LOGARITHMIC RELAXATION OF ELECTROCHEMICAL INSULATING-TO-CONDUCTING CONVERSION AT POLYANILINE FILMS - INTERPRETATION BY ELECTRIC PERCOLATION

Electrochemical switching of conducting polymers coated on an electrode is associated with hysteresis called the slow relaxation, the memory effect or the first scan effect, which are exemplified by linear dependence of the anodic peak potential on the logarithm of the time of cathodic electrolysis. The faradaic charge of the voltammetric current at the polyaniline-coated electrode was used as a measure of the slow relaxation. Reproducible linearity between the charge and the logarithm of the electrolysis time was obtained at different thicknesses of the films when the time was less than 10(3) s. After then, the charge was constant, indicating termination of the slow relaxation. The ratio of the minimum charge to the maximum varied logarithmically with the thickness of the films. The slope of the charge vs. log(t) plot was related to the thickness of the film to the power 1.3. A model of the slow relaxation was presented on the concept that conducting domains without electric connection to the electrode are rear-ranged by thermal fluctuation to recover the electric contact with the electrode. The average volume of the conducting domains in contact with the electrode was expressed as a function of the molar fraction of the conducting species. It was combined with a kinetic equation under the assumption of rapid mass transport or rapid electron exchange. Then, the relative amount of the redox charge exhibited a linear relation to the logarithm of the electrolysis time. The theory explained quantitatively the experimental behavior of the logarithmic relaxation, the termination of the relaxation and the dependence on the film thickness.