Dry-wet phase inversion block copolymer membranes with a minimum evaporation step from NMP/THF mixtures

Block copolymer (BCP) membranes are a very promising new type of membranes, but often have a difficult fabrication method that involves a long evaporation step prior to phase inversion. In this work, we study new novel BCP phase inversion recipes for the fabrication of asymmetric membranes with a thin, ordered isoporous selective layer on top of a highly interconnected porous support layer. A key aim is to shorten the evaporation time while simultaneously allowing the formation of even thinner selective top layers. Asymmetric membranes were fabricated via the combination of polystyrene-block-poly(4-vinyl pyridine) self-assembly, solvent evaporation and liquid induced phase separation. Using a solvent mixture of THF and NMP, a selective top layer of just 60 nm thick was formed with an ordered honeycomb-like pore structure. The formed structure depended on several parameters, such as THF/NMP ratio, polymer concentration of the polymer solution and the duration of solvent evaporation. When a high THF/NMP ratio was used (more THF than NMP) the solvent evaporation step could be reduced to only 1 s, a clear advantage when considering scale up of this approach. The THF/NMP ratio also influenced the morphology of the support layer, which translated into a variety of permeabilities (270–1320 L m−2 h−1 bar−1). Filtration experiments showed that the different top layer structures result in different filtration performance, with more ordered pores resulting in more selective filtration.