Effect of formamide additive on the structure and gas permeation performance of polyethermide membrane

Asymmetric polyetherimide (PEI) membranes have been prepared by phase-inversion process from casting solution containing n-methyl-2-pyrrolidone as a solvent and formamide (FM) as an additive. The effects of FM concentration on the structure and gas separation performance of the PEI membranes have been investigated and analyzed in terms of the casting solution viscosity, phase diagram, and solubility parameter difference. With the increase of FM concentration, the casting solution becomes thermodynamically unstable and easier to cause casting solution demixing, but the precipitation rate decreases because of the increased casting solution viscosity. The experimental results reveal that the structure of the dense skin layer strongly depends on the content of FM in the casting solution, and the gas permeance of the PEI membrane is markedly improved by the addition of FM. After coated with silicone rubber, the membrane prepared from the casting solution with 28.0 wt.% of PEI and 3.0 wt.% of FM exhibits the H2 permeance of 24.6 GPU (1 GPU = 10−6 cm3/cm2 s cmHg), combining with the O2/N2 and H2/N2 separation factors of 7.0 and 153.0, respectively.