CAREER: Understanding Plasticity In Polymer Glasses at The Molecular Level by Computer Simulation and Solid-State NMR Spectroscopy

尽管玻璃聚合物具有相当重要的工业意义，但它的可塑性仍然代表着材料科学和凝聚态物理学的一个主要前沿。尽管聚合物玻璃在大应变下的力学性能的现象学模型确实存在，但其潜在的机制尚不清楚。这项拟议的工作旨在在纳米尺度上识别塑性的基本过程，从而揭示分子结构和塑料性能之间的关系。为了实现这一目标，采用了包括分子模拟和实验工作在内的综合方法。建模工作将阐明由缠绕的高分子链引起的拓扑约束在决定由塑性变形激活的基本松弛过程的空间扩展中所起的作用。计算机模拟的第二个目标将是分子玻璃的物理老化和塑性变形之间的相互作用。作为模拟的补充，实验工作还将重点放在分子纠缠的作用上。相容的聚合物共混系统提供了通过简单地改变组成来改变缠结密度的机会。这将被用于系统的固态核磁共振研究，研究由塑性变形产生的分子排列的数量和特征如何取决于纠缠密度。此外，还将探索纠缠和剪切激活体积之间的联系，剪切激活体积是塑性响应的关键参数，与其基本过程的空间扩展有关。总而言之，这些研究的结果有可能将对无定形聚合物固体中的可塑性的理解提高到一个新的水平。对分子结构和塑料性能之间关系的了解将促进新型聚合物材料和应用的优化和发展，为进一步研究打开许多机会。由于其介于物理、化学和材料科学之间的跨学科性质，拟议中的研究项目将为来自几个不同系的学生提供大量参与的机会，包括研究生和本科生。反过来，这将为核磁共振光谱学、电子、分子和连续谱尺度的计算机建模、聚合物科学和固体力学等各种领域创造一个极具启发性的学习环境。尽管玻璃态聚合物在当今的技术中普遍存在，但其机械行为的许多重要方面还没有被很好地了解。本项目试图确定此类材料延展性的分子起源。为此，将使用先进的磁共振光谱学以及计算机模拟。这项工作所获得的见解将有助于指导新型结构材料以及医疗和光电子应用的发展。

The plasticity of glassy polymers, in spite of its considerable industrial importance, continues to represent a major frontier of materials science and condensed matter physics. Although phenomenological models of the mechanical properties of polymer glasses at large strains do exist, the underlying mechanisms are unkown. The proposed work aims at identifying the elementary processes of plasticity at the nanometer length scale, thus uncovering the relationship between molecular structure and plastic properties. In order to meet this goal, a comprehensive approach is adopted, including molecular simulation as well as experimental work. Modeling efforts will shed light on the role of the topological constraints caused by the entangled polymer chains in determining the spatial extension of the elementary relaxation processes that are activated by plastic deformation. A second target of computer simulation will be the interplay between physical aging and plastic deformation in molecular glasses. Experimental work, in complement to the simulations, will also focus on the role of molecular entanglements. Compatible polymer blend systems offer the opportunity to vary the density of entanglements by simply varying composition. This will be exploited for a systematic solid-state NMR study of how the amount and character of molecular alignment that results from plastic deformation depends on the entanglement density. In addition, the connection between entanglements and the shear activation volume, a key parameter of the plastic response that is linked to the spatial extension of its elementary processes, will be explored. Together, the results from these studies have the potential to take understanding of plasticity in amorphous polymer solids onto a new level. Knowledge of the relation between molecular structure and plastic properties will foster the optimization and development of novel polymer materials and applications, opening many opportunities for further research. By virtue of its interdisciplinary nature between physics, chemistry and materials science, the proposed research program will open a multitude of opportunities for students from several different departments, graduate as well as undergraduate, to become involved. In turn, this will generate a highly stimulating learning environment for fields as diverse as NMR spectroscopy, computer modeling at electronic, molecular and continuum length scales, polymer science, and solid mechanics. %%%Although glassy polymers are ubiquitous in today's technology, many important aspects of their mechanical behavior are not yet well understood. The present project seeks to establish the molecular origins of the ductility of such materials. Advanced magnetic resonance spectroscopy as well as computer simulations will be used for this purpose. The insight gained from this work will be helpful to guide the development of novel materials for structural as well as medical and optoelectronic applications.