Electronically conducting polymer charge carriers

As intensive studies on the ECPs have been carried out for almost 30 years, a vast knowledge of the methods of preparation and the physico-chemical properties of these materials has accumulated [5-17]. The electrochemistry ofthe ECPs has been systematically and repeatedly reviewed, covering many different and important topics such as electrosynthesis, the elucidation of mechanisms and kinetics of the doping processes in ECPs, the establishment and utilization of structure-property relationships, as well as a great variety of their applications as novel electrochemical systems, and so forth [18-23]. In this chapter, a classification is proposed for electroactive polymers and ion-insertion inorganic hosts, emphasizing the unique feature of ECPs as mixed electronic-ionic conductors. The analysis of thermodynamic and kinetic properties of ECP electrodes presented here is based on a combined consideration of the potential-dependent differential capacitance of the electrode, chemical diffusion coefficients, and the partial conductivities of related electronic and ionic charge carriers. 

The metallic state of the conducting polymers has been confirmed by optical measurements and thermoelectric power, but only above 200 C by dc conductivity because of semiconducting transport betw een the metallic regions. The intrinsic anisotropy of such a metallic conduction that could specify dimensionality of the electronic states, is rather difficult to measure by dc conductivity for the same reason. The spin dynamic techniques might be a powerful tool to characterize microscopic conduction of charge carriers with spin. Such studies were recently reported for several systems with ESR and NMR as a function of frequency. This... 

Electroactive polymers can be divided into three broad classes electronically conducting polymers, such as poly(pyrrole), in which conduction is associated with motion of charge carriers along the chains redox polymers, such as poly(vinylfer-rocene), in which conduction is associated with cross-exchange reactions between discrete redox sites and polymer electrolytes, such as Nafion, in which conduction is associated with ion motions within the film. Examples of polymers from all three classes have been used in bioelectrochemical applications. 

Depending on the type of charge transport carrier, conductive polymers can be classified into two main groups of ionically and electronically conductive polymers (Fig. 6.1). Polyethylene oxide with lithium perchlorate (LiCl04) is an example of an... 

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. 

Most successful receptor-molecule carriers are conducting polymers. Among them are substances with metallic conductivity. These are e.g. polypyrrole, polyaniline and polythiophenes. Their molecules contain multiple conjugate double bonds which are the reason for electron mobility in the molecule. A different type of conductance exists in redox polymers, where redox centres are inserted into the polymer. Charge carriers can be exchanged between such centres. 

When a polymer of (Xe > o-jon, such as polypyrrole in the oxidized state, is subjected to changes of the applied electrode potential, during the transient state electric fields develop in the polymer matrix and then disappear as electronic and ionic charge carriers migrate to new equilibrium positions [19,27,31,184]. The analyses may be based on the concepts derived for redox polymers under the condition that the hopping mobility of the electrons exceeds the counterion mobility. It has been shown [31] that in this case the system behavior is again diffusiona in character. The coated electrode behaves like a porous metal electrode with pores of limited depth. Numerous experimental reports on this behavior of conducting polymers have appeared in the literature the first was probably that of Bull et al. in 1982 [214]. 

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