Dynamics of amorphous materials and dynamical heterogeneity

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For two years we have investigated glass transitions and the dynamics of the α-process in polystyrene (PS), poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) by dielectric relaxation spectroscopy and thermal expansion spectroscopy. The results obtained in our study are summarized as follows :PS : The profile of dielectric loss vs. frequency becomes broader as the film thickness decreases. This suggest that the depression of Tg is associated with the broadening of the relaxation times of the α-process. Relaxation measurements showed that the thickness dependence of the relaxation times of the α-process becomes much prominent near Tg compared with that at higher temperatures. This can be accounted for by assuming the existence of the characteristic length scale which increases as temperature approaches to Tg.PVAc : The temperature T_α at which the dielectric loss has a maximum due to the α-process in temperature domain at 100Hz has no molecular weight dependence in the Mw-range from 1.2×10^5 to 2.4×10^5. From this result, it is expected that Tg of thin PVAc films decreases with decreasing film thickness and the functional form of Tg with respect to thickness has almost no Mw-dependence.PMMA : The temperature T_α of the α-peak in the dielectric loss at 40Hz has been measured. It is found that the temperature T_α remains constant from the bulk thickness to a critical thickness d_c and below d_c it decreases rapidly with decreasing thickness. On the other hand, the temperature T_β of the β-process corresponding to T_α of the α-process decreases slightly with decreasing film thickness even for relatively larger thickness. Below the same critical thickness, T_β decreases rapidly with decreasing film thickness. The prominent thickness dependence of T_α and T_β has been discussed from the view point of confinement of polymer chains and cooperativity of the dynamics.