Polypyrrole and polypyrrole/wood-derived materials conducting composites: A review

Sasso, C., Beneventi, D., Zeno, E., Chaussy, D., Petit-Conil, M., and Belgacem, N. (2011). "Polypyrrole and polypyrrole/wood-derived materials conducting composites: A review," BioRes. 6(3), 3585-3620.

Abstract

Wood and cellulose derivatives, in both fibrous and water-soluble macromolecular form, are emerging as outstanding candidates for organic electronics applications due to their large-scale availability, low cost, and easy processability. Paper and wood fibre-based derivatives are considered to be materials of choice as supports for communication world-wide. The interest in producing inexpensive and universally available conducting polymer/cellulose fibres substrates resides in the possibility of creating new materials that can be used for a broad range of advanced applications. For instance, PPy/cellulose fibres composites can be used for the preparation of energy storage devices thanks to the conjugation of the high specific area of cellulose fibres and the electrochemical properties of PPy. Other possible applications of such composites are in the area of the antistatic materials, sensors, electromagnetic interference shielding materials, smart packaging, and tissues. Concerning the woody polymers, some of them (i.e. cellulose derivatives) also exhibit biocompatibility, as well as film-forming properties and transparency. In combination with the electrical properties of PPy, these features make PPy/macromolecular cellulose composites suitable for applications as displays, lighting, and photovoltaics. Due to their chemical structure, macromolecular wood derivatives have been proposed with success as enhancing conductivity additives in Py polymerisation. The aim of the present review is to provide an overview of PPy chemistry and of the most relevant advances attained in the production of PPy/wood derived materials conducting composites.

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POLYPYRROLE AND POLYPYRROLE/WOOD-DERIVED MATERIALS CONDUCTING COMPOSITES: A REVIEW

Claudia Sasso,^a,b,§ Davide Beneventi,^a,* Elisa Zeno,^b Didier Chaussy,^aMichel Petit-Conil,^b and Naceur Belgacem ^a

Keywords: Polypyrrole; Paper; Conductivity; Wood derived materials

Contact information: a: LGP2, UMR 5518 CNRS-Grenoble-INP, 461 rue de la Papeterie, Domaine Universitaire, BP 65, 38402 St. Martin d’Hères, France; b: Centre Technique du Papier, Domaine Universitaire, BP. 251, 38044 Grenoble CEDEX 9, France; §: now with EDF – DPI – DAIP – UFPI – Dampierre-en-Burly Nuclear Power Plant, France.

*Corresponding author: davide.beneventi@pagora.grenoble-inp.fr

Polypyrrole

The aqueous synthesis of polypyrrole as an insoluble black precipitate, formerly known as pyrrole black, from an acidic pyrrole/H₂O₂ solution, was documented for the first time by Angeli (1916). However, PPy semiconducting properties have been highlighted only in the pioneering works of McNeil et al. (1963) and Dall’Olio et al. (1968) who synthesised PPy by the pyrolisis of tetraiodopyrrole and electrolytic oxida-tion of pyrrole in a H₂SO₄ solution, respectively. At first, the electrochemical synthesis, as well as the ability to obtain highly conducting PPy films, triggered an enormous interest (Diaz et al. 1979, 1981; Kanazawa et al. 1981; Genies et al. 1983) and the cationic radical synthesis in water or organic solvents was disregarded, owing to the insolubility of PPy precipitates and their low processability (Angeli 1916; Salmon et al. 1982). More recently, the use of anionic surfactants during pyrrole chemical polymerisation (Lee et al. 1995, 1997; Oh et al. 2002) made it possible to obtain soluble polypyrrole that can be easily used for casting conducting PPy films. These works demonstrated that soluble and water dispersible PPy can be obtained by chemical polymerization, thus opening the way to a new broad range of applications. For this reason, the latest literature on the elaboration and use of chemically synthesised PPy is very abundant. In order to match with the aim of this review and to provide a background for the comprehension of PPy/wood derived materials elaboration, this section focuses only on fundamental aspects related to PPy aqueous chemical synthesis.

