X-ray Photoelectron Spectroscopic Studies of Carbon Fiber Surfaces. 25. Interfacial Interactions between PEKK Polymer and Carbon Fibers Electrochemically Oxidized in Nitric Acid and Degradation in a Saline Solution

The interfacial interaction between surface-treated pitch-based carbon fibers and poly(ether-ketone-ketone) (PEKK) was investigated using core and valence band X-ray photoelectron spectroscopy (XPS) by using a thin film approach that allowed the buried interface to be investigated by XPS. The extent of interaction of the surface-treated fiber and the matrix material has a significant effect on the strength of the ultimate composite. This paper reports the interfacial reaction between fiber and matrix and the stability of the interface in simulated environmental conditions by exposing the PEKK-coated fibers to a saline solution. The surface chemistry of the carbon fibers was modified by electrochemical oxidation via galvanostatic treatment in nitric acid. Interfacial chemical reaction was observed mainly between the carbonyl group in PEKK and the hydrogen-bridged structure (HBS) on the fiber surface to form C−O−C bonding. When the interface was exposed to a saline solution, it was found that the PEKK polymer was degraded by this solution, together with some additional X-ray damage. It was found that there was a change in the surface functionality that suggested the replacement of C O bonds at the interface to COOH and/or C−OH bonds. The degradation of the interfacial chemistry between the fiber and PEEK in saline solution resulted mainly from the absorption of water and the formation of COOH functionality. The valence band spectra were interpreted by spectra generated from molecular orbital calculations.