Where R can be H o r any alkyl group CH3 CH2CH3 etc

Fig. 2. Hydrolysis and condensation reactions in sol-gel processing.

condensed polymers (2). The ratio of water to metal alkoxide, in which rw = [water]/[metal alkoxide], can also be used to tailor the gel network. For rw < 4, the structure is primarily that of linear polymers, whereas if rw > 4, the structure is primarily crosslinked, three-dimensional (3D) networks (5). Another critical factor controlling gel structure is pH. Silicate sols prepared under acidic conditions (acid catalyzed) form primarily linear polymers, and on drying, a dense gel forms with relatively small pores (Fig. 3A). On the other hand, silicate sols prepared under basic conditions (base catalyzed) form primarily branched clusters, and on drying, the gel network is more open, with a larger pore network (Fig. 3B) (3).

As hydrolysis and condensation polymerization reactions continue, the sol becomes a gel. The gelation point is defined as the time at which the silica matrix forms a continuous solid (6) or the point at which the material can support a stress elastically (1). After the sol-to-gel transition, there is a sharp increase in viscosity and the polymer structure becomes rigid. The sol-to-gel transition is irreversible, with essentially no change in volume. The material after gelation is composed of two distinct phases: amorphous silica particles (5-10 nm in diameter) and an interstitial liquid phase (6). This material is described as a "wet" gel, as long as the interstitial liquid phase (i.e., pore liquid) is maintained. As the gel ages, polycondensation continues, increasing the connectivity of the network, and its structure and properties continue to change.

It is possible to prepare sol-gel materials in a wide range of geometries, from bulk monolith materials (dimensions >1 mm) to thin films (thickness of

Fig. 3. (A) Acid catalysis in sol-gel processing of silica leads to primarily linear polymers, and upon drying, a dense gel forms with relatively small pores. (B) Base catalysis in sol-gel processing of silica leads to primarily branched clusters, and on drying, the gel network is more open, with a larger pore network. (Adapted from ref. 3.)

Fig. 3. (A) Acid catalysis in sol-gel processing of silica leads to primarily linear polymers, and upon drying, a dense gel forms with relatively small pores. (B) Base catalysis in sol-gel processing of silica leads to primarily branched clusters, and on drying, the gel network is more open, with a larger pore network. (Adapted from ref. 3.)

tens to hundreds of nanometers). In the fabrication of these materials, if the pore liquid is allowed to evaporate, the gel begins to dry. The drying process is accompanied by considerable weight loss and volume shrinkage. Typically, gel drying is accomplished under ambient conditions. As the interstitial liquid evaporates from the pores, there are large capillary forces, causing pore collapse and material shrinkage. The dried gel is described as a "xerogel" (see Fig. 1). For silica sol-gel materials, xerogels are typically <50% of their original volume. Alternatively, solvent extraction can be performed using supercritical drying. This supercritically dried gel is called an "aerogel" (see Fig. 1). Under these conditions, pore liquid can be removed without pore collapse and subsequent shrinkage, resulting in a lightweight material with relatively large pore sizes and high pore volume.

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