Nanoscale characterisation of thickness and properties of interphase in polymer matrix composites

An overview is presented of the properties and effective thickness of the interphase formed between fibres and polymer matrices. Chemical and physical characterization of the interphase is discussed to portray molecular interactions comprising the interphase layers in silane-treated glass-fibre composites. The gap between physico-chemical investigation on one side and bulk material testing on the other side is bridged by implementation of novel techniques, such as nanoindentation, nanoscratch tests, and atomic force microscopy (AFM), which have been successfully used for nanoscopic characterization of the interphase in the past few years. Salient differences are identified between the major findings of these studies in terms of hardness/modulus of the interphase relative to the bulk matrix material. While there is a significant "fibre stiffening" effect that may cause misinterpretation of the interphase hardness very close to the fibre, the formation of both a softer and a harder interphase is possible, depending on the combination of reinforcement, matrix, and coupling agent applied. This is explained by different interdiffusion behaviour, chemical reactions, and molecular conformation taking place at the interphase region in different composite systems. The effective interphase thickness is found to vary from as small as a few hundred nanometers to as large as 10 µm, depending on the constituents, coupling agent, and ageing conditions.