Confinement effects on glass transition temperature, transition breadth, and expansivity: Comparison of ellipsometry and fluorescence measurements on polystyrene films

Using ellipsometry, we characterized the nanoconfinement effect on the glass transition temperature (T-g) of supported polystyrene (PS) films employing two methods: the intersection of fits to the temperature (T) dependences of rubbery-and glassy-state thicknesses, and the transition mid-point between rubbery-and glassy-state expansivities. The results demonstrate a strong effect of thickness: T-g (bulk) - T-g (23 nm) = 10 degrees C. The T-range needed for accurate measurement increases significantly with decreasing thickness, an effect that arises from the broadening of the transition with confinement and a region below T-g where expansivity slowly decreases with decreasing T. As determined from expansivities, the T-g breadth triples in going from bulk films to a 21-nm-thick film; this broadening of the transition may be a more dramatic effect of confinement than the T-g reduction itself. In contrast, there is little effect of confinement on the rubbery-and glassy-state expansivities. Compared with ellipsometry, T-g's from fluorescence agree well in bulk films but yield lower values in nanoconfined films: T-g (bulk)- T-g (23 nm) = 15 degrees C via fluorescence. This small difference in the Tg confinement effect reflects differences in how fluorescence and ellipsometry report average T-g with confinement. With decreasing nanoscale thickness, fluorescence may slightly overweight the contribution of the free-surface layer while ellipsometry may evenly weight or underweight its contribution.