Confinement Effect on the Effective Viscosity of Plasticized Polymer Films

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, $M_{\mathrm{w}}$, from 13.7 to 2,100 kg/mol was investigated. Our result shows that for the thin films with $M_{\mathrm{w}}$ \textless \textasciitilde 100 kg/mol, the addition of DOP causes the effective viscosity to decrease by a factor of \textasciitilde 4, independent of $M_{\mathrm{w}}$. But for the higher $M_{\mathrm{w\thinspace }}$films, the effective viscosity of the DOP added films creeps towards that of the neat films with increasing $M_{\mathrm{w}}$. A model assuming the effective viscosity to be dominated by enhanced surface mobility for the lower $M_{\mathrm{w}}$ films, but surface-promoted interfacial slippage for the higher $M_{\mathrm{w}}$ films is able to account for the experimental observations.