Design of Rigid Biphenyl-Pyridine Copolymers with Twisted Structures for High-Performance Epoxy Composites

Many toughening strategies have been employed to address the inherent brittleness of epoxy resin (Ep), but at the cost of strength and modulus. Herein, a series of poly(arylene pyridine) copolymers (MTAP-X) were synthesized via superacid-catalyzed polyhydroxyalkylation and blended directly with epoxy precursors in varying ratios. Both MTAP-X copolymers and epoxy composites were characterized using 1H NMR, FT-IR, TGA, DSC, DMTA, SEM, and AFM, as well as mechanical and impact testing. The results demonstrate that the twisted aromatic molecular structures of MTAP-X, controlled by the m-terphenyl to p-terphenyl ratio, enhances both the toughness and mechanical properties of epoxy composites by increasing free volume and incorporating rigid biphenyl units. As the proportion of meta-terphenyl in triphenyls increases from 30% to 60%, MTAP-X copolymers significantly enhance the tensile strength, Young’s modulus and toughness of the epoxy composites. Although further increasing the meta-terphenyl content improves toughness, it compromises the mechanical strength of the epoxy composites. At an optimal loading of 7.5 wt % MTAP-50 copolymer, the KIC, GIC, impact strength, tensile strength, and Young’s modulus increase by 92.87%, 220.39%, 93.14%, 59.66%, and 46.61%, respectively, compared to pure Ep. The toughening mechanism is attributed to the ability of adjacent phenyl groups in the MTAP-X copolymers to rotate toward each other through σ-bonds, thereby increasing free volume. Additionally, strong interfacial interactions between the pyridine ring, suspended from the polymer backbone, and the Ep contribute to the toughening effect. The rigid copolymer backbone further reinforces the epoxy network, enhancing the overall strength and durability of the Ep.