Direct measurement of glass transition temperature in exposed and buried adsorbed polymer nanolayers

We employ a fluorescence bilayer method to directly measure the glass transition temperature (T_g) of the irreversibly adsorbed layer of polystyrene (PS) buried in bulk films as a function of adsorption time, t_ads. This bilayer geometry allows for the examination of interfacial effects on T_g of the adsorbed nanolayer. In the presence of a free surface, we observe a substantial reduction in T_g from bulk that lessens with t_ads as a result of increased chain adsorption at the substrate. Submerging the adsorbed layer and effectively removing the free surface results in a suppression of the T_g deviation at early t_ads, suggesting chain adsorption dictates T_g at long t_ads. Annealing in the bilayer geometry promotes recovery of bulk T_g on a time scale reflecting the degree of adsorption. Our data are quantitatively rationalized via the free volume holes diffusion model, which explains adsorbed nanolayer T_g in terms of the diffusion of free volume pockets toward interfacial sinks.