Stress-Activated Conjugation-Delocalization Potential for Strengthening and Toughening of Heterocyclic Aramid Fibers

Abstract Rigid-rod aramid fibers face a formidable challenge in achieving simultaneous strengthening and toughening, as conventional incorporation of flexible ether-containing diamines improves deformability at the expense of tensile strength. Herein, the concept of a “stress-activated conjugation–delocalization potential” is proposed as a design pathway that resolves this dilemma by regulating tensile-induced conjugation–delocalization while retaining flexible linkages that ensure solution processability and toughness. Specifically, linking ether segments to the electron-donating amide-NH position causes electron-rich crowding that suppresses such delocalization and weakens load transfer; relocating them to the electron-withdrawing amide-C═O position instead redistributes local electron density and lowers the orbital-overlap barrier, enabling strain-induced planarization and enhanced π-delocalization under tension, and thus efficient intrachain load transfer. Furthermore, incorporating these optimized linkages via a block-sequence topology constructs a rigid–flexible hierarchical architecture that promotes cooperative chain packing and orientation and enables this potential to be further expressed at the aggregate level, thereby amplifying the macroscopic mechanical synergy. Experiments validate this design. Relative to the unmodified heterocyclic aramid fiber, the fiber incorporating the diphenyl ether at the amide-NH position exhibits inferior tensile strength, whereas its counterpart with the ether at the amide-C═O position exceeds it, reversing the conventional trend and outperforming the NH-positioned fiber by 23.5% in strength and 37.8% in toughness. This work establishes an electronic-structure-guided principle for the synergistic strengthening and toughening of high-performance rigid-chain polymers.