined chromatographically on a column (1 12 X 0.6 cm) filled with alumina a t 65°C. A modification of this method was described by Krasikova and co-workers [66,67]. The structural analysis of polysiloxanes by GC and thermal analysis was developed by Franc et al. [68]. Note that in trying t o establish the structure of a polymer, it is also advisable t o use data on its elemental composition. Methods of elemental analysis involving the use of GC have been described [8, 69, 701. The reaction on polyniers t o physical factors such as radiolysis and ultraviolet (W) light, coupled with subsequent GC analysis of the volatile decomposition products, is an additional means for characterizing polymers. The radiation-chemical method can be used for the quantitative identification of polymers. Berezkin et al. [71] studied the dependence of the range of gaseous radiolysis products on the polymer structure in the series polyethylene, polypropylene, polyisobutylene, which differ, among other things, in the concentrations and positions of the methyl groups. A weighed polymer sample was placed in a glass ampoule, which was evacuated t o mni, sealed and irradiated with y-rays (from a cobalt-60 source) at a dose rate of 1.7 1OI6 eV/g sec. The gaseous radiolysis products were analyzed chronlatographically. The method has a high sensitivity t o hydrocarbon substituents in the main chain. The sensitivity t o lateral branchings in the main chain permits, in particular, low- and high-pressure polyethylenes t o be distinguished. The effect of radiation on copolymers of vinyl acetate with ethylene makes it possible t o determine the quantitative composition of the copolymer. When a polymer sample is subjected t o 100 Mrad of y-radiation (721 , low-molecular-weight hydrocarbons, carbon monoxide, carbon dioxide and hydrogen are formed. The amount of carbon monoxide not detected in the decomposition products of pure polyethylene is proportional t o the vinyl acetate content in the copolymer. The accuracy of the determination is 1%. In investigations on the degradation of polymers under the effect of electrons [73], it was shown that in branched polyethylene, ethyl and butyl radicals are located after each 100 carbon atoms. On irradiation of polypropylene, methane is formed, which indicates the absence of recombination of the methyl radicals under the conditions used. Low-volatility compounds can be analyzed by the photodecomposition method [74, 751 A mercury-saturated sample was placed in a silica capillary tube, which was evacuated and sealed. The sample was then irradiated from a mercury source. The chroinatograms obtained after irradiation are simple and reproducible. The method can be used
-
*
References p. 156
156
REACTION GC OF POLYMERS
successfully in standard determinations, and also for estimating the photodecomposition stability of the polymer. The combination of photodecomposition with GC is used for estimating UV adsorption in polymers [76]. Samples of polymer film in a silica vessel were irradiated from a standard UV lamp, then the degree of decomposition of the film was estimated by analyzing, at definite time intervals, the gaseous decomposition products. A column (200 X 0.635 cm) filled with poly(propy1ene glycol) on Chromosorb W at 100°C was used. The chromatograms of films of acrylonitrocellulose containing 54-55% of poly(methy1 methacrylate), 24-26% of dinitrocellulose, 19-2 1% of dioctyl phthalate and 1% of absorber have seven main peaks, varying in height. The decomposition of the polymer is proportional to the efficiency of the absorber. Of the several absorbers investigated, 2,2'-dihydroxy-4-methoxybenzophenone (Cyasorb UV-24) was the most efficient. Thus, the investigation and analysis of polymers by reaction GC make it possible to determine a number of important characteristics of high-molecular-weight compounds: the proportions of the initial monomeric compounds, the distribution of monomers in the polymer chain, the content of the separate functional groups, etc. Unfortunately, the development of these methods has not been given sufficient attention until recently. The level of application of reaction GC methods in polymer analysis is considerably lower than their apparent potential. In our opinion, one of the factors restricting the application of chromatographic methods is the lack of systematic manuals on the use of gas chromatography in the functional group analysis of organic compounds, including polymers.
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63 G. M. Akhmedli and A. N. Perelman, Vysokomol. Soedin., 8 (1966) 61. 64 G. W. Heylmun and J. E. Pikula,J. Gas Chromatogr., 3 (1965) 266. 65 G. W.Heylmun, R. L. Bujarski and H. B. Bradley,J. Gas Chromatogr., 2 (1964) 300. 66 V. M. Krasikova and A. N. Kaganova, Zh. Anal. Khim, 25 (1970) 1409. 67 V. P. Mileshkevitch, V. M. Krasikova, A. H. Kachanova, A. V. Kerlin and T. N. Pratova, Author’s Certificate, No. 395,769, Applied 6.7.71; published 25.1.74. 68 J. Franc, K. PlaEer and F. Mike& Collect. Czech. Chem Commun., 32 (1967) 2242. 69 F. Salzer, Microchem J.,16 (1971) 145. 70 H. Kelker, in H. G. Struppe (Editor), Aspects of Gas Chromatography, Akademie-Verlag, Berlin, 1971, p. 37. 71 V. G. Berezkin, Yu. A. Kalbanovksy and E. A. Kyazimov, Vysokomol. Soedin.,Ser. A , 9 (1967) 2566. 72 R. M. Kamath and A. Barlow, Anal. Chem., 37 (1965) 1266. 73 R. Salovey and J. V. Pascale, J. Polym. Sci, Part A-2, (1964) 2041. 74 R. S. Juvet, 11.and L. P. Turner,Anal. Chew., 37 (1965) 1464. 75 R. S. Juvet, Jr., R. L. Tanner and J. C. Y. Tsao, J. Gas Chromatogr., 5 (1967) 15. 76 H. C. Su and J. L. Cameron, Anal. Chem., 39 (1967) 949.
