Transport modeling in ionomeric polymer transducers and its relationship to electromechanical coupling

Abstract

Ionomeric polymer transducers consist of an ion-conducting membrane sandwiched between two metal electrodes. Application of a low voltage (<5V) to the polymer produces relatively large bending deformation (>2% strain) due to the transport of ionic species within the polymer matrix. A computational model of transport and electromechanical transduction is developed for ionomeric polymer transducers. The transport model is based upon a coupled chemoelectrical multifield formulation and computes the spatiotemporal volumetric charge density profile to an applied potential at the boundaries. The current induced in the polymer is computed using the isothermal transient ionic current associated with surface charge accumulation at the electrodes induced by nonzero volumetric charge density within the polymer. The stress induced in the polymer is assumed to be a summation of linear and quadratic functions of the volumetric charge density. Euler-Bernoulli beam mechanics are used to compute the bending deflection of the transducer to an applied potential. The diffusion coefficient and permittivity of the polymer is identified from the measured current density to a step change in the applied potential. A comparison between the measured data and the predicted response demonstrates that this model accurately predicts the current discharge due to the applied potential at voltages over the range of 50–500mV. Furthermore, the measured strain response is accurately predicted by determining the two parameters of the mechanics model that relates volumetric charge density to induced stress. The coupled model with parameters identified from the step response analysis is used to predict the harmonic response of the current and the bending strain. Comparisons between measured data and simulations illustrate that the coupled transport-mechanics model accurately predicts the magnitude and trends associated with the current response and strain output. Excellent agreement is obtained at excitation periods above approximately 1s while good agreement is obtained at longer excitation periods. The transport model highlights the importance of the asymmetry that develops at large applied potentials and long excitation periods in the volumetric charge density due to the fixed anionic species in the polymer.