High molecular weight poly(vinyl pyrrolidone) induces hierarchical surface morphology in poly(vinylidene fluoride) membrane and facilitates separation of oil-water emulsions

Preparation of membranes with hierarchical surface morphology without delaying phase inversion process or increasing time lag between casting and contacting to non-solvent bath is reported. Poly(vinylpyrrolidone) (PVP) acts as a pore forming polymer in membrane preparation via the non-solvent induced phase inversion process. Herein, we report that molecular weight (Mn) of PVP greatly influences the surface morphology of poly(vinylidene fluoride) (PVDF) membrane. PVP of Mn ≥ 360 kDa induces hierarchical globular morphology on PVDF/poly[(methyl methacrylate)-co-(vinylbenzyl chloride)] copolymer membrane, without delaying phase inversion process or increasing time lag between casting of polymer solution and contacting to non-solvent. PVP (Mn ≥360 kDa) exhibited sole effect on the morphology, which did not depend on presence or absence of copolymer nor on the Mn of PVDF. Post modification of the blend membrane with polyethyleneimine gave superhydrophilic and underwater oleophobic membrane. The membrane could separate surfactant-free and surfactant-stabilized oil-water emulsions under applied pressure of 0.35 bar without need of frequent cleaning. A permeate water flux of about 250 L m⁻² h⁻¹ with > 99% rejection of oil and flux recovery ratio as high as > 99% was achieved. The membrane modules also showed similar antifouling property during oil-water separation to that of membrane coupons. PVDF/copolymer undergoes nano-phase separation in the casting solution in presence of PVP (Mn ≥360 kDa), formed nano-clusters which remained on the membrane surface. Novel effect of PVP on the surface morphology in continuous-mode membrane preparation, easy post modification, and low flux reduction (15–25%) during separation of oil-water emulsions highlight the potential of the membrane in practical applications.