Cross-linked starches swelling

The starch granule gelatinization temperature increases in proportion to the extent of cross-linking, while the swelling power decreases (Fig. 4.38). Starch stability remains high at extreme pH values (as in the presence of food acids) and under conditions of shear force. Cross-linked starch derivatives are generally used when high starch stability is demanded. 

In contrast to monophosphates, starch phosphate diesters contain cross-links between two or more starch chains. This covalent linkage in the granule produces a starch product which swells less but is more resistant to heat, agitation, and acid than natural starch. 

Gels used for the stationary phase can be hydrophilic, for separations in aqueous and other polar solvents, or hydrophobic, for use in non-polar or weakly-polar solvents. Agar, starch, polyacrylamide and cross-linked dextrans possess hydroxyl or amide groups and are thus hydrophilic. They swell... 

A slightly different approach is to deliver the active drug in a dry powder carrier system, for example microcrystalline cellulose, hydroxyethyl starch, cross-linked dextran, microcrystalline chitosan, carbomer, pectin, or alginic acid. The polymer absorbs water upon contact with the nasal mucosa and swells to become a viscous gel, often demonstrating bioadhesive properties. Such systems can remain in the nasal cavity for as long as six hours. For example, the bioavailability in rats of the somatostatin analogue, octreotide, was shown to be enhanced by the co-administration of alginic acid and cross-linked dextran as dry powders. 

Gels used for the stationary phase can be hydrophilic, for separations in aqueous and other polar solvents, or hydrophobic, for use in non-polar or weakly-polar solvents. Agar, starch, polyacrylamide and cross-linked dextrans possess hydroxyl or amide groups and are thus hydrophilic. They swell in aqueous media and in such solvents as ethylene glycol and dimethylform-amide. Bio-Cel (a co-polymer of acrylamide and jV.W -methylene-bisacryl-amide) and Sephadex (dextran cross-linked with epichlorhydrin) arc two commercially available gels made in bead form. Cross-linking produces a... 

Figure 4-42 Values of Yield Stress of Starch-Xanthan Dispersions Relative to those of the Starch-Water Dispersions (YSA SO) and Relative Mean Granule Diameters (D/DO) Plotted against Values of c[j) of Xanthan Gum waxy maize (WXM), cross-linked waxy maize (CWM), and cold water swelling (CWS).

Achayuthakan et al. (2006) studied vane yield stress of Xanthan gum-stareh dispersions. The intrinsic viscosity of Xanthan gum was determined to be 112.3 dl/g in distilled water at 25°C. In addition, the size of the granules in the dispersions of the studied starches waxy maize (WXM), cross-linked waxy maize (CWM), and cold water swelling (CWS) were determined. The values of yield stress of the starch-xanthan dispersions relative to those of the starch-water dispersions (YSA"S0) and relative mean granule diameters (D/DO) plotted against values of c[ ] of xanthan gum are shown in Figure 4-42. With the values of YS/YSO being less than 1.0, there was no synergism between CLWM starch and xanthan gum. 

Meng, Y. and Rao, M. A. 2005. Rheological and structural properties of cold-water-swelling and heated cross-linked waxy maize starch dispersions prepared in apple juice and water. Carbohydrate Polymers 60 291-300. 

These dressings are sheets of three-dimensional networks of cross-linked hydrophilic polymers (polyethylene oxide, polyacrylamides, polyvinylpyrrolidone, carboxymethylcellulose, modified corn starch). Their formulation may incorporate up to 96% bound water, but they are insoluble in water and they interact by three-dimensional swelling with aqueous solutions. The polymer physically entraps water to form a solid sheet and they have a thermal capacity that provides initial cooling to the wound surface. A secondary dressing is required. 

Lightly cross-linked polyacrylamide is used to make superabsorbents of water. Astarch-g-polyacryla-mide/clay superabsorbent composite has been synthesized [27] by graft copolymerization reaction of acrylamide, potato starch, and kaolinite micropowder (< 1 pm) followed by hydrolysis with sodium hydroxide. Such a superabsorbent of compositin 20% kaolinite, 20% potato starch, 60% acrylamide, 2% initiator (ceric amonium nitrate), and 0.04% cross-linker (N,N-methylenebisacrylamide) is found to absorb 2250 g H20/g at room temperature at swelling equilibrium. 

Carbinatto F. M., De Castro A. D., Evangelista R C., Cuiy B. S. F. (2014). Insights into the swelling process and drug release mechanisms from cross-linked pectin/high amylose starch matrices, f hgnrt cL, 9, 27-34. 

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