Thermal and viscoelastic properties of alginate/poly(vinyl alcohol) blends cross-linked with calcium tetraborate

Binary blends of alginate (Alg) with poly(vinyl alcohol) (PVA) were cast in film form from mixed aqueous polymer solutions by solvent evaporation. The thermal transition and viscoelastic behavior of the Alg/PVA blends were investigated by differential scanning calorimetry and dynamic mechanical analysis. Irrespective of composition, blend samples gave a single glass transition temperature (Tg) situated between the Tgs of PVA and alginate per se. The melting point of the PVA component was depressed systematically with increasing alginate content in the blends. These observations indicate that the polymer blend of Alg/PVA is capable of forming a thermodynamically miscible phase in the amorphous state. An additional treatment of as-cast samples with calcium tetraborate solution gave rise to a marked elevation of their Tgs. This may be ascribed to the possible, simultaneous occurrence of a chelate complexation of alginates with Ca2+ cation and a borate ion-aided cross-links between PVA chains, resulting in the formation of an `IPNʹ structure.