Photoinduced deformation of crosslinked liquid crystalline polymers and soft actuators

Abstract

Photoresponsive polymeric materials are a type of functional materials that can absorb photo energy and undergo intra- or inter-molecular physical or chemical reactions. As a result, the materials exhibit macroscopic property changes when exposed to light, such as shape, color or refractive index. They attract more and more attention for the reason that light is an environmental-friendly and remotely controllable stimulus source. Liquid crystals exhibit cooperative motion of molecules in changing the molecular alignment by external stimuli. That means that only a small amount of energy is needed to bring about the alignment change in the whole system. Crosslinked liquid crystalline polymers (CLCPs) can be prepared by crossliking conventional liquid crystal polymers into a network, which possess both the order of liquid crystals and the elasticity of elastomers. The cooperative effect of liquid crystals coupled with polymer network structure gives rise to the characteristic properties of CLCPs. The initial ordering of mesogens can be fixed by the crosslinkers in CLCPs, which might cause a quick change in shape due to a fast order-disorder transition. CLCPs have been a hot topic because large deformation can be induced by changing the alignment of mesogens in CLCPs by external stimuli such as electric fields, changes in temperature, humidity and light. Incorporation of photochromic moieties into CLCPs provides the materials with photo controllable properties. Photoresponsive CLCPs can generate sophisticated movements including contraction/expansion, bending and twisting which convert photo energy into mechanical energy directly. Therefore, the CLCPs can be fabricated into various soft smart actuators by rational design, which brings about broad applications in artificial muscles, microrobots, microvalves, sensors and so on. In this review, the photoresponsive mechanism is firstly elucidated involving photochemical phase transition and photo realignment. Then, we detail the historical development of photoresponsive CLCPs, with emphasis on light- driven mechanical responses concerning two dimensional and three dimensional movements and surface property changes caused by deformation. We also summarize the recent progress of light-driven soft actuators made of photoresponsive CLCPs. We end with an outlook of existing challenges and opportunities.