Molecular origins of the rheology of high-sugar gellan systems
In aqueous/low-sugar systems it is considered that gellan gels are formed through the formation of double helices which further associate to form highly aggregated three-dimensional structures. In this study, large and small deformation measurements have been carried out on deacylated gellan gels in the presence of increasing levels of cosolute and the results are discussed in terms of the changing nature of the gel network. Large deformation measurements show a significant decrease in the gel modulus at 60% cosolute compared to gels with no added cosolute, accompanied by an increase in yield strain and yield stress and a decrease in the calcium required for maximum modulus.
This argues for limited aggregation of the polymer chains in the presence of the cosolute. Small deformation measurements show that in 80% cosolute cooling profiles are typical of a material that undergoes vitrification. The Williams—Landel—Ferry/ free-volume theory has been successfully applied to these high-sugar gellan gum systems. The application of this theory requires a lightly cross-linked system where the contribution to elasticity is mainly entropic. The importance of the polymer lies in the introduction of a new phase, the rubbery region, which vitrifies readily upon cooling, resulting in a higher glass-transition temperature than for the sugar alone. The variation of the relaxation distribution function with the time scale of the measurement shows a striking similarity between the biological materials studied here and synthetic polyisobutylene. We therefore envisage the gellan high-sugar system as a polymeric network of flexible disordered coils supported by occasional junction zones.
Key wordsGellan Sugar Vitrification Distribution function Activation energy
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