Slow dynamics near glass transitions in thin polymer films

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 expansion spectroscopy. The relaxation rate of the alpha process increases with decreasing film thickness at a given temperature above the glass transition. This increase in the relaxation rate with decreasing film thickness is much more enhanced near the glass transition temperature. The glass transition temperature determined as the temperature at which the relaxation time of the alpha process becomes a macroscopic time scale shows a distinct molecular weight dependence. It is also found that the Vogel temperature has a thickness dependence, i.e., the Vogel temperature decreases with decreasing film thickness. The expansion coefficient of the free volume alpha(f) is extracted from the temperature dependence of the relaxation time within the free volume theory. The fragility index m is also evaluated as a function of thickness. Both alpha(f) and m are found to decrease with decreasing film thickness.