Nanostructure of Poly(<i>N</i>‑isopropylacrylamide) Brush at the Air/Water Interface and Its Responsivity to Temperature and Salt

Nanostructure and transition of the poly­(<i>N</i>-isopropylacrylamide (PNIPAm) brush at the air/water interface were investigated by π-<i>A</i> isotherm and X-ray reflectivity, and an interesting behavior was observed with the change in temperature and salt. The polymer monolayer of poly­(<i>n</i>-butyl acrylate)­(P<i>n</i>BA)-<i>b</i>-PNIPAm on the water surface showed a transition between carpet-only/carpet+brush structures as a function of brush density, which was controlled by compression/expansion, as was the case for ionic brush systems. The brush stretching factor was about 50%, which was slightly less than that for a strongly ionic brush. The number of water molecules inside the brush layer was estimated to be 11–13 per repeating unit of PNIPAm chain. This value is very close to the number of hydrated water molecules reported, which means that all the water molecules inside the brush layer were hydrated water. With elevating temperature, the PNIPAm brush shrank, and the number of water molecules in the brush layer was reduced to 3. These observations certainly indicated a dehydration process. Interestingly, a part of the PNIAPm chain formed a “hydrophobic PNIPAm layer” on the carpet layer under the P<i>n</i>BA hydrophobic layer. A similar transition was observed also by the addition of salt to the water subphase. Although the formation of “hydrophobic PNIPAm layer” was not observed in this case, shrinking of the brush was observed with increasing salt concentration, and finally it became a carpet-only structure, which contained no water molecules. This salt effect was found to be ion specific, and its effectiveness was in the order of F^– > Cl^– > Br^–, which is in agreement with the Hofmeister series

pH-Sensitive Adsorption Behavior of Polymer Particles at the Air–Water Interface

pH-Sensitive adsorption of polymer particles bearing poly­[2-(dimethylamino)­ethyl methacrylate] hairs at the air–water interface was investigated using a surface tensiometer, a Langmuir–Blodgett trough, and an X-ray reflectometer. We clarified that the particles are adsorbed at the interface at basic pH; by contrast, at acidic pH, only a small number of particles are adsorbed, whereas the majority are dispersed in the water phase. X-ray reflectometry analysis revealed that a particle monolayer was formed at the air–water interface, which was packed densely under increasing surface pressure, as determined by the electron density profile change. The contact angles of the particles at the air–water interface were estimated to be 29° and 34° at pH 3 and 10, respectively, by direct visualization of the air–water interface position of the particles using a polycyanoacrylate trapping method