Effects of residual solvent on gas separation properties of polyimide membranes

The main aim of this study was to determine the effects of residual solvent on the gas sorption and transport properties of polyimide dense membranes. For this purpose, the membranes were made by casting a solution of polyimide on glass plates, followed by treatment at 25 and 150 °C for different periods of time. The membranes obtained were characterized by UV/vis scanning spectrophotometry, dynamic mechanical analysis, differential scanning calorimetry, and positron annihilation lifetime spectroscopy. The permeability and solubility coefficients of the membranes were determined at 35 °C by permeametry and microbalance measurements, respectively. They were found to be dependent on the amount of residual solvent in the membranes. The results show that the permeability, solubility, and diffusivity of polyimide membranes increase with increasing amounts of residual solvent. The difference in separation behavior is derived from changes in small molecular segment motion, free-volume hole size, and free-volume distribution. For membranes with low residual solvent content, separation performance could be affected significantly by antiplasticization; however, for membranes with higher residual solvent content, the dominant factor is plasticization.