Polyaniline Fractal Nanocomposites

Encyclopedia of Nanoscience and Nanotechnology

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Polyaniline Fractal Nanocomposites Reghu Menon, A. K. Mukherjee Indian Institute of Science, Bangalore, India

CONTENTS 1. Introduction 2. Preparation of Conducting Polymers and Composites 3. Conducting Polymer Nanocomposites 4. Conducting Polyaniline Fractal Nanocomposites 5. Applications 6. Conclusions Glossary References

1. INTRODUCTION Typical composites consist of two components, in which the host material determines mechanical and processing properties of the system and the guest material imparts special physical and chemical properties to the intricately mixed host–guest blend. The host material is supposed to have a wide range of facile processing options so that the guest material can be easily incorporated into the host system to obtain the desired properties. Toward this end, polymers are considered to be excellent host systems, because a wide range of physical and chemical properties can be observed in polymers; moreover, they can be easily processed and blended with a variety of materials. Hence, several polymer composites have been extensively studied for the past few decades for use in various applications [1]. Some of the well-known composites are glass, carbon, or metal fiber reinforced polymers [2] and elastomers reinforced with various types of particles like carbon [3]. In previous generation polymer composites, the size of the guest material was usually on the order of microns. In such composites, it was difficult to optimize the physical properties, because intimate and homogenous mixing of guest–host materials is hard to achieve; moreover, the processibility is not versatile enough for subtle applications. Hence it is highly desirable to have nanoscopic and even molecular level mixing of guest–host materials, so that it is easier to finetune the physical and chemical properties, and this could pave the way for several innovative applications. ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

In recent years, the advent of well-defined nanoparticles of metallic, magnetic, semiconducting, and insulating materials has facilitated the preparation of a wide range of well-tailored nanocomposites with specific properties [4–6]. Moreover, it is possible to attain much lower values for the percolation threshold (according to classical percolation theory, the volume fraction of spherical particles required to make quasi-contact with each other in a composite is nearly 16%, which is considered to be the threshold for percolative processes among particles) in composites containing nanoscopic materials rather than mesoscopic ones [7]. This is mainly due to the self-organizational properties of nanoparticles, as shown by Michels et al. for carbon nanoparticle composites [8]. Furthermore, it is possible to obtain an aspect ratio (ratio of the length to the diameter of the particle, which is 1 for a spherical particle) above 1 for nanoparticles. It is well known that by increasing the aspect ratio of particles it is possible to bring down the value of the percolation threshold (fc , which means that the quasicontact between particles can be attained at a much lower volume fraction of filling than the conventional 16% for spherical particles [9–11]. The advantage of attaining a lower volume fraction of filling and percolation threshold is that the pristine mechanical and processing properties of host matrix can be retained in the blend, and the design of composites becomes easier with lower cost. Although conventional polymer composites are highly tractable with excellent mechanical properties, host polymer matrices such as polyethylene have rather poor electrical and thermal properties. In typical composites, the host matrix plays the role of inert support for the active guest material, which effectively determines the electrical and optical properties of composites. Moreover, in previous generation composites, inorganic or ceramic guest materials were not that compatible with the organic polymer host matrix, and this usually led to the formation of inhomogeneous composites with poor mechanical and physical properties. In recent years several organic/polymeric materials (for example, conducting polymers, ferroelectric polymers, photoconductive polymers, light-harvesting polymers, photorefractive polymers, and nonlinear optical materials) have shown interesting electrical and optical properties (analogous to those in inorganic materials) [12–14], and these materials are much Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (715–729)

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Polyaniline Fractal Nanocomposites

more compatible with the polymer matrix. Hence a new opportunity has come for making nanoscopic and molecular level polymer composites that show a wide range of solidstate physical properties. The conducing polymer consists of polyconjugated chains [( C C C C C C )n ] with alternating single (-bonds) and double bonds (-bonds). The -electrons are highly delocalized and easily polarizable, and these features play important roles in the electrical and optical properties of polyconjugated systems. Moreover, the intrinsic quasi-one-dimensional nature and the extent of both intra- and interchain delocalization of -electrons play significant roles in the structural, electrical, and optical properties of polyconjugated systems. The chemical structure of some of the well-known conducting polymers including polyacetylene (PA), polyaniline (PANI), polypyrrole (PPy), polythiophenes (PTs), poly(p-phenylene vinylene), poly(p-phenylene) (PPP), polythienylene vinylene (PTV), and others, are shown in Figure 1 [13]. Polyconjugated chains are stiff and less tractable, unlike conventional polymers, which are highly processible. Usually polyconjugated systems are made processible by substituting solublizable side groups to the main chain or with the help of modified counterions in dopant species. For example, in the case of polythiophenes, the alkyl group substitution makes them soluble, and in the case of polyaniline, the surfactant counterion in protonic acids makes it soluble in several organic solvents [15]. Hence by co-dissolving conjugated and host polymers in a suitable solvent, it is possible to make conducting polymer nanocomposites in an insulating polymer matrix. Also, chemical modification of guest materials such as inorganic semiconductor nanoparticles (e.g., cadmium, selenide, and titanium dioxide) [16], fullerene (C60 )

trans-polyacetylene trans-PA

S S

S

cis-polyacetylene cis-PA R

R

p o ly i s o t h ia n a p h t h e n e P I T N

R

R

S

R

S

R

polydiacetylene P D A

S

S

R

R

p o ly ( 3 - a lk y lth io p h e n e ) P 3 A T

2. PREPARATION OF CONDUCTING POLYMERS AND COMPOSITES

NH

poly(1,6-hepadiyne) NH

NH

p o ly p y r r o le N

N

poly( p- phenylene) PPP

N

p o ly p a r a p y r id in e N

poly( p- phenylenevinylene) PPV

S

N

p o ly a n ilin e P A N I OR

S S

OR

S polythiophene PT S

[17], or carbon nanotubes [18, 19], can disperse well in conjugated polymer solutions, so that nanocomposites of these materials in conjugated polymer matrices can be prepared. These types of composites have interesting optical properties and application in optoelectronic devices. In this chapter, a general introduction to conducting polymer nanocomposites is given, with some detailed descriptions for several of them. From the fundamental science point of view, the main objective of this chapter is to tie up the concepts of nanocomposites, conducting polymers, morphology, fractal structures, percolation process, and charge transport in novel nanosystems. In conducting polymer nanocomposites many of the above-mentioned ideas come together in a very interesting manner unlike in nanoparticle systems. In one of the most exciting types of conducting polymer nanocomposites the chains self-assemble to form an interpenetrating fibrillar fractal network, which is rather unique in nanosystems. In such a nanoscale fibrillar fractal network, the electrical charge transport properties have several interesting features. However, the optical and magnetic properties are not that different from those observed in bulk systems, because it is well known that in conducting polymers the optical and magnetic properties are mainly governed by molecular level features that are nearly identical in both macro- and nanosystems. Among, the various conducting polymers, only for surfactant counterion processed polyaniline is this nanoscale fibrillar fractal network formed well in a controllable fashion and also being investigated systematically. In this unique nanoscale fibrillar fractal network the conducting polyaniline chains are self-assembled by subtle interactions of the polymer chain, surfactant counterion, and solvent. Although a wide range of nanostructures, from a fibrillar fractal network to phase-segregated aggregates, can be obtained by changing the host polymer matrix and processing parameters, among them the most interesting morphological and electrical properties are observed only with the nanoscale fibrillar fractal network. Hence this chapter is largely devoted to the structural and electrical charge transport properties of this special type of polyaniline fractal nanocomposites. Moreover, they have several interesting electrical and optical applications, as mentioned later in the chapter.

