New insights into thermal conductivity of uniaxially stretched high density polyethylene films

Thermal conductivity of drawn polyethylene (PE) fibers has been extensively studied, while thermal conductivity for the stretched PE sheets still remains unknown. In addition, no consensus conclusions have been reached on the thermal conduction mechanisms for drawn PE. In this work, melt processed PE sheets are subjected to uniaxial stretching under different stretch ratios. The in-plane and out-of-plane thermal conductivity of stretched PE has been measured and the crystalline structure has been analyzed through 2-dimensional wide-angle X-ray diffraction (2D-WAXD) and scanning electron microscopy (SEM). Results show that both in-plane and out-of-plane thermal conductivities of stretched PE increase with increasing stretch ratios. The enhancement of thermal conductivity has been ascribed to the formation of interlocking shish-kebab superstructures induced by the stretching process. A new model is proposed to demonstrate the thermal conductivity improvement seen for the uniaxial-stretched PE. To the best of our knowledge, it is the first time that the mechanisms of thermal conductive enhancement were revealed for uniaxially stretched semi-crystalline PE sheet along with the understanding of the associated process-structure-property relationship. This work may provide key information towards design and manufacturing of polymer-based materials for thermal management applications.