Membrane characterisation by SEM, TEM and ESEM: The implications of dry and wetted microstructure on mass transfer through integrally skinned polyimide nanofiltration membranes

SEM and TEM imaging of dry membranes revealed that this type of polyimide membrane has three microstructurally distinct polyimide layers, not the two indicated in prior literature. Furthermore, TEM images reveal nano-sized pore-like features 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 at the working pressure of the ESEM (5.50 Torr), the microstructure of the membranes changes; it is wispy and thus quite different to the more solid polymer nodules and interconnected polymer network observed in the dry membranes. Thus, transport and separation mechanisms based on the structure of the dry membranes may not be accurate. Overall, these results indicate that the current theory used to describe polyimide membrane mass transfer and separation performance (where it is based on dry membrane microstructures), most likely needs to be rethought.