Kokshenev Valery B, Borges Pablo D, Sullivan Neil S
Departamento de Física, Universidade Federal de Minas Gerais, Brazil.
J Chem Phys. 2005 Mar 15;122(11):114510. doi: 10.1063/1.1855877.
The primary relaxation time scale tau(T) derived from the glass forming supercooled liquids (SCLs) is discussed within ergodic-cluster Gaussian statistics, theoretically justified near and above the glass-transformation temperature T(g). An analysis is given for the temperature-derivative data by Stickel et al. on the steepness and the curvature of tau(T). Near the mode-coupling-theory (MCT) crossover T(c), these derivatives separate by a kink and a jump, respectively, the moderately and strongly SCL states. After accounting for the kink and the jump, the steepness remains a piecewise conitnuous function, a material-independent equation for the three fundamental characteristic temperatures, T(g), T(c), and the Vogel-Fulcher-Tamman (VFT) T(0), is found. Both states are described within the heterostructured model of solidlike clusters parametrized in a self-consistent manner by a minimum set of observable parameters: the fragility index, the MCT slowing-down exponent, and the chemical excess potential of Adam and Gibbs model (AGM). Below the Arrhenius temperature, the dynamically and thermodynamically stabilized clusters emerge with a size of around of seven to nine and two to three molecules above and close to T(g) and T(c), respectively. On cooling, the main transformation of the moderately into the strongly supercooled state is due to rebuilding of the cluster structure, and is attributed to its rigidity, introduced through the cluster compressibility. It is shown that the validity of the dynamic AGM (dynamically equivalent to the standard VFT form) is limited by the strongly supercooled state (T(g) < T < T(c)) where the superrigid cooperative rearranging regions are shown to be well-chosen parametrized solidlike clusters. Extension of the basic parameter set by the observable kinetic and diffusive exponents results in prediction of a subdiffusion relaxation regime in SCLs that is distinct from that established for amorphous polymers.
从玻璃形成过冷液体（SCLs）中得出的主要弛豫时间尺度τ(T)，在遍历团簇高斯统计中进行了讨论，该统计在玻璃化转变温度T(g)附近及以上具有理论依据。对Stickel等人关于τ(T)的陡度和曲率的温度导数数据进行了分析。在模式耦合理论（MCT）交叉温度T(c)附近，这些导数分别通过一个扭结和一个跃变，将适度和强烈的SCL状态区分开来。在考虑了扭结和跃变之后，陡度仍然是一个分段连续函数，找到了一个与材料无关的关于三个基本特征温度T(g)、T(c)和Vogel-Fulcher-Tamman（VFT）T(0)的方程。两种状态都在以一组最少的可观测参数自洽参数化的类固体团簇异质结构模型中进行了描述：脆性指数、MCT减速指数以及Adam和Gibbs模型（AGM）的化学超额势。在阿仑尼乌斯温度以下，动态和热力学稳定的团簇出现，其大小在T(g)以上约为七到九个分子，在T(c)附近及以下约为两到三个分子。在冷却过程中，适度过冷状态向强烈过冷状态的主要转变是由于团簇结构的重建，这归因于通过团簇压缩性引入的其刚性。结果表明，动态AGM（在动力学上等同于标准VFT形式）的有效性受到强烈过冷状态（T(g) < T < T(c)）的限制，在该状态下，超刚性协同重排区域被证明是参数化良好的类固体团簇。通过可观测的动力学和扩散指数扩展基本参数集，可预测SCLs中的亚扩散弛豫区域，该区域与非晶态聚合物中建立的区域不同。
