Microstructure and thermo-responsive behavior of poly(N-isopropylacrylamide) brushes grafted in nanopores of track-etched membranes

We report on the surface-initiated polymerization by aqueous free radical polymerization (FRP) and atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM) from nanopore walls of poly(ethyleneterephthalate) (PET) membranes. By combining atomic force microscopy and transmission electron microscopy coupled with an electron energy loss detector, we show that grafting polymers from the walls is an efficient process leading to functional and switchable membranes. The thickness of PNIPAM brushes grafted within small and large nanopores could be measured by a template-based method, indicating that confinement impacts strongly the polymerization process. Finally, the temperature-modulated permeability properties of these responsive polymeric systems are evaluated. Depending on the membrane pore size, two types of permeation control mechanisms are observed. For large pore membranes (Φ ∼ 330 nm), expanded PNIPAM chains conformations (at T < LCST) result in reduced effective pore size and therefore lower permeabilities relative to collapsed macromolecules chain conformations. In contrast, for narrow pore membranes (Φ ∼ 80 nm), the PNIPAM layer grafted at the surface of the membrane becomes the rate-controlling factor; the expanded PNIPAM brush represents greater degrees of hydration in the surface layer and therefore gives rise to higher permeabilities than the collapsed conformation. In this situation, the overall permeability is thus comparable to that of a hydrogel membrane.