Commercial Applications of Sol-Gel-Derived Hybrid Materials
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Commercial
Applications of Sol-Gel-Derived Hybrid Materials Barry Arkles
Introduction Sol-gel processing readily yields both inorganic and hybrid organic–inorganic materials. Commercial applications of solgel technology preceded the formal recognition of this technology as an important field of study. Likewise, successful commercial hybrid organic–inorganic polymers have been part of manufacturing technology since the 1950s. As our understanding of polymer chemistry, processing, and structure–property relationships has grown, materials scientists have learned to create hybrid materials that display unique properties. Those materials that demonstrate viability in the marketplace either possess singular properties that enable new end-use applications or are well outside the costperformance envelope of existing commercial materials. Looking at current hybrid polymers that have won niches in the marketplace suggests directions for future successes. The sol-gel-derived materials discussed here represent only a portion
of successful hybrid materials.1 An important premise in this article is that a “commercial” product is one that is both offered for sale and used in the regular production of a device or item in general commerce. There appear to be many hybrid materials with significant commercial potential that have not yet achieved commercial status. No generally accepted definition has been determined for the bewildering variety of materials that are described as “hybrid organic–inorganic polymers.” A narrow definition is that (a) they consist of clear regions or morphologies in which organic structures (C, H, N, O) dominate as well as separate regions in which distinct structures imposed by heteroatoms dominate, and (b) they exhibit physical properties which are not a linear or geometric average of these regions. For example, poly(vinyltrimethylsilane) and poly(trimethylsilylpropyne), two polymers of interest in permselective
(differential-permeation) membrane technology, are not considered hybrid polymers because they have no distinct region that is associated with the silicon heteroatom. On the other hand, dimethylsiloxanebisphenol A carbonate block polymers are considered hybrid polymers, since they display independent glass-transition temperatures associated with the inorganic and organic regions. The minimum dimensions in which clear distinction of physical properties associated with a region or domain appear are in the range of about 1 nm. For example, in block polymers with siloxane linkages, this correlates with 4–6 units. In nanocomposites, this correlates roughly to the dimensions of a silsesquioxane cube. A well-considered discussion of the definition of hybrid organic–inorganic structures in this context has recently been presented.2 The only inorganic polymers that have achieved commercial status, or that have been seriously considered for commercial applications, have been derived from the Group IVA and IVB elements, of which silicon is preeminent. Most commercial hybrid polymers derive from the intr
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