Polypyrrole Subject

Although the electrochemistry of conducting polymers is now a quite mature subject, there is still considerable debate over most of the basic processes. In part, the issues have been clouded by the diversity of different polymers that have been studied. It is often assumed that conclusions drawn from data on a certain polypyrrole, for example, can be extended... 

The only polymer which has been subjected to systematic studies of mechanical properties is polypyrrole, and even here there are few data. Increasing the polymer molecular weight would be expected to lead to improved toughness, particularly in the very short-chain materials such as Kovacic polyphenylene. 

In recent years the electrochemistry of the enzyme membrane has been a subject of great interest due to its significance in both theories and practical applications to biosensors (i-5). Since the enzyme electrode was first proposed and prepared by Clark et al. (6) and Updike et al. (7), enzyme-based biosensors have become a widely interested research field. Research efforts have been directed toward improved designs of the electrode and the necessary membrane materials required for the proper operation of sensors. Different methods have been developed for immobilizing the enzyme on the electrode surface, such as covalent and adsorptive couplings (8-12) of the enzymes to the electrode surface, entrapment of the enzymes in the carbon paste mixture (13 etc. The entrapment of the enzyme into a conducting polymer has become an attractive method (14-22) because of the conducting nature of the polymer matrix and of the easy preparation procedure of the enzyme electrode. The entrapment of enzymes in the polypyrrole film provides a simple way of enzyme immobilization for the construction of a biosensor. It is known that the PPy-... 

Electrically conducting polymers are quite different systems to the above elec-troinitiated chain polymerizations since they are formed by an unusual step-growth mechanism involving stoichiometric transfer of electrons. The polymers are obtained directly in a conductive polycationic form in which charge-compensating counter anions from the electrolyte system are intercalated into the polymer matrix [173], Exact mechanistic details remain the subject of discussion, but Scheme 4, which shows polypyrrole formation is plausible. Polythiophene is similar where S replaces NH in the ring. 

Figure 16.17. Thermal stability of polypyrrole film prepared at —20°C and subjected to heat treatment at various temperatures, (a) conductivity and --0-- Cl content measured by EPMA and (b) distribution of Cl measured by EPMA across the film section. Adapted from Synth. Mel. 14, 61 (1986),with permission of Elsevier Science S.A., Lausanne.

Conjugated polymers have been the subject of great interest, both theoretically and experimentally, since the discovery of conductivity in doped polyacetylene in the seventies [1]. Many works have been devoted to their synthesis, characterizations and properties [2]. They have found many apphcations, particularly in the field of optoelectronics, as light-emitting diodes (LEDs), field-effect transistors (FETs), solar cells, etc., due to the semi-conducting behavior of the conjugated backbone [3]. Among them, polythiophenes (PTh) and polypyrroles (PPy) have been extensively studied because of their synthesis versatility and environmental stability [4-6]. Their functionalization permits the combination of their... 

Nanocomposites of gold/polypyrrole have also been prepared by electrochemical methods. The composite system can be prepared by simultaneous reduction of AUCI4 and autopolymerization of pyrrole. The polymer coating can be removed by subjecting the composite to ultrasonic waves to obtain elemental gold nanoparticles of 2 nm in diameter. 

Conductive polymers such as polypyrrole (PPy), polyaniline (PAni), and polythiophen (PTh) have been the subject of much research owing to their wide applications in biosensors, electrochemistry, and electrocatalysis [191, 192]. Recently, conductive polymers have been also investigated as ORR electrocatalysis in three different ways (1) utilizing conductive polymers as ORR electrocatalysts on their own, (2) incorporating non-precious metal complexes into the conductive polymer matrix, and (3) employing conductive polymers as a nitrogen/carbon precursor material for pyrolyzed M-N c/C catalysts [105]. 

Arsenic analysis at polymer electrodes has also been the subject of investigation. Upt e into a polypyrrole film can be used for preconcentration of arsenic species and subsequent analysis by HPLC/ICP/MS 63). 

The ability of the material to withstand transient voltage excursions outside the reversible window is also a function of time and, mainly, electrolytic solution composition. In the literature on the subject there is no common opinion concerning the region of potentials where poly pyrrole is chemically stable. Christensen and Hamnet [116], for example, have found that overoxidation of polypyrrole in aqueous 1 M NaC104 starts with potentials higher than 700 mV versus SCE. Nov ... 

The porosity of electroactive polymers has also been the subject of many investigations. An experiment has been described in which the ion exchange property of the polymer was used to modify the morphology [29]. In this study polypyrrole with tosylate as the counterion was ion exchanged with KCl to take up chloride as the counterion. Replacement of the bulky tosylate ion with the much smaller chloride ion resulted in greater porosity of the polymer and an increase of its fractal dimension. 

Carrier densities and carrier mobilities were obtained for polypyrrole via the Hall effect technique. The apparatus was also used to measure conductivities. The analysis was performed for polypyrrole containing three different counterions p-toluenesulfonate, perchlorate, and tetrafluoroborate. The carrier density was found to vary by one order of magnitude, from 1019 to 1020 cm-3. The carrier mobility, on the other hand, remained constant with respect to counterion, at 1 cm2V-lsec-l. In order to ensure that any solvent from preparation of the polymer was not affecting the results, one polymer, polypyrrole-p-toluenesulfonate, was subjected to vacuum dehydration for varying periods of time, followed by re-analysis. It was found that the carrier density changes very little with dehydration. There was no affect on the carrier mobility. 

We now consider various types of charge carriers that can be found in electronically conducting polymers. As previously noted both experimental and theoretical evidence suggest that the precise nature of charge carriers present in conjugated polymer systems depends to a very large extent on the type of polymer. We discuss two representative polymer materials, polyacetylene and polypyrrole, which have been the subject of considerable study. 

Another route to achieve melt and solution processability is to substitute long-chain alkyl groups at the 3-position of the heterocycle. This is the subject of an excellent review [2] with 68 references on polythiophene, but only five on polypyrrole. A state-of-the-art review of processable polythiophenes is included in Volume 3. The principal drawback of this approach is that there is a significant increase in the cost of the product. 

Figure 10.11 Cross-section through a film of polypyrrole. The film was first embedded in an epoxy resin, then microtomed and finally subjected to a two-stage etching process to generate the required relief. The film can be seen standing proud of the embedding material in addition, internal texture is also apparent within the polypyrrole itself.

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