Chapter 7
Pyrolysis gas chromatography Pyrolysis gas chromatography (GC) is an indirect method in which the substance under study is characterized on the basis of the GC of the volatile products of its pyrolysis. By qualitative and quantitative analysis of the products formed in the pyrolysis of polymers, one can arrive at conclusions about the structure and composition of the initial polymer system. Thermal degradation and subsequent analysis of the degradation products has long been used for the qualitative and quantitative analysis of polymeric compounds and to establish their structures [ 1-41 . The use of the GC analysis of pyrolysis products has increased the practical value of the method for the investigation of polymers considerably, because only certain of the products contained in the complex mixture formed are characteristic of a given polymer. In the separation of pyrolysis products, use is made of standard chromatographic equipment, while the pyrolysis unit is a small independent attachment to the standard chromatograph. The cost of the attachment is moderate, being only 10-20% of that of the entire chromatograph. At present, pyrolysis GC is widely used for analyzing high-molecular-weight compounds; the method itself and its practical applications are being continuously developed and extended. Thus, for instance, according to the Soviet abstract journal Referutivnyi Zhurnal Khimiya, 15 publications on pyrolysis GC were abstracted in 1960, about 40 in 1965 and about 70 in 1970. Over 400 articles were published during the period between 1960 and 1968 [ 5 ] . Pyrolysis GC, however, also has some limitations associated with the complexity of the chemical reactions in pyrolysis and with the great influence of secondary reactions. In addition, the composition of the pyrolysis products depends on the specific conditions of pyrolysis (temperature, duration, sample size, carrier gas velocity, etc.). Therefore, investigations with this technique require strict standardization of the pyrolysis conditions. Processes of thermal degradation of polymers have not been studied sufficiently so far. In the general case, it is impossible to predict the quantitative composition of the volatile decomposition products formed in pyrolysis on the basis of the structure of the polynier and the conditions used in its pyrolysis. The opposite problem, which is of great interest, namely establishing the composition and structure of a polymer from its pyrolysis products, has not been solved theoretically either. Therefore, at present the researcher’s task is to establish the empirical correlation between the structure of a polymer and the range of the products formed in its pyrolysis. In practice, different compositions of the pyrolysis products are obtained, depending on the nature of the polymer and the pyrolysis conditions. Sometimes the composition of the volatile products is simple. Thus, for instance, in the pyrolysis of copolymers of styrene and methyl methacrylate under certain conditions the pyrolysis products consist mainly of the corresponding monomers [6]. However, more often, random ruptures of macromolecules occur due to the effect of high temperature on polymers, and a complex References p. 190
PYROLYSIS GC
160
'
-4-c -c+ccc4-c+c+c+c+c-c 1*-I
I '
C I
' I '
14;
I l l
I
I
I
I C I
I
I
I C I
I c
-
I
Fig. 7.1. Diagram illustrating the formation of polypropylene pyrolysis products. 1 = Propane; 2 = isobutane; 3 = 2-methylpentane; 4 = 2,4-dimethylpentane; 5 = 2,4-dimethylheptane; 6 = 2,4,6trimethylheptane; 7 = 2,4,6-trimethylnonane.
mixture of volatile products based on separate fragments of the initial molecules is formed; this permits conclusions to be drawn about the structure and composition of the initial polymers by determining the quantitative and qualitative composition of the products. Thus, 2,4-dimethylpentane, propane, pentane, 2-methylpentane, 2,4,6trimethylnonane, 2,4,6-trimethylheptane, isobutane, ethane, 2,6-dimethyInonane and 2,4-dimethylheptane were identified (in order of decreasing yield) as the main products of the pyrolysis of polypropylene (after their hydration) [7]. The origin of these main products is illustrated in Fig. 7.1. It should be emphasized that pyrolysis GC is often very sensitive to structural differences in polymers. Chromatograms (pyrograms) of the pyrolysis products from test substances may show both qualitative and quantitative differences. depending on the similarity of the chemical structures and the choice of the pyrolysis and chromatographic separation conditions. Thus, for instance, chromatograms of the pyrolysis products of phenolformaldehyde resins obtained on the basis of 3-methylphenol (Fig. 7.2a) and 3,5-dimethylphenol (Fig. 7.2b) differed widely in the qualitative composition of the pyrolysis products [8], and for low-density (Marlex-6002) and high-density (Okiten G-03) polyethylenes Deur-Siftar [9] found only a difference in the proportions of the separate products (Fig. 7.2c, d). Because the method is highly sensitive to the specific features of the structure of the test substances, pyrograms are sometimes called 'fingerprints' and are widely used for polymer identification. Therefore, investigations in which there is no need to identify the pyrolysis products (they are in the majority) [ 101 are often said to be carried out by the 'fingerprint' method [ l I ] ; this also emphasizes the empirical nature of the method. At present, the method is most widely used for polymer identification, determination of the composition of copolymers and mixtures of homopolymers and for studying the structure of polymers.
