Surface Dynamics of Branched Polystyrene Melts

Thermally stimulated fluctuations at the surface of a melt of long-branched polystyrene chains have been studied for the first time. The surface fluctuations were probed using x-ray photon correlation spectroscopy (XPCS), a recently-developed technique already applied to study the surfaces of melts of linear polystyrene chains. The variation of surface relaxation time with in-plane scattering vector, q(in-plane) can be fit using a continuum hydrodynamic theory of thermally stimulated capillary waves with a nonslip boundary condition. Assuming the theory is applicable, apparent viscosities of the films may then be inferred from the XPCS data. For a melt of linear chains, the viscosity inferred from XPCS data in this manner is the same as that measured from a bulk sample using conventional rheometry. In contrast, for each of five different sorts of well-defined branched chains, the viscosity inferred from surface relaxation times using the hydrodynamic continuum theory is higher than that measured in a bulk sample. The source of this discrepancy is under investigation.

Acknowledgements: NSF support (CBET 0730692) is gratefully acknowledged. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, under contract No. W-31-109-ENG-38.