Thermomechanical Properties of Nematic Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are materials formed by cross-linking crystal mesogens into a flexible polymer network, and they display soft behavior and undergo large, reversible strains. The mesogenic order determines material properties, causing coupling between temperature, liquid crystalline order, and deformation, which leads to temperature-based actuation. LCEs have important applications in soft robotics and medical devices, so attempts have been made to theoretically model their behavior in order to develop new use cases. One such model, developed by Lee (2021), identifies regions of liquid crystal orientation and has agreed with initial experimental data (Lee et al., 2023). This thesis aims to characterize the behavior of isotropic-genesis polydomain LCEs across various temperatures, strain rates, and crosslinking densities and further test the model by comparing the experimental data against it. Tensile tests were run across five strain rates (10⁻¹/s, 5×10⁻²/s, 10⁻²/s, 5×10⁻³/s, 10⁻³/s), three temperatures (26◦C, 55◦C, 90◦C), and two crosslinking densities (50 mol%, 25 mol%). A custom tensile rig with a heated chamber made by Lee (2021) was modified for the purpose of this thesis to allow for digital image correlation and trials across temperatures. These tensile tests revealed that stiffness increased with faster strain rates, and, as temperature increased, soft behavior was reduced at 55◦C and vanished at above the nematic transition temperature. Additionally, residual strain decreased with increasing temperature, at ∼1.5 at 26◦C, ∼0.75 at 55◦C, and ∼0.1 at 90◦C. Reducing the crosslinking density more than doubled the strain at failure and drastically increased the region of soft behavior. Experimental data across three strain rates (10⁻²/s, 5×10⁻³/s, 10⁻³/s), three temperatures, and at 50 mol% crosslinking density were compared against the model developed by Lee (2021). The soft behavior of the LCE was generally well characterized by the model, however, the model deviated from experimental data above two strain, as the Neo Hookean-based model was unable to capture strain hardening. Since higher temperature trials were run to lower strains, the model was able to better capture the full behavior of the LCE at higher temperatures, even with the loss of soft behavior at 90◦C. This model is therefore a useful tool for modeling LCE soft behavior across various temperatures.