EQUIPMENT AND EXPERIMENTAL PROCEDURE The equipment used in pyrolysis GC is extremely diverse and so are the designs of pyrolytic cells, specially prepared cells often being used. According to Levy [12], there
161
EQUIPMENT AND EXPERIMENTAL PROCEDURE
c
I
Time (min)
p 2 0 0 "C
I
I
8
4
50 -2OOOC
I
--.50k
Fig. 7.2. Chromatograms of the pyrolysis products of (a) phenolformaldehyde resin based on 3-methylphenol, (b) phenolformaldehyde resin based on 3,5-dimethylphenol, (c) lowdensity polyethylene (Marlex 6002) and (d) highdensity polyethylene (Okiten C-03).
are almost as many different designs of pyrolysis devices as there are investigations on pyrolysis GC, which often impedes the comparison of the results of different workers. According to the principle of operation, pyrolytic systems can be classified into two different types: static (enclosed) and dynamic (continuous flow) [13, 141. In a static pyrolyzer, the sample is heated in an enclosed volume for a long period, then all of the volatile pyrolysis products formed or part of them, are introduced into the chromatograph. This principle was applied more widely in early work [ 1 5-1 71 on the use of pyrolysis GC for the investigation of polymers. In these investigations, pyrolysis was carried out in a special installation and the pyrolysis products were collected and analyzed using a standard gas chromatograph. It is more convenient to carry out polymer pyrolysis in a sealed glass ampoules and to analyze the pyrolysis products chromatographically after breaking the ampoule in the flow of carrier gas before the entrance to the column [18]. This method was applied in the analysis of copolymers of acrylonitrile with styrene References p. 190
162
PYROLYSIS CC
over a wide range of composition [ 191. A batch of the polymer (5-10 mg) was placed in an ampoule, which was pumped out to a residual pressure of 10 mmHg. The pyrolysis was carried out for 20-30 min at 500°C. It was found that the composition of the copolymers can be determined from the peaks of hydrocyanic acid and toluene, the amount of which in the pyrolysis products is proportional to the content of acrylonitrile and toluene in the copolymer. A similar version for small samples was used successfully for the quantitative analysis of hydroxyethyl groups in hydroxyethyl starch [20]. A sample of the test substance (1 nig) was pyrolyzed in a sealed capillary (9.0 X 0.1 cm) in a vacuum at 400°C for 10 min. The pyrolysis products were investigated chromatographically. A linear dependence between the peak height of acetaldehyde and the amount of oxyethyl groups in the sample was established. This method is useful when the use of small samples is impossible because of the inhomogeneity of the sample, for studying the mechanism and kinetics of thermal or thermo-oxidative degradation when one has to investigate the composition of the volatile reaction products at low degrees of conversion, etc. [21]. The method yields the best reproducibility of temperatures and pyrolysis time, but it has the major disadvantage that the primary products of thermal degradation can enter into various inter- and intramolecular reactions because pyrolysis requires a long time. Because of these transformations, it is very difficult to draw conclusions about the possible structure of the initial polymer from the composition of the pyrolysis products. This disdavantage can be considerably reduced if the volatile products are, say frozen out in a trap in order to remove them from the hot zone. Despite the limitation, static systems are still in use [22,23]. In continuous-flow pyrolytic systems, the sample is rapidly heated in a continuous flow of carrier gas. The volatile pyrolysis products are diluted by the carrier gas and quickly carried away from the heated zone to the separation column. The main drawback of this method is the comparatively poor reproducibility of the heating conditions. A pyrolyzer is usually connected directly to the sample introduction system or parallel to it, and in some instances the pyrolysis element is introduced directly into the chromatographic column in order to increase the efficiency of the subsequent chromatographic separation. This method has a number of advantages over pyrolysis in an ordinary static system: the investigation time is considerably reduced because the pyrolysis and sample introduction are combined in a single short-term operation and when pyrolysis is carried out in the flow of carrier gas, the role of secondary processes can be substantially reduced. The dynamic pyrolytic systems most frequently used can be divided into two main classes according to the principle of sample heating: (1) Systems with a special heating element rapidly heated to a high temperature on which the pyrolysis sample is placed. In this type of pyrolyzer, the temperature of the pyrolysis chamber walls is much lower than the pyrolysis temperature. This class includes two types of pyrolyzer, in which the heating element is either (a) an electrically heated conductor (filament) or (b) a rod of a ferromagnetic material heated with a high-frequency current to the Curie point. ( 2 ) Systems with a pyrolysis chamber of the tubular-oven type whose walls are heated to the pyrolysis temperature.
