Permeation of Butane Isomers through 6FDA-DAM Dense Films Omoyemen Esekhile, Wulin Qiu, William J. Koros School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332-0100 Correspondence to: W. J. Koros (E-mail: bill.koros@chbe.gatech.edu) Received 22 June 2011; revised 20 July 2011; accepted 2 August 2011; published online 8 September 2011 DOI: 10.1002/polb.22351 ABSTRACT: Permeation of mixtures of butane isomers through 6FDA-DAM (2,2-bis(3,4-carboxyphenyl) hexafluoropropane dianhydride–diaminomesitylene) dense films is shown to deviate from the dual-mode transport model, even if one accounts for nonideal bulk flow effects. Two hypotheses are proposed to explain the deviations observed. Both hypothe- ses relate to an apparently much slower exchange rate of the bulky i-butane molecule versus n-butane isomer between Henry’s Law and Langmuir sorption sites in the rigid matrix. The first hypothesis suggests a change in the equilibrium isotherm for normal butane, due to the presence of i-butane. A model is considered to account for the change in local equilibrium and to improve the description of experimental data compared to the dual-mode transport model accounting for bulk flow. The second hypothesis considers an effect on the accessible unrelaxed volume associated with the Lang- muir capacity of one penetrant due to the presence of the second penetrant. We introduce an empirical parameter representing this effect for each penetrant in the n/i-butane system. Models based on either hypothesis show some improvement to the dual-mode transport model accounting for bulk flow, in making mixed gas predictions; however, the second model gives a much better fit to experimental data. Further experimentation is required to provide a physical basis for the empirical factor in the second model, thereby allowing generalization of the observations reported here. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1605–1620, 2011 KEYWORDS: free volume; gas permeation; membranes INTRODUCTION Despite its potential importance in energy- efficient separations, limited information exists on the transport behavior of butane isomers through glassy poly- meric membranes. Pure gas studies indicate that a n/i-bu- tane selectivity of 21 with n-butane permeability of 3.7 Bar- rers can be achieved using 6FDA-DAM (2,2-bis (3,4-carboxyphenyl) hexafluoropropane dianhydride–diami- nomesitylene) dense films. 1 These experiments, however, were performed using pure gases on the upstream with vacuum downstream, which may give overly optimistic per- formance expectations. This article extends the prior study to a diverse array of gas mixture compositions to character- ize separation performance of the membrane under more realistic conditions. The dual-mode sorption model is generally used to describe gas sorption in glassy polymers. This model (eq 1) as proposed by Michaels et al. 2 identifies two sorp- tion modes in glassy polymers. The first idealized mode involves sorption into the dissolved phase of the polymer, also known as Henry’s sorption mode. The other idealized mode involves sorption into the limited microvoids, which can be described as Langmuir sorption sites, in a ‘‘hole- filling process’’ for a mixture of components ‘‘i’’ and ‘‘j’’, viz., C i ¼ C Di þ C Hi ¼ k Di p i þ C 0 Hi b i p i 1 þ b i p i þ b j p i ; (1) where C i is the total sorption capacity, C D is the sorption in the Henry’s environment, C H is the sorption in the Langmuir environment, k D is the Henry’s law solubility coefficient, C 0 H is the Langmuir sorption constant, which is a measure of the unrelaxed volume in the polymer, b is an affinity constant, which measures the tendency of molecules to sorb in the Langmuir environment of the polymer, and p i and p j refer to the partial pressure (or fugacity) of components i and j, respectively. The Langmuir sorption constant, C 0 H , can be expressed as a function of the unrelaxed fractional free volume (FFV) of the polymer, 3–6 viz., C 0 H ¼ V g  V l V g   q  ; (2) where V g is the specific volume of the polymer in the glassy state, V l is the specific volume of the polymer in a hypotheti- cal liquid state, and q * is the liquid-like molar density of the penetrant in the Langmuir region. V C 2011 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE PART B: POLYMER PHYSICS 2011, 49, 1605–1620 1605 WWW.POLYMERPHYSICS.ORG FULL PAPER