Aqueous Phase Py polymerisation

Polymerisation mechanism

Polypyrrole can be obtained mainly through cationic radical, electrochemical, or vapour-phase polymerisation. Among these methods, the cationic radical synthesis is the only way allowing the production of high quantity massive conducting polymer and the most promising for industrial applications.

Py polymerisation occurs through the oxidation of the monomer, via a pseudo-polycondensation mechanism (Fig. 1) (Planche et al. 1994; Reinisch et al. 1979).

Fig. 1. Pyrrole polymerisation mechanism

In the initiation step, monomers are oxidised and radical cations are formed. Then, these radical species couple through deprotonation, forming soluble bipyrroles.

Then, in the propagation step, bipyrroles are oxidised again, and oligomers with radical cations are produced. The further coupling of these species leads to the formation of higher oligomers and of an extended conjugated polymer (Skotheim and Reynolds 2007). Owing to the solubility of Py oligomers with polymerization degree DP_n < 30 (Appel et al. 1999), polymerisation starts in the continuous liquid phase and, when Py oligomers are no longer soluble, solid PPy clusters form in the polymerisation medium. The polymerisation will continue mainly at the surface or inside the solid polymer particles, depending on the activation of surface sites and on the diffusion of the reactive species within the formed polymer. In this case, diffusion deals with the movement of the initiator toward the monomer (and vice-versa) but also with the self-diffusion of the monomer towards the system (solvent, monomer, polymer,…).

Polymerisation kinetics

The kinetics of Py aqueous cationic radical polymerisation depends on the oxidant, the monomer/oxidant ratio, and the temperature. For short-time polymerisation, i.e. less than 1 hour (or mild oxidants), it is generally found that the reaction follows pseudo-first-order kinetics, i.e. , where [Py] is the monomer concentra-tion at time t, [Py]₀ is the initial Py concentration, and k is the kinetic rate constant (Ferrero et al. 2006; Planche et al. 1994; Cavallaro 1999; Stankovic et al. 1995).

Even though long (up to 12-24 hours) polymerisation times have often been applied (Song et al 2004; Oh et al. 2001; Lee et al. 2000), the kinetics of such systems have been poorly analysed in the literature. The principal reason is that the complex reaction mechanism of PPy is not yet fully understood. This is even more of an issue for long polymerisation times, during which different side reactions can occur. The complexity in determining long-time pyrrole polymerisation rates derives principally from the polyphasic system and the difficulty in evaluating Py functionality.

During polymerisation, different species can act as oxidants, in addition to the oxidising agent voluntary introduced for the polymerisation. It was reported by Novak (1992) that during aqueous polymerization, PPy chains can be oxidised by OH^– ions, and also they can undergo some direct reactions with oxygen. Other species, deriving mainly from the dissociation of the principal oxidant (as HSO₄^– for ammonium peroxodisulfate, APS), are supposed to attack the monomers (Blinova et al. 2007). This is particularly true for long polymerisation times.

Moreover, pyrrole polymerisation does not proceed linearly. At the beginning of the polymerisation, the electrical charge within the pyrrole ring allows linear α-α’ linkages to form, leading to linear molecules. With the polymer growth, the monomer charge is differently distributed in the ring, thus allowing the activation of β’ sites. So α-β’ linkages between the growing chains are created, and the molecules can develop three-dimensional structures. Moreover, to counterbalance the charge, the added monomers (or chains) can have different inclinations, giving rise to distorted arrangements, such as cones or helices (Schmeisser et al. 1993; Appel et al. 1999; Naarmann et al. 1998). These facts render the functionality of the pyrrole ring and the polymerisation degree of the final polymer difficult to estimate, which, as schematized in Fig. 2, is reflected by the generation of insoluble cross-linked structures with reduced conjugation length and electron conductivity. Few data on PPy molecular weight (Oh et al. 2001) indicate that in the presence of a conductivity enhancer and a long polymerisation time, i.e. di(2-ethylhexyl) sulfoccinate and 20 h, respectively, a DP_n of 303 can be attained. Other studies (Lee et al. 1995; Song et al. 2000) did not succeed in estimating the DP_n of PPy synthesised in the presence of dodecylbenzenesulfonic acid and concluded that PPy was composed mainly by high oligomers with a degree of polymerisation too low to be detected by viscosity and dynamic light scattering measurements.