OR OR

p o ly ( 2 ,5 - d ia lk o x y - p - p h e n y le n e v in y le n e

polythienylene vinylene PTV n

polyalkylflourene

C H

Figure 1. Structures of various conjugated polymers.

CmH2m+1

Usually the synthesis of conjugated polymers is carried out by one of two procedures: chemical synthesis or electropolymerization. The chemical route is further subdivided into catalytic polymerization, noncatalytic polymerization, precursor methods, and organometallic coupling. Various synthetic routes for the preparation of some of the well-known conjugated polymers are given below [20]. PA can be synthesized by the following routes: catalytic polymerization, noncatalytic polymerization, catalytic polymerization of monomers other than acetylene, and precursor methods. For the catalytic polymerization routes, five types of catalysts can be used to yield PA in the form of a uniform film. These are Zeiglar–Natta catalyst [Ti(o-n-C4 H9 4 and (C2 H5 3 Al], Luttinger catalyst (NaBH

Polyaniline Fractal Nanocomposites

and nickel chloride) [21], metathesis catalyst [WCl6 and (C6 H5 4 Sn in toluene] [22], single component catalyst [Cp2 Ti(PMe3 2 ] [23], Rh and Re catalysts [e.g., Rh(COD)Cl and Rh(NBD)Cl, where COD is cycloocta-1,5-diene and NBD is bicyclo[2.2.1]hepta-2,5-diene] [24]. For the noncatalytic route, irradiation of acetylene gas with ultraviolet or ionization radiation has shown to produce PA [25]. Monomers such as 1,3,5,7-cyclooctatetraene with a metathesis catalyst W[OCH(CH2 Cl)2 ]n Cl6−n -(C2 H5 2 AlCl (n = 2 or 3) also yield PA [26] through the ring open polymerization reaction. The precursor route makes use of a prepolymer that can be thermally converted to PA [27]. The process basically consists of three steps. The first step is the synthesis of the monomer 7,8-bis(trifluoromethyl)tricyclo[4.2.2.02 5 ]deca3,7,9-triene by the thermal reaction between hexafluorobut2-yne and cyclooctatetraene. The second step is the polymerization reaction carried out in presence of ring open metathesis polymerization initiators to form a high molecular weight precursor polymer. The third step is the thermal conversion of the precursor polymer to PA. PPP can be synthesized by the following routes: direct oxidation of benzene, direct synthesis, organometallic coupling, precursor methods, and electropolymerization. The procedure of direct oxidation of benzene consists of dehydro coupling of benzene nuclei by catalyst–oxidant systems, leading to the formation of C C bonds. The reagent for the polymerization is either a binary system of a Lewis acid such as AlCl3 and an oxidant such as CuCl2 [28], CuCl [29], MnO2 , PbO2 , and NO2 [30] or a simple reagent with both Lewis acid and oxidizing properties such as FeCl3 , MoCl5 , AsF5 , and SbF5 [31–36]. The direct synthesis route makes use of Bergman cyclization of enediynes followed by thermal treatment of an enediyne yielding a PPP derivative with a molecular weight of 1500–2000 [37]. The organometallic synthesis is basically classified into two categories: the Yamamoto– Colon route or Suzuki coupling. The first method makes use of Ni(II) complexes such as NiCl4 2(bpy), where bpy is 2,2 -bipyridine, and NiBr2 (PPh3 2 [38–43] whereas the second method involves coupling of Pd catalysts with various bromobenzene derivatives with benzene boronic acid [44, 45]. Precursor-based synthesis of PPP has also been carried out. Some of the precursors are Marvel’s precursor [46, 47], ICI precursor [48], and Grubb’s precursor [49, 50]. PPP has also been synthesized through anodic electrochemical oxidation. The electropolymerization has been carried out in various media and electrolytes. For example, Rubinstein [51, 52] used a strongly acidic medium—an HF/H2 SO4 two-phase system. Organic aprotic media such as dichloromethane, nitrobenzene, nitromethane, phenylacetonitrile, acetonitrile, and propylene carbonate, along with various simple and complex electrolytes such as P2 O2 , CuCl2 , LiBF4 + CuCl2 , and LiAsF2 + CuCl2 [53–61], have also been used for the electrosynthesis of PPP. The basic synthesis procedures for PT can be considered in parallel with those for PPP. PT and poly-(3alkylthiophene) (P3AT) can be prepared through direct oxidative coupling with various oxidizing agents such as Fe(III), Mo(V), and Ru(III) [62–66]. Polyalkoxythiophenes were prepared by Cu(ClO4 2 oxidation of bithiophenes [67]. The organometallic coupling reactions can be basically classified into two catagories: Ni-catalyzed coupling and

