| Abstract
| - Much attention has been directed to the development ofsophisticated membranes that canregulate the permeability in response to environmental changes. Inthis study, pH-sensitive membraneswere prepared by grafting of poly(acrylic acid) (PAAc) onto aporous Nuclepore membrane. The filtrationrate of a membrane with a PAAc graft density of 0.30μg/cm2 was 28 times higher at pH 2.4 than atpH5.4. Atomic force microscopy (AFM) was employed to make clear howthe PAAc graft chains regulatethe filtration rate. The thickness of PAAc graft layers wasdetermined from the force curve in bufferedsolutions of various pHs. It was found that the graft layerthickness was several tens of nanometers atpH 2.6 and increased to 20−430 nm at pH 7.6, depending on the PAAcgraft density. The PAAc chainsgrafted on the membrane surface dynamically changed their configurationin response to the mediumpH. In addition, AFM images demonstrated that the graft chainsshrank and precipitated on the surfaceof the membrane and the wall of pores at acidic pHs, thereby openingthe pores of the membrane, whereasthey hydrated and thus effectively closed the pores at neutral andalkaline pHs. The hydrodynamicpermeation in conjunction with AFM observation of the graft layersallowed us to conclude that the PAAcgraft chains dynamically opened and closed the pores in response to themedium pH, functioning as amolecular valve to regulate the permeationcharacteristics.
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