Silica formation

This process is highly suitable for rubbers with poor solubility. In this process, the rubber sheet is soaked in TEOS or quite often in TEOS-solvent mixture and the in situ sUica generation is conducted by either acid or base catalysis. The sol-gel reaction is normally carried out at room temperature. Kohjiya et al. [29-31] have reported various nonpolar mbber-silica hybrid nanocomposites based on this technique. The network density of the rubber influences the swelling behavior and hence controls the silica formation. It is very likely that there has been a graded silica concentration from surface to the bulk due to limited swelling of the rubber. This process has been predominantly used to prepare ionomer-inorganic hybrids by Siuzdak et al. [48-50]. 

The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

Patel et al. [70] in a recent publication have explored the adhesive action of the mbber-siUca hybrid nanocomposites on different substrates. The rubber-silica hybrid nanocomposites are synthesized through in situ silica formation from TEOS in strong acidic pH within acryhc copolymer (EA-BA) and terpolymer (EA-BA-AA) matrices. The transparent nanocomposites have been apphed in between the aluminum (Al), wood (W), and biaxially oriented polypropylene (PP) sheets separately and have been tested for peel strength, lap shear strength, and static holding power of the adhesive joints. 

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

Fig. 3.4 Schematic drawings illustrating the nucleation of silica precursor and possible mechanisms of reactions leading to silica formation. (A) The precursor hydrolysis. THEOS nucleates on a polysaccharide macromolecule through hydrogen bonds that are formed with hydroxy groups in the biomacromolecule. Per-...

In contrast, such approaches have been much more developed with proteins, termed silicateins, that have been extracted from some silicified sponges [38]. The success of these approaches probably originates from the fact that the reactivity of these proteins towards silica precursors differs significantly from the processes occurring in diatoms. Whereas silaffins and poly-amines activate silica formation via electrostatic interactions due to the presence of positively-charged ammonium... 

Poulsen, N., Sumper, M. and Kroger, N. (2003) Silica formation in diatoms Characterization of native silaffin-2 and its role in silica morphogenesis. Proceedings of the National Academy of Sciences of the United States of America, 100, 12075-12080. 

Systematic investigations into the kinetic aspects of silica formation in microemulsions have been undertaken by only a few researchers (23,24,29). Various forms of useful qualitative and semi-quantitative kinetic information are available, however. For example, size exclusion chromatography (SEC) was used by Espiard et al. (22) to monitor TEOS concentration as a function of time for different R values (R = 4, 6, 10). An initial period of rapid reaction (approximately first 6 h) was observed,... 

In each of these formulas additional free OH groups are available on the silicon so that it is possible to crosslink more than two polysaccharide chains. Silicon may function as a biological crosslinking agent in connective tissue. Silaffins, small polypeptides containing polyamine side chains of modified lysine residues, apparently initiate silica formation from silicic acid in diatoms.0... 

Fig. 9 Synthesis scheme for surface modification of silica. Formation of (A) a monomeric phase and (B) a polymeric phase. (From Ref. 112.)...

Other than accelerating the formation of spherical nanoparticles of silica from metastable silicic acid solutions, the silaffins appear to have no structure-directing activity. If they are responsible for silica formation in the living diatom, as seems quite likely, control of the higher order architecture of the resulting silica apparently must be determined by the pre-formed shape of the silica deposition vesicle (the envelope within which the silica grows) serving as a complex three-dimensional mold. 

In the presence of the silica formation of a carbanion ion takes place easily, thus enhancing the fragmentation reaction at lower temperature. 

De La Rocha C. L., Brzezinski M. A., and DeNiro M. J. (1997) Fractionation of sihcon isotopes by marine diatoms during biogenic silica formation. Geochim. Cosmochim. Acta 61, 5051-5056. 

Kroger N. and Sumper M. (2000) The biogeochemistry of silica formation in diatoms. In Biomineralization (ed. E. Baeuerlein). Wiley, Weinheim, Germany, pp. 151-170. 

Biswas, P, Li, X.M., and Pratsinis, S.E., Optical wave-guide preform fabrication— silica formation and growth in a high-temperature aerosol reactor, J. Appl. Phys., 65, 2445, 1989. 

Diatoms were also investigated for the mechanism of silicon transport that is an integral part of the silicification process. As the environmental concentrations of dissolved silicon are rather low, diatoms must have an efficient transport system. Orthosilicic acid must not only be transported into the cell, but also transported intracellularly into the SDV where silica formation occurs. A protein of the diatom C. fusiformis was characterized that transports silicon from seawater into the cell. This discovery was accomplished by cloning and characterizing the DNA that codes for this protein. However, silicon transporter proteins of this particular type are not necessarily involved in intracellular transport. 

Percent Signal Assignable to Silica Formation Resulting from Lattice Degradation. ... 

P. Kipkemboi, A. Fogden, V. Alfredsson, and K. Flodstrom, Triblock Copolymers as Templates in Mesoporous Silica Formation Structural Dependence on Polymer Chain Length and Synthesis Temperature. Langmuir., 2001, 17, 5398-5402. 

Kipkemboi, P. Fogden, A. Alfredsson, V. Flodstrom, K. Triblock copolymers as templates in mesoporous silica formation structural dependence on polymer chain length and synthesis temperature. Langmuir 2001, 17 (17), 5398-5402. 

Special methods of incorporation alkyl peroxide bridging to vinyl containing siloxane polymers used for conductive applications improved curing characteristics decreased by the presence of carbon black crosslinking based on platinum catalyzed polyvinylmethyl siloxane and polymethyl hydrogen siloxane is not inhibited by carbon black in situ silica formation in fonnation of PDMS nanocomposite, montmorillonite was delaminated in polymer prior to crosslinking ... 

Floess. J. K.. Kiefer, J. H.. Kumaran. S. S., and Fotou, G. P. (1997) A Kinetic Model for Fumed Silica Formation in a Rame Reactor, paper presented at the AlChE Meeting, Los Angeles. CA. November 1997. 

Recently, dendrimers loaded with Cu ions were used as template for silica formation [68]. As is well known,poly (propylene)imine dendrimer (DAB-Am-64) serves as a nanoreactor for well controlled nanoparticle formation [69]. However, when such dendrimers alone are used as silica templates [68], it does not lead to high porosity (BET surface area is 290 m /g) and does not provide ordering (Fig. 8). The latter factor is important for optical properties, but of no concern for catalytic applications. However, as silica cast over Cu-DAB-Am-64 lacks interpenetrating pore system and easy access to CuO particles, this destroys any hopes for catalytic applications as well. 

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