Asymmetric Bilayer Muscles: Cooperative Actuation, Dynamic Hysteresis, and Creeping in NaPF6Aqueous Solutions

Abstract

Three bilayer muscles [polypyrrole–paraphenolsulfonic acid/polypyrrole–dodecylbenzensulfonic acid (PPy–HpPS/PPy–DBS) asymmetric bilayer, PPy–HpPS/tape, and PPy–DBS/tape] were characterized during potential cycling in NaPF6aqueous solutions. In parallel, the angular displacement of the muscle was video-recorded. The dynamo-voltammetric (angle–potential) and coulo-dynamic (charge–potential) results give the reaction-driven ionic exchanges in each PPy film. Electrochemical reactions drive the exchange of anions from the PPy–HpPS layer and cations from the PPy–DBS layer. This means that both layers from the asymmetric bilayer follow complementary volume changes (swelling/shrinking or shrinking/swelling), owing to complementary ionic exchanges (entrance/expulsion) driven by the bilayer oxidation or reduction. The result is a cooperative actuation; the bending amplitude described by the asymmetric bilayer muscle is one order of magnitude larger than those attained from each of the conducting polymer/tape muscles. The cooperative actuation almost eliminates creeping effects. A large dynamical hysteresis persists, which can be attributed to an irreversible reaction of the organic acid components at high overpotentials.