Connecting Dynamics and Thermodynamics to Predict Mobility and Glassiness

这项研究将导致对某些非常广泛使用的材料如何表现以及为什么它们有时会表现不佳的新理解。有些固体材料是结晶的；食盐和糖就是两个熟悉的例子。在这种情况下，构成固体的原子或分子的位置是有规律的，组织是高度有序的，它们的结构和性质不随时间而改变——它们处于平衡状态。在许多其他情况下，固体是玻璃状的，就好像液体状态的颗粒被固定在原地一样。窗户玻璃只是一个例子；它是由含有硅和氧的小分子组成的。很大一部分特殊设计的材料，通常涉及被称为聚合物的大分子，也处于玻璃固体状态。尽管几个世纪以来，玻璃固体一直是我们周围环境的一部分，但它们的行为仍有一些方面尚未得到很好的理解。日常生活中的例子包括防护涂层、光刻胶、纳米颗粒增强聚合物、膜和过滤器。在每一种情况下，玻璃状固体分子的很大一部分在界面附近；不同种类的实验得出的结论是，这些分子的行为与周围的分子非常不同。一个相关的问题是，当玻璃固体性能被精心设计以优化性能时，随着材料的老化，其性能可能会以不希望的方式发生变化。这些情况反映了一个事实，即玻璃状固体不像结晶固体，处于不平衡状态；它们的性质会随着时间的推移而改变。这些更改会影响它们的性能，从而导致问题。这个项目将产生模拟玻璃固体的新方法，这将解释相邻表面对分子如何堆积的影响。这些模型将与实验数据一起工作，并产生新的方法来理解和预测当界面存在时玻璃材料行为的变化方式。摘要材料的动态松弛是通过节段运动局部开始的；它在短时间和长度尺度上的进展驱动了更长更大的反应。动力学行为也反映了决定热力学性质的局部结构和能量特征。这里提出的研究将通过使用和推进源自PI研究小组的热力学（局部相关晶格）和动态（合作自由体积）模型来揭示这些领域之间的深层联系。这套新工具将用于预测玻璃材料在各种条件下的行为。两个特别关注的领域涉及界面区域的存在和用于模拟长时间老化影响的不同实验途径的影响。本研究涉及相关的项目：(a)将为界面区域开发一个模型，从非常接近（nm）的表面到恢复体行为的距离。它将预测局部密度、迁移率（可移动层厚度）、段松弛时间和协同运动的变化长度尺度，作为与界面的距离、温度和薄膜厚度的函数。(b)将引入纳米复合材料的新模型，该模型反映了界面区域（颗粒旁边）和基体之间的差异。根据热力学性质，该模型将预测分段动力学和协同长度尺度随温度、压力和纳米颗粒负载的变化。(c) (a)和(b)的进展将导致将玻璃形成物的热力学和动态特性与其长期稳定性联系起来的新方法。这项工作还将使人们了解这些连接如何依赖于玻璃性的实验路径。该项目针对的材料具有非常广泛的功能，这项研究将使人们对其分子性质如何与特定性质联系在一起产生新的见解，从而使它们独特地适合于它们的应用。这项研究产生的新模型将是可访问的，并且对于将热力学性质与动态行为联系起来通常是有用的。该项目将进一步成功地吸引和鼓励女性参与STEM，并将继续与研究生和本科生合作，为公共知识库增加聚合物相关科学的新资源。PI还将继续积极努力，向公众介绍物理科学的基本概念。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical abstractThis research will lead to new understanding of how certain very widely-used materials behave and why they sometimes misbehave. Some solid materials are crystalline; table salt and sugar are two familiar examples. In these cases the atoms or molecules that make up the solid are regularly positioned and highly organized, and their structure and properties do not change with time - they are at equilibrium. In many other cases the solids are glassy, as if the particles in the liquid state were stuck in place. Window glass is just one example; it is made of small molecules containing silicon and oxygen. A large fraction of specially designed materials, which often involve very large molecules known as polymers, are also in the glassy solid state. Although glassy solids have been part of our surroundings for centuries, there are still aspects of their behavior that are not well understood. Examples from daily life include protective coatings, photoresists, polymers reinforced with nanoparticles, membranes, and filters. In each of these cases a significant fraction of the glassy solid molecules are near an interface; different kinds of experiments have led to the conclusion that these molecules can behave very differently from neighboring molecules ensconced in the bulk. A related problem is that, where glassy solid properties are carefully designed to optimize performance, as the material ages its properties can change in undesirable ways. These situations reflect the fact that glassy solids, unlike crystalline solids, are not at equilibrium; and their properties can shift over time. These changes can affect their performance, and therefore cause problems. This project will produce new ways of modeling glassy solids that will account for the influence of a neighboring surface on how molecules pack. The models will work together with experimental data and lead to new methods for understanding and predicting the ways that glassy material behavior can change when interfaces are present.Technical abstractDynamic relaxation of material begins locally through segmental motion; its progression over short time and length scales drives the longer and larger response. Dynamic behavior also reflects the local structural and energetic characteristics that determine thermodynamic properties. The research proposed here will reveal deep connections between these realms by using and advancing thermodynamic (Locally Correlated Lattice) and dynamic (Cooperative Free Volume) models originated in the PI's research group. This new set of tools will be applied to predict glassy material behavior over a wide range of conditions. Two areas of particular focus involve the presence of interfacial regions and the effect of different experimental pathways used to mimic the effects of long-time aging. This research involves interrelated projects:(a) A model will be developed for the interfacial region from very near (nm) the surface to distances where bulk behavior is recovered. It will lead to predictions for local density, mobility (mobile layer thickness), segmental relaxation times, and the changing lengthscale of cooperative motion, as functions of distance from the interface, temperature, and film thickness.(b) A new model will be introduced for nanocomposites that reflects differences between the interfacial region (next to the particles) and the matrix. This model, informed by thermodynamic properties, will lead to predictions for segmental dynamics and changes in cooperative length scales as functions of temperature, pressure, and nanoparticle loading.(c) Advances from (a) and (b) will lead to new ways for connecting the thermodynamic and dynamic properties of glass formers to their long term stability. This work will also lead to insight about how those connections depend on the experimental path to glassiness.The materials targeted by this project serve an extremely wide range of functions, and this research will lead to new insight about how their molecular nature links to the particular properties that make them uniquely suited to their applications. The new models resulting from this research will be accessible and generally useful for linking thermodynamic properties to dynamic behavior.The PI will further successful efforts to involve and encourage STEM participation among women and will continue working with graduate and undergraduate students to add new resources on polymer-related science to the public knowledge base. The PI will also continue energetic efforts to introduce fundamental concepts in physical science to the general public.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.