Upcycling Waste Polyethylene Into Ultra‐High Thermal Conductivity Composites Through Interface Engineering and Shear‐Induced Orientation

ABSTRACT Polyethylene (PE) is the most widely produced and discarded polymer, yet current recycling approaches often yield low‐value products with inferior performance and risk secondary pollution. Here, we present a scalable recycling strategy that integrates interface engineering and orientation structuring to transform recycled polyethylene (rPE) into value‐added materials with ultrahigh thermal conductivity (κ). In situ grafting of maleic anhydride (MAH) strengthens interfacial hydrogen bonding between rPE‐g‐MAH (rPM) and introduced graphite nanosheets (GNPs), increasing the interaction energy from −256.51 to −581.50 kcal/mol. During high‐shear twin‐roll milling, GNPs acquire a highly planar orientation (orientation factor f = 0.91) and are tightly bound by rPM, forming continuous, efficient phonon transport pathway with an interface thermal resistance ( R) as low as 7.03 × 10 −12 m 2 K·W −1 . Benefiting from interfacial design and strong anisotropic alignment, the resulting laminate exhibits exceptional in‐plane thermal conductivity (κ∥) of 44.5 W m −1 K −1 and cross‐plane thermal conductivity (κ⊥) of 2.82 W m −1 K −1 , achieving temperature reductions of 25°C for the LED lamp and 31°C in the CPU cooling tests. This straightforward yet effective strategy not only enables the sustainable upcycling of PE but also establishes a viable route to highly oriented polyolefin/inorganic composites with ultrahigh thermal conductivity.