Gel-formation process

Strazhesko, Strelko, Vysotsky, and co-workers (49-51) investigated the polymerization of monosilicic acid. Polysilicic acid is more acidic than monosilicic acid, and the dissociation constant of the acidic centers on the surface of polysilicic acid is 2 to 3 orders of magnitude greater than that of the monomer (49-51). Strazhesko, Vysotsky, and co-workers (52-55) showed that the isoelectric point represents not only the minimal rate of the gel formation process but also its syneresis. As a result, mechanically strong silica gels were obtained that had a maximum specific surface area. 

The molar ratio of reacting silanes TEOS APTES MTES (or PTES) in a starting mixture was equal to 1 1 1. The syntheses were effected in the presence of ethanol since our preliminary experiments showed that absence of a solvent led to appearance of an interface between phases during gel formation. Elemental analysis of these phases (in the case of MTES) showed that in terms of their composition they almost do not differ from each other. Thus, the upper phase contained 18.5 wt.% of C, 5.2 wt.% of N, and 4.7 wt.% of H while the content of these elements in the lower phase made up 17.0, 4.8, and 4.4 wt.% respectively. The causes for such a separation into layers are not known but the fact observed suggests an idea that it is necessary to exercise a more close control over gel formation processes especially in those systems where such a process is accompanied by a strong opalescence. The systems under study are just systems of this kind. 

Analysis of double bonds content in oligopiperylene macromolecules showed that for all catalysts applied in synthesis process insignificant change of unsaturation in forming polymer products was observed (Table 5.2). Addition into the catalyst of ether as modifier, in particular of (C6H5)20 in ratio 1 1 mole / mole reduces both electrophilic catalyst activity (kp is decreased) and possibility of gel-formation processes proceeding (cross-linking). 

For embedding of cells and for injectable hydrogels, gel precursors that cure under mild conditions have to be selected. Gel formation processes that are due to physical changes like temperature, pH, or ionic strength and chemical reactions like Michael addition, Schiff base, or disulfide bond formation are appropriate (cf. Section 3.8.2.5). 

Precursor Precursor concentration in the feed (wt%) Solvent Temperature of the gelation process (°C) Appearance of the samples after the gel formation process References... 

The tunabiUty of dendritic structures means that subtle modifications can easily be made to the dendritic architecture. Such modifications can modulate gel-phase properties without completely disrupting the gel formation process. 

Figure 5. Photographs of reaction mixture during gel formation process in...

What kind of viscoelastic behavior can be observed in samples that are intermittently removed during tiie gel formation process ... 

Many other agents promoting oxidative coupling catalyze the gelation process. It has therefore been proposed that an oxidative coupling process could be involved (45). The gel formation process is in all circumstances a three-stage one complex formation—> intermolecular bridge formation—>gel formation. 

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