Thermal molecular motion in polystyrene thin and ultrathin films by dynamic viscoelastic measurement

Segmental motion in polystyrene (PS) thin and ultrathin films supported on substrates was studied by dynamic viscoelastic measurement. A polymer film, which has the thickness comparable to or less than twice of radius of gyration of an unperturbed chain (2R _g), is defined as an ultrathin film. In the case of PS, α _a-relaxation process corresponding to the segmental motion was generally observed at approximately 380 K. Even for both the PS thin and ultrathin films, the α _a-absorption peak was clearly observed. A rheological analysis reveals that the α _a-relaxation behavior for the thin films with the thickness of about 200 nm is the same as that of the bulk sample. On the contrary, in the case of the PS ultrathin films, the α _a-absorption peak on temperature-loss modulus (E) curve is broadened toward both lower and higher temperature sides. This can be interpreted by taking into account that the segmental motion in the vicinity of surface and interface is detectable for such ultrathin films, which should be faster and slower than that in the bulk, respectively. And, it is found that the apparent activation energy (ΔH) for the α _a-relaxation in the ultrathin films becomes smaller than the bulk value probably due to the surface effect. Finally, an interfacial effect on the ΔH was studied by using different substrates. When the interaction between PS and substrate becomes stronger, the ΔH value for the α _a-relaxation in the ultrathin films increases. The results imply that the segmental motion in the polymer ultrathin films is strongly influenced by surface and interfacial effects.