Department of Physics, Syracuse University, Syracuse, NY 13244;
Google Brain, Google, Mountain View, CA 94043.
Proc Natl Acad Sci U S A. 2017 Oct 3;114(40):10601-10605. doi: 10.1073/pnas.1703927114. Epub 2017 Sep 19.
Nanometrically thin glassy films depart strikingly from the behavior of their bulk counterparts. We investigate whether the dynamical differences between a bulk and thin film polymeric glass former can be understood by differences in local microscopic structure. Machine learning methods have shown that local structure can serve as the foundation for successful, predictive models of particle rearrangement dynamics in bulk systems. By contrast, in thin glassy films, we find that particles at the center of the film and those near the surface are structurally indistinguishable despite exhibiting very different dynamics. Next, we show that structure-independent processes, already present in bulk systems and demonstrably different from simple facilitated dynamics, are crucial for understanding glassy dynamics in thin films. Our analysis suggests a picture of glassy dynamics in which two dynamical processes coexist, with relative strengths that depend on the distance from an interface. One of these processes depends on local structure and is unchanged throughout most of the film, while the other is purely Arrhenius, does not depend on local structure, and is strongly enhanced near the free surface of a film.
纳米级薄玻璃状薄膜与它们的块状对应物的行为明显不同。我们研究了大块和薄膜聚合物玻璃形成体之间的动力学差异是否可以通过局部微观结构的差异来理解。机器学习方法表明，局部结构可以作为成功预测大块系统中粒子重排动力学的基础。相比之下，在薄玻璃状薄膜中，我们发现尽管动力学非常不同，但处于薄膜中心的粒子和靠近表面的粒子在结构上无法区分。接下来，我们表明，在薄玻璃状薄膜中，已经存在于大块系统中且与简单的促进动力学明显不同的无结构依赖性过程，对于理解薄膜中的玻璃状动力学至关重要。我们的分析表明，玻璃状动力学存在两种共存的动力学过程，其相对强度取决于与界面的距离。其中一个过程取决于局部结构，并且在整个薄膜中基本不变，而另一个过程是纯粹的 Arrhenius 型，不依赖于局部结构，并且在薄膜的自由表面附近被强烈增强。
