Designing integrated Janus membrane based on a double-cross-linked polyphenol hydrophilic layer for complex emulsion separation

•A novel double-cross-linked hydrophilic coating comprising TA–PEI and TA–Cu2+ was developed to construct integrated Janus membranes.•Coating considerably promotes membrane surface hydrophilicity and stability.•The integrated Janus membranes were employed to separate the complex dye emulsion containing surfactant.•Prepared network shows high water flux (1061 L m−2 h−1) and improved rejection (99%).•Proposed facile scale-up method exhibit a remarkable potential for practical applications. Efficient separation materials with multifunctional and switchable properties are urgently needed to treat submicron-scale oily droplets. Herein, tannic acid (TA) and polyethylenimine (PEI) were blended with poly(vinylidene fluoride) (PVDF) and used to coat a polydimethylsiloxane (PDMS)-modified non-woven fabric immersed in a CuSO4/H2O2 coagulation bath. TA and PEI form a phenolamine hydrophilic network that migrates to the surface and firmly binds to the PVDF substrate. CuSO4 catalyses the H2O2 oxidation, promoting the self-polymerisation of phenol hydroxyl group for forming a carboxyl group. Furthermore, the remaining Cu2+ ions complex with TA to form a metal–phenol cross-linked network. A double-cross-linked hydrophilic layer with polyphenols as connection points was formed on the PVDF surface. The hydrophilic contact angle of the membrane reached 0°. A rough structure with PDMS micro-/nanoparticles was constructed on the PVDF underside via surface nanocasting and the hydrophobic contact angle of the membranes reached 165°. In experiments, each side of the obtained Janus membranes demonstrated surface superhydrophilic and superlipophilic properties and a high flux and excellent separation efficiency for various oil–water emulsions. The integrated Janus membranes are suitable for unidirectional liquid collection, oil–water emulsion separation, cationic pigment adsorption and dealing with complex dye–emulsion pollutants. The integrated structure formed by the hydrophilic layer and the membrane enhances the fouling resistance of the membrane and the removal rate of pollutants reached >99 %. This research presents a simple approach to prepare Janus membranes with asymmetric wettability.