A polyionized hydrogel polymer (sodium polyacrylate-grafted poly(vinylidene fluoride) (PAAS-g-PVDF)) is fabricated via an alkaline-induced phase-inversion process. PAAS-g-PVDF coatings exhibit unprecedented anti-adhesion and self-cleaning properties to crude oils under an aqueous environment. A PAAS-g-PVDF-coated copper mesh can effectively separate a crude oil/water mixture with extremely high flux and high oil rejection driven by gravity, and is oil-fouling-free for long-term use.