Characterization of cryogenically slightly crosslinked biomedical poly(vinyl alcohol) gels

Abstract

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 degrees C) followed by thawing (12 h at 20 degrees 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 M-C (using the Flory-Rehner approach) were calculated. Values of alpha and M-C considerably decrease with the growth of the number of freezing/thawing cycles n(C). The modulus of elasticity E, tensile strength sigma(B), and elongation at brake epsilon(B) were determined from experimental stress-strain relationships of swollen gels. The E and sigma(B) and values considerably increase with n(C): up to 6-8 times for E and almost by an order for tensile strength sigma(B). More concentrated water solutions provide almost two times greater E and sigma(B) values. Strength-deformation 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 degrees C) and water sorption (at 25 degrees 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.