Unusual stretching–twisting of liquid crystal elastomer bilayers

Abstract

Liquid crystal elastomers (LCEs), as a unique class of smart soft materials combining the properties of liquid crystals and hyperelasticity, are capable of rapid, anisotropic, and reversible deformations in response to mechanical, thermal or optical stimuli. Here, we report a hitherto unknown stretching-induced twisting behavior of LCE bilayer strips. Under uniaxial stretching, we reveal that due to the spontaneous mismatch strain arising from interlayer anisotropy, the bilayer strips exhibit notable twisting deformations. We develop an LCE bilayer strip model based on semi-soft elasticity to quantitatively understand and predict such intriguing tension-twisting response. Based on our experiments and theoretical analyses, we systematically explore how the liquid crystal director orientation, geometric dimensions and material parameters of the strips would affect the twisting behavior. We find that when the alignment of directors of bilayer are symmetric about the stretching direction, a larger deviation angle of the initial directors results in a more significant twisting deformation. Additionally, a longer, narrower and thicker strip has a more pronounced twisting effect. Furthermore, the material anisotropy encourages the twisting, while the feature of semi-soft elasticity discourages it. The findings not only reveal the tension-twisting coupling behavior of LCE bilayer strips, but also offer new insights into the design of LCE actuators, intelligent structures and soft robots.