Influence of Diffusivity and Sorption on Helium and Hydrogen Separations in Hydrocarbon, Silicon, and Fluorocarbon-Based Polymers

• Published in: Macromolecules 2014-05, Vol.47 (9), p.3170-3184
• Main Authors: Smith, Zachary P, Tiwari, Rajkiran R, Dose, Michelle E, Gleason, Kristofer L, Murphy, Thomas M, Sanders, David F, Gunawan, Gabriella, Robeson, Lloyd M, Paul, Donald R, Freeman, Benny D
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Miscellaneous; Organic polymers; Physicochemistry of polymers; Properties and characterization
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma402521h

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 dramatic. 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.