Gas Solubility in Perfluoropolymers and Hydrocarbon-Based Polymers for Applications in Hydrogen Separation

Polymer membranes are currently used for a number of applications in hydrogen separation such as ammonia purge gas recovery, oxo chemical synthesis, and refinery off-gas purification.  Because of these applications, much research has focused on studying hydrogen permeation in polymers.  However, little is known concerning hydrogen solubility and diffusivity, and the aim of this paper is to address the fundamental contributions of solubility and diffusivity to hydrogen permeability in several families of polymeric materials.  Additionally, the unique sorption characteristics of perfluoropolymers, which are often attributed to perfluoropolymer plasticization resistance, are addressed at a fundamental level by comparing hydrogen/helium separations for perfluoropolymer and hydrocarbon-based polymers.

Using a magnetic suspension balance, hydrogen sorption isotherms between -20°C and 70°C were determined between 0 and 60 bar for Matrimid^®, polysulfone, poly(dimethylsiloxane), Teflon^® AF 2400, and a polyimide and its corresponding thermally rearranged polymers.  At 35°C, the thermally rearranged polymers showed the highest hydrogen sorption, and polysulfone showed the lowest hydrogen sorption.  For the glassy polymers, slight dual-mode curvature was observed and was most pronounced at the lowest temperatures.  Conversely, poly(dimethylsiloxane), a rubbery polymer over the range of temperatures considered, showed no indication of dual-mode curvature.  Enthalpies of sorption were calculated from these isotherms.  Matrimid^® showed the most exothermic enthalpy of sorption and poly(dimethylsiloxane) showed a slightly endothermic enthalpy of sorption.

Helium sorption isotherms at 35°C were determined for the aforementioned samples.  To introduce a broader framework to this study, hydrogen and helium sorption isotherms at 35°C were also determined for additional perfluoropolymers:  Teflon^® AF 1600, Hyflon AD60, and Nafion N117.  For glassy, hydrocarbon-based polymers, H₂/He solubility selectivities were between approximately 3 and 4, whereas the glassy perfluoropolymers had H₂/He solubility selectivities of approximately 1.5.  Diffusivity selectivities were relatively unchanged between the glassy perfluoropolymers and hydrocarbon-based polymers.  Therefore, the unique hydrogen/helium separation properties for perfluoropolymers are a result of their reduced H₂/He solubility selectivity.