Substrate Effects on Glass Transition and Free Surface Viscoelasticity of Ultrathin Polystyrene Films

Abstract

We describe results from experiments in which spontaneous embedment of 20 nm silica particles was used to probe under-layer effects on the free surface dynamics and the glass transition temperature (Tg) of polystyrene. Both 13 and 20 nm thick polystyrene (PS) films were prepared and placed on different under-layer substrates, which in turn were supported on a silicon wafer (Si) substrate. The under-layer substrates used were PS, poly(2-vinylpyridine) (P2VP), and poly(methyl methacrylate) (PMMA) with thicknesses ranging from 13 to 350 nm. The particle height change during the embedment was monitored using an atomic force microscope. For the PS films supported on the PS and P2VP under-layers, experimental temperatures varied from Tg – 10 K to Tg + 5 K. In the case of the PMMA under-layer, experimental temperatures varied from Tg – 10 K to Tg + 10 K. The Hutcheson and McKenna model [Phys. Rev. Lett. 2005, 94 (7)] was applied to the particle embedment depth to obtain the surface rheological temperatures [Eur. Phys. J. E 2007, 22 (4), 281−286] and the Tg. It is found that the dynamics of the top-layer PS films were faster than the bulk material below the macroscopic Tg and slower above it for all under-layers considered. The Tg of both the 20 and 13 nm top-layer PS films were found to be essentially independent of under-layer thickness and reduced by less than 7 °C. Upon replacing PS under-layers with the same thickness of P2VP and PMMA as the under-layers, the Tg of the 20 nm PS top-layer films changed by less than 5 K.