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.