Establishment of multistage gradient modulus intermediate layer between fiber and matrix via designing double “rigid-flexible” structure to improve interfacial and mechanical properties of carbon fiber/resin composites

The gradient modulus intermediate layer between CF and epoxy by designing double “rigid-flexible” structure using CNTs and polyamide (PA) on the CF surface was firstly established, and the effects of modulus intermediate layer on interfacial and mechanical properties of CF composites were investigated. The constructing of double “rigid-flexible” structure on the CF surface can greatly increase the polar functional groups, roughness and wettability of the CF surface as well as the thickness of intermediate layer, which could balance the modulus of fiber and resin to achieve the best matching. The interfacial shear strength (IFSS), interlaminar shear strength (ILSS), flexural strength, and impact strength of CF composites increased by 75.6%, 44.1%, 41.3%, and 34.6%, respectively. In addition, the interfacial failure behaviors and strengthening mechanisms of composites have been studied. This novel design and method built a multistage gradient modulus intermediate layer of composite with strong physicochemical interaction and good wettability, which has greatly potential for high-performance composite.