The Role of Polymeric Additives on the Morphology and Performance of a Porous Layer of Nanofiltration Composite Membranes Based on Polyether Sulfone

The role of polymeric additives such as PVP and PEG is studied with respectto the morphology of PES porous layer as a sublayer of nanofiltration compositemembranes based on PES/PA. Results show that by phase inversion process of quaternary systems comprised of four components of polymer/solvent/nonsolvent/additive and the diffusion of intertwined polymers some changes occur inmembrane morphology with changes in their concentration. With addition of PVP,tear-like pores, finger-like and channel-like morphology change to enlarged channelcavities and by adding more PVP, membrane morphology changes further and spongyregions are extended in the membrane. Presence of PEG in casting solution delayedthe precipitation time. By adding PEG, the solution viscosity is increased which is followedby decreases in diffusion rates of solvent/non-solvent in coagulation bath.Therefore, membrane morphology shifts to small pores and spongier region. Anothereffect of increased PEG content would be deformed PA layer formation in PES sublayerwhich affects membrane performance. However, PVP as an additive does notchange membrane salt rejection very much while it leads to higher fluxes. A membranewith 2.5 percent PVP would perform by 40 percent flux increases, while amembrane with 5% PVP shows flux reductions even below the initial value. Contraryto PVP, the PEG content of 20 percent leads to 4 folds flux increases and in a membranewith 50 percent PEG, there is a flux increase by 7 folds and drop in salt rejectionoccurs by 50 percent and 70 percent, respectively