Polystyrene films supported on silicon substrates form an important model system for studying the effects of confinement on polymer flow. X-ray scattering is a useful tool for such studies.

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Structural and Dynamical Studies of Thin Polymer
Films Using X-ray Scattering Sunil K. Sinha
(UCSD) and Laurence Lurio (NIU)   DMR-0209542
Polystyrene films supported on silicon substrates
form an important model system for studying the
effects of confinement on polymer flow. X-ray
scattering is a useful tool for such studies.
Coherent x-ray beam
Scattered X-rays
For thick polymer films the capillary wave
dynamics is very well described by classical
hydrodynamics. An important result of the present
study is the observation of deviations from
predicted hydrodynamic behavior when the film
thickness approaches the size of the polymer
coils. This behavior can only be explained as the
combination of two effects. 1) For polymer films
which are very thin the polymers are not as
entangled as they are in the bulk material. This
results in an increased mobility. 2) Parts of
the polymer chain get stuck or adsorbed on to the
bottom substrate. This gives the polymeric liquid
a resistance to shear which normal liquids do not
have.
Si Substrate
Polymer films are rough because thermal energy
excites waves on their surfaces called capillary
waves. The shape of the rough surface slowly
changes over time. By following the motion of
the surface, the hydrodynamic properties of the
film can be studied. The figure on the right is
a schematic of how a polymer surface fluctuates.
On the left is a time sequence of measured x-ray
scattering images from a polymer surface.
Analysis of the changes in the x-ray patterns
with time reveals the film dynamics
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Structural and Dynamical Studies of Thin Polymer
Films Using X-ray Scattering" Sunil K. Sinha
(UCSD) and Laurence Lurio (NIU)   DMR-0209542
Education This work formed part of the Ph.D.
thesis of UCSD graduate student Zhang Jiang. In
addition, it was the basis of the Masters thesis
of NIU graduate student Jarrett Stark, and formed
part of the post-doctoral training of Dr.
Xuesong Jiao. NIU undergraduate engineering
student Yagnesh Trivedi, designed the vacuum and
thermal enclosure used for parts of these
studies. Dr Jiao is presently a staff scientist
at the Advanced Photon Source. Jarrett Stark,
after completing his Masters degree in 2005, is
working in the semiconductor division of the
United States Patent Office
Societal Impact Understanding the mechanical,
elastic, and flow properties of polymers at the
molecular scale is important for applications
such as coatings, insulation, adhesives,
packaging material and in prosthetic devices.
The results of the present work will provide a
basis for an improved theoretical understanding
these phenomena One example of possible
applications is IBMs millipede project, which
aims to replace magnetic hard drives on computers
with arrays of small holes melted in polymer
films. The success of such a device would depend
on understanding molecular scale polymer flow and
melting.