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

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    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
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