Investigation on the formation mechanisms of hydrogels made by combination of γ-ray irradiation and freeze-thawing

Hydrogels based on poly(vinyl alcohol) (PVA)/water-soluble chitosan (ws-chitosan)/glycerol were prepared by gamma-ray irradiation, freeze-thawing, and combination of gamma-ray irradiation and freeze-thawing, respectively. The influence of freeze-thawing cycles, the irradiation doses, and the sequence of freeze-thawing and irradiation processes on the rheological, swelling, and thermal properties of these hydrogels was investigated to evaluate the formation mechanisms of hydrogels made by combination of irradiation and freeze-thawing. For hydrogels made by freeze-thawing followed by irradiation, the physical crosslinking is destroyed partially while chemical crosslinking is formed by irradiation. However, the chemical crosslinking density reduces with the increase of freeze-thawing cycles. Hydrogels made by irradiation followed by freeze-thawing bear less degree of physical crosslinking with the increase of irradiation dose for the increased chemical crosslinking density. It is found that these hydrogels own larger swelling capacity and better transparent appearance than those made by freeze-thawing followed by irradiation. Moreover, the former hydrogels have larger mechanical strength than the latter at low freeze-thawing cycles. (c) 2008 Wiley Periodicals, Inc.