Molecular weight dependence of reductions in the glass transition temperature of thin, freely standing polymer films

TLDR

The study investigates how reductions in the glass transition temperature of freely standing polystyrene films depend on polymer molecular weight in a chain‑confinement dominated regime. Using transmission ellipsometry, the authors examined freely standing polystyrene films of six molecular weights (5.75 × 10³–9.1 × 10³) to characterize T_g reductions as a function of thickness and molecular weight. They observed T_g reductions up to 80 °C at thin film thicknesses and found evidence for a mobility mechanism distinct from bulk cooperative motion that is suppressed in the bulk.

Abstract

We have used transmission ellipsometry to perform a comprehensive study of the glass transition temperature ${T}_{g}$ of freely standing polystyrene films. Six molecular weights ${M}_{w},$ ranging from $575\ifmmode\times\else\texttimes\fi{}{10}^{3}$ to $9100\ifmmode\times\else\texttimes\fi{}{10}^{3},$ were used in the study. For each ${M}_{w}$ value, large reductions in ${T}_{g}$ (as much as $80\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ below the bulk value) were observed as the film thickness h was decreased. We have studied in detail the dependence of the ${T}_{g}$ reductions on ${M}_{w}$ in a regime dominated by chain confinement effects. The empirical analysis presented is highly suggestive of the existence of a mechanism of mobility in thin freely standing films that is inhibited in the bulk and distinct from the usual cooperative motion associated with the glass transition.