Synthesis and Characterization of Crosslinked Polyimide Membranes for GAS Separation Applications

Synthesis and Characterization of Crosslinked Polyimide Membranes for Gas Separation Applications

Ruilan Guo¹, James E. McGrath², Kyle Gaines², 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

Gas separation using polymer and polymer-based membranes has rapidly grown into an area of technological importance and usefulness, because of the progresses made by synthetic chemists in designing and synthesizing new sophisticated macromolecular structures.  Aromatic polymers, such as polyimides, are robust and thermally stable polymers that have shown excellent promise as gas separation membranes.  Unfortunately, most of the polymeric membranes suffer from plasticization at elevated pressures induced by the sorbed penetrant molecules such as CO₂, which significantly spoils the membrane performance in that the permeability of all components in the feed increases and the selectivity decreases dramatically.  Further complicating the use of polymer membranes is that they are limited to relatively low thermo-oxidative stability and poor resistance to solvent and chemical contaminants, as compared with inorganic ceramic membranes.  Covalently crosslinked polyimides have been reported to show promise for the suppression of CO₂-induced plasticization with simultaneously improved thermo-oxidative and mechanical stability and chemical resistance.  Such crosslinked materials may represent a new platform of polymer membrane materials for a broad range of critical separations (e.g., olefin/paraffin separation in petrochemical industry, hydrogen purification, etc.). 

Here we report the synthesis and characterization of a series of thermally curable, phenylethynyl-terminated polyimides derived from 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylidane (DAPI) with various commercial aromatic dianhydrides via the ester-acid solution imidization route.  Thermally curable ethynyl endgroups were introduced using an endcapping agent, i.e., 4-(phenylethynyl)phthalic anhydride (PEPA), or 3-ethynylaniline (PEA).  DAPI, a building monomer of commercial Matrimid^® polyimide, is a bulky aromatic diamine with isomeric structure.  When incorporated into polymer backbone structure, the noncoplanar nature of DAPI disrupts efficient chain packing and thus provides excess free volume in the films rendering high gas permeability in DAPI-based linear polymers.  The high initial permeability of the linear system is expected to be able to mitigate the reduction in the gas permeability after crosslinking.  Telechelic polyimide oligomers were prepared with a wide range of molecular weights (3K-15K) by adjusting monomers feeding ratio and the amount of end-capping agent to provide a control on the crosslink density in the resulting crosslinked films.  The synthesis and characterization of phenylethynyl-endcapped polyimide oligomers with well-controlled molecular weights and the corresponding high molecular weight linear controls will be discussed.  Those telechelic polyimide oligomers were thermally cured under optimized curing conditions (solution concentration, heating rate, curing temperature and curing time) to produce robust, defect free crosslinked polyimide membranes for gas separation applications. The crosslinked membranes were characterized comprehensively in terms of chemical structure, gel fraction and thermal properties.  Preliminary results on the gas transport properties of the resulting crosslinked membranes will be presented. 

References

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