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

We report the first measurements of the glass transition temperature Tg for thin freely standing polystyrene (PS) films. We have used Brillouin light scattering to measure Tg for freely standing films of different thicknesses. We find that Tg decreases linearly with film thickness h for h≤700 Å, with a reduction of 70 K for a film with h=290 Å. These measurements characterize unambiguously the effects of the free surface on Tg 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.