Characterization and modeling of extensional and bending actuation in ionomeric polymer transducers

Abstract

Ionomeric polymer transducers have received considerable attention in the past ten years due to their ability to generate large bending strain and moderate stress at low applied voltages. Bending transducers made of an ionomeric polymer membrane sandwiched between two flexible electrodes deform through the expansion of one electrode and contraction of the opposite electrode due to cation displacement. This is similar to a bimorph-type actuation. In this study we report actuation through the thickness of the membrane, leading to the potential of a new actuation mechanism for ionomeric polymer materials. Several experiments are performed to compare bending actuation with extensional actuation. A novel fabrication process previously developed by the authors, called the direct assembly process, is used to fabricate ionic polymer transducers with controlled electrode dimensions and morphology. In the first experiment, the actuators are cut in a beam shape and are allowed to bend in a cantilever configuration. In the second set of experiments, bending is constrained by sandwiching the membranes between two solid metal plates and force is measured across the thickness of the actuator. A bimorph model is used to assess the effect of electrode thickness on the strain. In the bimorph model, the electrode is assumed to be the 'active area' that generates strain due to charge displacement. An electromechanical coupling model that relates strain to charge is assumed. This model contains a linear and a quadratic term that acts at the active area and produces volumetric strain. The quadratic term in the strain generates a zero net bending moment for ionic polymer transducers with symmetric electrodes, while the linear term is canceled in extensional actuation for symmetric electrodes. The model successfully predicts the bending response from parameters computed using experimental thickness results. The prediction is particularly precise in estimating the trends of nonlinearity as a function of the amount of asymmetry between the two electrodes.