  The structural aspects of polyacrylamide thin films annealed at degradation
threshold temperature have been studied as a function of annealing time using
in situ X-ray reflectivity technique in vacuum. We observe significant decrease
of thickness and increase of density with annealing time for all the films. The
dynamical behavior of the changes was modeled in terms of two distinct
exponential decay functions, following our earlier observation of two different
time scales for the chemical modification pathways, and was found to be in
excellent agreement with the data. The diffusion coefficients of the polymer
chains corresponding to the two modes are found to be different by an order of
magnitude. It was found that the two dynamical modes correspond to the
formation of two degradation products at two different rates. The larger time
constants for both the modes in case of thickness reduction compare to the
chemical changes was explained in terms of inter-chain entanglement and
attachment of the polymer with the substrate.
