Synthesis of patterned PVDF ultrafiltration membranes: Spray-modified non-solvent induced phase separation

Patterned membranes have been attractive for the mitigation of fouling in membrane processes, but existing patterning methods are rather in-effective in terms of scalability and performance due to pore-size reduction and patterning at the non-selective membrane side. Polyvinylidene fluoride (PVDF) was used now to synthesize patterned membranes via the spray-modified non-solvent induced phase separation (s-NIPS), a recently developed technique that avoids the current drawbacks of patterned membranes. However, PVDF intrinsically suffers from a slow phase separation, while very fast fixation of the patterns is crucial with this synthesis method. Therefore, the casting solution was optimized towards an accelerated process of phase inversion by systematically studying the effect of polymer concentration, addition of poly(vinylpyrrolidone) (PVP), addition of non-solvent (H2O) and use of different casting solvents for adequate patterning of the membranes. SEM analysis and pure water permeance of the synthesized membranes assessed the homogeneity and pattern formation for each membrane. The most optimal casting solution composition comprised of 20 wt% PVDF, 6.7 wt% PVP and 1 wt% H2O which resulted in homogenously patterned PVDF membranes. The optimized patterned membrane showed a 9 fold increase in PWP as compared to the reference flat membrane thanks to the addition of PVP and H2O as well as the additional surface area. In the filtration of proteins, the water permeance of the membranes improved drastically (+140%) upon patterning with only a moderate loss of BSA rejection (from 90% to 71%) as compared to the corresponding flat membrane. Realisation of patterning via the s-NIPS method resulted in a higher effective membrane surface area combined with an increased membrane porosity. Reduced flux decline of the patterned membrane (52%) as compared to the flat membrane (62%) during continuous BSA filtration also proved the anti-fouling potential of the created patterns.