Anomalous Structural Changes and Gas Transport Properties in Ultrathin Films of Polymers of Intrinsic Microporosity

Solution-processable polymers of intrinsic microporosity (PIMs) have been explored as a next-generation material for the synthesis of CO₂ capture membranes. Herein, we highlight the critical need for understanding the transition from bulk PIM materials (50-150 μm) to their thin films (<3 μm). 6FDA-DAM and PIM-1 were chosen as archetypal PIMs, and their thin-film composite (TFC) membranes were prepared via a spin-coating method. Interestingly, the PIM-based TFC membranes always exhibited lower gas permeabilities compared to the model predictions based on their bulk films. Moreover, such deviations became more pronounced by reducing the selective layer thickness to several tens of nanometers. Two-dimensional grazing-incidence wide-angle X-ray scattering (2D GIWAXS) analyses reveal that the PIM thin films significantly differ from those of bulk films by showing thickness-dependent anisotropic microstructures potentially due to fast solvent evaporation and confinement effects during film preparation. Also, physical aging significantly affects the microstructures and the CO₂ capture performance of the aged PIM films, which should be decoupled from the effects of film thickness.