Mechanisms leading to improved mechanical performance in nanoscale alumina filled epoxy

There have been several reports of improved ductility and toughness in brittle thermoset polymers due to the addition of equiaxed nanoparticles [Ash BJ, Siegel RW, Schadler LS. Mechanical behavior of alumina/poly(methyl methacrylate) nanocomposites. Macromolecules 2004;37:1358–69; Kinloch AJ, Lee JH, Taylor AC, Sprenger S, Eger C, Egan D. Toughening structural adhesives via nano-and micro-phase inclusions. J Adhes 2003;79:867–73; Kinloch AJ, Mohammed RD, Taylor AC, Eger C, Sprenger S, Egan D. The effect of silica nanoparticles and rubber particles on the toughness of multiphase thermosetting epoxy polymers. J Mater Sci 2005;40:5083–6; Johnsen BB, Kinloch AJ, Mohammed RD, Taylor AC, Sprenger S. Toughening mechanisms of nanoparticle-modified epoxy polymers. Polymer 2007;48:530–41]. The mechanisms leading to this improvement, however, are incompletely understood. In this paper, a model system of nanoscale alumina filled bisphenol A based epoxy with two interface conditions was used to highlight the mechanisms leading to significant improvements in both ductility (max 39%) and modulus (max 18%). Tensile test showed that crack deflection, debonding and plastic deformation of the debonded matrix (plastic void growth) operated for the untreated particles with a relatively weak interface, but that a stronger interface led to enhanced crack deflection and microcracking, and, as a result, a further improvement in properties.