Anisotropic Thermal Conductivity in Deforming Polymers

David C. Venerus

Department of Chemical and Environmental Engineering and

Center of Excellence in Polymer Science and Engineering

Illinois Institute of Technology

Chicago, IL 60616

Polymer processing flows involve a strong coupling of mechanical and thermal effects that often have a significant impact on the final properties of the material.  While a great deal of effort has focused on developing mechanical, or rheological, constitutive equations, much less is known about thermal transport in deforming polymer systems.  Recently, both experimental and theoretical activity on energy transport in deforming polymeric materials have increased.  Theoretical results indicate that the thermal conductivity in such systems is anisotropic, and support, analogous to the well-known stress-optic rule, the validity of a stress-thermal rule where the thermal conductivity and stress tensors are linearly related.  In this study we have developed a method to measure the thermal diffusivity in deforming polymers.  The method is based on an optical technique known as Forced Rayleigh Scattering.  This sensitive and non-invasive technique is shown to be capable of quantitative measurements of anisotropic thermal diffusivity in both static and dynamic (relaxing) polymers subjected to deformations.  Results will be presented for a polymer melt in step-shear strain flows and a cross-linked elastomer in uniaxial extension.  Thermal diffusivity data are complemented by measurements of stress and birefringence so that evaluations of the stress-optic and stress-thermal rules can be made.  We find that the thermal diffusivity is enhanced in the flow (or stretch) direction compared to the equilibrium value and that the stress-thermal rule is valid for the modest deformations achieved in this study.