Reductions of the glass transition temperature in thin polymer films: Probing the length scale of cooperative dynamics

We report measurements of the glass transition temperature, T-g, in free standing polymer films in a low M-n limit where chain confinement effects are not observed. The measured T-g values are accurately described by a layer model incorporating a mobile surface layer with a size determined by the length scale of cooperative dynamics. The analysis leads to a surface T-g value and length scale of cooperative motion near bulk Tg which quantitatively agree with independently determined values. The model and parameters provide a framework within which all previous measurements of T-g values in thin supported films may be understood and provides values for the length scale of cooperative motion over an extended range of temperatures below the bulk T-g value.