EFFECTS OF SOLUTION CONCENTRATION ON THE GELATION OF AQUEOUS POLYVINYL-ALCOHOL SOLUTION

The concentration dependence of cryogenic gelation for aqueous solutio n of poly(vinyl alcohol) was studied by measuring the apparent gel fra ction G and the swelling ratio Q of the gel formed by freezing and tha wing. It was found that for the gelation process there were three dist inct regions of solution concentration bounded by two concentrations C -gel and C-gel. The gel started to form at C = C-gel, while no visibl e gel could be detected even upon repeated freezing and thawing of the extremely dilute solutions of C < C-gel. The entire solution was gela tinized as a whole in the high concentration region of C > C-gel. In the intermediate concentration region, C-gel < C < C-gel, which cover s three orders of magnitude in concentration, gel and sol phases coexi st. Both concentration dependencies of G and and show two branches joi nted at a concentration very close to the overlap concentration C. Th e curve of G . Q versus C shows a sharp cusp. In case the sharp cusp c oncentration is really the value of C, gelation offers a precise meth od to determine the overlap concentration. (C) 1995 John Wiley and Son s, Inc.