Suppressing the Entanglements of Ultrahigh-Molecular-Weight Polyethylene via Controlling the Adhesion Effect in a POSS-Modified Support

Polyhedral oligomeric silsesquioxane (POSS) nanoaggregates can serve as a modifier of the MgCl2-supported Z-N catalyst, thus controlling the polymerization behavior and modifying the microstructure of the synthesized polymer. In this study, a series of POSS-modified MgCl2-supported Z-N catalysts were explored to reveal the relationship between the entanglement formation of ultrahigh-molecular-weight polyethylene (UHMWPE) and the adhesive effect control of hydroxy groups in POSSs. X-ray photoelectron spectroscopy (XPS) and CO in situ FTIR low-temperature adsorption technology were employed to investigate the catalytic surface structure. It was evidenced that POSSs can coordinate with the MgCl2 (110) plane, acting as adhesive of a cage structure to be in situ embedded into the MgCl2-based support. Therefore, the loaded TiCl4 active sites were isolated in the molecular scale on the MgCl2-based support and the entanglements were greatly suppressed thereof. The ″adhesion effect″ of the MgCl2 support can be controlled by adjusting the type (i.e., hydroxyl content) of POSS introduced, thereby suppressing entanglements to a greater extent. The MgCl2 support modified with POSS with no hydroxy groups endowed it with the optimum entanglement reduction effect. As a consequence, the as-synthesized polyethylene exhibits an overall promotion of the impact resistance (+14.1%), tensile strength (+13.9%), Young's modulus (+16.8%), and break elongation (+10.5%), compared to those of UHMWPE synthesized from the POSS-free catalyst. In addition, the POSS-modified MgCl2-based catalyst shows more stable catalytic activity and responsiveness of molecular weight with hydrogen modulation, which exhibits great prospects of industrial application in polyethylene production.