Effect of Chain Architecture on Surface Fluctuations in Thin Polymer Films

Thermally stimulated fluctuations always present on the surfaces of polymer melts and compliant solids dictate key properties of those surfaces including adhesion, wetting and friction. While these fluctuations are a surface phenomenon, it is already known that they reflect the mobility of the polymeric material not only near the surface, but also deeper in a film. If a melt film becomes thin enough, the architecture of the chains becomes important for determining the surface friction and wetting through the surface fluctuations.  The surface fluctuations of annealed melt films of 6k cyclic polystyrene (CPS), its linear analog, and a long-branched chain were measured using X-ray photon correlation spectroscopy (XPCS) for films of various thicknesses. The surface fluctuations of the 6k linear PS melt films 17 nm and thicker and the 6k cyclic melt films 28 nm and thicker can be described using a hydrodynamic continuum theory (HCT) that assumes the film is characterized by the bulk viscosity. When a film of CPS is 24 nm or thinner, the behavior can no longer be captured using the HCT with bulk viscosity. The surface fluctuations behave as though the film has an effective viscosity higher than the bulk value. The thickness at which such confinement effects are seen for the 6k CPS chains is larger than that for the linear analogs. Confinement effects for long-branched chains appear at thicknesses, relative to R_g, that are even larger. Acknowledgements: Use of the Advanced Photon Source at Argonne National Laboratory was supported by the DOE’s Office of Science under Contract DE-AC02-06-CH11357. This work was supported by NSF Grants CBET-0730692 and CBET-0731319.