Nanostructure of Poly(<i>N</i>‑isopropylacrylamide)
Brush at the Air/Water Interface and Its Responsivity to Temperature
and Salt
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Abstract
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<sup>–</sup> > Cl<sup>–</sup> >
Br<sup>–</sup>, which is in agreement with the Hofmeister series