CAREER: Free Surface Mobility and its Role in the Formation of Exceptionally Stable Glasses

技术概述：近年来，利用物理气相沉积（PVD）技术已生产出具有高密度和特殊动力学稳定性的分子玻璃。在这些实验中，衬底温度保持在玻璃化转变温度Tg以下，其中体弛豫动力学非常缓慢。为了使这些PVD玻璃克服阻碍大块玻璃在低温下达到这种接近平衡状态的动力学障碍，分子必须在气相沉积过程中获得增强的迁移率。据推测，这种增强的流动性是由靠近空气界面的一层增加的流动性引起的。在材料研究部固态和材料化学项目的支持下，这一假设将通过使用PI和其他人开发的纳米粒子探针技术研究有机分子玻璃表面的动力学特性来验证。对该技术提出的改进将允许同时研究大块玻璃及其自由表面的性质，使其成为研究表面性质与稳定玻璃形成之间关系的理想方法。这项技术将应用于广泛的有机分子，以研究这种现象是否普遍适用于所有的玻璃，或者是特定分子结构的化学效应。这些研究对于推进我们对玻璃化转变现象的基本理解将是重要的。非技术概述：玻璃是一种非平衡固体，其结构类似于平衡液体，在我们的日常生活中无处不在，并广泛应用于电子和医疗行业。尽管如此，由于玻璃内部分子运动极其缓慢，开发新的有用的玻璃材料或改进已知玻璃材料的性能已被证明是困难的。例如，为了通过老化（玻璃自然变得更致密的过程）来制造高密度的玻璃，人们必须等待几十万年。然而，最近的一项发现表明，可以在几个小时内生产出具有高度理想性能的玻璃，例如密度增加和稳定性提高。据推测，这是由于大多数有机和聚合物玻璃的空气/玻璃表面存在一层几纳米厚的层，其行为像液体而不是非平衡固体。我们提出的研究旨在了解液体层的起源及其对玻璃结构的影响。了解液体层的性质可以帮助设计和生产具有改进性能的有机电子，制药，润滑和涂层技术的材料。此外，我们将把这些研究与教育工作结合起来，旨在向广大读者介绍玻璃聚合物动力学和结构的概念。聚合物广泛应用于日常生活中。防弹玻璃、橡皮泥和轮胎等产品都是具有类似设计概念的材料，但性能差异很大。我们将设计实验模块来强调化学成分、结构和动力学对聚合物系统最终性能的重要性。这些实验将以可调整的技术细节水平呈现，以便所有背景的学生都可以从中学习。该项目还将与宾夕法尼亚大学的机构和费城地区中小学的科学教师密切合作，开发、实施和传播这些模块。为了接触到更多的观众，这些模块的视频将在网上提供。

Technical SummaryMolecular glasses with high densities and exceptional kinetic stabilities have been recently produced by means of physical vapor deposition (PVD). In these experiments, the substrate temperature was held at a temperature below the glass transition temperature, Tg, where the bulk relaxation dynamics are extremely slow. In order for these PVD glasses to overcome the kinetic barriers preventing bulk glasses from reaching such near-equilibrium states at low temperatures, molecules must have access to enhanced mobility during vapor deposition. It is hypothesized that this enhanced mobility is caused by a layer of increased mobility close to the air interface. With support from the Solid State and Materials Chemistry program in the Division of Materials Research, this hypothesis will be tested by studying the dynamical properties of the surface of organic molecular glasses using a nanoparticle probe technique developed by the PI and others. The proposed improvements to this technique will allow simultaneous studies of the properties of the bulk glass and its free surface, making it an ideal method for investigating the correlation between surface properties and stable glass formation. This technique will be applied on a broad range of organic molecules to investigate whether this phenomena is universal for all glasses, or a chemical effect specific to particular molecular structures. These studies will be important in advancing our fundamental understanding of the glass transition phenomena. Non-Technical Summary: Glasses, out of equilibrium solids with structures that resemble that of equilibrium liquids, are ubiquitous in our daily life, and are widely used in the electronic and medical industries. Despite this, developing new useful glassy materials, or improving the properties of known glassy materials has proven to be difficult due to extremely slow molecular motion within the glass. For example, in order to make high-density glasses via aging, the process by which a glass naturally becomes more dense, one would have to wait a few hundred thousand years. A recent discovery, however, shows that glasses with highly desirable properties, such as increased density and stability, can be produced in a few hours. It is hypothesized that this is caused by the presence of a layer at the air/glass surface of most organic and polymeric glasses that is a few nanometers thick and behaves like a liquid rather than an out of equilibrium solid. Our proposed studies aim at understanding the origins of the liquid-layer and its effect on the structure of glasses. Understating the properties of the liquid-layer can help design and produce materials with improved properties for organic electronic, pharmaceutical, lubrication and coating technologies. Furthermore, we will combine these studies with educational efforts aimed at introducing concepts of glassy polymer dynamics and structure to a wide audience. Polymers are widely used in everyday life. Products such as bullet proof glass, silly putty and tires are examples of materials with similar design concepts, but widely varying properties. We will design experimental modules to highlight the importance of chemical composition, structure and dynamics on the final properties of a polymeric system. These experiments will be presented with adjustable levels of technical detail so that students of all backgrounds can learn from them. The PI will also work closely with institutions at the University of Pennsylvania and science teachers from elementary and high schools in the Philadelphia area to develop, implement, and disseminate these modules. To reach an even larger audience, videos of these modules will be made available online.