Heterogeneous and Aging Dynamics in Single and Stacked Thin Polymer Films

Abstract

The glass transition and the a-process above the glass transition temper- ature, Tg, and the aging dynamics below Tg were investigated for single and stacked thin polymer films. First, the glass transition dynamics of stacked thin films of polystyrene (PS) and poly(2-chlorostyrene) (P2CS) were measured using differ- ential scanning calorimetry and dielectric relaxation spectroscopy. The Tg for as-stacked thin PS films is much less than that of the bulk sample. However, after annealing at high temperatures above Tg, the stacked thin films exhibit glass transition at a temperature almost equal to the Tg of the bulk system. The a-process dynamics of stacked thin films of P2CS show a very slow time evolution from single thin film-like dynamics to bulk-like dynamics during the isothermal annealing process. The temperature dependence of the relaxation time for the a-process changes from Arrhenius-like to a Vogel-Fulcher-Tammann dependence with an increase in annealing time. Secondly, the aging dynamics of P2CS ultrathin films with thicknesses less than 10 nm were investigated using dielectric relaxation spectroscopy. The imaginary part of the dielectric susceptibility, e 00 , for P2CS ultrathin films with a thickness of 3.7 nm increases with an increase in isothermal aging time, but this is not the case for P2CS thin films thicker than 9.0 nm. This anomalous increase in e 00 for the ultrathin films is strongly correlated with the