Topological Changes In Associative Polymer Networks Due to Mechanical Stress

该奖项支持可逆聚合物网络的理论和计算研究和教育。当受到剪切或应力作用时，这些聚合物网络表现出有趣的非线性行为。例如，当对模型远螺旋聚合物施加均匀应力时，观察到强烈的剪切变薄。该体系屈服并形成两个或多个剪切带。此外，还发现应力剖面的流变波动表现出自组织的临界行为。在机械应力下，根据应力水平的不同，可以观察到缓慢老化、蠕变、剧烈破坏和断裂。压力停止后，系统会慢慢愈合。这些聚合物体系由亲水聚合物和疏水端基组成。在水溶液中，由于疏水末端自组装成胶束，它们形成网络结构。人们认为，应力诱发了一种新的微观网络结构，这种结构可能具有空间和时间组织。利用一种新的分子动力学/蒙特卡罗混合模型，PI将研究这些微观拓扑变化，并确定它们如何与宏观非线性响应联系起来。网络的拓扑结构由其连通性矩阵的性质来表征，例如谱密度或度分布。在可逆网络中，随着时间的推移，胶束会破裂，新的胶束会形成。这些过程的过渡速率将从模拟中得到，并用于构建描述网络动力学的主方程，并可用于进一步的理论研究。计算结果将与实验结果进行比较。PI还将进行模拟，并将模拟数据与物理相关的三嵌段共聚物的实验进行比较，这些共聚物显示出与生物聚合物网络非常相似的流变行为，例如人类细胞中的交联肌动蛋白网络或植物细胞中的纤维素网络。特别是，在相对较低的应变值下，它们都表现出应变硬化。生物系统在施加的机械应力下发挥其功能。PI将调查网络拓扑和动态变化的作用。该项目将为研究生和本科生提供宝贵的培训机会。它被整合在圣地亚哥州立大学的数学/物理和生物跨学科教育项目中。作为这个项目的一部分，P.I.教授一门关于合成和生物聚合物的课程，在这门课程中，她的研究小组的成果将被讨论。圣地亚哥州立大学招收了大量的少数民族学生，他们中的许多人参加了这门课程，参与了这个项目，或者是PI研究小组的一员。该奖项支持长链状分子，聚合物的理论和计算研究，这些分子可以在其末端之间形成连接。聚合物的末端不愿与水接触。在水中，这些分子形成网状结构。当这些网络受到机械应力时，实验显示出有趣的行为。例如，当系统缓慢变形时，它会突然屈服。在更高的变形中，分子以不同的速度运动，从而出现条带。在停止变形后，系统会记住它很长一段时间，但最终会慢慢恢复到原来的状态。认为应力诱发了一种新的微观网络结构。PI将使用模拟来研究这种行为，并描述分子在网络中的组织方式。这项研究将促进对实验的理解。PI还将研究与生物系统（如动物和植物细胞中的支架）中的聚合物网络行为相似的聚合物。生物系统在施加的机械应力下执行其功能，PI将研究聚合物在网络中组织方式变化的作用。在自然界中，分子之间的键不断断裂和重组的聚合物网络比比皆是。例如植物细胞中的纤维素网络和人类细胞中的原纤维网络。这些聚合物网络的特殊机械性能，例如它们在压力停止后放松回原始结构和重组的能力，使这些细胞能够发挥其生物功能。暂态网络的这些自修复特性有助于设计自修复的智能纳米材料和传感器。该项目将为研究生和本科生提供宝贵的培训机会。它被整合在圣地亚哥州立大学的数学/物理和生物跨学科教育项目中。作为这个项目的一部分，P.I.教授一门关于合成和生物聚合物的课程，在这门课程中，她的研究小组的成果将被讨论。圣地亚哥州立大学招收了大量的少数民族学生，他们中的许多人参加了这门课程，参与了这个项目，或者是PI研究小组的一员。

TECHNICAL SUMMARYThis award supports theoretical and computational research and education on reversible polymer networks. These polymer networks show intriguing nonlinear behavior when subjected to shear or stress. For instance strong shear thinning has been observed when a uniform stress is applied to a model telechelic polymer. The system yields and forms two or more shear bands. In addition, it is found that rheochaotical fluctuations in the stress profile exhibit self-organized critical behavior. Under mechanical stress, slow aging, creep, dramatic failure, and rupture are observed, dependent on the stress level. After cessation of stress, the system slowly heals. These polymer systems consist of hydrophilic polymers with hydrophobic end-groups. In aqueous solution they form network structures since the hydrophobic ends self-assemble into micelles. It is believed that stress induces a novel microscopic network structure that might have spatial as well as temporal organization. Using a novel Molecular Dynamics/Monte Carlo hybrid model, the PI will study those microscopic topological changes and determine how they connect to the macroscopic nonlinear response. The topology of the network is characterized by the properties of its connectivity matrix, for example spectral density or degree distribution. In reversible networks micelles break apart and new ones form over time. The transition rates for these processes will be obtained from simulations and employed to construct a master equation that describes the network kinetics and can be used for further theoretical investigation. The results will be compared with experiments. The PI will also perform simulations and compare the simulation data to experiments on physically associating triblock copolymers that show rheological behavior very similar to that observed in biopolymer networks, for example a cross-linked actin network in a human cell or a cellulose network in a plant cell. In particular, all exhibit strain stiffening at relative low values of strain. Biological systems perform their function under applied mechanical stress. The PI will investigate the role of changes in network topology and dynamics. This project will provide valuable training opportunities for graduate and undergraduate students. It is integrated in an interdisciplinary educational program in math/physics and biology at San Diego State University. As part of this program the P.I. teaches a course on synthetic and biopolymers, in which results of her research group are discussed. San Diego State University enrolls a large number of minority students, many of whom take this course, participate in this program, or are part of the PI's research group. NONTECHNICAL SUMMARYThis award supports theoretical and computational research on long chain-like molecules, polymers, that can form junctions between their ends. The ends of the polymers prefer not to be in contact with water. In water, these molecules form network structures. Experiments show interesting behavior when these networks are subjected to mechanical stress. For instance, when slowly deformed the system suddenly yields. At higher deformations bands appear in which the molecules move at different speeds. After stopping the deformation, the system remembers it for a long time, but eventually slowly returns back to its original state. It is believed that stress induces a novel microscopic network structure. The PI will use simulation to study this behavior and to characterize the way the molecules are organized in the network. The research will advance understanding of experiments. The PI will also study polymers that behave similarly to networks of polymers that occur in living systems like the scaffolding in animal and plant cells. Biological systems perform their function under applied mechanical stress and the PI will investigate the role of changes in the way the polymers are organized in the network.Polymer networks in which bonds between molecules constantly break and recombine abound in nature. Examples are the cellulose network in plant cells and the fibril network in a human cell. The special mechanical properties of these polymer networks, for example their ability to relax back to their original structure and restructure after cessation of stress, enable these cells to perform their biological functions. These self-healing properties of transient networks are instrumental to the design of self-repairing smart nanomaterials and sensors.This project will provide valuable training opportunities for graduate and undergraduate students. It is integrated in an interdisciplinary educational program in math/physics and biology at San Diego State University. As part of this program the P.I. teaches a course on synthetic and biopolymers, in which results of her research group are discussed. San Diego State University enrolls a large number of minority students, many of whom take this course, participate in this program, or are part of the PI's research group.