| Abstract
| - A polyhedral oligomeric silsesquioxane (POSS) containing one epoxy group and sevenisobutyl groups per molecule was incorporated into an epoxy network following a two-stage process. Inthe first stage, POSS was reacted with an aromatic diamine, employing a 1:1 molar ratio of both reactants.The distribution of species at the end of reaction, determined by size exclusion chromatography (SEC),was close to the ideal one. In a second step, this precursor was reacted with the stoichiometric amountof an aromatic diepoxide to generate an organic−inorganic hybrid material containing 51.8 wt % POSS.A primary liquid−liquid phase separation process occurred at the time of adding the diepoxide to thePOSS−diamine precursor. This led to a macrophase separation into epoxy-rich and POSS-rich regions,possibly derived from the incompatibility of the isobutyl groups attached to the POSS with the aromaticepoxy−amine network. A secondary phase separation occurred in the epoxy-rich phase in the course ofpolymerization, producing a dispersion of small POSS domains. Both modulated local thermal analysis(LTA) and differential scanning calorimetry (DSC) showed that most POSS-rich domains were amorphous.A small fraction of POSS crystals was also detected. A postcure cycle led to an increase in the glasstransition temperature and the disappearance of crystallinity. A reference network was synthesized byreplacing POSS by phenyl glycidyl ether (PGE) in equimolar amounts. The resulting network washomogeneous but exhibited a lower glass transition temperature than the POSS-modified network. Asboth networks had the same topology, the higher Tg observed for the POSS-modified epoxy may beassociated with the hindering of polymer chain motions by their covalent bonding to POSS clusters. Themost important concept arising from these results is that a phase separation process may take placewhen employing a POSS bearing organic groups that are not compatible with the epoxy network.
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