Slow dynamics near glass transitions in thin polymer films - art. no. 011803

The alpha process (segmental motion) of thin polystyrene films supported on glass substrate has been investigated in a wider frequency range from 10(- 3) Hz to 10(4) Hz using dielectric relaxation spectroscopy and thermal expa nsion spectroscopy. The relaxation rate of the alpha process increases with decreasing film thickness at a given temperature above the glass transitio n. This increase in the relaxation rate with decreasing film thickness is m uch more enhanced near the glass transition temperature. The glass transiti on temperature determined as the temperature at which the relaxation time o f the alpha process becomes a macroscopic time scale shows a distinct molec ular weight dependence. It is also found that the Vogel temperature has a t hickness dependence. i.e., the Vogel temperature decreases with decreasing film thickness. The expansion coefficient of the free volume alpha (f) is e xtracted from the temperature dependence of the relaxation time within the free volume theory. The fragility index m is also evaluated as a function o f thickness. Both alpha (f) and m are found to decrease with decreasing him thickness.