X-ray photoelectron spectroscopic study of carbon fiber surfaces. 23. Interfacial interactions between polyvinyl alcohol and carbon fibers electrochemically oxidized in nitric acid solution

The paper examines the interaction of a carbon fiber surface, surface-treated by galvanostatic electrochemical oxidation in nitric acid solution, with poly(vinyl alcohol) (PVA). The surface chemistry of the fiber before and after interaction was examined by core and valence band X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscopy (AFM). Various surface treatment conditions were applied, with the previously reported hydrogen-bridge oxide structure (HBS) being the predominant species under most conditions, with much smaller amounts of surface hydroxide, carbonyl, and carboxyl groups. The amount of surface roughening increased with the level of surface oxidation. The surface of the fibers became seriously damaged when the oxidation was conducted for prolonged periods of galvanostatic treatment (more than 60 s at 0.5 A). Interfacial chemical interaction was observed between the oxidized carbon fiber surface and the PVA, by examination of a film of PVA on the fiber surface that was sufficiently thin to allow the interface region to be examined. The valence band XPS spectrum of PVA is predicted by a band structure calculation, complemented by molecular orbital calculations based upon the polymer repeat unit. The valence band spectrum is seen to be an effective fingerprint of the PVA structure. An interfacial reaction was found to occur between the oxidized fiber and PVA involving reaction between the hydroxide group in PVA and the HBS oxide on the fiber surface. The results have a wider relevance to size molecules containing -OH functionality and adjust the interfacial bonding in composites.