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

The surface height fluctuations of melt films of densely branched comb polystyrenes of thicknesses greater than 55nm and at temperatures more than 23 C above the $T_{g,bulk}$ can be rationalized using the hydrodynamic continuum theory (HCT) known to describe melts of unentangled linear and cyclic chains. Film viscosities ($\eta_{XPCS})$ for three combs inferred from fits of the HCT to X-ray Photon Correlation Spectroscopy (XPCS) data are the same as bulk viscosities ($\eta_{bulk})$ measured with rheometry. For the comb most like a star polymer and the comb closest to showing bulk entanglement behavior, $\eta_{XPCS}$ is greater than $\eta_{bulk}$. However, the values of $\eta_{XPCS}-\eta_{bulk}$ 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 very dense. While data of relaxation time versus $T$ for cyclic chains virtually collapse to a single curve when $T_{g,bulk}$ is accounted for, that is not the case for combs. $T_{g,bulk}$ and specific chain architecture both play important roles in determining the surface fluctuations. Acknowledgements: NSF CBET 0730692, CBET-0731319, DURIP W911NF-09-1-0122.