Tetramethoxysilane silica precursor

Silica precursors, tetraethoxysilane and tetramethoxysilane (TEOS and TMOS) are able to solvate some organic polymers. This enables the silica precursor to polymerize in the environment of an organic polymer solution. The number of polymers that form solutions with sol-gel formulations is, however, limited. Some initially soluble polymers tend to precipitate during gelation when a change in solvent composition leads to phase separation. 

For silica, much of the work in this area was carried out in the 1980s and the reader is referred to the series of proceedings volumes from the Materials Research Society entitled Better Ceramics through Chemistry. 63 To illustrate typical precursor structures, and the role of reaction chemistry on precursor structure, Fig. 2.3 presents capillary gas chromatography results for the hydrolysis of tetramethoxysilane [TMOS Si(OCH3)4].72 Typical polymeric species formed (Fig. 2.3a) include linear and cyclic structures. 

Silica-based monolithic columns (Figure 9) are generally prepared using sol-gel technology. This involves the preparation of a sol solution and the gelation of the sol to form a network in a continuous liquid phase within the capillary. The precursors for the synthesis of these monoliths are normally metal alkoxides that react readily with water. The most widely used are alkoxysilanes such as tetramethoxysilane (TMOS) and TEOS. 

Almost at the same time (1996), Tanaka and co-workers [33] and Fields [34] expanded the research field of continuous polymer support by the introduction of (derivatized) inorganic silica rods, which were prepared by sol-gel process of silane precursors [tetramethoxysilane (TMOS) or dimethyloctadecylchlorosilane (ODS)] in the presence of poly(ethylene oxide) (PEO) as porogen. 

An aqueous solution of salt monomer 9PME was mixed with tetramethoxysi-lane, yielding a gel, which was then vacuum-dried forming a precursor powder composed of silica gel and the salt monomer. This was subjected to high-pressure polycondensation under 235 MPa at 230 °C for 5 h, producing polyimide-silica hybrid molding. By varying the ratio of tetramethoxysilane to the salt... 

Inorganic-organic hybrid silica was prepared via co-condensation of tetraethoxy- or tetramethoxysilane (TEOS or TMOS) and aminoalkoxysilane with appropriate amounts of ethanol (or methanol), water, and ammonia. The amine functional groups in the silica were converted to N-diazeniumdiolate NO donors via exposure to high pressures of NO (5 atm) under basic conditions. Control over both the structure and concentration of the silane precursors (i.e., tetraalkoxy-and aminoalkoxysilanes) and specific synthetic conditions allowed for the preparation of NO donor silica particles of widely varying sizes (20-500 nm), NO payloads (50-1,780 nmol/mg), maximum amounts of NO released (10-5,500 ppb/mg), half-lives (0.1-12 h), and NO release durations (up to 30 h)... 

The precursors used in the sol-gel technique are mainly silicon alkoxides that possess high degree of purity. Many of these silicon derivatives are used in various types of solvents and catalysts to produce silica aerogels. Each of these produces varying degrees of physical, thermal, mechanical and optical properties in the aerogels. Some of these precursors used are Tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), polyethoxydisilane (PEDS), methyltriethoxysilane (MTES), and many others (Rao et al. 2003, 2005 Soleimani Dorcheh and Abbasi 2008 Wei et al. 2011). However, they are very expensive and hazardous materials that are known to cause blindness. 

Sol-gel nanocompwsite materials (FexOy-SiCh) were obtained using atkoxide and aqueous routes (Raileanu et al., 2003). The structure and properties of the prep)ared p>articles were comp)ared for different precursors of silica (tetramethoxysilane, methyltriethoxysilane, colloidal silica solution, etc.). 

The continuous demand for new materials have moved to the investigation of polyimides containing metal atoms. Although many polyimide formulations have been reported containing inorganic fillers, of instance silica, it seems that not more than 10% of silica can be incorporated to the composite material at molecular level. Therefore, methods have been outlined to get hybrid polyimides by the sol-gel process. It implies the combination of polyimide with classical precursors of silica employed in the sol-gel methods, particularly tetraethoxysilane and tetramethoxysilane, which lead to silicon oxide through hydrolysis and polycondensation. 

Overcoming the Alcohol Release Problem. TMOS, Si(OMe)4, is currently used as a precursor rather than TEOS, Si(OEt)4. Methanol is then released that has a polarity closer to water and is less harmful than ethanol. However, even when the neat alkoxide is used as a preciusor, concentrations of methanol up to 8 M could be reached via the fall hydrolysis of tetramethoxysilane Si(OCH3)4. Such an amoimt of alcohol can be harmful for enzymes and cells. The enzymatic activity of bovine liver catalase in a TMOS-based silica matrix actually shows a 70% reduction in the presence of 5 vol% methanol (Miller, 1996). It has also been shown that concentrations of methanol larger than 0.5 M lead to the lysis of Escherichia coli bacteria (Coiffier, 2001). It must therefore be eliminated before adding biomolecules. 

The epoxy-silica-ormosil (ESOR) precursor was obtained from tetramethoxysilan (TMOS) and 3-glycid oxypropyl trimethoxysilane (GLYMO) with molar ratio 1 1. First, TMOS was hydrolyzed at room temperature (7j ) for 2 h (TM0S CH30H H20 CH3COOH = 15.0 12.8 7.2 2.4), then GLYMO was added and stirred for 3 h at 2. ... 

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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