Polymer-polymer interactions in dextran systems using thermal analysis

Miscibility of carbohydrate polymers with similar chemistry at low water activity (a,) was studied using dextran mixtures as model systems. Dextrans with weight-average molecular weights (M(w)) ranging from 970 to 2,000,000 were used. Mixtures were prepared both by blending in powder form, and by dissolving in water and mixing, followed by freeze-drying. All mixture samples were equilibrated at a(w) = 0.33, and their glass transition temperatures (T(g)) were determined by thermal analysis. For pure dextrans, T(g) increased with M(w) up to a critical M(w) approximately between 23,000 and 30,000; reaching a plateau at higher M(w). The physical blend of dextrans in powder form resulted in dual T(g) even after melting of the blends, indicating immiscibility; whereas for dextrans that were dissolved in water and mixed, only one T(g) was observed, indicating miscibility. T(g) of miscible systems was better related to the number-average molecular weight than weight-average molecular weight. (c) 2005 Elsevier Ltd. All rights reserved.