Thermally gelation

Other examples illustrating the effect of substituent distribution on properties include (/) enzymatic stabiUty of hydroxyethjlceUulose (16,17) (2) salt compatibihty of carboxymethylceUulose (18,19) and (J) thermal gelation properties of methylceUulose (20). The enzymatic stabUity of hydroxyethylceUulose is an example where the actual position of the substituents within the anhydroglucose units is considered important. Increasing substitution at the C2 position promotes better resistance toward enzymatic cleavage of the polymer chain. Positional distribution is also a factor in the other two examples. 

Properties. MethylceUulose [9004-67-5] (MC) and its alkylene oxide derivatives hydroxypropylmethylceUulose [9004-65-3] (HPMC), hydroxyethylmethylceUulose [9032-42-2] (HEMC), and hydroxybutyknethylcellulose [9041-56-9] (HBMC) are nonionic, surface-active, water-soluble polymers. Each type of derivative is available in a range of methyl and hydroxyalkyl substitutions. The extent and uniformity of the methyl substitution and the specific type of hydroxyalkyl substituent affect the solubifity, surface activity, thermal gelation, and other properties of the polymers in solution. 

Thermal gelation, 13 73 Thermal gel stabilization, 23 71 Thermal generation, of Ag+ intermediate complexes, 19 355... 

Cai, R., Arntfield, S. (1997). Thermal gelation in relation to binding of bovine serum albumin-polysaccharides systems. Journal of Food Science, 62, 1129-1134. 

Thermal gelation The polymeric solution is cast hot. Cooling causes precipitation... 

Precipitation of the cast liquid polymer solution to form the anisotropic membrane can be achieved in several ways, as summarized in Table 3.1. Precipitation by immersion in a bath of water was the technique discovered by Loeb and Souri-rajan, but precipitation can also be caused by absorption of water from a humid atmosphere. A third method is to cast the film as a hot solution. As the cast film cools, a point is reached at which precipitation occurs to form a microporous structure this method is called thermal gelation. Finally, evaporation of one of the solvents in the casting solution can be used to cause precipitation. In this technique the casting solution consists of a polymer dissolved in a mixture of a volatile good solvent and a less volatile nonsolvent (typically water or alcohol). When a film of the solution is cast and allowed to evaporate, the volatile good solvent evaporates first, the film then becomes enriched in the nonvolatile nonsolvent, and finally precipitates. Many combinations of these processes have also been developed. For example, a cast film placed in a humid atmosphere can precipitate partly because of water vapor absorption but also because of evaporation of one of the more volatile components. 

Perhaps the simplest solution-precipitation membrane preparation technique is thermal gelation, in which a film is cast from a hot, one-phase polymer/solvent solution. As the cast him cools, the polymer precipitates, and the solution separates into a polymer matrix phase containing dispersed pores filled with solvent. Because cooling is usually uniform throughout the cast film, the resulting membranes are relatively isotropic microporous structures with pores that can be controlled within 0.1-10 i m. 

A schematic diagram of a commercial-scale thermal gelation polymer precipitation process is shown in Figure 3.16. The hot polymer solution is cast onto a water-cooled chill roll, which cools the solution, causing the polymer to precipitate. The precipitated film is passed through an extraction tank containing methanol, ethanol or isopropanol to remove the solvent. Finally, the membrane is dried, sent to a laser inspection station, trimmed and rolled up. 

Tashiro, K., Ikeda, S., Sekiguchi, T., Shoji, S., Makazu, H., Funatsu, T., Tsukita, S., A particle and biomolecules sorting micro flow system using thermal gelation of methyl cellulose solution. Micro Total Analysis Systems, Proceedings 5th i.TAS Symposium, Monterey, CA, Oct. 21-25, 2001, 471-473. 

Figure 3 Thermal gelation of methylcellulose in aqueous solution (100 mPa s at 20°C and heating rate 0.25°C min-1). Reprinted with the permission of the Dow Chemical Co., Midland, MI.

Sarkar, N. (1979). Thermal gelation properties of methyl and hydroxypropyl methylcellulose. /. Appl. Polym. Sci. 24 1073-1087. 

Pluronic copolymers with molecular weight of 11,500 have been used as a viscosity modifier and form a clear gel at 20 to 25% w/w in water. A solution of Pluronic undergoes a thermal gelation process in which a mixture of the polymer and water forms a clear solution at low temperature and then becomes a gel when raised to room temperature (above 21°C). A blend of Pluronic and lecithin is sold as Organogel for topical gel formulations. 

FIGURE 7.32 Thermal gelation of hydroxypropyl methylcellulose 60SH (2 wt%) in water T = the temperature at which viscosity starts decreasing T2 = the temperature at which viscosity starts increasing. [Graph reconstructed from data by Shin-Etsu Chemical, METO-LOSE, 1993.]... 

Gelation Gelation Spray evaporation Thermal gelation evaporation... 

Wu, M. C., Lanier, T. C., and Hamann, D. D. 1985a. Rigidity and viscosity changes of croacker actomyosin during thermal gelation, y. FoodSci. 50 14-19. 

Thermal gelation of a protein is influenced by many factors, such as the protein concentration, pH, and presence of salts. Protein-Protein interactions are generally favored at conditions that reduce the net charge on the molecules, that is, pH values near the isoelectric point (Boye et al., 1995). Monovalent and divalent salt ions screen electrostatic interactions between charged protein molecules (Bryant... 

The thermal gelation temperature for methylcellulose decreases as a function of concentration. The presence of additives can increase or decrease the thermal gelation temperature. The presence of drugs can influence the properties of methylcellulose gels. In addition, the viscosity of methylcellulose solutions can be modified by the presence of drugs or other additives. Aqueous solutions of methylcellulose can be frozen and do not undergo phase separation upon freezing. 

Figure 1. Schematic diagram showing the formation of a microporous membrane by thermal gelation of a polymer solution exhibiting a miscibility gap at certain conditions of temperature and composition.

In this paper the discussion was concentrated mainly on the phase separation in polymer solutions due to the change of the composition of the mixture. The basic relations are also valid for phase separations induced by temperature changes, that is thermal gelation and can be applied to glass and metal alloys as well as to polymers. 

Loppinet B, Stiakakis E, Vlassopoulos D, Fytas G, Roovers J (2001) Reversible thermal gelation in star polymers an alternative route to jamming of soft matter. Macromolecules 34 8216-8223... 

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