Ansiotropy in k and cp induced by deformation in polymers: experimental methods, current understanding and application to numerical methods

Abstract

The thermo-physical properties of polymers such as thermal conductivity and heat capacity are affected by molecular orientation induced by deformation. These properties influence the optimization of fabrication processes and the performance of polymeric materials during use. In this talk, we introduce two complementary experimental methods to characterize the anisotropy in thermal conductivity and its relationship to stress and deformation in polymers subjected to uniaxial extension. The first method, Forced Rayleigh Scattering (FRS) [1], allows directional measurement of thermal diffusivity. The second method, Infrared Thermography (IRT) [2], allows characterization of the deviations from the un-deformed value of different components of the thermal conductivity tensor. Surprisingly, we find: 1) universality of a linear relationship between anisotropy in thermal conductivity and stress known as the stress-thermal rule and 2) that, in contrast to the analogous stress-optic rule, the validity of the stress-thermal rule extends beyond finite extensibility [1]. Additionally, we present a transient method using Infrared Thermography to investigate the dependence of heat capacity on deformation [3]. We find that the heat capacity increases with stretching in lightly cross-linked natural rubber. Using a simple thermodynamic analysis based on classical rubber elasticity an energetic contribution to the stress is found to be responsible for the changes in heat capacity. We discuss the implications of our findings for the assumption of purely entropic elasticity. A growing trend in the design and tuning of polymer manufacturing processes is the use of numerical simulations for the complex non-homogeneous and non-isothermal flows involved. However, while there has been a significant amount of work to include more complete rheological constitutive models into these simulations [4], the characterization and implementation of material thermo-physical properties and their connection to the micro-structural orientation remains a challenge. Our work is presented as a stepping-stone for the development of a molecular-to-continuum methodology for the simulation of industrially relevant flows in polymer manufacturing.