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.
Highlights
Glassy dense polymer membranes are commonly used in CO2 gas separations
In our previous work we showed that the CO2 concentration profile in a glassy polyimide matrix follows the CO2 density profile and
The extent of plasticization is dependent on the CO2 concentration in the polymer matrix, and becomes more severe as the pressure increases in the gaseous CO2 phase [10]
Summary
Glassy dense polymer membranes are commonly used in CO2 gas separations. It is well known that high concentrations of CO2 induce plasticization phenomena in these glassy polymer membranes [1,2,3,4,5,6]. Glassy polymers especially suffer from severe plasticization in high pressure applications, e.g. CO2/CH4 separation [9]. The effect of sc-CO2 and its fluid properties on the membrane performance and the plasticization behavior is not much studied, while it does have a large impact on the performance in high pressure applications (e.g. sc-CO2 extractions or enhanced methane recovery) [12,13,14,15,16,17]. Previous work shows that the CO2 density is the most important fluid property influencing plasticization phenomena in polymer membranes and the CO2 permeability behavior at supercritical conditions [12, 18]. Shamu et al [18] concluded that not the phase transition from gaseous to supercritical CO2, but the quasi-phase transition within the supercritical region is the crucial transition influ encing the CO2 permeability behavior in polymer membranes
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