Assessment of Structure-Solubility Relationship for Copolyimide Gas Separation Membranes Via Molecular Simulation

Aromatic polyimides are a promising group of polymers, due to their outstanding thermal and chemical stability, to be used as polymeric gas separation membranes.  Polyimides are synthesized by the reaction of a dianhydride and a diamine in a solvent and by dehydration of the resultant polyamic acid by imidization. Copolyimides are obtained when more than two monomers are used. Copolyimides allow us to optimize the separation properties by using different dianhydride and diamine combinations.  Hundreds of different copolyimide structures can be proposed, however, the experimental analysis of the separation properties of all these structures is a tedious and time-consuming task. 

In this study, a combination of molecular dynamics (MD) and Monte Carlo (MC) simulation methods was applied to estimate the structural properties (d-spacing, fractional free volume, radius of gyration and glass transition temperature), solubility coefficients, and solubility selectivities of small gases (CO₂, CH₄, O_2, N₂) in different copolyimide membranes, for which no experimental data are available: 6FDA/BTDA-DABA, 6FDA/BTDA-pBAPS, 6FDA-pBAPS/DAM, 6FDA-pBAPS/DABA, and 6FDA-pBAPS/mPDA.  The selection of these copolyimide structures was based on our previous study [1] in which the permeability coefficients of H₂, O₂, He, CO₂, N₂, and CH₄ gases for more than 2200 possible copolyimide structures were estimated by the group contribution method of Alentiev et al. [2].  The predicted permselectivities of the above mentioned copolyimides were located above the polymer upper bound of Robeson [3] for CO₂/CH₄ and O₂/N₂ separation applications.

This methodology was first validated by predicting the membrane properties for two polyimides (6FDA-DAM and 6FDA-ODA) for which experimental data were available in the literature. The estimated properties for 6FDA-DAM and 6FDA-ODA were in excellent agreement with the experimental data, with a maximum deviation of ~5%.  Among the five copolyimides, 6FDA/BTDA-DABA exhibited two distinct d-spacing values in the simulated X-ray patterns, provided the highest CO₂, CH₄, O₂ and N₂ solubility coefficients and lowest CO₂/CH₄ and O₂/N₂ solubility selectivities indicating that fractional free volume of 6FDA/BTDA-DABA is highest. This may be attributed to the BTDA-DABA linkage of this copolyimide. To better understand this finding, BTDA-DABA and 6FDA-DABA polyimides were constructed and their structural properties were estimated. Results indicated that the fractional free volume of BTDA-DABA polyimide is much larger than those of 6FDA-DABA.

Acknowledgment

This work was partially supported by Turkish Scientific and Technological Research Council (Grant No. 106M339) and National Center for High Performance Computing (Grant No. 10932010).

Abbreviations

6FDA: 4,4-hexafluoro isopropylidene diphthalicanhydride; ODA: 4,4-oxydianiline; DABA: 3,5-diamino benzoic acid; BTDA: 3,3-4,4-benzophenone tetracarboxyclic dianhydride; DAM: 2,4,6-trimethyl-m-phenylene diamine.

References

[1]   S. Halitoglu, S.B. Tantekin Ersolmaz, Prediction of gas permeability coefficients of copolyimides by group contribution methods, presented at the NAMS meeting, May 12-16, 2007, Orlando, Florida.

[2]   A. Y. Alentiev, K.A. Loza, Y. Yampolski, J. Memb. Sci. 167, 91-106  (2000).

[3]   L. M. Robeson, J. Memb. Sci., 62, 165-185 (1991).