(643d) CO2 Separation and Plasticization Properties of Copolyimides Containing Sulfone Groups

Polyimide membranes have been widely applied for gas separations due to their attractive permeability, selectivity, and processing characteristics. Their use could be further expanded for CO₂ capture in natural gas purification, coal gasification, and flue gas treatment processes. The principle issue in these applications is the plasticization of the polyimide membranes at high partial pressures of CO₂, which can lead to reduced membrane selectivity and unpredictable membrane properties.

To investigate structure-plasticization relationships, three sulfonated copolyimides were selected: 6FDA/BTDA-pBAPS, 6FDA-pBAPS/DABA, and 6FDA-pBAPS/mPDA. The selection of the 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 well above the upper bound of Robeson diagrams for CO₂/CH₄ and O₂/N₂separation applications. Proceeding, in another previous study of us [3] the copolyimides were synthesized and characteristics of membranes were determined both experimentally and via molecular simulation methods. The physical characteristics of simulated membranes matched well with the experimental ones and also sorption characteristics were reported.

In this work Molecular Dynamic (MD) and Monte Carlo (MC) simulations were carried out to investigate the relationship between structural properties and CO₂-induced plasticization behavior of copolyimides and CO₂/CH₄separation. Polymer Consistent Force Field (PCFF) was used to simulations. Simulated sorption coefficients agreed well with the experimental results obtained from gravimetric sorption (IGA) measurements. Diffusion coefficients of copolyimides were determined using MD simulation runs and permeabilities and perm-selectivities of each copolyimide membrane were evaluated up to 30 bars to investigate the plasticization resistance of each copolyimide.

Among the three copolyimides, 6FDA/BTDA-pBAPS provided the highest CO₂ and CH₄ gas sorption coefficients indicating that fractional free volume of 6FDA/BTDA-pBAPS is highest in the swollen state.  FFV calculations confirm that CO₂ sorption-induced swelling is the highest in 6FDA/BTDA-pBAPS, while the FFV of the three copolyimides were similar before sorption. The sorption-induced increase of the FFV of 6FDA-pBAPS/DABA is the smallest which also agrees with the FAV analysis. This may be due to its rigid backbone indicated by its high Tg. Sulfone group (S) in pBAPS diamine is the prefential sorption site in all three copolyimides, but its affinity to CO₂ is highest in 6FDA/BTDA-pBAPS and lowest in 6FDA-pBAPS/DABA. Certain groups and regions in copolyimides where CO₂affinity is higher is located.

Abbreviations: 6FDA: 4,4-hexafluoro isopropylidene diphthalicanhydride; DABA: 3,5-diamino benzoic acid;  BTDA: 3,3-4,4-benzophenone tetracarboxyclic dianhydride; pBAPS: bis [4-(4-aminophenoxy) phenyl] sulfone; mPDA: 1,3-phenylenediamine.

Acknowledgment: This work was supported by TUBITAK trough grant no. 113M336.

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

2. Alentiev, A. Y.; Loza, K. A.; Yampolski, Y. "Development of the methods for prediction of gas permeation parameters of glassy polymers: polyimides as alternating co-polymers", J. Memb. Sci., 167, 91-106 (2000)