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Abstract
We report a molecular simulation study for gas permeation in two membranes constructed from polymers of intrinsic microporosity (PIM-1 and PIM-7). With rigid ladder polymer chains, the membranes posses approximately 47.7 and 46.6% fractional free volumes (FFVs) in PIM-1 and PIM-7, respectively. The voids in the membranes have a diameter up to 9 Å and are largely interconnected. The sorption and diffusion of four gases (H2, O2, CH4 and CO2) were calculated by Monte Carlo and molecular dynamics simulations. The solubility coefficients increase in the order of H2 < O2 < CH4 < CO2, while the diffusion coefficients increase in the following order: CH4 < CO2 < O2 < H2. The simulation results agree well with experimental data, particularly for the solubility coefficients. The solubility and diffusion coefficients correlate well separately with the critical temperatures and effective diameters of gases. These molecular-based correlations can be used in the prediction for other gases. As attributed to the microporous structure, PIM-1 and PIM-7 outperform most glassy polymeric membranes in sorption and diffusion. PIM-1 has larger solubility and diffusion coefficients than PIM-7 because the cyano groups in PIM-1 lead to a stronger affinity and a larger FFV. The simulated solubility, diffusivity and permeation selectivities of CO2/H2, CO2/O2 and CO2/CH4 are consistent with experimental data. The quantitative microscopic understanding of gas permeation in the PIM membranes is useful for the new development of high-performance membranes.
Acknowledgements
The authors are grateful to Prof P.M. Budd for providing the experimental density value of the PIM-7 membrane and the National Research Foundation of Singapore on the Competitive Research Programme (R-279-000-261-281) for financial support.