Thermal Conductivity in Deforming Polymers
of Chemical and Environmental Engineering and
of Excellence in Polymer Science and Engineering
Institute of Technology
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.