(170c) Helium and Hydrogen Diffusion and Sorption in Polymers for Membrane Applications

Polymer films can be used to selectively separate molecules for applications in membrane separations.  For certain gas pairs, perfluoropolymers exhibit unusual separation characteristics compared to hydrocarbon-based polymers.  One particularly relevant example is the separation of helium from hydrogen, where perfluoropolymers have consistently higher combinations of permeability and permselectivity than hydrocarbon-based polymers.  Several authors have suggested that these differences in separation performance are attributed to differences in helium and hydrogen solubility between perfluoropolymers and hydrocarbon-based polymers; however, this hypothesis has been difficult to test due to the low solubility of helium and hydrogen in polymers.  The aim of this study is to critically evaluate this hypothesis by experimentally determining helium and hydrogen sorption using a magnetic suspension balance.  This balance has the ability to determine gas solubility more accurately than currently available methods.

Nine polymers, which cover a broad range of glass transition temperatures, free volume, backbone structures, and permeabilities, were considered in this study, including four hydrocarbon-based polymers, four perfluoropolymers, and poly(dimethylsiloxane).  The hydrocarbon-based polymers include a Matrimid^® polyimide, HAB-6FDA polyimide and corresponding thermally rearranged (TR) polymer, and polysulfone.  The perfluoropolymers include Nafion^® N117, amorphous Teflon AF^® 2400, amorphous Teflon AF^® 1600, and Hyflon^® AD60.  For all of the samples considered, the perfluoropolymers outperformed hydrocarbon-based polymers.  Diffusion coefficients for all of the samples were determined from the solution diffusion model, which defines permeability as the product of diffusivity and solubility.  Besides Nafion^® N117, which showed a high helium/hydrogen diffusivity selectivity of approximately 8.5, there was little variability in diffusivity selectivity between hydrocarbon-based polymers and perfluoropolymers.  Conversely, perfluoropolymers had much higher helium/hydrogen solubility selectivities than the hydrocarbon-based polymers.  In general, the perfluoropolymers had helium/hydrogen solubility selectivities near 0.7, and the hydrocarbon-based polymers had much lower solubility selectivities near 0.3.  Therefore, perfluoropolymers outperform hydrocarbon-based polymers because of their unique helium/hydrogen solubility selectivities.