Molecular Engineering of Strong, Flame‐Retardant and Closed‐Loop Recyclable Crosslinked Polyamide for All‐Aramid Composites

ABSTRACT Aramid fiber reinforced composites (AFRCs) are widely used in military, aerospace, and transportation applications. However, the non‐recyclability and flammability of traditional epoxy‐AFRCs remain significant challenges that limit their future developments. Herein, a flame‐retardant and closed‐loop recyclable crosslinked network based on poly‐m‐phenylene isophthalamide (PMIA) is reported for novel AFRCs. The deliberately incorporated phosphorus (P) element in thermally stable PMIA molecular chains, and the selected P and nitrogen riched aldehyde‐cyclophosphazene crosslinkers endows the crosslinked P‐containing PMIA (CCPMIA) with exceptional flame‐retardant properties. The tailor‐made P containing PMIA segments and optimized crosslinking ratio with crosslinkers also enable CCPMIA to exhibit excellent mechanical strength. The good interfacial compatibility between CCPMIA and para ‐aramid fibers confers comparable interfacial shear strength and mechanical properties of CCPMIA‐AFRCs to those of traditional epoxy‐AFRCs, while the highly flame retarding CCPMIA matrix significantly enhances the flame retardancy of CCPMIA‐AFRCs with high limited oxygen index of 39.8% and the highest UL‐94 rating of V‐0. Additionally, the recycling of the CCPMIA matrix and para ‐aramid fibers without deterioration on intrinsic properties is achieved by exploiting the catalytic effect of water and heat on the reversible reaction of imine bonds, offering a facile way for the recycling of imine‐based materials.