(382a) Comb Polymers: Film Surface Fluctuation Dynamics and Surface Segregation in the Limit of 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 linear and unentangled cyclic chains. Film viscosities (η_XPCS) for three combs inferred from fits of the HCT to X-ray Photon Correlation Spectroscopy (XPCS) data are the same as bulk viscosities (η_bulk) measured with rheometry. For the comb most like a star polymer and the comb closest to showing bulk entanglement behavior, η_XPCS > η_bulk. However, the values of η_XPCS - η_bulk are much smaller than those seen for less densely branched polystyrenes. We conjecture that the smaller magnitude of η_XPCS - η_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.

We have also considered the effect of the dense branching on surface segregation in blends of linear and comb-branched polystyrenes.  The comb-branched chains are strongly preferred at the surface and the experimentally determined surface segregation can be compared with self-consistent field theory calculations.

 Acknowledgements: NSF CBET 0730692 & CBET-0731319 and DURIP W911NF-09-1-0122.