Grafting carbon nanotubes densely on carbon fibers by poly(propylene imine) for interfacial enhancement of carbon fiber composites

Due to the weak interfacial bonding caused by the poor wettability and adsorption between carbon fibers (CFs) and matrix, CFs cannot fully exert their excellent mechanical properties in carbon fiber reinforced polymer composites. To settle this problem and bring a significant increase of interfacial strength in the composites, chemical grafting method was applied to fabricate the hierarchical reinforcement structure containing carbon nanotubes (CNTs) and CFs using poly(propylene imine) (PPI) molecules as bridges, which had a giant amount of amino groups at the periphery of molecules. CNTs were grafted on the surface of CF homogeneously and firmly with an extremely high grafting density. The surface free energy increased about 66% compared to the acid-treated carbon fiber, due to the obvious change of surface morphology and the increase of polar functional groups. The interfacial shear strength (IFSS) of the CNTs-grafted CF composite greatly increased by as much as 109% compared with that of CF composite without functionalization. The mechanism of this tremendous enhancement was also illustrated in details.