Measuring anisotropic thermal conduction in polyisobutylene following stepshear strains

The connection between polymer chain orientation and several macroscopic pr oper ties in a polymer melt was studied using mechanical and optical techni ques. Anisotropic thermal conductivity following shear deformation was meas ured wing forced Rayleigh light scattering, the refractive index tensor is followed using birefringence measurements, and the stress was measured mech anically in a parallel-plate rheometer: The thermal diffusivity measured in the from, and neutral directions increased and decreased, respectively, im mediately following the deformation. These quantities then relaxed to the e quilibrium value on the time-scale of the stress-relaxation memory. Compari son of the difference between measured flow and neutral direction thermal d iffusivities with the analogous flow-induced birefringence in the same defo rmation provided indirect evidence for a linear relation between stress and thermal diffusivity at two different values of strain. Mechanical measurem ents were used to characterize the memory of the fluid.