Thermoviscoelasticity of polydomain liquid crystal elastomers regulated by soft elasticity

Abstract

Liquid crystal elastomers (LCEs) are elastomeric networks with rod-like mesogens that reorient under load. In polydomain LCEs, this reorientation drives a polydomain-to-monodomain transition that produces a soft-elastic plateau. The intricate coupling between this soft elasticity and the polymer-network viscoelasticity gives rise to a path-dependent thermoviscoelastic response, central to applications of polydomain LCEs in damping, impact protection, and tough adhesives. However, the physics governing this response under complex thermomechanical histories remains insufficiently studied. In this work, we present a combined experimental and theoretical study of polydomain LCEs under three uniaxial loading protocols: single-cycle loading-unloading, stress-free recovery from various pre-stretches, and multi-cycle loading with progressively increasing amplitude. We develop a finite-deformation constitutive model that combines two parallel mechanisms: rate-independent, temperature-dependent soft elasticity from mesogen reorientation, and time- and temperature-dependent viscoelasticity from various sources. Calibrated with a single parameter set, the model quantitatively reproduces all three protocols and resolves the individual contribution of each mechanism. Across the three protocols, a temperature-dependent soft-elastic limit governs the low-rate response and the long-time recovered stretch, while polymer-network viscoelasticity controls the rate-dependent deviation and the cycle-wise accumulation of residual stretch away from this low-rate limit. A thermal recovery test above the nematic-isotropic transition temperature confirms that all hysteresis and residual deformation are reversible, ruling out irreversible internal damage. The combined experimental-theoretical framework provides mechanistic understanding and a predictive basis for the design of polydomain LCE components under complex thermomechanical histories.