717 Pd-catalyzed coupling. The former can be subdivided into four methods, viz., the Yamamoto method [68], the Colon method [69], the McCullogh method [70], and the Rieke method [71]. The latter can be further subdivided into two routes: the Curtis method [72] and the Stille method [73]. The most conductive PTs have been prepared electrochemically in rigorously anhydrous aprotic solvents of high dielectric constant and low nucleophilicity such as acetonitrile [74], benzonitrile [75], nitrobenzene [76], and propylene carbonate [77] in the presence of small anions derived from strong − − − acids such as ClO− 4 , PF6 , BF4 , and AsF6 associated with lithium and tetraalkylammonium cations [77–80]. Partial crystallinity has been observed in poly-3-methylthiophene grown in acetonitrile with CF3 SO− 3 [81, 82]. For the synthesis of PPy, two routes have been used: chemical synthesis and electropolymerization. Because pyrrole is one of the most easily oxidized monomers, a variety of oxidizing agents are available for the preparation of pyrrole such as FeCl3 , Fe(NO3 3 , Fe(ClO4 3 , Fe2 (SO4 3 , K3 Fe(CN)6 , FeBr3 , CuCl2 , and CuBr2 [83–90]. Effective catalytic processes have also been favored for mass production at low cost and extensive post-treatment steps. CuCl/AlCl3 /O2 is a typical system [91] applied to synthesize PPy chemically. The electrochemical synthesis method has also been adopted for the preparation of PPy, and it is basically an oxidative process. Therefore, it is of fundamental importance that the electrode does not oxidize to compete with pyrrole. Most PPy has been prepared with inert electrodes, such as platinum [92], gold [93], and glassy carbon [94]. Among the organic solvents, acetonitrile and propylene carbonate have been most extensively used for their poor nucleophilic characters, along with electrolytes such as tetraethylammonium tetrafluoroborate and tetrabutylammonium hexafluorphosphate [95, 96]. High-quality films have been obtained even with nucleophilic aprotic solvents such as dimethylsulfoxide, dimethylformamide, and hexamethylphosphoramide [97–99]. Because this chapter is mainly concerned with PANI and its composites, a detailed description of the synthesis of PANI is given here. The anhydrous salt of LiCl was dissolved while 125 ml of 1.0 M HCl was stirred in to prepare a 2.0 M salt solution. Dissolution of the salt is exothermic. Aniline (C6 H5 NH2 , 5.0 ml, 0.0548 mol) added to 75 ml of the 2.0 M salt/HCl solution which was then cooled while being stirred at 0  C in an ice bath to obtain a homogeneous solution. Ammonium peroxydisulfate [(NH4 2 S2 O8 , 2.75 g, 0.0126 mol] dissolved in the remaining 50 ml of the 2 M HCl solution and cooled to 0  C, and then added to the aniline/salt/HCl solution with constant stirring for 1 min. A homogeneous solution was obtained. The temperature remained between 0 and 3  C throughout the entire polymerization reaction. By using the potential profiling technique [100, 101], which involves placement of a platinum electrode and a saturated calomel reference electrode in the reaction vessel to measure the potential changes during the reaction, the open circuit potential Voc  was continously monitored. The synthesis was discontinued when Voc reached a value of 0.43 V. The approximate time taken to reach 0.43 V was 3 h. An intense blue-green precipitate with a coppery glint was formed in each case. This precipitate was collected on a Buchner funnel with water (9 cm

718 diameter, Whatman no. 4 filter paper) using an aspirator. It was washed approximately 10 times with 1.0 M HCl (80 ml portions) until the filtrate became colorless. Care was taken to ensure that the precipitate cake was always covered by a thin layer of the washing solution and that it was never allowed to develop cracks during this washing process, which would impair the washing efficiency. This as-synthesized emarldine hydrochloride was partially dried for 15 min by sucking air through the filter cake in the Buchner funnel, then transferred to a Petri dish, and dried for 24 h in a desiccator under dynamic vacuum. Similar to the preparation of conducting polymers, the preparation for composites is also divided into two categories: electrochemical route and chemical route. For smallscale preparation, the electrochemical route is used whereas the chemical route allows large-scale synthesis of composites. The electrochemical route encompasses two methods: electrode coating or codeposition. The former method uses three electrodes (reference, working, and counter) in a one-compartment cell containing the electrolyte and the monomer solution. To obtain blends, the working electrode is coated with a film of insulating polymer, before the anodic deposition of conducting film. The codeposition method is similar to the electrode coating method, but the insulating polymer host is dissolved in the electrolyte solution, which also contains the monomer of the conductive polymer. As the conductive polymer film is anodically deposited on the surface of the electrode, it becomes soaked with the insulating polymer solution. Blends of poly(vinyl chloride) (PVC) with pyrrole [102], thiophene [103], and 3-methylthiophene [104] can be prepared by the electrochemical method. There are two methods for the chemical synthesis of conductive polymer blends: mechanical mixing of the blend components (PANI–PVC blend [105]) and polymerization of the conductive polymer in the insulating polymer matrix. An oxidizing agent is used to polymerize the heterocyclic monomer that is either embedded in the insulating polymer matrix or mixed with it in a common solvent. Various PPy blends with latex [106], poly(2-vinyl pyridine) [107], and others are prepared by this method. This chapter also focuses on blends of PANI; thus, the procedures for preparation of various PANI blends is given in detail. Conductive blends of PANI with lowdensity polyethylene (PE), high-density PE, polystyrene, and polypropylene [108] can be prepared by melt processing of PANI doped with dodecylbenzenesulfonic acid (DBSA) in a miniature counter-rotating double-screw extruder; this thermoplastic blend, with conductivity values of ∼0.1 S/cm, is commercially known as Neste complex. A PANI–PVC blend is also prepared by the above-mentioned procedure and is commercially known as Versicon [109]. Blends of PANI with cellulose acetate can be prepared by two methods. In one of the methods, a fine powder of chemically prepared PANI is suspended in an acid solution of insulating polymer, and the films obtained by casting this solution are brittle in nature. The electrode coating method was also used and it improved the adhesion of PANI to an optically transparent electrode, facilitating spectroelectrochemical measurements [110]. Fibrils of PANI were prepared by the polymerization of aniline in a gel

Polyaniline Fractal Nanocomposites

of poly(acrylic acid) using FeCl3 as oxidant [111]. Scanning electron micrography revealed a diameter of 50 nm for the fibrils having a length of 1–5 m. Monofilaments of conductive fibers from a blend of PANI and poly(pphenylene-terephthalamide) were obtained at different concentrations of PANI through the process of wet-spinning from a solution of the component polymers in sulfuric acid into a 1 N sulfuric acid solution [112]. Electroactive polymer blends of PANI and polystyrene or poly(methylmethacrylateco-butadiene-co-styrene) (MBS) are prepared via various techniques [113]. The methods used were in-situ oxidative polymerization in aqueous dispersion of the insulating host, coagulation of latex of the thermoplastic in an acidic dispersion of PANI, or dry blending (casing from a solution containing both components). A blend of polyaniline and nitrilic rubber has been prepared by coating a platinum working electrode with a thin film of nitrilic rubber (29% acrylonitrile) and polymerizing aniline by the potentiodynamic method. Because aniline polymerization must be performed at low pH, nitrilic rubber was chosen for its resistance to acid attack [114]. The melt mixing procedure was used to disperse PANI doped with DBSA in poly(styrene-co-butadieneco-styrene) (SBS rubber) [115] and thermoelastic polymer poly(styrene-ethylene/butylene-styrene) [108]. Conductivity values of 2 and 0.1 S/cm at volume fractions of PANI of 50 and 2%, respectively, can be obtained [108, 115]. PANI doped with poly(p-styrene sulfonic acid) can also be viewed as a polymer blend and can be electrochemically deposited onto a working electrode surface using an aqueous solution containing p-styrene sulfonic acid as an electrolyte [116].

