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 degrees C above the bulk T-g can be rationalized using the hydrodynamic continuum theory (HCT) known to describe melts of unentangled linear and cyclic chains. Film viscosities (eta(XPCS)) inferred from fits of the HCT to X-ray photon correlation spectroscopy (XPCS) data are the same as those measured in hulk rheometry (eta(bulk)) for three combs. For the comb most like a star polymer and the comb closest to showing bulk entanglement behavior, eta(XPCS) > eta(bulk). These discrepancies are much smaller than those seen for less densely branched polystyrenes. We conjecture that the smaller magnitude of eta(XPCS) - eta(bulk) for the densely grafted combs is due to a lack of interpenetration of the side chains when branching is most dense. Both T-g,T-bulk and the specific chain architecture play key roles in determining the surface fluctuations.