In Situ Synthesized Self-Reinforced HDPE/UHMWPE Composites with High Content of Less Entangled UHMWPE and High Gradient-Distributed Oriented Structures

In this study, tailored polyethylene reactor blends with the in situ embedding of a weakly entangled ultra-high-molecular-weight polyethylene (UHMWPE) were synthesized by the polyhedral oligomeric silsesquioxane modified Ziegler–Natta (ZN) catalysts using a two-stage cascade polymerization technique. Holistic improvement of strength/stiffness/toughness was realized by the common injection modeling owing to the enhanced formation of an orientated structure which was verified by the patterns of two-dimensional (2D) small-angle X-ray scattering and 2D wide-angle X-ray diffraction, as well as the fracture morphology. 30 wt % of the less entangled UHMWPE was successfully incorporated into the high-density polyethylene (HDPE) matrix to achieve a synchronously increased tensile strength (52.4 MPa, +97.7%), Young’s modulus (604.2 MPa, +43.6%), and impact resistance (74.4 kJ/m², +675%), compared with those of the benchmarked HDPE. However, dissipation of a shish-kebab structure was pronounced in the depth direction of the HDPE spline reinforced with the disentangled UHMWPE. This dissipation was proved to partially sacrifice the strength and stiffness. Importantly, the impact strength is greatly enhanced (+140%) due to the cocrystallization effect of the disentangled linear UHMWPE chains with the HDPE matrix. The HDPE matrix reinforced with the weakly entangled UHMWPE presents wider gradient distribution and orientation degree distribution of oriented structures (including shish-kebab and stacked lamella) along the depth direction. Thus, the balance of strength/stiffness/toughness was synergistically improved with the wider gradient distribution of oriented structures. This in situ polymerization method with the weakly entangled UHMWPE offers a promising routine for achieving the high-performance polyethylene commodity.