Plasticization behavior of crown-ether containing polyimide membranes for the separation of CO₂

This paper describes the influence of crown-ether ring size and rigidity on the gas separation performance of CO₂/N₂ and plasticization behavior of crown-ether containing Matrimid® 5218 polyimide membranes. The crown-ethers provide the membrane material with good affinity for CO₂ due to the polar ether segments. Three different crown-ethers were used that differ in ring size and in rigidity (DB21C7, DB18C6 and 18C6). The gas separation performance of the crown-ether containing membranes was evaluated using pure gas and mixed gas conditions at pressures up to 40 bar. Thermal analysis (TGA and DSC) shows that the incorporation of the crown-ethers in the polyimide matrix was successful and that the crown-ethers have good compatibility with the polymer matrix. Gas sorption in the crown-ether containing membranes shows an enhanced solubility selectivity due to the enhanced interaction with CO₂ compared to the pristine Matrimid®. However, both solubility and permeability of CO₂ and N₂ are decreased in these crown-ether containing membranes over the whole pressure range. When the molecular size of the crown-ether decreases (DB21C7 > DB18C6 > 18C6) the permeability increases. In addition, the incorporation of the more rigid crown-ethers (DB18C6 and DB21C7) causes rigidification of the polymer matrix contributing to the lower permeability of DB18C6 and DB21C7-based membranes. Pure gas CO₂ permeation measurements show a typical plasticization behavior for pristine Matrimid®, while for the crown-ether containing membranes the extent of plasticization is significantly reduced. Next to that the plasticization pressure is shifted to higher CO₂ feed pressures. No plasticization was observed for the crown-ether containing membranes in the mixed gas conditions, as a result of the lower solubility of the crown-ether containing membranes and competition effects of CO₂ and N₂. The differences in size and rigidity of the different crown-ethers showed no significant influence on the plasticization resistance of crown-ether containing membranes. Altogether, the operating window of these crown-ether containing membranes is increased with respect to Matrimid® showing their potential to operate at higher CO₂ partial pressure without compromising selectivity.