Enhanced in-plane thermal conductivity of ultrahigh molecular weight polyethylene films via a new design of a two-step biaxial stretching mode

The thermal conductivity of bulk polymeric films can be generally improved by introducing interlaced lamellar or "shishkebab" crystals along the machine direction (MD) through uniaxial stretching or high-pressure extrusion. However, the thermal pathway along the transverse direction (TD) is disrupted to limit the enhancement of in-plane thermal conductivity as the draw ratio increases. In this paper, a mesh-like crystal structure of ultrahigh molecular weight polyethylene (UHMWPE) films is achieved through a two-step biaxial stretching mode to construct a planar-oriented crystal network. The in-plane thermal conductivity increases to 7.3Wm/K at a total draw ratio of 25. This mesh-like crystal network structure was investigated through scanning electron microscopy (SEM) and 1-dimensional wide-angle X-ray diffraction (1D-WXRD). The evolution mechanism of the crystal network structure is proposed on the basis of single-temperature biaxial stretching modes with different draw ratios. The construction of additional order along the TD to form a planar-oriented crystal network structure by biaxial stretching can provide new insight into improving the in-plane thermal conductivity of bulk polymeric films.