Construction of Microporous Polyimides with Tunable Pore Size and High CO2 Selectivity Based on Cross-Linkable Linear Polyimides

Polyimides with intrinsic microporosity have been well developed and explored in the field of gas adsorption and separation. However, most linear polyimides without rigid and distorted segments do not have porous structure owing to the densely packed polymer chains. Two cross-linked polyimides (PEDA-PI-CL and PEQDA-PI-CL) were synthesized from 3,3′-bis(2,3,5,6-tetrafluoro-4-vinylphenoxy)-4,4′-biphenyldiamine (TFVBPA) with 2,5-bis(3,4-dicarboxyphenoxy)-4′-phenyl dianhydride (PEDA) and 2,5-bis(3,4-dicarboxyphenoxy)-4′-phenylethynyl phenyl dianhydride (PEQDA) through polycondensation and thermal cross-linking reactions. Here, the cross-linked structure is utilized to construct microporous polyimides because it can restrict the movement of polymer segments and prevent the polymer chains from dense packing. The micropore size, microporous structure, and CO2 adsorption performance can be finely tuned by increasing the cross-linking density from PEDA-PI-CL to PEQDA-PI-CL. The micropore size reduces from 1.06 to 0.54 nm and CO2 adsorption increases from 6.32 to 9.88 wt %. Meanwhile, the CO2/N2 and CO2/CH4 selectivity values of PEQDA-PI-CL at 273 K/1 bar are 76.62 and 12.40, respectively, which is better than many other common microporous polyimides. These results reveal that increasing cross-linking density in cross-linked polyimides is favorable for preparing microporous polyimide networks with smaller pores and higher CO2 adsorption selectivity.