Department of Polymer Science, The University of Akron, Akron, Ohio 44325, USA.
J Chem Phys. 2009 Nov 21;131(19):194511. doi: 10.1063/1.3266508.
The mechanism behind the steep slowing down of molecular motions upon approaching the glass transition remains a great puzzle. Most of the theories relate this mechanism to the cooperativity in molecular motion. In this work, we estimate the length scale of molecular cooperativity xi for many glass-forming systems from the collective vibrations (the so-called boson peak). The obtained values agree well with the dynamic heterogeneity length scale estimated using four-dimensional NMR. We demonstrate that xi directly correlates to the dependence of the structural relaxation on volume. This dependence presents only one part of the mechanism of slowing down the structural relaxation. Our analysis reveals that another part, the purely thermal variation in the structural relaxation (at constant volume), does not have a direct correlation with molecular cooperativity. These results call for a conceptually new approach to the analysis of the mechanism of the glass transition and to the role of molecular cooperativity.
分子运动在接近玻璃转变时急剧减缓的背后机制仍然是一个巨大的谜团。大多数理论将这种机制与分子运动的协同作用联系起来。在这项工作中，我们从集体振动（所谓的玻色峰）估算了许多玻璃形成系统的分子协同作用长度尺度 ξ。得到的值与使用四维 NMR 估计的动态不均匀性长度尺度很好地吻合。我们证明 ξ 直接与结构弛豫对体积的依赖性相关。这种依赖性仅代表了减缓结构弛豫的机制的一部分。我们的分析表明，另一个部分，即结构弛豫的纯热变化（在恒体积下），与分子协同作用没有直接关系。这些结果需要对玻璃转变机制和分子协同作用的作用进行概念上的新分析。
