Optical Polarizing Studies of Cellulose Acetate Membranes Prepared by Phase-Inversion

Abstract

Anisotropic spherical cells dispersed in an isotropic medium were observed, using polarizing microscopy, in asymmetric cellulose acetate (CA) membranes. Membranes CA-400 and CA-316 were prepared from ternary casting solutions of CA-acetone-formamide and CA-acetone-magnesium perchlorate/water respectively. The content of nonsolvent, formamide, was varied 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 nonsolvent content. The membranes CA-316 were subjected to a heat treatment—annealing—at varying temperatures yielding membranes CA-316-50, CA-316-68 and CA-316-86. The crystallinity observed was dependent on the temperature of this heat post-treatment. The selective permeation properties are correlated with the ratio of isotropic to anisotropic phases at the membrane skin surface. This ratio is in turn dependent on the casting solution composition-type and concentration of nonsolvent and on the annealing treatment. The results were interpreted considering that a lyotropic mesophase develops during the evaporation of the solvent after spreading the solution for film preparation. The ternary system is initially isotropic but as the polymer concentration (C) increases due to solvent evaporation, anisotropic droplets may appear at some value of C ≈ C*, where C* is the critical concentration for the isotropic to liquid crystal phase transition in the CA/acetone (binary) system.