Molecular modelling of polyimides with intrinsic microporosity: from structural characteristics to transport behaviour

Abstract

Molecular dynamics (MD) simulations and the Monte Carlo (MC) method were adopted to investigate the characteristics of polyimide (PI)-derived membranes with intrinsic microporosity. Two PI-derived membrane models, PIM-PI-1 and PIM-PI-8, were constructed to analyse the effect of the spiro-centres on the membrane structure and gas transport behaviour. The torsional angle, wide angle X-ray diffraction, free volume and cavity size of these membranes were analysed to characterize the membrane structure. The sorption and diffusion behaviours of various gases in the membranes were also investigated to evaluate the transport properties of the membranes. The spiro-centres stiffened the PI-derived membrane, resulting in the formation of a larger and more effective free volume in the PIM-PI-1 and PIM-PI-8 membranes. The free volume did not have a significant influence on gas sorption, however the nature of the gas species was the predominant factor affecting gas solubility in the membrane matrix. The diffusion analysis revealed that the increased effective free volume of the PIM-PI-1 and PIM-PI-8 membranes provided more pathways for gas molecules to transfer from one cavity to another. The increased diffusivity in these PI-derived membranes results in an increase in gas permeability. In addition, the solubility of gases, including CO₂ and CH₄, in the membrane increased, further improving gas permeability. The quantitative results from the molecular simulation technique agreed well with the experimental data. The combined MD and MC techniques proved to be a promising tool for the characterization of the membrane structure and gas separation processes of PI-derived membranes with spiro-centres.