Liquid Crystal Elastomers Prepared by Thiol–Ene Photopolymerization Amenable to Surface-Enforced Alignment

Abstract

Liquid crystal elastomers (LCEs) assimilate the anisotropy of liquid crystalline phases with the rubber elasticity of polymer networks. Numerous recent reports detail the preparation of LCEs by sequential chain extension reactions of diacrylate liquid crystalline monomers followed by the photopolymerization of liquid crystalline oligomers. While these reactions are widely utilized, sample fabrication can take hours or even days. Other reports detail the preparation of LCEs via direct thiol−acrylate photopolymerization via free radical chain transfer reactions. While this is a rapid and straightforward approach to preparing LCEs, the topology of the polymer network composition is heterogeneous, with wide variance in molecular weight between cross-links and extensive prevalence of pendant groups. Here, we detail the preparation of LCEs via thiol−ene photopolymerization. The compositions introduced here are amenable to surface-enforced alignment, which enables the high-fidelity inscription of complex director profiles. Compared to prior thiol-based LCE compositions, direct preparation of LCE by thiol−ene photopolymerization is rapid, forms largely homogeneous polymer networks, and has wide tunability in cross-link density.

Enhanced Electromechanical Output in Liquid Crystal Elastomers Prepared by Thiol‐ene Photopolymerization

“…[49] Green diamonds represent materials prepared via thiol-ene photopolymerization. [52] modulus in each direction. However, the ratio of Young's modulus (e.g., "modulus contrast") is consistent (3.2 for LCE-c versus 3.1 for LCE-r).…”

“…Expanding on the recent report, we add a liquid crystalline, allyl ether functionalized monomer to affect the mechanical properties of the LCE. [52] This material system greatly improved the electromechanical response in surface-aligned thiol-ene LCEs by lowering Young's modulus without loss in modulus contrast. Further, this composition considerably improved cyclic recoverability due to eliminating yield and necking.…”

“…As a starting point, we canvased the literature for Young's modulus values of LCEs aligned by surfaceanchoring. [48][49][50][51][52] In particular, we isolate Young's modulus perpendicular to the direction of alignment (i.e., the direction of electromechanical deformation) as well as Young's modulus contrast (i.e., the ratio of the parallel and perpendicular Young's modulus) as representations of mechanical anisotropy. This analysis is summarized in Figure 2.…”

“…Our recently developed materials chemistry utilizing thiol-ene polymerization emerges as a promising approach to make very soft LCEs, with some of the lowest reported Young's modulus values perpendicular to the direction of alignment for a surface-aligned material. [52] LCEs prepared from "free-radical chain transfer" (e.g., certain thiol-acrylate reactions) also make soft LCEs with much higher modulus contrast. This approach was utilized in our prior report.…”