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In the first type of pyrolytic reactor, the pyrolysis of the sample takes place on a filament (coil) that is rapidly heated by an electric current. This type of pyrolytic cell is called a cell with a filament, or a filament-type cell. The heated coil is placed in a continuous-flow chamber whose walls have a temperature that does not exceed that of the subsequcnt chromatographic separation of the pyrolysis products. When working with such cells, the test substance is deposited on a metal (usually platinum or nichronie) filament. After the introduction of the coil with the test substance into the flow of carrier gas, hermetization of the cell and attainment of the pre-assigned regime, the coil is heated. The pyroylsis products formed are carried t o the chromatographic column by the flow of carrier gas, separated and then detected. This type of cell is simple in design, the sample under investigation can be heated to the pre-assigned temperature within a comparatively short time, the pyrolysis is carried out in the flow of carrier gas and the heating surface is small, reducing the role of secondary reactions of the pyrolysis products. The following requirements are imposed on the design of this type of cell: ( I ) the volume of the cell must be the minimum possible, because an increase in volume reduces the efficiency of the subsequent chromatographic separation; ( 2 ) additional heating of the cell walls is necessary in order to prevent the possible condensation of part of the pyrolysis products on the cold walls of the cell, or the cell must be placed in the chromatograph thermostat; and (3) rapid and convenient replacement of coils with the substance under investigation is necessary. Fig. 7.3a illustrates a typical design of a glass pyrolytic cell [24] . Cells of similar design are described in papers by Janik [25], Jones and Moyles [26,27] and Mlejnek [28]. These workers used a chromatograph with an ionization detector, because the total volume of the products formed on the coil in pyrolysis is not sufficient, in the case of very small samples, for detection by a sinipler but less sensitive detector (katharometer). Franc and Blaha [29] used a pyrolytic cell in which a platinum net was used instead of a coil. This enabled them t o increase the size of the sample without increasing the weight of the polymer per unit of the heated wire surface, and thus they were able to record the pyrolysis products with a katharometer. The use of metal Filaments and coils (platinum, nichronie, etc.) as the support for the polymer sample (film) in pyrolysis is not the best method because of the possible catalytic activity of the metals. It has been shown [27] that if the weight of the polymer sample exceeds 1 mg, the filament material exerts a distinct effect on the composition of the products formed, and the composition of the pyrolysis products is simpler when using a gold-plated filament than with a nichronie coil. With the use of microgram samples, the character of the pyrograms of polystyrene and poly(methy1 methacrylate) is independent of the coil material (nichrome, platinum, gold-plated platinum). Dimbat and Eggertsen [30] greatly reduced the catalytic effect o f the surface of a platinum coil by depositing a glass coating on it (melting glass microbeads). I n certain instances the sample t o be pyrolyzcd should be placed not directly on t o the coil but rather in a boat made o f mica, quartz or some other inert material. When the pyrolysis o f rubber is carried out in a mica boat heated by a nichrome coil, more reproducible and characteristic (ie., more distinct) pyrograms are obtained than when the sample t o be pyrolyzed is deposited directly on the coil [31]. References p. 190
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Fig. 7.3. Pyrolytic cells. (a), Filament-type glass cell. 1 = Nichrome coil; 2 = tungsten electrodes; 3 = sorbent; 4 = column; 5 = carrier gas inlet; 6 = groundglass joint; 7 = asbestos layer; 8 = glass insulator. (b), Curie-point cell of Pye design. 1 = Carrier gas inlet; 2 = ferromagnetic wire; 3 = quartz tube; 4 = gasket; 5 = induction coil. (c), Samples of ferromagnetic wire for pyrolysis.
The heating element in filament-type cells can be made in the form of a cup [32] , plate [33], saucer [34] or ribbon [35,36], the horizontal surface of which carries the test sample. The pyrolyzer described by Fischer [37] is convenient for work with different technical materials. The pyrolyzer is made in the form of a small unit connected t o the current and carrier gas sources by flexible cables and the pyrolysis products are
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introduced into the chromatograph evaporator through an injection needle. The pyrolyzer is equipped with detachable containers for samples and detachable and replaceable heating elements of different design suitable for pyrolyzing samples of various types (powders, liquid, viscous, soluble and insoluble, etc.). One of the most important properties of the pyrolyzers is their temperature characteristic. Two factors are important: the temperature increase time (heating time, HT) of the sample and the reproducibility of the kinetic curve of temperature variation. For filament-type cells, the heating time is usually a few seconds. Fig. 7.4 shows curves for the cell described by Fischer [37]. Under these conditions, especially for microgram samples, sample pyrolysis is often virtually completed even during heating at temperatures below the equilibrium temperature [38]. To speed up the attainment of the equilibrium temperature (pyrolysis temperature), various heating schemes have been proposed for filament-type cells which permit the heating time to be reduced to tenths [39,40] and even hundredths of a second [41]. Fig 7.4b shows filament temperature variation curves [41] obtained with the use of (1) a constant voltage and (2) a constant-voltage source plus an additional source of powerful discharge. The kinetic sample heating curves are reproduced comparatively well on a single cell and not always satisfactorily on different cells on the same type [ 1 I ] . The polymer is introduced into a filament-type pyrolytic cell basically by three methods: (1) from solution by depositing it on a heated surface and evaporating the solvent (for soluble substances); (2) small samples of identical shape are placed inside the coil; and (3) the sample is placed in a boat or a special container inserted in the coil. A polymer