Competition between polymerization and dissolution of poly(3-methylthiophene) films

Films of electrically conducting polymer, poly(3-methylthiophene), are dissolved in monomer-free solutions at positive potentials to become thin, whereas they are polymerized in monomer-rich solutions at the same potentials as for the dissolution. A question arises whether they are dissolved or polymerized in solutions including a given concentration of the monomer when a positive potential is applied to the film. Conditions of the competition between the polymerization and the dissolution were searched at potentiostatic experiments for poly(3-methylthiophene) films at various concentrations of monomers and potentials. The dissolution prevailed over the polymerization as the concentration decreased and the potential was less positive. Chronoamperometric currents exhibited oscillation under the competition conditions. The oscillation was explained in terms of the Lotka-Volterra model for a simple oscillation reaction, in which competitive species were the conducting polymer and the monomer.