Much attention has been directed to the development of sophisticated membranes that can regulate the permeability in response to environmental changes. In this study, pi-I-sensitive membranes were prepared by grafting of poly(acrylic acid) (PAAc) onto a porous Nuclepore membrane. The filtration rate of a membrane with a PAAc graft density of 0.30 mu g/cm(2) was 28 times higher at pH 2.4 than at pH 5.4. Atomic force microscopy (AFM) was employed to make clear how the PAAc graft chains regulate the filtration rate. The thickness of PAAc graft layers was determined from the force curve in buffered solutions of various pHs. It was found that the graft layer thickness was several tens of nanometers at pH 2.6 and increased to 20-430 nm at pH 7.6, depending on the PAAc graft density. The PAAc chains grafted on the membrane surface dynamically changed their configuration in response to the medium pH. In addition, AFM images demonstrated that the graft chains shrank and precipitated on the surface of the membrane and the wall of pores at acidic pi-Is, thereby opening the pores of the membrane, whereas they hydrated and thus effectively closed the pores at neutral and alkaline pHs. The hydrodynamic permeation in conjunction with AFM observation of the graft layers allowed us to conclude that the PAAc graft chains dynamically opened and closed the pores in response to the medium pH, functioning as a molecular valve to regulate the permeation characteristics.