Comparing Local Glass Transition by Fluorescence with Physical Aging by Ellipsometry Within Rubbery-Glassy PnBMA/PS Bilayer Films

Expanding our ellipsometry analysis, we were recently able to isolate the impact of a single glassy-rubbery interface on the physical aging response of glassy polystyrene (PS) within bilayer films of poly(n-butyl methacrylate) (PnBMA) atop PS, avoiding competing free surface effects. Surprisingly, no change in the glassy PS aging response was observed down to layer thicknesses of 150 nm PnBMA / 75 nm PS, the thinnest we were able to measure. At first glance, these results seem at odds with our group’s previous findings of large local glass transition temperature T_g(z) changes near glassy-rubbery polymer interfaces in semi-infinite systems. Here we investigate this directly by performing fluorescence measurements to determine the PS T_g response within PnBMA/PS bilayer films with equivalent geometry to those used for the physical aging studies. We have confirmed that the PnBMA/PS interface imparts a T_g reduction to the underlying PS layer that is larger than that caused by the free surface in single layer PS films. For example, 75 nm PS on silica capped by 150 nm PnBMA shows an ≈5 K decrease in T_g relative to bulk. We compare fluorescence measurements of the local T_g(z) profile within 150 nm PnBMA / 75 nm PS bilayer films with the physical aging results obtained by ellipsometry.