EFFECT OF POLYDISPERSITY ON SURFACE MOLECULAR-MOTION OF POLYSTYRENE FILMS

Surface molecular motions of monodisperse polystyrene (PS) films, thei r binary and ternary blend films, and commercially available polydispe rse PS films were investigated on the basis of scanning force microsco pic (SFM) measurements at 293 K. The monodisperse PSs were synthesized by a living anionic polymerization. The binary and the ternary PS ble nds 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, M-n, lower than ca. 30k, it was revealed that the s urface was in a glass-rubber transition state even at room temperature due to excess free volume induced by the surface localization of chai n end groups. SFM measurements revealed that the surfaces of the binar y and the ternary PS blend films were in a glass-rubber transition sta te even at room temperature, when a component with M-n lower than ca. 30k existed. A more vigorous surface molecular motion for the binary a nd 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 weigh t distribution was broad and the somewhat lower molecular weight compo nent was mixed, the active surface molecular motion showing a glass-ru bber transition state was remarkably depressed at room temperature in comparison with the case for the monodisperse PS film with correspondi ng M(n)s. The difference in surface thermal molecular motion between t he monodisperse and the commercial polydisperse PS films might be expl ained on the basis of the chemical structure of the chain end groups. Also, for the case that the lower molecular weight component was not p resent in the system in spite of the broad molecular weight distributi on, the surface molecular motion corresponding to a glass-rubber trans ition state was not observed at room temperature.