Characterization of water vapor permeable membranes

Air permeability and water vapor permeability of naphion membranes were measured using specially built instruments. Air permeability was almost zero. Water vapor permeability was zero during an incubation period. After the incubation period, the permeability became large and then gradually decreased with time. The behavior has been attributed to the contributions of chemical and mechanical forces to the net flux through the membrane. A model consistent with these results has been presented to explain absorption and transport of water vapor through naphion membranes.

Instruments capable of accurately measuring very small gas permeability and water vapor permeability have been described and used to investigate naphion membranes. Naphion membranes show negligible air permeability. The permeability of water vapor is appreciable after an initial incubation period in which the permeability is zero. The large permeability observed after the incubation period gradually decreases with time. It is proposed that the vapor maintains an equilibrium solute concentration on one side of the membrane. The solute gradually migrates to the other side. The incubation period is observed until the concentration on the far side of the membrane reaches a non-zero value. Beyond the incubation period, the flux of solutes has contributions from the chemical force due to chemical interaction of water with the membrane and the mechanical force due to the pressure gradient. The chemical permeability is large at the beginning, but small at the end. On the other hand, mechanical permeability is smaller at the beginning.