Study of cryostructuration of polymer systems. XVII. Poly(vinyl alcohol) cryogels: Dynamics of the cryotropic gel formation

Freeze-thaw treatment of concentrated (>5 g/dL) aqueous solutions of polyvinyl alcohol) (PVA) (MW 115,000; DD ≃ 100%) resulted in the formation of opaque gels. The extent of such a cryostructuration process was exhibited in the rheological properties of similar PVA cryogels. The gels' strength depended on the initial polymer concentration in the solution to be frozen and on the conditions of a cryogenic influence. The key factor was the defrostation dynamics: the slower the thawing rate, the stronger the cryogel sample formed, provided other parameters of the process were identical. The observation for the kinetics of the freeze-thaw-induced gel formation revealed the extreme character of the temperature dependence of the efficacy of PVA cryotropic gelation, the maximum point being in the vicinity of -2°C. It was shown that the effect of the strengthening of PVA cryogels prepared by means of a single-cycle cryogenic treatment could be reached either with use of as slow as possible thawing regimes, or by the prolonged frozen storage of the samples at high subzero temperatures.