Crosslinking-free preparation of thermoplastic triblock liquid crystal elastomer actuators

Abstract

A previous study of liquid crystal elastomers (LCEs) synthesized through post-functionalization of the styrene–butadiene–styrene (SBS) triblock copolymer revealed two important advantages. (1) The starting material SBS is a commercially available thermoplastic elastomer. (2) The stretchability stemming from the microphase separation in SBS allows the monodomain actuator to be prepared by stretching the LCE in the nematic phase, followed by one-step chain crosslinking, in contrast with the usual two-step crosslinking. Herein, we unveil an even more appealing advantage of SBS-based LCEs: no crosslinking step is required in making the monodomain actuator. Our study found that by just stretching the LCE at an elevated temperature with mesogens in the isotropic phase and the microphase-separated polystyrene (PS) nanodomains in the rubbery state, cooling the LCE under strain leads to macroscopic alignment of mesogens self-locked by glassy PS nanodomains acting as physical crosslinks. Our investigation suggests that the occurrence of two events on cooling – the spontaneous alignment of mesogens in an anisotropic medium while entering the nematic phase and the rubbery-to-glassy state transition of PS nanodomains – does the work of transforming the strained sample into an LCE actuator. This facile, crosslinking-free processing represents a general mechanism extendable to LCEs derived from similar tri- or multi-block copolymers. Moreover, the thermoplastic nature of LCE actuators having no chemical crosslinking means reprocessability, which we used to demonstrate intricate nonlinear shape changes and movements.