Skin–Core Structural Homogeneity and Stress-Induced Ordering Enable Extrinsic Toughening in Rigid-Chain Heterocyclic Aramid Fibers

Achieving both high strength and high toughness of materials remains inherently challenging, especially for rigid-chain polymer fibers, where intrinsic chain rigidity and high molecular orientation further exacerbate the tradeoff. Here, we report a heterocyclic aramid (PBIA) fiber that achieves a tensile strength of 34.18 cN/dtex and an exceptional toughness of 117.17 MJ/m3, markedly outperforming commercial Kevlar 29 and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers. To reveal the structural origins of this synergistic enhancement, we systematically characterized the hierarchical microstructure and fracture behavior of PBIA fibers in comparison with Kevlar 29 and PBO fibers using in situ synchrotron X-ray scattering and tensile scanning electron microscopy. Unlike the typical heterogeneous skin–core morphology in Kevlar 29 and PBO, PBIA fibers feature a relatively homogeneous skin–core structure with minimal variations in crystallinity and orientation. This structural uniformity results in a distinctive internal “thin-necking” fracture mode during tensile deformation without any skin–core delamination. To the best of our knowledge, this fracture mechanism has been reported here for the first time in high-performance polymer fibers. Moreover, upon stretching, PBIA fibers undergo stress-induced structural ordering, characterized by increased crystallinity and orientation, which further retard crack propagation. This synergy between structural homogeneity and stress-induced ordering in PBIA fibers enables extrinsic toughening, thus achieving simultaneous improvements in both strength and toughness.