EFFECTS 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-gel*. 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-gel*. In the intermediate concentration region, C-gel < C < C-gel*, which covers three orders of magnitude in concentration, gel and sol phases coexist. Both concentration dependencies of G and and 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. (C) 1995 John Wiley and Sons, Inc.