Thermal behavior of water in crosslinked dextran

The phase behavior of the system water/SephadexG25 in a wide range of water content We (g water/g dry Sephadex) has been investigated by differential scanning calorimetry. At low degree of hydration (Wc < 0.38), the system is homogeneous and is characterized by a glass transition whose temperature-c omposition dependence is well described by the Gordon-Taylor equation. When We is higher than 0.38, all water in excess with respect to 38% crystalliz es an cooling to low temperature, i.e., the system contains two phases: sep arated freezable water in equilibrium with an amorphous phase of water-plas ticized Sephadex at constant composition. Depending on the hydration level, separated water exists in two different states. Clusters of small size, pr esent at We < 0.8, exhibit a freezing and melting behavior deviating from t hermodynamic equilibrium, and bulk-like water properties are observed only at a water content higher than 0.8. In the hydration range 0.67 less than o r equal to Wc less than or equal to 1.48, calorimetric curves show crystall ization phenomena and multiple melting peaks which have been correlated wit h the presence of a wide distribution of crystallite sizes. On repeated coo lings, Che pore structure of the hydrated polysaccharide network is damaged by mechanical stresses generated by ice formation.