film is obtained either by immersing the coil (filament) in a dilute solution, or the polymer solution (ca. 1%) is deposited on one or two turns in the middle of the coil with the aid of a soft brush or a microsyringe and the solvent is evaporated; sometimes an infrared lamp is used to speed up drying [26]. A considerable advantage of the filament-type cells is the possibility of stepwise pyrolysis [42, 431 . In stepwise pyrolysis, as distinct from single-stage pyrolysis, the same sample is pyrolyzed at several successively increasing temperatures (for instance, at 300,400 and 500°C) during the same time period (usually 10 sec) and the pyrolysis products formed at each temperature are chromatographed. Another disadvantage of the filament-type cells is the possibility of very rapid heating, the disadvantage being the change in filament resistance during operation and, occasionally, poor reproducibility of the thermal conditions. In the second type of pyrolyzer, designed by Simon and Giacobbo [44,45], the test sample is deposited on a wire of ferromagnetic material, which is placed in a quartz tube in the flow of carrier gas. The wire is rapidly heated to the Curie point of the ferromagnetic material with the aid of a high-frequency electromagnetic field. At this temperature, the ferromagnetic properties of the wire change so that it is no longer heated by the field. Thus, the surface temperature of the wire rapidly increases to the Curie point and remains constant. The design of the Pye pyrolyzer is shown in Fig. 7.3b 1461. The pyrolysis temperature can be changed discretely from 300 to 1000°C, depending on the ferromagnetic material used as the sample support; Table 7.1 lists the compositions References p. 190
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166 (a)
I
0
I
I A 1 2 Time(sec)
I
Time(sec)
Fig. 7.4. Kinetics of temperature variations in different cells. (a), Filament-type cell: 1 = 300°C; 2 = 500°C; 3 = 800°C; pyrolysis time = 10 sec. (b), Filament-type cell: 1 = with constant-voltage source, HT = 10 sec; 2 = with constant-voltage source and additional source of special powerful discharge, H T = 15 msec, diameter of heated platinum wire = 0.25 mm, pyrolysis tcrnperature = 800°C. (c), Curie-point cell for certain ferromagnetic materials with wire diameter of 0.5-0.6 mm: 1 = Co-Ni (60:40); 2 = Fe(Zn); 3 = Fe; 4 = Co-Ni (33:67); 5 = Ni-Fe (60:40); 6 = Ni-Cr-Fe (51: 1:48); 7 = Ni-Fe (45:55); 8 = Ni; generator frequency = 0.45 MHz. (d), Curie-point cell for wire of diameter (1) 0.05 and (2) 0.5 mm: pyrolysis time = 1 sec, HT = 0.02 and 0.1 sec. (e), Curie-point cell for wire of diameter 0.5 mm: 1 = with 30-WPhilips generator, HT = 1.3 sec; 2 = with 2.5-kW generator, HT = 120 msec.
and Curie temperatures of some ferromagnetic materials. The heating curves for wires of different ferromagnetic materials are given in Fig. 7 . 4 ~ . The heating time of the wire is usually from 1 sec (ref. 47) to a few tenths of a second, depending on the pyrolysis conditions for the Curie-point pyrolyzer, although sometimes it may be as short as two or three hundredths of a second [43,48].
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TABLE 7.1
CURIE POINTS 01: SOME FERROMAGNETIC MATERIALS I l l ] Elements
Composition (%)
Curie point ("C)
Fe-Co Fe Fe-Ni
50:50 -
980
30: 70 40: 60 49:51
610 590 5 10 440 420 420 900 660 358
55:45 Fe-Ni-Cr Fe-Ni-Mo Ni-Co
48:51: 1 17:19:4 40: 60
61: 3 3 Ni
-
I70
The kinetics of the variations in the wire temperature depend on the diameter of the wire and the power output of the high-frequency generator. Fig. 7.4d [48] depicts the curves of temperature variation with time for two wires o f diameter 0.05 and 0.5 mm, and Fig. 7.4e shows similar curves (obtained on another cell) for a 0.5-mm wire with the use of high-frequency generators of different power output [41]. The effect of the parameters of the high-frequency generator and the wire diameter on the time of its heating has been studied comprehensively by Buhler and Simon [49] . The sample is deposited on a ferromagnetic wire mainly in the form of a film from a solution by immersing the wire in the solution t o a depth of 1-3 cm or with the aid of a microsyringe. In order to deposit equal absolute amounts of the polymer, one must use a microsyringe. To obtain better reproducibility in depositing the polymer solution with a microsyringe, the end of the wire on to which the sample is deposited is bent, curled or made as a helical plate (Fig. 7 . 4 ~[46] ). For bent and curled wires, the heating time increases to 2-3 sec (ref. 43). A Curie-point pyrolyzer can also be used with insoluble polymers, the samples being pyrolyzed in the form of solid pieces. Such a sample, the size of which may reach 0.1-0.5 mg, is placed in a specially made depression in the wire. Sometimes the wire is filed off and bent, and the sample is clamped between the filed planes. In order to increase the amount of the sample t o be pyrolyzed, which is used in the form of a piece of up t o 1 mg in weight, it was suggested [50] that a wire of diameter 0.5 mm be wound as a dense coil around a wire of the same diameter and a piece of the wire be tucked under the lower end of the coil formed, the length of the coil being 10 rntn. A polymer sample in the form of a thin plate is placed inside the coil. The advantages of the Curie-point cells are as follows: (1) the pyrolysis temperature is readily determinable, self-maintained and reproducible; ( 2 ) the heating time of the sample is comparatively small (down t o hundredths of a second); (3) the volume of the pyrulytic cell is very small [0.2 ml (ref. 47)] ; (4) the secondary processes can be considerably reduced; and ( 5 ) it is possible to standardize the pyrolysis conditions and t o carry out reproducible wotk o n commercial cells in different laboratories. Refcrences p. 190
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PYROLYSIS GC
