Membrane separation technology has recently emerged as a potential alternative technique to remove higher hydrocarbons (C₃₊) from natural gas¹. For economic reasons, membranes for this application should be organic vapor selective materials such as poly(dimethylsiloxane) (PDMS) or ultra-high free volume polymers such as poly(1-trimethylsilyl-1-propyne) (PTMSP). These polymers, often called solubility selective polymers, sieve penetrant molecules based strongly on relative penetrant solubility in the polymer. Traditionally, membrane separation performance is estimated from pure gas permeation measurements. However, this approach is not accurate in certain cases, especially in glassy polymers. For example, the presence of CO₂ in cellulose acetate induces membrane plasticization and decreases the CO₂/CH₄ selectivity as the CO₂ partial pressure increases². In some cases, the selectivity of a vapor over a permanent gas in a mixture is actually higher than that estimated from pure gas measurements, which is attributed to the high solubility of vapor in the polymer, which partially blocks the permeation pathway of the smaller, less soluble permanent gas³. To accurately estimate the membrane separation performance, the mixed gas studies are required. This paper presents the n-C₄H₁₀/CH₄ mixed gas permeability, solubility, and diffusivity in vapor selective polymers at various temperatures from -20^oC to 50^oC. The dilation isotherms are also reported to complement the mixture sorption data. In rubbery PDMS, the presence of larger, more condensable n-C₄H₁₀ considerably improves the CH₄ solubility, permeability, and diffusivity in the polymer. On the other hand, the n-C₄H₁₀ sorption and transport properties in mixtures are unaffected by the presence of CH₄ and are similar to those observed under pure gas conditions. The overall n-C₄H₁₀/CH₄ permeability selectivity in mixtures increases as n-C₄H₁₀ activity increases and as temperature decreases, and the selectivity is lower than that estimated from pure gas measurements. The overall selectivity in PDMS is mainly governed by solubility selectivity. This report represents the first combined presentation of gas mixture permeability, solubility, and diffusivity in PDMS, which is the most widely used vapor separation polymer. Another polymer whose properties will be discussed is poly(1-trimethylsilyl-1-propyne) (PTMSP), an ultra-high free volume, solubility selective, glassy polymer. These studies help illustrate the fundamental mechanism of competitive permeation, sorption, and diffusion in glassy polymers. (1)        Baker, R. W. Membrane Technology and Applications; McGraw-Hill: New York, 2000.

(2)        Lee, S. Y.; Minhas, B. S. AICHE Symposium Series 1970, 84, 93-101.

(3)        Pinnau, I.;  Casillas, C. G.;  Morisato, A.; Freeman, B. D. Journal of Polymer Science: Part B: Polymer Physics 1996, 34, 2613-2621.