A nonionic polymer-brush-grafted PVDF membrane to analyse fouling during the filtration of oil/water emulsions

Abstract Hydrophilic polymer brushes on membrane surfaces can alleviate fouling during the separation of oil/water emulsions; the charged moieties on the polymer brushes presumably interact with charged surfactants and weaken the membrane fouling behaviour. However, the oil-repellence mechanism of nonionic polymer brushes has not been thoroughly examined. In this study, a nonionic poly(N-acryloyl morpholine)-brush-grafted-poly(vinylidene fluoride) membrane (PVDF-g-PACMO) was fabricated via surface-initiated atom transfer radical polymerisation (ATRP) to separate various surfactant-stabilised emulsions. PVDF-g-PACMO membranes with high hydrophilicity and high permeation were successfully fabricated by controlling the duration of ATRP. Furthermore, fouling models of oil/water emulsions stabilised by anionic, nonionic, and cationic surfactants were examined. Multicycle filtration experiments revealed that the anionic surfactant-stabilised emulsions severely fouled the similarly charged PVDF-g-PACMO membranes owing to electrostatic repulsion, whereas the nonionic and cationic surfactant-stabilised emulsions exhibited reduced irreversible fouling. In particular, an unusual relief in irreversible membrane fouling was observed with respect to the cationic surfactant-stabilised emulsions, presumably because of strong demulsification and steric effects (surfactant barrier), which repelled the penetration of oil droplets into the internal pores and averted pore blockage. These observations suggest that oppositely charged surface potentials of the PVDF-g-PACMO membranes compared to that of the oil/water emulsions assist in mitigating irreversible fouling. The mechanisms examined herein described the relationship between the surface properties of the oil/water emulsions and the tendency for irreversible fouling, thereby providing valuable guidance for sustainable separation of oil/water emulsions.