Poly(vinyl alcohol) was used as base material for hydrogel preparation. The water absorption, gel fraction, and density of different gels prepared by the freezing/thawing method were studied.

Poly(vinyl alcohol) gels, prepared by the freezing/thawing technique, were studied. Poly(vinyl alcohol) water solutions were exposed to 1-3 subsequent cycles of freezing (12 h at -20 °C) followed by thawing (12 h at 20 °C). Water content (weight and volume fraction) and degree of swelling α at the equilibrium state were determined. Average molecular weights of polymer chains between crosslinks M C (using the Flory-Rehner approach) were calculated. Values of α and M C considerably decrease with the growth of the number of freezing/thawing cycles n C .The modulus of elasticity E, tensile strength σ B , and elongation at brake ε B were determined from experimental stress-strain relationships of swollen gels. The E and σ B and values considerably increase with n C : up to 6-8 times for E and almost by an order for tensile strength σ B . More concentrated water solutions provide almost two times greater E and σ B values. Strengthdeformation characteristics for gels prepared at n C = 2-3 are acceptable for their application in potential drug delivery systems.To assess the stability of crosslinked structures, gels were subjected to subsequent drying (at 25, 60, and 105 °C) and water sorption (at 25 °C) cycles. Reduction of the swelling degree and respective calculated M C values as well as lessening of the initial rate of water sorption after each drying cycle indicate the formation of additional crosslinks.

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