Liquid Crystalline Hydrogel Actuator with Positive Thermal Expansion over Order–Disorder Phase Transition

Combining a liquid crystal elastomer (LCE) and a hydrogel opens a new avenue for the development of materials for soft actuators. Herein, we report a novel liquid crystalline hydrogel (LCH) consisting of a cross-linked main-chain LC polyester as the LCE and cross-linked poly(vinyl alcohol) (PVA) as the hydrogel, whose actuator, in contrast to existing main-chain LCE actuators, features perpendicular alignment of LC mesogens with respect to the stretching direction and exhibits positive thermal expansion in the stretching direction over the LC–isotropic phase transition. This peculiar actuating behavior arises from a reorientation process of mesogens, switching from parallel alignment right after stretching to perpendicular alignment after thermal equilibrium of the LCH in hot water followed by cooling, which appears to occur due to a rearrangement of the LCE domains during the thermal relaxation as well as the anisotropic swelling of the surrounding hydrogel domains and thus is unique to the LCH actuator. Unlike the hydrogel actuator, whose deformation is mostly based on a water-diffusion-induced volume change and thus is generally slow, the LCH actuator is driven by the order–disorder phase transition of mesogens and is much faster, with an actuation rate in seconds. Moreover, the LCH actuator not only brings together the reversible actuation of the LCE and the water-controlled shape memory effect of the PVA hydrogel but also is capable of optical welding, which facilitates the actuator design and fabrication.