3. CONDUCTING POLYMER NANOCOMPOSITES Conducting polymer nanocomposites can be prepared in several ways. Either conjugated polymers can be dispersed in a host matrix such as conventional polymers, inorganic glassy materials, porous materials, on layered structures, or molecular/nanoparticle guest materials such as C60 , carbon nanotubes, titanium dioxide, cadmium selenide, or biological materials, can be dispersed in a conjugated polymer matrix. Moreover, the morphology of the composite can be easily altered by varying the self-organization/self-assembly features of the guest–host matrix, and this also significantly affects its physical properties. Typically, undoped conjugated polymers are semiconductors with bandgaps varying from 1 to 4 eV. The conductivity can be varied by 10 orders of magnitude, from insulator to metal, by doping, as shown in Table 1 [27, 75, 117–154]. The electrical and optical properties can be fine-tuned by varying the -electron delocalization, interchain interactions, or disorder. Hence it is possible to obtain a wide range of interesting electrical and optical properties in conducting polymer nanocomposites. Of the various types of conducting polymer nanocomposites, the doped conjugated polymers in the insulating polymer matrix have been the most extensively studied. In these composites, either the dispersion of insoluble conducting polymer particles or the formation of a tenuous interpenetrating network of soluble conducting polymers in an insulating polymer matrix can be achieved. In the latter,

719

Polyaniline Fractal Nanocomposites Table 1. Room temperature conductivity of various doped conducting polymers [117–154]. Polymer

Dopant

Polyacetylene (Naarmann route) Polyacetylene (liquid crystal) Polyacetylene (Shirakawa) Polyacetylene (Durham) Polypyrrole

I2

105

I2

1 2 × 104

I2

103

I2

10–102

Poly(3-methylthiophene) Poly(3-methylthiophene) Polythiophene

Poly(3-octylthiophene) Poly(thieneylthiophene) Poly(p-phenylene) Poly(2-butoxy-5-methoxy– 4-phenylene vinylene) Poly(2,5-diheptyl1,4-phenylene vinylene) Poly(phenylene vinylene) Polyaniline

Conductivity (S/cm)

[Ti(Obu)4 –AlEt3 ] I2 FeCl3 AsF5 Rb PF6 HClO4 p-Toluenesulfonic acid HClSO3 ClO−4 , BF6 , TsO, and HSO4 PF6 ClO4 SO3 CF3 PF6 BF4 ClO4 AsF6 FeCl3 I2 AsF5 Naphthyl K+e FeCl3

1–100 1–100 1–100 1–100 1–100 200–1500 4 × 102 200–230 2 × 102 50–200

SbF5

2 8 × 104

FeCl3 HCl/CSA/DBSA/MSA H3 PO4 Diphenyl phosphate Phosphoric acid

35 101 –220 58 65 40

the aspect ratio of the conducting polymer network can be controlled by varying the processing conditions, because the nanoscopic level self-assembly of the conducting polymer chains depends on molecular scale interactions. In this way, it is possible to attain very low percolation thresholds and also to observe a novel charge transport mechanism in these nanoscopic networks of conducting polymer chains. Moreover, these conducting networks have interesting applications, as discussed in Section 4. Hence this chapter is largely focused on the morphology and charge transport properties of conducting polymer networks in an insulating polymer matrix, although a brief overview of several other types of conjugated polymer nanocomposites is given. Over the past 10 years, several types of conducting polymer nanocomposites with interesting electrical properties have been prepared, for example, poly(3-octylthiophene)/ polyethylene (P3OT–PE), polyacetylene/polyvinyl butyral (PA–PVB), poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT–PSS) or polyvinyl pyrrolidone (PEDOT–

1975 750 30–100 370 270 10–100 97 1–180 315 500 50 5 7 × 102

PVP), and polyaniline–camphor sulfonic acid/polymethyl methacrylate [PANI–CSA/PMMA] [155–158]. Several of these composites show a finite conductivity at volume fractions of conducting polymers as low as 1%, which is much below the classical percolation threshold of 16% conducting material in the matrix. To a large extent, the distribution of conducting networks in the insulating matrix depends on the interaction between conducting and insulating polymers, which in turn is quite sensitive to the processing parameters. For example, in solution and gel processing routes the conducting polymer can get adsorbed to the connected gel network, and this leads to the formation of a two-dimensional network of conducting pathways in the insulating matrix [159]. Similarly, the chemical functionality of the guest–host matrix can influence the morphology from fractal networks to phase-segregated granular type formations. The structure of the conducting polymer chain plays a significant role in composite formation. The connectivity and conductivity of network are influenced by the flexiblity

720 and mobility of the conducting polymer chains in the matrix. Either via the molecular level interaction of functional groups in guest–host polymer chains or via cross-linking of the guest and host polymers through covalent bonds, it is possible to control the extent of interpenetration in the network. In this way, the domain size, the nature of the interfacial region, and morphology can be controlled. Some typical examples are given in the following paragraphs. PA is the simplest of all of the conducting polymers because of its chemical structure, and doped films of PA have shown conductivity on the order of 105 S/cm, typical to that observed in conventional metals. However, PA is intractable and unstable at ambient atmosphere. Recently, fairly stable blends of PA were synthesized by blending it with PVB as the host insulating polymer matrix, with conductivity values of ∼1 S/cm [156]. PEDOT is another well-known conducting polymer because of its stability, semitransparent conducting properties, wide range of bandgap tuning (1.4–2.5 eV), and processibility. PEDOT when doped with poly(styrene sulfonic acid) (PSS) yields a soluble polyelectrolyte complex with positively charged PEDOT chains and negatively charged PSS ions, and it forms a fine dispersion in several solvents including water. The conductivity of PEDOT–PSS composites can be enhanced by several orders of magnitude (from 10−4 to 102 S/cm) under special processing conditions as mentioned in the literature [160]. In recent years, PEDOT composites have received a lot of attention because of their special electrical and optical properties [148]. Conducting polymer blends of PPy can be prepared by several different techniques. For example, PPy–Nafion and PPy–PMMA are made by interfacial polymerization [161, 162], PPy–nylon and PPy–cellulose [163, 164] are made by vapor-phase polymerization of pyrrole in the insulating polymer matrix containing the required oxidant, PPy–PVC and PPy–poly(vinyl alcohol) can be prepared by solutionphase polymerization of monomer, host polymer, and oxidant in a common solvent [165, 166], and PPy–PVC and PPy–Aramid can be prepared by electropolymerization of the monomer adsorbed into the host matrix [167]. All of these techniques usually yield PPy composites at ∼1 S/cm. Nanocomposites of poly(2-methoxy, 5-(2 -ethyl-hexoxy)– p-phenylene vinylene) (MEH–PPV) in PE can be prepared by the solution or gel processing route [168]. Both MEH–PPV and PE are dissolved in xylene at elevated temperatures and form a gel upon cooling down to room temperatures. Free-standing films of MEH–PPV–PE, containing a volume fraction of MEH–PPV as low as 0.1%, can be prepared by stretch orienting the gel. These films show a high degree of anisotropy in polarized absorption and photoluminiscence along the direction of tensile drawing with ratios in excess of 150:1 and 60:1, respectively. The MEH–PPV chains are highly oriented in the PE matrix and the delocalized -electronic states are over 100 Å, which corresponds to several tens of monomer units [168, 169]. During the last decade, much work has been carried out in the field of conjugated polymer composite-based photosensors and photovoltaic devices [170]. For example, blends

