Polymer Glass, Melt, and Mixture Thermodynamics in the Bulk and in Thin Films

技术摘要材料研究部和化学部为该奖项提供资金。它支持理论研究和教育，包括应用统计热力学方法来解决基础和应用问题。聚合物薄膜在电子、光学、保护膜等领域具有潜在的应用前景；此外，它们是纳米复合材料中普遍存在的成分。结构完整性的维护是关键，因此必须确保操作条件不接近玻璃化转变温度。来自各种实验技术的证据表明，相对于体积值，支撑基板和自由表面都可以对聚合物薄膜的玻璃化转变温度产生深远的影响。然而，对这些结果的解释是有争议的。理论可以通过发展具有明确定义的假设的明确阐明的模型，并应用它们来确定随之而来的物理行为，从而在解决争议方面发挥重要作用。该项目的一个重点是建立在PI之前的工作基础上，该工作产生了一个热力学性质模型，该模型似乎能够预测与实验数据相匹配的独立聚合物薄膜的玻璃化转变。PI的目标是利用这个和相关的模型来预测聚合物薄膜中的玻璃化转变和相分离。一个重要的问题是，是否有可能足够好地捕捉到热力学行为——不仅是自由能，还有熵和焓对它的贡献——这样，关于任何混合物的预测都可以只用纯组分实验数据作为输入。PI的基于晶格的聚合物流体和混合物理论分析了几十种聚合物和混合物，在纯组分性质和热力学行为之间建立了有趣的联系。PI计划探索组分的策略选择是否会导致聚合物共混物和溶液中混合的焓和熵的控制，从而允许仅基于纯组分性质对混相进行真正的预测。在研究过程中建立的理论模型具有广泛应用的潜力。将特征纯组分行为与混合热力学的焓和熵贡献联系起来的能力将为新材料的开发提供途径。这项研究的另一个成果将是一个热力学模型，用于描述聚合物及其混合物在从体到膜的过程中局部和平均行为是如何变化的。这项研究的影响将通过研究生和博士后的参与，通过出版物和他们在国家会议上的演讲来扩大。PI在研究和教学指导方面的努力使研究生和研究生队伍以及教员中的妇女人数增加；预计这种影响将持续下去。材料研究部和化学部为本奖项提供资金。一个理论家长久以来的抱负是用概念上透明的工具来解决复杂的问题。PI将专注于研究领域，从玻璃聚合物薄膜的研究到新的，未经测试的，含聚合物混合物的热力学性质。初步证据表明，来自同一基本集合的工具可能在这一兴趣范围内有效。当一个简单的工具在解决一系列复杂问题时显示出令人信服的效果时，它不仅揭示了问题的一些基本要素，而且还阐明了看似截然不同的问题之间可能存在的关联程度。一种典型的小分子液体，冷却后可以变成无定形的玻璃或晶体。通常被认为是“玻璃”的东西，比如在窗户和饮水罐里发现的东西，其主要成分是二氧化硅和其他小分子。然而，像聚合物这样的大链状分子也能形成玻璃状材料。在许多常见的例子中，聚碳酸酯眼镜和挡风玻璃。聚合物体系变成玻璃的温度低于这个温度被称为“玻璃化转变温度”。最近的实验研究表明，当聚合物形成非常薄的薄膜时，10到100纳米厚，其中一纳米是十亿分之一米，自由表面或支撑底层的存在可能导致玻璃化转变的变化，这种变化可能相当剧烈-高达30摄氏度。有一些证据表明，在玻璃温度随薄膜厚度的明显变化中，可能有不止一种物理驱动力在起作用。PI的调查开始了一个思想实验：想象一下，从大块样品中提取“一片”材料，形成一个超薄板。两个自由表面的形成产生了聚合物节段层，其与其他节段的相邻相互作用已大大减少。通过用数学术语描述其含义，PI发现缺少相互作用导致薄膜玻璃化转变温度低于体样品的温度。试验计算结果与实验结果基本吻合。这种对相邻相互作用如何影响聚合物样品物理行为的关注也激发了PI对两种聚合物混合时会发生什么的研究。在这种情况下，由于另一种片段的存在，聚合物片段不会面临缺失的相互作用，而是面临改变的相互作用。预测不同类型的聚合物可能混合的程度的能力在新材料的设计中是至关重要的，也将导致更好地操纵已经使用的不太了解的系统。将上述两种问题的结果结合起来，将允许PI以薄膜和块状形式“创建”混合物，并有可能控制不同材料混合的程度，作为样品厚度的函数。这项研究产生的科学发现将通过研究生和博士后的参与，通过出版物和他们在国家会议上的演讲来扩大。此外，PI在研究和教学指导方面的努力使研究生和研究生以及教员中的妇女人数有所增加；预计这种影响将持续下去。

TECHNICAL SUMMARYThe Division of Materials Research and the Division of Chemistry contribute funds to this award. It supports theoretical reearch and education that involves the application of statistical thermodynamic methods to problems of both fundamental and applied interest.Polymeric thin films have potential applications in areas such as electronics, optics, and protective membranes; in addition, they are a ubiquitous component in nanocomposites. Maintenance of structural integrity is key, thus it is essential to ensure that operating conditions do not approach the glass transition temperature. Evidence from a variety of experimental techniques indicates that a supporting substrate and a free surface can each have a profound impact on the glass transition temperature of a polymeric thin film, relative to the bulk value. However, the interpretation of these results is controversial. Theory can play an essential role in resovlving the controversy through the development of clearly elucidated models with well-defined assumptions, and the application of them to determine what physical behaviour would ensue. A focus of this project builds on the PI's previous work that yielded a model of thermodynamic properties that appears able to predict shifts in the glass transitions of freestanding polymeric films that match with experimental data. The PI aims to exploit this and related models in order to predict both glass transitions and phase separation in polymer films.An important question is whether it is possible to capture the thermodynamic behavior well enough - not just the free energy, but also the entropic and enthalpic contributions to it - so that predictions regarding any mixture may be made using only pure component experimental data as input. The PI's lattice-based theory of polymer fluids and mixtures in analyzing dozens of polymers and blends has yielded intriguing connections between pure component properties and thermodynamic behaviour. The PI plans to explore whether strategic choices of components will result in control of both the enthalpy and entropy of mixing in polymeric blends and solutions, allowing for true predictions regarding miscibility based on pure component properties, alone. Theoretical models developed in the course of the research have potential for widespread use. The ability to correlate characteristic pure component behaviour with the enthalpic and entropic