Approaches to Tailoring the Structure and Properties of Polyethylene
Li Pi Shan, Colin
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Alternative methods to control the molecular weight and short chain branching distribution of polyethylene were investigated. The ability to produce polyolefins with multimodal microstructural distributions using single catalyst/single reactor set-up is very attractive and could, in principle, be used to produce polyolefin resins with advanced molecular architecture. In this thesis, resins with controlled microstructures were produced, characterized and properties tested in order to develop a better understanding of polymerization structure-property relationships. Copolymerizations of ethylene and 1-hexene were carried out with an in-situ supported metallocene catalyst. Copolymers were produced with different alkylaluminum activators and the effect on molecular weight and short chain branching distributions was examined. It was found that different activator types produce polymer with unimodal and narrow molecular weight distributions but with very different short chain branching distributions. Each activator exhibits unique comonomer incorporation characteristics to produce bimodal short chain branching distributions with the use of a single activator. By using individual and mixed activator systems, it is possible to control the short chain branching distributions of the resulting copolymers while maintaining narrow molecular weight distributions. To further investigate the capabilities of this in-situ supported catalyst system, an experimental design was carried out to study the effect of polymerization conditions on the catalyst activity and microstructure of poly(ethylene-co-1-octene). The parameters investigated were: polymerization temperature, monomer pressure, chain transfer to hydrogen, comonomer/ethylene feed ratio and concentration of alkylaluminum. The effect of each parameter on the catalyst activity, comonomer incorporation and molecular weight distribution was investigated. The results obtained were not typical of a conventional single-site catalyst. The copolymerization system was sensitive to all of the parameters and many interactions were evident. The most prominent effect was the catalyst response to temperature. As the temperature was decreased, the short chain branching distributions of the copolymers became broad and bimodal. Overall, it was found that a wide range of microstructures could be produced, ranging from copolymers with low and high 1-octene content with unimodal to broad short chain branching distributions, and from low to high molecular weight with narrow to broad molecular weight distributions. To examine the effect of these broad short chain branching distributions on the polymer properties, a series of poly(ethylene-co-1-hexene) resins with very distinct, and in some cases bimodal crystalline distributions, were synthesized. The attractive feature of the resins in this study is that their molecular weight distributions are similar but each possesses a different short chain branching distribution, thus effectively minimizing the effect of molecular weight on the properties investigated. It was found that the tensile properties of a copolymer could be controlled by the ratio of the crystalline species present in the sample. In this study, a balance of stiffness and toughness was exhibited by a copolymer containing a large proportion of crystalline material and a small fraction of material of lower crystallinity. A series of poly(ethylene-co-1-octene) resins with tailored molecular weight and short chain branching distributions were synthesized with a heterogeneous metallocene catalyst in a two-stage polymerization process. Blends of high molecular weight copolymer and low molecular weight homopolymer and reverse blends of low molecular weight copolymer and high molecular weight homopolymer were produced. The physical properties of these resins were tested for their dynamic mechanical (tensile) and rheological properties. Increasing the copolymer content in the blend resulted in a decrease in stiffness. However, the energy dampening properties of these blends benefit from the presence of the copolymer. It was also confirmed that the melt flow properties of polymers mostly depend on their molecular weight distribution. Regardless of the comonomer content, the melt viscosities decreased with the addition of low molecular weight polymer.