(177ac) Examining Free Volume and Flexibility of Polymers of Intrinsic Microporosity (PIMs) from Molecular Dynamics Simulations

Gas separation membranes are essential in industrial applications, providing benefits such as energy efficiency, cost-effectiveness, and versatility. Polymers of Intrinsic Microporosity (PIMs) have gained considerable attention due to their unique microporous structure and exceptional permeability-selectivity. Particularly, PIMs have a highly interconnected distribution of microporous voids, which significantly enhances the passage of penetrants through the membrane compared to regular polymers. Despite their many benefits, the performance of PIM-based membranes is often compromised by aging and plasticization phenomena, which lead to decreased separation efficiency and overall performance. Both aging and plasticization are closely linked to chain and backbone flexibility and contribute to a change in the polymer microstructure. The relaxation phenomena of these membranes are influenced by segmental dynamics occurring at short lengths and time scales, making it challenging to characterize them using experimental techniques alone. In this study, we use molecular dynamics (MD) simulations to elucidate the origin and impact of local dynamics in PIM-based gas separation membranes. Our aim is to unravel the underlying mechanisms that compromise membrane performance by examining chain mobility and intermolecular interactions. To ensure an accurate representation of the molecular structure, we begin by parameterizing PIM systems using Ambertools. We then utilize the simulated polymerization algorithm, Polymatic, to generate and validate membrane structures, ensuring reproducibility and agreement with experimental properties. We explore different PIMs with various functionalities, including PIM-1, PIM-Py, and PIM-COOH, all of which are industrially relevant variants of PIMs. By studying membranes with different functional groups, we aim to understand how these variations impact molecular structure, chain packing, free volume structure, and penetrant transport. The bond autocorrelation function will be used to analyze chain dynamics, while VACUUMMS, the void analysis software, will be used to compute free volume. Insights gained from our study will deepen our understanding of aging and plasticization phenomena in PIM-based membranes and will shed light on the role of chain mobility at the nanoscale. Ultimately, this knowledge will inform the design of more robust and durable membrane materials, paving the way for developing the next generation of high-performance gas separation membranes that are aging and plasticization resistant.