What happens at the ionomer-electrode interfaces and how it influences sensing and actuation in ionic polymer metal composites

The process of metal electrode deposition used in the fabrication of ionic polymer metal composites (IPMCs) results in the formation of a highly heterogeneous layer at the ionomer-electrode interfaces, whose conductive and dielectric properties may significantly differ from both the ionomer and the metal. Charge redistribution in the vicinity of such layers is often considered to be a determinant of IPMC charge dynamics and, thus, a key contributing factor to both actuation and sensing. In this chapter, we propose a refined modeling framework to understand what happens at these interfaces and predict the mechanics and electrochemistry of IPMCs. We describe the kinematics of an IPMC in terms of its mechanical deformation, the concentration of mobile counterions neutralizing the ionomer, and the electric potential. The chemoelectromechanical constitutive behavior is obtained from a Helmholtz free energy density, which accounts for mechanical stretching, ion mixing, and electric polarization. We demonstrate the approach in the analysis of three representative instances, namely: electrical response of IPMCs with fully covered electrodes at blocked null deformations; sensing dynamics of IPMCs with partially covered electrodes; and static actuation of IPMCs with perfect electrodes.