| Soft sensors and actuators are made from hydrated ionic polymer-metal composites (IPMCS). When a small voltage is applied across an IPMC, large bending deformations are produced. The bending response, which is produced by mechano-electrochemical interactions, varies for different cations and water content of the ionic polymer. It is the goal of this work to identify the important physical phenomena that contribute to the observed mechano-electrochemical bending response of IPMCs when a small voltage is applied across them. Due to the nano-scale of the ionomer and electrode structure, it is difficult to make direct observation of physical changes in the IPMC. As a result, one has to deduce underlying phenomena from macro-scale observations. To this end, a series of experiments are performed, which characterize the mechanical, chemical, and electrical behavior of the IPMC. The experiments quantify the hygroscopy, bending stiffness, shear modulus, surface resistance, capacitance, and coupled mechano-electrochemical bending response of the IPMC for various cations and hydration levels. To differentiate the effects of these coupled phenomena, a generalized IPMC model is introduced. The model consists of a porous-elastic material with electrolyte-filled pores, a porous electrode, and irreversible transport of cations and water. The involved transport phenomena are analyzed using classical irreversible thermodynamics. In terms of the model parameters, the steady-state solution of the IPMC model suggests that the equilibrium bending deformation of the IPMC is the result of electro-static and hydraulic-osmotic forces. The electrostatic forces are proportional to capacitive charge on the IPMC. The important parameters affecting the hydraulic-osmotic forces are the cation size and the osmotic coefficient of the different cations. The transient solution indicates that the rate of charging, hydraulic permeability and water diffusion coefficients are the important factors governing the transient bending response of the IPMC. |
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