Polyaniline Fractal Nanocomposites

of conjugated polymers as the electron donor with C60 as the acceptor have shown very interesting photoinduced charge transfer processes [171–174]. A nanocomposite of a conducting polymer and C60 has a high interfacial area for efficient charge separation through the control of morphology of phase separation in the interpenetrating network of donor and acceptor. Polyblends of PPV and C60 have shown a conversion efficiency of 2.9% [175]. Also, interpenetrating networks of two conjugated polymers, for example, MEH–PPV as a donor and a cyano group-substituted PPV as an acceptor have a quantum efficiency of 15% [175]. Furthermore, quite efficient large area photodiodes have been fabricated from these molecular-scale composites. Also, poly(3-octylthiophene) and fullerene composites have shown interesting photophysical properties [176]. Recently, composites of PPV derivatives and carbon nanotubes have improved the performance of devices, because the presence of nanotubes can enhance the mobility of carriers [19]. Conducting polymer nanocomposites with inorganic nanoparticles have been prepared and have several interesting physical properties. For example, PPy–ZrO5 , PPy– Fe2 O3 , and PANI–TiO2 , the dielectric properties can vary significantly [5]. Nanocomposites of PANI or PPy with Fe2 O3 have shown interesting magnetic properties [5]. For example, laser action has been reported in films of MEH–PPV–TiO2 in polystyrene [177]. It has been observed that the presence of TiO2 nanocrystals significantly narrows the emission spectrum of MEH–PPV. For MEH–PPV/CdS the presence of CdS has improved the quantum yield for charge separation [178]. CdSe dispersed in MEH–PPV has enhanced the photoluminecence characteristics of the latter owing to efficient charge transfer [179]. It has been observed that the incorporation of semiconductive inorganic nanocrystals such as CdS or CdSe in MEH–PPV-based devices has improved the device performance. Conducting polymers such as PPy or PANI, can be incorporated into the porous structure of zeolite systems. Composites of PPy with zeolites such as faujasite and mordenite [180, 181] have conductivity on the order of 10−4 S/cm. PANI has been incorporated in layered zeolites such as V2 O5 · nH2 O [182] (known as oxide bronze) to form a layered conducting polymer–zeolite nanocomposite system, in which the conductivity is nearly 4 orders of magnitude larger with respect to the pristine oxide, with a room temperature conductivity as high as 0.1 S/cm. Conducting polymer composites have been prepared with various types of elastomers too. For example, blends of poly(3-methyl thiophene)/nitrile rubber has been prepared by the electrode coating method, with a conductivity of ∼0.1 S/cm [183]. Composites of poly(3,4dialkylpyrrole) or poly(3-alkylthiophene) with natural or nitrile rubber have conductivity as high as 1 S/cm [183]. Among the various conducting polymer nanocomposites, only for surfactant counterion-processed PANI systems has a systematic and rigorous investigation been carried out. In particular, for the PANI–CSA/PMMA fractal nanocomposite, a detailed study on the morphology and charge transport has been done, as described in Section 4.

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Polyaniline Fractal Nanocomposites

4. CONDUCTING POLYANILINE FRACTAL NANOCOMPOSITES PANI is ideal for making conducting polymer nanocomposites owing to its ease of processing and inexpensive method of synthesis with high yield, good stability, and wide variation of conductivity. The conductivity of PANI can be varied by 10 orders of magnitude by altering the protonation level, with a high value of conductivity of ∼500 S/cm by doping with camphor sulfonic acid (CSA). The chemical structures of protonated PANI (emarldine salt) and CSA are shown in Figure 2. Emarldine base is a semirigid polymer that is sparingly soluble in a few solvents such as N -methylpyrrolidone and concentrated sulfuric acid. However, by doping emarldine base with functionalized protonic acids (e.g., CSA or DBSA) the conducting form of PANI can be made soluble in several solvents such as meta-cresol or xylene. Functionalized protonic acids have a chemical structure consisting of H+ (M-R)− where the counterion consists of an ionic group such as SO− 3 and a functional group such as dodecylbenzyl or camphor [15]. The role of the counterion (M-R)− is to act as a surfactant in which M− attaches ionically to the positively charged polymer and the R group imparts solubility in solvents and also makes it processible. This counterion-induced processibility of PANI in various organic solvents has opened up the possibility of codissolving it with a host polymer matrix in a suitable solvent, and this yields a nanoscopic/molecular level control on the morphology of the composite. This has significantly improved the electrical and optical properties of PANI blends. Moreover, the conducting PANI blends can be easily cast from solution to obtain free-standing films and fibers. Conducting PANI nanocomposites have been prepared with some well-known polymers such as polymethylmethacrylate, nylon, polycarbonate, polystyrene, polysulfone, polyvinyl acetate, polypropylene, polyvinyl chloride, and acrylonitrile–butadiene–styrene copolymer [15]. In each of these polyblends the morphology of the PANI network is slightly different, because the molecular level interactions come into play and this determines the nanoscale structures. In some of the polyblends guest–host chains are highly interpenetrating; whereas in others a phase-segregated structure can be observed. For example, polyblends with ultra-high molecular weight polyethylene, polyvinyl alcohol, or polyacetonitrile (PAN) are optically clear and transparent for low volume fractions of PANI, because the typical width of PANI interconnects in the network are less than 200 Å; as a result the scattering loses are less [184]. Moreover, these polyblends can be stretched to high draw ratios, and this makes the PANI chains well aligned within the matrix. This has been verified by X-ray diffraction and polarized absorption spectra studies [15]. These types of polyblends can be used in flexible infrared (IR) polarizers. PANI chains form wellordered conducting lines in a PAN matrix because the chains can be easily aligned by tensile drawing, and the scattering losses are less in the IR range [184]. The phase-segregated morphology in the PANI–PAN blend is being facilitated by the polar CN groups in the host polymer, which is a classic example to show how the molecular level interactions can alter the morphology and physical properties of these systems. Whereas, in the less polar PMMA host matrix, the

interpenetrated nanostructure can be controlled by varying the processing parameters such as the surfactant counterions, molecular weight of PMMA, and solvents, and this helps to investigate the interesting correlation between morphology and physical properties. This correlation is rigorously studied for PANI–PMMA composites, as discussed in Section 4.1.

4.1. Morphology of PANI–PMMA Fractal Nanocomposites Cao and co-workers [185–188] were the first to perform a detailed systematic study of the nature and properties of surfactant counterion-processed PANI composites. Although composites with several host polymer matrices have been prepared, those with PMMA are the best in terms of very fine controls on composition, morphology, and physical properties. For example, by varying the molecular weight of PMMA, the diffusion of PANI–CSA in PMMA (hereafter PANI–PMMA) can be controlled, and this affects the formation of nano- or microstructures during the slow liquid– liquid phase separation process at temperatures around 50  C. Hence, the dynamics of the diffusion of PANI–CSA in PMMA plays a significant role in the formation of the selfassembled interpenetrating fibrillar network that determines its physical properties. For example, in optimally processed composites with nanoscopic level intricate connectivity, it is possible to obtain conductivity values of ∼1 S/cm for a 1% volume fraction of PANI–CSA in PMMA, which is one of the amazing properties of these systems. The best results are obtained by using modest molecular weight PMMA with

H N

N

N

N

n

H (a) H

H

N

N

N

N

H

H

A-

A-

n

(b)

CH3

CH3 CH2-SO3H O

(c)

Figure 2. Chemical structures of (a) emarldine base, (b) emarldine salt, and (c) 10-camphor sulfonic acid.

