The influence of casting solution structure on the microporosity of polyetherimide gas separation membranes prepared by the coagulation post-leaching method

The microporosity and performance of polyetherimide (PEI) membranes produced by the coagulation post-leaching technique were investigated. Computational chemistry and chemical structure principles indicate that a lithium cation solvated in NMP (N-methyl-2-pyrrolidinone) forms a complex with two NMP molecules. This complex was determined to be stable in NMP and isopropanol but not in methanol or water. Increasing the additive (LiNO3) concentration in the casting solution lead to a greater number of micropores in the 0.7–1.2 nm range, while the peak in the micropore distribution remained unchanged at 1.0 nm. Molecular mechanics calculations indicate that the PEI polymer chain can form coils having an inner section of 0.7 nm×1.0 nm. Micropore measurements and computational chemistry results suggest that the inherent size of the PEI polymer coils remains unchanged on the addition of LiNO3 but the number of polymer coils increases. A linear relation was found to exist between the permeance of the membranes to air and micropore volume. Larger micropores were present at higher LiNO3 concentrations and caused a decline in the actual separation factor for these membranes.