Impact of polymer topology on physical aging of thin film composite membranes based on PIM-1, cPIM-1 and associated blends

An overview is provided of the influence of polymer topology on the physical aging of PIM-1 thin film composite (TFC) membranes (1-3 µm selective layer) measured in gas permeation studies over the past few years. A range of topologically distinct PIM-1 samples are compared firstly with each other, then in polymeric blends, and then with other literature. Both initial permeability (1 day) and long-term aging rates (up to 1 year) can be attributed to structural components present within the overall microstructure of the polymer. The rigidity and structural regularity of a predominantly di-substituted PIM-1 polymer proved to facilitate high initial CO₂ permeability in TFCs followed by rapid aging rate (β_P = 1.0) to produce an increasingly non-selective membrane over 28 days. By contrast, TFCs prepared from branched PIM-1 polymers, which have a readily measurable and lower glass transition temperature (T_g = 419 ºC) compared to a di-substituted PIM-1 polymer (Tg = 442 ºC), exhibit lower initial permeabilities followed by much slower aging rates, remaining highly selective for up to one year. Branched PIM-1 polymers, prepared from tetrachloroterephthalonitrile (TCTPN), which contain a greater proportion of small loop structures, show a very slow aging rate (β_P = 0.22−0.25), whereas those prepared from tetrafluoroterephthalonitrile (TFTPN) tend to exhibit a faster aging rate (β_P = 0.67−0.69). Open branch points in polymer chains provide added segmental mobility within the thin film active layer on the support surface, which translates into better chain packing and lower free volume initially in the formed TFCs (1 day aging). If a branch point is confined within a small loop or larger overall ring structure, it will have less mobility to densify further as polymer chain backbones
come into closer contact during long term aging. Thin film nanocomposite (TFN) membranes cast from blends of a di-substituted PIM-1 with colloidal network rich PIM-1 fillers can completely halt permeability aging for up to one month, but then subsequently resume aging at a faster rate (β_P = 1.8−2.8) to more than compensate for this initial aging improvement. TFNs prepared from blending
a branched PIM-1 polymer with a colloidal network rich Cardo-PIM-1 filler can produce better long-term aging performance (up to 1 year). Consideration is also given to how the level of hydrolysis impacts aging of carboxylated PIM-1 (cPIM-1) TFCs.