Salts, polymeric silicic acid

Two inorganic water-soluble polymers, both polyclcctrolytcs in their sodium salt forms, have been know ll for some time poly(phosphoric acid) and poly(silicic acid). A more exciting inorganic water-soluble polymer with nonionic characteristics has been reported. This family of phosphazene polymers is prepared by the ring-opening polymerization of a heterocyclic monomer (6) followed by replacement of the chlorine atoms in Ihe resultant polymer. 

Figure 9, this new silica sol is elongated. For example, the particles have a diameter of about 10 nm and a length of 50-100 nm. To prepare such elongated silica sols, a calcium salt and an aqueous sodium hydroxide solution are added to an active silicic acid or an acidic silica sol, and the mixture is heated in an autoclave at 100-150 °C for several hours. Thus, particle polymerization takes place in nonuniform directions, and an elongated silica sol (31) can be formed. 

Some indication of at least the maximum size of the silicate species in a solution of sodium silicate of 3.3 ratio SiOjrNajO has been obtained from the nature of the corresponding silicic acid ester (69). Silicic acid of low molecular weight is so unstable in aqueous solutions that any attempt to isolate it by evaporation of water, even at ordinary temperature, results in rapid polymerization to a gel. The direct esterification of silicic acid therefore remained impractical until the discovery of Kirk (70) of a method for transferring silicic acid of low molecular weight from aqueous solution to solution in an alcohol. This transfer is accomplished by extraction of the acid with a suitable polar organic solvent, simultaneously saturating the aqueous phase with sodium chloride in order to salt the silicic acid into the organic phase. An alcohol such as rt-butyl alcohol is then added, and esterification is effected by azeotropic distillation of water from the alcohol solution (29). 

Goto (157) was one of the first to examine the polymerization of silicic acid in the absence of salts and in the pH range 7-10, where colloidal particles are formed and no aggregation or gelling can occur as at lower pH. He used the equation... 

One difference between gels made from stabilized colloidal silica and gels from silicic acid is that the commercial colloids are made in hot, slightly alkaline conditions that remove water and micropores from the interior of the particles. On the other hand, the particles made by polymerization of silicic acid, usually below pH 7 and at less than 80 C, and often in the presence of sodium salts, may be microporous. This point has been discussed by Vysotskii et al. (221), who suggest that the pores may be accessible to water but not large molecules. 

Figure 1.4. The polymerization behaviour of silicic acid to form silica. In basic solution (A), particles in the sol grow in size with a decrease in their number, while in acid solution or in the presence of flocculating salts (B), particles aggregate into three-dimensional networks and form gels. Reproduced from ref. (16) by permission of John Wiley Sons, Inc.

There is thus good experimental evidence that silicate melts are ionic liquids containing relatively free cations and mixtures of polymeric silicate anions. In a previous chapter Kleppa has reviewed what is known of the mixing properties of simple molten salts. The applications of these principles to melts containing a large number of different polyanions requires the introduction of methods developed by organic polymer chemists (Flory, 1936, 1952). Before describing the polymer models which have been applied to silicate melts it will be useful to review briefly the use of the terms acidic and basic as applied to oxides or melts. 

In one example, a quaternary ammonium salt of dimethylaminoacry-lamide (Q modified monomer of acryUc resin) was ion-bonded to silicate layers, while ethyl acrylate (EA) and acrylic acid (Aa) were copolymerized in the clay gallery. The ratio between the EA and the Aa was 10 1 (molar ratio). Eour kinds of acrylic resin-clay nanocomposites were polymerized. Their clay contents were 1, 3, 5 and 8 wt % on the basis of the solid acryhc resins. Suspensions with greater than 3 wt % clay addition acted as pseudoplastic fluids. Transparent acrylic resin-clay nanocomposite films cross-Hnked by melamine... 

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