Tailoring Size and Shape of beta-Sheet Nanocrystals for Crosslinking and Reinforcement of Elastomers

非技术性摘要。弹性体是一种橡胶材料，全球每年生产2亿吨。它们的应用范围从日常用品(例如轮胎)到国防(例如海军舰艇的声纳穹顶)和生物医学(例如动脉支架的涂层)。这个由NSF支持的研究/教育团队由一名化学家(Jia)和一名物理学家/化学工程师(Foster)领导，将生物系统的灵感与化学和物理原理的教训结合起来，开发下一代弹性体。一个中心焦点是将增强弹性体的硬颗粒的尺寸减小到纳米级。研究小组将推进我们对这些强化元素如何产生坚固、僵硬和可伸展的弹性体以及具有程序能力来分散能量的基本理解。这些科学知识可以用于许多应用，例如安全、耐用和省油的轮胎。开发的弹性体也有可能直接应用于使医疗设备更安全。在研究工作的同时，该计划将培养这一跨学科领域的本科生和研究生。该团队将开展外联活动，旨在吸引国内人才投身科技事业，特别是在聚合物相关领域，使用弹性体的各个方面作为主要内容材料。操纵原子和分子形成具有精确控制大小和形状的分级结构的能力是纳米科学的核心。β-片状纳米晶存在于天然和合成弹性体中，起到交联剂的作用并提供增强作用。然而，在这两种情况下，它们的形态截然不同。在天然弹性体(即丝绸)中，它们是三个维度都小于10纳米的颗粒。在合成弹性体(例如，聚氨酯)中，它们被发现是纤维状的，在氢键方向上的尺寸最长，从数百纳米到微米不等。在丝绸中，纤度的控制归因于特定的氨基酸序列和精致的制丝工艺。对于合成系统来说，控制β片状纳米晶的尺寸和长径比是一个尚未解决的挑战。这项研究致力于在软材料的一个重要领域--弹性体--进行纳米科学的中心探索，以实现否则无法实现的材料特性。这项研究的科学方法是多方面的，涉及分子、超分子和纳米尺度的合成、表征和机械研究。在他们最近成功地将一系列低聚(β-丙氨酸)接枝聚异丁烯的n-β-片状纳米晶的最长尺寸降低到远低于100 nm的基础上，研究小组将扩大他们在没有复杂的氨基酸序列的情况下调节β-片状纳米晶的尺寸和长宽比的能力，并阐明这两种形态相反的β-片状纳米晶的增强特性。纳米晶界面处的聚合物刷将是另一个焦点，因为它对形态控制至关重要，在增强方面也可能发挥重要作用。计划研究的具体目标是：(1)合成单分散低聚(β-丙氨酸)接枝聚异丁烯，形成具有更小纵横比的颗粒纳米晶以及形成纤维纳米晶的颗粒纳米晶。(2)表征β-片状纳米晶的结构和形貌，包括附着在纳米晶表面的聚合物刷。(3)阐明颗粒状和纤维状纳米晶的增强特性，研究其增强机理。

NON-TECHNICAL SUMMARY. Elastomers are rubbery materials that are produced on the scale of 200 million tons annually worldwide. Their applications range from daily goods (for example tires) to defense (for example sonar domes of navy ships) and to biomedicine (for example coatings of artery stents). This NSF-supported research/educational team led by a chemist (Jia) and a physicist/chemical engineer (Foster) combines inspirations from biological systems and lessons from chemical and physical principles to develop the next generation of elastomers. A central focus is to reduce the size of hard particles that strengthen the elastomers to the nanometer scale. The research team will advance our fundamental understanding of how these reinforcing elements produce elastomers that are strong, stiff, and extensible and that have programmed capability to dissipate energy. This scientific knowledge can be used for a number of applications, e.g. tires that are safe, durable, and fuel-efficient. The elastomers developed can also potentially be directly applied to make medical devices safer. Parallel to the research effort, the program will train undergraduate and graduate students in this interdisciplinary area. The team will carry out outreach activities aimed at attracting domestic talent to careers in science and technology and particularly in polymer-related areas using aspects of elastomers as the primary content materials.TECHNICAL SUMMARY. The ability to manipulate atoms and molecules to form hierarchical structures with precisely controlled size and shape is central to nanoscience. Beta-sheet nanocrystals exist in both natural and synthetic elastomers and function as crosslinks and provide reinforcement. However, their morphologies are drastically different in these two circumstances. In natural elastomers (i.e., silks), they are particulates with all three dimensions smaller than 10 nm. In synthetic elastomers (e.g., polyurethanes), they have been found to be fibrous with the longest dimension, in the hydrogen-bonding direction, ranging from hundreds of nanometers to microns. In silks, the size control is attributed to specific amino acid sequences and an exquisite reeling process. Controlling the size and aspect ratio of beta-sheet nanocrystals is an unresolved challenge for synthetic systems. This research pursues this central quest of nanoscience in an important area of soft materials, elastomers, to realize material properties otherwise unattainable. The scientific approach of the research is multifaceted, involving synthesis, characterization, and mechanical studies across the molecular, supramolecular, and nanometer scales. Based on their recent success in reducing the longest dimension of n-beta-sheet nanocrystals in a series of oligo(beta-alanine)-grafted polyisobutylenes to well below 100 nm, the research team will expand their ability to regulate the size and aspect ratio of the beta-sheet nanocrystals without an elaborate aminoacid sequence and to elucidate the reinforcing characteristics of the two morphologically contrasting beta-sheet nanocrystals. The polymer brush at the interface of the nanocrystal will be another focus as it is critical for the morphological control and likely plays an important role in reinforcement as well. The specific objectives of the planned research are to: (1) synthesize monodisperse oligo(beta-alanine)-grafted polyisobutylenes that form particulate nanocrystals with still smaller aspect ratios as well as those that form fibrous nanocrystals. (2) characterize the structures and morphologies of the beta-sheet nanocrystals including the polymer brush attached to the nanocrystal surface. (3) elucidate the reinforcing characteristics and study the reinforcing mechanisms of the particulate and fibrous nanocrystals.