On The Relationship Between the Electric Double Layer and Actuation in Ionomeric Polymer Transducers
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ON THE RELATIONSHIP BETWEEN THE ELECTRIC DOUBLE LAYER AND ACTUATION IN IONOMERIC POLYMER TRANSDUCERS Barbar J. Akle and Donald J. Leo Center for Intelligent Material Systems and Structures (CIMSS) Department of Mechanical Engineering Virginia Tech, 310 Durham Hall Blacksburg, VA 24061, U.S.A. ABSTRACT Ionic polymer transducers are soft actuators that perform large bending deflections when voltages on the order of 1-5 V are applied across their thickness. Previous work showed that actuation performance of ionic polymer transducers is strongly correlated with the capacitance due to surface charge accumulation. Increasing the capacitance of the actuator increases the motion of the charges and increases the strain produced under the application of an electric field. Ionomeric transducers consist of an ionomer, such as Nafion (a product of DuPont), sandwiched between two high surface area electrodes. An electric double layer is formed on the interface between the cathode and the adsorbed positive ions. A novel plating technique which was previously developed is used to vary the morphology of the polymer-electrode interface to investigate the parameters of importance to the formation of the electric double layer. Electromechanical transducer tests are performed as a function of electrode morphology to correlate surface charge accumulation with the deflection generated by the transducer. INTRODUCTION Ionic polymers membranes are materials that exhibit ionic conductivity and ion selectivity. Several researchers have shown that these polymer membranes can function as electromechanical actuators and sensors [1]. Typically, an ionic polymer transducer is made by depositing an electrode, typically consisting of platinum and gold, onto an ionomeric membrane and saturating the membrane with water. In this state the membrane will bend towards the anode when a voltage (1-5 V) is applied across its thickness. Similarly, the polymer will generate charge when deformed. Ionic polymer transducers have the advantage of being able to generate large strains under small applied voltages, are compliant and thus compatible with conformal structures, and have a very high sensitivity to motion when used in charge sensing mode [2]. In order for the transduction to occur in Nafion membranes, the cations within the membrane must be dissociated by saturation with an appropriate solvent. Typically, the solvent used has been water. Water is a volatile solvent, leading to dehydration and the corresponding loss in performance of these actuators when operated in air. Bennett and Leo have previously shown that the hydration problem can be overcome by using ionic liquids as the solvent in these transducers [3]. Ionic liquids offer increased environmental stability and a larger electrical operation window compared to water but suffer from high viscosity. Transducers solvated with ionic liquids have slower strain rates compared to water hydrated samples. Previous results demonstrate that actuation performance of ionic polymer transducers is strongl
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