Demonstration of Glass Transition Temperature Depression in Thin Supported Polystyrene Films Using Internal Standard

Clear evidence for glass transition temperature (Tg) depression in $\sim$ 5 nm thick atactic polystyrene (Mw = 212 kg/mol) films supported on silicon substrates is demonstrated by ellipsometry in vacuum [1]. Transition in polystyrene droplets formed by dewetting is used as an internal reference. Both temperature-modulated [2] and linear temperature scanning techniques are utilized; measurements are performed at $10^{-6} - 10^{-8}$ torr residual gas pressure. The method is sensitive enough to observe glass transition in 1 $-$ 2 nm thick supported polystyrene films. Our recent study shows appreciable reduction of Tg in less than 10 $-$ 20 nm thick samples; Tg versus thickness function is found to follow a step-like curve originally reported by [3]. The curve is characterized by moderate (about 17 K) constant Tg depression for thickness less than 7 $-$ 8 nm. References: [1]. M. Y. Efremov, S. S. Soofi, A. V. Kiyanova, C. J. Munoz, P. Burgardt, F. Cerrina, and P. F. Nealey, Rev. Sci. Instrum., 79, 043903 (2008). [2]. M. Y. Efremov, A. V. Kiyanova, and P. F. Nealey, Macromolecules, 41, 5978 (2008). [3] T. Miyazaki, K. Nishida, and T. Kanaya, Phys. Rev. E, 69, 061803 (2004).