Modeling non-idealities in ionic polymer metal composites

Within the family of electroactive polymers, ionic polymer metal composites (IPMCs) stand out for their low driving voltage, operability in water, and biocompatibility. These characteristics make them attractive as soft actuators for applications in underwater robotics and biomedical engineering. However, their use is currently limited by the lack of predictability of their chemoelectromechanical behavior. Part of this unpredictability is associated with the complex microstructure of the ionic membrane forming the core of the IPMC. The membrane consists of a porous, negatively charged polymer network, which is neutralized by a solution of mobile counterions. In the literature, migration of counterions in the membrane is typically described through the Poisson-Nernst-Planck system of equations. To the best of our knowledge, non-ideal behaviors of the ionic saturating solution have never been considered in the IPMC literature. Here, we make a first step toward studying the effect of non-idealities on the mechanics and electrochemistry of IPMCs. We investigate four non-ideal behaviors of ionic solutions: 1. solvation (that is, the interaction between solute and solvent molecules due to ionic or dipole bonds); 2. electrostatic interactions that affect the mixing of ions in the solution, causing ions of one sign to be surrounded by ions of the opposite sign; 3. physical interactions between ions of opposite signs; and 4. steric effects associated with the finite volume of the membrane pores. Through numerical simulations, we demonstrate the role of each of these contributions on the formation of an electric double layer in a charged membrane near to an electrode, toward developing more realistic models to describe the mechanics and electrochemistry of IPMCs.