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 $575\times {10}^3$ to $9100\times {10}^3,$ were used in the study. For each $M_w$ value, large reductions in $T_g$ (as much as $80°\mathrm{C}$ 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.