Synthesis and physicochemical characterization of gellan gels

Thermally stable hydrogels have been prepared starting from aqueous gellan physical gels by partial self-cross-linking of the polysaccharide chains (formation of essentially interchain ester bonds, with no added bridging moieties) with the aid of the water-soluble 1-ethyl-3-[3-(dimethylammino)propyl]-carbodiimide (EDC). The structure of the hydrated networks-physicochemical gels-was analyzed on the basis of the results of small-angle X-ray scattering experiments. Here a molecular model, which is composed of a bundle of 48 gellan double helices aligned in parallel, was proposed to represent the ordered domains in the hydrogels. The rigid bundles formed by associated gellan double helices constitute the junction zones which sustain the overall gel structure displaying solidlike properties. Relatively large cavities supported by rigid bundles inside the gels absorb water quickly as indicated by kinetic water uptake data. A network structure formed by rigid microfibrils of associated gellan double helices connected by flexible joints of disentangled short gellan chains-In which most of the chemical cross-linking should have taken place-is consistent with the rheological and swelling behavior of the new polymeric materials.