Abstract
The morphologies of polymeric integrally skinned asymmetric gas‐separation membranes are typically visualized as a thin gas‐selective skin region supported by a nonselective, low‐resistance porous structure. The validity of this visualization for defect‐free and defective membranes was investigated via scanning electron microscopy (SEM) in combination with traditional gas‐permeation measurements for previously reported ultrathin defect‐free hollow‐fiber membranes. Hollow‐fiber membranes were formed via a dry‐jet, wet‐quench process with a spinning solution composed of Matrimid® polyimide and components of varying volatility. For all the defect‐free membranes formed, SEM images revealed a prominent dense skin layer across the fibers' entire outer circumference. Skin‐thickness estimates from 36 SEM images and N~2~, O~2~, and He pure gas‐permeation measurements agreed to within 9, 7, and 24% for defect‐free membranes with 150‐, 300‐, and 600‐nm skin thicknesses, respectively. Defective membranes with O~2~/N~2~ gas selectivities of Knudsen values, nominally 50 and 80% of dense film values, exhibited a well‐developed nodular, partially developed nodular, and uniform dense layer (no nodules) skin morphologies, respectively. These results provide in‐depth experimental evidence for the existence of a distinct dense integral‐skin layer and suggest that the presence of nodules in the membrane's final skin morphology correlate to gas‐permeation defects. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 399–411, 2003