Enhancing gas separation performance of polyimide with Tröger’s bases: Unveiling the impact on polymer and carbon molecular sieve membranes

The potential of carbon molecular sieve membranes (CMSM) for gas separation is widely recognized, and the key precursor for these membranes is polyimide (PI). This study focuses on enhancing the gas separation properties of CMSMs by introducing a unique structural element, Tröger’s bases, into polyimides, resulting in the creation of five distinct polyimides containing Tröger’s bases (PI-TBs). The incorporation of Tröger’s bases has a profound impact on the arrangement of polymer chains within the PIs, leading to a substantial improvement in gas permeability without compromising selectivity. Following carbonization at 600 °C, the resultant CMSMs exhibit significantly enhanced gas permeability while preserving, and in some cases, improving gas selectivity. Among the various types of diamines considered, the 6FDA variant of PI-TB stands out, boasting outstanding gas permeability with a H2 permeability of 4991.0 Barrer and a H2/CH4 selectivity of 30.7; this level of performance surpasses the 2015 Robeson’s upper bound. Furthermore, thermal analysis coupled with mass spectrometry (TG-MS) reveals that the pyrolysis of Tröger’s bases commences prior to that of polyimide. This sequential degradation process facilitates the formation of pyridine N and promotes the π-stacking of PI chains; both of which are instrumental in maintaining excellent gas selectivity. These findings provide valuable insights into the roles of Tröger’s bases in enhancing the gas separation performance of PIs and CMSMs, offering a promising avenue for further advancements.