Fine-tuning gas separation performance of intrinsic microporous polyimide by the regulation of atomic-level halogen substitution

The molecular structure of polymers is essential to membrane separation performance. However, the inherent relationship between molecular structure and gas separation performance of polymeric membranes is unclear. Herein, the halogen atoms (F, Cl, and Br) were introduced into intrinsic micropore polyimide (PIM-PI) to precisely regulate its interchain cavity and elucidate the structure-property relationship. Based on the pristine BAPI, the halogen atoms were precisely designed at the ortho position of the amino group and synthesized three PIM-PIs, namely BFPI, BCPI, and BBPI, respectively. Then, it can be found that the packing structure of polymer chains and fine-tuning of fractional free volume (FFV) exhibits a regular trend in halogen-containing PIM-PIs. In addition, as the van der Waals volume of halogen atoms increases, the gas permeability of halogen-containing PIM-PIs exhibits an increasing trend. Specifically, compared with BFPI, the CO2 permeability of BCPI and BBPI is increased by 48 % and 100 %, respectively. Our findings may elucidate the structure-property relationship of microporous polymer membranes and provide a brand-new insight to design an advanced high-performance membrane.