Giovambattista Nicolas, Angell C Austen, Sciortino Francesco, Stanley H Eugene
Center for Polymer Studies and Department of Physics, Boston University, Massachusetts 02215, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2005 Jul;72(1 Pt 1):011203. doi: 10.1103/PhysRevE.72.011203. Epub 2005 Jul 19.
We perform molecular dynamics (MD) simulations using the extended simple-point-charge (SPC/E) model for water to study the structural relaxation through the glass transition region. We follow the same standard protocol used in differential scanning calorimetry experiments. Specifically, we cool liquid configurations at different cooling rates to produce a glass, and then we heat the glass back to the liquid state. We also study aging effects in the glass before heating. We find that MD simulations can reproduce the phenomenology observed in experiments. We review the Tool-Narayanaswamy-Moynihan (TNM) phenomenological approach, introduced to describe the behavior of the specific heat upon heating glass to the liquid phase. The TNM approach requires, as an ansatz, an expression for the temperature dependence of the relaxation time. We compare the simulation results with the prediction of the TNM approach supplemented with two well-known expressions for the relaxation time: the Narayanaswamy-Moynihan (NM) and the Adam-Gibbs-Scherer (AGS) expressions. We find that, in the case of slow cooling rates, our simulations are well represented by the TNM approach, but only if the AGS expression is adopted. We also find that the TNM approach fails in the case of fast cooling rates for both NM and AGS expressions. Several attempts to provide more freedom to the fitting procedure by allowing the fitting parameters to depend on temperature, cooling, and/or heating rates do not improve the agreement between the simulation data and the TNM predictions.
我们使用扩展的简单点电荷（SPC/E）水模型进行分子动力学（MD）模拟，以研究通过玻璃化转变区域的结构弛豫。我们遵循差示扫描量热法实验中使用的相同标准方案。具体而言，我们以不同的冷却速率冷却液体构型以产生玻璃态，然后将玻璃态加热回液态。我们还研究了加热前玻璃态中的老化效应。我们发现MD模拟可以重现实验中观察到的现象学。我们回顾了Tool-Narayanaswamy-Moynihan（TNM）唯象学方法，该方法用于描述将玻璃加热到液相时比热的行为。TNM方法作为一个假设，需要一个弛豫时间的温度依赖性表达式。我们将模拟结果与补充了两个著名的弛豫时间表达式的TNM方法的预测进行比较：Narayanaswamy-Moynihan（NM）表达式和Adam-Gibbs-Scherer（AGS）表达式。我们发现，在慢冷却速率的情况下，只有采用AGS表达式时，我们的模拟才能很好地由TNM方法表示。我们还发现，对于NM和AGS表达式，在快冷却速率的情况下，TNM方法都失败了。通过允许拟合参数依赖于温度、冷却和/或加热速率来为拟合过程提供更多自由度的几次尝试并没有改善模拟数据与TNM预测之间的一致性。
