Roughness-enhanced thermal-responsive surfaces by surface-initiated polymerization of polymer on ordered ZnO pore-array films

The correlation between stimuli-responsive wettability switching and surface morphology of poly(N-isopropylacrylamide) (PNIPAm)-modified ZnO pore-array surface is studied. PNIPAm is grafted to the ZnO pore-array surface by surface-initiated polymerization. The effects of thickness of the PNIPAm layer and surface morphology on the thermally responsive switching behavior of the PNIPAm-modified films were studied considering the influences 3D capillary effect (Wenzel's model) and air trapping effect (Cassie's model). The air trapping effect can be controlled by the pore size, amount of grafted PNIPAm polymer, and shape of the pore edge. The surface roughness and expanded pore edge with mushroom-like texture of S2 film amplify the thermally responsive wettability switching between hydrophilicity and hydrophobicity. When the pore structure is completely filled, the switching properties get lower. The thermally responsive switching behavior is enhanced with increasing molecular chain length of the PNIPAm. By controlling the ZnO pore-array structure and the amount of grafted PNIPAm layer, the S2 film shows excellent reversibility for more than 3 cycles and a quick transformation between hydrophilicity and hydrophobicity.