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 fibe rs and poly(ether-ketone-ketone) (PEKK) was investigated using core and val ence band X-ray photoelectron spectroscopy (XPS) by using a thin film appro ach 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 re ports the interfacial reaction between fiber and matrix and the stability o f the interface in simulated environmental conditions by exposing the PEKK- coated fibers to a saline solution. The surface chemistry of the carbon fib ers was modified by electrochemical oxidation via galvanostatic treatment i n nitric acid. Interfacial chemical reaction was observed mainly between th e carbonyl group in PEKK and the hydrogen-bridged structure (HBS) on the fi ber surface to form C-O-C bonding. When the interface was exposed to a sali ne solution, it was found that the PEKK polymer was degraded by this soluti on, 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 resulte d mainly from the absorption of water and the formation of COOH functionali ty. The valence band spectra were interpreted by spectra generated from mol ecular orbital calculations.