Interface and Chain Confinement Effects on Tg Manuscript

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

James Forrest, Kari Dalnoki-Veress and John Dutcher,
Phys. Rev. E 56, 5705-5716 (1997).

ABSTRACT

We have used Brillouin light scattering and ellipsometry to measure the 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 films, films supported on a SiO_x surface with the other film surface free (uncapped supported), and films supported on a SiO_x surface and covered with a SiO_x 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 < R_EE (the average end-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_EE values. We observe a large M_w dependence of the T_g 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, T_g is reduced only slightly (< 10 K) from the bulk value, with only small differences in T_g (< 4 K) 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 SiO_x.