20: Review of Liquid Crystal Elastomers

Chapter 20 reviews liquid crystal elastomers (LCEs), first discussed by Finkelmann. These materials can be used as robotic actuators via inducing a nematic–isotropic phase transition in them upon a temperature increase, which causes them to shrink, as described in a thorough review on these intelligent multi-functional materials by Brand and Finkelmann. LCEs have been made electroactive by creating a composite material that consists of monodomain nematic LCEs and a conductive phase such as graphite or CP that are distributed within their network structure. The actuation mechanism of these materials involves phase transition between nematic (cholesteric, smectic) and isotropic phases over less than a second. The reverse process is slower, taking about 10 seconds, and requires cooling the LCE back to its initial temperature as the LCE expands back to its original size. The mechanical properties of LCE materials can be controlled and optimized by effective selection of the liquid crystalline phase, density of cross-linking, the flexibility of the polymer backbone, the coupling between the backbone and liquid crystal group, and the coupling between the liquid crystal group and the external stimuli.