Investigation of the fundamental differences between polyamide-imide (PAI) and polyetherimide (PEI) membranes for isopropanol dehydration via pervaporation

Torlon^® 4000TF polyamide-imide (PAI) and Ultem^® 1010 polyetherimide (PEI) membranes were fabricated and studied for isopropanol dehydration by pervaporation. The properties of these two materials and the membranes fabricated were tested and compared through different characterizations (DSC, TGA, Goniometer, X-ray diffraction, gas permeation, and water sorption). Compared with PEI dense membranes, PAI dense membranes show a much higher separation factor (up to 3000 at 60 °C) and comparable flux. Higher hydrophilicity, narrower d-space and higher water uptake of PAI membrane confirmed by above characterizations all contribute to its higher pervaporation performance. The pervaporation results of the asymmetric membranes show that the polymer concentration for membrane casting is very important. For both PAI and PEI membranes, dope concentrations equal to or higher than their critical concentrations are essential to produce useful pervaporation membranes. In addition, heat treatment is needed to reduce defects and enhance separation performance. Different operation modes were also studied. The separation using a membrane with porous structure facing against the feed solution shows a much higher separation factor with only a slight decrease of flux. This important phenomenon is explained in terms of the balance between two major contradictory effects: concentration polarization and dense layer swelling.