  Density of the polyacrylamide ultrathin films has been studied using X-ray
reflectivity technique. Two sources (one powder and another aqueous solution)
of polyacrylamide were used to prepare spin coated films on silicon substrate.
Light scattering measurements show that the polymer chains were unentangled in
a concentrated (4 mg/ml) as well as in a dilute (2 mg/ml) solution prepared
from the powder, whereas the solution (4 mg/ml) prepared by diluting the
solution source shows entangled chain morphology. Three sets of films of
different thicknesses were prepared using the three solutions by spin coating
on silicon substrates. Comparison of X-ray reflectivity data for as prepared
and dry films reveals that the shrinkage of the films decreases with increasing
thickness. Average electron densities of the films were found to follow a trend
of higher density for thinner films with a maximum increase of about 12%
compared to the bulk. The densities of all the films irrespective of the nature
of entanglement and concentration of their source were found to increase with
spin speed of coating and attain saturation at higher speed. Absence of
correlation between shrinkage and density data and the fact that the densities
of all the films follow a master curve irrespective of their origin suggest
that the higher density of the films result from the higher orientation of
chains as a consequence of an interplay between stretching and stronger
attractive interactions of polar nature.
