Control of surface structure of graphite fibres for improved composite interfacial properties

Abstract

The properties of carbon fibres modified by aqueous electrochemical synthesis of pyrrole have been determined by using a dynamic contact angle analyser, scanning electron microscopy, and Fourier transform infrared spectroscopy (FTIR). Electrochemical process parameters such as initial pyrrole concentration, electrolyte concentration, applied voltage, electrolyte type, and reaction time were systematically varied and their impacts on the polypyrrole–carbon fibre interphase surface free energy and morphology ascertained. The surface free energies of the polypyrrole–carbon fibre interphases were obtained using single fibre filaments. SEM analysis of the interphases revealed several distinct surface structures, including smooth, porous, granular, microspheroidal, and plateletlike morphologies. Non-coated, commercially surface oxidised carbon fibres have smooth surface morphology with occasional longitudinal striations. FTIR analysis of the polypyrrole interphases confirmed that the counterions derived from the electrolytes were incorporated into the film. The surface free energies of the electrochemically formed polypyrrole–carbon fibre interphases (0·06–0·075 J m_-2) were determined to be up to 40% higher than those of surface oxidised but unsized carbon fibres (0·05 J m^-2). This improvement in the surface free energies of the polypyrrole–carbon fibre interphases suggests easy wettability by polymer matrices such as epoxy resin and polyimide (surface free energies around 0·047 and 0·045 J m^-2 respectively).