An epoxy resin, cured with an anhydride, has been modified by the addition of silica nanoparticles.The particles were introduced via a sol-gel technique which gave a very well dispersed phase of nanosilica particles which were about 20 nm in diameter. Atomic force and electron microscopy showed that the nanoparticles were well dispersed throughout the epoxy matrix. The glass transition temperature was unchanged by the addition of the nanoparticles, but both the modulus and toughness were increased. The measured modulus was compared to theoretical models, and good agreement was found. The fracture energy increased from 100 J/m 2 for the unmodified epoxy to 460 J/m 2 for the epoxy with 13 vol% of nanosilica. The fracture surfaces were inspected using scanning electron and atomic force microscopy, and the results were compared to various toughening mechanisms proposed in the literature. The toughening mechanisms of crack pinning, crack deflection and immobilised polymer were discounted. The microscopy showed evidence of debonding of the nanoparticles and subsequent plastic void growth. A theoretical model of plastic void growth was used to confirm that this mechanism was indeed most likely to be responsible for the increased toughness that was observed due to the addition of the nanoparticles. IntroductionEpoxy polymers are widely used for the matrices of fibre-reinforced composite materials and as adhesives. When cured, epoxies are amorphous and highly-crosslinked (i.e. thermosetting)polymers. This microstructure results in many useful properties for structural engineering applications, such as a high modulus and failure strength, low creep, and good performance at elevated temperatures.However, the structure of such thermosetting polymers also leads to a highly undesirable property in that they are relatively brittle materials, with a poor resistance to crack initiation and growth.Nevertheless, it has been well established for many years that the incorporation of a second microphase of a dispersed rubber, e.g. Hence rigid, inorganic particles have also been used, as these can increase the toughness without affecting the glass transition temperature of the epoxy. Here glass beads or ceramic (e.g. silica or alumina) particles with a diameter of between 4 and 100 μm are typically used, e.g. [9][10][11][12][13][14].However, these relatively large particles also significantly increase the viscosity of the resin, reducing the ease of processing. In addition, due to the size of these particles they are unsuitable for use with infusion processes for the production of fibre composites as they are strained out by the small gaps between the fibres.More recently, a new technology has emerged which holds promise for increasing the mechanical performance of such thermosetting polymers. This is via the addition of a nanophase structure in the polymer, where the nanophase consists of small rigid particles of silica [15][16][17][18]. Such nanoparticle-modified epoxies have been shown to not only increase further the toug...

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