Chain Movements at the Topmost Surface of Poly(methyl methacrylate) and Polystyrene Films Directly Evaluated by In Situ High-Temperature Atomic Force Microscopy.

Abstract

The surfaces of polymeric materials are thermodynamically unstable, and the glass-transition temperature (Tg) is significantly lower than that in the bulk material. However, the mobility of the chains at the top of the surface has never been directly evaluated. In this study, the movements of the topmost chains of poly(methyl methacrylate) (PMMA) and polystyrene (PS) bulk films were observed in situ at high temperatures with atomic force microscopy in tapping mode. PMMA and PS chains started moving at ∼97 and ∼50 °C, respectively, which were slightly and significantly below the values of their bulk Tg (PMMA, 108 °C; PS, 104 °C), respectively. The activation energies of the apparent diffusion constants of PMMA and PS, derived by particle image velocimetry analysis, were 193 and 151 kJ mol-1, respectively, and reasonable for the glass transition. Movements of isolated PMMA chains deposited on a PMMA film by the Langmuir-Blodgett technique were also observed and confirmed to be essentially the same as those on the PMMA film surface.