Closed-Loop Chemically Recyclable Liquid Crystal Elastomers via Selective Hydrolysis of Boronic Ester Crosslinks

Abstract

Liquid crystal elastomers (LCEs) are promising soft actuators, yet their permanently crosslinked networks severely limit end-of-life recyclability. Although dynamic covalent bonds have enabled reprocessability, closed-loop chemical recycling that allows high-yield recovery of reusable molecular building blocks while preserving actuation performance remains challenging. Here, we report a closed-loop chemically recyclable LCE constructed from commercially available reagents via a two-step synthesis. The network is crosslinked by boronic ester bonds formed between a vicinal-diol-bearing liquid crystal oligomer and 1,4-phenylenediboronic acid (PBBA). By varying the PBBA content, the thermal, mechanical, and actuation properties of the LCEs can be systematically tuned. The optimized monodomain LCEs exhibit reversible thermal actuation with good cycling stability. Under mild solvent-assisted hydrolysis conditions, the boronic ester crosslinks are selectively cleaved, while the thioether backbone remains intact, enabling high-yield recovery of both PBBA and the LC oligomer. The recovered components can be directly reconstituted into renewed LCE actuators with thermal, mechanical, and actuation properties closely matching those of the pristine materials. A cascading closed-loop recycling demonstration further shows that the same recovered building blocks can be successively transformed from a 2D film actuator to a 3D tubular actuator and then to a thermally responsive smart surface. This work provides a practical component-level closed-loop recycling strategy for sustainable LCE actuators.