Polysaccharides chemical cross-linking

Oil-field chemistry has undergone major changes since the publication of earlier books on this subject Enhanced oil recovery research has shifted from processes in which surfactants and polymers are the primary promoters of increased oil production to processes in which surfactants are additives to improve the incremental oil recovery provided by steam and miscible gas injection fluids. Improved and more cost-effective cross-linked polymer systems have resulted from a better understanding of chemical cross-links in polysaccharides and of the rheological behavior of cross-linked fluids. The thrust of completion and hydraulic fracturing chemical research has shifted somewhat from systems designed for ever deeper, hotter formations to chemicals, particularly polymers, that exhibit improved cost effectiveness at more moderate reservoir conditions. 

Dextrans are bacterial and yeast polysaccharides made up of (a 1—>6)-linked poly-D-glucose all have (al—>3) branches, and some also have (al—>2) or (al—>4) branches. Dental plaque, formed by bacteria growing on the surface of teeth, is rich in dextrans. Synthetic dextrans are used in several commercial products (for example, Sephadex) that serve in the fractionation of proteins by size-exclusion chromatography (see Fig. 3-18b). The dextrans in these products are chemically cross-linked to form insoluble materials of various porosities, admitting macromolecules of various sizes. 

Nanoparticles have been prepared from polysaccharides, proteins, and amphiphilic macromolecules by inducing their aggregation followed by stabilization either by heat denaturation or chemical cross-linking. The former can be done by water-in-oil emulsion system or in aqueous environments. The cross-... 

Before proceeding with the details of polysaccharide gels, it is worthwhile to examine the C NMR characteristics of gel samples of chemically cross-linked synthetic polymers in which the extent of the cross-links can be varied more easily. 

The polysaccharide can exist in a number of chemical forms. It is composed of uronic acids. If the acid groups are in the acid form (-COOH), the polysaccharide is called alginic acid, which is water insoluble. If the acid groups are in the car-boxylate form (-COO ) it is as the alginate or sodium salt (-COONa) or sodium alginate, which is water soluble. If the sodium ions are replaced by a divalent metal ion such as calcium, barium, or strontium, the polysaccharide is cross-linked by the metal ions to form gels. 

Work by the present author in collaboration with a major group in Rome, interested in the chemical cross-linking of polysaccharides, can be used both to illustrate the approach, and to display the form of tiib critical gel spectrum. (We note that while the majority of critical do seem to reflect this behaviour, it does not appear to be universal, as shown by Ilavsky and co-workers. There are also systems that can be regarded as critical gels, if at all, only by twisting the definition, Miith appear to give the same shape of spectrum). 

Chen (1994) reacted wood with epicholorohydrin, using triethylamine as a catalyst. Weight loss due to decay by G. trabeum in a 12-week exposure test was less than 3 % for a WPG of 11 %. Some of this weight loss was found to be due to loss of epicholorohydrin. IR and chemical analysis data was presented, which was interpreted as indicating that cross-linking of cell wall polymers had occurred, with reference to other work where this had been found with polysaccharides. However, it is not clear from the evidence presented that such a cross-linking reaction had indeed occurred. 

Depending upon chemical structure and the conformations that are possible, polysaccharides in solution may develop secondary structures such as helices, tertiary structures formed from junction zones or by double helix or triple helix unions and even quaternary structures from the cross linking of tertiary structures. Polysaccharides thus mimic proteins and nucleic acids, which are specific types of sugar-phosphoric acid copolymers. 

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