Doping of carbon fiber into polybenzimidazole matrix and mechanical properties of structural carbon fiber-doped polybenzimidazole composites

Carbon fibers (CF) were oxidized and reduced to introduce OH, COOH, and NH2, on which polybenzimidazole (PBI) was in situ polymerized. The CF reinforced PBI composites (CF/PBI) were characterized by 1H nuclear magnetic resonance spectroscopy and thermal gravimetric analysis. The effect of oxidation and/or reduction of the CF on the chemical states of typical elements such as C, N, and O were investigated by means of X-ray photoelectron spectroscopy. The PBI and CF/PBI powders were hot-press sintered in a vacuum oven to generate structural samples, and the mechanical properties of the structural samples were evaluated using a mechanical properties testing machine. Besides, a scanning electron microscope was used to observe the microscopic features of the structural samples. It was found that COOH, COH, and NO2 could be introduced onto the surfaces of the CF using H2O2 and H2SO4/HNO3 as the oxidizing agents, while the NO2 on the surface of the CF could be reduced to NH2 by SnCl2·2H2O/HCl. This would make it feasible for PBI to be grafted to the surface of CF via in situ polymerization. Hence the mechanical properties of the structural CF/PBI were significantly improved.