Factors affecting pyrrole polymerisation

An oxidising agent is a chemical compound capable of withdrawing electrons. It must enable the polymerisation initiation and propagation. Typical oxidants for Py polymerisation are metal transition salts (Chao and March 1998), halogens (Neoh et al. 1989, Kang et al. 1986), and persulfates (Khulbe et al. 1982; Malinauskas 2001). Depending on its oxido-reduction potential, the oxidant affects the polymerisation kinetics and consequently PPy conductivity and morphology (Xing and Zhao 2007). Among the oxidants, iron chloride (FeCl₃), and ammonium peroxodisulfate (APS, (NH₄)₂S₂O₈)_, are two of the most widely studied. Generally, the former, having a redox potential lower then the latter (persulfate: 2.01 eV; Fe(III): 0.77 eV, both in standard conditions), induces slower and more controlled reactions (Xing and Zhao 2007; Mandal and Mandal 1999).

The oxidant performance in terms of PPy final conductivity depends strictly on the Py/oxidant ratio (Armes et al. 1987). An excess of oxidant is necessary to guarantee the electron withdrawal from Py, but this excess must not be higher than a certain level in order to avoid degradation of PPy chains (over-oxidation).

Fig. 2. Schematic representation of reticulated PPy and possible chemical and conformational defects

In chemical synthesis, the doping process occurs simultaneously with the chain oxidation. In fact, the decomposition of the oxidant, particularly with salt-like oxidants, frequently gives rise to ions that act as dopant. In the case of FeCl₃ and APS, the dopants are chloride (Cl^–, ClO₄^–,…) and sulfate (SO₄^–) derivative ions, respectively (Beneventi et al. 2006; Blinova et al. 2007).

It was recently shown that the use of macromolecules in PPy synthesis led to an improvement in the polymer conductivity. These macromolecules are co-dopants, or additives (normally called dopants), and they induce also morphological/chemical modification of the polymer (see the conductivity enhancers section).

Determining the optimal polymerisation time means controlling the balance between the development of conjugated system (which length is associated with the conductivity), and its over-oxidation (i.e. the disruption of conjugation) (Novak 1992; Thieblemont et al. 1994; Dall’Acqua 2004). In fact, it was found that by modifying only the polymerisation time, orders of magnitude variations in conductivity were detected. Indeed, the importance of polymerisation time in obtaining highly conducting PPy is correlated to the electrochemistry of the oxidant. Oxidants with high redox potential would quickly oxidise Py, but they also rapidly degrade the newly formed PPy chains and promote PPy interchains cross-linking. In this case, time can greatly affect PPy conductivity. By contrast, the use of less aggressive oxidants makes time a secondary parameter.

It is usually accepted that the best PPy conductivities are obtained with lower temperatures (Fig. 3) (Lee et al. 1997). Temperature influences both the polymerisation kinetics (consequently the regular polymer development), and the number of side reactions (Kudoh et al. 1998). At low temperature, the kinetics are slowed down, and a quite linear PPy is obtained. The charge carrier movement, in this case, is favoured due to the regularity of conjugation.

By contrast, when temperature is increased, the kinetics are accelerated. In this case, many sites in the Py ring are activated, leading to extensive PPy cross-linking and to a three-dimensional polymer growth. In such an irregular structure, charge mobility is impeded. With temperature, the number of side reactions (between oxidant species present in the polymerisation reactor and the PPy chains) increases, thus inducing the possibility of interruption of conjugation (Kassim et al. 2002).

Conductivity enhancers.

Additives can be used during Py polymerisation with the aim of providing specific properties to the polymer. These additives, usually surfactants or polymeric stabilizers, can improve PPy conductivity, thermal and air stability, polymerisation yield, and PPy particles morphology.

