CORROSION OF CONDUCTING POLYMERS IN AQUEOUS-MEDIA

The corrosion of conducting polymers (polypyrrole and polythiophene, each with two derivatives) was investigated in aqueous buffers (pH 1-13). The corrosion rate was determined experimentally by potentiodynamic discharge of the residual redox capacity of the conducting polymer after exposure to the corrosion medium for some time. The corrosion reactions were found to proceed in two steps, both following a pseudo-first-order mechanism. The initial rapid process is due to an electrochemical mechanism. The cathodic dedoping is balanced by an anodic overoxidation reaction, even at relatively negative potentials. A rather slow second process is caused by chemical attack of nucleophiles dissolved in the solid at the remaining radical cationic centers. The pseudo-first-order rate constants cover a range of 3 x 10(-5) -2 x 10(-2) s-1, from polypyrrole to polythiophene, for the fast process. Anodic overoxidation is rate determining. Corrosion increases with increasing pH, which is attributed to a negative shift of the overoxidation potentials. The rate constants could be evaluated from the exponential decay of the corrosion potential with time. In contrast with polypyrrole, the polymer backbone conjugation for polythiophene is not interrupted because -SBAR- --> -SO2-, and recharge is possible to some extent.