Thermodynamical Modeling of the Electromechanical Behavior of Ionic Polymer Metal Composites

Ionomeric polymer transducers are a class of smart materials which exhibit electromechanical coupling when subjected to low voltage (< 5 V) excitation. Generally these materials are soft actuators exhibiting large bending strains (> 5%) but correspondingly low force output. The mechanisms producing electromechanical coupling have so far not been completely understood. It is clear from experimental and theoretical investigations that diffusion and migration of ionic species within the polymer are the main cause for electromechanical coupling. For this reason we have developed a thermodynamically based mechanical model - using chemo-electrical inputs - which is able to predict the mechanical output i.e., deformation, bending, etc. for a given applied voltage to the IPMC strip. The chemo-electrical transport model is capable of computing the charge density profile in space and time as well as the current flux for applied electric fields. Based upon thermodynamic laws, the mechanical model has been developed to describe the strain within the material. The mechanical stress in this model is accomplished by two terms of the charge density, a linear and a quadratic one. The linear term represents the volume displacement caused by the charge migration while the quadratic term stands for the electrostatic forces caused by charge imbalances in the material. In this paper, numerical investigations of the electromechanical model as well as displacement measurements have been performed. A comparison of numerical and experimental investigations shows a very good correlation. This confirms the quality and the validity of the developed model.