contributions to mixture thermodynamics will yield routes to the development of new materials. Another outcome of this research will be a thermodynamic model for how both the local and averaged behaviour of polymers and their blends changes in going from bulk to film. The impact of this research will be magnified through the involvement of graduate students and postdoctoral fellows, both via publications and also through their presentations at national meetings. The PI's efforts in research and teaching mentorship have resulted in an increase in the number of women in the ranks of graduate and postgraduate students and in faculty; it is expected that such effects will continue. NONTECHNICAL SUMMARYThe Division of Materials Research and the Division of Chemistry contribute funds to this award. An enduring ambition for a theorist is to solve complex problems with conceptually transparent tools. The PI will focus on research areas from studies on glassy polymer thin films to thermodynamic properties of novel, untested, polymer-containing mixtures. Preliminary evidence indicates that tools from the same basic set may be effective across this span of interests. When a simple tool is shown to be convincing in tackling a series of complicated questions, it not only reveals something about the essential elements of the problems, but also illuminates the extent to which seemingly very different problems may be related.A typical small molecule liquid, when cooled, can turn into an amorphous glass or a crystal. What is usually thought of as 'glass,' as found in windows and drinking vessels, has as its majority component silicon dioxide, with other small molecules mixed in. However, large chain-like molecules such as polymers also form glassy materials. Among the many common examples are polycarbonate eyeglasses and windshields. The temperature below which a polymer system turns into a glass is called the 'glass transition temperature'. Recent experimental studies have revealed that when polymers are formed into very thin films, ten to one hundred nanometers thick, where a nanometer is a billionth of a meter, the presence of either a free surface or a supporting underlayer may result in a shift in the glass transition that can be quite dramatic - up to thirty degrees Celsius.There is some evidence that more than one physical driving force may be at play in the evident shifts of glass temperature with film thickness. The PI's investigation begins a thought experiment: imagine that a 'slice' of material is extracted from a bulk sample to form an ultrathin slab. The formation of two free surfaces creates layers of polymeric segments for whom neighbouring interactions with other segments have been greatly diminished. By describing the implications in mathematical terms the PI finds that missing interactions result in a film glass transition temperature lower than that for the bulk sample. Test calculations are in reasonable agreement with experiments.This focus on how neighbouring interactions affect the physical behaviour of a polymeric sample also motivates the PI's studies on what happens when polymers of two types are mixed. In that case a polymer segment will not face missing interactions, but altered ones, due to the presence of another kind of segment. The ability to predict the extent to which polymers of different types may mix is crucial in the design of new materials, and will also lead to better manipulation of poorly-understood systems already being used. Combining results for the two kinds of problems described above will allow the PI to 'create' mixtures in both thin film and bulk form, with the potential for controlling the extent to which different materials may mix as a function of sample thickness. The scientific discoveries arising from this research will be magnified through the involvement of graduate students and postdoctoral fellows, both via publications and also through their presentations at national meetings. In addition, the PI's efforts in research and teaching mentorship have resulted in an increase in the number of women in the ranks of graduate and postgraduate students and in faculty; it is expected that such effects will continue.