A structure-property study for the effect of methyl substituents on the gas separation of spirodifluoranthene-based polymers of intrinsic microporosity

A simple methyl substitution on the structural building blocks of a PIM can alter its chain stacking behavior. Hence, we have introduced differing amounts of methyls on bulky and rigid spirodifluoranthene (EN) configured as two fluorenyl groups perpendicular to anthracene to obtain a series of spirodifluoranthene-based PIMs with increased intrinsic microporous, as demonstrated by nitrogen adsorption and enhanced gas permeability. The methyl substitution increased the free volume of polymer membranes, but the free volume does not increase in proportion to the number of methyl groups, and the increment is relatively slight at high methyl substitution amounts. Also, the effect of methyl substitution was closely associated with the molar contents of EN structures in PIMs. The series of polymers exhibited excellent CO2/N2, CO2/CH4, and O2/N2 gas separation performances, ranking among state-of-the-art polymeric materials for gas separation. Particularly, PIM-OMEN-40 exhibited the highest CO2 permeability, up to 12,431 barrer, which corresponds to 2.94 times higher than that of PIM-1 and along with comparable gas selectivity. Even after more than 100 days of aging, the series of PIMs still showed higher gas permeability and selectivity than fresh PIM-1, demonstrating great potential in separation areas for CO2 capture and storage, natural gas and biogas upgrading of prime importance to energy and the environment.