Solute- and temperature-responsive "smart" grafts and supported membranes formed by covalent layer-by-layer assembly

Polymers like poly(N-isopropylacrylamide) (PNIPAM) exhibit lower critical solution temperature (LCST) behavior. A variety of reports have shown that brush grafts of PNIPAM on surfaces exhibit similar temperature responsiveness. We recently described an alternative synthetic approach to such surfaces that affords surfaces with similar LCST-like behavior. We also noted how such surfaces' wettability can change in response to the identity and concentration of solutes. Here we show that this synthetic procedure can be extended to glass surfaces and to more complex surfaces present in porous glass frits. Functionalized glass surfaces exhibit solute-dependent wetting behavior analogous to that previously reported. We further show that the resulting responsive nanocomposite grafts on such frits exhibit "smart" responsive permeability with a greater than 1000-fold difference in permeability to water versus aqueous solutions of sodium sulfate. This "smart" permeability is ascribed to the solute-dependent wettability behavior of the responsive PNIPAM component of the nanocomposite graft, which is sensitive both to the identity and concentration of the solute anion and to temperature.