The Role of Grafting Mechanism on the Self-Assembly and Properties of Polymer Nanocomposites

非技术性总结：将无机纳米颗粒（NP）添加到聚合物（即塑料材料）中可以产生具有高度改进的性能的杂化材料。一个经典的例子是，虽然橡胶（例如橡皮擦）的耐磨性极差，但添加二氧化硅纳米颗粒会使耐磨性大大提高，以至于汽车可以使用超过50，000英里的轮胎行驶。 虽然这些性能的改善是推动聚合物纳米复合材料领域发展的原因，但固有的问题是纳米颗粒在化学上与聚合物（烃材料）如此不同，因此具有类似于水-油悬浮液的彼此分离的强烈趋势。因此，仅可通过聚合物和NP的紧密混合实现的期望的、有利的性质改进变得难以常规实现。这里的工作提出通过将聚合物链化学接枝到纳米颗粒上来解决这个问题，以便无机颗粒表面可以“屏蔽”聚合物，从而获得混溶性。特别是，这项工作集中在两个策略，这种嫁接：要么嫁接完全形成的链的表面，或增长的链从表面一块一块。这些策略中的哪一种可以更好地控制颗粒-聚合物的相容性，从而最大限度地改善性能是本研究的重点。 除了这一技术重点外，拟议的工作还将强调学生的广泛的跨学科教育，以及通过吸引高中和本科生参加PI实验室的工程研究活动，招募和保留女性和少数民族学生参加STEM课程。该提案的重点是聚合物接枝纳米颗粒（NP）在聚合物基质中的自组装，目的是影响所得混合（或复合）材料的行为。这种物理状况与例如橡胶轮胎和环氧树脂（其用于电子封装和计算机芯片应用）的改进性能有关。本文将通过三个试验台的探索，将理论与实验有机地结合起来，对这一基础问题进行研究。特别是，重点将放在链接枝到NP表面上的机制对最终材料性能的作用上，即，无论链是预先形成并附着到表面上的，还是链是从表面一个接一个地生长的。预计前一种机制将在NP表面上产生更均匀的聚合物覆盖。这些差异的表面覆盖的组装形成的结果和由此产生的性能是这项工作的最终重点。这些研究活动与广泛的教育和外联活动相结合。一个主要重点将是在本科和研究生两级招收代表性不足的学生（包括妇女和少数民族）。 计划在学年期间增加目前的夏季高中研究计划和本科生研究经验计划，目标是招募（并希望留住）妇女和少数民族进入STEM工作。

NON-TECHNICAL SUMMARY: The addition of inorganic nanoparticles (NPs) to a polymer (i.e. a plastic material) can result in hybrid materials with highly improved properties. A classic example is that, while rubber (as e.g. in an eraser) has extremely poor wear resistance, the addition of silica nanoparticles causes the wear resistance to go up so much that a car can drive with tires that last over 50,000 miles. While such property improvements are what drive developments in the field of polymer nanocomposites, the inherent problem is that the nanoparticles are chemically so different from the polymer (a hydrocarbon material) and thus have a strong tendency to separate from each other akin to a water-oil suspension. Thus, the desirable, favorable property improvements that can only be realized by the intimate mixing of the polymer and the NPs become hard to realize on a routine basis. The work here proposes to remedy this problem by chemically grafting polymer chains onto the NPs so that the inorganic particle surface can be "shielded" from the polymer, thus allowing miscibility to be obtained. In particular, the work focuses on two strategies for this grafting: either grafting fully formed chains to the surfaces, or growing the chains piece-by-piece from the surface. Which of these strategies allows better control over particle-polymer miscibility and therefore improve properties the most is the focus of this research. In addition to this technical emphasis, the proposed work will also stress the broad, interdisciplinary education of students and the recruitment and retention of women and minority students into STEM programs by attracting high-school and undergraduate students into engineering research activities in the PI's laboratory.TECHNICAL SUMMARY: The focus of this proposal is on the self-assembly of polymer-grafted nanoparticles (NPs) in a polymer matrix with the goal of impacting the behavior of the resulting hybrid (or composite) material. This physical situation has relevance to improved performance in, for example, rubber tires and epoxies (which are used in electronic packaging and computer chip applications). This underpinning question will be studied through the exploration of three test-beds by judiciously combining theory and experiments. In particular, emphasis will be placed on role of the mechanism by which chains are grafted onto NP surfaces on the final material properties, i.e., whether the chains are preformed and attached to the surface or if the chains are grown link-by-link from the surface. It is expected that the former mechanism will yield a more uniform polymer coverage on the NP surface. The consequence of these differences in surface coverage on the assemblies formed and the resulting properties is the ultimate focus of this work. These research activities are coupled to extensive education and outreach activities. A major focus will be to recruit underrepresented students (both women and minorities) at both the undergraduate and graduate levels. It is planned to grow both the current summer high-school research program, and also the research-experiences-for-undergraduates program during the schoolyear, with the goal of recruiting (and hopefully retaining) women and minorities into STEM efforts.