THE RESISTANCE TOWARDS GAS-TRANSPORT OF THE SUBLAYER OF ASYMMETRIC PPO HOLLOW FIBER MEMBRANES DETERMINED BY PLASMA-ETCHING

Plasma-etching is a powerful tool in the determination of the resistance of the sublayer of skinned asymmetric hollow fiber membranes of poly(2,6-dimethyl-1, 4-phenylene oxide). Using a resistance model and data provided by etched hollow PPO fiber membranes it is possible to predict the ultimate selectivity of the total membrane for helium over nitrogen and oxygen over nitrogen. From this model and from permeability measurements it is shown that the gas resistance of the sublayer of asymmetric membranes with a selective layer has a large effect on the ultimate overall selectivity of the membranes. In order to improve the membrane performance the resistance of the sublayer has to be minimized. Etching experiments were performed on follow fiber membranes spun from PPO-types with different intrinsic viscosities. The resistance of the sublayer decreased markedly with increasing intrinsic viscosity of the PPO. To a lesser extent the membrane preparation (the length of the air gap) influences the gas resistance of the sublayer.