In this paper, glass transitions of polystyrene (PS) in confined geometry such as ultrathin state, surface and interface were studied. Dynamic mechanical analysis was successfully used to characterize polymer ultrathin films supported on substrates. This method allowed us to uncover the effects of the free surface and the substrate interface on the segmental dynamics in PS films with various thickness. As the film thickness decreased, the distribution of relaxation times for the segmental motion became broader, a change mainly due to surface and interfacial effects. The results obtained in this study imply that two contrasting effects exist: the effect of the free surface accelerates the segmental motion, whereas interfacial interaction produce the opposite effect. This was also confirmed by independent experiments related to glass transition temperature at the surface and the interface by scanning force microscopy and time- and space-resolved fluorescence spectroscopy, respectively.
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