Network density control in epoxy-silica hybrids by selective silane functionalization of precursors

Following previous work on the compatibilization of organic-inorganic hybrids through coupling reactions with the precursor components, the present study evaluates the relative efficiency of different types of coupling agents on the morphology and properties of epoxy-silica hybrids. In particular, this investigation compares the effects of introducing trialkoxysilane functional groups at the chain end (using amine- and mercapto-silanes) with similar types grafted in the middle of the chain of the constituent resin (using an isocyanate silane). The use of coupling agents with a basic character (amine silane type) brings about the formation of denser networks in both constituent phases of the resulting epoxy-silica hybrid, which is manifest through a large increase in the T-g and a more extensive suppression of the molecular relaxations within the glass transition regions. Increasing the number of alkoxysilane functional groups at the chain end, with the use of a bis-aminosilane, has a relatively minor effect on the morphology and dynamic mechanical spectra of the resulting epoxy-silica hybrids. It was also found that while the incorporation of small amounts of a high molecular weight epoxy resin causes considerable plasticization of the organic phase, much larger amounts of organic (aliphatic) co-agent within the siloxane phase are required to deteriorate those properties that are related to the inorganic character of the hybrid material. (c) 2005 Wiley Periodicals, Inc.