Mechanism and Molecular Interactions in the Transport of Water through Membranes

摘要

Changes in waveguide properties of several cellulose acetate membranes and one polyimide membrane were measured as a function of their exposure to varying levels of relative humidity. The volume fraction of water in the films and the occupied pore volumes were determined from refractive index and thickness changes. The dependence of the refractive index on water absorption is related to a competition between two processes: 'pore filling' with no film expansion and 'free expansion' where the film expands to accommodate the added water volume. The hydration properties of these membranes are affected by degree of acetylation, casting temperatures and annealing treatments. The hydration characteristics of a polyimide membrane are compared to cellulose acetate membranes. Raman measurements of unannealed CA and CTA membranes and annealed CA membranes using the waveguide technique showed that OH groups in unannealed polymers are all hydrogen bonded and are highly accessible to absorbed water.