Optimization of interface microstructure of high‐strength‐high‐modulus polyimide fibers composites utilizing waterborne polyamide and hybrid sizing agent

Abstract The weak interface adhesion between high‐strength‐high‐modulus polyimide (PI) fibers and epoxy (EP) resin matrix has seriously hampered the application of PI/EP composites. Herein, a waterborne polyamide (WPA) sizing agent was used to modify PI fibers to enhance the interface adhesion strength between the PI fibers and EP resin matrix. The surface characteristics of PI fibers were investigated to determine chemical composition, morphology, roughness, wettability, interfacial shear strength (IFSS) and interlinear shear strength. The results indicated that the WPA sizing agent significantly enhanced the interface adhesion properties between the PI fibers and EP resin matrix with a 132% and 22.35% increment obtained for IFSS and interlaminar shear strength (ILSS) compared with unsized PI/EP composites. Then, the hybrid sizing agent generated by WPA and waterborne EP sizing agent exhibited the highest interface adhesion with 168.65% and 28.62% increment obtained for IFSS and ILSS compared with unsized PI/EP composites. Based on these results, a sizing mechanism with chemical interaction and mechanical interlocking was proposed, which provides an effective and feasible method to enhance the interface adhesion between the PI fibers and EP resin matrix.