INTERFACE AND CHAIN CONFINEMENT EFFECTS ON THE GLASS-TRANSITION TEMPERATURE OF THIN POLYMER-FILMS

We have used Brillouin light scattering and ellipsometry to measure th e glass transition temperature T-g of thin polystyrene (PS) films as a function of the film thickness h for two different molecular weights M-w. Three different film geometries were studied: freely standing fil ms, films supported on a SiOx surface with the other film surface free (uncapped supported), and films supported on a SiOx surface and cover ed with a SiOx layer (capped supported). For freely standing films T-g is reduced dramatically from the bulk value by an amount that depends on both h and M-w. For h less than or similar to R-EE (the average en d-to-end distance of the unperturbed polymer molecules), T-g decreases linearly with decreasing h with reductions as large as 60 K for both M-w values. We observe a large M-w dependence of the T-g reductions fo r freely standing films which provides the first strong evidence of th e importance of chain confinement effects on the glass transition temp erature of thin polymer films. For both the uncapped and capped suppor ted films, T-g is reduced only slightly (<10 K) from the bulk value, w ith only small differences in T-g (<4 K) observed between uncapped and capped supported films of the same thickness. The results of our expe riments demonstrate that the polymer-substrate interaction is the domi nant effect in determining the glass transition temperature of PS film s supported on SiOx.