Anisotropic thermal conduction in polymer melts in uniaxial elongation flows

Anisotropic thermal conduction was measured in two amorphous polymers that were quenched immediately after being subjected to uniaxial elongation in the molten state. The quenching is performed so that the flow-induced orientation is retained and the samples are essentially in a stress-free state. A novel optical technique based on forced Rayleigh scattering is used to measure the two independent components of the thermal diffusivity tensor as a function of strain and strain rate. The thermal diffusivity is found to increase in the direction parallel, and decrease in the direction perpendicular, to the direction of elongation. Thermal diffusivity data along with measurements of the tensile stress at the point of quenching were used to evaluate the stress-thermal rule, which is analogous to the well-known stress-optic rule. The stress-thermal rule was found to be valid for both polymers over a range of strains and strain rates. Since the quenched samples have orientation only, it appears that the primary source of anisotropy in thermal conductivity is the anisotropy of polymer chain orientation.