Viscosity and Surface-Promoted Slippage of Thin Polymer Films Supported by a Solid Substrate
2015
Thermally activated flow dynamics of polystyrene films supported by silicon is studied for a wide range of film thickness (h₀) and molecular weights (Mw). At low Mw, the effective viscosity of the nanometer thin films is smaller than the bulk and decreases with decreasing h₀. This is due to enhancement of the total shear flow by the augmented mobility at the free surface. As Mw increases, with h₀ becoming smaller than the polymer radius of gyration (Rg), the effective viscosity switches from being substrate-independent to substrate-dependent. We propose that interfacial slippage then dominates and leads to plug flow. The friction coefficient is found to increase with h₀ providing h₀/Rg < ∼1, demonstrating a surface-promoted confinement effect.
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