(178ab) CO2-Induced Plasticization in Copolyimides Containing a Sulfone Group

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/N₂ separation applications. This work, to the best of our knowledge, is the first to synthesize these three copolyimide structures.

The copolyimide synthesis is carried out by one-pot rule. Dimethyl acetamide is used as solvent and 1,2-dichlorobenzene is used as an azeotrop agent to remove H₂O during imidization step. Imidization is determined by FTIR analysis of polyamic acid and polyimide. The copolyimide is characterized by GPC, DSC, DMA, and wide angle XRD analyses. The O₂, N₂, CO₂ and CH₄ permeability coefficients of three copolyimide films are measured at 35°C at up to 10 atm feed pressure..

We also compared our experimental results on structural, mechanical and separation properties with those estimated via molecular simulation methods. We used a combination of molecular dynamics and Monte Carlo simulation methods to investigate the relationship between structural properties and CO₂-induced plasticization behavior of copolyimides.

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 (41.5%) in 6FDA/BTDA-pBAPS. The FFV of the three copolyimides are 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. N and O1 sites are also the preferential sorption sites for CO₂ for all three copolyimides. Furthermore 6FDA-pBAPS/DABA includes an extra sorption site of O3 in DABA diamine monomer which provides more rigidity to the polymer chain. Simulated sorption coefficients agree well with the experimental results obtained from gravimetric sorption (IGA) measurements. Permeability coefficients obtained from group contribution method and experiments are on the same order of magnitude.

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.

¹Halitoglu, 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.

²Alentiev, 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., 2000, 167, 91-106.