NSLS-II Seminar

Grazing incidence small angle scattering (GISAXS) and x-ray photon correlation
spectroscopy (XPCS) studies from silicon supported polystyrene (PS) molten films
having different thickness and different molecular weights of PS will be presented.
GISAXS measurements were made in x-ray standing wave geometry designed to enhance
the scattering from the interior of the film compared to surface and substrate scattering.
Melt films thicker than 4Rg show bulk like density fluctuations. The thinner films exhibit
a peak in structure factor S(q) near q = 0 which grows with decreasing thickness. This
peak is attributed to a decreased interpenetration of chains resulting in an enhanced
compressibility [1]. The effect of confinement is also evident from the wide angle
measurements where a liquid scattering ring corresponding to the side chains of the
polymers is observed in thick films. In thinner films the intensity in the scattering ring is
strongly concentrated along the surface normal direction indicating alignment of the side
chains parallel to the surface.
XPCS studies on polymer molten films reveal a transition from single through a stretched
to another single exponential regime as the temperature is decreased from well above to
near the bulk glass transition temperature. A universal scaling of the dynamics was
discovered over a wide range of film thicknesses, temperatures, and molecular weights
(except in the multiple relaxation regime). The observation is understood in a manner in
which the capillary wave relaxation behavior progresses through a reptation dominated
regime at high temperature (in the so-called terminal zone), followed by an intermediate
or rubbery regime and then a regime dominated by Rouse-like modes of the nonentangled
chain segments just above the bulk Tg. In the latter regime (known as the
transition zone), the relaxation of capillary waves becomes independent of the molecular
weight for above the critical value for entanglement