Surface chemistry and adhesion in carbon fiber reinforced epoxy microcomposites

In an extensive series of experiments, PAN based carbon fiber was subjected to electrochemical oxidation under a wide variety of conditions. In this part of the study, the activity and chemical composition of fibers oxidized in sulfuric acid were characterized by cyclic voltammetry (CV), diffuse reflectance infrared spectroscopy (DRIFT) and X-ray photoelectron spectroscopy (XPS). Interfacial adhesion was measured in epoxy microcomposites by fragmentation. The results proved that the type and amount of functional groups formed on the surface of the fiber depends very much on the conditions of oxidation. In sulfuric acid, a large number of sulfoxides also form besides the usual functional groups containing carbon and oxygen. The surface concentration of all functional groups increases with the intensity of oxidation, but to a different extent. Only a few of them increase interfacial interaction. Sulfur-containing groups and quinoidal compounds are not very reactive, while carboxyl groups seem to be the most active in the improvement of adhesion. Chemical reactions take place on the surface of the fiber during the curing of the resin, which depend on the chemical composition of both the surface and the resin system. The chemical activity of the fiber can be characterized by cyclic voltammetry, because the number of electroactive and reactive groups increase with the intensity of oxidation in a similar way. Although the removal of a weakly adherent layer during oxidation may contribute to the improvement of adhesion, chemical coupling plays a similarly important role.