The mechanical properties and fracture behaviour of epoxy-inorganic micro- and nano-composites

Hybrid materials have been formed using an epoxy polymeric matrix and a range of inorganic particles, including mica and organically-modified montmorillonites (‘organoclays’), with various concentrations of the silicate modifier up to about 30 wt.% depending upon the viscosity increase induced by the presence of the silicate. Wide-angle and small-angle X-ray scattering plus transmission electron microscopy were used to identify the morphologies produced, which included particulate, intercalated and ordered exfoliated. The modulus of these composites increased with the weight fraction of silicate. The morphology had a small effect on the measured modulus; the nano-composites with the ordered exfoliated microstructure showing the highest values of the modulus for a given volume fraction of silicate. The fracture toughness, Kc, and the fracture energy, Gc, initially increased as the weight fraction of the silicate was increased, but then decreased at relatively high concentrations. The measured moduli and toughnesses were compared to theoretical predictions. The measured moduli values showed very good agreement with the predicted values, whilst the agreement for values of the measured fracture energy, Gc, with the predicted values, based upon a crack deflection toughening mechanism, were less convincing. Indeed, analysis of the fracture surfaces using scanning electron microscopy showed that the main toughening effect of the silicate particles is due to plastic deformation of the epoxy matrix around the particles.