Structure evolution in epoxy-silica hybrids: sol-gel process

The evolution of heterogeneous structure during polymerization in the epoxy-silica hybrid was followed by small-angle X-ray scattering using a position-sensitive detector. The organic-inorganic hybrid was composed of an epoxide-amine system and the silica formed by the sol-gel process from tetraethoxysilane (TEOS), Silica structure evolution is determined by catalytic conditions and the way of preparation: one- or two-stage process. The one-stage polymerization was base-catalyzed by an amine used as a cross-linker of the epoxide. The reaction results in formation of large overlapping polysiloxane clusters from the very beginning of the reaction. During polymerization more branched domains gradually appear within the structure. The polymer shows a compact structure with fractal dimension increasing during the polymerization to D-m = 2.5. The two-stage procedure consisting in acid prehydrolysis of TEOS and basic catalysis in the second step leads to an acceleration of gelation. Primary particles are formed in the first step followed by aggregation into clusters in the second step. The inner structure of the clusters described by a fractal dimension does not change during the polymerization. The diffusion-limited cluster-cluster reaction may be responsible for a more open structure with a fractal dimension D-m = 1.7. (C) 1998 Elsevier Science B.V. All rights reserved.