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SYNTHESIS AND PROCESSING: GENERAL METHODS 19 Porous Sol-Gel Composites. An important characteristic of sol-gel-derived materials is the very large percentage (30 to 70) of microporosity in the materials after drying. Consequently, a wide range of physical properties and microstructural control can be achieved with materials of the same chemical composition by varying the volume fraction, size distribution, and connectivity of the microporosity. Microporous structures can be impregnated with passive and active organic, polymeric, and inorganic materials to produce a wide range of structural, optical, and electromagnetic composites with submicron dimensions (Ulrich, 1988a). Porous gel glass-organic polymer composites reveal significant differences between nanoscale and bulk polymers. For example, it is well known that as the size of a particle decreases, the surface area-to-volume ratio increases, such that a 10-nm particle has a surface area-to-volume ratio 10,000 times that of a 0.1-nm particle. The thermodynamics of surfaces differ significantly from that of the interior of a material, raising the question as to whether differences can be observed between the bulk polymer and the polymer impregnated into 10-nm pores of the silica gel. In Table 2, data for polymethylmethacrylate (PMMA) prepared under identical conditions are shown with one difference. In the second case the porous gel was impregnated with monomer and PMMA polymerized in the gel. A significant reduction of the glass transition and curing temperatures was observed. The density of the polymer phase was measured to be the same in both cases. These transparent composites have relatively high strengths and elastic modulus as well as low densities (Pope and Mackenzie, 1985, 1986). Table 2 Glass Transition and Curing Temperature for PMMA Sample Glass Transition Temperature (°C) Curing Temperature (°C) PMMA (bulk) 97.2 157.1 PMMA (in silica gel) 81.9 120.5 Volatile-Host Method Another approach for near-net-shape processing requires an organic polymer as well as an inorganic polymer. In one method, known as the volatile-host method, a plastic filament is dipped into an alkoxide solution
