Structural aspects and phase behaviour in deacylated and high acyl gellan systems

The physical properties of deacylated and high acyl gellan systems were exa mined using small deformation rheology, optical rotation and differential s canning calorimetry. It is demonstrated that the drop in strength of deacyl ated gellan structures at high levels of counterions is caused by the forma tion of ordered nuclei which require low temperatures ( < 50 degrees C) to stabilise a low functionality network whereas the conformational transition leading to nuclei occurs at approximate to 70 degrees C. High acyl gellan is capable of gelation in the absence of added cations with structures show ing similar cooling and heating profiles. It appears, however, that increas ing levels of salt encourage sparse cross-linking between the acyl-free par ts of the chain. These intermolecular associations remain stable during hea ting and support a network of increasingly flexible chains as the acylated helices are gradually melting. Thus the structure shows heating profiles wi th rubbery characteristics (i.e. increasing storage modulus), also observed in conditions of suppressed aggregation in high-sugar deacylated gellan ne tworks. Chains of the two gellan variants are sterically incompatible and, depending on the level of added salt, yield single-phase systems, or compos ites with disparate phase behaviour. Finally, blending-law analysis support s the hypothesis that the entrapment of solvent in a polymeric phase is gov erned by the topology and the morphology of its network. (C) 1999 Elsevier Science Ltd. All rights reserved.