Polypyrrole

3 Principal ICPs Derivatives Used in Electroanalysis 2.3.1 Polypyrrole 

PPy is one of the most frequently used ICPs in electroanalysis [29]. This is due both to the ease of electropolymerization (also possible in aqueous solutions and within a wide pH range) and to the relatively high stability of the p-doped polymer under ambient enviromnental conditions. Furthermore, many studies reveal that this material is biocompatible [30], which constitutes a fundamental feature when considering sensors for some clinical applications. 

Among different ICPs, pristine PPy is characterized by very low band-gap values, resulting in the p-doping process occurring at relatively low potentials. The neutral polymer can be directly oxidized by oxygen present either in air or in aqueous solution. Although a majority of PPy applications in electroanalysis 

The lineshape of the spectrum obtained from oxidised and neutral forms of polypyrrole has aroused much interest. Albery and Jones [89], using the in-situ semi-annular tube, have presented a spectrum which they asserted to 

The work of Nechtstein and co-workers with in-situ cells demonstrates the further information available from such techniques and, alongside more recent conductivity data on the polypyrrole system [93], suggests that further examination of the bipolaron view of polymer conductivity is necessary. 

Because the oxidation potential of the polymer is lower than that of the monomer, the polymer is electrochemically oxidized into a conducting state, kept electrically neutral by incorporation of the electrolyte anion as a counter-ion. This is an essential since precipitation of the unoxidized, insulating polymer would stop the reaction. Both coulometric measurements and elemental analysis show approximately one counter-ion per four repeat units. An important feature is the fact that the polymerization is not reversible whereas the oxidation of the polymer is. If the polymer film is driven cathodic then it is reduced towards the undoped state. At the same time neutrality is maintained by diffusion of the counter-ions out of the film and into the electrolyte. This process is reversible over many cycles provided that the film is not undoped to the point where it becomes too insulating. It is possible to use it to put new counter-ions into the film, allowing the introduction of ions which are too nucleophilic to be used in the synthesis. The conductivity of the film for a given degree of oxidation depends markedly on the counter-ion, varying by a factor of up to 105. 

Polymerization of pyrrole on a platinum electrode in water requires an applied potential of at least 0.6 V v SCE. Okano et al.129) showed that this potential is lower on an illuminated n-Ti02 surface and that simultaneous electrolysis and irradiation through a mask could be used to generate conducting patterns with a line width of 45 pm. 

Pyrrole was first polymerised in 1916 [239, 240] by the oxidation of pyrrole with H202 to give an amorphous, powdery product known as pyrrole black, which was 

A large number of different pyrrole-based polymers have now been electrochemically synthesised, using a variety of conditions, and these are summarised in Table 2, although it should be noted that the size of this field and its rate of growth mean that it is impossible to make such a table completely comprehensive, and that reports of related new materials, particularly of copolymers incorporating pyrrole are continually appearing in the literature. Water-soluble polypyrroles have also recently been reported [246], 

Although pyrrole can be chemically synthesised [239-241,267,268], electropolymerisation is easily achieved and is the most common preparative method. This was first reported by Dall Olio et al. [242] who prepared brittle polymer films on Pt by 

Monomer (concn.) Electrode/solvent3 Electrolyte (concn.) 

Mixed solvent systems are shown as e.g. acn-aq (0.01 M) where the number in parentheses indicates the concentration of the lesser constituent 

Major polymer applications nonnietallic conductors, EMI shielding, battery electrodes, sensors, electronic displays, optoelectronic systems, capacitors, controlled release agents for other components 

Important processing methods Langmuir-Blodgen teclinique of monolayer production, solution polymerization over the substrate, electrochemical anodic polymerization, chemical oxidation of pyrrole in carbon black suspension 

Typical fillers silica, tin oxide, carbon black 

Typical concentration range carbon black - 10-85 ul%, silica - l-3ul% 

Anxiliary agents support materials such as PMMA chemical oxidant determines the size of particles in polypyrrole/silica nanocomposites  

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Selvan S T ef a/1998 Gold-polypyrrole oore-shell partioles in diblook oopolymer mioelles Adv. Mater. 10 132... 

Conductivities of polymers of technological interest such as polypyrrole and polythiophene are typically 1000 cm in the doped state, and the conductivity can be tuned by reversibly doping and undoping the polymer. Derivatives of these and other polymers have achieved even higher conductivities. 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Polypyrroles. Highly stable, flexible films of polypyrrole ate obtained by electrolytic oxidation of the appropriate pyrrole monomers (46). The films are not affected by air and can be heated to 250°C with Htde effect. It is beheved that the pyrrole units remain intact and that linking is by the a-carbons. Copolymerization of pyrrole with /V-methy1pyrro1e yields compositions of varying electrical conductivity, depending on the monomer ratio. Conductivities as high as 10 /(n-m) have been reported (47) (see Electrically conductive polymers). 

Because of its physical properties, polypyrrole has been cited as a unique building block for intelligent polymeric materials, ie, it has characteristics which make it capable of sensing, information processing, and response actuation (48). 

Common conductive polymers are poly acetylene, polyphenylene, poly-(phenylene sulfide), polypyrrole, and polyvinylcarba2ole (123) (see Electrically conductive polymers). A static-dissipative polymer based on a polyether copolymer has been aimounced (124). In general, electroconductive polymers have proven to be expensive and difficult to process. In most cases they are blended with another polymer to improve the processibiUty. Conductive polymers have met with limited commercial success. 

