Time-dependent plasticization behavior of polyimide membranes at supercritical conditions

Abstract The time-dependent CO2-induced plasticization behavior of glassy Matrimid® 5218 polymer membranes at supercritical conditions up to 120 bar was investigated. Glassy polyimide membranes were conditioned with both gaseous CO2 and liquid-like sc-CO2. The plasticization behavior during permeation and sorption was correlated with the intrinsic membrane properties and the CO2 fluid properties. In the gaseous region the CO2 concentration increased slightly over time, while in the liquid-like sc-CO2 region the CO2 concentration remained constant over time and showed no hysteresis, indicating an induced glass transition. Contrary to the CO2 sorption the CO2 permeability showed more pronounced time-dependent behavior which increases with feed pressure because of polymer membrane plasticization. Despite the strong time-dependency, the CO2 permeability was independent of the feed pressure in the liquid-like sc-CO2 region. This difference in time-dependent behavior between sorption and permeation is due to the presence of a concentration gradient during permeation experiments. In addition, the permeability showed significant hysteresis. Exposure to liquid-like sc-CO2 resulted in a highly plasticized membrane and changed the permeation behavior at all subsequent feed pressures, due to slow polymer chain relaxation rates. Clearly, these relationships proof that the permeation history is a critical aspect for time-dependent plasticization phenomena at high CO2 pressures.