Polymer-solvent cluster coupling drives permeation pathways through PIM-1 membrane

Transport of liquids through nanoporous polymer membranes is commonly interpreted using viscosity-based or solution–diffusion models. Here, using pressure-driven molecular dynamics simulations of a PIM-1 membrane in contact with water–ethanol mixtures, we reveal a fundamentally different transport mechanism governed by collective intermolecular interactions and structural reorganization of the polymer matrix. We show that ethanol permeability in mixtures can exceed that of pure ethanol despite the higher viscosity of the mixtures. Most strikingly, both flux and intrinsic permeance exhibit a pronounced non-monotonic dependence on mixture composition, with a maximum at intermediate ethanol-rich compositions (0.75 M fraction of ethanol). This anomalous behavior correlates with the formation of transient ethanol clusters under confinement and with composition-dependent pore enlargement inside the membrane. By isolating the intrinsic membrane permeance, we demonstrate that transport is controlled not by bulk viscosity, but by the coupling between solvent clustering and time-dependent structural rearrangements of the polymer matrix, which continuously modify pore sizes and reorganize permeation pathways. These findings establish solvent clustering and fluctuating pore architecture as key determinants of transport in nanoporous polymer membranes and provide a molecular framework for understanding non-monotonic permeation in complex solvent mixtures.