DIFFUSION IN GAS SEPARATION MEMBRANE MATERIALS - A COMPARISON AND ANALYSIS OF EXPERIMENTAL CHARACTERIZATION TECHNIQUES

Typically, materials with high-performance transport properties such a s zeolites, carbon molecular sieves, or hyper rigid polymers are inher ently difficult or impossible to characterize by steady-state membrane permeation experiments used for conventional polymers. Diffusion coef ficients determined by transient sorption, a measurement easily perfor med on brittle media, are analyzed here and compared to those determin ed by steady-state permeation/sorption and transient permeation for a glassy polymer and a carbon molecular sieve. Average and local diffusi on coefficients are extrapolated to zero upstream partial pressure to eliminate effects caused by concentration dependence. Good agreement b etween the techniques was observed for the glassy polymer. On the othe r hand, carbon molecular sieves, possessing a more complex morphology, exhibit a greater difference in diffusion coefficients determined by the various techniques. Nevertheless, comparison of the analysis techn iques is shown to provide potentially valuable insights into the morph ological features of such carbon molecular sieves. (C) 1998 John Wiley & Sons, Inc.