The enzyme can be immobilized on the electrode by several techniques (53). The simplest method, first used in 1962, is to trap an enzyme solution between the electrode surface and a semipermeable membrane. Another technique is to immobilize the enzyme in a polymer gel such as polyacrylamide which is coated on the electrode surface. Very thin-membrane films can be obtained by electropolymerization techniques (49,54,55) using polypyrrole, polyindole, or polyphenylenediamine films, among others. These thin films (qv) offer the advantage of improved diffusion of substrate and product that... 

Significant variations in the properties of polypyrrole [30604-81-0] ate controlled by the electrolyte used in the polymerization. Monoanionic, multianionic, and polyelectrolyte dopants have been studied extensively (61—67). Properties can also be controlled by polymerization of substituted pyrrole monomers, with substitution being at either the 3 position (5) (68—71) or on the nitrogen (6) (72—75). An interesting approach has been to substitute the monomer with a group terminated by an ion, which can then act as the dopant in the oxidized form of the polymer forming a so-called self-doped system such as the one shown in (7) (76—80). 

In all cases of electrochemicaHy or chemically polymerized unsubstituted polypyrrole, the final polymer is intractable in both the conducting and insulating forms. In contrast, a broad number of substituted polythiophenes have been found to be processible both from solution and in the melt. The most studied of these systems ate the poly(3-alkylthiophenes) (P3AT). 

Fig. 5. Evolution of optical spectra of polypyrrole during electrochemical doping.

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

Fig. 6. Thermal stability of epoxy-encapsulated poly(pyrrole tosylate) film,, 0.5 Q/sq, and polypyrrole-coated textiles, D, 20 Q/sq, with exposure to...

Entrapment of biochemically reactive molecules into conductive polymer substrates is being used to develop electrochemical biosensors (212). This has proven especially useful for the incorporation of enzymes that retain their specific chemical reactivity. Electropolymerization of pyrrole in an aqueous solution containing glucose oxidase (GO) leads to a polypyrrole in which the GO enzyme is co-deposited with the polymer. These polymer-entrapped GO electrodes have been used as glucose sensors. A direct relationship is seen between the electrode response and the glucose concentration in the solution which was analyzed with a typical measurement taking between 20 to 40 s. 

Poly(phenylenepyrazoles), 5, 300 Polypyrazoles, 5, 300 N-substituted, 5, 300 Polypyrazolines, 5, 300 Poly(pyrazol-l-yl) borates as ligands, 5, 225, 235 Polypyrroles applications, 4, 376 Polypyrrole tetrafluoroborate conductors, 1, 355 Polysaccharides as pharmaceuticals, 1, 152 Poly-2,5-selenienylenes applications, 4, 971 Polysilacyclopentanes, 1, 609 Polysufides macrocyclic... 

The polymers which have stimulated the greatest interest are the polyacetylenes, poly-p-phenylene, poly(p-phenylene sulphide), polypyrrole and poly-1,6-heptadiyne. The mechanisms by which they function are not fully understood, and the materials available to date are still inferior, in terms of conductivity, to most metal conductors. If, however, the differences in density are taken into account, the polymers become comparable with some of the moderately conductive metals. Unfortunately, most of these polymers also have other disadvantages such as improcessability, poor mechanical strength, instability of the doped materials, sensitivity to oxygen, poor storage stability leading to a loss in conductivity, and poor stability in the presence of electrolytes. Whilst many industrial companies have been active in their development (including Allied, BSASF, IBM and Rohm and Haas,) they have to date remained as developmental products. For a further discussion see Chapter 31. 

The polymers which have stimulated the greatest interest are the polymers of acetylene, thiophene, pyrrole and aniline, poly-p-phenylene, polyphenylvinylene and poly-l,6-heptadiyne. Of these materials polypyrrole has been available from BASF under the trade name Lutamer P160 since 1988. 

The poor stability on exposure to air and water, particularly at elevated temperatures, which results in a reduction in conductivity, also poses problems. In the case of polypyrrole it has been found that conductivity can, however, be maintained either by the drastic measure of storing under the protective layer of the inert gas argon or embedding polypyrrole film in a matrix of an epoxide resin-glass-fibre composite. 

Polypyrrole, poly thiophene, polyfuran, polycarbazole, polystyrene with tetrathi-afulvalene substituents, polyethylene with carbazole substituents, and poly-oxyphenazine as electrochemically active polymers for rechargeable batteries 97CRV207. 

Synthesis, properties, and applications of polypyrrole as a conducting polymer 97UK489. 

At this point it might be appropriate to comment on the conflicting requirements of the synthesis. The large interest which other conjugated polymers such as polypyrrole, polyanilinc or poly(parattracted originates, firstly, from their attractive physical properties, but also from the sim-... 

Polymetric matrix Polydiallyldimethylammonium bromide [9] Polypyrrole [10[ Poly (MA -dimethyb-S -pyrrolidinium bromide [11J Styrene-divinyl benzene copolymers [4] Polyacrylamide [12]... 

FIGURE 4-13 Structures of common polymeric coatings (a) Nafion, (b) polyvinyllferro-cene (c) polyvinylpyridine id) polypyrrole. 

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