Modeling of thickness effect and polarization saturation in electrostrictive polymers

Thanks to their lightweight, low cost and high flexibility, electroactive polymers (EAPs) have attracted the attention of both research and industrial communities. In particular, electrostrictive polymers have been widely used for the development of large strain actuators such as artificial muscles. However, the polarization and strain abilities of such materials are highly dependent on the sample thickness, for example because of a non-uniform space charge distribution caused by interface effects. The purpose of this paper is to propose a model taking into account the thickness effect on the actuation abilities of dielectric electrostrictive polymers, based on a continuous variation over the depth of the permittivity that can be seen as a surface effect. It is shown that this approach allows taking into account several effects, such as permittivity maximization for an optimal thickness. In addition, the paper introduces a simple relationship between the equivalent permittivity and the polarization saturation that limits the strain, allowing a good performance assessment of the actuation abilities of such devices. Dielectric measurements made on 20 ∼ 220 μm thick films demonstrated the validity of the proposed model, showing maximal permittivity around 100 μm, and strain ability assessments carried out on films ranging from 20 to 95 μm exhibited the highest strain response for the same thicknesses for low electric field excitations, and for lower thicknesses as the electric field is increased because of saturation effect, which was well predicted by the pseudo-empirical approach.