Characterization of the network structure of dextran glycidyl methacrylate hydrogels by studying the rheological and swelling behavior

This paper reports the results and the structural interpretation of theological, swelling, and analytical sol fraction (w(s)) measurements performed on dextran glycidyl methacrylate (dex-gma) hydrogels as a function of the dex-gma concentration (c) and the degree of gma substitution (DS). Besides the analytical determination, the sol fi action was also calculated from the elasticity of the hydrogels using a theoretical model. This model takes into account the presence of sol chains and dangling ends, the polydispersity of the dex-gma chains, the functionality of the junctions, and the nonaffinelike behavior of the dex-gma network. It assumes the absence of intramolecular cross-links and of physical entanglements. Fundamental in the discussion is the reason why the w(s) values determined analytically are systematically lower than the w(s) values calculated using this theoretical model. Besides possible influences from the unknown functionality of the junctions and from a nonaffinelike deformation behavior of the network, the presence of intramolecular cross-links (especially for hydrogels with a low dex-gma concentration and a high DS) may contribute to the observed differences between the measured and the calculated sol fraction. Structural information is also revealed from the influence of the DS on the elasticity if plotted against the network concentration (i.e., (1 - w(s))c) of the hydrogels. Clearly, a DS increase increases quantitatively the network fraction. However, for high DS values, a DS increase does not increase the number of intermolecular cross-links per unit of mass present in the network fraction. Also the abundant presence of intramolecular cross-links, especially for dex-gma hydrogels with a high DS, may contribute to this phenomenom. Contrary to the elastic properties, considering hydrogels with the same network concentration, the DS does have a definite influence on the swelling properties of the network fraction of the gels. This was attributed to the dependence of the polymer-solvent interaction parameter on the DS

Calcium induced skim-milk gelation during heating as affected by pH

International audienceAbstractMilk gels (acid or rennet) are used by dairy industry to produce dairy products such as yogurt and cheese. Enrichment of milk with calcium salts and heat treatment are known to produce “calcium-milk coagulum” as a new type of milk gels, due to reduction of milk protein charges through calcium binding. The combination of heat treatment and calcium addition to milk results in gel structures, but the effect of calcium addition and pH adjustment during heating of milk is still unclear. The role of added calcium and decreasing pH were investigated by addition of calcium chloride (30 mM) to reconstituted skim milk followed by pH adjustment by hydrochloric acid and sodium hydroxide (4.6 < pH < 6.6 investigated), followed by heating at 90 °C for 10 min and overnight storage at 22 °C. In parallel, samples with no addition of calcium chloride were produced under the same conditions. The time and temperature to reach the gelation point, as detected by dynamic measurements of storage modulus (G′), were decreasing as pH decreased without addition of calcium, while calcium addition made gelation time and temperature independent of pH except for pH 4.6. Heat treatment combined with calcium addition was found, using confocal laser microscopy, to provide a fine and dense gel structure for skim milk with higher pH, while at pH lower than 5.6, the gel structure was similar to the structure of acid-induced gels. The last observation helps to establish a pH limit for production of calcium gels