(539b) Molecular Simulation of CO2-Induced Plasticization in 6FDA-Based Polyimides in the Presence of Residual Solvent

Polyimide membranes have a strong potential 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. Previous experimental studies on gas permeabilities of polyimide membranes showed that the presence of the residual solvent in the membrane has a significant effect on the membrane free volume size distribution, and it alters the gas sorption behavior and gas permeability. While a small amount of solvent in membrane acts as an anti-plasticizing agent, larger amounts exhibit an opposite effect. Molecular simulation methods were previously used to investigate the effect of the residual solvents on free volume changes of the polyimides.¹

In this study, we investigated the change in the CO₂ sorption capacity of 6FDA-DAM and 6FDA-ODA as a function of residual solvent type and amount. A low-boiling-point solvent, tetrahydrofurane (THF) (b.p. 66ºC), and a high-boiling-point solvent, n-methyl-2-pyrrolidone (NMP) (b.p. 202ºC), were selected for this purpose. For each polyimide, two model matrices containing 4% either NMP or THF molecules were constructed. Following an equilibration process, the solvent molecules were removed gradually to obtain new matrices with 2% and 1% solvent content and finally with no solvent. The matrices were equilibrated again after each removal step.

The CO₂-induced plasticization effect was reproduced by applying sorption-relaxation cycles until the CO₂ concentration converges: At each cycle, the polymer matrix was loaded with CO₂ through Grand Canonical Monte Carlo simulations at the considered pressure, and then molecular dynamics simulation runs in the NPT ensemble were applied to obtain an equilibrated matrix. The cycle was repeated until CO₂ concentration converges. The change in the linkage flexibility due to the presence of solvent and its effect on CO₂ sorption capacity and plasticization were analyzed.

The structural properties of solvent-free 6FDA-DAM and 6FDA-ODA polyimides were estimated in good agreement with the experimental data. Estimated sorption coefficients of these gases in both polymers are within the same order of magnitude with the experimental data in the literature. Simulation revealed also that CO₂ solubility  is solely a function of the FFV of the plasticized polyimides, and the increase in the fractional free volume and the flexibility of the diamine-dianhydride linkage are correlated.

Radial distribution function (RDF) analyses showed that THF affects the nitrogen in the imide ring of 6FDA-ODA and hinders chain packing; resulting in high fractional free volume (FFV) and sorption capacity. Between the four sorption sites (the oxygen and nitrogen in the imide, the fluorine in the -CF₃ group, and the carbon in –CH₃ group of the diamine) in 6FDA-DAM, the oxygen in the imide is the preferential one for both CO₂ and NMP indicating that NMP sorption at and around this site would lower the sorption capacity of CO₂. Consequently, residual solvents reduce CO₂ sorption capacity and alter CO₂/CH₄ selectivity of both polyimides. Relationship between backbone rigidity and plasticization resistance is altered in the presence of solvents.

Abbreviations: 6FDA: 4,4-hexafluoro isopropylidene diphthalicanhydride; ODA: 4,4 oxydianiline; DAM: 2,4,6-trimethyl-m-phenylene diamine.

¹Chang, K. S.; Tung, C. C.; Lin, C. C.; Tung, K. L. Residual Solvent Effects on Free Volume and Performance of Fluorinated Polyimide Membranes: A Molecular Simulation Study J. Phys. Chem. B. 2009, 113, 10159.