Kinetics of sorption and permeation of water in glassy polyimide

A vapor permeation experiment for water–ethanol mixtures was carried out using asymmetric Ube polyimide hollow-fiber membranes, which exhibit high selective permeability for water vapor, under the conditions of T=413 K, upstream gas pressure P_h=1.5×10⁵∼2.95×10⁵ Pa and downstream gas pressure P_l=400 Pa. To represent gas separation properties of the Ube polyimide membrane with a high transition temperature (570 K), the contribution of Henry's law part and Langmuir part modes on the diffusion through the membrane is studied on the basis of the dual-mode transport models. The results show that Henry's law penetrant controls the diffusion in the membrane. For the separation of water–ethanol mixtures by permeation through Ube polyimide membranes, the water trapped in microcavities can be assumed to be totally immobilized under the operating conditions applied here.