OPTICAL POLARIZING STUDIES OF CELLULOSE-ACETATE MEMBRANES PREPARED BYPHASE-INVERSION

Anisotropic spherical cells dispersed in an isotropic medium were obse rved, using polarizing microscopy, in asymmetric cellulose acetate (CA ) membranes. Membranes CA-400 and CA-316 were prepared from ternary ca sting solutions of CA-acetone-formamide and CA-acetone-magnesium perch lorate/water respectively. The content of nonsolvent, formamide, was v aried to yield membranes CA-400-22, CA-400-27 and CA-400-32 that show a decreasing number of larger anisotropic cells with the increase of n onsolvent content. The membranes CA-316 were subjected to a heat treat ment-annealing-at varying temperatures yielding membranes CA-316-50, C A-316-68 and CA-316-86. The crystallinity observed was dependent on th e temperature of this heat post-treatment. The selective permeation pr operties are correlated with the ratio of isotropic to anisotropic pha ses at the membrane skin surface. This ratio is in turn dependent on t he casting solution composition-type and concentration of nonsolvent a nd on the annealing treatment. The results were interpreted considerin g that a lyotropic mesophase develops during the evaporation of the so lvent after spreading the solution for film preparation. The ternary s ystem is initially isotropic but as the polymer concentration (C) incr eases due to solvent evaporation, anisotropic droplets may appear at s ome value of C approximate to C, where C* is the critical concentrati on for the isotropic to liquid crystal phase transition in the CA/acet one (binary) system.