Local Polymer Dynamics in Melts and Glasses

合成聚合物的许多重要性质是由聚合物的局部节段动力学控制的。这些参数包括玻璃化转变温度（Tg）、黏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光谱将用于研究局部节段动力学与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。目的是了解为什么不同的可观测物的年龄不同，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。对聚合物玻璃长期性能的预测取决于这些问题的答案。第二个项目探讨了聚合物熔体的空间非均质动力学及其对低分子量物质扩散系数的影响。重要的是要理解为什么这些非均质区域在某些熔体中比其他熔体更重要，以便能够准确预测技术上重要的扩散系数。这些区域的寿命将被探索，因为这决定了这些区域如何影响更大的长度尺度运动。第三个项目使用变场核磁共振光谱来研究聚乙烯和无规聚丙烯熔体。这两个简单的系统对于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是理想的。预计这些结果将有助于理解支化聚乙烯在工业上重要的流动特性。第四个项目使用分子动力学轨迹来测试二维核磁共振和中子自旋回波实验的解释。这项研究可能会从这些实验观察结果中得出更可靠的微观推论。聚合物在我们社会中的应用是基于其广泛的性质，这些性质可以通过改变重复单元的化学性质而获得，例如，一些结构产生橡胶材料，而另一些结构产生硬塑料。这一提议探讨了化学结构如何控制一些重要的性质。这种基本的理解将被应用到一些工业感兴趣的问题中，并将为解决更广泛的实际问题提出策略。＊＊＊

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. ***