Synthetic saponite clays as additives for reducing aging effects in PIM1 membranes

Polymers of intrinsic microporosity represent one of the most promising polymeric materials for gas separation applications. Their very rigid and contorted backbone induces unusually high free volumes and high internal surface area, with high gas permeabilities and moderate ideal selectivity, especially for O₂/ N₂ and CO₂/N₂ pairs with values lying above Robeson's upper bound. However, the high FFV of PIM1 tends to be short-lived, soon collapsing to leave fewer transport pathways and reduce gas permeability. One way to tackle this problem is the addition of fillers within the polymeric matrix. Here we report the use of synthetic clays named saponites as fillers to slow down the physical aging of PIM1 membranes. Mixed matrix membranes (MMMs) based on two different saponite samples (one completely inorganic and one functionalized with a surfactant) have been obtained, and their permeation performances have been studied in the course of one year to explore physical aging effects over time. Without filler, PIM1 exhibits the classical aging behavior of polymers of intrinsic microporosity, namely, a progressive decline in gas permeation. On the contrary, with saponites, MMMs present a plateau after 1 week within the aging showing that the fillers slow down the aging of PIM1 membranes in the long term. After one year of aging, the total reduction for CO₂ permeability for native PIM1 was 80%, whereas for the MMMs it was 53% and 59% for the inorganic and the functionalized saponite, respectively. Interactions between the fillers and the polymeric matrix in addition to aging effects have been also monitored through SS-NMR spectroscopy. The ¹³C spin−lattice relaxation time (T₁) measurements reveal that PIM1 chains intercalation between T−O−T lamellar sheets could be one of the mechanisms responsible for PIM1 slowing down aging. Chains confinement between lamellar sheets could play a significant role in reducing the densification of chains, while maintaining small free volumes.