Color and Volume Change in PPy(DBS)

Abstract

The optical−mechanical−electrochemical coupling in polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), is studied during electrochemical switching. Cations enter this material during reduction, leading to an increase in volume. A special experimental geometry is used in which the top surface of the PPy film is covered by a transparent ion barrier, which constrains the charge-compensating cations to enter the film only at the edges. The cation concentration determines the volume of the polymer and is also directly related to the oxidation level of the polymer, which in turn determines its color. During electrochemical reduction, the edges of the film lighten, and this light color travels as a front to the center of the film. At the same time, the height of the film increases when and where the color lightens, and the increase in height is directly proportional to the change in color intensity. The height and the intensity attain their maximum values as the front passes and do not increase further thereafter, indicating that the ion concentration in the fully reduced state has a maximum value that is not exceeded. The color and volume changes are associated with only the first pair of peaks in the cyclic voltammogram, which represent only a fraction of the consumed charge in a typical electrochemical scan.