Isomorphic α-Crystal and Amorphous Multiscale Structural Evolution Enables Heat-Resistant and High-Strength Semiaromatic Copolyamide Fibers

The mechanical and thermal performance of polyamide fibers emerges from the coupled interplay of molecular chemistry and condensed-state morphology; accordingly, property optimization demands codesign of chain architecture and hierarchical organization. We therefore selected poly(hexamethylene adipamide) (PA66) as a matrix and introduced semiaromatic terephthalamide (6T) units via copolymerization to form PA66-co-PA6T. High-strength fibers were produced by melt spinning followed by postdrawing to the maximum draw ratio and subjected to multiscale characterization of chain conformation, hydrogen-bond networks, crystalline polymorph, lamellar long period, and orientational hierarchy. Within a three-phase framework, we identify a compositional window near ∼10 mol % 6T, where the melt retains sufficient mobility to enable isomorphic crystallization of the high-strength α polymorph, while postdrawing selectively enhances amorphous orientation. The resulting coassembled architecture-α-crystals embedded in a highly oriented amorphous network-delivers a tensile strength of 7.46 cN/dtex and exhibits markedly improved strength retention after thermo-oxidative conditioning relative to neat PA66. These results establish a structure–property map for heat-resistant, high-strength copolyamide fibers and provide actionable design rules for next-generation high-performance polyamides.