Effects of chain ends and densities on the glass transition of polymer thin films probed by linear and cyclic polystyrene

Abstract

There are substantial debates on the origin of glassy dynamic changes of nanoconfined polymers. We measure the glass transition temperature ( T g ) of linear and cyclic polystyrene thin films, and find that the T g of cyclic polystyrene nearly remains constant when the film thickness is decreased. The T g of analogous linear polystyrene, however, substantially decreases with decreasing film thickness. When the thin films are patterned into line gratings with a height of about 69 nm, T g -reduction is observed in both linear and cyclic polystyrene. The magnitude of T g -reduction of cyclic polystyrene is about 7 K regardless the molecular weight, whereas the T g -reduction of linear polystyrene ranges from about 20 K to 6 K depending on the molecular weight. These experimental results indicate that the effect of chain ends alone cannot explain the mechanism of confinement-induced T g depression, and the decreased chain density at interfaces may play an overwhelm role. • T g of linear polystyrene thin films decreases with decreasing thickness. • T g of cyclic polystyrene thin films remains constant when thickness is decreased. • T g reduction is observed in linear and cyclic polystyrene in nanostructures. • Decreased chain density at interfaces play an overwhelm role in T g reduction.