The shortcomings of the Curie-point cells include the fact that only strictly fixed temperatures can be used and hence it is impossible to carry out stepwise pyrolysis; also, the procedure is greatly complicated when working with insoluble polymers. In addition, the known designs of Curie-point pyrolyzers have no provision for heating the cell walls to prevent possible condensation of heavy pyrolysis products on the cold walls. In pyrolytic cells of the second type the sample to be pyrolyzed is introduced into the zone of a tubular reactor that has been previously heated to a high temperature, where the pyrolysis takes place [51]. The advantages of this type of reactor are better standardization of the thermal conditions and the possibility of operating with small and large samples; its drawback is the strong effect of secondary reactions of the pyrolysis products due to the increased time of their hold-up in the heated zone. An increase in the size of the polymer sample makes it possible to use, together with GC for studying volatile pyrolysis products, other physicochemical methods for the analysis of both the volatile pyrolysis products and the residue of the pyrolyzed polymer sample (weight, chemical, spectra and other methods). Various methods for introducing the sample into a pyrolyzer of the tubular-reactor (oven) type have been described. The sample can be introduced into the pyrolysis zone with the aid of a magnet [ 5 2 ] ,by dropping it vertically or by introducing it directly into the reactor with the aid of a special injector for solid samples [53]. Of considerable interest is the design of the pyrolytic cell of a tubular-reactor type proposed by Alishoyev et al. [54]. The sample is placed in a quartz boat located in a detachable holder, which is introduced, with the aid of a piston rod, into a previously heated quartz pyrolytic tube, where the pyrolysis takes place. To reduce the hold-up time of the volatile pyrolysis products in the heated zone, the part of the quartz tube after the boat is made in the form of a capillary. For pyrolysis, the boat containing the sample is placed at the beginning of the capillary. In front of the pyrolyzed zone, the quartz pyrolytic tube has a zone for stepwise-controlled pre-heating of the sample, which is used when it is necessary to remove the volatile products contained in the sample, such as solvent residues. The boat can be annealed in air in a special electric furnace in order to clean it of the sample (by burning the involatile residue) after the pyrolysis, if such cleaning is possible. The shifting of the boat into the pre-heating zone, the pyrolysis zone, the boat-cleaning furnace and the loading position is effected very simply and conveniently without the need to remove the boat from its holder or to take off the holder. The tightness of the system and the operating regime of the device are not disturbed. It should be noted that all of the above pyrolytic devices suffer from a serious drawback: while relatively good producibility of results can be achieved on a single device, devices of the same model manufactured by the same firm often show poor reproducibility. Until about 1970 it was thought that the best reproducibility as regards the composition of pyrolysis products was achieved on the Curie-point pyrolyzer [55]. However, a comparison of results obtained in 18 laboratories on the same samples [56] showed that Curie-point cells give the same data scattering as cells of the other types. Also, considerable inter-laboratory non-reproducibility of results was revealed. Later, second [57] and third [58]correjation tests were made in order to establish the reproducibility of the results obtained by the ‘fingerprint’ method. The correlation tests
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were arranged by the Pyrolytic Gas Chromatography Sub-group of the Chromatography Discussion Group of the Institute of Petroleum, London. The second correlation test, which was carried out when standardizing certain experimental conditions, showed that inter-laboratory reproducibility reduces abruptly with increasing complexity of the sample. A number of possible causes of errors were established: (1) the presence of a residual solvent in the test sample; (2) the wide range of pyrolysis temperatures; (3) soiling of the equipment by samples during previous pyrolysis; and (4) a considerable difference in sample size. In the third correlation test, polymers of four widely differing groups were analyzed; fibrous materials, facing materials, rubbers and mouldable polymers. The experimental conditions were standardized much better than in the second test. Standardization embraced the pyrolysis temperature and time, the column temperature and efficiency, the method of preparation and size of the sample, the conditions of chromatography and recording of the pyrograms; some freedom in the choice of the pyrolyzer was allowed. The better standardization of the pyrolysis and chromatography conditions resulted in much more reproducible data. The results of all of the correlation tests suggested that in the future it would be essential to establish the main method of sample preparation and the pyrolysis and chromatography procedures. It is also necessary to develop a draft pyrogram atlas. Investigation into the possible causes of inter-laboratory non-reproducibility of the results when using high-frequency and filament-type pyrolyzers was carried out by Levy et al. [59]. It was confirmed that the true pyrolysis temperature may differ considerably (by up to several hundred degrees) from the pre-assigned temperature depending on the heating time. In order to obtain reproducible results on different pyrolyzers with different heating times, the time during which a definite part, for instance half, of the initial sample, is pyrolyzed, must be much longer (preferably by an order of magnitude) than the heating time. As a result of a comparative study of the temperature characteristics of high-frequency and filament-type pyrolyzers, we consider it necessary to draw attention to a number of specific features of high-frequency pyrolyzers that may reduce the reproducibility: (1) The power output of the high-frequency oscillator is of great importance. For oscillators with power outputs of 1500 and 30 W,the heating time differs by one order of magnitude. (2) Sample pyrolysis involves power consumption. In the vicinity of the equilibrium temperature, however, the wire consumes less high-frequency power, and the true pyrolysis temperature is found to be below equilibrium. (3) Ferromagnetic materials with the same Curie temperature but of different compositions have different heating times. (4) The heating times for different points of the wire depend on its position in the high-frequency coil (along the length and across the width of the coil). Filament-type pyrolyzers are free from these drawbacks. In addition, they may have a shorter heating time than high-frequency pyrolyzers. Therefore, we believe that better reproducibility of the results can be obtained in a filament pyrolyzer with a very short heating time (about 10 msec).
