Changes in the thermal properties of polymeric materials induced by molecular orientation: Experimental methods, current understanding and strategies for the application to numerical methods

Abstract

The thermo-physical properties of polymers such as thermal conductivity and heat capacity influence the optimization of fabrication processes and the performance of polymeric materials during use. Remarkably, these properties are strongly affected by molecular orientation induced by deformation [1-3]. This talk introduces two complementary experimental methods to characterize the anisotropy in thermal conductivity and its relationship to stress and deformation in polymers subjected to uniaxial extension. 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. 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, the characterization and implementation of material thermo-physical properties and their connection to the micro-structural orientation remains a challenge that has motivated the development of a molecular-to-continuum methodology for the simulation of industrially relevant flows in polymer manufacturing. A portion of this methodology combines the thermal conductivity/stress response with two recent constitutive equations for linear (Rolie Poly) and branched (eXtended Pom-Pom) polymers to obtain predictions for the anisotropy in thermal conductivity. A few examples of interesting and relevant flows and the thermal transport predictions will be given