Dynamic Molecular Behavior on Thermoresponsive Polymer
Brushes
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Abstract
The surface dynamics of individual
surfactant and polymer molecules
on thermally responsive polymer brushes (poly(<i>N</i>-isopropylacrylamide),
PNIPAAM) were studied using high throughput single molecule tracking
microscopy. The probe molecules universally exhibited intermittent
hopping motion, in which the diffusion switched between mobility and
confinement with a broad distribution of waiting times; this was analyzed
in the context of a continuous time random walk (CTRW) model described
using “waiting time” and “flight length”
distributions. We found that the surface mobility, which was affected
by waiting times and flight lengths, of both probe molecules increased
abruptly with temperature above the 32 °C lower critical solution
temperature (LCST) transition of the PNIPAAM brush. In particular,
above the LCST, where the polymer brush collapsed into a more hydrophobic
dense polymer film, the effective diffusion coefficients and mobile
fraction of probe molecule increased, suggesting that mobility was
inhibited by penetration into the brush at lower temperatures. Waiting
times at lower temperature were twice as long as at higher temperatures,
and the longest flight length increased from 0.9 to 1.8 μm.
Moreover, we found that the high density of strong binding sites available
on the swollen PNIPAAM brush led to long waiting times and a high
probability of readsorption, which resulted in short flight lengths,
while the absence of strong binding sites on collapsed PNIPAAM films
led to short waiting times and long flights