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

Abstract

Using ellipsometry, we characterized the nanoconfinement effect on the glass transition temperature (T _gof supported polystyrene (PS) films employing two methods: the intersection of fits to the temperature (Tdependences 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: $\ensuremath T_{{\rm g}}({\rm bulk})-T_{{\rm g}}(23{\,\mbox{nm}})= 10 ^{\circ}$ 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^° C via fluorescence. This small difference in the T _g 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.