Residual Solvent Effects on Free Volume and Performance of Fluorinated Polyimide Membranes: A Molecular Simulation Study

In this study, molecular simulation techniques were used to investigate the residual solvent effects on the free volume and performance of 6FDA-mPDA polyimide (PI) membranes. A molecular dynamics (MD) simulation was used to analyze how the residual solvent in the 6FDA-mPDA PI membrane affects the fluctuation and flexibility of the polymer segment and the free volume. The gas sorption in the membrane was analyzed by a Monte Carlo (MC) technique. The energy analysis of the MD simulation indicates that the presence of solvent molecules tends to favor the fluctuation, and flexibility of polymer segments tended to be encouraged due to the presence of solvent molecules through the energy analysis by MD simulation. The free volume analysis revealed that the free space in the membrane would be enlarged by extracting the solvent from the membrane. The gas sorption behavior, analyzed by the MC technique, showed that gas solubility increased in the lower residual solvent membranes. This was caused by the free volume released by extracting the solvent, thus providing more suitable sites for gas sorption. The sorption ability was also affected by the intermolecular attractive energy, while the free volumes inside the membranes were similar. In addition, the residual solvent effect on gas sorption would be eliminated at higher pressure. From the thermal motion analysis of gas molecules, it was found that the effective thermal motion of gaseous molecules improved as residual solvent Molecules remained in the membrane matrix, but this did not have an effective influence on gas permeation.