Electrochemically induced chemomechanical bending of bilayered ion-exchange membranes

A bilayered ion-exchange membrane consisting of poly(styrene-co-4-vinylpyridinium ion) (anion-exchange membrane) and a gel-like mixture of poly(vinyl alcohol) and poly(acrylic acid) (cation-exchange membrane) was prepared. The bilayered membrane strip, one end of which is fixed, is placed between two carbon electrodes in a cell which contains KCl solution of 0.01 mol/kg(-1) molality. If the electric field is applied from the cation-exchange layer side to the anion-exchange layer side, the cation-exchange layer becomes the K+ form and is elongated. On the contrary, if the electric field is reversed, dissociation of water into H+ and OH- occurs at the interface between the two layers. Consequently, the carboxyl group becomes the acid form, resulting in contraction of the layer. The anion-exchange layer, which contains a strong base group, becomes either the Cl- form or the OH- form with changing the electric field, but there is no detectable change in volume. Thus if the electric field applied to the membrane is periodically reversed, periodical bending of the bilayered membrane is observed.