Electromechanical Coupling in Ionic Polymer-Metal Composites

Ionic polymer-metal composites (IPMCs) are smart materials which function as soft sensors and actuators. When a small DC voltage (1–5 V) is applied to an IPMC in a cantilever configuration, ion and solvent transport through the thickness of the polymer membrane causes the transducer to bend towards the anode. For device development and use in engineering applications, actuation is often described at a higher level in terms of an electromechanical coupling between the ionic charge distribution and the stresses developed in the IPMC. In this work we derive a set of relationships describing the coupling response by starting with basic considerations of polymer microstructure and local interactions during actuation. A micromechanical modeling framework is employed in order to account for the material microstructure. Using a generalized expression for electrostatic cluster pressure which takes into account clusters recombining to form larger cluster upon expansion, we define an effective local stiffness which varies with both solvent uptake and charge density in the boundary layers. An equilibrium relationship between solvent uptake and charge density is determined by considering the free energy of the homogenized polymer as the sum of elastic, electrostatic, and chemical components. Stress developed in the boundary layers is then calculated from changes in local stiffness and solvent uptake with respect to charge density. The resulting relationship for electromechanical coupling is found to be in good agreement with previous empirical models, thus serving as a model validation and demonstrating why certain forms for electromechanical coupling can be used to explain a variety of experimental observations. Specifically, we see that stress developed in the boundary layers is well described as a quadratic polynomial in charge density due to the form of the electrostatic cluster pressures.