Effect of Free Surfaces on the Glass Transition Temperature of Thin Polymer Films

We report the first measurements of the glass transition temperature $T_g$ for thin freely standing polystyrene (PS) films. We have used Brillouin light scattering to measure $T_g$ for freely standing films of different thicknesses. We find that $T_g$ decreases linearly with film thickness $h$ for $h\le 700Å$, with a reduction of 70 K for a film with $h=290\mathrm{}Å$. These measurements characterize unambiguously the effects of the free surface on $T_g$ of thin polymer films. Results are compared to similar results for supported PS films [Keddie et al., Europhys. Lett. 27, 59 (1994)], and we find that their measured values are influenced strongly by the substrate.