Molecular Design of Aramid Copolymer for Enhanced Toughness and Retention Performance.

Aramid fibers are valued for their high strength, stiffness, and thermal stability, but their intrinsic chain rigidity restricts toughness and fatigue resistance. In this study, aramid copolymers containing heteroaromatic 6-(4-aminophenoxy)-pyridine-3-amine (APA) and extended aromatic 4,4'-bis-(4-aminophenoxy)-biphenyl (BABP) were synthesized by solution polycondensation and processed into fibers through wet spinning. The incorporation of APA improved ductility through dynamic hydrogen bonding, whereas BABP reinforced stiffness and strength through strong π-π stacking and rigid biphenyl alignment. The combination of these comonomers with 3,4'-oxidianiline (3,4'-ODA) further enhanced drawability and molecular orientation, resulting in a synergistic improvement in toughness and durability. Under cyclic loading, APA- and BABP-based fibers retained up to 78% and 64% of their initial tenacity, respectively, compared with 34% for the 3,4'-ODA fiber. Corresponding energy retention exceeded 50% in APA and 35% in BABP, accompanied by lower hysteresis losses. These results reveal that targeted comonomer design effectively balances strength, ductility, and fatigue resistance by tuning hydrogen bonding and π-π stacking. The findings provide molecular-level insights for developing next-generation aramid fibers capable of sustained performance in demanding mechanical and thermal environments.