Combined use of the free volume and coupling theories in the glass transition of polysaccharide/co-solute systems

The structural properties of the glass dispersion in agarose, κ-carrageenan and deacylated gellan with co-solute (glucose syrup) at 80.0% (w/w) solids were studied. Investigative techniques were small-deformation stress relaxation and dynamic oscillation on shear. Vitrification was monitored between −2 and −50 °C with continuous thermal runs and isothermal frequency or time sweeps obtained at constant temperature intervals. The time–temperature superposition principle was utilized to compose master curves. The Williams, Landel and Ferry equation was able to pinpoint the network Tg for these systems as the turning point from the predictions of the free volume theory in the transition region to those of the reaction rate theory at the glassy state. Further insights into the physics of intermolecular interactions at the vicinity of Tg were obtained using the coupling model of molecular dynamics in the form of the Kohlrausch, Williams and Watts function. The model described well the spectral shape of the local segmental motions in polysaccharide/co-solute samples at the short-time region of the stress–relaxation master curve. Analysis provided the intermolecular interaction constant and apparent relaxation time, which are valuable parameters for the elucidation of structural morphology at Tg.