Effect of physical aging on sorption and permeation of small molecules in polyimide membranes

The gas transport properties of Polyimide membranes were examined as a function of physical aging time. A clear decrease of the gas permeability coefficient was observed. Interpretation of the permeation properties using results from differential scanning calorimeter and dynamic mechanical analyzer indicated that the smaller segmental motion of the polymer chains would be more restrict with aging time. Physical aging reduced the measured solubility coefficients and calculated diffusivity coefficients. Furthermore, the effect of physical aging on selectivity coefficients of polymeric membrane was controlled by the change of diffusivity selectivity. A modified three parameter fit model is obtained and predicts the long-term physical aging behavior with good agreement with experiments.

Physical aging is an important phenomenon that needs to be understood for practical application of gas separation glassy polymer membranes. As a result of a free volume decrease during physical aging in the glassy state, polyimide membranes showed significant reduction in their gas permeability over the time scale of investigation. The rate of permeability decreasing was strongly dependent on the gas size. DSC and DMA results shown that the smaller segmental motion of the polymer chains would be more restricts with aging time. Both solubility and diffusivity coefficients decrease with increasing aging time. The effect of physical aging on gas selectivity of polyimide membrane was controlled by the change of diffusivity selectivity. Additionally, a three parameter fit model can well predict physical aging behavior of polyimide membranes.