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 a at the equilibrium state were determined. Average molecular weights of polymer chains between crosslinks MC (using the Flory–Rehner approach) were calculated. Values of a and MC considerably decrease with the growth of the number of freezing/thawing cycles nC.
The modulus of elasticity E, tensile strength sB, and elongation at brake eB were determined from experimental stress–strain relationships of swollen gels. The E and sB and values considerably increase with nC: up to 6–8 times for E and almost by an order for tensile strength sB. More concentrated water solutions provide almost two times greater E and sB values. Strength-deformation characteristics for gels prepared at nC = 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 MC 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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