Study of cryostructuration of polymer systems. XIV. Poly(vinyl alcohol) cryogels: Apparent yield of the freeze-thaw-induced gelation of concentrated aqueous solutions of the polymer

Poly(vinyl alcohol) (PVA) cryogels, which are formed as a result of freeze- thaw treatment of concentrated solutions of the polymer, were studied in re spect to the amount of gel and sol fractions in these heterogeneous macropo rous gel materials depending on the conditions of the thawing step of simil ar cryotropic gelation. It was shown that the yield of gel fraction (the ef ficiency of the gelation process) was not quantitative; this was controlled by the initial PVA concentration in the solution to be frozen, and to a hi gher extent, by the thawing rate, when the yield increased with slowing of the defrostation process. The sol fraction could be extracted from the PVA cryogels by their rinsing with water at room temperature; the extraction of the sol was accompanied with the variations of the swelling parameters of the gels (initial slight upswelling and subsequent gradual deswelling), as well as with analogous, in their character, variations of the gel strength. It was also demonstrated that at the evaluation of the fusion enthalpies o f PVA cryogels with the aid of the Eldridge-Ferry equation a consideration of the values of gel-fraction yield gave rise to the significantly higher D elta H values than in traditional cases commonly used for the thermoreversi ble gels, where such an yield was not taken into account. (C) 2000 John Wil ey & Sons, Inc.