Liquid crystal elastomers (LCEs) are smart (or active or multifunctional) materials containing small crystal-forming molecules that can orient or disorient. The stimulus, e.g. light, heat, or electric current, will prompt an action at the molecular level that will result in the macroscopic deformation. Moreover, the trigger relies on different physical mechanisms, which makes these materials ideal candidates for various applications, for instance soft actuators, micro-valves, or artificial muscles. The open questions in this research field cover different areas: chemistry for synthesizing LCEs, physics for identifying specific small-scale mechanisms, and mechanics for optimizing and predicting the macroscopic behavior. Main-chain LCEs have a high potential for innovative applications due to their performance in terms of shape change and their great flexibility in the preparation procedure.
Their thermo-mechanical behavior exhibits multiple phenomena linked to the change of order of the liquid crystals in the microstructure, such as soft elasticity and the nematic-isotropic transition. These molecules provide LCEs with a shape memory ability through the nematic-isotropic transition between the low-temperature ordered nematic state and the high-temperature disordered isotropic state. Stretching or heating these elastomers can induce or prevent the orientation of the liquid crystals. In addition, the polymer chain dynamics change with the orientation of the liquid crystals.
We aim at determining the structure-property relations pertaining to the rate-dependent viscoelastic and dynamic behavior of LCEs.
I started exploring the mechanical behavior of smart elastomers three years ago. The study aimed at determining the influence of the viscoelasticity of the material on the soft elasticity phenomenon. With the assistance of undergraduate students, I characterized the viscoelasticity with a master curve of the storage modulus, and the phase transition with uniaxial tensile tests at various strain rates and temperatures. Using the time-temperature superposition principle, we showed that the stress and strain values present a linear viscoelastic behavior. For these LCEs, the soft elasticity of the phase transition belongs to the transient part of the behavior. This also reveals the contribution of relaxation mechanisms originating from the liquid crystal orientation. These conclusions provided strong experimental evidence to support a modeling approach of the effective behavior of LCEs.