722 meta-cresol as the solvent. The extent of polarity of the surfactant counterion, host polymer, and solvent and processing conditions such as temperature and solution viscosity play significant roles in nanoscopic level morphology and the associated physical properties. Transmission electron micrographs of PANI–PMMA nanocomposites containing volume fractions (f  of 0.25 and 0.5% of PANI–CSA are shown in Figure 3. These micrographs clearly show the distribution pattern of PANI, as expected for a percolating medium, with the formation of “links” (PANI–CSA fibrils), “nodes” (intersection point of two fibrils), and “blobs” (bundled-up dense regions), especially for 0.5% PANI–CSA. The links have diameters of ∼100–500 Å. However, as f is lowered to 0.25%, the number of links connecting the blobs rapidly decreases, as shown in Figure 3b. This suggests that as the links thin down, by lowering the volume fraction of PANI–CSA, the network falls apart to disconnected blobs. What exactly triggers this instability and makes the network unsustainable is not yet clearly understood. It seems that there is some critical diameter

Figure 3. Transmission electron micrographs of PANI–CSA/PMMA blends for volume fractions, (a) f = 0 005 and (b) f = 0 0025 of PANI– CSA. Scale: 1 cm = 50 nm. Reprinted with permission from [186], R. Menon et al., Phys. Rev. B 50, 13931 (1994). © 1994, American Physical Society.

Polyaniline Fractal Nanocomposites

around 100 Å for the links to hold the network together; and below this number the links become fragile and a significant volume fraction of PANI–CSA tends to stay within the blobs. Hence, in this processing route, with several optimized parameters such as the molecular weight of PMMA, the percolation threshold volume fraction (fc  is between 0.5 and 0.25% of PANI–CSA and the conductivity near the percolation threshold is ∼10−3 S/cm, as shown in Figure 4 [186]. These values are quite remarkable with respect to the composites consisting of intractable PANI particles dispersed in PMMA, in which the conductivity at fc ∼8% is ∼10−8 S/cm [186]. Moreover, it seems that the processing parameters can be optimized further, and this could yield PANI nanocomposites with fc ∼0.1%, and conductivity at fc of ∼1 S/cm. Detailed investigations of transmission electron micrographs of PANI–PMMA composites have been carried out for a wide range of compositions of PANI–CSA [186]. When these micrographs are examined at different magnifications, the structure of the interconnected fibrillar network appears to be self-similar at all length scales, and this is especially so near the percolation threshold. This finding is consistent with the view that all systems exhibit self-similarity near the percolation threshold due to its fractal structure, which has been verified by numerical analysis of transmission electron micrographs. The numerical analysis is performed by mapping the mass distribution as a function of distance in the micrographs [186]. Usually in a homogeneous medium, the mass M r) increases as a function of distance (r from a chosen origin; for example, in two dimensions, M r ∝ r 2 . Similarly, for a fractal structure the mass distribution is given by M r ∝ r D , where D is the fractal dimension. Typically, M r) is estimated by scanning and digitizing

Figure 4. (a) Conductivity versus volume fraction of PANI–CSA in PMMA. (b) Conductivity as a function of difference from critical volume fraction. Reprinted with permission from [186], R. Menon et al., Phys. Rev. B 50, 13931 (1994). © 1994, American Physical Society.

723

Polyaniline Fractal Nanocomposites

the surface area of the network as a function of successively larger scanned areas, from any arbitrary starting point in the micrograph, which are then sampled and averaged out, as given in [185]. The log–log plots of surface area versus distance yield straight lines, and the slope of these lines depends on the volume fraction of PANI–CSA in the composite. For example, the typical values obtained from the above analysis for D with f = 3 7, 1.96, and 0.96% are 1.99, 1.75, and 1.53, respectively [186]. Hence, by decreasing f of PANI–CSA toward the percolation threshold, the value of D also decreases, and this suggests the presence of a self-similar structure with holes on every length scale. The estimate of D for the two-dimensional scanned slice of the three-dimensional object is ∼1.5. The fractal dimension of the three-dimensional network at the percolation threshold (i.e., fc ∼0.5%) can be estimated by adding 1 to the value of D obtained from the above analysis for the two-dimensional slice; that is, 1 + 1 5 = 2 5. This number suggests that the network is open, porous, and self-similar on every length scale. Hence, this yields a large surface area for a fractal network, which is quite useful for several applications, as described in Section 5.

4.2. Electrical Properties of PANI–PMMA Fractal Nanocomposites The typical values of room temperature conductivity and resistivity ratios r r =  (4.2 K)/ (300 K)] for PANI– PMMA composites at various f values of PANI–CSA are presented in Table 2. The conductivity versus f plot is shown in Figure 4. The smooth and continuous increase in conductivity as a function of f is rather special to these systems. This suggests that as the connectivity of conducting channels gets denser, the movement of charge carriers becomes easier. When one compares this with classical percolating systems, in which the conductivity increases abruptly at the percolation threshold and then quickly saturates at higher values of f , it is possible to get very fine control of the value of conductivity as a function of f in PANI–PMMA nanocomposites. Hence the conductivity can be easily varied by nearly 10 orders of magnitude as a function of f . The precise determination of a percolation threshold can be obtained by fitting the data to the well-known “scaling

law” of percolation theory [i.e.,  f  ≈ T f − fc t , where T is a conductivity constant and t is the critical exponent (usually t ≈ 1 and 2 for two- and three-dimensional systems, respectively)] [189]. The fit is shown in Figure 4b. At 10 K, t = 1 99 ± 0 04 (fc = 0 3 ± 0 05%), which is close to t = 2 for three-dimensional systems, although the room temperature data give t = 1 33 ± 0 02 (fc = 0 3%), which can be attributed to thermally assisted hopping transport of charge carriers among the weakly connected parts of network. The temperature dependence of t is an interesting feature of PANI–PMMA nanocomposites, which is not usually observed in classical percolating systems. The temperature dependence of conductivity of PANI– PMMA composites for a wide range of f values (0 002 ≤ f ≤ 1) is shown in Figure 5 [186]. The temperature coefficient of resistivity (TCR) for 100% PANI–CSA is quite weak with the typical metallic positive TCR above 150 K, and the resistivity minima vary according to the extent of disorder present in the system [190]. Upon increasing the dilution of PANI–CSA with PMMA, although the negative TCR became stronger at low temperatures, the intrinsic metallic positive TCR of PANI–CSA at temperatures above 200 K was observed even in samples containing volume fractions of PANI–CSA as low as f ∼0.3%, which is remarkable. For f > 0 006, the temperature dependence of conductivity was weak for T > 50 K, as shown in Figure 5. The temperature dependence of conductivity is characterized by the resistivity ratios (r ) at various volume fractions as shown in Table 1. For f > 0 006, the network is well connected and the barrier due to disorder is rather weak at T > 50 K. However, at very low temperatures, the charge carriers have to hop across the random disorder potential barriers, and the exponential temperature dependence of conductivity is indicative

