Fukao K, Miyamoto Y
Faculty of Integrated Human Studies, Kyoto University, Kyoto 606-8501, Japan.
Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics. 2000 Feb;61(2):1743-54. doi: 10.1103/physreve.61.1743.
The glass transition temperature T(g) and the temperature T(alpha) corresponding to the peak in the dielectric loss due to the alpha process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in T(g) was observed with decreasing film thickness, while T(alpha) was found to remain almost constant for d>d(c) and to decrease drastically with decreasing d for d<d(c). Here, d(c) is a critical thickness dependent on molecular weight. The relaxation time tau(alpha) of the alpha process, which was measured as the frequency at which the dielectric loss realizes its peak value at a given temperature, was found to have a d dependence similar to that of T(alpha). The relaxation function for the alpha process was obtained by using the observed frequency dependence of the peak profile of the dielectric loss. The exponent beta(KWW), which was obtained from the relaxation functions, decreases as thickness decreases. This suggests that the distribution of relaxation times for the alpha process broadens with decreasing thickness. The thickness dependence of T(g) is directly related to the distribution of relaxation times for the alpha process, not to the relaxation time itself. The value of the thermal expansion coefficient normal to the film surface was found to increase with decreasing film thickness below T(g), but to decrease with decreasing film thickness above T(g). These experimental results are discussed in the context of a three-layer model in which within thin films there are three layers with different mobilities and glass transition temperatures.
通过对支撑在玻璃基板上的聚苯乙烯薄膜进行介电测量，已同时确定了玻璃化转变温度T(g)以及与α过程引起的介电损耗峰值相对应的温度T(α)作为薄膜厚度d的函数。还研究了给定温度下介电损耗峰值随频率的变化。观察到T(g)随薄膜厚度减小而降低，而对于d>d(c)，T(α)几乎保持恒定，对于d<d(c)，T(α)随d减小而急剧下降。这里，d(c)是取决于分子量的临界厚度。α过程的弛豫时间tau(α)，被测量为在给定温度下介电损耗达到其峰值的频率，发现其对d的依赖性与T(α)相似。通过使用观察到的介电损耗峰值轮廓的频率依赖性获得了α过程的弛豫函数。从弛豫函数得到的指数beta(KWW)随着厚度减小而减小。这表明α过程的弛豫时间分布随着厚度减小而变宽。T(g)的厚度依赖性与α过程的弛豫时间分布直接相关，而不是与弛豫时间本身相关。发现垂直于薄膜表面的热膨胀系数的值在低于T(g)时随薄膜厚度减小而增加，但在高于T(g)时随薄膜厚度减小而减小。这些实验结果在三层模型的背景下进行了讨论，在该模型中，薄膜内存在具有不同迁移率和玻璃化转变温度的三层。
