Stress-charge coupling coefficient for thin-film polypyrrole actuators – Investigation of capacitive ion exchange in the oxidized state

Abstract

This work quantifies the actuation behavior of thin electrodeposited polypyrrole films on rigid substrates. During in situ cantilever bending experiments in aqueous perchloric acid, the films remain clamped to their substrate and the film stress is inferred from a small bending of the cantilever. The potential range under study is 0.4–0.8 V versus the standard hydrogen electrode. Within this potential region, the film is in the oxidized state and the actuation arises from capacitive electrode processes. For the volumetric capacitance, c*, we find 0.24±0.01F/mm3. The stress-charge coupling coefficient, ξ, which is defined as the stress variation per volumetric charge density, emerges as −153±11mV. The results are robust, reproducible and independent of the film thickness or the potential scan rate. The experimental value of ξ is supported by an independent estimate, based on a micromechanical model in combination with literature data for the partial molar volume of solvated perchlorate anions and the elastic constants of polypyrrole.