Progress towards a phenomenological picture and theoretical understanding of glassy dynamics and vitrification near interfaces and under nanoconfinement
Progress towards a phenomenological picture and theoretical understanding of glassy dynamics and vitrification near interfaces and under nanoconfinement
复制标题
DOI：
10.1063/1.5129405
复制
发表时间：
2019-12-28
影响因子：
4.4
通讯作者：
Simmons, David S.
中科院分区：
文献类型：
--
作者：
Schweizer, Kenneth S.;Simmons, David S.
界面纳米级附近的动力学和玻璃化变化的性质（通常称为玻璃化转变的“纳米限制”效应）在四分之一个世纪以来一直是一个悬而未决的问题。我们首先分析过去十年的实验和模拟结果，以构建整体现象学图景。主要特征包括：在计量和化学相关的开始之后，近界面弛豫时间遵循与体弛豫的分数幂律解耦关系；弛豫时间随着距界面的距离以双指数方式变化，内在的动态长度尺度在低温下似乎饱和；激活势垒和玻璃化温度 T-g 以空间指数方式接近体行为；所有这些行为在数量上取决于界面的性质。我们证明，各个系统的薄膜平均 T-g 的厚度依赖性为区分不同理论提供了较差的基础，因此我们根据上述动态梯度特性评估了它们的优点。基于熵的理论似乎与现象学表现出明显的不一致。各种自由体积驱动理论与观察结果的一致性各不相同，其中调用协作运动的方法最有希望。弹性协作非线性朗之万方程理论似乎捕获了现象学的最大部分，尽管重要的方面仍有待解决。完整的理论理解需要改进与模拟和实验的对抗，这些模拟和实验在可访问的 1-ps 到 1 年的时间窗口内探测空间异质动力学，最小化可调节参数的使用，并认识到对化学和界面的丰富的定量依赖性。由 AIP Publishing 许可发布。
The nature of alterations to dynamics and vitrification in the nanoscale vicinity of interfaces-commonly referred to as "nanoconfinement" effects on the glass transition-has been an open question for a quarter century. We first analyze experimental and simulation results over the last decade to construct an overall phenomenological picture. Key features include the following: after a metrology- and chemistry-dependent onset, near-interface relaxation times obey a fractional power law decoupling relation with bulk relaxation; relaxation times vary in a double-exponential manner with distance from the interface, with an intrinsic dynamical length scale appearing to saturate at low temperatures; the activation barrier and vitrification temperature T-g approach bulk behavior in a spatially exponential manner; and all these behaviors depend quantitatively on the nature of the interface. We demonstrate that the thickness dependence of film-averaged T-g for individual systems provides a poor basis for discrimination between different theories, and thus we assess their merits based on the above dynamical gradient properties. Entropy-based theories appear to exhibit significant inconsistencies with the phenomenology. Diverse free-volume-motivated theories vary in their agreement with observations, with approaches invoking cooperative motion exhibiting the most promise. The elastically cooperative nonlinear Langevin equation theory appears to capture the largest portion of the phenomenology, although important aspects remain to be addressed. A full theoretical understanding requires improved confrontation with simulations and experiments that probe spatially heterogeneous dynamics within the accessible 1-ps to 1-year time window, minimal use of adjustable parameters, and recognition of the rich quantitative dependence on chemistry and interface. Published under license by AIP Publishing.