Mechanisms of Equilibration in Block Copolymer Micelles

纳米结构材料是由一类被称为嵌段共聚物的极其大分子的先进塑料组成的，在各种新兴技术中越来越重要。例如，用于防止排斥反应的支架药物洗脱涂层；合成机油粘度调节剂，提高燃油经济性；将治疗剂输送到特定细胞（如癌症细胞）的载体；薄膜使重量更轻，不易燃的锂电池。在所有这些以及更多的应用中，纳米结构都是通过“自下而上”的自组装过程创建的，通过这种过程，分子被精心设计以产生预期的结构。然而，一个具有广泛重要性的基本问题是了解自组装本身的过程。特别是，有必要知道所得到的纳米结构是否是最有利的平衡结构，或者实际上系统是否已经成为结构困在所谓的“亚稳态”状态。有了这些知识，就有可能在最短的时间内定制一个商业过程来生产最有用的纳米结构，可靠且可复制。在这个项目中训练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发表技术演讲和海报，以及指导有才华的本科生。来自大双城的高中生，尤其是女性和少数族裔，将通过“聚合物日：你创造它，你打破它”（polymer Day: You Make It, You Break It）接触到聚合物科学，这是一个更广泛的“发现STEM”为期一周的夏令营的动手部分。一个新的版本，美国印第安材料周，将被开发，以服务于一个严重不足的群体在STEM领域。这是本提案的总体目标，阐明分子水平的机制，其中嵌段共聚物纳米结构实现平衡。通过主要关注溶液组合，即胶束，决定单链交换障碍的分子因素将被量化，然后集体运动，如融合或破碎，将得到解决。该方法的基石是时间分辨小角中子散射，它提供了链交换动力学的无与伦比的定量测量。集体运动将需要额外的工具，例如荧光。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进行结构表征也很重要。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，相对容易设计UCST和LCST系统，以及非常广泛的可访问温度范围。本研究旨在回答八个问题：(i)链交换壁垒对质量的功能依赖是什么。（ii）电晕块长度对功能的依赖性是什么？（iii）类似三嵌段凝胶的链交换与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)什么因素控制胶束破碎和融合的速率？（vi）胶束如何与聚集数相平衡？（vii）不同胶束的混合物如何达到平衡？这些屏障如何随浓度变化，从稀体系到熔体？

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?