(579c) Tuning Ether Motifs in Polymers Membranes for CO2/N2 Separation

Polymer membranes suffer from design challenges due to the negative correlation between two principal features of separation performance (permeability and selectivity). We aim to develop polymer membranes that will achieve both high CO₂ permeance as well as high CO₂/N₂ and CO₂/O₂ selectivity. (PEO)-based polymers have been leading membrane materials for CO₂/N₂ separation. The ether oxygen moiety is a unique functional group that exhibits affinity towards CO₂ but not N₂, which leads to high CO₂ solubility and CO₂/N₂ solubility selectivity. We hypothesize increasing the ether oxygen content would increase the CO₂ solubility and CO₂/N₂ solubility selectivity. First, we used the group contribution (GC) method based on perturbed-chain statistical associating fluid theory equation of state. By invoking simple combination rules for the chemical functional groups, the GC method allows the properties of any molten polymers to be easily calculated from their chemical structure without the use of any adjustable parameters. We have investigated five different polymer materials that range in O:C ratio, including polyethylene (PE, O:C=0), polytetramethylene oxide (PTMO, O:C=0.25), polyethylene oxide (PEO, O:C=0.5), poly(1,3-dioxolane) acrylate (PDXLA, O:C=0.67), and polyoxymethylene (POM, O:C=1). Our results show strong agreement in the absolute CO₂ and N₂ solubility and the solubility selectivity with the experimental data for PE, PEO, and PTMO. In addition, we predict higher solubility selectivity for the two new candidate polymers, PDXLA and POM. Next, the promising candidates are further studied with computationally intensive atomistic molecular dynamics (MD) simulations to predict their gas diffusivity. The detailed investigation with MD simulations showed POM has higher CO₂/N₂ diffusivity selectivity compared to commonly used PEO. By combining CO₂/N₂ solubility and diffusivity selectivity, we reveal high CO₂/N₂ separation performance for POM, supporting our hypothesis that increasing O:C ratio leads to better performing polymers. Furthermore, we investigated the solvation environments of the gas molecules and showed that the chain termination with CO₂-philic and N₂-phobic functional groups enhances polymers’ separation performance. The framework outlined in this work helps us down select polymers with high CO₂/N₂ solubility selectivity to conduct complete and targeted computational investigations on the new and improved polymer membranes.