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.
