Non-linear high Tg polyimide-based membranes for separating CO2/CH4 gas mixtures

A novel series membranes based on non-linear all-aromatic polyimides (PIs) was investigated with the aim to understand how the PI backbone geometry and local electrostatics govern gas transport and the ability to separate CO2/CH4 mixtures. Non-linear 3-ring aromatic diamines, with exocyclic bond angles varying between 120 and 134°, enable the design of high Tg (>276 °C) PIs. A polar 1,3,4-oxadiazole diamine (ODD) (μ = 3D) monomer and a non-polar m-terphenyl diamine (TPD) reference monomer were synthesized and coupled with 3 dianhydrides, i.e. ODPA, ODDA, and 6FDA. In 6FDA-based membranes CO2 permeabilities (PCO2) are the highest of the series. The 6FDA-ODD membrane shows excellent membrane performance with high PCO2 values at all feed pressures. Up to 12 bar (6 bar CO2) none of the membranes reached their plasticization pressure. The non-linear backbone geometry promotes CO2 permeability, whereas the presence of an electrostatic dipole moment associated with the 1,3,4-oxadiazole heterocycle governs CO2/CH4 separation selectivity.