Organic -inorganic hybrid nanocomposites from cubic silsesquioxanes.

Abstract

This dissertation is dedicated to nanostructure-processing-property relationship studies using well-defined functionalized cubic silsesquioxane (cube) nanocomposites to establish a comprehensive picture of nanocomposite behavior. In our approach, various functional groups are appended to cubes to mimic segments of common polymer structures followed by crosslinking reactions that lead to well-defined networks. The cubes behave as invariant, nano-sized inorganic crosslinkers uniformly distributed in polymer matrices, while various tether building parameters are selectively varied and their effects on physical properties are concordantly investigated in relation to changes in nanostructures. The network architectures can be directly analyzed via cube core dissolution by HF followed by GPC analysis. Our initial efforts focus on the studies of cube nanocomposites prepared be reacting octaglycidyldimethylsiloxyoctasilsesquioxane (<italic>OG</italic>) and octa(dimethylsiloxyethylcyclohexenylepoxide) silsesquioxane (<italic> OC</italic>) with diaminodiphenylmethane (<italic>DDM</italic>). These studies suggest that tether structures and properties dominate global thermomechanical properties. Macroscopic relaxation is minimized when bifurcated tethers form, but linear tether formation can be favored over bifurcated tether formation by controlling crosslinking chemistry. Cyclic vs. linear tether segments exhibit improved thermomechanical properties. Mechanical property and blending studies demonstrate non-linear structure-property relationships which appear to be true nanocomposite behavior. Toughening of OC/DDM epoxy nanocomposites by core shell rubber particles demonstrates the concept of composite modification at nano and macro length scales. The results suggest that multiple properties can be improved simultaneously and independently. Studies of epoxy and imide cube nanocomposite prepared from octaaminophenylsilsesquioxane (OAPS) also suggest that tether segmental structures and motions determine the global properties. Crosslink density, tether architecture, length and processing conditions such as choice of cure temperatures also affect nanocomposite macroscopic behavior greatly. In conclusion, this work provides basic standards and guidelines for designing, synthesizing and processing cube nanocomposites, and characterizing nanostructures and properties. This study starts a new category of nanocomposites with excellent control of nanostructures and properties.