Novel semi-alicyclic polyimide membranes: Synthesis, characterization, and gas separation properties

To develop membrane materials for gas separation, a series of semi-alicyclic polyimides was synthesized using one-step thermal solution imidization from an alicyclic dianhydride with non-planar twisted structure, 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA). All synthesized polyimides exhibited amorphous structures and superior thermal stability, with high glass transition temperatures (242–288 °C) withstanding high operation temperature and pressure. They also showed excellent solubility in many polar organic solvents commonly used in the fabrication of gas separation membranes. The gas permeation properties of the polyimide membranes were measured for six representative gases (H₂, CO₂, O₂, CO, N₂, and CH₄). The gas permeabilities and selectivities of polyimide membranes were significantly influenced by the chemical structure of the diamines, which could be explained reasonably by the kinetic diameter of gases, the fractional free volumes, and d-spacing values of the polyimides. Two DOCDA-based polyimides showed very high selectivities for H₂/CH₄ and CO₂/CH₄ and slightly low permeabilities for H₂ and CO₂, which performances were comparable to the commercial polyimide materials such as P84^® and Matrimid^® used in gas separation field.