posted on 2012-02-21, 00:00authored byEva Bittrich, Sina Burkert, Martin Müller, Klaus-Jochen Eichhorn, Manfred Stamm, Petra Uhlmann
Temperature-sensitive poly(N-isopropylacrylamide)
(PNIPAAm) brushes with different molecular weights Mn and grafting densities σ were prepared by the
“grafting-to” method. Changes in their physicochemical
properties according to temperature were investigated with the help
of in situ spectroscopic ellipsometry and in situ attenuated total
reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Brush
criteria indicate a transition between a brush conformation below
the lower critical solution temperature (LCST) and an intermediate
to mushroom conformation above the LCST. By in situ ellipsometry distinct
changes in the brush layer parameters (wet thickness, refractive index,
buffer content) were observed. A broadening of the temperature region
with maximum deswelling occurred with decreasing grafting density.
The brush layer properties were independent of the grafting density
below the LCST, but showed a virtually monotonic behavior above the
LCST. The midtemperature ϑhalf of the deswelling
process increased with increasing grafting density. Thus grafting
density-dependent design parameters for such functional films were
presented. For the first time, ATR-FTIR spectroscopy was used to monitor
segment density and hydrogen bonding changes of these very thin PNIPAAm
brushes as a function of temperature based on significant variations
of the methyl stretching, Amide I, as well as Amide II bands with
respect to intensity and wavenumber position. No dependence on Mn and σ in the wavenumber shift of these
bands above the LCST was found. The temperature profile of these band
intensities and thus segment density was found to be rather step-like,
exceeding temperatures around the LCST, while the respective profile
of their wavenumber positions suggested continuous structural and
hydration processes. Remaining buffer amounts and residual intermolecular
segment/water interaction in the collapsed brushes above the LCST
could be confirmed by both in situ methods.