Glass Transition and Aging Dynamics in Single and Stacked Thin Polymer Films

The glass transition and aging dynamics of single and stacked thin films of polystyrene (PS) and poly(2-chlorostyrene) (P2CS) were investigated using differential scanning calorimetry and dielectric relaxation spectroscopy. The glass transition temperature Tg of as-stacked thin films of PS has a strong depression from that of the bulk samples. 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 dynamics of the α-process of stacked P2CS thin films show a time evolution from single thin film-like dynamics to bulk-like dynamics during the isothermal annealing process. The relaxation rate of the α-process becomes smaller with increase in the annealing time. The time scale for the evolution of the α-dynamics during the annealing process is very long compared with that for the reptation dynamics. At the same time, the temperature dependence of the relaxation time for the α-process changes from Arrhenius-like to Vogel-Fulcher-Tammann dependence with increase of the annealing time. The aging dynamics of P2CS thin films with thickness less than 10 nm were also investigated using dielectric relaxation spectroscopy. The imaginary part of the dielectric susceptibility ε" for P2CS thin films with a thickness of 3.7 nm increased with an increase in isothermal aging time, while this was not the case for P2CS thin films thicker than 9.0 nm. This anomalous increase in ε" for the ultrathin films is strongly correlated with the presence of a mobile liquid-like layer within the thin films.