Molecular Motions in Flowing Semi-dilute Polymer Solutions

聚合物是大的链状分子，是工业相关产品（即塑料，涂料和纤维）和柔性光伏和显示器等设备的最先进材料的基石。对于这些应用中的许多应用，必须对聚合物进行加工，以达到其最终性能；例如，为了制造坚固的纤维，“拉伸”过程用于排列聚合物链。类似地，涂层流动可以控制半导体聚合物中的分子堆积（从而控制电荷传输）。在这两种情况下，聚合物在液体溶液中通过施加强流来操纵。然而，了解流动究竟如何影响分子运动仍然具有挑战性。这一挑战的出现是因为长聚合物链相互渗透，因此任何给定的分子都可以直接或通过溶剂流动与许多其他分子相互作用。拟议的研究将使用新的计算算法来理解聚合物运动，分子水平流体动力学和应用流动之间的联系。通过展示如何使用浓度和强流动来调整分子结构和相应的材料性质，这种理解将成为聚合物溶液加工中分子控制的基础。该项目还将通过跨学科培训和对学生的指导来培养下一代科学家。此外，该项目还将开展外展活动，将互动实验/计算机模拟融入实践活动，旨在促进代表性不足的学生参与STEM领域。聚合物溶液通常以半稀释的方式处理。区域，单个线圈明显重叠。半稀溶液的特点是显著的溶剂介导的流体动力学相互作用（HI）和拓扑挂钩。目前的理论能够捕获平衡特性（弛豫时间，分子结构），但尚未扩展到流动溶液。一种新的构象平均方法能够有效地模拟大型系统，从而探索HI和链拓扑结构如何影响流动中高度不平衡的聚合物构象。该提案将解决两个目标：(i)了解浓度和流体动力学相互作用如何影响流动的半稀聚合物溶液中的链动力学；（ii）捕获非线性聚合物结构和剪切流如何导致分子“钩”。第一个目标将描述强流对水动力筛分的扭曲，并将平衡标度概念扩展到非平衡系统。第二个目标将使用拓扑不变量来表征钩相互作用，以及它们如何影响分子构象的分布。因此，拟议的工作将为聚合物的流动控制溶液加工提供重要的见解，从流动涂层到喷墨或3D打印。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。

Polymers are large, chain-like molecules that are the building blocks of both industrially-relevant products (i.e. plastics, coatings, and fibers) and state-of-the-art advanced materials for devices such as flexible photovoltaics and displays. For many of these applications, the polymer must be processed with an eye towards its final properties; for example, in order to create strong fibers, a "drawing" process is used to align polymer chains. Similarly, coating flows can control molecular packing (and thus charge transport) in semiconducting polymers. For both situations, polymers are manipulated in a liquid solution by applying strong flows. However, it remains challenging to understand how exactly flow influences molecular motions. This challenge arises because long polymer chains interpenetrate, so that any given molecule interacts with many others both directly and via solvent flows. The proposed research will use new computational algorithms to understand the connection between polymer motions, molecular-level fluid dynamics, and applied flows. This understanding will serve as the foundation for molecular control in polymer solution processing, by showing how concentration and strong flows can be used to tune molecular structure and correspondingly material properties. This project will also serve to train the next generation of scientists, through the interdisciplinary training and mentorship of students. In addition, this project will involve the development of outreach events that incorporate interactive experimental/computer simulation into hands-on activities designed to promote the participation of underrepresented students in STEM fields.Polymer solutions are typically processed in the ?semi-dilute? regime, where individual coils overlap significantly. Semidilute solutions are characterized by significant solvent-mediated hydrodynamic interactions (HI) and topological ?hooking?. Current theory is capable of capturing equilibrium properties (relaxation time, molecular structure) but has not been extended to flowing solutions. A new conformational-averaging procedure is capable of efficiently simulating large systems, enabling exploration of how HI and chain topology affect highly out-of-equilibrium polymer conformations in flow. This proposal will address two aims: (i) Understand how concentration and hydrodynamic interactions affect chain dynamics in flowing semi-dilute polymer solutions and (ii) capture how non-linear polymer architectures and shear flows lead to molecular ?hooking?. The first aim will characterize the distortion of hydrodynamic screening by strong flows and extend equilibrium scaling concepts to out-of-equilibrium systems. The second aim will use topological invariants to characterize hooking interactions, and how they affect the distributions of molecular conformations. The proposed work will thus provide insight important for flow-controlled solution processing of polymers, ranging from flow coating to inkjet or 3D printing.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.