Solution-processable methyl-substituted semi-alicyclic homo- and co-polyimides and their gas permeation properties

Because of their excellent thermal, chemical and physical properties, as well as high gas selectivity, polyimides are promising polymers for gas separation membranes. However, their commercial use is limited due to their relatively poor solubility and permeability. Here, 5-(2,5-dioxotetrahydrofuryl)-3-methyl-cyclohexane-1,2-dicarboxylic anhydride (DOCDA) was used to improve the solubility of the polyimides, and various methyl-substituted diamines were introduced to increase their permeability, due to the effect of the bulky methyl groups. Polyimides based on DOCDA (DOCDA-PIs), synthesized via one-step solution polymerization, exhibited high solubility in common organic solvents and improved gas permeability due to the non-planar structure of DOCDA and the presence of methyl groups, which disturbed chain packing, an effect that was correlated with increased fractional free volume (FFV) and d-spacing. Co-polyimides based on DOCDA, DOCDA-coPIs, exhibited high gas permeability and CO2/CH4 and O2/N2 selectivity, comparable to commercial polymeric membranes. [Display omitted] •Alicyclic polyimides containing different methyl substituent groups were synthesized by the one-step imidization.•lDOCDA-PIs showed excellent solubility in organic solvents thanks to the incorporation of asymmetric alicyclic groups.•The methyl substituents had a crucial effect on gas permeability.•The DOCDA-coPIs had permeability and selectivity values similar to commercial polymeric membranes.