The efficiency of an additive in enhancing conductivity is strictly associated with its chemical/electrochemical characteristics. Anionic additives display better performances than their cationic or non-ionic homologues (Omastova et al. 1998; Shen and Wan 1998, Zhang et al. 2006). This can be attributed to the fact that negatively charged molecules interact with PPy by counterbalancing their positive charge and remain trapped within PPy clusters. Moreover, they act as counterions (as dopants), thus supplying an additional doping action and improving the conductivity. On the other hand, cationic and non-ionic additives, due to their positive or null charge, have repulsive and weak attractive ion-dipole interactions and do not act as PPy counterions. Therefore, the final electrical properties of PPy are not altered. Among the anionic additives, alkyl sulfonates, particularly alkyl benzene sulfonates, are the most effective additives in term of conductivity improvement (Shen and Wan 1998; Kudoh 1996; Oh et al. 2001). PPy pellets with conductivity up to 100 S/cm have been obtained by synthesising the polymer with APS and toluenesulfonic or butylbenzenesulfonic acids (Song et al. 2004).

It was demonstrated that molecular weight and steric hindrance affect the additive’s electroactivity towards PPy (Shen and Wan 1998). Additives with lower molecular weight (or shorter alkyl chain length) can diffuse more easily in the medium, thus well approaching the growing PPy chains and counterbalancing the polymer charge. Moreover, with a minor steric hindrance, they allow PPy short range order arrangement, thus resulting in an elevated final conductivity (Song et al. 2004).

A positive contribution to conductivity will be noticed until a certain additive concentration, before the tendency is inversed. In fact, for each system an optimal concentration leading to a maximum in conductivity can be found. Beyond this value, the system becomes overcharged and the presence of non-conducting elements hinders the conductivity: PPy is thus over-oxidised and degraded, and the additive hindrance towards PPy impedes the charges movement (Omastova et al. 2004).

Being incorporated to PPy, additives induce an increase of the polymerisation yield. Moreover, anionic additives (i.e. aromatic sulfonates or benzene sulfonate surfactants) are permanently incorporated into PPy, and their hydrophobic moieties protect PPy-additive ionic bonds with a subsequent improvement of PPy thermooxidative and moisture stability (Kudoh 1996, Omastova et al. 2003, Ansari 2006) . In Table 1 the most frequently used additives with relative PPy pellets conductivity are reported.

Table 1. The Most Widely Used Additives and Relative PPy Pellets Conductivity

PPy particle size, shape and compactness can be modified by changing the additive type (Shen and Wan 1998) and the surfactant concentration (Prissanaroon et al. 2000). In most of the cases, globular, cauliflower-like PPy is obtained (Omastova et al. 1998; Lu and Pich 2004), thus showing that anionic additives (surfactants) mainly act as conductivity enhancer with a limited action on the PPy nanostructure. On the other hand, despite a null doping action, cationic surfactants (i.e. dodecyl- and cetyltrimethyl-trimethylammonium bromide) above the critical micellar concentration can form lamellar mesophases with the anions of the oxidising agent that act as templates for the formation of PPy nanofibers, as shown in Fig. 4 (Zhang et al. 2006).

PPy processability improvers

Among all the advantages of using additives, the increase of PPy processability has been one of the most attractive features. In fact, the PPy obtained after chemical synthesis is in the form of a black solid, insoluble in most of the solvents and unsuitable for moulding. Poor processability is, in fact, the key factor limiting the large-scale use of PPy.

Fig. 4. PPy morphologies obtained by varying cetyltrimethylammonium bromide concentrations (left side pictures) (a)>(b)>(c), and by using different chain length surfactant salts (right side pictures): cetyltrymethylammonium bromide (a), dodecyltrymethylammonium bromide (b), octyltrymethylammonium bromide (c). Reprinted with permission from (Zhang et al. 2006). Copyright 2006, John Wiley and Sons.

Additives can help to overcome this problem in two principal ways: the first deals with the increase of PPy solubility, whereas the second concerns the production of PPy nanoparticles dispersible in many solvents or blends. The PPy insolubility originates from the strong intra- and inter- chains interactions, which impede polymer solvatation. When large molecule additives are used, they insert within the conducting polymer chains, thus reducing their interactions, and opening the PPy backbone (Lee et al. 1995). To render PPy soluble in different solvents, various additives can been used.

