Modifying Surface Fluctuations of Polymer Melt Films with Substrate Modification

Abstract

Deposition of a plasma polymerized film on a silicon substrate substantially changes the fluctuations on the surface of a sufficiently thin melt polystyrene (PS) film atop the substrate. Surface fluctuation relaxation times measured with X-ray photon correlation spectroscopy (XPCS) for ca. 4 R g thick melt films of 131 kg/mol linear PS on hydrogen-passivated silicon (H–Si) and on a plasma polymer modified silicon wafer can both be described using a hydrodynamic continuum theory (HCT) that assumes the film is characterized throughout its depth by the bulk viscosity. However, when the film thickness is reduced to ∼3 R g, confinement effects are evident. The surface fluctuations are slower than predicted using the HCT, and the confinement effect for the PS on H–Si is larger than that for the PS on the plasma polymerized film. This deviation is due to a difference in the thicknesses of the strongly adsorbed layers at the substrate which are impacted by the substrate surface energy.