Liquid Crystalline Hydrogel Capable of Thermally-induced Dual Actuation

Abstract

Stimuli-responsive shape-changing materials, particularly hydrogel and liquid crystal elastomer (LCE), have demonstrated significant potential for applications across various fields. Although intricate deformation and actuation behaviors have been obtained in either hydrogels or LCEs, they typically undergo reversible shape change only once (e.g., one expansion plus one contraction) during one heating/cooling cycle. Herein, we report a study of a novel liquid crystalline hydrogel (LCH) and the achievement of dual actuation in a single heating/cooling cycle by integrating the characteristics of thermoresponsive hydrogel and LCE. The dual actuation behavior arises from the reversible volume phase transition of poly(N-isopropylacrylamide) (PNIPAM) and the reversible order-disorder phase transition of LC mesogens in the LCH. Due to a temperature window separating the two transitions belonging to PNIPAM and LCE, LCH actuator can sequentially execute their respective actuation, thus deforming reversibly twice, during a heating/cooling cycle. The relative actuation degree of the two mechanisms is influenced by the mass ratio of PNIPAM to LCE in the LCH. Moreover, the initial shape of a bilayer actuator made with an active LCH layer and a passive polymer layer can be altered through hydration or dehydration of PNIPAM, which further modifies the dual actuation induced deformation. This work provides an example that shows the interest of developing LCH actuators.