Polymer Film Surface Fluctuation Dynamics in the Limit
of Very Dense Branching
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Abstract
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