Conductive Polymers: Electroplating of Organic Films

Electron-conductive polymers were first reported in 1971 by Shirakawa and co-workers [1]. They synthesized conducting polyacetylene and found that it had a considerably high conductivity relative to other organic compounds, 10 S cm 1 [1–3]. Since 1971, various conducting polymers and their synthesis mechanisms have been studied actively by many researchers [4]. The conductivities of these polymers are shown in Figure 16.1, compared with those of various inorganic and organic compounds and materials. Generally speaking, because conducting polymers have electrons that are delocalized in p-conjugated systems along the whole polymer chain, their electron conductivity is much higher than that of other polymers that do not have a p-conjugated system, as shown in Figure 16.1. Furthermore, in 1979, Diaz and co-workers [5, 6] synthesized conductive polypyrrole film by electropolymerization (see Fig. 16.2). Electropolymerization is similar to electroplating of metal films. Its polymerization reactions occur electrochemically; that is, electrons go through the electrode–electrolyte interface when a potential is applied to an electrode, as shown in Figure 16.2. In the case of an insoluble polymer generated on the electrode, a film of conducting polymer is thus obtained easily. Essentially, regarding electron transfer, all electrochemical reactions are classifiable into oxidative or reductive reactions. Electrochemical polymerizations can be classified in a similar way. In almost all cases, electropolymerized polymers have been synthesized by electrooxidation. It seems, however, that films obtained by this method contain branching or cross-linking defects [7–9]. On the other hand, electroreductive polymerization performed by using a zero-valent nickel complex is sensitive to the halogen atom in the monomer, and therefore it yields polymers with well-defined linkages among the monomer units [10–13]. In the past four decades, many types of organic devices with conducting polymer films have been studied. Among the processes used to form these films, electropolymerization is one of the most interesting methods of preparing thin functional polymer films. Electropolymerized films directly deposited on various electrodes, for example, metals and semiconductors, have various functionalities and applications, as shown in Table 16.1. Also, it has been determined that electroinactive nonconducting polymer films are formed by electropolymerization [14]. When the electropolymerization solution contains nucleophilic reagents, such as, NaOH, NaHCO3, or Na2CO3 with monomer reagents, polymerization occurs first, and then film propagation automatically stops, resulting in a changing nonconductive state caused by a nucleophilic attack reaction. Since the electrode of the film made by the process described above responds to the pH value, it can be applied to micro–pH sensors [15] and also to biosensors with a pH change enzymatic reaction [16–18]. This chapter introduces the functional design of electropolymerized films and their applications for polymer batteries, electric devices, and chemical sensors.