GAS Separation Membranes Based On Thermally Rearranged (TR) Polymers: Structure-Property Studies

Gas Separation Membranes Based on Thermally Rearranged (TR) Polymers: Structure-Property Studies

Ruilan Guo¹, James E. McGrath², David Sanders³, Zachary Smith³, Benny D. Freeman³

¹Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA

²Macromolecules and Interfaces Institute, Department of Chemistry, Virginia Tech, Blacksburg, VA 24061, USA

³Department of Chemical Engineering, The University of Texas, Austin, TX 78758, USA

Membranes have become a competitive option to conventional gas separation technologies (e.g., cryogenic distillation, adsorption on solid supports and amine “scrubbing”) with substantial potential for further growth. There have been widespread gas separation applications utilizing polymeric membranes, such as air separation, removal of CO₂from natural gas, and syngas ratio adjustment in petrochemical processes, etc.  Membrane mediated gas separation is a highly interdisciplinary field requiring new chemistries to prepare novel macromolecular structures, and complex characterizations including the physics and transport behavior of sub-micron thin films to relate microscopic molecular structure to macroscopic membrane properties such as permeability and selectivity.  The most promising macromolecules for high performance gas separations are considered to be those with rigid backbone structures having high free volume fraction with narrow size distribution. 

Recent developments with so-called thermally rearranged (TR) polymeric membranes offer major potential for advancements in the flux or permeation of the gases, the separation efficiency and resistance to CO₂-induced plasticization.  In general, TR polymers are thermally derived from aromatic polyimide precursors containing ortho-hydroxy groups via molecular rearrangement, i.e., imide-to-benzoxazole conversion.  We have demonstrated that the efficiency and feasibility of this thermal conversion strongly depend on the chemical structure of the polyimide precursors (monomer combinations, ortho-functionality, etc.), which, in turn, strongly affect the transport properties of the resulting TR films.  This talk explores some of the experimental efforts that have looked at the synergistic effects of molecular design and synthetic variables as well as membrane fabrication on the transport properties of TR polymer membranes.  In particular, we report herein an efficient and versatile synthetic route to prepare a series of aromatic polyimides varying in ortho-functional groups via chemical derivatization.  The derivatized polyimides have been used as new precursors for thermal rearrangement process to afford high performance TR polymer gas separation membranes.  A detailed discussion on the effects of ortho-functional groups (in terms of size and chemical nature) of the polyimide precursors on the feasibility and efficiency of TR conversion and the transport properties of the resulting TR membranes will be presented in this talk based on our systematic studies on the design, synthesis, characterization and performance verification of a series of functional polyimide precursors and their corresponding TR polymer membranes. A structure-property correlation established in these new TR materials is expected to provide practical guidance in the design of a new generation of TR polymer membranes with tailored gas transport properties and enable better prediction of the membrane properties. 

References

1. Baker, R.W. “Natural Gas Processing with Membranes: An Overview.” Ind. Eng. Chem. Res. 2008, 47, 2109-2121.
2. Park, H.B. et al. Polymers with cavities tuned for fast selective transport of small molecules and ions. Science, 2007, 318, 254-258.
3. Sanders, D.; Smith, Z.; Ribeiro, C.; Guo, R.; McGrath, J.E.; Paul. D.R.; Freeman, B.D. Permeability, diffusivity, and free volume of thermally rearranged polymers based on 3,3'-dihydroxy-4,4'-diamino-biphenyl diamine (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), J. Membrane Science, 2012, accepted.
4. Smith, Z.; Sanders, D.; Ribeiro, C.; Guo, R.; Freeman, B.D.; Paul. D.R.; McGrath, J.E.; Swinnea, S. Gas Sorption and Characterization of Thermally Rearranged Polyimides based on 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) Hexafluoropropane Dianhydride (6FDA), J. Membrane Science, 2012, accepted.