Simultaneously improving interfacial adhesion and toughness of carbon fiber reinforced epoxy composites by grafting polyethyleneimine on the carbon fiber surface to construct a rigid‐flexible interface.

Chemical grafting is commonly employed to functionalize carbon fiber (CF) surface to enhance the interfacial properties of CF reinforced polymer composites (CFRP) by forming a robust interface. However, an overly rigid interface layer can result in relatively poor interface toughness. To balance interfacial strength and toughness, this paper proposes an effective method to simultaneously strengthen and toughen the interface of CF/epoxy composites by grafting branched polyethyleneimine (PEI) on CF surface using hexachlorophosphazene (HCCP) as active intermediate, which features a rigid ring structure and six active P‐Cl groups. The introduction of PEI provided a large number of amine and imine groups on CF surfaces, which were contributive to improving surface wettability and reactivity of CF, thus resulting in strong chemical bonding between CF and epoxy matrix. Additionally, the long and flexible molecular skeleton of PEI constructed a rigid‐flexible composite interface, which could effectively reduce stress concentration and absorb impact energy. After surface modification with PEI, the interfacial shear strength and fracture toughness of CF/epoxy composite were increased to 89.9 MPa and 200.3 J m−2, by 64.3% and 149.7%, respectively, in compared with desized CF reinforced epoxy composites. Highlights: A rigid‐flexible interface was constructed between CF and epoxy resin.The interfacial strength and toughness of CFRP were synergistically improved.IFSS and interfacial toughness increased by 64.3% and 149.7%, respectively. [ABSTRACT FROM AUTHOR]