International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
J Phys Chem B. 2010 Aug 26;114(33):10734-9. doi: 10.1021/jp104520f.
Deeply supercooled fragile liquid is known to be dynamically heterogeneous, where super Arrhenius behavior of shear viscosity and alpha and beta relaxation processes have been observed. To clarify origins of these behaviors, we have investigated correlations between microscopic molecular diffusion and macroscopic hydrodynamics of vapor-deposited toluene films by using time-of-flight secondary ion mass spectrometry. The molecules are intermixed gradually at around 70 K on the film deposited at 15 K, which is followed by an abrupt film morphology change at around the calorimetric glass-transition temperature (T(g)) of 117 K. For the film deposited at 100 K, intermixing of the molecules occurs at temperatures over 100-130 K, but no abrupt increase in diffusivity is recognizable at T(g). This result can be explained as dynamical heterogeneity or decoupling between translational diffusion and viscosity. The self-diffusion is thought to occur in a sub-T(g) region as a precursor state of the glass-liquid transition; that is, individual motion in the solid-like state evolves into cooperative motion of liquid-like state at T(g). The supercooled liquid nucleates at 147 K, although the crystal can grow even at 117 K provided that nuclei preexist. The origins of unusual glass transition behaviors of amorphous solid water are also discussed in comparison with this standard.
深度过冷的脆性液体是动态不均匀的，在这种液体中已经观察到剪切黏度的超 Arrhenius 行为和 α 和 β 弛豫过程。为了澄清这些行为的起源，我们通过飞行时间二次离子质谱法研究了蒸气沉积的甲苯薄膜的微观分子扩散和宏观流体动力学之间的相关性。在 15 K 下沉积的薄膜中，分子在大约 70 K 时逐渐混合，随后在大约 117 K 的量热玻璃化转变温度（Tg）处突然发生薄膜形貌变化。对于在 100 K 下沉积的薄膜，分子在 100-130 K 以上的温度下混合，但在 Tg 处无法识别扩散率的突然增加。这一结果可以解释为平移扩散和黏度之间的动力学不均匀性或解耦。自扩散被认为发生在亚 Tg 区域作为玻璃-液体转变的前体状态；也就是说，固态中的个体运动在 Tg 处演变成液态的协同运动。过冷液体在 147 K 时成核，尽管只要有晶核存在，晶体甚至可以在 117 K 时生长。与该标准相比，还讨论了非晶态固体水异常玻璃化转变行为的起源。
