Anisotropic Thermal Conductivity of Poly(lactic acid) Polymers Subjected to Uniaxial Elongation

The processing and application of polymeric materials involves thermal transport. In many cases, the thermal conductivity is anisotropic due to flow-induced anisotropy of the polymer microstructure. The relationship between the microstructure and thermal conductivity is, however, poorly understood. A major reason for this is the scarcity of reliable thermal conductivity data, particularly for anisotropic systems. Here, we use a robust optical technique to measure the components of the thermal diffusivity tensor for several grades of poly(lactic acid) polymers that have been subjected to uniaxial elongation in the molten state. These data are complemented with tensile stress measurements, and with thermodynamic property measurements that include density, heat capacity, and crystalline fraction. With these data, we examine for amorphous and semi-crystalline polymers the validity of the stress-thermal rule, which gives a linear relation between the thermal conductivity and stress tensors.