(356a) Hydrogen Sorption in Polymers

For over 30 years, dense polymer membranes have been used for industrial applications in hydrogen separation, including syngas ratio adjustment, refinery off-gas purification, and ammonia purge gas recovery [1].  These membranes operate based on the solution-diffusion model, where permeability is defined as the product of an effective diffusion coefficient and an effective sorption coefficient.  Determining sorption of H₂, however, is difficult owing to its highly non-condensable nature and, therefore, low sorption in polymer films.  This study seeks to quantify how H₂ sorption changes during conversion of a thermally rearranged (TR) polymer and assess differences between H₂ sorption in TR polymers and in other polymers reported in the membrane literature.

TR polymers are synthesized from reactive polyimide precursors, which contain hydroxyl groups ortho-position to the diamine [2-4].  These polyimide precursors are synthesized through traditional polyimide chemistry [5, 6], cast as films, and converted to TR polymers in the solid state via a thermally induced reaction.  For this study, we focus on a polyimide precursor prepared from 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA).  This polyimide, along with three partially converted HAB-6FDA TR polymers were tested in this study.  The partially converted TR polymers were converted 39%, 60%, and 76% by heating the polyimide precursor under N₂ for different amounts of time and at different temperatures.  To avoid polymer degradation, which occurs at sufficiently high temperatures, samples were not converted above 76%.  Samples were characterized using ¹HNMR, TGA-MS, DSC, FT-IR, and XRD, and sorption was determined using a Rubotherm Magnetic Suspension Balance.

H₂ sorption in the HAB-6FDA polyimide is similar to that of Matrimid™, which is a polyimide used industrially for gas separations.  Interestingly, thermal rearrangement induces significant changes in sorption capacity for HAB-6FDA.  Compared to the polyimide, H₂ sorption increases by a factor of approximately 2.6 for the 76% converted HAB-6FDA TR polymer.  This dramatic increase in sorption (i.e., a factor of 2.6) is largely driven by non-equilibrium free volume in the polymer and matches the increase in sorption observed for N₂, O₂, and CH₄ in the same material.  Surprisingly, over the same extent of conversion, CO₂ sorption only increases by a factor of approximately 1.7.  This difference is likely attributed to a quadrupolar moment that exists in CO₂ but not in H₂, N₂, O₂, and CH₄ [7].  This quadrupolar moment would encourage favorable interactions with polymer carbonyl groups [8] that are found in the HAB-6FDA polyimide, but are lost during the thermal rearrangement process.  When compared to Matrimid™, polysulfone, AF2400, and the other partially converted TR polymers, H₂ sorption was highest in the 76% converted HAB-6FDA TR polymer. 

The enthalpy of sorption was also determined as a function of conversion for HAB-6FDA.  The HAB-6FDA polyimide had a similar enthalpy of sorption to Matrimid™.  However, the enthalpy of sorption decreased in magnitude as the polyimide was converted to its TR polymer.  This change in the enthalpy of sorption likely results from increasing free volume within the polymer film [9].  For the 76% converted HAB-6FDA TR polymer, the enthalpy of sorption decreased in magnitude to values similar to AF2400, which is another high free volume polymer.

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2.         Park, H.B., C.H. Jung, Y.M. Lee, A.J. Hill, S.J. Pas, S.T. Mudie, E. Van Wagner, B.D. Freeman, and D.J. Cookson, Polymers with cavities tuned for fast selective transport of small molecules and ions. Science, 2007. 318(5848), 254-258.

3.         Tullos, G. and L. Mathias, Unexpected thermal conversion of hydroxy-containing polyimides to polybenzoxazoles. Polymer, 1999. 40(12), 3463-3468.

4.         Tullos, G.L., J.M. Powers, S.J. Jeskey, and L.J. Mathias, Thermal conversion of hydroxy-containing imides to benzoxazoles: Polymer and model compound study. Macromolecules, 1999. 32(11), 3598-3612.

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6.         Ohya, H., V.V. Kudryavtsev, and S.I. Semenova, Polyimide membranes: Applications, fabrications, and properties. 1996, Amsterdam: Gordon and Breach Publishers.

7.         Lin, H., E. Van Wagner, J. Swinnea, B.D. Freeman, S.J. Pas, A.J. Hill, S. Kalakkunnath, and D. Kalika, Transport and structural characteristics of crosslinked poly(ethylene oxide) rubbers. Journal of Membrane Science, 2006. 276(1-2), 145-161.

8.         Koros, W.J., Simplified analysis of gas/polymer selective solubility behavior. Journal of Polymer Science: Polymer Physics Edition, 1985. 23(8), 1611-1628.

9.         Broom, D.P., Hydrogen storage materials. 2011, London: Springer.