Self‐Reinforcement of Polyethylene Enabled by the Additive‐Induced Chain Orientation

Self‐reinforcement of polyolefins, achieved through flow‐induced internal alignment rather than incorporation of foreign fillers, represents a key strategy for material enhancement and sustainable recycling. Herein, we report a facile method that employs trace amounts of a high‐aspect‐ratio additive (CH‐1) to realize significant self‐reinforcement in high‐density polyethylene (HDPE) during injection molding. Although CH‐1 does not modify the linear viscoelasticity of the melt, it plays a crucial role under processing conditions. During shear, CH‐1 promotes the breakdown of the elastic network and accelerates the alignment of molecular chains along the flow direction. Upon cooling, molecular chains tend to recover their entropic state in the absence of external forces, while CH‐1 effectively retards this relaxation process. This dual functionality extends the orientation window and markedly enhances the formation of Shish–Kebab structures, leading to a 272% increase in impact strength. Moreover, elevated content of low‐molecular‐weight fractions accelerates chain relaxation and thereby diminishes the effectiveness of CH‐1. This study provides mechanistic insight into flow‐induced crystallization and offers a practical pathway toward high‐performance HDPE production.