| Although the sol-gel process for producing ceramic oxides was first demonstrated over a century ago, significant research and applications of this process have developed only in the past several decades. The sol-gel process involves the hydrolysis and condensation of metal alkoxides (usually tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS)) in acidic or basic aqueous solutions with an appropriate cosolvent to produce a solvent swollen inorganic oxide gel. Slow evaporation of the solvent results in a porous oxide, called a xerogel, and shrinkages of 70-80% are routine during this drying process. Because xerogel production can be conducted at ambient temperatures and pressures, hybrid inorganic-organic composites can be synthesized by the incorporation of organic polymers into the sol-gel solution, and the organic polymer can then be entrapped in the developing inorganic network as the sol-gel reaction proceeds. Polymers such as poly(dimethylsiloxane) and poly(methyl methacrylate), as well as other high performance polymers have been used to produce inorganic-organic composites with unique properties via the sol-gel process.;Two significant limitations in the synthesis of sol-gel composites are the limited number of polymers that are compatible with this process and the ubiquitous shrinkage that results during the drying step. To overcome the polymer compatibility problem, we have designed simultaneous interpenetrating networks (SIPN's) where both the inorganic network formation and organic polymer formation occur simultaneously. We have extended this process to address the shrinkage problem by synthesizing tetraalkoxysilanes, and more recently, poly(silicic acid esters) bearing polymerizable alkoxide groups. By using these new siloxane derivatives, a polymerizable cosolvent, and a stoichiometric quantity of water, all components contribute to either the inorganic network or the organic polymer, and no drying or shrinkage associated with drying occurs. Using this new technique, a wide variety of inorganic-organic composites with unique properties have been synthesized. |
