Reducing the Tg-Confinement Effect in Polystyrene Films by Use of Carbon Substrates and Changing the Direction of the Effect in Polystyrene-Carbon Model Nanocomposites

Using ellipsometry and fluorescence, we have shown that single-layer polystyrene (PS) films supported on carbon substrate exhibit suppression of the classic T_g-confinement effect due to favorable polymer-substrate interactions. Additionally, a bilayer film/fluorescence study has shown that in a bulk PS film, the 15 nm layer at the carbon interface exhibits a Tg nearly 10 K higher than bulk. The ability of PS to favorably interact at the interface with the graphitic substrate (versus silicon dioxide) was demonstrated qualitatively via a dewetting study. Because PS does not naturally wet the native oxide layer on silica, a thin 30-nm-thick, 30 kg/mol PS film readily dewets when annealed at 433 K. However, dewetting of supported thin PS films is greatly suppresses using substrates with favorable interfacial interaction with PS. Model nanocomposites were also made by sputtering a graphitic layer above PS films supported on carbon, sandwiching the film between two graphitic surfaces and providing a consolidated film with constant interlayer distance between the two hard surfaces after annealing. Single-layer PS films in this model nanocomposite geometry exhibit enhanced T_g with decreasing nanoscale thickness due to pi-pi bonding interactions of PS with the graphitic substrates.