Poly /siliceous

Pyroxenes are poly silicates ( inosilicates in the mineralogical classification), crystallizing, respectively, in the monoclinic clinopyroxenes spatial groups C2lc, Pljlc, P2ln) and orthorhombic systems orthopyroxenes spatial groups... 

The production of silicate glass is believed to be a transcondensation of the siloxane linkages in silica. A comparable poly(silicic acid) is produced when silicon tetrachloride is hydrolyzed. 

Many minerals can be described as salts of poly silicic acids, with Si04 tetrahedra sharing corners with one another. Their structure is discussed in Chapter 13. 

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. 

Schwarz and Knauff (64) approached the problem of the poly-silicic acids quite differently, by preparing linear partial hydrolysis products of methyl silicate,... 

Combinations of cationic starch and anionic microparticles are useful commercial systems. Shear-sensitive flocculation occurs, allowing microscale reflocculation in the formed paper sheet, which improves dewatering and retention.63,75,76 The microparticles can be colloidal silica, aluminum silicate, poly(silicic acid) or bentonite of specific size and surface area.77 79 Cationic, anionic or polymeric aluminum-containing compounds can be additional components. A three-part coacervate system uses a high molecular weight anionic polyacrylamide, cationic starch and silica.80 Cooking cationic starch in the presence of an anionic silica hydrosol was reported to improve drainage and retention.81... 

It is important to recognize that asbestos is not a single substance, but is the generic name for a family of six related poly silicate fibrous minerals of which one (chrysotile) belongs to the serpentine family and five (actinolite, amosite, anthophyllite, crocidolite, and tremolite) belong to the amphibole family. These minerals differ from each other in physical and chemical properties, and each mineral can exist in a wide range of fiber sizes. These differences between fiber type and, more importantly, fiber size (length and diameter) are believed to be important determinants of the health risks posed by asbestos. 

A powerful technique for investigating these different species is Si-NMR (see Fig. 9.1). Different lines can be observed in the Si-NMR spectrum of a silicate solution corresponding to the differently positioned Si nuclei in the (poly)silicate ions. The highest values for the chemical shift (5 = -71.5 ppm with respect to tetra methyl silane) are found for the monomeric silicate units while the resonances of fully condensed silicon atoms (i.e. Si-(0-Si)4 tetrasiloxy silane) are to be found at the lowest values for the chemical shift (5 = -110 ppm). 

Hoebbel et al. carefully studied the state of polysilicate anions in TBAS solution.181 Their results indicated that there mainly existed monomeric, dimeric, trimeric, tetra-meric, pentameric, and heptameric silicate ions and poly silicate anions in the form of double 3-, 4-, and 5-membered rings in TBAS solution. Compared with the TMAS and TEAS solutions that mainly contain double 3- and 4-membered-ring-type silicate ions under some conditions, the distribution of these species in TBAS solution is quite even. However, the distribution of these species in solid crystalline structures is quite different to that in solution. It was found that there are a large number of double 5-membered rings in the solid crystalline structure obtained from the corresponding solution. Studies indicated that there are some similarities between the properties of TPAS and TBAS solutions. Owing to space limitation, studies on TPAS solution will not addressed here. 

An important question to be explained by the chemical mechanism is why base catalysis often leads to condensed structures. In 1950, Aelion et al. (13) pointed out that the condensation reaction in base-catalyzed systems was faster than with acid catalysis and that the microstructure of the final product was different. Many of these facts are due to the opposite effects of, for example, substituents, on silicon on the stabilization of the transition state in base- and acid-catalyzed reactions (15). For the base-catalyzed condensation reaction to take place, a silicon atom has to be attacked by a deprotonated silanol oxygen the nucleophile. The acidity of the silanol proton increases as the basicity of the other groups bonded to the silicon decreases. This feature implies that poly silicic acid is a stronger acid than Si(OH)4 (20). Therefore, monomers react preferentially with higher polymerized species. 

Figure 2. NMR spectrum of a sodium silicate solution in D20 with a ratio of R = Na Si =0,14, The spectrum shows the resonance of the principal building units of a poly silicate. This and all of the other spectra were run on a Bruker WM 250, Conditions measuring time 1 h 8-s pulse length 0,8-s acquisition time.

The third type is a hybrid of the first two with poly-silicic acid chains joining preformed colloidal silica particles. Hybrid gels are formed by neutralization of mixtures of silicates and colloidal silica sols. 

These both formulae are in good compatibility with the poly silicate presented in the model proposed by Glasser et al. [68] and the second with C-S-H composed of the dimers only. 

Scratchproof coatings on poly(methyl methacrylate) can be produced by application of a 50 50 mixture of poly(silicic acid) and poly(tetrafluoro-ethylene-co-hydroxyalkyl vinyl ether). It is not known why coatings as scratchproof as glass can be produced in this manner. 

Inagaki, S. Fukushima, Y Kuroda, K., Synthesis of highly ordered mesoporous materials from a layered poly silicate. Journal of the Chemical Society, Chemical Communications (8), 680-682, 1993. 

This section deals with the chemistry of the reaction and gives some recommended analytical procedures. Application of the reaction to determining the nature of poly-silicic acids and colloidal particles is reserved for Chapter 3. 

The term colloidal silica here refers to stable dispersions or sols of discrete parti-cks of amorphous silica. By arbitrary definition, the term excludes solutions of poly-silicic acid in which the polymer molecules or particles are so small that they are not stable. Such solutions, which are usually obtained by acidifying sodium silicate solutions or by hydrolyzing silicon esters or halides at ordinary temperatures, have been discussed in Chapter 3 as precursors of colloidal particles. 

Aggregation and gelling have already been discussed in detail in Chapter 3 as one of the mechanisms of polymerization in the case of extremely small particles of poly-silicic acid. In this chapter only the conversion of sols of relatively uniform discrete particles to gels is considered. The main discussion of gels is reserved for Chapter S. 

The presence of residual toxic chemical species, such as the electrolyte (HF) itself and the etch by-products silicon tetrafluoride (Sip4) and hexafluorosilicic acid (H2S1F6), all water soluble, must be considered after anodization (Koynov et al. 2011) and, likewise, any residual electrolyte additives. The secondary reaction of H2Sip6 with water in the aqueous environment of the pores may produce metasilicic acid (H2Si03) - this can polymerize to a less-soluble (poly-silicate) gel (and ultimately a solid), which may entrap the residual toxic species mentioned above (Koynov et al. 2011). The complete removal of all toxic species is a prerequisite for most applications and is particularly important for the biomedical field. 

Complete hydrolysis yields poly silicic acids. Monosilicic acid is formed in dilute solution and when the hydrochloric acid set free according to the equation ... 

Muller, W.E.G., SchloBmacher, U., Wang, X.H., Boreiko, A., Brandt, D., Wolf, S.E., Tremel, W., Schroder, H.C., 2008c. Poly(silicate)-metabolizing silicatein in siliceous spicules and silicasomes of demosponges comprises dual enzymatic activities (silica-polymerase and silica-esterase). FEBS J. 275, 362-370. 

Inorganic polymers do not contain any carbon in their chains, and they can be formed by the elements of the group IV of the periodic table. Cements, poly silanes, and poly silicates below are typical examples of these types of polymers ... 

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