Anomalous Structural\nChanges and Gas Transport Properties\nin Ultrathin Films of Polymers of Intrinsic Microporosity

Solution-processable polymers of\nintrinsic microporosity\n(PIMs)\nhave been explored as a next-generation material for the synthesis\nof CO<sub>2</sub> capture membranes. Herein, we highlight the critical\nneed for understanding the transition from bulk PIM materials (50–150\nμm) to their thin films (<3 μm). 6FDA-DAM and PIM-1\nwere chosen as archetypal PIMs, and their thin-film composite (TFC)\nmembranes were prepared via a spin-coating method. Interestingly,\nthe PIM-based TFC membranes always exhibited lower gas permeabilities\ncompared to the model predictions based on their bulk films. Moreover,\nsuch deviations became more pronounced by reducing the selective layer\nthickness to several tens of nanometers. Two-dimensional grazing-incidence\nwide-angle X-ray scattering (2D GIWAXS) analyses reveal that the PIM\nthin films significantly differ from those of bulk films by showing\nthickness-dependent anisotropic microstructures potentially due to\nfast solvent evaporation and confinement effects during film preparation.\nAlso, physical aging significantly affects the microstructures and\nthe CO<sub>2</sub> capture performance of the aged PIM films, which\nshould be decoupled from the effects of film thickness.