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Polymer Film Surface Fluctuation Dynamics in the Limit of Very Dense Branching
The surface fluctuation dynamics
of melt films of densely branched
comb polystyrene of thickness greater than 55 nm and at temperatures
23–58 °C above the bulk <i>T</i><sub>g</sub> can be rationalized using the hydrodynamic continuum theory (HCT)
known to describe melts of unentangled linear and cyclic chains. Film
viscosities (η<sub>XPCS</sub>) inferred from fits of the HCT
to X-ray photon correlation spectroscopy (XPCS) data are the same
as those measured in bulk rheometry (η<sub>bulk</sub>) for three
combs. For the comb most like a star polymer and the comb closest
to showing bulk entanglement behavior, η<sub>XPCS</sub> >
η<sub>bulk</sub>. These discrepancies are much smaller than
those seen
for less densely branched polystyrenes. We conjecture that the smaller
magnitude of η<sub>XPCS</sub> – η<sub>bulk</sub> for the densely grafted combs is due to a lack of interpenetration
of the side chains when branching is most dense. Both <i>T</i><sub>g,bulk</sub> and the specific chain architecture play key roles
in determining the surface fluctuations