Time-dependent plasticization behavior of polyimide membranes at supercritical conditions

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-CO_2. 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.