  The effect of annealing on the dynamical behavior of swelling for ultrathin
polyacrylamide films deposited on silicon substrates have been studied using
X-ray reflectivity technique. The spin coated polyacrylamide films of similar
thicknesses were annealed at various temperatures below and above the glass
transition temperature of the polymer. The electron density of the films was
found to increase systematically on annealing. The swelling dynamics of the
annealed films were found to have systematic dependence on the temperature of
annealing. The interaction between the substrate and the polymer molecules was
found to play important role in the swelling dynamics of the annealed films
unlike our earlier observation with as coated films. The chain segments
attached directly to the substrate were believed to have restricted freedom of
movements compared to the ones that are at a distance from the substrate and
relatively free. Accordingly, the dynamical behavior of swelling was modeled in
terms of the combination of a free and a restricted component and was found to
be in excellent agreement with the data. The diffusion coefficients
corresponding to the restricted polymer segments were an order of magnitude
smaller than those of the free segments and the fraction of the same was found
to increase with annealing at higher temperatures. The overall reduction of
swellability of the films was explained in terms of the increase of density of
the films and the segmental attachment to the substrate on annealing.

Soft Condensed Matter;Materials Science