Adhesion of thermoplastic matrices to carbon fibers: Effect of polymer molecular weight and fiber surface chemistry

A structure property study between the interfacial adhesion and the polymer molecular weight, fiber chemistry and processing temperature in thermoplastic composites was undertaken. In the first phase of the study, four different molecular weight grades of pure bisphenol A polycarbonate (BPA-PC) and three different consolidation temperatures were used. Molecular weight and processing temperatures were found to have significant effect on interfacial adhesion, with a major increase in adhesion between the low molecular weight grade, which is below the critical molecular weight and the next higher molecular weight grade and a monotonous increase, but to a lesser degree above the critical limit. Further, the study shows that the interface might affect the segregation of polymer chains by their molecular weight resulting in a higher concentration of low molecular weight chains at the interface and consequent loss in mechanical properties [1]. In the second phase of the study, Hercules IM7 carbon fiber with four different levels of proprietary oxidative surface treatment and subsequent hydrogen passivation were employed to determine the effects of fiber chemistry on the interfacial adhesion. It was observed that, differing amount of surface oxygen functionalities have no effect on the level of adhesion, indicating insignificant polar or hydrogen bonding interactions between the fiber and the matrix. The level of adhesion was also found to be very weakly dependent on mechanical interlocking between the fiber and the matrix. In the third phase of the study, BPA-PC and polymethyl methacrylate (PMMA) were grafted on the surface of carbon fiber to study the effect of grafted chain length and entanglement on improvement in adhesion over ungrafted fiber composites. Grafting polymer chains to the fiber surface produced a penetrable interface, which improved adhesion levels by 25-100% over the ungrafted composites. The improvement in adhesion was independent of molecular weight of the grafted chains.