Inorganic silica polymers hydrolysis

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... 

Silica hydrolysis-condensation of alkoxysilane-terminated chains of various polymers (PB, poly(propylene)-glycol, poly (ethylene)-glycol) leads to the formation of silica nodules. Hybrid (organic-inorganic) nanocomposite rubbery materials are obtained in this way. These materials have been swollen by various deuterated solvents, in order to probe the order induced in the polymer matrix under stress [80]. 

Direct copolymerization of VFA can be carried out either with BVU, in the presence of silica particles, or with VTS, which must be pre-grafted onto silica surfaces. Both reactions can be used to immobilize large amounts of PVFA-co-PVAm on silica surfaces. The second described direct co-polymer-ization procedure is exhibited by the covalently bonded polyelectrolyte layer on the inorganic substrate. The swelling capacity of the PVFA/silica hydrogels can be controlled by the degree of hydrolysis of the PVFA. 

The sol-gel process performed in low concentrated polymer-solvent solutions is another attractive route to develop hybrid membranes because it allows an in situ dispersion of metal-based nanoparticles within the polymeric matrix, achieving a suitable interfacial morphology between the continuous and the dispersed phase. Silica particles and polyimide have been frequently used to produce these hybrid membranes [107,108]. In general, hydrolysis and condensation reactions are involved in the sol-gel process, when alkoxides are involved in the formation of the dispersed phase. The advantage of using this method is the formation of an inorganic network largely interconnected with the polymeric materials mainly with noncovalent interactions [109]. In Fig. 7.10 a... 

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