Effect of Spacer and Mesogen in Side-Chain Liquid Crystal Elastomer Structure on Reversible Actuation Behavior

A liquid crystal elastomer (LCE) actuator is capable of displaying reversible shape change through order-disorder phase transition, and it is generally prepared by aligning the mesogens (often through mechanical stretching) and then cross-linking polymer chains. Herein, a series of four side-chain LCEs are synthesized by grafting side-group mesogens onto the middle block of the styrene-butadiene-styrene (SBS) triblock copolymer. These LCEs differ either in the length of the flexible spacer linking mesogen and chain backbone or in the mesogen used in their chemical structures. By means of polarized infrared spectroscopic and X-ray diffraction (XRD) measurements, the effects of spacer and mesogen on stretching-induced orientation of mesogens are investigated. The results show that varying the length of spacer or changing the mesogen has a profound effect on the orientation direction (parallel or perpendicular to the stretching direction), orientation degree (order parameter), and orientation stability to large strain. The characteristic orientation behaviors of the side-chain LCEs are retained in their respective actuators, i.e., stretched films subjected to photo-cross-linking and thermal equilibrium in the isotropic state, and determine their reversible actuation upon heating to the isotropic phase and cooling to the LC phase. In particular, the results confirm that in order for a side-chain LCE actuator to exhibit the unusual thermally induced auxetic-like shape change, i.e., its strip contracts in both length and width on heating and extends in both directions on cooling, the LCE must have a high and stable perpendicular orientation of mesogens that can compete with the conformational change of the main chain backbone aligned parallel to the stretching direction.