Thermal properties of water insoluble alginate films containing di- and trivalent cations

Water insoluble alginic acid (Alg) films were prepared by replacing the sodium ions (Na+) of sodium alginate (NaAlg) with di- and trivalent cations, such as Cu2+, Al3+, Fe3+, etc. Dope concentration and degree of substitution (DS) were changed in order to prepare various types of alginate films. Glass transition temperature (T-g), heat capacity (C-p) and C-p difference at T-g (Delta C-p) of the Alg films in the dry state were investigated by differential scanning calorimetry (DSC). With increasing ionic radius of cations, T-g of dry films decreased. In contrast, C-p's of alginate films in the glassy state and Delta C-p increased. The non-freezing water content (W-nf) in alginate films in the wet state was also estimated by DSC using enthalpies of melting and crystallization of water. W-nf in the films increased with increasing ionic radius. These results indicated that alginate molecules form a more tightly cross linked structure with decreasing ionic radius. Structural change of alginate films during ion exchange was also investigated based on the change of T-g, C-p and W-nf values as a function of DS.