Branched Polymers: Dynamics and Transport Mechanisms

长期以来，对模型支链聚合物液体（即具有精确控制分子结构、具有良好表征的节段微结构和窄分子量分布的聚合物）的应力松弛动力学的基础研究一直被认为是理解分子拓扑结构如何影响合成聚合物的输运性质和加工流动行为的工具。拟议研究的目的有三个方面。首先，量化结构对扩散、应力松弛和单组分支化聚合物及其线性链共混物近表面组成的影响。其次，确定带电分析物的拓扑结构如何影响其在聚合物凝胶和溶液中的电泳特性。最后，了解分子结构如何影响聚合物在瞬态剪切和拉伸流动中的非线性流变行为。本研究采用阴离子技术和DNA自组装技术合成模型对称星型和非对称h型聚合物结构。卢瑟福后向散射光谱（RBS）与机械流变学相结合，量化了臂长和臂长不对称对支链分子自扩散系数和粘弹性（线性和非线性）性能的影响。这些测量很重要，因为它们同时允许对动态稀释分析进行测试，并揭示控制静态和高度变形聚合物液体中分支点扩散的基本过程。RBS还将与次级离子质谱法（SIMS）结合使用，以量化聚合物基质中支化/线性添加剂的组成。这些实验的结果将与自一致的现场模拟和最近提出的响应理论的预测进行比较，以确定添加剂如何/为什么在聚合物中迁移。这种迁移对聚合物表面功能化和增塑剂设计的影响将被详细探讨。通过自组装合成的支链DNA将用于可视化、量化和模拟聚合物凝胶和纠缠溶液中具有复杂拓扑结构的聚电解质的电泳。仅在美国，每个分子具有多个长侧分支的聚烯烃的年产量就超过200亿磅。这些聚合物的合成成本很低，但将它们制成有用物品（如汽车内饰板、杂货袋和饮料容器）的最佳方法却很少显而易见。这种复杂性来自于缺乏对侧分支如何影响聚合物流动特性的基本理解，以及这些特性如何反过来影响加工。因此，通常需要数月昂贵的试错实验来修改现有的聚合物加工设备，以适应具有少量长侧分支的聚合物。该研究旨在利用阴离子合成和DNA自组装合成具有良好分子拓扑结构的理想支链聚合物。第一组的支链聚合物将在本项目中用于研究分子结构对流动特性的影响。由DNA自组装产生的支链分子将用于可视化分子运动，并设计出新的、有效的DNA测序方法。除了对聚合物加工科学和技术的直接影响外，这项拟议的研究预计将在至少三个方面影响教育。首先，提出的使用DNA的可视化实验将为各级学生的聚合物物理教学提供重要的可视化组件和/或演示工具。其次，将参与研究的研究生和本科生团队将接受综合学科的综合教育：聚合物物理、合成化学、流体动力学、聚合物加工、光学与光谱学、分子生物学和分子理论。最后，PI和他的学生将通过一个面向K-12学生、伊萨卡的科学教师和当地工业的外展计划，将项目中创造的知识传播给学生和当地工业。该项目由康奈尔材料研究中心管理，为影响年轻学生学习科学的方式以及当地公司如何利用大学研究提高竞争力提供了独特的机会。

TECHNICAL SUMMARY Fundamental studies of stress relaxation dynamics of model branched polymer liquids (i.e. polymers with precisely controlled molecular architectures, well-characterized segmental microstructures, and narrow molecular weight distributions), have long been recognized as instrumental for understanding how molecular topology affects transport properties and processing flow behavior of synthetic polymers. The objective of the proposed research is three-fold. First, to quantify the effect of architecture on diffusion, stress relaxation, and near-surface composition profile of single-component branched polymers and their blends with linear chains. Second, to determine how topology of charged analytes influence their electrophoretic properties in polymer gels and solutions. Finally, to understand how molecular architecture affects nonlinear rheological behavior of polymers in transient shear and extensional flows. The proposed research employs anionic techniques and DNA self-assembly to synthesize model symmetric star and asymmetric H-shaped polymer structures. Rutherford backscattering spectroscopy (RBS) is used in conjunction with mechanical rheometry to quantify the effect of arm length and arm length asymmetry on the self-diffusion coefficient and viscoelastic properties (linear and non-linear) of branched molecules. These measurements are important because they simultaneously allow the dynamic dilution ansatz to be tested and reveal the fundamental processes that govern branch-point diffusion in quiescent and highly deformed polymer liquids. RBS will also be used in conjunction with secondary ion mass spectrometry (SIMS) to quantify the composition profile of branched/linear additives in polymer hosts. Results from these experiments will be compared with predictions from self-consistent field simulations and a recently proposed response theory to determine how/why additives migrate in polymers. Implications of such migration for polymer surface functionalization and plasticizer design will be explored in detail. Branched DNA synthesized by self-assembly will be used to visualize, quantify, and model electrophoresis of polyelectrolytes with complex topologies in polymer gels and entangled solutions. NON-TECHNICAL SUMMARYAnnual production of polyolefins with multiple long side branches per molecule exceeds 20 billion pounds in the United States alone. These polymers are inexpensively synthesized, but the best procedures for shaping them into useful articles (e.g. interior panels for automobiles, grocery sacks, and beverage containers) are rarely obvious. The complexity comes from a lack of fundamental understanding of how side branches affect polymer flow properties, and how these properties in-turn affect processing. As a result, months of expensive trial-and-error experimentation are often required to modify existing polymer processing equipment to accommodate polymers with even small amounts of long side branches. The research proposed seeks to synthesize ideal branched polymers with well-defined molecular topologies using anionic synthesis and DNA self-assembly. Branched polymers in the first group will be used in this project to investigate the effect of molecular architecture on flow properties. Branched molecules created by DNA self-assembly will be used to visualize molecular motions and to devise new, efficient methods for sequencing DNA. In addition to its direct impact on science and technology of polymer processing, the proposed study is expected to impact education in at least three ways. First, the proposed visualization experiments using DNA will provide an important visual component and/or demonstration tool for teaching polymer physics to students at all levels. Second, the team of graduate and undergraduate students who will execute the study will receive comprehensive education in a unique combination of subjects: polymer physics, synthetic chemistry, fluid dynamics, polymer processing, optics and spectroscopy, molecular biology, and molecular theory. Finally, the PI and his students will disseminate knowledge created in the project to students and local industry via an outreach program for K-12 students, science teachers in Ithaca, and local industry. This program, administered through Cornell Center for Materials Research, provides unique opportunities for influencing how young students learn science and how local companies take advantage university research for enhancing their competitiveness.