Viscous fragility of concentrated maltopolymer/sucrose mixtures

The rheological properties of low moisture content glass-forming solutions of a maltopolymer and sucrose were measured over a range of temperatures. The results showed near-Newtonian behaviour throughout the measureable regime. Shear and dynamic viscosity data were combined and modelled as a function of temperature by a restricted Vogel-Fulcher-Tammann (VFT) equation. The fragility parameter (F1/2) ranged from 0.6 to 0.7, which represents moderate to fragile behaviour. Fragility decreased with increased moisture content for some carbohydrate compositions, but experienced little change for others. Carbohydrate compositions with lower weight-average molecular weight and/or lower degrees of polydispersity were more fragile. Some basis for expecting a polydispersity dependence can be found in empirical relations used to model the glass transition temperature and melt viscosity of carbohydrate polymers, which employ number-average and weight-average dependencies, respectively.