Actuation and Charging Characteristics of Ionic Liquid-Ionic Polymer Transducers

Ionic polymer transducers (IPTs) are soft sensors and actuators which operate through a coupling of micro-scale chemical, electrical, and mechanical interactions. The use of an ionic liquid as solvent for an IPT has been shown to dramatically increase transducer lifetime in free-air use, while also allowing for higher applied voltages without electrolysis. In this work we model charge transport in an ionic liquid IPT by considering both the cation and anion of the ionic liquid as mobile charge carriers, a phenomenon which is unique to ionic liquid IPTs compared to their water-based counterparts. The electrochemical behavior of the large ionic liquid ions is described by use of a modified Nernst-Planck equation which accounts for steric effects in double layer packing. The method of matched asymptotic expansions is applied to solve the resulting system of equations, and analytical expressions are derived for the nonlinear charge transferred and capacitance of the IPT as a function of the applied voltage. The boundary layer ionic concentration and charge density profiles and the leading order dynamics are also computed for the ionic liquid IPT. A simple equivalent circuit model is constructed in order to facilitate a comparison with experimental results. The implications of these model results in regards to actuation and charging performance characteristics of ionic liquid IPTs are noted.