Reducing Nematic–Isotropic Transition Temperature Improves Phototropic Actuation in Printable Liquid Crystal Elastomers

Abstract

Soft, phototropic polymer networks have broad potential use as stimuli-responsive, shape-morphing, and actuating materials. Despite their promising capabilities, phototropic materials that convert light-based stimuli into mechanical work have remained limited by low strain and force output. Azobenzene-functionalized liquid crystal elastomers (azoLCEs) are one common class of phototropic materials with high work densities and compatibility with additive manufacturing. While improvements in azoLCE photoactuation have been made, they tend to employ methods that require significant increases in operating temperature, hindering their broad applicability. In this work, we explore reducing the nematic–isotropic transition temperature (T ni) of azoLCEs as a route to enhancing their phototropic actuation performance. Along with characterization via differential scanning calorimetry, wide-angle X-ray scattering, and dynamic mechanical analysis, we demonstrate that azoLCEs with reduced T ni produce phototropic actuation strains of up to 30% and stress outputs of up to 12.5 kPa from 15 mg samples. We showcase this improved performance in 3D printed, multistimuli-responsive material systems, where photoresponsive azoLCE features capable of larger photoresponsive actuation induce a greater extent of global shape change. To our knowledge, this work reports the largest reversible, nonphotothermal, phototropic actuation strain achieved in LCEs. Overall, it provides an alternative route to improving the light-responsive actuation of LCEs, enabling the creation of improved actuators for applications such as adaptive and biocompatible stimuli-responsive materials, artificial muscles, and soft robotics.