Thermal and rheological properties of poly(vinyl alcohol) and water-soluble chitosan hydrogels prepared by a combination of γ-ray irradiation and freeze thawing

Poly(vinyl alcohol) (PVA)/water-soluble chitosan (ws-chitosan) hydrogels were prepared by a combination of gamma-irradiation and freeze thawing. The thermal and rheological properties of these hydrogels were compared with those of hydrogels prepared by pure irradiation and pure freeze thawing. Irradiation reduced the crystallinity of PVA, whereas freeze thawing increased it. Hydrogels made by freeze thawing followed by irradiation had higher degrees of crystallinity and higher melting temperatures than those made by irradiation followed by freeze thawing. ws-Chitosan disrupted the ordered association of PVA molecules and decreased the thermal stability of both physical blends and hydrogels. All the hydrogels showed shear-thinning behavior in the frequency range of 0.2-100 rad/s. Hydrogels made by freeze thawing dissolved into sol solutions at about 80 degrees C, whereas those made by irradiation showed no temperature dependence up to 100 degrees C. The chemical crosslinking density of the hydrogels made by irradiation followed by freeze thawing was much greater than that of hydrogels made by freeze thawing followed by irradiation. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 109: 3825-3830,2008.