SEPARATION AND PURIFICATION TECHNOLOGY, vol.178, pp.90-104, 2017 (SCI-Expanded)
Three novel co-polyimides containing a sulfone group (pBAPS) were synthesized and characterized in pursuit of gas separation membrane materials with high CO2/CH4 separation performance. The co-polyimides, 6FDA/BTDA-pBAPS (3:1), 6FDA-pBAPS/mPDA (3:1) and 6FDA-pBAPS/DABA (3:1), were identified as promising structures in our previous study based on a group contribution method. The molecular weight of the co-polyimides varied between 50 and 100 kDa. The TGA analysis of the co-polyimides showed that 6FDA/BTDA-pBAPS and 6FDA-pBAPS/mPDA were stable up to a degradation temperature of higher than 500 C-omicron. However, two-step (similar to 300 C-omicron and 460 C-omicron) degradation curve was observed for 6FDA-pBAPS/DABA. The glass transition temperatures and densities of the co-polyimides are within 276-287 C-omicron and 1.387-1.419 g/cm(3) range, respectively. Measured permeability coefficients for O-2, N-2, CO2, and CH4 gases were in good agreement with the predictions of the group contribution method with an average absolute deviation of 0.59, confirming that group contribution methods could be used for fast screening of co-polyimides. The highest CO2/CH4 and CO2/N-2 selectivities were obtained for 6FDA/BTDA-pBAPS as 51.9 and 28.6, respectively, with a CO2 permeability of 7 Barrer. Molecular simulation techniques also were used to better understand the structure/property relationships. Preferential CO2 sorption sites in co-polyimides were investigated through the analysis of the radial distribution function (RDF), and the available vacancy inside the simulation cell was detected by accessible free volume analysis. RDF results showed that the main interaction with CO2 arises from the sulfur groups in the pBAPS diamine which increase the sorption coefficient of CO2. Moreover, the carboxyl group in DABA diamine enables the formation of hydrogen bonds which results in closed packing and decrease in the sorption and permeability coefficients. Experimental and simulated sorption coefficients are in good agreement with each other. (C) 2017 Elsevier B.V. All rights reserved.