Small-angle X-ray scattering and rheological characterization of alginate gels. 2. Time-resolved studies on ionotropic gels

Gelation was observed by time-resolved small-angle X-ray scattering and rhe ology on 10 mg/ml Ca-alginate gels prepared by in situ release of Ca2+ from CaEGTA or CaCO3 with total Ca2+ concentration in the range 10-20 mM. This was carried out for alginates having a fraction of alpha -L-GulA (G) of F-G = 0.39 and 0.68, respectively, obtained by the selection of alginates isol ated fi om two different brown algae, Ascophyllum nodosum and Laminaria hyp erborea stipe. Correlation between the rheological data and SAXS data shows that a large fraction of the lateral association precedes the formation of a continuous network through the sample cell. Following the initial associ ation of chain segments in junction zones, the analysis using two-component broken rod model indicates the formation of larger bundles, and that the r elative weight of these bundles increases with increasing time. The molecul ar model for the bundles is proposed by associating 2-16 units (G-blocks) c omposed of 14 (1 --> 4) linked residues of alpha -L-GulA in parallel accord ing to the available crystallographic data. The storage modulus increases as the bundles composed of associated alginat e chains grow during the gel formation. The gel elasticity is mainly sustai ned by single chains in the alginate sample with a low fraction of alpha -L -GulA. The alginates with a high fraction of alpha -L-GulA associate into t hicker bundles which join to Form a network. Here the gel elasticity seems to be due to the flexible joints between bundles, since the fraction of sin gle chains is extremely low. (C) 2000 Elsevier Science B.V. All rights rese rved.