Polymer ultrafiltration membranes with high water flux have long been pursued to improve filtration efficiency and reduce costs of industrial separation processes such as water treatment and food processing. However, increasing water flux is usually accompanied by decreased rejection property in these membranes. In this paper, we show that by using a polymer functionalized with hydrophobically ionizable groups as additive, polymer ultrafiltration membranes with an increasing concentration of the additive from the active layer to the supporting layer are produced in the membrane casting process due to slow hydration of the additive. The hydrated additive becomes hydrophilic after the membrane formation. The resulting membranes exhibit dramatically enhanced water permeability while maintaining excellent separation property. We demonstrate that polyethersulfone/tris(2,4,6-trimethoxyphenyl)polysulfone- methylene quaternary phosphonium chloride (PES/TPQP-CI) membrane exhibits a water permeability of up to 14.61 m(-2) h(-1) kPa(-1), which is 35 times that of PBS membrane. The membrane also shows excellent anti-biolouling property. The work provides a new methodology for designing high-performance membranes for improving separation efficiency and exploring other novel applications. (C) 2014 Elsevier B.V. All rights reserved.