Interfacial confinement-induced β-phase enrichment in fiber-reinforced PVDF composite membranes for oil-in-water emulsion separation

Conventional poly (vinylidene fluoride) (PVDF) membranes often exhibit limited permeation and progressive fouling during oil-in-water emulsion separation. Herein, a fiber-reinforced PVDF composite membrane was fabricated by integrating a low-density polyethylene/polyamide 6 island-in-the-sea nonwoven scaffold, nonsolvent-induced phase separation, and selective fiber opening, without piezoelectric particles or high-voltage post-poling. Removal of the low-density polyethylene ‘sea’ component exposed an interconnected polyamide 6 ultrafine-fiber network within the porous PVDF matrix, improving pore accessibility and continuous water transport. Interfacial interactions, low-temperature coagulation, and structural reconstruction promoted enrichment of the polar β phase to above 80%. Density functional theory calculations further indicated that β-phase PVDF possessed higher polarity and stronger interactions with the fiber components than the α phase. The optimized membrane achieved emulsion permeances of 2600–3200 L m −2 h −1 bar −1 and separation efficiencies of 97–99%. Under pulsed hydraulic filtration, reproducible voltage signals of approximately 0.8 V confirmed an in situ pressure-induced piezoelectric response. During a 72 h test with a sodium dodecyl sulfate-stabilized vacuum-pump-oil emulsion, the membrane retained approximately 81% of its initial permeance and maintained about 95% separation efficiency.