Rigid-Flexible Integrated Epoxy Toughener: Achieving Simultaneous Enhancement of Strength, Toughness, and Glass Transition Temperature via π–πF Conjugation Effect

Epoxy resins (ERs) are frequently employed in high-performing composites for their superior comprehensive performances. However, their inherent brittleness limits broader applications, and current toughening strategies often face a trade-off between strength, toughness, and thermal stability. Herein, we developed a rigid-flexible integrated epoxy toughener (2,3,5,6-tetrafluorobenzenethiol-grafted diallyl bisphenol A diglycidyl ether, abbreviated as 4FDA), integrating rigid tetrafluorobenzene structure and flexible short alkyl chains into one epoxy monomer. Various contents of 4FDA were used to modify conventional epoxy curing systems. The results show that the 4F-7.5 system achieves simultaneous enhancement of T g -DSC (137.8°C, 7.6°C increase), toughness (7.8% of elongation at break, 43% increase), and tensile strength (91.5 MPa, 17% increase) compared with the unmodified epoxy (EP) system. SEM analysis demonstrates a crazing-shear band toughening mechanism. Furthermore, carbon fiber composites based on the 4F-5 matrix exhibit a 17.1% improvement in interlaminar shear strength (ILSS). The improved performance is credited to the synergistic effect of π–π F conjugation strengthening effect (from the rigid tetrafluorobenzene structure) and flexible short-chain toughening effect (from flexible short alkyl chains). This work provides a simple, green, and adjustable approach to fabricate high-performing epoxy matrix with balanced thermal and mechanical performances, promising applications in aerospace, automotive, and electronic packaging.