Surface Mobile Layer, Mobility and Glass Transition Dynamics of Polymer Films

摘要

It has been broadly observed that the glass transition temperature, Tg, of some polymer films can become substantially smaller than the bulk when the film thickness, h, is decreased to the nanometer range . Most workers believe that the Tg reduction is caused by a mobile layer located at the free surface of the film . In this experiment, we measure the viscosity, η, of thin polystyrene (PS) films coated on silicon – the system most studied for Tg reduction. We observe a clear positive correlation between the temperatures where the viscosity of the films begin to diverge and the Tg of the films . By examining the mobility of the films, M (≡ h~3/(3η)), we uncover a physical mechanism whereby the surface mobile layer modifies the overall dynamics of the films14