Investigation of Molecular Mechanisms for Anisotropic Thermal Transport in Polymers

聚合材料的加工涉及机械和热传递过程的复杂相互作用。尽管它很重要，但人们对流动聚合物中的热输运知之甚少。然而，众所周知，流动诱导的聚合物链段的各向异性导致应力与应变速率和导热系数的非线性依赖。完整的热导张量的定量测量是在几种类型的聚合物材料受到明确定义的，伸长变形。这些测量是在熔融、玻璃状和交联状态下的聚合物上进行的，使用基于强迫瑞利散射的敏感和非侵入性光学技术。从以前的研究中，已经观察到一些聚合物体系的导热张量和应力张量之间的线性关系，或应力-热规律。这一具有相当深远意义的观察结果伴随着两个意想不到的结果。首先，应力-热规律似乎在应力水平上是有效的，而应力-光学规律（它给出了方向与应力的线性依赖关系）失效了。其次，应力-热规律中的前因子，当用聚合物模量归一化时，对聚合物化学反应相对不敏感。这些观察结果与目前试图将热导率与聚合物中的分子取向联系起来的模型不一致。热导率测量是由应力、双折射和声速测量补充的，以阐明聚合物材料中各向异性热传输的潜在分子机制。高分子材料是现代社会不可分割的组成部分，有许多不同的应用。聚合物材料的微观结构及其取向在加工过程中发生的变形对材料的行为有深远的影响，这些行为在加工过程和最终产品中都表现出来。流动聚合物中各向异性热导率的定量测量揭示了取向和应力的相对重要性，这极大地扩展了加工方法的知识库，并且对于推进宏观性质与分子结构相关的理论模型至关重要。在芝加哥科学与工业博物馆展出的一个展览展示了强迫瑞利散射背后的原理，使大量不同的观众了解了光学和热传输在材料加工中的有趣应用。

CBET- 1336442Venerus The processing of polymeric materials involves a complex interaction of mechanical and thermal transport processes. Despite its importance, thermal transport in flowing polymers is poorly understood. It is well established, however, that flow-induced anisotropy of polymer chain segments leads to a non-linear dependence of stress on strain rate, and to anisotropy in thermal conductivity. Quantitative measurements of the complete thermal conductivity tensor are made on a several types of polymeric materials subjected to well-defined, elongational deformations. These measurements are made on polymers in molten, glassy and cross-linked states using a sensitive and non-invasive optical technique based on forced Rayleigh scattering. From previous studies a linear relationship between the thermal conductivity tensor and stress tensor, or the stress-thermal rule, has been observed for several polymeric systems. This observation, which has rather profound implications, is accompanied by two unexpected results. First, the stress-thermal rule appears to be valid at stress levels where the stress-optic rule, which gives a linear dependence of orientation on stress, fails. Second, the pre-factor in the stress-thermal rule, when normalized by the polymer modulus, is relatively insensitive to polymer chemistry. These observations are not consistent with current models that attempt to relate thermal conductivity to molecular orientation in polymers. Thermal conductivity measurements are complemented by measurements of stress, birefringence, and speed of sound, to elucidate the underlying molecular mechanisms for anisotropic thermal transport in polymeric materials. Polymeric materials are an integral component of modern society with numerous and diverse applications. The underlying micro-structure of polymeric materials and its orientation by deformations that occur during processing have profound effects on material behavior, which are manifested both during processing and in final products. Quantitative measurements of anisotropic thermal conductivity in flowing polymers that reveal the relative importance of orientation and stress significantly expand the knowledge base for processing methods, and are crucial for advancing theoretical models relating macroscopic properties to molecular structure. An exhibit displayed at the Museum of Science and Industry in Chicago demonstrating the principles behind forced Rayleigh scattering exposes a large and diverse audience to interesting applications of optics and thermal transport in materials processing.