Structure and morphology of freeze/thawed PVA hydrogels

The structure and morphology of poly(vinyl alcohol) (PVA) gels prepared by repeated cycles of 8 h freezing at -20 degrees C and 4 h thawing at +25 degrees C were examined. Long-term morphological changes of such gels were determined upon swelling in water at 37 degrees C for 6 months. The preparation conditions were examined by varying such parameters as the number of freezing and thawing cycles, the concentration of aqueous solution, and the PVA molecular weight. The overall structure and stability were examined in terms of water content, fractional PVA dissolution, degree of crystallinity, and crystal size distribution. An increase in the number of freezing and thawing cycles served to reinforce existing crystals within the structure. Increased initial concentrations of aqueous PVA solutions resulted in hydrogels that contained initially higher crystallinity and added stability upon swelling, an increase in the PVA molecular weight resulted in crystals of higher lamellar thickness and a broadening of the crystal size distribution due to an increase in PVA chain length. Secondary crystallization was more pronounced for more loosely cross-linked samples. An increase in the free volume and mobility within the network allowed for additional crystallization to proceed during swelling. Overall, freeze/thawed PVA gels of intermediate molecular weight ((M) over bar(n) = 64 000) and increased number of freezing and thawing cycles showed enhanced stability during swelling at 37 degrees C for a 6-month period of time demonstrating their appropriateness for long-term biomedical applications.