The Implications of Microstructure on the Performance of Integrally Skinned Polyimide Nanofiltration Membranes
Due to their excellent resistance to a range of solvents, integrally skinned polyimide membranes have been used to achieve selective separations in a range of industrial and lab-scale chemical operations. These include: homogeneous catalyst recycle, petrochemical dewaxing, solvent exchange and chiral resolutions. However, despite the widening scope of use of these membranes, there is still little understanding of how the microstructure defines their separation performance. In particular, the following questions have yet to be answered:
• What are their microstructure?
• What is the transport mechanism through these membranes (i.e. how do we reliably predict the separation that these membranes give)?
• How do solvents affect their microstructure?
As a first step towards answering these questions, integrally skinned nanofiltration membranes were fabricated by phase inversion using Lenzing P84 polyimide. The microstructure of these membranes, dry and in solvent, were investigated by SEM, TEM and ESEM (where appropriate).
SEM and TEM imaging revealed that this type of polyimide membrane have three microstructurally distinct polymide layers, not two as traditionally thought. Furthermore, TEM images reveal nano-sized pores in the polyimide structure, which indicate that the transport mechanism is probably neither only solution-diffusion nor only pore-flow. ESEM imaging showed that when saturated in ethanol, these membranes also exhibit considerable structural swelling. Therefore, the relationship between changes in microstructure and separation performance of the membrane may need to account for the changes in all three layers. Overall, these results indicate that the current theory used to describe polyimide membrane separation performance must be rethought.