The Influence of Microstructure on Boundary Layer Interactions in Ionic Polymer Transducers

Abstract

Ionic polymer transducers (IPTs), also known as ionic polymer-metal composites (IPMCs), are smart sensors and actuators which operate through a coupling of micro-scale chemical, mechanical, and electrical interactions. It is known that ion movement, when a voltage is applied, causes stresses which lead to a net bending movement of a cantilevered transducer towards the anode. However, it is not well understood how these stresses arise, and it is not known how the material microstructure affects the observed macroscopic bending response. In this work, we apply a micromechanics modeling framework to analyze how assumptions of the material microstructure of an IPT affect local interactions and the resulting boundary layer stresses which lead to actuation. In the micromechanics framework, local equilibrium consists of a balance between internal cluster pressure and the stress developed in the polymer backbone. Here, we derive a generalized expression for the electrostatic cluster pressure and show how it depends on microstructure and microstructural evolution in the boundary layers. It is proposed that the boundary layer stiffness varies locally with changes in solvent uptake and charge density; this relationship is defined from micromechanics equilibrium conditions and includes effects of the generalized electrostatic cluster pressure. By assuming a relationship between ion and solvent movement, we then examine how boundary layer stresses are affected by assumptions of the material microstructure. The results and implications of the model are compared with recent experimental observations as well as other models of IPT actuation.