Single-step coating of polyethylenimine on gradient nanoporous phenolics for tight membranes with ultrahigh permeance

Ultrafiltration membranes with relatively small pores are highly expected in various separation cases. As a flexible manner to efficiently tighten the pores, post modification provides a robust platform for preparing the desired membranes. However, the endeavor to pursue high permselectivity of the membranes has been made by paying close attention to the modification layers while the effects of the substrate membranes are generally overlooked. In this work, tight membranes with ultrahigh permeance were produced by coating polyethylenimine (PEI) on gradient nanoporous phenolic substrates. Cationic PEI was tightly deposited on the negatively charged phenolics via electrostatic deposition, thus reducing the effective pore sizes to ∼2.6 nm through a single-step coating. The gradient porous nanostructures still remained after PEI coating in the phenolic membranes. The coated membranes exhibited ∼4–45 times higher permeance than counterparts with similar rejections. As the cationic PEI was distributed on the membrane surface and pore wall after coating, the positively charged membranes thus produced were able to efficiently separate cationic dyes from water.