Asymmetric hollow fibers by polyimide and polybenzimidazole blends for toluene/iso-octane separation

Among the various separation technologies available, pervaporation has been regarded as a very promising one for aromatic/aliphatic hydrocarbons separations. Works in the past have already identified many polymers as suitable membrane materials for these targeted separations. As an effort toward practical application, this current work was aimed at the formation of asymmetric polymeric membranes. The best performance achieved in this study has been a separation factor of ∼200 and a flux of ∼1.35 kg/m2-h in separating toluene/iso-octane (50/50 wt.%) at a downstream pressure of ∼10 mbar. These membranes were fabricated from blends of commercially available polybenzimidazole (PBI) and polyimide Matrimid using dry-jet wet-spinning followed by non-solvent induced phase inversion. Information from DSC and SEM analyses confirmed the molecular miscibility of the two polymers within the range of their weight ratio chosen. The blue shift for the characteristic peak of phthalimide carbonyl group in Matrimid revealed in FTIR evidenced the formation of hydrogen bonding between Matrimid and PBI, which is an important factor helpful for miscibility. From pervaporation performance tests results, it was observed that increasing PBI content gave rise to a higher separation factor for the selective transport of toluene, while total flux was correspondingly reduced. The effect of PBI on separation performance was attributed to its stronger polarity, tighter and more rigid structure, as well as enhanced anti-swelling property. It was also found that a higher spinneret temperature and a longer air-gap have positive influences on the pervaporation performance. Tensile strength test results revealed that all the hollow fibers held reasonable mechanical properties.