References p. 190
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Some other methods of decomposition have been described, for instance, submersion of a U-shaped chamber with a sample in a bath with a metal melt [60], induction heating of a sample mixed with a powder of a ferromagnetic metal with the use of hgh-frequency currents [51], 0-irradiation [61] and decomposition in an electric discharge [43, 621. Pyrolysis under the effect of a pulsed laser beam [41, 631 appears to be a promising method. Folmer and Azarraga [63] used a ruby laser with a pulse duration of 400 ysec and a wavelength of 6943 A. They claimed that the heating of a sample and the formation of volatile degradation products take ca. 150 sec, while the temperature reached is 3000-3700°C. The sample cooling rate is as high as the heating rate. They believe for this reason that the role of secondary reactions must be reduced to a minimum. Interaction between the laser beam and the polymer sample is, of course, much more complicated than in thermal degradation, but the nature of this interaction has not yet been sufficiently studied. Therefore, the use of the term 'pyrolysis' here is largely a matter of convenience. Folmer and Azarraga [63] pyrolyzed some polymers in a filament-type cell (pyrolysis temperature 1000°C) and in a cell of the tubular-reactor type (8OO0C), and their pyrograms were compared with those of the same polymers obtained on degradation under the effect of a laser beam. Indeed, in laser pyrolysis the pyrograms are more specific. However, the comparison carried out is not completely valid, because pyrolysis in the other two cells was conducted at excessive, and not the optimal temperatures, and this may naturally have reduced the specificity. Pyrolysis under the effect of a laser beam has also been applied in studies on some amino acids [64] , copper and nickel salts of organic acids [65] and in the rapid analysis of hydrocarbons in shales [66]. The method yields reproducible results only when a polymer is not transparent or when the sample is a mixture of a polymer with carbon [67], which is a considerable limitation. The proposed type of pyrolyzer is extremely interesting, but the principal regularities and the nature of the processes of polymer degradation under the effect of a laser beam, as well as the fields of application of this method, have been studied insufficiently. Fanter el al. [68] suggested carrying out disintegration of transparent polymers under the effect of the energy of a pulsed laser beam, placing the samples, in the formof a thin film, on the flat surface of a rod made of cobalt glass. The beam energy is absorbed by the glass, which is in close contact with the sample. The contact is achieved by pressing the film against the moderately heated rod. Using polyethylene and polystyrene as examples, they studied the effect of the laser beam energy and film thickness on the formation of disi.ntegration products. The composition of the decomposition products is usually closer to that of the pyrolysis products at 1000-1500"K, rather than to the composition of the pyrolysis products at 4000- 10,00O0K, although the latter temperature is adopted as the heating temperature of the systems absorbing the pulsed laser beam. Vanderborgh and Ristau [69] assumed that the low reproducibility of the results in the disintegration of transparent polymers under the effect of a laser beam is due to the fact that a crater is usually formed in the sample surface and that the variations in the composition of the polymer disintegration products result from the difference in the absorption of the radiation energy by the initial surface and the floor of the crater. Therefore, they pyrolyzed thin polymer plates, piercing them with a laser beam. It was
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found that with increasing radiation energy the fraction of light disintegration products increased while that of the heavy products decreased. In this method of disintegration, the absolute amounts of the volatile products formed may vary, whereas the relative amounts remain comparatively constant, provided that the pulse characteristics of the laser are strictly constant. In conclusion, it should be mentioned that all of the above basic types of pyrolyzer (filament, Curie-point and tubular-oven types) are manufactured industrially and applied in laboratory practice, and for each type of pyrolyzer there are fields of application in which it has advantages over the other types. The merits and demerits of each type have been discussed above. Filament-type cells are most widely used, although Curie-point cells are finding ever-increasing application [ 101 . It should be noted that any type of pyrolyzer can be used in work with the ‘fingerprint’ method, it being necessary only to choose the optimal experimental conditions and obtain reproducible results. In investigating the mechanism and kinetics of degradation processes, it is advisable to reduce the role of secondary reactions and to pyrolyze thin films. In comparative studies of a series of polymers differing in class, composition, structure or other characteristics, the researcher’s tasks are as follows: (1) to obtain pyrograms that generally meet the imposed requirements, i.e., to establish the relationship
Y j = f (X, Zi)* where Y j is the peak value, or a combination of peak values, for instance the ratio of the values of two peaks on the pyrograms of the polymer series studied, X is the polymer characteristic being investigated [for instance, when determining the composition of copolymers (see below), X is the content of one of the components] and Zi is the experimental parameters; (2) to choose, from among a series (the maximum number of terms in which is equal to the number of peaks on the pyrograms or to the number of combinations) of relationships of the type in eqn. 7.1, the optimal one, Le., a dependence that is most clearly defined and also reproducible and that ensures the maximum accuracy of calculation. It is from this point of view that one should consider the effect of the various experimental parameters on the composition of the pyrolysis products. In order to obtain reproducible results and characteristic pyrograms, one must choose the optimal experimental parameters, which must then be strictly standardized, as thermal degradation of a polymer is often sensitive to even small changes in the pyrolysis conditions. In addition to the type of cell, the determining experimental parameters are (1) the temperature and time of pyrolysis, (2) the size and shape of the sample being pyrolyzed, ( 3 ) the nature and velocity of the carrier gas and (4) the conditions of chromatographic separation. The optimal pyrolysis temperature is determined by the analytical task, the nature of the polymer being investigated and the design of the pyrolytic cell. The optimal pyrolysis temperature is the temperature at which the composition of the *fW, Z j ) # constant. References p. 190
