Thermo-reversible gels

Nishinari, K., Koide, S., and Ogino, K. 1985. On the temperature dependence of elasticity of thermo-reversible gels. J. de Physique 46 793-797. 

In contrast, low-methoxy (LM) pectins gel in the presence of divalent cations, especially calcium, by the egg box mechanism proposed for alginates. Moreover, calcium pectinate gels prepared at neutral pH are heat stable, whereas acidic pH gels are thermo-reversible. Gel strength depends on the extent of esterification (levels from 30% to 50% are optimal), the distribution of ester groups on the chain, and the average molecular weight. LM pectins have been used traditionally in antidiarrheal formulations with kaolin. HM pectins were evaluated in controlled release matrix formulations. Pectin microspheres were reported to improve ophthalmic bioavailability of piroxicam in rabbits compared with commercial piroxicam eye drops. ... 

Thermo-reversible gels are formed in water at concentrations above the overlap concentration. 

Interaction of xanthan with guar gum in solution leads to enhanced viscosity, whereas elastic thermo-reversible gels are obtained with LEG and konjac mannan sofP (Figure 26.5). Since xanthan may contain cellulases, it cannot be used with cellulose derivatives. 

Formulating Considerations For best results, should be hydrated in hot water, preferably above 80°C. Thermo-reversible gels melt and reset at 50-55°C Fortn Supplied White to tan powder... 

Figure 1. Tensile strength (stress at break) versus year of some mam-made industrial fibers. Recent highlights are the discovery of high-performance fibers derived from liquid-crystalline aramids, first commercialized by Du Pont under the registered trade name Kevlar and the discovery of the unusual tensile drawing characteristics of thermo-reversible gels of high molecular weight polyethylene, which resulted in ultra-high strength fibers sold by DSM/Toyobo (Dyneema) and, under licence from DSM, by Allied (Spectra).

THERMO-REVERSIBLE) GELS OF FLEXIBLE CHAIN MOLECULES... 

The structure of thermo-reversible gels of high molecular weight polyethylene, discussed in the following sections, is revealed by the self-explanatory wide-angle X-ray pattern and micrographs in Figures 13 and 14. 

It should be pointed out that thermo-reversible gels can be obtained from solutions of relatively low molecular weight species, and at concentrations well below , if these molecules precipitate as high-aspect ratio entities, such as fibrillar crystals or whiskers. The latter gels, however, invariably are brittle and not suitable for the applications discussed below. 

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