Mobility Gradient of Polystyrene in Films Supported on Solid Substrates

In this review, we show the distribution of glass transition temperature (T _g) in monodisperse polystyrene (PS) films coated on silicon oxide layers along the direction normal to the surface. Scanning force microscopy with a lateral force mode revealed that surface T _g (\( T_{\rm{g}}^{\rm{s}} \)) was lower than the corresponding bulk T _g (\( T_{\rm{g}}^{\rm{b}} \)). Interestingly, the glass transition dynamics at the surface was better expressed by an Arrhenius equation than by a Vogel–Fulcher–Tamman equation. Interdiffusion experiments for PS bilayers at various temperatures, above \( T_{\rm{g}}^{\rm{s}} \) and below \( T_{\rm{g}}^{\rm{b}} \), enabled us to gain direct access to the mobility gradient in the surface region. T _g at the solid substrate was examined by fluorescence lifetime measurements using evanescent wave excitation. The interfacial T _g was higher than the corresponding \( T_{\rm{g}}^{\rm{b}} \). The extent of the elevation was a function of the distance from the substrate and the interfacial energy. The T _g both at the surface and interface was also studied by the coarse-grained molecular dynamics simulation. The results were in good accordance with the experimental results. Finally, dynamic mechanical analysis for PS in thin and ultrathin films was made. The relaxation time for the segmental motion became broader towards the faster and slower sides, due probably to the surface and interfacial mobility.