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-w, from 13.7 to 2100 kg/mol was investigated. Our result shows that for the thin films with M-w < similar to 400 kg/mol the addition of DOP causes the effective viscosity to decrease by a factor of similar to 4, independent of M-w. But for the higher M-w, films, the effective viscosity of the DOP added films creeps toward that of the neat films with increasing M-w. A model assuming the effective viscosity to be dominated by enhanced surface mobility for the lower M-w films but surface-promoted interfacial slippage for the higher M-w films is able to account for the experimental observations.