Effects of Pre-Stretch on Thermomechanical Behaviors of Nematic Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are rubbery polymer networks with liquid crystal mesogens linked to their polymer chains. They are capable of generating large deformation and decent amount of mechanical work upon light illumination or temperature change, making them ideal candidates for new applications including soft robots, metamaterials, and other stimuli-responsive devices. This talk will present our recent progress in the fundamental understanding of several thermomechanical behaviors of nematic LCEs under both mechanical load and prescribed temperature. We will focus on the experiment and theoretical modeling of monodomain nematic liquid crystal elastomers fabricated by two-step polymerization, where the second step applies a mechanical pre-stretch to align the polymer chains and mesogens to form the monodomain in the LCE. Experimental results suggest nontrivial effects of the applied pre-stretch on the spontaneous deformation of the LCE at room temperature as well as the nematic-isotropic phase transition temperature of the mesogen. By contrast, the shape of a high-temperature, isotropic-phase LCE is completely independent of the pre-stretch but is determined solely by the first-step polymerization that leads to a polydomain material. To understand these phenomena, we establish a continuum theory based on classical free energy models that incorporate the entropic elasticity of polymer networks, the long-range directional molecular interaction, the additional coupling between elasticity and nematic order, and the non-ideality of the polymer network due to crosslinking. We discuss the agreement between the theory and experiment. 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.