Mechanistic insight into highly efficient gas permeation and separation in a shape-persistent ladder polymer membrane

Abstract

A fully atomistic simulation study is reported to provide mechanistic insight into the superior performance experimentally observed for a polymer membrane (Carta et al., Science, 2013, 339, 303–307). The membrane namely PIM-EA-TB is produced by a shape-persistent ladder polymer of intrinsic microporosity (PIM) with rigid bridged bicyclic ethanoanthracene (EA) and Tröger's base (TB). The simulation reveals that PIM-EA-TB possesses a larger surface area, a higher fraction free volume and a narrower distribution of torsional angles compared to PIM-SBI-TB, which consists of less rigid spirobisindane (SBI). The predicted surface areas of PIM-EA-TB and PIM-SBI-TB are 1168 and 746 m² g⁻¹, close to experimental values of 1120 and 745 m² g⁻¹, respectively. For five gases (CO₂, CH₄, O₂, N₂ and H₂), the solubility and diffusion coefficients from simulation match well with experimental data, except for H₂. The solubility coefficients decrease in the order of CO₂ > CH₄ > O₂ > N₂ > H₂, while the diffusion coefficients increase following CH₄ < CO₂ < N₂ < O₂ < H₂. In terms of the separation for CO₂/N₂, CO₂/CH₄ and O₂/N₂ gas pairs, PIM-EA-TB exhibits higher permselectivities than PIM-SBI-TA, in good agreement with experiment. From a microscopic perspective, this simulation study elucidates that the presence of bridged bicyclic units in PIM-EA-TB enhances the rigidity of polymer chains as well as the capability of gas permeation and separation, and the bottom-up insight could facilitate the rational design of new high-performance membranes.