Blending rigid polyimide containing bulky isopropyl groups with flexible polyacrylonitrile to improve CO2/CH4 separation

Polymer blending is a simple and cost-effective method to improve the gas separation performance of polyimide membranes, thereby potentially overcoming the inherent permeability/selectivity trade-off effect. However, its application still faces challenges, since only a few polymers are miscible with polyimides, and the mechanical property of the membranes usually reduces significantly after polymer blending. Herein, we prepare membranes by blending polyimide containing bulky isopropyl substituents with a small amount of polar polymers such as polyacrylonitrile (PAN). The resultant blend membranes using PAN as additive not only possess excellent gas separation performance but also maintain good mechanical property. For example, when 0.8 wt% PAN is added, the corresponding blend membrane displays a high CO 2 permeability of 632.2 Barrer and a high CO 2 /CH 4 selectivity of 23.4 under humidified condition, close to the 2008 Robeson upper bound, which is attributed to the formation of micro-phase separation structure within the blend membranes. Moreover, the tensile strength of the blend membranes using 0.8 wt% PAN as additive is only 12 % lower than that of the pure polyimide membrane. In addition, the blend membranes using PAN as additive show remarkable stability against physical aging and outstanding long-term operation stability, suggesting their great prospect for natural gas purification and biogas upgrading.