Surface characterization of carbon fibers by inverse gas chromatography at low pressures
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J.L. Fierro Instituto de Cata´lisis y Petroquı´mica, Consejo Superior de Investigaciones Científicas, Tres Cantos, Madrid, Spain
M.C. Gutierrez Instituto de Te´cnica Aeroespacial, Instituto Nacional de Técnica Aeroespacial, Torrejo´n de Ardoz, Madrid, Spain
J.L. Oteo Instituto de Cera´mica y Vidrio, Consejo Superior de Investigaciones Científicas, Arganda del Rey, Madrid, Spain (Received 20 September 2001; accepted 19 November 2001)
Carbon fiber surfaces were analyzed by inverse gas chromatography (IGC) and x-ray photoelectron spectroscopy (XPS). IGC measurements were carried out at infinite and finite dilution by using neutral and specific probes. At infinite dilution the dispersive component of the surface free energy and the acid–base indexes were obtained. At finite dilution the energy distribution functions were calculated. Three carbon fibers were analyzed, one untreated and two fibers treated with different sizings. The fibers have similar dispersive components of surface free energy but their acid–base characteristics are markedly different. The untreated fiber has an acidic surface, and the sized fibers have a surface with high base character. The energy distribution functions show different peaks assigned to active sites existing on the carbon fiber surfaces. The use of acid or basic probes provides different energy distribution functions with good correlation with the base and acid character of the fiber surface. Changes in surface heterogeneity revealed by energy distribution functions were correlated also with surface chemical composition derived from high-resolution XPS measurements. I. INTRODUCTION
Modern composite materials specially used in aerospace applications are mainly fabricated by carbon fibers and different types of organic matrices (such as epoxy, phenyl-ester, poly-ether-ether-ketone, ciano-ester, etc. It is well known in these materials that for a given fibermatrix system, the mechanical properties are influenced to a great extent by the fiber–matrix interface.1,2 In the interface region any stress given to the composite is transmitted from the matrix to the fibers, which are responsible for supporting such stress. Therefore, the performances of the composite material are determined by the characteristics of the interface. For a given matrix– fiber system, the interface is dependent on the physicochemical interactions existing between matrix and fiber. These interactions are carried out through the surface of each material (matrix and fiber) when they are put in contact during composite processing. It is then very important to characterize the surface of fibers and/or matrices to understand the interface formed in the final composite material. J. Mater. Res., Vol. 17, No. 2, Feb 2002
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Different methods have been employed to characterize the surface of carbon fibers, for example, Fourier transform infrared (FT-IR),3 x-ray photoelectron spectroscopy (XPS),4 scanning electron microscope (SEM),5 wetting and electrokinetic measurements
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