Study On Tuning Of Polyethylene Microstructure Via Chain Transfer And The Application Of Resultant Branched Polyethylene

Study on the tuning of polyethylene (PE) microstructure is significant since it plays a crucial role on the properties, performances and applications of PE. Chain transfer reaction is an effective and convenient method to tune PE microstructures. The dissertation studied the influence of chain transfer reaction on ethylene polymerization and the microstructures of resultant PE through adding chain transfer agent (CTA) into polymerization system, the preparation of HBPE fibers and HBPE-based composite fibers via electrospinning and the consecutive tuning of the molecular weight distribution of PE utilizing chain transfer to CTA.Four PE samples with different microstructures were prepared from ethylene polymerization mediated by a-diimine Ni complex system at different temperatures. GPC and DSC results indicated that temperature had a remarkable effect on the microstructure and thermal properties of PE, respectively. It was found that the molecular weight of PE reduced from222.34to77.33kg/mol with temperature increasing from4℃to36℃. Utilizing the competence among the reactions of chain transfer, chain propagation and chain walking,14types of PE samples were obtained at different temperatures. The molecular weight of PE reduced with the increase of diethyl zinc (ZnEt2) concentration which showed a larger reduction on the molecular weight of PE obtained at32℃than that at22℃. However, the presence of ZnEt2showed modest influence on the branching distribution of PE, which might be due to slow chain transfer to ZnEt2relative to chain propagation and chain walking.Hyperbranched polyethylene (HBPE) shows excellent solubility in organic solvents at room temperature.21types of HBPE fibers,11types of HBPE/MWCNT fibers and23types of HBPE/PSt fibers were obtained employing solution electrospinning. Detailed effects of solvent type, concentration, voltage, rotate speed of collector, etc. on the fiber morphologies were investigated. Results indicated that THF and chloroform are suitable for using as the solvents of HBPE solution for electrospinning. HBPE fibers were obtained with notable cross-links or agglomerations via electrospinning due to the low Tg of HBPE. High voltage favored the formation of thin PE fibers as well as tortile fibers. Only the solution with concentration higher than4wt%can provide long HBPE fibers through electrospinning. Also, HBPE/MWCNT and HBPE/PSt composite fibers were obtained successfully via electrospinning. The influences of various parameters of the solution and electrospinning on the fiber morphologies were studied.PE samples (31types) were prepared from ethylene polymerizations mediated by rac-Et(Ind)2ZrCl2/MAO system in the presence of varying concentration of triethyl aluminum (TEA). At different temperatures, TEA showed different influences on the ethylene polymerization and PE microstructures. The commonality is that the molecular weight of PE reduced with the increase of TEA concentration.1H NMR indicated that the presence of TEA reduced the branching degree of PE. The dependence of quantity of PE chains on TEA concentration implied that high temperature was beneficial for the occurrence of chain transfer to TEA. Ethylene polymerization results obtained at different AlMAO/Zr ratios suggested that partial TEA participated the activation of catalytic precursor and the presence of TEA could enhanced the activity remarkably. The ethylene polymerizations in the absence or presence of TEA produced distinctively different results with the extension of polymerization time. In the absence of TEA, the molecular weight of PE reduced with the extension of time while the number of PE chains remained constant. In the presence of TEA, both the molecular weight of PE and the number of PE chains increased with the extension of time. The results indicated that more and more TEA participated in the chain transfer reactions with the extension of time.PE (9types) with different molecular weight distributions were prepared successfully from ethylene polymerization mediated by rac-Et(Ind)2ZrCl2/MAO by tuning the time and equivalent of TEA addition. The molecular weight, molecular weight distribution, thermal properties, crystalline morphology and rheological property of the samples were studied scientifically. Molecular weight and PDI of PE samples could be adjusted gradually in ranges of40.87～4.49kg/mol and2.35～7.23, respectively, by tuning the time of TEA addition. On the other hand, molecular weight and PDI of PE samples could be adjusted gradually in ranges of40.87～6.12kg/mol and2.35～11.55, respectively, by tuning the equivalent of TEA addition. The experimental results confirmed that the molecular weight and molecular weight distribution of PE could be adjusted effectively and conveniently via controlling the time and equivalent of TEA addition. Dynamic rheological tests indicated that the PE samples with broad/bimodal distribution have reduced viscosities.PE samples with PDI of2.52,3.61,7.30,10.65and12.69were obtained with a-diimine Ni complex/rac-Et(Ind)2ZrCl2/MAO as the binary catalytic system in the presence of varying concentrations of ZnEt2. Composition analysis of the resultant polymers showed that the polymers produced by the binary catalytic system consist of linear PE fraction with relatively low molecular weight from Zr catalyst and branched PE fraction with relatively high molecular weight from Ni catalyst. PE samples from binary catalytic system showed distinctively different crystalline morphologies compared with that from single zirconocene catalyst. The influence of BMPE on the mechanical behaviors of HDPE was studied by blending. It was found that10wt%content of BMPE showed notable influences on the storage modulus, elongation at break and tensile strength of HDPE.