Stiffening Liquid Crystal Elastomers with Liquid Crystal Inclusions

Liquid crystal elastomers (LCEs) are promising building blocks for soft robots, given their large, programmable, reversible, and stimuli-responsive shape change. Enhancing LCEs' stiffness and toughness has been a longstanding desire previously explored by reinforcing them with fillers, crystalline microdomains, and interpenetrating polymer networks. While promising, these methods adversely affect molecular order and thermal strain. Here, a significant enhancement of the stiffness of LCEs is reported by loading them with low molecular weight liquid crystals (LMWLCs) without sacrificing thermal strain and molecular order. While pristine LCEs rapidly transition to a soft elastic plateau when strained from poly- to monodomain, LC-loaded samples (LC-LCEs) first experience a pronounced linear elasticity, followed by a soft elastic plateau at higher stresses. Further thermomechanical and X-ray analysis confirm the emergence of an additional mesophase in polydomain LC-LCEs, which evolves to short-range smectic (cybotactic) during the poly- to monodomain transition. Monodomain LC-LCEs show between 6.5- and 9.0-fold stiffness enhancement with improved molecular order and thermal strain. Their work densities are more than double that of pristine LCEs, with active thermal stroke of up to 25% under loads of over 2000 times their weight. Such remarkable behaviors are attributed to the interplay between post-polymerization phase separation of LCs and their strain-enhanced smectic ordering. The results suggest that LMWLC inclusion can be a simple yet robust method to significantly improve the mechanical properties of LCEs.