Effects of carbon dioxide-induced plasticization on the gas transport properties of glassy polyimide membranes

The time dependence of carbon dioxide (CO2) transport properties, such as permeability, solubility, and diffusivity, in glassy polyimide membranes was investigated in terms of membrane preparation protocols (i.e., casting solvent and thermal treatment). The polyimide used was 6FDA-TeMPD (4,4-(hexafluoroisopropylidene) diphthalic anhydride) (6FDA)-2,3,5,6-tetramethyl-1,4-phenylene-diamine (TeMPD). The time dependence of CO2 permeability in the as-cast 6FDA-TeMPD membranes prepared from tetrahydrofuran and dichloromethane showed typical CO2-induced plasticization at pressures over 10 atm. The critical plasticization pressure at which CO2-induced plasticization begins to affect the gas permeability shifted from nearly 10–30 atm after heat treatment. The increase in CO2 permeability upon plasticization is mostly caused by an increase in CO2 diffusivity. Furthermore, we found that regardless of the membrane preparation protocol, there is a critical CO2 diffusivity of 73 ± 5 × 10−8 cm2/s at the plasticization pressure in 6FDA-TeMPD membranes.