Mechanisms of Equilibration in Block Copolymer Micelles

非技术概述纳米结构材料是由一类极大的分子形成的，称为嵌段共聚物的先进塑料，在各种新兴技术中越来越重要。例子包括支架上的药物洗脱涂层，以防止排斥;合成机油中的粘度调节剂，以提高燃料经济性;用于将治疗剂输送到特定细胞（如癌症）的载体;使重量更轻的膜，不易燃锂电池。在所有这些应用中，以及更多的应用中，纳米结构是通过自组装的“自下而上”过程产生的，其中分子被仔细设计以产生预期的结构。然而，一个具有普遍重要性的基本问题是理解自组装过程本身。特别是，它是必不可少的，以了解所产生的纳米结构是否是最有利的，平衡的，或是否实际上系统已成为结构陷入所谓的“亚稳态”状态。有了这些知识，就有可能在尽可能短的时间内定制一个商业化的过程，以可靠和可重复的方式生产最有用的纳米结构。在本项目中培训的研究生将获得化学合成和材料表征方面的广泛技能。他们还将有广泛的机会向外部观众展示技术讲座和海报，以及指导有才华的本科生。来自双子城的高中生，特别是女性和代表性不足的少数民族，将通过聚合物日接触到聚合物科学：你制造它，你打破它，这是一个更广泛的发现STEM为期一周的夏令营的实践组成部分。一个新的版本，美国印第安材料周，将被开发，以服务于一个严重代表性不足的群体在STEM领域。本提案的首要目标是阐明嵌段共聚物纳米结构实现平衡的分子水平机制。通过主要关注解决方案组件，即，胶束中，决定单链交换障碍的分子因素将被量化，然后集体运动，如融合或断裂，将得到解决。该方法的基石是时间分辨小角中子散射，它提供了一个无与伦比的，定量测量链交换动力学。集体运动将需要额外的工具，如荧光。通过小角度X射线散射、动态光散射和低温透射电子显微镜的结构表征也将是重要的。离子液体溶剂的使用带来了多种优势，包括精确调节热力学相互作用的能力，设计UCST和LCST系统的相对容易性，以及非常宽的可达温度范围。本研究旨在回答八个问题：（一）链交换壁垒的功能依赖于什么？ 质量. (ii)电晕块长度的函数依赖性是什么？(iii)链交换和类似的三嵌段凝胶的弛豫时间之间的关系是什么？(iv)交换如何取决于胶束形态？(v)什么因素控制胶束碎裂和融合的速率？(vi)胶束如何平衡聚集数？(vii)不同胶束的混合物如何平衡？（viii）这些障碍如何演变与浓度，从稀释系统熔化？

Non-Technical Summary Nanostructured materials that are formed by a class of extremely large molecules, advanced plastics called block copolymers, are of growing importance in a variety of emerging technologies. Examples include drug-eluting coatings on stents to prevent rejection; viscosity modifiers in synthetic motor oils to boost fuel economy; vehicles for delivery of therapeutic agents to specific cells, such as cancers; membranes to enable lighter weight, non-flammable lithium batteries. In all of these applications, and many more, the nanostructure is created through the "bottom-up" process of self-assembly, whereby the molecules are carefully designed to produce the intended structure. However, a fundamental problem of widespread importance is to understand the process of self-assembly itself. In particular, it is essential to know whether the resulting nanostructure is the most favorable, equilibrium one, or whether in fact the system has become structure-trapped in a so-called "metastable" state. With this knowledge, it will be possible to tailor a commercial process to produce the most useful nanostructure, reliably and reproducibly, in the shortest possible time. Graduate students trained in this project will acquire a broad suite of skills in chemical synthesis and materials characterization. They will also have extensive opportunities to present technical talks and posters to external audiences, as well as to mentor talented undergraduates. High school students from the greater Twin Cities, particularly women and underrepresented minorities, will be exposed to polymer science through Polymer Day: You Make It, You Break It, a hands-on component of a broader Discover STEM week-long summer camp. A new version, American Indian Materials Week, will be developed, to serve to a drastically underrepresented group in STEM fields. Technical Summary It is the overarching goal of this proposal to elucidate the molecular-level mechanisms by which block copolymer nanostructures achieve equilibrium. By focusing primarily on solution assemblies, i.e., micelles, the molecular factors that dictate the barriers to single chain exchange will be quantified, and then collective motions, such as fusion or fragmentation, will be addressed. The cornerstone of the approach is time-resolved small-angle neutron scattering, which provides an unrivaled, quantitative measure of chain exchange kinetics. Collective motions will require additional tools, such as fluorescence. Structural characterization by small-angle X-ray scattering, dynamic light scattering, and cryogenic transmission electron microscopy will also be important. The use of ionic liquid solvents brings multiple advantages, including the ability to tune thermodynamic interactions precisely, the relative ease of designing both UCST and LCST systems, and the remarkably broad accessible temperature range. The research will aim to answer eight questions: (i) What is the functional dependence of chain exchange barriers on quality. (ii) What is the functional dependence on corona block length? (iii) What is the relationship between chain exchange and the relaxation time of an analogous triblock gel? (iv) How does exchange depend on micelle morphology? (v) What factors control the rates of micelle fragmentation and fusion? (vi) How do micelles equilibrate with respect to aggregation number? (vii) How do mixtures of different micelles equilibrate? (viii) How do these barriers evolve with concentration, from dilute systems to melts?