Table 2. Room temperature conductivity [(300 K)] and resisitivity ratio [(4.2 K)/(300 K)] of PANI–CSA/PMMA blends at various volume fractions (f  of PANI–CSA. f 1 0.67 0.33 0.12 0.04 0.012 0.008 0.003 0.001

(300 K)(S/cm)

(4.2 K)/(300 K)

200–400 110 21 9 1.8 0.22 0.12 0.003 10−4

1.3–10 13 19 30 210 2200

Source: Reprinted with permission from [186], R. Menon et al., Phys. Rev. B 50, 13931 (1994). © 1994, American Physical Society.

Figure 5. Resistivity versus temperature at various volume fractions. Reprinted with permission from [186], R. Menon et al., Phys. Rev. B 50, 13931 (1994). © 1994, American Physical Society.

724 of thermally activated hopping transport among localized states at various energy levels. Usually in disordered systems, the low temperature dependence of resistivity due to hopping transport is exponential in behavior {i.e.,  T  = 0 exp T0 /T  , where  = 1/ d + 1 and d is the dimensionality of the system, and T0 = q/kB N EF L3c , where q is a numerical coefficient, N EF  is the density of states at the Fermi level, Lc is the localization radius of states (rate of fall-off of the envelope of the wavefunction of localized states) near the Fermi level}. As temperature decreases, the average hopping length r ∼ Lc T0 /T   increases as T −1/4 ; hence, this type of transport is usually referred as variable range hopping (VRH). In VRH, a carrier just below the Fermi level jumps to a state just above it, and the farther it jumps the greater is the choice of states available, and usually it jumps to a state for which the energy !E ∼ 4 2r 3 N EF " required is as small as possible. Whereas in nearest-neighbor hopping [ = 0 exp A/kB T ] with a constant activation energy A, the average hopping length is on the order of the mean separation between localized states, and it does not vary with temperature. The value of T0 in VRH gives an estimate of how far the system has moved to the insulating side. Moreover, the localization length can be determined from T0 , which gives some idea about the length scale of the localized wavefunctions and the extent of disorder present in the system [13]. The values of  and T0 give some idea about the hopping transport mechanism in disordered systems. However, for the convoluted and multiply connected PANI–CSA network, the complex fractal morphology plays a significant role in the hopping mechanism. For example, in these systems the value of  systematically increases from 0.25 to 1 upon varying f from 0.012 to 1, which is not expected in the conventional VRH model. Nevertheless, this behavior is expected in systems in which the fractal structure modifies the wavefunctions of the localized states, and accordingly  varies as the fractal dimensionality of the system changes. Details about the charge transport mechanism in fractal structures, as in PANI–PMMA, are described in [186]. Hence, conducting polymer nanocomposites offer the possibility of investigating novel charge transport mechanisms in fractal network structures. Among various charge transport properties, magnetoresistance (MR), the change of resistance as a function of magnetic field, which especially occurs at low temperatures, is well known for providing additional information on the nanoscopic scale charge transport mechanism. In particular, this gives insight into the underlying relationship between nanostructure and scattering and relaxation mechanisms of charge carrier dynamics. The classical transverse MR is mainly due to the bending of the charge carrier trajectory by a Lorentz force, and it is proportional to the square of the field, with the proportionality constant expressed as a function of charge transport scattering time [185]. In typical metals the dominant contribution to the weak positive MR (usually less than a 5% increase in resistance at 4.2 K) is due to the classical orbital motion of charge carriers. Moreover, the MR gives information about the second derivative of the density of states at the Fermi level. Because MR probes the local charge carrier dynamics in conducting systems, the MR

Polyaniline Fractal Nanocomposites

data can be used to determine the nanoscopic transport property parameters, for example, the elastic and inelastic scattering length and scattering time. Furthermore, the MR results are supplementary to conductivity data; hence it is essential to check the internal consistency of the models used to understand the charge transport properties. The MR data at 4.2 K for PANI–PMMA nanocomposites are shown in Figure 6 [187]. The normalized variation in MR is plotted as a function of H2 for PANI–PMMA samples with 0 015 < f < 0 004 that are just above the percolation threshold, and in these the fractal structure is observed to be prominent. The H2 dependence of the positive MR at low fields is due to the shrinkage in the overlap of wavefunctions of localized states in the presence of a magnetic field, which is usually observed in VRH transport. The positive MR decreases considerably by lowering f from 0.015 to 0.004, as shown in Figure 6. The MR shows a maximum at f ∼ 0 015 at 4.2 K. As the system approaches the percolation threshold, the connectivity of the network decreases; as a result the carriers find it increasingly difficult to hop from site to site in the presence of the magnetic field, and the value of MR decreases considerably for f < 0 015. Whereas for f > 0 015 the connectivity increases at higher volume fractions, and the effect of field on hopping transport gradually decreases; hence the value of MR is less. This shows that MR is an ideal probe to investigate the role of nanoscopic level morphology in the charge transport properties in conducting polymer nanocomposites. The detailed analysis of MR results is described in [186]. Thermoelectric power [or thermopower S] is another phenomena to explore the charge transport mechanism in metallic and semiconducting systems. In the thermoelectric effect or Seebeck effect, a temperature gradient at the

Figure 6. Magnetoresistivity versus square of magnetic field for f = 0 015 •, 0.012 ∗, 0.01 (+), 0.008 (
), and 0.006 (). The inset shows magnetoresistivity versus volume fraction at 4.2 K. Reprinted with permission from [186], R. Menon et al., Phys. Rev. B 50, 13931 (1994). © 1994, American Physical Society.

