Interface and chain confinement effects on the glass transition temperature of thin polymer films

We have used Brillouin light scattering and ellipsometry to measure the glass transition temperature Tg of thin polystyrene (PS) films as a function of the film thickness h for two different molecular weights Mw. Three different film geometries were studied: freely standing films, films supported on a SiOₓ surface with the other film surface free (uncapped supported), and films supported on a SiOₓ surface and covered with a SiOₓ layer (capped supported). For freely standing films Tg is reduced dramatically from the bulk value by an amount that depends on both h and Mw. For hREE (the average end-to-end distance of the unperturbed polymer molecules), Tg decreases linearly with decreasing h with reductions as large as 60 K for both Mw values. We observe a large Mw dependence of the Tg reductions for freely standing films which provides the first strong evidence of the importance of chain confinement effects on the glass transition temperature of thin polymer films. For both the uncapped and capped supported films, Tg is reduced only slightly (<10K) from the bulk value, with only small differences in Tg (<4K) observed between uncapped and capped supported films of the same thickness. The results of our experiments demonstrate that the polymer-substrate interaction is the dominant effect in determining the glass transition temperature of PS films supported on SiOₓ.