Electrochemical Polymerization and Film Deposition

Most 2,5-unsubstituted pyrroles and thiophenes, and most anilines can be polymerized by electrochemical oxidation. For pyrroles, acetonitrile,54 or aqueous55 electrolyte solutions are normally used, while the polymerization of thiophenes is performed almost exclusively in nonaqueous solvents such as acetonitrile, propylene carbonate, and benzonitrile. 0 Polyanilines are generally prepared from a solution of aniline in aqueous acid.21 Platinum or carbon electrodes have been used in most work, although indium-tin oxide is routinely used for spectroelectrochemical experiments, and many other electrode materials have also been employed.20,21 

Cyclic voltammetry is most commonly used to investigate the polymerization of a new monomer. Polymerization and film deposition are characterized by increasing peak currents for oxidation of the monomer on successive cycles, and the development of redox waves for the polymer at potentials below the onset of monomer oxidation. A nucleation loop, in which the current on the reverse scan is higher than on the corresponding forward scan, is commonly observed during the first cycle.56,57 These features are all illustrated in Fig. 3 for the polymerization of a substituted pyrrole.58 

Polymerization at constant current is most convenient for controlling the thickness of the deposited film. Charges of ca. 0.3, 0.2, and 0.08 C cm-2 are required to produce 1 fim of polypyrrole,59 poly(3-methylthio-phene)60 (no data are available for polythiophene), and polyaniline 43 respectively. Although these values can reasonably be used to estimate the thicknesses of most electrochemically formed conducting polymer films, it should be noted that they have considerable (ca. 30%) uncertainties. For each polymer, the relationship between charge and film thickness can 

In most cases, oligomers are initially generated in solution,61-64 but most rapidly precipitate onto the electrode surface and/or couple with adsorbed chains, and become oxidized 62,63,65 As a result, an oxidized (p-doped) polymer film is deposited on the electrode surface with, in most cases, high faradaic efficiency. Since ca. 0.3 electrons are required to dope the film to the polymerization potential, the overall polymerization + deposition process consumes ca. 2.3 electrons per monomer unit. 

Although the mechanisms discussed above are still topics of debate, it is now firmly established that the electrodeposition of conducting polymers proceeds via some kind of nucleation and phase-growth mechanism, akin to the electrodeposition of metals.56,72-74 Both cyclic voltammetry and potential step techniques have been widely used to investigate these processes, and the electrochemical observations have been supported by various types of spectroscopy62,75-78 and microscopy.78-80 

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