Dynamics of amorphous materials and dynamical heterogeneity

两年来，我们用介电弛豫光谱和热膨胀光谱研究了聚苯乙烯（PS）、聚醋酸乙烯（PVAc）和聚甲基丙烯酸甲酯（PMMA）中的玻璃化转变和α-过程的动力学。本文的研究结果总结如下：PS：随着薄膜厚度的减小，介质损耗随频率的变化曲线变宽。这表明Tg的降低与α-过程弛豫时间的延长有关。弛豫测量表明，在Tg附近，α-过程的弛豫时间对厚度的依赖性比在高温下更为明显。这可以通过假设存在随着温度接近Tg而增加的特征长度尺度来解释。PVAc：在100Hz温度域α-过程导致介质损耗最大的T_α温度在1.2×10^5 ~ 2.4×10^5的mw范围内与分子量无关。由此可知，PVAc薄膜的Tg随膜厚的减小而减小，Tg对膜厚的函数形式几乎没有mw依赖性。测量了40Hz时介电损耗α峰的T_α温度。发现温度T_α在体厚至临界厚度d_c范围内保持恒定，在d_c以下随厚度的减小而迅速降低。另一方面，随着膜厚的减小，α-过程的T_α对应的β-过程的温度T_β略有降低，即使膜厚相对较大。在相同的临界厚度下，T_β随膜厚的减小而迅速减小。从聚合物链约束和动力学协同性的角度讨论了T_α和T_β的显著厚度依赖性。

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.