Fundamental Structure/Property Studies of Gas Separation Membrane Polymers

Work in 2012-2013 focused on studying gas permeability, diffusivity and solubility of polyimides and thermally rearranged (TR) polymers based on 3,3’-dihydroxy-4,4’diaminobiphenyl (HAB) and 2,2’-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). To determine the effect of temperature on gas sorption, we studied H2 sorption in the HAB-6FDA polyimide and 3 partially converted HAB-6FDA TR polymers. Increasing conversion increased gas solubility by approximately a factor of 2.6, which is similar to that of other nonpolar gases. Furthermore, in comparing H2 sorption in HAB-6FDA with that of other polymers commonly considered for membrane applications, the HAB-6FDA TR polymers had the highest H2 sorption. Similar studies are under way with other gases relevant for gas separations applications. We also investigated the effect of the size of the reactive leaving group and synthesis route on transport properties in TR polymers. In general, bulkier reactive leaving groups contribute to increased gas permeability in TR polymers compared to TR polymers synthesized from hydroxyl-functional leaving groups. Furthermore, we investigated samples with identical leaving groups but synthesized from two different routes. These tests showed that the leaving group on the polyimide has more of an effect on gas transport than that of synthesis route. We are also measuring mixed gas performance of these polymers and have found, in many cases, situations where CO2/CH4 mixed gas selectivity is actually higher than the selectivity calculated based on pure gas permeability values. The fundamental basis for this observation is under investigation. Fundamental Structure/Property Studies of Gas Separation Membrane Polymers Freeman – University of Texas at Austin Hydrogen Fuel Cells 2 DOE Hydrogen and Fuel Cells Program 2013 Annual Merit Review and Peer Evaluation Meeting To broaden the framework of this study, we investigated H2 sorption in other polymers commonly studied for membrane applications (c.f., Figure 1B). Matrimid, which is of the same family of polymers as HAB-6FDA (i.e., polyimides), had nearly identical gas sorption to that of the HAB-6FDA polyimide. Amorphous Teflon (AF 2400) had H2 sorption between that of the TR 350 60min and TR 400 60min sample. Polysulfone had the lowest H2 sorption, and poly(dimethylsiloxane) (PDMS), which was the only rubbery polymer considered, had a slightly endothermic enthalpy of sorption, as indicated by the negative correlation between gas solubility and 1000/T. From these results, accurate diffusion coefficients for H2 sorption in these polymers can be determined. As we have shown for other gases in TR polymers, increases in gas permeability from thermal rearrangement for H2 are largely a result of increases in gas diffusivity. Effect of Reactive Leaving and Synthesis Route Group on Transport Properties Thermal rearrangement occurs at elevated temperatures for precursor polyimides with reactive ortho-position functional groups. We have previously reported that there is an effect of precursor synthesis route and/or the reactive leaving group on TR polymer transport properties. To decouple these two variables, we pursued a unique organic synthesis approach to synthesize TR polymer precursors from identical synthesis routes but with different reactive Progress Report Hydrogen Sorption Polymers for Membrane Applications Hydrogen separation is used in many industrially significant gas separation applications such as oxo-chemical synthesis, refinery off-gas purification, and syngas ratio adjustment. However, because H2 has a small kinetic diameter (2.89Å) and one of the lowest critical temperatures, Tc (33K), the effect of polymer structure on hydrogen diffusion and solubility is difficult to determine because solubility is so low. Challenges in determining these fundamental gas transport parameters have hindered the polymer membrane field from advancing our understanding of polymer structure on hydrogen transport. To investigate H2 sorption, we used a high precision magnetic suspension balance to determine H2 sorption as a function of temperature in a series of HAB-6FDA TR polymers with increasing conversion. As shown in Figure 1A, gas solubility increased by approximately a factor of 2.6 between the polyimide and most highly converted TR polymer (TR 450 30min), so the relative change in H2 sorption as a function of thermal rearrangement is similar to that of other nonpolar gases such as O2, N2, and CH4. Notably, the increase in H2 sorption does not track with increases in gas sorption for quadrupolar gases, such as CO2, whose solubility increases by a factor of 1.7 between the HAB6FDA polyimide and TR 450 30min sample. Figure 1. Hydrogen solubility in (A) HAB-6FDA as a function of thermal rearrangement, and (B) additional polymers frequently used for membrane applications. In Figure A, Polyimide stands for the HAB-6FDA polyimide, and partially converted TR polymers are represented by the temperature and time at which they have been rearranged. For example, TR 450 30min indicates a sample of HAB-6FDA that has been converted at 450°C for 30min. For Figure B, AF 2400 is amorphous Teflon AF 2400 and PDMS is poly(dimethylsiloxane). A 0.04 0.06 0.08 0.1 0.3 0.5 2.6 2.8 3 3.2 3.4 3.6 3.8 4 S (c m 3 ( ST P) /c m 3 ( po ly m er ) a tm ) 1000/T (K) TR 450 30min TR 400 60min