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characteristic products fie., the products from whose peaks quantitative measurements are made or definite conclusions are drawn in a qualitative assessment of the pyrograms) ensures the maximum accuracy of determination or is most specific (in qualitative assessment). In determining the composition of copolymers, the monomer peaks are generally used as the characteristic peaks. According to McCormick [70] and Gatrell and Mao [71], in the pyrolysis of acrylate copolymers the relative yield of monomers passes through a maximum as the pyrolysis temperature increases. Walker and Wolf [72] studied the effect of temperature on the yield of volatile products in the pyrolysis of 2,4,10- and 2,4,1l-trimethyldodecaneson a filament-type pyrolytic cell (heating time 15 msec) and on a Curie-point cell (heating time 120 msec). With a few exceptions, the amount of light pyrolysis products increases (up to Cs hydrocarbons) with an increase in temperature, while the amount of heavy products decreases, although the nature of changes in the amount of the separate products is different, depending on the type of the cell. The pyrolysis time is different from one case to another. In determining the pyrolysis time, researchers sometimes proceed from the fact that the peak areas of the pyrograms did not change with increasing time, i.e., the pyrolysis is complete. The size of the sample being pyrolyzed affects the yield and composition of the pyrolysis products. In studying the dependence of the composition of the pyrolysis products on the sample size, Jones and Moyles [27] showed the advantages of working with microgram samples. They compared two polystyrene pyrograins obtained from a milligram and a microgram sample using identical pyrolysis conditions. The appearance of additional peaks on the pyrogram for the milligram sample indicated an increased role of secondary reactions with the larger sample [24]. Barlow e l aZ. [73] concluded that only for very thin films is the course of pyrolysis independent of the film thickness. However, the preparation of thin film is very difficult, especially when insoluble substances are involved. Therefore, investigators often pyrolyze macro-samples in the form of solid pieces; indeed, when using the ‘fingerprint’ method, the form of the sample (sheet, powder, liquid, etc.) is of no radical importance and it is only necessary to ensure that pyrolysis is carried out under reproducible conditions. Voight [74] established that in working with a 2-mg sample 50% variations in the sample size only slightly affect the composition of the pyrolysis products. The effect of the sample size in the range from 2.5 mg to 50 gg on the yield and composition of the pyrolysis products in a filament-type cell was studied by Alishoyev et al. [75]. Using a solid piece of natural rubber as the sample, it was shown that the pyrolysis of microgram amounts produces a higher specific yield of volatile products and a higher relative content of heavy fractions than pyrolysis of milligram samples; the effect of the sample size on the yield and composition of the pyrolysis products is especially pronounced for microgram samples (Fig. 7.5). Therefore, in order to obtain more reproducible results in working with samples in the form of a solid piece, it is advisable t o use samples of 1 mg or more, unless special limitations are imposed. The pyrolysis of several solid pieces do not come into contact with each other proceeds independently. Therefore, a pyrolysis sample should be in the form of a single piece of a standard shape. The results obtained are easy to interpret.
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P/P 0.4
0.2
1
2
Sample slze (mg)
Fig. 7.5. Effect of sample size on (1) the specific total yield of volatile pyrolysis products and (2) on the ratio of the light and heavy products formed (Sll/S.,).P / p . ratio of total area of peak to sample size; S,,/S,, ratio of area of peak 11 to that of peak 4 in pyrograms of natural rubber (see ref. 75).
The transition of the pyrolysis products formed inside a solid piece into the gas phase must be preceded by their diffusion to the sample surface, which is proportional to the ratio of the surface area of the sample to its volume or (which produces a similar effect) to the transition of the surface layers of the sample into the gas phase. This process requires a certain time and increases the hold-up time of the primary pyrolysis products in the heated zone, which leads to their greater fragmentation. The ratio of surface area to volume is different for samples of different shape and size and therefore, in order to obtain reproducible results, the sample shape should also be standardized. It has been found that the nature and flow-rate of the carrier gas largely determine the pyrolysis conditions. In the pyrolysis of atactic polypropylene in nitrogen and hydrogen atmospheres, the volatile pyrolysis products are formed at a temperature about 200°C higher in the nitrogen than in the hydrogen atmosphere [76]. The flow rate of the carrier gas determines the hold-up time of the pyrolysis products in the heated zone and thus can affect their composition. It has been noted [77] that a decrease in the flow rate of the carrier gas from 60 to 40 ml/min doubled the content of benzene in polystyrene pyrolysis products. This effect, however, is not always observed. In the pyrolysis of atactic polypropylene in the range 320-935°C the amounts of the products formed did not change with the flow rate of the carrier gas [78]. When choosing the conditions for a chromatographic separation, one must take into account the character of the pyrolysis products formed. In the absence of information on the composition of the pyrolysis products, one can use universal phases (silicone oils and elastomers, Apiezon lubricants, etc.) that operate over a wide temperature range. If the composition of the pyrolysis products is known tentatively, the rules for selecting a suitable liquid stationary phase do not differ from the corresponding rules in analytical GC. As
