CAREER: Dynamics in Nanostructured Polymer Materials

聚合物膜是解决重要社会需求的潜在可持续解决方案，如能源储存和清洁水。本项目所研究的材料在小尺度上具有复杂的结构，在大尺度上具有独特的性能，这是没有这种结构的材料所没有的。根本原因尚不清楚。这项工作最重要的潜在影响是对聚合物物理的基本理解，这些物理涉及到分子系结和界面柔软对局部动力学的影响，以及界面和更大结构的集体动力学的性质，以及这些过程如何影响膜性能。这种理解将通过设计新材料、在国家实验室和PI实验室进行先进实验以及运用理论来实现。由于纳米结构在许多先进的功能软材料中普遍存在，因此这种理解和正在开发的实验技术可以扩展到广泛的其他材料。这些知识将有助于解决现有纳米结构聚合物材料的局限性，并在许多技术应用中提高性能和耐久性，例如膜基分离、食品包装、电池电解质和电容器电介质。该项目还为国家服务，通过参与先进的研究工作和对中学生的教育推广，培养了一支由研究生和本科生组成的多元化团队。该项目的教育目标是增加聚合物和工程社区的多样性，将从地方一级开始，但将在全国范围内实施。研究所产生的知识将被纳入PI所教授的课程，并将记录外展计划，以便在其他大学复制，并通过科学期刊和会议广泛传播。将通过调查和跟踪来评估综合研究和教育工作对增加长期STEM职业多样性的影响。本CAREER项目的主要研究目标是在广泛的长度和时间尺度上对强分离嵌段共聚物（bcp）的动力学有更深入的了解。强分离嵌段共聚物形成具有尖锐界面的可预测纳米结构，并将小分子运输与机械性能分离。这种脱钩的根本原因尚不清楚。与系缚和/或约束有关的界面效应可能是改变每个相性质的原因。另一方面，低频弹性等特性可以从结构本身产生。只有有限的努力致力于了解强分离bcp的动力学，特别是在与纳米结构大小相当的长度尺度上。首先，x射线光子相关光谱（XPCS）将用于在大范围的时间尺度上对这种长度尺度进行询问，以测量结构动力学，如颗粒旋转和表面波。其次，将使用选择性氘化和中子自旋回波光谱，辅以介电光谱来实现系留对局部（段）动力学的影响。第三，将研究局部和介观长度尺度上的动力学与宏观性质之间的联系。将评估机械、流变学和小分子运输特性。实验将在有机械对比的玻璃-橡胶嵌段共聚物和没有机械对比的橡胶-橡胶嵌段共聚物上进行，以区分BCP颗粒运动和表面波。还将评估界面柔软度对局部节段动力学的影响。除了检查机械对比的影响外，成分和加工将用于检查形貌和晶粒尺寸对动力学的影响。结构动力学的实验测量将在软玻璃流变模型的背景下进行分析，在Rostiashvili的聚合物动力学理论的背景下进行段动力学分析，并在包括有效介质理论在内的几种理论的背景下进行宏观性能分析。对动力学是如何在如此大的长度和时间尺度上传递的基本理解，将使纳米结构聚合物材料的智能设计成为可能，并增强对重要的、特定应用性能（如渗透率和韧性）的解耦。研究工作与教育工作相结合，通过教学、指导和教育外展来影响STEM的多样性。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

PART 1: NON-TECHNICAL SUMMARYPolymer membranes are potential sustainable solutions for important societal needs, such as energy storage and clean water. The materials studied in this project have complex structure on small scales that result in unique properties on much larger scales not seen in materials without such structure. The underlying cause is not well understood. The most significant potential impact of the proposed work is fundamental understanding of polymer physics related to the effect of molecular tethering and interface softness on local dynamics, as well as the nature of collective dynamics of interfaces and larger structures and how these processes impact membrane properties. This understanding will be achieved by designing new materials, conducting advanced experiments at national laboratories and in the PI's lab, and using theory. Since nanostructures are pervasive in many advanced functional soft materials, this understanding and the experimental techniques being developed can be extended to a broad range of other materials. This knowledge will help address limitations of existing nanostructured polymeric materials and improve performance and durability in many technological applications, such as membrane-based separations, food packaging, battery electrolytes, and capacitor dielectrics. This project also serves the nation by training a diverse team of graduate and undergraduate students through their participation in the advanced research effort and educational outreach to middle-school students. The project's educational goal of increasing diversity in the polymer and engineering communities will begin at a local level but be designed for implementation nationally. The knowledge generated by the research will be incorporated into the curriculum taught by the PI, and the outreach program will be documented for replication at other universities and broadly disseminated through scientific journals and conferences. The impact of the integrated research and educational efforts on increasing diversity in long-term STEM careers will be evaluated with surveys and tracking. PART 2: TECHNICAL SUMMARYThe main research objective of this CAREER project is to develop a deeper understanding of the dynamics of strongly-segregated block copolymers (BCPs) across a broad range of length and time scales. Strongly-segregated block copolymers form predictable nanostructures with sharp interfaces, and they decouple small molecule transport from mechanical properties. The underlying causes of this decoupling are not well understood. Interfacial effects associated with tethering and/or confinement could be responsible for modifying the properties of each phase. On the other hand, properties like low-frequency elasticity can emerge from the structure itself. Only limited effort has been dedicated to understanding dynamics in strongly segregated BCPs, especially on length scales comparable to the size of the nanostructures. First, X-ray photon correlation spectroscopy (XPCS) will be used to interrogate on such length scales across a wide range of time scales to measure structural dynamics such as grain rotations and surface waves. Second, the effect of tethering on local (segmental) dynamics will be achieved using selective deuteration and neutron spin echo spectroscopy, complemented by dielectric spectroscopy. Third, the connection among dynamics on local and mesoscopic length scales with macroscopic properties will be investigated. Mechanical, rheological, and small-molecule transport properties will be evaluated. Experiments will be performed on a glass-rubber block copolymer with mechanical contrast and on a rubber-rubber block copolymer without mechanical contrast in order to differentiate BCP grain motion from surface waves. The effect of interface softness on local segment dynamics will also be evaluated. In addition to examining the effect of the mechanical contrast, composition and processing will be used to examine the effect of morphology and grain size on dynamics. Experimental measurements of structural dynamics will be analyzed in the context of the Soft Glassy Rheology model, segmental dynamics in the context of Rostiashvili's theory of polymer dynamics, and macroscopic properties in the context of several theories including effective medium theory. Fundamental understanding of how dynamics is transmitted across such a wide range of length and time scales will enable intelligent design of nanostructured polymeric materials with enhanced decoupling of important, application-specific properties, such as permeability and toughness. The research effort is integrated with an educational effort to impact diversity in STEM through teaching, mentoring, and educational outreach.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.