Separation of Oil-in-Water Emulsions Using Hydrophilic Electrospun Membranes with Anisotropic Pores

It has been challenging to separate oil from oil/water emulsions with droplet size less than 1 μm using conventional porous membranes. Membranes with small pores are preferred, but the trade-off is a dramatic reduction of volumetric flux. Here, we prepared membranes from electrospun polycaprolactone (PCL) fibers with high porosity (∼88%). When the membranes were stretched uniaxially at different strain levels, the pores became anisotropic with an aspect ratio (pore length/width) up to 5.3 ± 3.0. To improve their wettability, we added Tween 80, a hydrophilic surfactant, to PCL solutions for electrospinning. The modified PCL membranes showed excellent mechanical properties with a tensile strength at 6.59 ± 1.67 MPa and the elongation at break up to 130 ± 21%, warranting their use as free-standing separators. We narrowed the pore gap while maintaining the high porosity by stretching the membranes. Scanning electron microscopy (SEM) images of the stretched membranes show changes of pore geometry without altering the fiber size and fiber network integrity with strain up to 80%. The anisotropic membrane could exclude oil from oil-in-water emulsion droplets with a diameter as small as 18 nm without reduction of the volumetric flux in comparison with the nonstretched one.