Ultrathin bimodal porous membranes with uniform small pores separated by laminated large pores for efficient water separation

Bimodal pore structure design of separation membranes is a promising way to simultaneously improve permeability and selectivity. However, the thickness limitation of bimodal porous membranes is an obstacle to further improving the separation efficiency. In this study, a novel bimodal porous structure with uniform small pores on the surface and a unique laminated large pore structure in the thickness direction was successfully constructed through biaxial stretching of gel films in ultra-high molecular weight polyethylene membranes. With an increase in the draw ratio to 12 × 12, the uniformly refined small pores increased the rejection rate of the carbon nanoparticle solution (particle size distribution of 60–400 nm) to 99.4%, and the unique laminated large pore structure enabled the permeance to reach 1141.8 L m-2h−1 bar−1. By further increasing the draw ratio to 16 × 16, the membrane thickness decreased to 0.91 μm and the permeance was further increased to 2008.2 L m-2h−1 bar−1 while the selectivity slightly decreased to 98.9%. Furthermore, a bimodal porous membrane with sufficient strength (237 MPa of 16 × 16) was efficient for long-term water separation. This study proposes a new design idea for ultrathin porous membranes, which can be further expanded to prepare various antifouling, self-cleaning, and multifunctional separation membranes.