Single Molecule Imaging for the Study of Heterogeneous Materials: Supercooled Liquids, Polymer Nanocomposites, and Ionic Liquids

我们可以认为一个容器的纯液体是均匀的，相同的性质（例如，密度，折射率和粘度）在整个样品容器。 然而，在分子水平上，液体不是均匀的，而是在分子如何相对于彼此排列方面高度变化。 在大多数温度下，分子都在运动。在任何给定的时刻，液体的某些区域可能比其他区域更稠密或更粘稠。 这种情况被称为“动态异质性”，在讨论聚合物的机械性能、蛋白质如何在水中移动等时，这是一个重要的考虑因素。在这个由化学系化学结构、动力学和机理-A（CSDM-A）项目资助的项目中，哥伦比亚大学的Laura考夫曼教授和她的学生正在使用一种先进的分子成像技术，该技术可以跟踪单个（分子探针）通过液体或聚合物材料的运动。 探针分子是强荧光的，这意味着当暴露于一种波长的光时，它会发出另一种特定波长的光。 当分子移动通过材料的不同区域时，发射的光允许分子被跟踪。 探测器揭示了动态的不均匀性，因为它的运动会因环境结构的变化而改变。考夫曼教授和她的学生们正在研究过冷液体（低于其凝固温度的液体，但不知何故仍然是液体），聚合物纳米复合材料（用于气体分离和电子元件）和离子液体中的动态异质性。大多数常见的离子系统都是固体，如食盐。 离子液体由正离子和负离子组成，但在室温下是液体。离子液体在“绿色”工业过程中具有潜在的应用价值。从事这项研究的学生正在学习与具体项目相关的广泛的实验技术和分析方法，这些方法也适用于许多科学和新兴技术领域。除了为研究生提供培训外，该项目还包括对K-8学生的宣传。外展活动的主题提供了一个切入点，化学概念，如相图和化学稳定性的概念。考夫曼教授和她的学生正在完善和扩展单分子成像方法，以详细说明小分子过冷液体和聚合物玻璃形成剂的动态不均匀性方面，随着新的探针分子和新技术的出现，来区分空间和时间的异质性。同时，这项工作的范围正在扩大到聚合物纳米复合材料和离子液体。新设计的拴系探针分子的制备和使用，以补充自由探针测量和歧视的动态变化，发生由于环境重排相关的探针易位。 系留探针提供了一种新的方法来表征这些复杂系统中不同动态区域的空间范围。该项目还采用了超分辨率实验，允许同时跟踪旋转和平移;这项研究旨在解决有关旋转-平移解耦现象知之甚少的问题，澄清它是否来自实验偏差。聚合物纳米复合材料的研究重点是界面层的动力学，以及界面层的性质如何随基质的变化而变化。分子探针的开发旨在阐明结构和动力学异质性之间的相互作用如何影响离子液体的性质。该奖项反映了NSF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。

We may think of a container of a pure liquid as uniform, the same properties (for example, density, refractive index and viscosity) throughout the sample container. At the molecular level however, the liquid is not uniform, but highly varied in how the molecules are arranged with respect to one another. At most temperatures, molecules are in motion. At any given moment, some regions of the liquid might be denser, or more viscous than other regions. This condition is called “dynamic heterogeneity,” and it is an important consideration in discussing the mechanical properties of polymers, how proteins move through water, etc. Dynamic heterogeneity is difficult to measure precisely - most measurements look at averages within a sample. In this project, funded by the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) program of the Chemistry Division, Professor Laura Kaufman of Columbia University and her students are using an advanced molecular imaging technique that can track the motion of a single (molecule probe) through a liquid or polymer material. The probe molecule is strongly fluorescent, meaning that when exposed to light of one wavelength, it emits light of another specific wavelength. The emitted light allows the molecule to be tracked as it moves through different regions of the material. The probe reveals dynamic heterogeneity as its motions are altered by changes in the structure of its environment. Professor Kaufman and her students are investigating dynamic heterogeneity in supercooled liquids (liquids colder than their freezing temperature but somehow still liquid), polymer nanocomposites (used in gas separations and electronic components), and ionic liquids. Most familiar ionic systems are solid, like table salt. Ionic liquids are composed of positive and negative ions but are liquid at room temperature. Ionic liquids may have potential for “green” industrial processes. The students engaged in this research are learning a broad range of experimental techniques and analysis approaches relevant to the specific project, These approaches are also applicable to many areas of science and emerging technologies. In addition to providing training for graduate students, this project also includes outreach to K-8 students. The outreach events feature topics that provide an entry point to chemistry concepts such as phase diagrams and the concept of chemical stability.Professor Kaufman and her students are refining and extending single molecule imaging approaches to detail aspects of dynamic heterogeneity in small molecule supercooled liquids and polymeric glass formers, with advances coming in the form of new probe molecules and new techniques to distinguish spatial and temporal heterogeneity. At the same time, the scope of this work is being extended to polymer nanocomposites and ionic liquids. Newly designed tethered probe molecules are prepared and used to complement free probe measurements and discriminate changes in dynamics that occur due to environmental rearrangement from those related to probe translocation. The tethered probes provide a new approach to characterizing spatial extent of regions of distinct dynamics in these complex systems. The project also employs super-resolution experiments that allow simultaneous tracking of rotations and translations; this study seeks to resolve questions regarding the poorly understood phenomenon of rotational-translational decoupling, clarifying if it emerges from experimental biasing. The study of polymer nanocomposites focuses on the dynamics of the interfacial layer, and how interfacial layer properties vary with changes in the host matrix. Molecular probes are being developed to clarify how the interaction between structural and dynamic heterogeneity influences the properties of ionic liquids.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.