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) over bar (w), ranging from 575 X 10(3) to 9100 X 10(3), were used in the study. For each (M) over bar (w), value, large reductions in T-g (as much as 80 degreesC 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) over bar (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.