Thermal, mechanical, and gas separation performance of polyimides and their thermally rearranged membranes with spirocyclic structures

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The rearrangeable polyimide (PI) membranes have recently attracted wide attention due to their outstanding gas separation performance. In this study, a novel diamine monomer with a spirocyclic structure, namely 2-amino-3′,6′-bis(4-amino-3-hydroxyphenoxy)spiro [isoindoline-1,9′-xanthene]-3-one (ABAHS) was synthesized. Then, it was polymerized with six kinds of dianhydride monomers respectively, followed by thermal imidization to prepare rearrangeable PI membranes. Subsequently, thermal rearrangement (TR) at 450°C was carried out, and the corresponding TR polymer membranes were obtained. The test results demonstrated that the spirocyclic structure endowed the six PI membranes with excellent mechanical and thermal properties. The glass transition temperatures ranged from 309°C to 340°C, and the tensile strength was of 80.7–140.9 MPa. Although the mechanical properties of the TR membranes significantly decreased after thermal treatment, the distance between polymer chains increased markedly. Meanwhile, the gas separation properties of the TR samples were significantly boosted compared to the original PI membranes. Specifically, the TR (ABAHS-6FDA) membrane exhibited the best gas permeation performance, with permeabilities for H 2 , CO 2 , O 2 , and N 2 of 972.93, 911.58, 279.27, and 41.08 Barrer, respectively. Meanwhile, the CO 2 /N 2 separation performance approached the 2008 Robeson limit, and the O 2 /N 2 was close to the 2015 Upper bound.