Connecting Dynamics and Thermodynamics to Predict Mobility and Glassiness

非技术摘要这项研究将使人们对某些非常广泛使用的材料的行为以及它们有时表现不佳的原因有新的理解。有些固体物质是结晶的，食盐和糖就是两个常见的例子。在这些情况下，构成固体的原子或分子处于规则的位置和高度有序的状态，它们的结构和性质不会随着时间的推移而改变--它们处于平衡状态。在许多其他情况下，固体是玻璃状的，就像处于液体状态的颗粒被粘在适当的位置上一样。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。特别设计的材料中，有很大一部分通常涉及被称为聚合物的非常大的分子，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，但它们的行为在某些方面仍然没有被很好地理解。日常生活中的例子包括保护性涂层、光致抗蚀剂、纳米颗粒增强的聚合物、薄膜和过滤器。在每一种情况下，都有相当一部分玻璃状固体分子靠近界面；不同类型的实验得出的结论是，这些分子的行为可能与散布在块体中的相邻分子非常不同。一个相关的问题是，在玻璃固体特性经过精心设计以优化性能的情况下，随着材料的老化，其特性可能会以不受欢迎的方式发生变化。这些情况反映了这样一个事实，即玻璃固体与晶体固体不同，它们并不处于平衡状态；它们的性质会随着时间的推移而变化。这些更改可能会影响它们的性能，因此会导致问题。该项目将产生模拟玻璃固体的新方法，该方法将考虑邻近表面对分子堆积的影响。这些模型将与实验数据一起工作，并导致新的方法来理解和预测当界面存在时玻璃材料行为可能发生变化的方式。技术摘要材料的动态松弛通过分段运动局部开始；它在短时间和长度尺度上的进展驱动更长和更大的响应。动态行为还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改进热力学(局部关联格子)和动力学(合作自由体积)模型来揭示这些领域之间的深层联系，这些模型源于PI的研究小组。这套新的工具将被用于在广泛的条件下预测玻璃材料的行为。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效应的不同实验途径的影响。这项研究涉及到相关的项目：(A)将开发一个模型，用于从表面非常近(纳米)到恢复整体行为的距离的界面区域。这将导致对局部密度、迁移率(迁移层厚度)、分段松弛时间和合作运动的变化长度的预测，作为到界面的距离、温度和薄膜厚度的函数。(B)将引入一个新的模型来反映界面区域(颗粒附近)和基质之间的差异。这个模型受热力学性质的影响，将导致对分段动力学和合作长度尺度随温度、压力和纳米颗粒装载量的函数变化的预测。(C)从(A)和(B)的进展将导致将玻璃成形器的热力学和动力学性质与其长期稳定性联系起来的新方法。这项工作还将有助于深入了解这些联系是如何依赖于玻璃的实验路径的。该项目所针对的材料具有极其广泛的功能，这项研究将导致对它们的分子性质与使其独特适合其应用的特定性质的联系有新的见解。这项研究产生的新模型将是可访问的，通常用于将热力学性质与动力学行为联系起来。PI将进一步成功地努力吸引和鼓励妇女参与STEM，并将继续与研究生和本科生合作，为公共知识库增加与聚合物相关的科学的新资源。PI还将继续努力向公众介绍物理科学的基本概念。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.