Blochowicz T, Gainaru C, Medick P, Tschirwitz C, Rössler E A
TU-Darmstadt, 64289 Darmstadt, Germany.
J Chem Phys. 2006 Apr 7;124(13):134503. doi: 10.1063/1.2178316.
The susceptibility spectra of ten molecular glass formers are completely interpolated by an extension of the generalized gamma distribution of correlation times. The data cover at least 15 decades in frequency and the interpolation includes both alpha peak and excess wing. It is shown that the line shape parameters and the time constant of the alpha relaxation are related to each other. Master curves are identified by a scaling procedure that involves only three parameters, namely, the glass transition temperature T(g), the fragility m, and the excess wing exponent at T(g). This holds independent of whether a further secondary relaxation peak is present or not. Above a crossover temperature T(x) this unique evolution of the line shape parameters breaks down, and a crossover to a simple peak susceptibility without excess wing is observed. Here, the frequency-temperature superposition principle holds in good approximation up to temperatures well above the melting point. It turns out that the crossover coincides with the temperature at which the low-temperature Vogel-Fulcher law starts to fail upon heating. Thus, the so-called Stickel temperature gets a more physical meaning as it marks a qualitative change in the evolution of the susceptibility spectra of glass formers. Moreover, the interrelation of the line shape parameters can explain why the "Nagel scaling" works in some approximation. Our study demonstrates that the excess wing in molecular glass formers is a secondary relaxation, which is linked to the alpha process in a unique way.
通过相关时间广义伽马分布的扩展，完全内插了十种分子玻璃形成体的磁化率谱。数据覆盖至少15个数量级的频率，且内插包括α峰和过量翼。结果表明，α弛豫的线形参数与时间常数相互关联。通过仅涉及三个参数的标度程序确定主曲线，这三个参数分别是玻璃化转变温度T(g)、脆性m和T(g)处的过量翼指数。这一结论与是否存在进一步的次级弛豫峰无关。在交叉温度T(x)以上，线形参数的这种独特演化失效，观察到向没有过量翼的简单峰磁化率的交叉。在此，频率 - 温度叠加原理在远高于熔点的温度下仍能很好地近似成立。结果表明，交叉与低温Vogel - Fulcher定律在加热时开始失效的温度一致。因此，所谓的Stickel温度具有了更实际的意义，因为它标志着玻璃形成体磁化率谱演化中的定性变化。此外，线形参数的相互关系可以解释为什么“Nagel标度”在某种近似下有效。我们的研究表明，分子玻璃形成体中的过量翼是一种次级弛豫，它以独特的方式与α过程相关联。
