Establishment of “Rigid–Flexible” Transition Interface between Carbon Fiber and Resin Matrix for Interfacial Adhesion Enhancement

A “rigid–flexible” transition interface structure was established between carbon fibers (CFs) and the resin matrix to improve interfacial adhesion. The CF surface was functionalized via polydopamine/polyethylenimine (PEI) codeposition, following chemical grafting of flexible polyurethane (PU) and branched PEI polymers. The surface characteristics of the functionalized CFs and the corresponding interfacial adhesion were investigated detailedly. The results manifested that the functionalization modification endowed the CF surface with abundant active groups, such as –NH₂ and –NH, and increased surface roughness and surface energy, which were favorable to promote the interfacial adhesion. The interfacial shear strength and interlaminar shear strength were increased by 49.4% and 43.2%, respectively. The flexural strength and flexural modulus were increased by 35.5% and 45.9%, respectively. The flexible interface, consisting of PU, established strong chemical bonds between CFs and the resin matrix, which could function as a tear resistance layer and transfer uniform stress, therefore significantly improving the interfacial interaction.