High-Speed Melt Stretching Enables Facile Fabrication of Mechanically Superstrong and Highly Thermo-Conductive Anisotropic Polyethylene/BN Nanocomposite Films

Fabrication of high-performance thermally conductive polymer nanocomposites through melt processing is crucial for large-scale industrial applications. However, this remains underdeveloped due to challenges with regulating nanofillers in viscous polymer melts. Herein, we propose a high-speed melt stretching strategy to prepare thermally conductive nanocomposite films of high-density polyethylene/boron nitride (PE/BN). By rapidly applying a maximum 80× stretch ratio to the high-temperature composite melt using a homemade two-drum extensional machine, two-dimensional BN nanosheets are highly oriented and redispersed within the PE matrix. This enables the production of nanocomposite films with superstrong mechanical properties, including high strength (>90 MPa) and modulus (>3 GPa). The horizontally aligned BN nanosheets simultaneously create fast pathways for phonon transport, resulting in a high in-plane thermal conductivity of 6.56 W/m K for the film with 40 wt % BN. This value is superior to previous reports on polymer/BN composites with similar filler fractions and ensures an excellent cooling capability of the film for commercial LED modules. Additionally, the melt-stretched PE/BN films demonstrate significantly improved thermo-mechanical and electrical insulating properties compared to their hot-pressed counterparts, attributed to the microstructural advantages. This work offers a viable melt processing approach for developing high-performance general plastic-based thermally conductive films toward electronic device applications.