Thermomechanical Coupling in Monodomain and Polydomain Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are rubbery polymer networks with liquid crystal mesogens linked to their polymer chains. These materials can generate large deformation and mechanical work upon temperature change or light illumination, making them ideal candidates for new applications including soft robots, metamaterials, and other stimuli-responsive devices. However, despite the rapid development of LCEs in new chemistry, manufacturing, and applications, the fundamental thermomechanical coupling between the microscopic temperature-controlled processes and the macroscopic mechanical behaviors of these materials remains poorly studied when combining theory and experiment. It remains further unclear how the modern fabrication methods such as the widely used "two-step polymerization" affect this thermomechanical coupling. This talk will present our recent progress in thermomechanical behaviors of nematic LCEs under both mechanical load and prescribed temperature. Through experiments with quantitatively controlled fabrication parameters, we show a nontrivial effect of the applied pre-stretch during polymerization on the thermomechanical behavior of nematic monodomain and polydomain LCEs. To understand the experimental observation, we establish continuum theories based on classical free energy models that incorporate the entropic elasticity of the polymer network and the long-range directional molecular interaction between mesogens. These efforts are hoped to help advance the fundamental knowledge of liquid crystal elastomers, bring together communities of relevant research fields, and further expand the large-scale applications of these materials.