Elastic free-standing RTIL composite membranes for CO2/N2 separation based on sphere-forming triblock/diblock copolymer blends
Date
2016
Authors
Wijayasekara, Dilanji B., author
Bailey, Travis S., advisor
Fisk, John D., committee member
Kipper, Matthew, committee member
James, Susan, committee member
Journal Title
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Volume Title
Abstract
The main focus of this dissertation was the development of a robust polymeric membrane material for separating CO2 from a gas mixture of CO2 and N2. Flu gas, which is mainly a mixture CO2 and N2, is the single largest form of anthropogenic CO2 emissions to the atmosphere. Capturing CO2 from flu gas is considered as a measure of controlling anthropogenic CO2 emissions. Existing CO2 capturing technologies for flu gas suffer from low efficiency and the low cost effectiveness. Adoption of membrane technology is comparatively the best route towards the economical separations. Challenges faced by existing CO2 separation membrane materials are the lack of high mechanical robustness and the processability required for fabrication of membrane units while maximizing their gas separation properties. We were able to form a novel membrane material that addresses each of these challenges. These novel membranes are based on highly swollen, self-standing films produced using sphere-forming PS-PEO diblock and PS-PEO-PS triblock copolymer blends. The intricate connectivity among spherical domains produced during melt-state assembly (prior to swelling), provides a framework that remains elastically tough even in the presence of large quantities of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTf2N) - a room temperature ionic liquid (RTIL) that has high selectivity for CO2 over N2. Further investigations on improving the robustness of these membranes and the gas separation properties were carried out based on two scenarios. First, potential of improving the thermal stability of these membranes by replacing the thermoplastic polystyrene with a thermoset moiety such as a chemically cross-linked polyisoprene (PI) was researched. Cross-linking chemistry utilized required a post-polymerization modification of PI and it was found that this oxidation modification of olefins on PI caused the decoupling of triblock copolymer in the blend and also substantially hindered melt-state self assembly. The membranes formed with this modification turned out to have inferior mechanical properties compared to the polystyrene based ones, most likely due to the above mentioned complications. Due to the time restrictions, this study was limited to just the identification of the existing challenges in the proposed strategy. Recommendations for addressing the challenges identified are also presented later in the dissertation. The second scenario for improving the performance of these membranes was to increase their productivity by improving both the CO2 permeability and maximizing the trans-membrane pressure differentials possible during operation. To accomplish this we focused on the development of an alternative matrix material (alternative for PEO) enriched with ionic groups. The goal was to increase matrix solubility in the RTIL (improved CO2 permeability) while simultaneously strengthening matrix-RTIL interactions for reduced leaching under higher pressure differentials. Synthetic routes to achieve this task involved a sequential polymerization of isoprene and ethoxy ethyl glycidyl ether (EEGE) monomers. Polymerization of EEGE to yield high molecular weight linear blocks proved to be extremely challenging due to the undesirable chain transfer reaction tendency of EEGE monomer. A great deal of research effort was spent characterizing various anionic reaction conditions and developing measures aimed at suppressing chain transfer. While ultimately unsuccessful, the results of these studies provide significant insight into the challenges of forming high molecular weight linear polyglycidols and will hopefully provide inspiration for the development of future synthetically successful strategies. A series of proof of concept experiments for transforming alcohol functionalities on this polymer system to imidazolium was also completed successfully. The dissertation concludes with a final project completed outside the main objective of the dissertation - a morphological characterization of a series of thermoplastic elastomers with unique molecular architectures. This work is reported separately in the appendix I.