59th Annual Report on Research 2014

The project, as defined by the proposal was concerned with the synthetic development of new polymeric materials and membrane structures aimed at the efficient separation of CO₂ from flue and other light gas mixtures. Our current dependence on fossil fuel-based power plants translates to billions of tons of CO₂ emitted into the atmosphere each year which suggests significant clean energy research expenditures must remain focused on CO₂ capture and removal technologies. Room temperature ionic liquids (RTILs) have emerged as promising candidates in this regard, with combinations of CO₂ permeability and CO₂/N₂ selectivity that consistently concentrate near, and often surpass, the historical upper bound for polymeric membranes.¹ However, industrially practical membrane configurations have yet to be realized,^2-5 due to an inability to translate the exceptional permeabilities and selectivities of liquid systems into robust membrane configurations suitable for industrial-scale processing of large gas volumes. Ultimately, the challenge lies in the formation of mechanically robust thin films (1-10 mm) that retain liquid-like transport behavior. Overcoming this obstacle was the motivating objective of the work supported by this grant.

Towards this end, the research activities have been focused on the synthesis of structured block copolymer-based RTIL composite membranes based on a tethered-micelle network paradigm.⁶ In the first year, most of the work was concentrated on the synthesis of poly(RTIL) block copolymer target molecules, and characterizing their thermodynamic phase behavior. The results of these initial first year studies performed in collaboration with the Gin and Noble groups from CU Boulder have resulted in three publications,^7-9 two supported by this work.

Importantly, the ACS PRF funding allowed our group to begin investigating these novel systems and work with these two of the leading experts in RTIL use for light gas separations. The collaboration permitted Vincent Scalfani, in his last year of his Ph.D work, to integrate collaboratively with students and post-docs from the Gin and Noble groups at CU Boulder prior to his successful defense. He now is an Assistant Professor at the University of Alabama.

Year 2 of the project changed its focus towards the ultimate formation of the tethered micelle membrane assemblies proposed. This tethered micelle network is based on the combination of AB diblock and ABA triblock copolymers as shown in the pictorial representation below.

Dilanji Wijayasekara was the primary student responsible for the project in its second year. The funding from the ACS PRF grant carried up to her penultimate year of her Ph.D. She will graduate in spring of 2015. She focused on membranes in which the A block was glassy polystyrene, physically crosslinking the system, and the ionic liquid soluble B block was selected to be poly(ethylene oxide). The initial results proved to be quite impressive, and have established the viability of our approach towards highly effective, mechanically sound membrane systems based on the structure above. Examples of the unswollen and swollen (with emimTf2N) self-standing membranes are shown below.

By changing the amount of ABA triblock copolymer (SOS) in the blends used to form the membranes, we could modulate the amount of RTIL uptake. While the mass percent is not very different, the swelling ratios (volume change) are different between these samples.

The exceptional quality of the gas separation performance is best captured on a Robeson plot, which includes the inverse relationship between selectivity and permeability for known materials. Operation near or beyond the boundary is indicative of outstanding performance.

However, possessing an excellent balance of both permeability and selectivity fails to capture the commercial or industrial use potential, as it gives no information with regard to the mechanical properties of the materials depicted in that plot. In general, many of the new materials cannot be easily integrated into existing membrane module technologies. The membranes developed here, in contrast, have exceptional mechanical properties.

The figures above give both the stress-strain behavior of these thin membrane films under both compression and tension. The noticable ability to reach strains over three times their length without hysteresis highlights their elastic nature. These results are now being prepared for the Journal of Membrane Science, and are supporting a new proposal in review with the National Science Foundation. We credit the ACS New Directions grant Program with enabling us to establish the promise of these tethered micelle networks in the gas separation arena.

1. Camper, D.; Bara, J.; Koval, C.; Noble, R., "Bulk-Fluid Solubility and Membrane Feasibility of Rmim-Based Room-Temperature Ionic Liquids", Industrial & Engineering Chemistry Research 2006, 45 (18), 6279-6283.

2. Bara, J. E.; Camper, D. E.; Gin, D. L.; Noble, R. D., "Room-Temperature Ionic Liquids and Composite Materials: Platform Technologies for CO2 Capture", Accounts of Chemical Research 2009, 43 (1), 152-159.

3. Bara, J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin, D. L.; Noble, R. D., "Guide to CO2 Separations in Imidazolium-Based Room-Temperature Ionic Liquids", Industrial & Engineering Chemistry Research 2009, 48 (6), 2739-2751.

4. Hasib-ur-Rahman, M.; Saij, M.; Larachi, F., "Ionic Liquids for CO2 Capture - Development and Progress", Chemical Engineering and Processing 2010, 49, 313-322.

5. Jones, C. W., "CO2 Capture from Dilute Gases as a Component of Moder Global Carbon Management", Annu. Rev. Chem. Biomol. Eng. 2011, 2, 2.1-2.22.

6. Guo, C.; Bailey, T. S., "Highly Distensible Nanostructured Elastic Hydrogels from AB Diblock and ABA Triblock Copolymer Melt Blends", Soft Matter 2010, 6, 4807-4818.

7. Scalfani, V. F.; Wiesenauer, E. F.; Ekblad, J. R.; Edwards, J. P.; Gin, D. L.; Bailey, T. S., "Morphological Phase Behavior of Poly(RTIL)-Containing Diblock Copolymer Melts", Macromolecules 2012, 45 (10), 4262-4276.

8. Wiesenauer, E. F.; Edwards, J. P.; Scalfani, V. F.; Bailey, T. S.; Gin, D. L., "Synthesis and Ordered Phase Separation of Imidazolium-Based Alkyl-Ionic Diblock Copolymers Made via ROMP", Macromolecules 2011, 44 (13), 5075-5078.

9. Wiesenauer, E. F.; Nguyen, P. T.; Newell, B. S.; Bailey, T. S.; Noble, R. D.; Gin, D. L., "Imidazolium-containing, hydrophobic-ionic-hydrophilic ABC triblock copolymers: synthesis, ordered phase-separation, and supported membrane fabrication", Soft Matter 2013, 9 (33), 7923-7927.