Preparation of semi-alicyclic homo- and blended polyimide membranes using alicyclic dianhydrides with kink structures and their gas separation properties

Abstract

Aromatic polyimides are promising membrane materials for gas separation due to their excellent gas separation properties. Herein, two soluble semi-alicyclic polyimides were synthesized via a one-step thermal imidization process with two semi-alicyclic dianhydrides possessing kink structures: bicyclo[2,2,2] oct-7ene-2,3,5,6-tetracarboxylic dianhydride (BCDA) and 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA), and a flexible aromatic diamine: 4,4′-Oxydianiline (ODA). Their homo- and blended membranes were prepared in various mixing ratios (100/0, 75/25, 50/50, 25/75, and 0/100) and their gas permeation properties were investigated for five gases (H2, CO2, O2, N2, and CH4) and selectivity for five gas pairs (H2/CH4, H2/N2, CO2/CH4, CO2/N2, and O2/N2). The homopolyimides (BCDA-ODA, DOCDA-ODA) exhibited amorphous structure without crystallinity and good solubilities in the casting solvents. BCDA-ODA showed larger d-spacing/FFV values, higher gas diffusivities/gas solubilities than DOCDA-ODA, resulted in higher gas permeabilities and lower gas selectivity, which were remarkably affected by feed temperature. Also, the gas permeability and selectivity of the blended membranes were dependent upon the BCDA/DOCDA mole ratio. The good gas separation performances of homo- and blended membranes were observed for H2/CH4, H2/N2, CO2/CH4, and O2/N2, which are comparable to those of commercial membrane materials such as P84®, PSF, CA, Matrimid®, etc.