Production of polyethylene (PE) of a relatively high molecular weight with improved properties and acceptable processability has been allocated to many research efforts. In the slurry polymerization, immobilization of homogeneous metallocene catalyst on a nano-sized support leads to improved mechanical and thermal properties in addition to controlled morphology and appropriate particle size distribution of product. Specific surface area of support particles can be an effective parameter affecting the immobilization process of catalyst and product properties. In this research the main purpose was to produce PE/nanosilica nanocomposite, using an in-situ polymerization technique, in a disentangled state.
A metallocene catalyst, such as zirconocene dichloride (Cp2ZrCl2), was immobilized on the surface of modified nano-fumed silica particles. Three grades of nano-fumed silica having specific surface areas of 380, 200 and 50 m2/g were used. First the surface of thermally pretreated nanosilica was chemically modified using methylaluminoxane. Then, by adding the catalyst, Cp2ZrCl2, immobilization reaction and activation of the catalyst were performed simultaneously. Finally ethylene polymerization was conducted using the prepared catalyst under the atmospheric pressure of monomer at 30°C.
The maximum polymerization yield was related to the heterogenized catalyst on nanosilica with a specific surface area of 200 m2/g. The results of solid state drawability and buildup of modulus in time sweep rheometry exhibited that the synthesized polyethylene is in the less entangled state. Reducing the concentration and density of the active sites on the heterogenized catalyst resulted in the reduced number of chain entanglements. Tensile test results showed that nanocomposite samples possess better mechanical properties compared to the pure polyethylene, an indication of appropriate distribution of silica nanoparticles into the polyethylene matrix which was evidenced using SEM images.
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