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

The physical properties of deacylated and high acyl gellan systems were examined using small deformation rheology, optical rotation and differential scanning calorimetry. It is demonstrated that the drop in strength of deacylated gellan structures at high levels of counterions is caused by the formation of ordered nuclei which require low temperatures (< 50°C) to stabilise a low functionality network whereas the conformational transition leading to nuclei occurs at ~70°C. High acyl gellan is capable of gelation in the absence of added cations with structures showing similar cooling and heating profiles. It appears, however, that increasing levels of salt encourage sparse cross-linking between the acyl-free parts of the chain. These intermolecular associations remain stable during heating and support a network of increasingly flexible chains as the acylated helices are gradually melting. Thus the structure shows heating profiles with rubbery characteristics (i.e. increasing storage modulus), also observed in conditions of suppressed aggregation in high-sugar deacylated gellan networks. Chains of the two gellan variants are sterically incompatible and, depending on the level of added salt, yield single-phase systems, or composites with disparate phase behaviour. Finally, blending-law analysis supports the hypothesis that the entrapment of solvent in a polymeric phase is governed by the topology and the morphology of its network.