Polyimides are attractive membrane materials for gas separation applications such as CO2 removal from natural gas or flue gas treatment processes, due to their good permeability, selectivity, and mechanical resistance. 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.
In this study we investigated structure-plasticization relationships of three sulfonated copolyimides: 6FDA/BTDA-pBAPS, 6FDA-pBAPS/DABA, and 6FDA-pBAPS/mPDA. These polymers were selected based on our previous studies in which the permeability coefficients of H2, O2, He, CO2, N2, and CH4 gases for more than 2200 possible co-polyimide structures were estimated by the group contribution method. These selected copolyimides were then synthesized and characteristics of the membranes were determined experimentally up to 10 bar. However, these studies were unable to deterimine the plasticization resistance of these materials, since the plasticization phenomenon occurs usually well above 10 bar. In this work we predicted the plasticization behavior of these materials up to 40 bar using molecular simulation methods.
We used Molecular Dynamic (MD) and Monte Carlo (MC) simulations to model the relationship between structural properties and CO2-induced plasticization behavior of copolyimides in CO2/CH4 separation. The physical characteristics of simulated membranes matched well with the experimental data Simulated sorption coefficients agreed well with the experimental results obtained from gravimetric sorption (IGA) measurements. Sorption simulations up to 40 bars were performed and fractional free volume increases of copolyimides were analyzed. Calculation of diffusion coefficients were carried out at the sorption pressures in order to calculate permeabilities. Finally we estimated the plasticization resistance of each copolyimide based on the variation of permeabilities as a function of CO2 pressure. Finally, plasticization pressures of polyimides were reported and compared.
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 Turkish Scientific and Technological Research Council (TUBITAK) trough grant no. 113M336.