Montmorillonite-thermoset nanocomposites via cryo-compounding

For organically modified montmorillonite (OMM)-epoxy nanocomposites, maximal montmorillonite dispersion is found to depend synergistically on the mechanical processing history of the resin mixture and the chemistry at the OMM surface. Specifically, Cloisite 30A (quaternary ammonium OMM) and I30.E (primary ammonium OMM), each containing surfactants with different catalytic effects on the curing chemistry of Epon 862, are compared. Irrespective of the OMM, conventional solvent-free processing methodologies, including sonication, result in an inhomogeneous distribution of OMM on the micron scale. Even though the primary ammonium alkyls within I30.E enhance intragallery reactivity, this only results in extensive swelling of tactoids (interlayer distance similar to 10-20 nm), and thus retention of layer-layer correlations, leading to 'hybrid' micron scale reinforcing particles, not nanoscale dispersion of individual layers. In contrast, sub-ambient temperature (cryo) compounding had substantial impact on the ability to reduce tactoid and agglomerate size and increase homogeneity of dispersion for Cloisite 30A. The reactivity near Cloisite 30A is similar to that in the bulk and thus localized gelation around the layer-stacks does not retard particulate refinement. In all cases, alteration of the global epoxy network structure was ruled out by FTIR and NMR measurements. For nanocomposites with similar OMM content, however, the final thermal-mechanical properties does not coherently relate to one characteristic of the morphology. The coefficient of thermal expansion (T > T-g) and hardness (T < T-g) depend only weakly on morphology, where as the glass transition temperature depends strongly on the extent of OMM dispersion and interfacial chemistry. In general, the inter-relationships between mechanical processing, OMM surface chemistry and the desired property enhancements are not linear and thus must be considered in light of a final application to evaluate the optimal 'nanocomposite' fabrication methodology to achieve maximal benefit. (c) 2006 Elsevier Ltd. All rights reserved.