Surfactants, such as alkyl benzene sulfonic acids, make PPy soluble in most of the organic weakly polar sovents (m-cresol, chloroform, n-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO) (Shen and Wan 1998a; Shen and Wan 1998b; Song et al. 2000; Lee et al. 1997).

More polar additives (such as di(2-ethylhexyl) sulfosuccinic acid) are necessary to improve PPy solubility in alcohols (Oh et al. 2001). Shen and Wan demonstrated that additives with an important steric hindrance or with a long alkyl chain are more successful in solvating PPy when compared with smaller homologues (Shen and Wan 1998a) (Table 2). In all the cases, the PPy solubility depends on Py/oxidant and Py/additive ratios.

PPy nanoparticles (30 to 300 nm) dispersible in many solvents can be obtained by polymerising Py with a suitable polymeric stabiliser.

Table 2. Different Alkyl Benzene Sulfonic Acids Used for Py Polymerization **

These stabilisers can be commercially available, i.e. polystyrene sulfonate (Qi and Pickup 1997), polyvinyl alcohol (Armes et al. 1987; Men’shikova et al. 2003), or “tailor made”, i.e. poly(2-vinyl pyridine-co-butyl methacrylate) (Armes et al. 1990). The stabiliser can be both adsorbed or segregated, during Py polymerisation, on PPy particles surface, and these core-shell particles are prevented from agglomerating by the additive’s steric hindrance (Simmons et al. 1995). In this way, PPy colloidal dispersions can be redispersed in solvents in which the additive is soluble. Nevertheless, an excess of stabilizer can induce the encapsulation of PPy by a non conductive shell with a subsequent drop in charge transfer between PPy particles and conductivity (Sasso et al. 2008).

Defects in PPy

Even if the concentration of charge carriers in the doped PPy (as in most of the conducting polymers) is higher (two-fold to five-fold) than that of the inorganic semiconductors, the conductivity is approximately of the same order of magnitude. This depends, above all, on the higher defects concentration. These defects trap the electrons (in a potential hole), and, in many cases, the energy required to pass over the defect barrier is so high that carriers are stopped, and they cannot contribute any more to conductivity.

In PPy, defects are formed prevalently during the polymerisation, and they deal with conformational and chemical aspects. Concerning the conformational defects, out-of-plane bonding leads to a reduced π-orbital overlap, thus creating an obstacle to the regular sequence of single and double bonds. Chemical defects mainly include 2-3’ and 2-4’ couplings, and the formation of carbonyl and hydroxyl groups due to overoxidation (Skotheim and Reynolds 2007). Possible defects in the PPy chain are illustrated in Fig. 2.

PPy films

Electrochemically Prepared PPy Films

Electrochemical PPy synthesis allows a direct film formation. It is indeed the most wide-spread and highly performing technique in term of conductivity. In fact, with this method it is possible to produce smooth, regular PPy films with outstanding conductivities (10² to 10³ S/cm).

PPy films can be obtained without adding plasticizers, but their limitations in mechanical properties and difficulties in handling impede their use. So, to produce self-standing films it is necessary to overcome the PPy stiffness. For this purpose, two main methods have been reported. The first deals with the additives implementation during the polymerisation, thus obtaining homogeneous PPy-based composites. The second concerns the direct monomer polymerisation on a non-conducting substrate, thus producing composites with conducting-insulating interpenetrated networks.

The Py electropolymerisation with additives is indeed the most widely studied method because PPy-based composites with good conductivities (10 to 10² S/cm) and improved mechanical properties can be obtained (Skotheim and Reynolds 2007). Only the elongation at break remains inadequate. For instance, by electropolymerising Py with different alkylbenzene sulfonic acids, films with Young’s Modulus ranging from 0.1 to 1 GPa, a maximum elongation at break of 4% and a maximum conductivity of 90 S/cm, were produced (Bay et al. 2002).

PPy/non-conducting polymer composites can also be produced by monomer polymerisation on a non conducting substrate, i.e. polyvinyl chloride (Paoli et al. 1985); polyvinyldenefluoride-trifluoroethylene copolymer (Niwa et al. 1987); polytetramethyl-bis-phenol A carbonate (Kalaycioglu et al. 1996); polystyrene (Wang et al. 1990); or polyimide (Tieke and Gabriel 1990). This leads to the formation of layered structures with heterogeneous PPy concentration along the composite (Fig. 5). The process is based on the Py diffusion into the polymer film through the channel formed by the matrix swelling. The polymerisation starts at the interface between the polymer and the electrode, then it grows within the film, forming a so-called conducting alloy (Niwa et al. 1987).

