The structure of the effective separating layer of asymmetric hollow fiber membranes with graded density skins

The structure of the effective separating layer of an asymmetric polysulfone hollow fiber membrane with a graded density skin was studied by SEM, TEM, and osmium tetroxide vapor adsorption and compared to that of a bilayer asymmetric hollow fiber membrane prepared from a solvent/nonsolvent mixture. The studies revealed the presence of micropores which transverse the effective separating layer of the membrane with graded density skin. The structural units which constituted the effective separating layer of this membrane are less tightly packed and distorted than are the entities forming the effective separating layer of its congener prepared from the traditional solvent/nonsolvent mixture. These results indicate that the enhanced free volume in an asymmetric hollow fiber membrane with a graded density skin exists in the effective separating layer as well as in the membrane interior.