725

Polyaniline Fractal Nanocomposites

ends of a sample gives rise to an electric field opposite to the direction of the temperature gradient [192]. Usually the sign of thermopower is consistent with a positive or negative charge of the carrier. The diffusion thermopower (Sd  in metallic systems is a function of the first derivative of density of states at Fermi level. Although the temperature dependence of thermopower [S T ] is supposed to be linear in metals, it is usually complicated by scattering processes. However, in high quality metallic conducting polymers a quasilinear temperature dependence of thermopower has been observed down to 8 K, which is quite remarkable when the extent of disorder present in these systems is considered. In several conducting polymer composites a linear temperature dependence of thermopower has been observed, but only down to 100 K [13]. The S T  of PANI–PMMA nanocomposites is shown in Figure 7 [193]. The thermopower at room temperature is nearly 8 V/K for all volume fractions, and it is positive for T > 80 K. The data show the typical metallic linear temperature dependence over a wide range of volume fractions of PANI–CSA in PMMA; however, there is a clear deviation from linearity below 100 K, and this deviation increases as f is lowered. The characteristic U-shaped dependence of S T  below 100 K indicates the onset of the breakup of the conducting network into disconnected regions, and the thermally activated hopping transport [Shop T  ∝ T 1/2 for VRH] across these regions dominates over the metallic diffusion thermopower [Sd T  ∝ T ]. Because thermopower is a zero current transport coefficient, it is relatively less sensitive to the electrical conduction paths between the disconnected regions, and this is mainly due to the fact that nanoscopic level thermoelectric voltages sum up together as long as the thermal gradients take place across the conducting paths. Hence investigating the low temperature U-shape in thermopower can give some insight into how well the conducting network is connected at the nanoscopic level. Thus S T  is another useful probe to investigate the correlation between

morphology and charge transport mechanism in conducting polymer nanocomposites.

5. APPLICATIONS Conducting polymer nanocomposites are used for a wide range of applications as indicated in the following [194]: 1. PANI nanocomposites have a wide range of applications for antistatic coating, electromagnetic interference shielding, and anticorrosive coatings. In particular, the fibrillar fractal network of PANI nanocomposite has high conductivity at low volume fractions of PANI and hence is ideal for these types of electrical applications. 2. PEDOT–PSS is most widely used by photographic film and electronics packaging industries. An antistatic coating of PEDOT–PSS on films (photographic/ electronics packaging) having a surface resistance less than 109 '/m2 , eliminates the risk of static charge buildup, which may damage the films. Interestingly, more than 100 million square meters of photographic films are coated by PEDOT–PSS every year. Also, PEDOT–PSS is widely used for transparent conducting electrodes [157]. 3. Conducting polymer-coated fibers such as polyester or nylon have applications in electromagnetically camouflaged nets in stealth devices [183]. 4. Conducting polymer composites with various nanocrystals have had interesting applications in light-emitting diodes, solar cells, lasing medium, and electrochromic devices. For example, MEH–PPV/perylene bis(phenethylimide)-based photovoltaic cells show an open circuit voltage of 0.58 V and their performance is better than that of MEH–PPV/CN–PPV-based photovoltaic devices [195]; PPV/carbon nanotube-based photovoltaic devices show a quantum efficiency of 1.8%, which is twice that of standard ITO-based devices, with an open circuit voltage of 0.9 V [19]. Nanocomposites based on MEH–PPV and TiO2 nanocrystals show higher quantum efficiency of 12% [16]. TiO2 dispersed in an MEH–PPV/polystyrene blend acts as a lasing medium that shows excited emission at wavelengths of 532 and 435 nm [177]. Blends of PPy and epichlorohydrin and ethylene oxide (Hydrin C) show excellent electrochromicity and are used in electrochromic devices [183]. 5. Networks of PANI blends filled with semiconducting polymers can be used as grid electrodes in a polymer grid triode [196]. The open structures of fibrillar fractal networks of PANI nanocomposite have large active surface area for devices. 6. Conducting polymer nanocomposites have shown potential for use in molecular sensors, biosensors, and molecular electronics applications [197, 198].

6. CONCLUSIONS Figure 7. Thermopower versus temperature for various concentrations (y of PANI–CSA in PMMA: y = 100% (•), 66.6% (♦), 33.3% (
), 9.09% (∗), 4.76% (+), 2.43% (), and 1.24% (). Reprinted with permission from [193], C. O. Yoon et al., Phys. Rev. B 48, 14080 (1993). © 1993, American Physical Society.

In conclusion, conducting polymer nanocomposites have broadened the horizon of conventional polymer composites. In usual polymer composites the physical and chemical properties of the composite are limited, and the volume fraction of guest material is above the percolation threshold

726 (16% volume fraction), which restricts the processibility and mechanical properties of the composite. Moreover, it is hard to attain uniform distribution of nano or meso particles in composites due to various problems with compatibility, and this usually leads to inhomogeneity in the physical properties. It is possible to overcome several of these shortcomings with the breakthrough in the self-assembled fibrillar fractal network of guest polymer chains in the host matrix. This is very well illustrated for the conducting PANI–CSA fibrillar fractal network in a PMMA matrix in which the nanoscale morphology and the associated physical properties can be controlled rather well. The connected network forms at a volume fraction of PANI–CSA as low as 0.3%, with conductivity values of ∼10−3 S/cm. The network is quite robust in that it remains well connected even after the removal of the host polymer. This large open area conducting fractal network is quite useful as an electrode in various applications, for example, in polymer grid triodes. It is possible to obtain excellent control of the electrical and optical properties of this network by subtle variations in the composition of the nanocomposite as demonstrated by the temperature dependence of conductivity, magnetoresistance, thermoelectric power, and optical transmission features. Further molecular scale engineering and functionalization of conjugated polymer chains, and fabrication of this type of self-assembled fractal networks can widen the scope of this category of nanostructured systems, which are rather unique compared with other types of nanosystems.

GLOSSARY Conducting polymer A long chain carbon polymer consisting of alternating single and double bonds, in which delocalized -electrons determine the physical and chemical properties. Fractal A system that looks self-similar at all length scales, in which the dimensionality has noninteger values [integer value means 1, 2 and 3 dimensions for Euclidean geometry]. Localization A phenomenon in which the wavefunctions associated with the electronic states get localized, to a few interatomic distances, by the random disorder potential fluctuations so that there is hardly any overlap with nearestneighbor wavefunctions. Lorentz force The force F experienced by a charge carrier q in electric E and magnetic field intensity B, and it is given by the following relation: F = q E + v × B where v is the velocity of charge. Magnetoresistance The variation of resistance as a function of magnetic field. It is mainly due to the bending of charge carrier trajectory by Lorentz force, which is usually proportional to the square of the field, with the proportionality constant expressed as a function of scattering time. Nanocomposite A nearly homogenous mixture of two or more components, in which the physical size of components is typically below 100 nm, and this imparts special sizedependent properties. Percolation threshold The minimum volume fraction of guest component in a composite, at which the physical properties increase by several orders of magnitude due to the connectivity of guest material. For example, for conducting

Polyaniline Fractal Nanocomposites

spherical particles, at around 16% volume fraction, the electrical conductivity increases by several orders of magnitude. Resistivity A basic property of a material that signifies the measure of resistance offered by it to the flow of charge carriers. Resistivity () is defined as  = R A/l, where R is the resistance, l is the length of the sample, and A is the cross-sectional area. The SI unit is ' · m. Thermopower In the Seebeck effect, a temperature gradient at the ends of a sample gives rise to an electric field in the opposite direction as the temperature gradient. It is a measure of the rate of change of potential difference across a material as a function of unit change in temperature gradient.

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