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

TLDR

Brillouin light scattering and ellipsometry were used to measure Tg of thin polystyrene films of two molecular weights across three geometries—freely standing, uncapped supported, and capped supported—as a function of film thickness. Freely standing PS films show a dramatic, Mw‑dependent Tg reduction up to 60 K for thicknesses below the polymer’s end‑to‑end distance, while supported films exhibit only minor (<10 K) decreases, indicating that polymer–substrate interactions dominate Tg in SiO₂‑supported films.

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 ${\mathrm{SiO}}_{x}$ surface with the other film surface free (uncapped supported), and films supported on a ${\mathrm{SiO}}_{x}$ surface and covered with a ${\mathrm{SiO}}_{x}$ 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 $h\ensuremath{\lesssim}{R}_{\mathrm{EE}}$ (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 ${M}_{w}$ 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 $(<10\mathrm{K})$ from the bulk value, with only small differences in ${T}_{g}$ $(<4\mathrm{K})$ 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 ${\mathrm{SiO}}_{x}.$