Estimating gas permeability and permselectivity of microporous polymers

• Four series of functionalized hypothetical PIMs were screened for CO2 gas separation.
• A novel screening technique is presented to estimate permeability/permselectivity.
• Increasing free volume was most effective in optimizing hypothetical membrane performance.
• PIMs exceed the Robeson Upper Bound with increased fractional free volume.

Experimental and computational investigations have shown that seemingly minor changes in the chemical structure can have a profound effect on the gas adsorption and separation properties of a polymeric membrane. However, the vast number of possible polymer functionalities makes the evaluation of candidate structures a daunting challenge. This study presents the first systematic screening for multiple series of amorphous porous polymers and elucidates several design principles, which will be useful in the design of new intrinsically microporous polymeric membranes for gas separation applications. Specifically, an efficient method for estimating the gas permeability and permselectivity of a polymeric sample by means of free volume theory, grand canonical Monte Carlo simulations, and the solution diffusion model is presented. This method for calculating permeability is orders of magnitude faster than other techniques involving molecular dynamics simulations and is shown to accurately calculate permeability and permselectivity values when compared to available experimental values. As an example, the method is applied to screen four series of polymers of intrinsic microporosity (PIMs) for CO2, CH4, and N2 permeability and CO2/CH4, and CO2/N2 permselectivity. Outstandingly, the gas separation performance of these PIMs was shown not to move parallel to the upper bound for each gas pair. Several porous polymers with high permeabilities and high solubilities were identified, which highlights the fact that permeable polymers with high solubilities coefficients are feasible and a promising route toward achieving industrially applicable gas separation materials.

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