Ngai K L, Casalini R, Capaccioli S, Paluch M, Roland C M
Naval Research Laboratory, Washington D.C. 20375-5320, Chemistry Department, George Mason University, Fairfax, Virginia, USA.
J Phys Chem B. 2005 Sep 22;109(37):17356-60. doi: 10.1021/jp053439s.
Upon decreasing temperature or increasing pressure, a noncrystallizing liquid will vitrify; that is, the structural relaxation time, taualpha, becomes so long that the system cannot attain an equilibrium configuration in the available time. Theories, including the well-known free volume and configurational entropy models, explain the glass transition by invoking a single quantity that governs the structural relaxation time. The dispersion of the structural relaxation (i.e., the structural relaxation function) is either not addressed or is derived as a parallel consequence (or afterthought) and thus is independent of taualpha. In these models the time dependence of the relaxation bears no fundamental relationship to the value of taualpha or other dynamic properties. Such approaches appear to be incompatible with a general experimental fact recently discovered in glass-formers: for a given material at a fixed value of taualpha, the dispersion is constant, independent of thermodynamic conditions (T and P); that is, the shape of the alpha-relaxation function depends only on the relaxation time. If derived independently of taualpha, it is an unlikely result that the dispersion of the structural relaxation would be uniquely defined by taualpha.
当温度降低或压力增加时，一种非结晶液体将发生玻璃化转变；也就是说，结构弛豫时间τα会变得极长，以至于系统在可利用的时间内无法达到平衡构型。包括著名的自由体积模型和构型熵模型在内的理论，通过引入一个控制结构弛豫时间的单一量来解释玻璃化转变。结构弛豫的色散（即结构弛豫函数）要么未被涉及，要么是作为一个并行的结果（或事后的想法）推导出来的，因此与τα无关。在这些模型中，弛豫的时间依赖性与τα的值或其他动态性质没有根本关系。这些方法似乎与玻璃形成体中最近发现的一个普遍实验事实不相容：对于给定材料在τα的固定值下，色散是恒定的，与热力学条件（T和P）无关；也就是说，α弛豫函数的形状仅取决于弛豫时间。如果独立于τα推导，结构弛豫的色散由τα唯一确定是不太可能的结果。
