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

非技术总结。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圆顶）和生物医学（例如动脉支架涂层）。这个由化学家（Jia）和物理学家/化学工程师（Foster）领导的由nsf支持的研究/教育团队结合了来自生物系统的灵感和化学和物理原理的经验教训，以开发下一代弹性体。一个中心焦点是减少硬颗粒的尺寸，以加强弹性体到纳米尺度。研究小组将推进我们对这些增强元素如何产生强、硬、可扩展的弹性体以及具有可编程的能量耗散能力的基本理解。这种科学知识可以应用于许多方面，例如制造安全、耐用和节能的轮胎。所开发的弹性体也可以直接应用于使医疗设备更安全。在研究工作的同时，该项目将在这一跨学科领域培养本科生和研究生。该小组将开展外联活动，以利用弹性体的各个方面作为主要内容材料，吸引国内人才从事科学和技术事业，特别是在与聚合物有关的领域。技术总结。操纵原子和分子形成精确控制大小和形状的层次结构的能力是纳米科学的核心。纳米晶体片存在于天然和合成弹性体中，作为交联并提供增强。然而，在这两种情况下，它们的形态是完全不同的。在天然弹性体（如丝绸）中，它们是所有三个维度都小于10纳米的颗粒。在合成弹性体（如聚氨酯）中，人们发现它们是纤维状的，在氢键方向上，最长的尺寸从几百纳米到微米不等。在丝绸中，尺寸控制归功于特定的氨基酸序列和精细的缫丝工艺。控制纳米晶片的尺寸和纵横比是合成系统中一个尚未解决的挑战。本研究在软材料弹性体这一重要领域中追求纳米科学的核心目标，以实现其他材料无法实现的材料特性。该研究的科学方法是多方面的，包括分子、超分子和纳米尺度的合成、表征和力学研究。基于他们最近成功地将一系列寡聚（β -丙氨酸）接枝聚异丁烯的n- β -片纳米晶体的最长尺寸减小到远低于100纳米，研究小组将扩大他们调节β -片纳米晶体的尺寸和纵横比的能力，而不需要复杂的氨基酸序列，并阐明两种形态对比的β -片纳米晶体的增强特性。纳米晶体界面上的聚合物刷将是另一个焦点，因为它对形态控制至关重要，并且可能在增强中也起着重要作用。计划研究的具体目标是：(1)合成单分散低聚（β -丙氨酸）接枝的聚异丁烯，形成具有更小长宽比的颗粒纳米晶体以及形成纤维状纳米晶体的纳米晶体。(2)表征纳米晶体的结构和形态，包括附着在纳米晶体表面的聚合物刷。(3)阐明颗粒纳米晶和纤维纳米晶的补强特性，研究其补强机理。

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.