Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites

The local microstructure can be altered significantly by various fibre surface modifications, causing property differences between the interphase region and the bulk matrix. By using tapping mode phase imaging and nanoindentation tests based on the atomic force microscope (AFM), a comparative study of the sized fibre surface topography and modulus as well as the local mechanical property variation in the interphase of E-glass fibre reinforced epoxy resin and E-glass fibre reinforced modified polypropylene (PPm) matrix composites was conducted. The phase imaging AFM was found a highly useful tool for probing the interphase with much detailed information. Nanoindentation experiments indicated the chemical interaction during processing caused by a gradient profile in the modulus across the interphase region of gamma-aminopropyltriethoxy silane (gamma-APS) and polyurethane (PU)-sized glass fibre reinforced epoxy composite. The interphase with gamma-APS/PU sizing is much softer than the PPm matrix, while the interphase with the gamma-APS/PP sizing is apparently harder than the matrix, in which the modulus was constant and independent of distance away from the fibre surface. The interphase thickness varied between less than 100 and approximate to300 nm depending on the type of sizing and matrix materials. Based on a careful analysis of 'boundary effect', nanoindentation with sufficient small indentation force was found to enable measuring of actual interphase properties within 100 nm region close to the fibre surface. Special emphasis is placed on the effects of interphase modulus on mechanical properties and fracture behaviour. The interphase with higher modulus and transcrystalline microstructure provided simultaneous increase in the tensile strength and the impact toughness of the composites. (C) 2002 Elsevier Science Ltd. All rights reserved.