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

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 575×10³ to 9100×10³, were used in the study. For each M_w value, large reductions in T_g (as much as 80°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.