glass transition dynamics and surface layer mobility in unentangled polystyrene films

Science 25 June 2010: Vol. 328. no. 5986, pp. 1676 - 1679 DOI: 10.1126/science.1184394

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Glass Transition Dynamics and Surface Layer Mobility in Unentangled Polystyrene Films

Zhaohui Yang,¹ Yoshihisa Fujii,¹ Fuk Kay Lee,^1,2 Chi-Hang Lam,² Ophelia K. C. Tsui^1,*

Most polymers solidify into a glassy amorphous state, accompanied by a rapid increase in the viscosity when cooled below the glass transition temperature (T_g). There is an ongoing debate on whether the T_g changes with decreasing polymer film thickness and on the origin of the changes. We measured the viscosity of unentangled, short-chain polystyrene films on silicon at different temperatures and found that the transition temperature for the viscosity decreases with decreasing film thickness, consistent with the changes in the T_g of the films observed before. By applying the hydrodynamic equations to the films, the data can be explained by the presence of a highly mobile surface liquid layer, which follows an Arrhenius dynamic and is able to dominate the flow in the thinnest films studied.

¹ Department of Physics, Boston University, Boston, MA 02215, USA.
² Department of Applied Physics, Hong Kong Polytechnic University, Hong Kong.

^* To whom correspondence should be addressed. E-mail: okctsui@bu.edu.