The chemical and physical aspects of the adhesion of carbon fibers to thermoplastic matrices

Radio frequency glow discharge oxygen and argon plasmas were used to modify the surfaces of PAN-based and mesophase pitch-based carbon fibers. X-Ray photoelectron spectroscopy indicated a significant increase in the amount of oxygenated functional groups on the surfaces of both types of fibers, with single bonded oxygen to carbon being the major type of functional group incorporated into the fibers' surface. Fiber wetting analysis indicated a large increase in the polar component of surface free energy of each fiber with either plasma treatment, as well as a large increase in the acid-base contribution to the theoretical work of adhesion between the fibers' surface and acid-base probe liquids. Oxygen plasma treatments increased the surface roughness of each fiber, as evaluated with scanning electron microscopy, in comparison to the untreated fibers, whereas argon plasma treatment caused no detectable change in the surface morphology of either fiber. There was no statistically significant change in fiber strength for either fiber with argon or oxygen plasma treatment. Stress transferability of the untreated and plasma treated carbon fibers were evaluated by the embedded single fiber test in poly(methyl methacrylate), poly(ethyl methacrylate), poly(methacrylonitrile), and poly(vinyl chloride), as these matrices offered various degrees of polar-dispersive and Lewis acid-base character to their total surface free energies. The presence of plasma induced oxygenated surface functional groups on both fibers was noted to improve the adhesion of these fibers to the matrices, in comparison to the untreated fibers. There was an excellent correlation of experimentally determined critical aspect ratios of the plasma treated fibers with the Lewis acid-base nature of the matrices. High temperature treatments of the plasma modified fibers, performed in argon/hydrogen atmospheres in order to reduce the plasma induced chemical changes to the fibers, were used to evaluate the effects of fiber surface roughening on adhesion of the fibers to the matrices. Oxygen plasma induced surface roughening contributed a minor role to the increased stress transferability of the pitch-based fiber embedded in the four matrices, but a moderate role for the PAN-based fiber.