Depending on the matrix, chemical interactions between PPy and the binder are formed during polymerisation. As a result, composites having properties that are different from the simple blend of the two polymers are produced (Kalaycioglu et al. 1996). With this technique, free-standing films having conductivity varying from 5 to 50 S/cm can be obtained (Kalaycioglu et al. 1996; Tieke and Gabriel 1990; Selampinar et al. 1994).

Despite the high conductivities reached by the films, the low quantities obtained in a given operation limit electropolymerisation use to the laboratory scale. Moreover, in the case of the alloy composites, the difficulty in controlling the PPy growth, mixing with the matrix, and the heterogeneity of the products obtained impede their large-scale production.

Fig. 5. PPy/non conducting film alloy: PPy growth occurs from the electrode to the film surface in a non homogeneous way. Reprinted with permission from (Niwa et al. 1987). Copyright 1987, American Chemical Society

Chemically Prepared PPy Films

To overcome the large-scale production problems encountered with the electrochemical PPy films production, several methods have been tested in the last decades. They were all based on the use of chemically synthesised PPy.

PPy appropriately modified (polymerisation with additives or chemical substitution) can be solubilised or dispersed in different solvents, and films can be obtained by casting technique or by spin coating in high quantities.

Depending on the solvent used, films obtained by spin coating PPy solutions on glass exhibit conductivities ranging from 10^-2 to 2 S/cm after solvent removal. In fact, the solvent modifies the interactions strength between the polymer and the counterions during the drying process, thus affecting the PPy chains stacking and the charge hopping between them. Song et al. (2000) demonstrated that, starting from the same PPy particles (1 S/cm), films obtained from dimethylsulfoxide and chloroform showed a difference in conductivity of two orders of magnitude, being the conductivity values 1.6×10^-2 S/cm and 1.7 S/cm for the two solvents, respectively. Similarly, not volatile additives can act as secondary dopants, remaining included in the PPy chains and modifying PPy stacking and conductivity (Lee et al. 2000).

In general, films obtained from soluble PPy are less rigid than those prepared electrochemically. They present a lower modulus, but improved elongation at break (Lee et al. 2000). Indeed, the enhanced flexibility of the PPy chain, produced by the additive insertion, favours the production of more flexible films. Compared to their homologues obtained electrochemically, they present a smoother surface (Song et al. 2000).

Despite the possibility of a large-scale production, this technique presents some drawbacks, mainly due to the low conductivity achieved and the extensive use of polluting organic solvents. Indeed, leaving aside the film forming stage, the synthesis of unsubstituted pyrroles can be carried out in water by the condensation of 1,4 dicarbonyl compounds with primary amines, also known as the Paal-Knorr method (Amarnath et al. 1991; Gilchrist 1998) or in vapour phase, where ammonia and furan (a furfural deriva-tive) vapors come into contact with an acidic catalyst, i.e. Al₂O₃ (Elming and Clauson-Kaas, 1952, Harreus, 2002). On the other hand, substituted pyrroles, which are used as precursors for the polymerisation of soluble PPy, are synthesised in organic media (Amarnath et al. 1991).

To limit the use of organic solvents, other techniques, involving the production of PPy/non-conducting polymer composites, have been proposed. In this way, besides the process environmental improvement, composites having specific properties, particularly in mechanical terms, can be obtained (Cassignol et al. 1999).

PPy direct synthesis on a preformed polymeric matrix allows the production of composites having improved mechanical properties through a low-polluting process. These composites can be achieved by dipping the conventional polymer film (containing the oxidant) onto the monomer solution (impregnation method) (Mandai and Mandai 1996; Wang and Fernandez 1992; Chakraborty et al.1999; Migahed et al. 2004; Yang et al. 1996; Bhattacharyya et al. 2000), or by exposing it to monomer vapours (vapour-phase method) (Bleha et al. 1999; Radhakrishnan et al. 1995). The use of volatile solvents such as tetrahydrofuran (THF) (Radhakrishnan et al. 1995; Chakraborty et al. 1999) or acetone (Wang and Fernandez 1992) is usually limited to the casting of the polymeric film.

