Water:alkoxysilane ratio

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

Hydrolysis of alkoxysilanes in water is reported to follow a first-order rate law [8, 13]. In the water-acetone mixture required for solubility of TMMS, there is a 15-fold molar ratio of water to TMMS. However, a plot of In ([TMMS],) vs. time does not result in a linear plot. This is because the consumption of water is significant. Therefore, a bimolecular, second-order rate law was employed as shown in equation (1) ... 

Traces of water from air are sufficient to promote the hydrolysis of =SiOEt groups in a thin polymer film and to give cross-linked material. Alkoxysilane groups hydrolyze rapidly under UV radiation with cationic photoinitiator, which was proved by NMR in the solid state (disappearance of resonances corre onding to OEt groups in cured materials). However, because of the low ratio of water to alkoxysilane, the condensation was relatively slow and might have not... 

Since only OH groups in the hydrolysis products are active toward condensation reaction to form =Si-0-Si= bonds, the stmcture or morphology of the resultant siloxane polymers should depend on the number of OH groups, x, in Si(OH) (OR)i. Namely, when x is 2, the hydrolysis may lead to linear siloxane polymers, and three-dimensional network polymers may be formed when x is 4. This reaction scheme teUs us that the fiber-drawing should be possible if Si(OR)4 is hydrolyzed with an imderstoichiometric amount of water (ideally, when the molar ratio of water to the alkoxysilane or r-value is 2) under the acidic condition. 

Fast Sol-Gel Reactions with Monomers other than MTMS. An intriguing question derived from the MTMS fast sol-gel process is whether the principle of "three hydrolyzable substituents applies only to a pure alkyl-trialkoxy monomer or perhaps it can be applied also to a mixture of tetra- and di- alkoxysilanes and may be even to multi-componet mixtures. A series of experiments was carried out with binary mixtures of tetramethoxysilane (TMOS) and dimethyldimethoxysilane (DMDMS), employing exactly the MTMS reaction procedure and casting process. The molar ratio of water to siloxane was kept at 1.5 (substoichiometric, in most cases). The results of these experiments are collected in Table IV. 

Sol-gel synthesis is the process of formation of porous, three-dimensional, integrated solid network (gel) of either discrete particles or network potymers from the conversion of monomers into stable suspension of colloidal solid particles or pol miers (sol) in a continuous liquid phase. Most popular precursors for the synthesis of colloids are metal alkoxides and alkoxysilanes. Tetramethoxysilane (TMOS) and Tetraethoxysilane (TEOS) are commonly used alkojq silanes, which form silica gel. The remarkable property of these silanes is that they readity react with water in the presence of shorter chain alcohol such as ethanol and ammonia to form monodispersed silica particles [7]. The size of silica particles formed is between 50 and 200 nm and depends on the silica ester used, type of alcohol, and molar ratios of water and alkoxysilane [32]. In this process, alcohol acts as a homogenizing solvent between alkoxides and water as both are immiscible but can be easily dissolved in alcohol. With the presence of this homogenizing agent, hydrolysis can be facilitated [33] due to the complete miscibility. However, aluminates borates and titanates often mixed with TEOS or TMOS are commonly used in sol-gel process. The hydrolysis of alkoxysilane proceeds according to Stober s process (Fig. 18.6). 

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