Effect of solution concentration on the gelation of aqueous polyvinyl alcohol solution

The concentration dependence of cryogenic gelation for aqueous solution of poly(vinyl alcohol) was studied by measuring the apparent gel fraction G and the swelling ratio Q of the gel formed by freezing and thawing. It was found that for the gelation process there were three distinct regions of solution concentration bounded by two concentrations C_gel and C. The gel started to form at C = C_gel, while no visible gel could be detected even upon repeated freezing and thawing of the extremely dilute solutions of C < C_gel. The entire solution was gelatinized as a whole in the high concentration region of C > C. In the intermediate concentration region, C_gel < C < C, which covers three orders of magnitude in concentration, gel and sol phases coexist. Both concentration dependencies of G and Q show two branches jointed at a concentration very close to the overlap concentration C^*. The curve of G?Q versus C shows a sharp cusp. In case the sharp cusp concentration is really the value of C^*, gelation offers a precise method to determine the overlap concentration. ©1995 John Wiley & Sons, Inc.