We have measured the effective viscosity of polystyrene films with a small (4 wt %) added amount of dioctyl phthalate (DOP) deposited on silica. A broad range of molecular weights, Mw, from 13.7 to 2100 kg/mol was investigated. Our result shows that for the thin films with Mw < ∼100 kg/mol the addition of DOP causes the effective viscosity to decrease by a factor of ∼4, independent of Mw. But for the higher Mw films, the effective viscosity of the DOP added films creeps toward that of the neat films with increasing Mw. A model assuming the effective viscosity to be dominated by enhanced surface mobility for the lower Mw films but surface-promoted interfacial slippage for the higher Mw films is able to account for the experimental observations.
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