Interpenetrated polymer network (IPN) composites, in which the PPy network is intimately interconnected with the matrix, are thus formed. Typical binder used are: poly(vinyl acetate) (Chakraborty et al. 1999); poly(vinyl alcohol) (Benseddik et al. 1995); poly(methyl methacrilate) (Mandai and Mandai 1996); poly(imide) (Tieke and Gabriel 1990); poly(vinyl methyl ketone) (Chakraborty et al. 1999); and poly(ethylene) (Radhakrishnan et al. 1995). As in the case of PPy/polymer alloys obtained electro-chemically, PPy diffuses within the channel formed by the swelling, and it grows within the matrix, forming a capillary three-dimensional network. But, in contrast to the above mentioned alloys, in this case more homogeneous films are obtained.

For these composites it was reported that the percolation threshold ranges between 2 and 10% (Mandai and Mandai 1996), which is much lower than the value predicted for conventional blends, namely 16% (Zallen 1983). The dendritic distribution of the conducting polymer is, in fact, much more anisotropic in this case if compared with that in a random blend.

The conductivity for this kind of composites is normally between 10^-4 and 1 S/cm, which is significantly lower than those obtained for their electrochemically synthesised homologues. Only Yang et al. (1996) obtained PPy/polypropylene (PP) composite films with conductivity up to 6.7 S/cm. A microporous PP membrane was laminated between two solutions: one containing Py and dodecyl benzene sulfonic acid, while the other was based on FeCl₃. Conductivities varying from 0 to 6.7 S/cm were obtained by modifying the polymerisation parameters (such as FeCl₃ and Py concentration, and polymerisation time). Such films presented remarkable mechanical properties: Young’s modulus of 0.4 to 0.8 GPa, and elongation at break of 90 to 200%.

Another technique that greatly reduces the requirement to use polluting solvents and at the same time makes it possible to produce freestanding well-conducting PPy composite films is the compression moulding of PPy/insulating polymer core/shell particles (Fig. 6) (Omastova et al. 1996, 1998, 1999; Jang and Oh 2005). PPy is synthe-sised in a water (Yang et al. 1996) or water/methanol (Omastova et al. 1999) solution containing the insulating polymer (or a surfactant), and the collected particles are then pressed at high temperature to obtain handling composites with conductivity up to 10 S/cm. The composite electrical properties can be increased by doping PPy with iodine during its synthesis, thus obtaining conductivities up to 116 S/cm (Jang and Oh 2005).

Finally, a method presenting several advantages in the large-scale PPy production over the above mentioned techniques is mixing. Preformed PPy particles are mixed to a melt/dissolved non-conducting polymer matrix (Hacarlioglu et al. 2003; Cassignol et al. 1999; Bhat et al. 1999; Balci et al. 1995), and after solvent drying, PPy/insulating polymer blends are obtained.

With this method, homogeneous conducting PPy-based films with no limitation in thickness and mechanical properties can be produced, depending on the matrix chosen. Concerning the conductivity, it was demonstrated that the particle size, particularly the particle tendency to agglomerate, greatly influences the percolation and consequently the conductivity of the final film (Cassignol et al. 1999). In fact, PPy particles produced with a steric stabiliser have a higher tendency to form clusters than particles obtained without additive. Once in the blend, the additive-issued PPy particles would agglomerate, thus increasing the percolation threshold in comparison with the additive-free particles. As a consequence, the dispersion method of the conducting particles in the matrix becomes essential to yield composite films with high conductivity. Vigorous stirring (with ultra-sound or turax) showed the best results (Fig. 7). Although the mixing technique leads to films with a lower conductivity (up to 10^-1 S/cm) compared to the electrochemically synthesised composites, it presents several advantages. From an industrial point of view, the described approach allows a massive production of PPy conducting films, employing more environmentally friendly conditions than the other techniques. Moreover, the process is simple and easy to implement (Table 3). A scheme of Py polymerisation procedures to produce the over mentioned film is shown in Fig. 8.

