Polypyrrole and Related Polymers

SIMON J. HIGGINS, PAULA. CHRISTENSEN, AND ANDREW HAMNETT 

FIGURE 7.5. Evolution of the optical properties (a,b) and film thickness(c) as a function of time for a polypyrrole film grown potentiostatically on a Pt sputter-deposited film electrode at +0.60 V (SCE) using an aqueous solution of pyrrole (0.025 M) and KNO3 (0.5 M). (From Ref. 19). 

Variation in Polypyrrole Optical Properties As a Function of Growth Conditions 

In addition to the applications of ICPs already mentioned (for a general review of and its applications, see [547]), PPy has been suggested as a matrix for a variety of sensor applications [548, 549,560-563]. (A special application where a sensor responds to the respective monomer was described by Vinokurov [564].) In a related concept, the size selectivity ( ion sieving ) of PPy membranes is utilized [565]. The influence of the electropolymerization conditions on the size-exclusion selectivity of various ICPs, includ- 

Polypynole films have been employed as carriers of catalytically active materials like finely dispersed metal particles [567], whose own catalytic properties in various electrochemical reactions have been studied [568]. These systems sometimes were investigated using spectroelectrochemical techniques. Mostly, measurements were aimed at elucidating catalytic aspects of the embedded catalyst. These reports are beyond the scope of this review. 

Diaz et al. [599] studied the electrooxidation of various aromatic monomers and oligomers, including pyrrole and its oligomers, to identify relationships between the number of repeat units in the substrate molecule and the oxidation potential as well as the UV-vi absorption maximum of the oxidation product (soluble as well as deposited on the electrode). As can be expected based on theoretical calculations alreacfy mentioned, the oxidation potential shifts to less positive values with an increasing number of pyrrole units in the educt, whereas the absorption maximiun shifts considerably to longer wavelengths. 

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Now. this is not usually a problem for producing polypyrrole and related polymers. since a typical film on the electrode weighs only some tens of milligrams, and an electrolyte containing, for example, 0.01 M pyrrole can support its formation without appreciable depletion of the monomer. Constant-current electrolysis is therefore often used for these polymers although it should be noted that the exact properties of the film can vary with preparation conditions, and with this methodology the exact electrode potenhal is not known. 

Environmental instability and/or thermal Instability is the major problem which has till now precluded a variety of important applications of organic conducting polymers. With the exception of p-doped polypyrrole and related polymers, which appear to have a relatively stable ambient state conductivity, highly conducting polymer complexes generally decrease in conductivity upon exposure to the... 

Properties of Polypyrrole and Related Polymers and their Electrochemically... 

The preparation of polypyrrole, polythiophene, polyaniline, and related conducting polymers demonstrates principles of electrochemical synthesis that are more widely applicable, and it is instructive to examine these in detail. 

The redox polymers, both of organic and inorganic origin (such as polyvinylpyridine modified by redox-active complexes of metals Prussian blue and related materials), can be considered as a version of electrodes of the second kind however, the equilibrium is usually estabhshed with respect to cations. Electron conducting polymers (polyanyline, polypyrrol, and so forth) also pertain, in the first approximation, to the electrodes of the second kind, which maintain equilibrium with respect to anion. Ion exchange polymer films on electrode surfaces form a subgroup of membrane electrodes. 

An electrically conductive polymer composite of polypyrrole and poly(ethyl methacrylate) has been prepared by an emulsion polymerization procedure [ 142]. In this case, the relation between conductivity and the polypyrrole content of the composite exhibited a percolation behavior, with conductivities as high as 6-7 S/ctn. Such composites might be amenable to melt processing for coating formation. Composite films consisting of polypyrrole or poly(N-ethylaniline) filler dispersed in a polyimide matrix have been described for potential use as corrosion control coatings for the A1 alloy AA 2024-T3 [143]. 

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