ABSTRACT
We have used Brillouin light scattering to make a detailed study of the behavior of the glass transition temperature T(g) in ultrathin, free-standing polystyrene films. The glass transitions were experimentally identified as near discontinuities in the thermal expansion. The effects of film thickness, molecular weight, and thermal history on the measured T(g) values have been investigated. While the size of the glass transition effects was comparable for all molecular weights, a complicated M(n) dependence suggested a separation of the results into two regimes, each dominated by a different length scale: a low M(n) regime controlled by a length scale intrinsic to the glass transition and a high M(n) region, where polymer chain confinement induced effects take over.
ABSTRACT
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 T(g) 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.
