Thermomechanical Coupling in Polydomain Liquid Crystal Elastomers

Abstract

Liquid crystal elastomers (LCEs) are made of liquid crystal molecules linked into rubber-like polymer networks. An LCE exhibits both the thermotropic property of liquid crystals and large deformation of elastomers. It can be monodomain or polydomain in the nematic phase and transforms to an isotropic phase at elevated temperature. These features have enabled various applications of LCEs in robotics and other fields. However, despite substantial research and development in recent years, thermomechanical coupling in polydomain LCEs remains poorly studied, such as their temperature-dependent mechanical response and stretch-influenced isotropic-nematic phase transition. This knowledge gap limits the fundamental understanding of the structure-property relationship, as well as future developments of LCEs with precisely controlled material behaviors. Here we construct a theoretical model to investigate thermomechanical coupling in polydomain LCEs, which includes a quasi-convex elastic energy of the polymer network and a free energy of mesogens. We study working conditions where a polydomain LCE is subjected to various prescribed planar stretches and temperatures. The quasi-convex elastic energy enables a “mechanical phase diagram” that describes the macroscopic effective mechanical response of the material, and the free energy of mesogens governs their first-order nematic-isotropic phase transition. Evolution of the mechanical phase diagram and the order parameter with temperature is predicted and discussed. Temperature-dependent mechanical behaviors of the polydomain LCE that have never been reported before are shown in their stress-stretch curves. These results are hoped to motivate future fundamental studies and applications of thermomechanical LCEs.