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 (<i>T</i>_g) of the irreversibly adsorbed layer of polystyrene (PS) buried in bulk films as a function of adsorption time, <i>t</i>_ads. This bilayer geometry allows for the examination of interfacial effects on <i>T</i>_g of the adsorbed nanolayer. In the presence of a free surface, we observe a substantial reduction in <i>T</i>_g from bulk that lessens with <i>t</i>_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 <i>T</i>_g deviation at early <i>t</i>_ads, suggesting chain adsorption dictates <i>T</i>_g at long <i>t</i>_ads. Annealing in the bilayer geometry promotes recovery of bulk <i>T</i>_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 <i>T</i>_g in terms of the diffusion of free volume pockets toward interfacial sinks