Glass transition dynamics of stacked thin polymer films

The glass transition dynamics of stacked thin films of polystyrene and poly(2-chlorostyrene) were investigated using differential scanning calorimetry and dielectric relaxation spectroscopy. The glass transition temperature T-g of as-stacked thin polystyrene films has a strong depression from that of the bulk samples. However, after annealing at high temperatures above T-g, the stacked thin films exhibit glass transition at a temperature almost equal to the T-g of the bulk system. The alpha-process dynamics of stacked thin films of poly(2-chlorostyrene) show a time evolution from single-thin-film-like dynamics to bulk-like dynamics during the isothermal annealing process. The relaxation rate of the alpha process becomes smaller with increase in the annealing time. The time scale for the evolution of the alpha 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 alpha process changes from Arrhenius-like to Vogel-Fulcher-Tammann dependence with increase of the annealing time. The fragility index increases and the distribution of the alpha-relaxation times becomes smaller with increase in the annealing time for isothermal annealing. The observed change in the alpha process is discussed with respect to the interfacial interaction between the thin layers of stacked thin polymer films.