Importance of small loops within PIM-1 topology on gas separation selectivity in thin film composite membranes

Abstract

Studies of blends of topologically distinct samples of the polymer of intrinsic microporosity PIM-1 demonstrate, for the CO₂/N₂ gas pair, the ability to vary dramatically the selectivity and aging behaviour of thin film composite (TFC) membranes. Polymerizations carried out in the absence of nitrogen gas produced polymers with higher degrees of branching, which formed further into loops and at higher temperatures into significant amounts of colloidal network PIM-1 (<100 nm). In contrast, polymerizations carried out with excessive nitrogen purging produced high molar mass, di-substituted PIM-1 polymer chains. Blending of these diverse samples and testing as TFC membranes, prepared with a 2 μm active layer by kiss-coating directly on a polyacrylonitrile support, identified small loops as a topological component within di-substituted polymer chains that is required to obtain above average gas separation. A refined polymerization process produced a topologically rich sample, best described on average as a multi-loop polymeric structure (with 3 small loops) containing very few chain ends (0–1 max), which when used in TFC membranes exhibited good long-term aging (120 days) performance: CO₂ permeance of 671 GPU and ideal CO₂/N₂ selectivity of 27. To put the level of optimization in context, a self-standing film (40 μm thickness) exhibited initial CO₂ permeability of 4835 barrer (121 GPU) and ideal CO₂/N₂ selectivity of 55.5. This work presents a new approach to delivering polymeric materials for use in thin film membranes, which is based on multi-loop polymeric structures that aid both permeance and selectivity performance.