Selected issues in liquid crystal elastomers and gels

In this perspective, we discuss select issues investigated in recent studies on liquid crystal elastomers (LCEs) and gels-soft solids having a mobile director. A considerable amount of knowledge accumulated during the past decade proves that LCEs are not simple extensions of liquid crystals (LCs) and elastomers. The strong coupling of orientational order of LCs and rubber elasticity of polymer networks leads to novel stimulus-response behavior that provides not only a wealth of academically interesting issues but also numerous potential applications including soft actuators, components of artificial muscles, and nonlinear optical devices. We mainly focus on the responses of LCEs swollen by a low molecular mass LC to the temperature variations and electric fields. The temperature response reveals that their macroscopic volume and shape are strongly coupled with nematic order; therefore, they behave as the anisotropic gels with temperature-responsive volume and shape. In fast response to electric fields, the swollen LCEs exhibit a macroscopic deformation as well as a significant change in birefringence. This pronounced electrooptical and mechanical effect suggests that they are promising materials for electrically driven soft actuator, and it also provides an important basis to understand a LCE-specific deformation mode induced during a 90 degrees rotation of director. We also highlight some characteristic textures and distortions driven by director realignment observed in the frustrated LCEs under mechanically constrained geometries. In addition, we discuss the phase transition and slow dynamics specific to polydomain LCEs having disordered directors. We summarize the unresolved and challenging issues regarding these topics.