Residual Solvent Effect On CO2-Induced Plasticization of Polyimide Membranes

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 free volume size distribution, and it alters the gas sorption behavior, hence the gas permeability. While small amounts of solvents in membrane act 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, 6FDA-ODA, 6FDA-DABA, 6FDA-25DPX and BTDA-DABA as a function of residual solvent type and amount. A low-boiling-point solvent, tetrahydrofurane (THF) (BP: 66ºC), and a high-boiling-point solvent, n-methyl-2-pyrrolidone (NMP) (BP: 197ºC), were selected for this purpose.  

First solvent-free polyimides are constructed and their sorption capacities and plasticization behavior are investigated. To reproduce the CO₂-induced plasticization effect, sorption-relaxation cycles were applied 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. Simulation results showed that the increase in the fractional free volume and the flexibility of the diamine-dianhydride linkage are correlated. Next, the procedure was repeated for model polyimides containing different amounts of NMP or THF up to 4% by weight. The change in the linkage flexibility due to solvent presence and its effect on CO₂ sorption capacity and plasticization are analyzed.

Abbreviations: 6FDA: 4,4-hexafluoro isopropylidene diphthalicanhydride; ODA: 4,4 oxydianiline; DABA: 3,5-diamino benzoic acid;  BTDA: 3,3-4,4-benzophenone tetracarboxyclic dianhydride; DAM: 2,4,6-trimethyl-m-phenylene diamine; 25DPX: 2,5-dimethyl-p-phenylenediamine.

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.