NONTECHNICAL Summary This award supports computational research and education to advance understanding of physical polymer aging, the process where the configuration of atoms relaxes over time leading to changes the properties of polymers. Polymers are long chain molecules made of several repeating units. When these chains are tangled and randomly arranged without any order, the polymer is said to be ‘amorphous’. Below the glass transition temperature (the point at which the polymer changes from soft and flexible to hard and glass-like), amorphous polymers are widely used in applications ranging from storage and packaging to separation technologies. Owing to their low cost and ease of manufacturing, polymer membranes have the potential to dramatically reduce the energy required for industrial separations, which currently account for a substantial fraction of global energy consumption. Polymers of intrinsic microporosity are especially promising membrane materials because their loosely packed molecular structure creates extra internal space (also known as free volume), which increases how quickly and efficiently gas molecules can move through the material. However, these polymers are not widely used in industrial applications, because they undergo physical aging, a slow and irreversible process in which the polymer relaxes and densifies over time. This relaxation leads to reduced separation efficiency, loss of mechanical integrity, and ultimately diminished membrane performance. At present, physical aging in high free volume polymers like polymers of intrinsic microporosity is captured indirectly through changes in membrane performance, and the underlying structural changes at the molecular level remain poorly understood. This CAREER award addresses this critical knowledge gap by uncovering how polymer chains rearrange during aging and how these rearrangements affect membrane performance. Molecular simulations will allow the PI to directly observe these small-scale