Emergence of the Robeson bound in non-equilibrium molecular dynamics simulations

Theoretical understanding of multicomponent transport processes in polymer membranes at the molecular level remains a significant scientific challenge due to the high computational cost and poor reproducibility of non-equilibrium molecular simulation methods, aiming at replicating experimental conditions and gradient-driven processes. In this study, we employ Dual Control Volume Molecular Dynamics (DCVMD) with fully atomistic, microsecond-scale simulations to elucidate gas separation mechanisms in porous polymer membranes. Using PIM-PI-8 as a case study, we develop a comprehensive workflow that includes sensitivity analysis and calculations of key transport and structural characteristics of the model membranes. This approach allows us to thoroughly characterize the uncertainties inherent in non-equilibrium simulations and explore the challenges associated with making quantitative predictions using this method. Expanding our analysis to a family of PIM polymers, our simulations successfully replicate several experimentally observed trends. These include a linear correlation between permeability and fractional free volume, as well as the relationship between perm-selectivity and sorption selectivity. Most importantly, we demonstrate that trade-offs between perm-selectivity and permeability emerge in non-equilibrium molecular simulations, qualitatively capturing one of the key patterns in experiments, known as the Robeson upper bound.