Intrinsically Flame-Retardant Liquid Crystal Elastomers With Flame-Triggered Actuation Enabled by Main-Chain P─C Bond Incorporation.

Abstract

Liquid crystal elastomers (LCEs) are promising soft actuators, but their practical use is constrained by the incompatibility between fire safety and reversible actuation. Extrinsic flame-retardant strategies can disrupt the liquid-crystal organization and network integrity required for deformation, whereas existing phosphorus-containing liquid crystal polymers (LCPs) are mainly rigid, aromatic thermoplastics designed for structural heat resistance rather than actuation. Here, we report a radical-mediated P─H/ene step-growth polymerization strategy for intrinsically flame-retardant organophosphorus LCPs and LCEs using hypophosphorous acid and diene-terminated mesogenic monomers. This main-chain P─C bond-forming strategy avoids rigid aromatic phosphorus units and affords phosphorus-containing liquid-crystalline systems with reduced phase-transition temperatures together with a high phosphorus content of up to 6.8 wt.%. The resulting LCEs retain thermotropic liquid-crystalline behavior, exhibit reversible thermoactuation, and show pronounced flame retardancy characterized by reduced heat release, self-extinguishing behavior, and phosphorus-promoted char formation. Notably, these organophosphorus LCEs also enable flame-triggered actuation within only 0.4 s while preserving structural integrity and reversible function after flame exposure. This work establishes a practical molecular design route toward intrinsically flame-retardant LCE actuators for thermally harsh environments.