The Contribution of Oligomerization Reaction Chemistry to the Thermomechanical Properties of Surface-Aligned Liquid Crystalline Elastomers

Liquid crystalline elastomers (LCEs) are stimulus-responsive materials that, when macroscopically aligned, undergo large, directional deformation amplified by the disruption of order within a polymer network. Recently, surface-enforced alignment has been used to prepare LCEs with spatial variation in the director orientation that enables the realization of complex 3-D shape transformation. Surface-enforced alignment is advantageous because it allows for high-resolution, nonlinear patterning of the nematic director within an LCE. When using surface alignment to enforce orientation within LCEs, multiple two-step reaction schemes have been commonly used, primarily aza-Michael oligomerization or thiol-Michael oligomerization with each addition reaction followed by photopolymerization of the residual acrylate-terminated oligomers. Here, LCEs were prepared by each polymerization technique with varied amine or thiol concentrations. The mechanical properties, thermomechanical response, reaction kinetics, and alignment were characterized and assessed for LCEs. This examination emphasizes the importance of the molecular weight between crosslinks over the reaction used to form the oligomer in determining the magnitude of the stimulus response of LCEs.