Poly(ethylene Lerephthalate) track etched ultrafiltration membranes (110 nm pore diameter) have been functionalized with the thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAAm) via surface-initiated Atom Transfer Radical Polymerization (SI-ATRP). The PNIPAAm chain lengths, i.e. degree of graft functionalization, inside the membrane pores could be controlled very well with polymerization time. Importantly, gas flow/pore dewetting permporometry measurements demonstrated that the pore diameter in the dry state could be reduced and that the narrow pore size distribution of the membranes was not changed during the grafting process. Both hydrodynamic pore diameters of the membranes and grafted hydrodynamic layer thickness on the pore walls as well as their response to temperature could be estimated by measuring water permeability and applying Hagen-Poiseuille law. Defined temperature-induced swelling/deswelling ratios of 2 had been observed. These data indicate that PNIPAAm chains in the brush state had been achieved. The ultrafiltration membrane pores could be switched between more open and more closed states. For example, the hydrodynamic pore diameter could be switched from 21 nm at 23 degrees C to 69 nm at 45 degrees C. For the same type of membrane the rejection of monomodal 21 nm silica nanoparticles could be switched from 99% at 23 degrees C to only 35% at 45 degrees C. The rejection for larger monomodal 35 nm silica nanoparticles was above 90% for every functionalized membrane irrespective of the temperature. For an exemplary functionalized membrane evidence for a switchable size-selective NP fractionation has been found. A mixture of the 21 and the 35 nm silica nanoparticles was ultrafiltered through the membrane and at 23 degrees C only the smaller ones could be found in the permeate whereas at 45 degrees C also the larger nanoparticles were able to pass the membrane. (C) 2013 Published by Elsevier By.