Physical characteristics of osmotic membranes of organic polymers

Abstract

AbstractThe mechanism of water transport across cellulosic membranes under high pressure differentials was studied in an attempt to explain the semipermeability of these films when used as osmotic membranes for ionic solutions. Diffusion coefficients and activation entropy values for the diffusion of water and other fluids through various polymer films were determined, and compressive deformation of cellulose acetate and cellophane films was measured. Activation entropy values for the diffusion of water across both cellulose acetate and cellophane films illustrated that water molecules are constrained to states of high order as they are transported across the membrane under high pressure differentials. Activation entropy values for the diffusion of methanol through cellulose acetate film failed to support the proposition that chains of this polymer may actually be crosslinked by hydrogen‐bonded water. It appeared, rather, that but one hydrogen of the water molecule was involved in hydrogen bonding to the polymer. A significant reorientation of the polymer in the pressure interval in which cellulose acetate film is induced into a semipermeable condition was indicated by pressure‐dependent fluctuations of diffusion coefficients and activation entropy values in this interval. The greater elastic modulus of wet cellulose acetate film under a compressive force applied normal to the film surface demonstrated a strong interaction between water and polymer chains in the cellulose acetate‐water structure. The capacity of the film for bound water was also increased by repeated compression while wet. Wet cellophane films deformed plastically under compression. Crosslinking polymer chains of cellophane film with cupric ions prevented plastic flow and, at the same time, increased the semipermeability of the film for sodium chloride solutions.