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Direct Measurement of Glass Transition Temperature in Exposed and Buried Adsorbed Polymer Nanolayers

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journal contribution
posted on 07.06.2016, 18:20 by Mary J. Burroughs, Simone Napolitano, Daniele Cangialosi, Rodney D. Priestley
We employ a fluorescence bilayer method to directly measure the glass transition temperature (Tg) of the irreversibly adsorbed layer of polystyrene (PS) buried in bulk films as a function of adsorption time, tads. This bilayer geometry allows for the examination of interfacial effects on Tg of the adsorbed nanolayer. In the presence of a free surface, we observe a substantial reduction in Tg from bulk that lessens with tads 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 Tg deviation at early tads, suggesting chain adsorption dictates Tg at long tads. Annealing in the bilayer geometry promotes recovery of bulk Tg 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 Tg in terms of the diffusion of free volume pockets toward interfacial sinks.