INVERSE ORGANIC-INORGANIC COMPOSITE-MATERIALS .3. HIGH GLASS CONTENT NONSHRINKING SOL-GEL COMPOSITES VIA POLY(SILICIC ACID-ESTERS)

A critical challenge in the design of inorganic-organic composites is control over the mixing between the two chemically dissimilar phases. The sol-gel process has been utilized for the homogeneous incorporatio n of organic polymers into an inorganic matrix. One major limitation i n the synthesis of sol-gel composites is the ubiquitous shrinkage that results from the evaporation of excess solvents and water. In our res earch, we have attempted to address this issue by developing in situ s ynchronous polymerization routes into these hybrid materials through t he use of tetraalkoxysilanes possessing polymerizable alkoxides. Since our initial reports on the use of these new sol-gel precursors for th e production of nonshrinking sol-gel composites, we have focused our e fforts toward the synthesis and application of poly(silicic acid ester s) bearing the same polymerizable alkoxides. This new approach allows for the synthesis of nonshrinking sol-gel composites with higher glass content than was previously possible with the tetraalkoxysilane deriv atives. Furthermore, the use of poly(silicic acid esters) provides a n ew means for controlling composite morphology through the size and sha pe of the poly(silicic acid ester). Molecular weights for these poly(s ilicic acid esters) can be varied from 3000 to 30 000 simply by increa sing the reaction time of the silicic acid synthesis. Also, particle s hape can be controlled by the pH of the reaction solution. A modified esterification process using THF or benzene for azeotropic removal of water has been developed for synthesizing poly(silicic acid esters) wi th thermally unstable unsaturated alcohols. These substituted poly(sil icic esters) have been used to produce transparent nonshrinking compos ites with a wide range of glass/polymer ratios.