Programming Mechanoresponse Behavior in Spiropyran-Incorporating Liquid Crystal Elastomers via Direct-Ink-Writing

Mechanoresponsive polymers have received significant attention, due to their potential application in stress and/or strain sensing and damage warning. Among them, spiropyran (SP)-based mechanoresponsive polymers, which can undergo ring-opening to the merocyanine form under mechanical stress, is the most established. The majority of SP-containing mechanoresponsive polymers investigated so far uses the isotropic polymer matrices such as silicone, polyurethane and poly(methyl methacrylate). In this work, we introduce SP into the anisotropic liquid crystal elastomer (LCE) matrix (SP-LCE) to investigate the linkage between LC alignment and mechanochromic behavior. In particular, the nematic director of SP-LCE can be efficiently controlled by direct-ink-writing method. Most interestingly, the mechanochromic responses of SP-LCEs can be significantly altered depending on the stretching direction with respect to the nematic director unlike conventional SP-based mechanoresponsive polymers. The ability to spatially program the mechanoresponse behavior in the SP-LCE monolith can offer the opportunity to develop a new class of strain sensors and anti-counterfeiting materials.