Photothermally driven liquid crystal polymer actuators

Liquid crystal polymers (LCPs) have emerged as a material of choice for soft actuators, for which applications have been envisioned in many areas. In contrast to the actuators based on polymer hydrogels whose reversible volume or size change relies on absorption and release of water molecules by the polymer related to a thermal phase transition of the polymer solution, LCPs exhibit macroscopic shape change as a result of LC-isotropic, or order-disorder, phase transition of the mesogens that are part of the polymer structure either in the main chain or as side groups. LCP actuators, in the form of a crosslinked network, can be triggered by various stimuli such as change in temperature, change in humidity, light and electric power. Of these, light is a particularly attractive stimulus owing to the attributes of remote, localized or patterned activation. Until now, the large majority of light-triggered LCP actuators, generally referred to as liquid crystal elastomers (LCEs) or liquid crystal networks (LCNs), are azobenzene-containing polymers, for which the order-disorder phase transition of the azobenzene mesogens is induced by the reversible trans-cis photoisomerization of the chromophore. In recent years, however, there has been growing interest in photocontrolled LCP actuators that involve no photochemical reactions but use simply a photothermal effect to control the order-disorder phase transition. This review is focused on such photothermally driven LCP actuators. Highlighted are examples of reported studies demonstrating the actuation modes and possible applications. We also discuss the advantages of using the photothermal effect and the possibilities of actuator designs. At the end, we provide an outlook for the development of this type of polymer actuator in the near future.