Rigid-flexible synergistic interphase constructed via segmental engineering for concurrent strengthening and toughening of AF/PEEK composites

The abrupt modulus mismatch and physicochemical incompatibility across the interface of aramid fiber reinforced polyetheretherketone (AF/PEEK) composites frequently induce stress concentration and micro-crack sensitivity under dynamic service conditions. Herein, a “rigid-flexible synergistic” interfacial engineering strategy is proposed to construct a stress-buffering interphase by tailoring the segmental stoichiometry of aqueous polyamic acid (PAA) sizing agents. Nanomechanical characterizations reveal that the optimized sizing transforms the abrupt boundary into a 549.8 nm modulus-gradient transition zone. It establishes a hierarchical energy-dissipation mechanism wherein rigid aromatic networks guarantee stress transmission, while aliphatic flexible segments dissipate excessive fracture energy through micro-elastoplastic yielding. Consequently, the modified composites successfully overcome the strength-toughness trade-off, delivering an 80.9% increase in interfacial shear strength, an 86.4% surge in interlaminar shear strength, 68.9% and 44.7% improvements in tensile and flexural strengths, respectively, as well as a 72.9% boost in impact toughness. Furthermore, the mechanically buffered boundary demonstrates exceptional dynamic tribological reliability under reciprocal sliding. This study provides a robust molecular engineering approach to concurrently optimize interfacial adhesion and toughness, offering broader insights into the design of highly durable composites for demanding service conditions.