PPy Applications

Owing to its typical poor solubility, today PPy can find a marginal application in anticorrosion coatings (Tallman et al. 2002). Nevertheless, recent works showed that new synthesis routes can open to PPy a broad range of potential applications in different fields thanks to its electrochemical and conducting properties.

Table 3. Electrical and Mechanical Properties of PPy-based Films Obtained by Electrochemical and Chemical Synthesis with Relative Possibility of High-volume Production

Fig. 8. Summary of possible techniques for preparing PPy films

PPy or PPy-based composites have been proposed as electrode materials for energy storage devices (electrochemical batteries and capacitors), electrocatalysts and biosensors, photo-luminescent materials, artificial muscles, gas separating membranes, anticorrosive coatings, electromagnetic shields, materials for organic electronic devices (transistor electrode or active channel), and so forth. In this section, the working principles (and the PPy function) of the most promising applications are presented.

Sensors

A sensor is a device that transforms the concentrations of analytes to other detectable physical signals (currents, absorbance, mass, or acoustic variables). In the case of PPy, the interactions between the analyte and the polymer correspond to oxido-reduction reactions. These redox reactions are associated with a change in PPy doping level and consequently to a change in PPy conductivity (Bai and Shi 2007). This conductivity variation is detected and transformed into analytical results. A possible technology for a gas sensor is proposed in Fig. 9.

It was reported that PPy can be used as a sensor for certain components in gas, such as inorganic (NO, CO₂, CO, NH₃, H₂S) (Hernandez et al. 2007; Liao et al. 2005; Waghuley et al. 2008) and organic molecules (acetone, methanol, ethanol) (Melo et al. 2005). pH or vapour sensors can also be produced. Moreover, immobilization of active molecules on PPy, such as glucose oxidase (Sung et al. 2004; Ekanayake et al. 2007), has been used to develop bioanalytical sensors for glucose, DNA, and proteins sensing (Ramanavicius et al. 2006).

Actuators

An actuator is a device capable of converting electrical energy into mechanical work. PPy is suitable to produce actuators because it can show a dimensional change. In fact, depending on its oxidation state, counter-ions are attracted or expelled from the PPy film in order to maintain charge neutrality, thus inducing a volume variation (Skotheim 2007; Madden et al. 2000). So, when an electric field is applied, the cations and the anions would move through the anode and the cathode, respectively, thus producing a mechanical deformation of the film. The most developed field of application of PPy-based actuators is in biomedical components for the production of artificial muscles (Otero and Cortes 2003).

Fig. 9. Possible technology for a PPy-based gas sensor

Batteries

A rechargeable battery is a device that can convert chemical energy into electrical energy and vice-versa. It is composed by two electrodes, positive and negative, and an electrolyte. The two electrodes ensure the electron movement, while the electrolyte acts as a buffer for the ion flows between the two electrodes. Due to its capacity for charge storage (Song and Palmore 2006; Killian et al. 1996), in a rechargeable battery, PPy has been proposed either as a positive (Wang et al. 2005) or a negative (Guo et al. 2005) electrode (Fig. 10). It has been studied also as an electrode and electrolyte for capacitors (Kim et al. 2006b).

Organic thin film transistors (OTFT)

A transistor is a three-terminal device capable of switching or amplifying an electrical current. A field-effect transistor (FET), the most common type of transistor, is composed by three parts: an insulator, a thin semiconducting layer (the active channel), and three electrodes. The source and the drain electrodes (made of n-doped Si in n-type FET) act as conventional electrodes, ensuring the flow of charge carriers to the device. The gate electrode (made of p-doped Si in n-type FET), as a function of its potential with respect to the source, can form a conducting or an insulating channel between the source and the drain electrode (Klauk 2006). The flow of electrons (or electron holes) from the source to drain is controlled by the voltage applied across the gate. In this kind of device, PPy can be used as electrode, but also as an active channel, depending on the targeted conductivity (Bjorklund and Liedberg 1986; Kou and Liou 1996).