Local Polymer Dynamics in Melts and Glasses

9732483 Ediger合成聚合物的许多重要性质由聚合物的局部链段动力学控制。这些参数包括玻璃化转变温度(Tg)、粘性流动的温度依赖性、小分子的扩散系数和物理老化速率。光学实验和核磁共振波谱将被用来研究局部链段动力学和这些重要性质之间的关系。其中一个项目将探索聚苯乙烯和聚碳酸酯物理老化的分子基础。我们的目标是理解为什么不同的可观测到的年龄不同，以及Tg以下的老化时间如何与Tg以上的平衡松弛时间相关。聚合物玻璃的长期性质的预测取决于对这些问题的回答。第二个项目探索聚合物熔体中的空间非均质动力学及其对低分子物质扩散系数的影响。重要的是要了解为什么这些非均相区在某些熔体中比其他熔体更重要，以便准确地预测具有重要技术意义的扩散系数。这些区域的寿命将被探索，因为这决定了更大长度尺度的运动将如何受到这些区域的影响。第三个项目使用变场核磁共振光谱学来研究聚乙烯和无规聚丙烯熔体。这两个简单的系统非常适合探索局部动力学和粘性流动之间的关系，以及理解导致玻璃化转变的聚合物运动。预计这些结果将有助于理解支化聚乙烯在工业上的重要流动特性。第四个项目使用分子动力学轨迹来测试2D核磁共振和中子自旋回波实验的解释。这项研究可能会导致从这些实验观测中得出更可靠的微观推断。%聚合物在我们社会中的用途是基于通过改变重复单元的化学特性可以获得的广泛的性质，例如，一些结构产生橡胶材料，而另一些结构产生硬塑料。这项建议调查了化学结构如何控制一些重要的性质。这一基本认识将应用于拟议工作中与工业有关的一些问题，并将提出解决更广泛实际问题的战略。***

9732483 Ediger Many important properties of synthetic polymers are controlled by the local segmental dynamics of the polymers. These include the glass transition temperature(Tg), the temperature dependence of viscous flow, the diffusion coefficients of small molecules, and the rates of physical ageing. Optical experiments and NMR spectroscopy will be used to investigate the relationship between local segmental dynamics and these important properties. One project will explore the molecular basis for physical ageing in polystyrene and polycarbonate. The goal is to understand why different observables age differently and how ageing times below Tg are related to equilibrium relaxation times above Tg. The prediction of long term properties of polymer glasses depends upon the answers to these questions. A second project explores spatially heterogeneous dynamics in polymer melts and their influence on the diffusion coefficients of low molecular weight species. It is important to understand why these heterogeneous regions are more important in some melts than others so that technologically important diffusion coefficients can be accurately predicted. The lifetimes of these regions will be explored because this determines how larger length scale motions will be influenced by these regions. A third project uses variable-field NMR spectroscopy to study polyethylene and atactic polypropylene melts. These two simple systems are ideal for exploring the relationship between local dynamics and viscous flow, and for understanding the polymer motions which are responsible for the glass transition. It is expected that these results will contribute to an understanding of the industially important flow properties of branched polyethylene. A fourth project uses molecular dynamics trajectories to test the interpretation of 2D NMR and neutron spin echo experiments. This investigation may result in more reliable microscopic inferences being drawn from these experimental ob servables. %%% The utility of polymers in our society is based upon the wide range of properties which can be obtained by changing the chemical identity of the repeat units, e.g., some structures produce rubbery materials while others produce hard plastics. This proposal investigates how some important properties are controlled by chemical structure. This fundamental understanding will be applied to some problems of industrial interest in the proposed work and will suggest strategies for solving a broader range of practical problems. ***