Abstract
The process (segmental motion) of thin polystyrene films supported on glass substrate has been investigated in a wider frequency range from to using dielectric relaxation spectroscopy and thermal expansion spectroscopy. The relaxation rate of the 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 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 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 and m are found to decrease with decreasing film thickness.
- Received 25 December 2000
DOI:https://doi.org/10.1103/PhysRevE.64.011803
©2001 American Physical Society