Evaluating fiber-matrix interaction in polymer-matrix composites by inverse gas chromatography

Inverse gas chromatography was used to determine the surface energy characteristics of process-modified vapor-grown carbon fibers. These fibers have several physical and mechanical properties, including very high thermal conductivity and high aspect ratios (l/d = 100-1000), which are of interest in composite applications. However, due to the low bulk density of the 'as-received' vapor-grown carbon fibers, and small physical size of the fibers, it was necessary to develop a sampling method which allowed for easy column preparation, but gave accurate, reproducible results. For this reason, the fibers were analyzed before and after modification by solvent washing to determine how removing polycyclic aromatic hydrocarbons from the surface would affect measurement statistics. Oxidation of the fiber surfaces was evaluated by comparing modified fibers with a standard fiber. In addition, the effects of silylation on surface properties was examined in order to reveal the presence of active hydrogens on the fiber surface. In this paper, surface energy measurements for various surface treatments are compared and described. Results indicate that the dispersive surface energies (gamma(s)(d)) of the fibers decrease when the fibers are oxidized due to disruption of the graphitic plains. Sampling methodology had a significant impact on reproducibility. Solvent washing of the fibers prior to packing of the column improved the relative standard deviation for the unoxidized fibers from 18.8% to 2.2% without dramatically altering the gamma(s)(d) of approximately 50 dynes cm(-1). Furthermore, derivitization by N,O-bis(trimethylsilyl)trifluoroacetamide reveals the presence of active hydrogens on the surface of the fibers. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.