Organic/Inorganic Hybrid Composites from Cubic Silsesquioxanes. Epoxy Resins of Octa(dimethylsiloxyethylcyclohexylepoxide) Silsesquioxane

Standard research protocols were developed to explore the use of octahedral silsesquioxane (cube) organic/inorganic nanocomposites as model systems for determining nanostructure−processing−property relationships to demonstrate that nanoscale structural manipulation of the organic component can significantly change macroscale physical properties. Comparison of octaglycidyldimethylsiloxyoctasilsesquioxane [(glycidyl-Me2SiOSiO1.5)8] (OG)/diaminodiphenylmethane (DDM) and octa(ethylcyclohexylepoxide)dimethylsiloxy silsesquioxane (OC)/DDM nanocomposite resins provide the first demonstration that well-defined nanostructures can be formed wherein linear organic tethers of known structure join only two cube vertices. HF dissolution of cubes followed by GPC analysis demonstrates that only linear tethers form in OC/DDM. TEM studies suggest that these nanocomposites are homogeneous at the nanometer scale, thus supporting the chemical analysis studies. The physical properties of these nanocomposites were then systematically assessed and the network tether architectures quantitatively analyzed to correlate the changes in nanostructure with macroscopic properties. TGA, DMA, room-temperature mechanical properties, and molecular modeling studies suggest that nanocomposite thermomechanical properties can be modified by changing the tether architecture/rigidity and predicted by molecular modeling. Surprisingly, OC/DDM elastic moduli increase from 2.2 to 3.3 GPa as the DDM content increases 2-fold beyond the maximum cross-link density into a high defect density region while the fracture toughness remains unchanged. An explanation for this behavior is proposed. This appears to be a true nanocomposite property. Blending provides an effective approach for modifying properties dominated by particular tethers. The results reported here offer several guidelines in designing cube hybrid nanocomposites and detailing future studies.