Dynamics of amorphous materials and dynamical heterogeneity

两年来，我们用介电弛豫谱和热膨胀谱研究了聚苯乙烯(PS)、聚醋酸乙烯酯(PVAc)和聚甲基丙烯酸甲酯(PMMA)的玻璃化转变和α过程的动力学。我们的研究结果总结如下：PS：介质损耗随频率的变化曲线随着薄膜厚度的减小而变宽。这表明，TG的降低与α过程的驰豫时间加宽有关。弛豫测量表明，与较高温度下相比，在Tg附近，α过程的弛豫时间随厚度的变化更为显著。这可以通过假设特征长度标度的存在来解释，该特征长度标度随着温度的增加而增加。PVAc：在100 Hz的温度域中，由于α过程而产生的介电损耗最大的温度T_α在1.2×10~2.4×10^5的mw范围内与分子量无关。从这一结果可以预期，PVAc薄膜的Tg随着薄膜厚度的减小而减小，Tg的函数形式与厚度几乎没有关系。PMMA：测量了在40 Hz的介电损耗中α峰的温度T_pAc。结果表明，从体厚到临界厚度dc，再到dc以下，温度T_α保持不变，并随厚度的减小而迅速下降。另一方面，即使在相对较大的厚度下，β工艺的温度T_α也随着薄膜厚度的减小而略有下降，该温度对应于α工艺的T_BET。在相同的临界厚度下，T_β随膜厚的减小而迅速减小。从高分子链的约束和动力学的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.