Quantifying glass transition behavior in ultrathin free-standing polymer films

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.