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(329b) Helium and Hydrogen Sorption, Diffusion, and Transport in Hydrocarbon, Silicon, and Fluorocarbon-Based Polymers

Authors: 
Freeman, B. D., The University of Texas at Austin
Smith, Z. P., The University of Texas at Austin
Dose, M., The University of Texas at Austin
Gleason, K. L., The University of Texas at Austin

The permeability-selectivity upper bounds show that perfluoropolymers have uniquely different separation characteristics than hydrocarbon-based polymers.  For separating helium from hydrogen, these differences are particularly large.  At a given helium permeability, the upper bound defined by perfluoropolymers has helium/hydrogen selectivities that are 2.5 times higher than that of the upper bound defined by hydrocarbon-based polymers.  Robeson hypothesized that these differences in transport properties resulted from the unusual sorption relationships of gases in perfluoropolymers compared to hydrocarbon-based polymers, and this paper seeks to test this hypothesis experimentally.  To do so, the gas permeability, sorption, and diffusion coefficients were determined at 35°C for hydrogen and helium in a series of hydrocarbon-, silicon-, and fluorocarbon-based polymers.  Highly or completely fluorinated polymers have separation characteristics above the upper bound for helium/hydrogen separation because they maintain good diffusivity selectivities for helium over hydrogen and they have helium/hydrogen sorption selectivities much closer to unity than those of hydrocarbon-based samples.  The silicon-based polymer had intermediate sorption selectivities between those of hydrocarbon-based polymers and perfluoropolymers.  Comparisons of hydrogen and helium sorption data in the literature more broadly extend the conclusion that helium/hydrogen sorption selectivity is rather different in hydrocarbon and fluorocarbon-based media.