Mechanical energy dissipation in polydomain nematic liquid crystal elastomers in response to oscillating loading

Abstract

Liquid crystal elastomers (LCEs) exhibit exotic mechanical behaviors such as reversible actuation, elevated loss tangent above the glass transition, and soft elasticity; however, LCEs have yet to be thoroughly investigated under high-strain cyclic loading. This study explores a main-chain polydomain nematic elastomer synthesized from a thiol-acrylate Michael-addition reaction in the nematic state. Low-strain (i.e., <0.2%) dynamic mechanical analysis and creep behavior was used to help link viscoelasticity and mesogen reorientation to high-strain cyclic loading. Specifically, the behavior was compared at discrete temperatures below the glass transition, in the nematic-rubbery regime, and in the isotropic-rubbery regime. Creep behavior in the nematic-rubbery regime was modeled using two characteristic relaxation times, which decreased with increasing temperature (τ1, τ2 (19 °C) = 140 s, 16 s, and τ1, τ2 (62 °C) = 2.4 s, 0.6 s). Samples were repeatedly loaded to 50 kPa at 5 cycles/min at each temperature for 350 cycles. In the rubbery nematic state at 62 °C, low threshold stress and short relaxation times allowed reversible domain reorientation between each cycle, resulting in a repeatable stress-strain curve with high hysteresis (220 kJ/m3). In contrast, the 19 °C and 39 °C conditions in the rubbery nematic state demonstrated ratcheting behavior in response to cyclic loading, which lowered the hysteresis values with increased cycling. The findings presented here represent the first investigation into mechanical energy dissipation in main-chain nematic LCEs under large oscillating stress conditions. Overall, this study helps to better understand the role LCEs can play in energy dissipating applications with high values of strain and varying temperature.