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 less than or equal to 700 Angstrom, with a reduction of 70 K for a film with h = 290 Angstrom. These measurements characterize unambiguously the effects of the foe surface on T-g of thin polymer films. Results are compared to similar results for supported PS films [Keddie er al., Europhys. Lett. 27, 59 (1994)], and we find that their measured values are influenced strongly by the substrate.