Gel networks

The important feature is that a three-dimensional gel network comes from the condensation of partially hydroly2ed species. Thus, the microstmcture of a gel is governed by the rate of particle (cluster) growth and their extent of crosslinking or, more specifically, by the relative rates of hydrolysis and condensation (3). 

Production of net-shape siUca (qv) components serves as an example of sol—gel processing methods. A siUca gel may be formed by network growth from an array of discrete coUoidal particles (method 1) or by formation of an intercoimected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore Hquid is removed as a gas phase from the intercoimected soHd gel network under supercritical conditions (critical-point drying, method 2), the soHd network does not coUapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

Polycondensation reactions (eqs. 3 and 4), continue to occur within the gel network as long as neighboring silanols are close enough to react. This increases the connectivity of the network and its fractal dimension. Syneresis is the spontaneous shrinkage of the gel and resulting expulsion of Hquid from the pores. Coarsening is the irreversible decrease in surface area through dissolution and reprecipitation processes. 

O Rosen, L Picullel. Interactions between covalently crosslinked ethyl(hydroxy-ethyl)cellulose and SDS. Polym Gels Networks 5 185-200, 1997. 

R Kishi, O Hirasa, H Ichijo. Fast responsive poly(N-isopropylacrylamide) hydrogels prepared by gamma-ray irradiation. Polym Gels Networks 5 145-151, 1997. 

Figure 5. Proposed 3-D associated gel network model. (Reproduced with permission from ref. 40. Copyright 1980 Society of Petroleum Engineers.)...

The use of other crosslinking metals developed simultaneously with the use of antimony, chromium, and boron(borate). Tiner, et al.(242) introduced titanium (IV) crosslinkers in 1975 as ammonium tetralactonate or bis(triethanolamine)bis(isopropyl)titanium(IV). Upon contact with water soluble titanium (IV) derivatives ordinarily form orthotitanic acid, Ti(0H)4, which rapidly forms oligimeric metatitanic acid, [Ti(0H)2] and titanium dioxide. Electron donors such as the hydroxyl groupsxof polysaccharides, if properly oriented, can participate in the sequence of titania reactions and a crosslinked gel network results. Various titanium metal crosslinkers remain in common use today. More will be said about titanium crosslinked gels later. 

Winter HH, Izuka A, De Rosa ME (1994) Polym Gels Networks 2 239... 

The gel point is defined as the point at which the entire solid mass becomes interconnected. The physical characteristics of the gel network depends upon the size of particles and extent of cross-linking prior to gelation. Acid-catalysis leads to a more polymeric form of gel with linear chains as intermediates. Base-catalysis yields colloidal gels where gelation occurs by cross-linking of the colloidal particles. 

The silica gel network readily retains water molecules. Therefore, the presence of hydrates or silicic acid is ascribed to be the support reagents responsible for mediating a chemical transformation.27... 

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