Deformation induced changes in the thermal properties of elastomers: experimental methods, current understanding and application to finite elements methods

Abstract

Deformation-induced molecular orientation of polymeric materials affects thermo-physical properties such as thermal conductivity and heat capacity. These properties influence not only the optimization of fabrication processes, but also the performance of polymeric materials during use. We introduce two complementary experimental methods to characterize the anisotropy in thermal conductivity and its relationship to stress and deformation in elastomers 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 this rule extends beyond finite extensibility [1]. Additionally, we present a transient method for Infrared Thermography technique to investigate the dependence of heat capacity on deformation [3]. We find that the heat capacity increases with stretching in lightly crosslinked natural rubber. Using a simple thermodynamic analysis based on classical rubber elasticity, we discuss the implications of our findings for the assumption of purely entropic elasticity, and the presence of an energetic contribution to the stress in deformed polymers. A growing trend in the design and tuning of polymer manufacturing processes is the use of finite elements simulations for the complex 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 that are connected to the micro-structural orientation remains a challenge. This 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