Surface molecular motions of monodisperse polystyrene (PS) films, their binary and ternary blend films, and commercially available polydisperse PS films were investigated on the basis of scanning force microscopic (SFM) measurements at 293 K. The monodisperse PSs were synthesized by a living anionic polymerization. The binary and the ternary PS blends were prepared by mixing monodisperse PSs with different molecular weights. The commercially available PSs were used as the polydisperse PS sample. In the case of the monodisperse PS film with number-average molecular weight, Mn, lower than ca. 30k, it was revealed that the surface was in a glass-rubber transition state even at room temperature due to excess free volume induced by the surface localization of chain end groups. SFM measurements revealed that the surfaces of the binary and the ternary PS blend films were in a glass-rubber transition state even at room temperature, when a component with Mn lower than ca. 30k existed. A more vigorous surface molecular motion for the binary and the ternary PS blend films compared with the bulk can be explained by the surface segregation of the lower molecular weight component. In the case of the polydisperse PS film, even though the molecular weight distribution was broad and the somewhat lower molecular weight component was mixed, the active surface molecular motion showing a glass-rubber transition state was remarkably depressed at room temperature in comparison with the case for the monodisperse PS film with corresponding MnS. The difference in surface thermal molecular motion between the monodisperse and the commercial polydisperse PS films might be explained on the basis of the chemical structure of the chain end groups. Also, for the case that the lower molecular weight component was not present in the system in spite of the broad molecular weight distribution, the surface molecular motion corresponding to a glass-rubber transition state was not observed at room temperature.
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