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Abstract The PVDF membranes were prepared via thermally induced phase separation (TIPS) method with saline water as quenching bath and applied to the separation of oil/water emulsions. Surface pore size, permeation performance and other properties of the PVDF membranes were finely tuned by adjusting salt concentration in quenching bath. The results show that with the addition of NaCl in quenching bath, the thermal transfer resistance increases in the TIPS process due to the obvious increase of bath viscosity and the slight decline of thermal conductivity of saline solution. All the PVDF membranes present bi-continuously fibrillar structure in top surfaces. The fibrils are fused together and get thicker, and leads to rougher top surfaces due to the decline of cooling rate with the addition of NaCl in quenching bath. At the same time, the PVDF membranes exhibit expanded pore size and higher permeation flux for both pure water and oil/emulsions. The PVDF membrane of S20 (prepared with 20 wt% NaCl aqueous solution as quenching bath) exhibits the largest mean pore size and the highest pure water permeation flux of 2481.1 L m−2 h−1 bar−1. However, overhigh NaCl content leads to the inverse trend. For example, the PVDF membrane of S30 (quenching bath containing 30 wt% NaCl) exhibits a lightly decreased pore size and pure water permeation flux compared to S20. All of the PVDF membranes exhibit good oil rejections above 90.0% for surfactant-stabilized oil-in-water emulsions and above 95.0% for surfactant-free oil-in-water emulsions. Highlights • PVDF membranes are prepared via TIPS method with saline water as quenching bath. • Pore size and permeation performances of PVDF membranes are finely tuned. • Thermal transfer resistance rises with the increase of NaCl concentration. • Pore size of PVDF membranes increases with the increase of salt concentration. • Permeability of PVDF membranes increases with the increase of salt concentration. [ABSTRACT FROM AUTHOR]

Permeability and mechanical strength normally present an opposite trend for separation membranes. There is a trade-off line of permeability vs. tensile strength for the prepared poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation (TIPS) method. In this work, TIPS-prepared PVDF membranes were fabricated with water (PVDF T-water) and air (PVDF T-air) as quenching bath for comparatively studying. PVDF T-air membranes present larger average pore size, higher permeability and better mechanical strength than those of PVDF T-water membranes due to the lower cooling rate of air bath. It is wonderful that all PVDF T-air membranes can break though the trade-off line of permeability vs. tensile strength of TIPS-prepared PVDF membranes. The optimal membrane (M-A30) shows superior performance, such as ultrahigh water flux of 3275 L m−2 h−1·bar−1 and tensile strength of 5.7 MPa. Additionally, this research provides a feasible method to regulate the morphology, pore size and permeability of TIPS-prepared PVDF membranes. • PVDF membranes (PVDF T-air) are prepared via TIPS method with air quenching bath. • PVDF T-air membranes overcome the upperbound of permeability vs. tensile strength. • PVDF T-air membranes have much higher permeability than PVDF T-water membranes. • Pore size increases with the increase of quenching temperature. [ABSTRACT FROM AUTHOR]

Janus membrane is an emerging separation material due to its unique separation performance. The asymmetric surface wettability on its two surfaces is crucial for a Janus membrane. In this work, robust Janus polyvinylidene fluoride (PVDF) membranes were prepared via thermal induced phase separation (TIPS) accompanied with spray-coating method. The two surfaces of TIPS-prepared PVDF membranes were rapidly deposited with polydopamine/polyethyleneimine (PDA/PEI) and micro-/nano-scale silica particles crosslinking with diphenyldichlorosilane respectively. The spray-coating endows the Janus membranes with superhydrophilicity and superhydrophobicity on its opposite surfaces. The optimized membrane of M-A0.9B0.2 presents an asymmetric surface wettability. Its hydrophilic surface has a water contact angle (WCA) of 11.2°, while the hydrophobic surface has a WCA of 169.9° as well as a sliding angle of <1°. The superhydrophilic surface allows water to quickly pass through the membrane and oil phase is trapped; while oil can penetrate through the membrane and water is rejected by the superhydrophobic surface. The Janus membranes show excellent separation performance with the highest permeation of 2060.5 L m−2 h−1·bar−1 accompanied with above 94.0% oil rejection for toluene-in-water emulsion and the highest permeation of 1769.0 L m−2 h−1·bar−1 accompanied with above 97.0% water rejection for water-in-toluene emulsion using the opposite surfaces. Image 1 • A robust Janus membrane with TIPS-prepared PVDF substrate. • Superhydrophilic and superhydrophobic opposite surfaces were constructed. • The Janus membrane can efficiently separate both O/W and W/O emulsions. [ABSTRACT FROM AUTHOR]