Effect of molecular structure on dynamic mechanical properties of polyethylene obtained with nickel–diimine catalysts (original) (raw)
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Polymer, 2003
In this work, five branched polyethylenes with different branching units were synthesized using bidentate nickel (II) catalyst containing adiimine ligands. For comparison, one linear polyethylene was also prepared using tridentate iron (II) catalyst containing a-diimine ligand. The crystalline structure of the polyethylenes was investigated using X-ray diffraction (XRD) and polarized optical microscope. The crystalline properties were also measured by differential scanning calorimeter (DSC). Viscoelastic properties of the polyethylenes were investigated using rheometric dynamic analyzer. The DSC and XRD results showed that highly branched polyethylenes exhibit no melting points and no predominating crystalline forms, while the linear polyethylene exhibits clear orthorhombic ð110Þ and ð200Þ reflections on XRD pattern and a clear melting point at 118 8C. The viscoelastic properties of the branched polyethylenes were very complicated due to the combined effect of the molecular weight difference and the degree of chain branching as well as the branching structure.
Macromolecules, 2017
The activation of a prototypical nickel(II) Brookhart catalyst by either methylalumoxane (MAO) or diethylaluminum chloride (AlEt 2 Cl) under a variety of conditions showed that a proper choice of the mode of activation is a powerful tool to modulate the polymer microstructure. In particular, use of AlEt 2 Cl instead of MAO resulted in the production of more branched polyethylenes with a higher content of long chain branches and even some "branches on branches". Characterization of these materials by NMR, thermal, X-ray diffraction, and mechanical analyses provided insight into the relationships between the microstructure and the crystallization behavior and the elasticity of the polymers. For these branched polyethylenes, a transition from plastomeric toward elastomeric behavior occurs for branch concentrations much lower than for ethylene− propylene copolymers and like those observed for ethylene copolymers with bulkier comonomers. For elastomeric materials, reduction of branch concentration implies two relevant advantages: (i) reduction of glass transition temperature becoming closer to that of polyethylene; (ii) more efficient radical cross-linking with reduction of degradation reactions. An additional advantage is, of course, a polymer production process involving only ethylene.
Molecular dynamics simulation for polyethylene crystallization: Effect of long chain branches
Polyolefins Journal, 2021
The influence of long branches on crystallization behavior has been studied by means of molecular dynamics simulations. Using two systems: polyethylene (PE) with long branches (LCB-PE) and PE without long branches (linear-PE) with the same molecular weight, we have examined the crystallization behavior of the two systems by molecular dynamics simulation. This paper explains the influence of long branches on the isothermal crystallization process and the non-isothermal crystallization process with similar initial interchain contact fraction (ICF) in terms of final ICF, crystal regions, crystallinity, concentration of tie chains and energy. It is found that the crystallization process is classified as two stages: the nucleation stage and the crystal growth stage. The existence of long branches is favorable for the first stage while unfavorable for the second stage. Knots that act as crystalline defects are excluded from the lamella, resulting in decreasing in regularity and crystallinity of molecular chains. From the perspective of potential energy and non-bond energy, LCB-PE has lower energy than linear-PE in the nucleation stage while the energy of linear-PE is lower than that of LCB-PE in the second stage. In short, the long branched chains inhibit the crystallization process.
Journal of Applied Polymer Science, 2004
A series of polyethylene (PE) samples were prepared in a slurry polymerization with bis(cyclopentadienyl) zirconium dichloride (Cp 2 ZrCl 2)/modified methylaluminoxane (MMAO) using a semibatch reactor. The samples had long-chain branch densities (LCBDs) of a 0.03-1.0 branch per 10,000 carbons and long-chain branch frequencies (LCBFs) up to a 0.22 branch per polymer molecule. The rheological and dynamic mechanical behaviors of these long-chain branched PE samples were evaluated. Increasing the LCBF significantly increased the 0 's and enhanced shear thinning. Long-chain branching (LCB) also influenced the loss modulus and storage modulus. Increasing the LCBF led to enhanced GЈ and GЉ values at low shear rates and broader relaxation spectrums. The samples exhibited thermorheologically complex behavior. LCB also played a significant role in the dynamic mechanical behavior. Increasing the LCBF increased the stiffness of the polymer and enhanced the damping or energy dissipation. However, LCB had little influence on the crystalline structure of the PE. The ␣and ␥-relaxations showed little dependence on the LCBF.
Macromolecules, 2010
This paper deals with the characterization of a broad range of linear polyethylenes (PE) by means of thermorheology. It is demonstrated, in which way the thermorheological behavior can be related to the comonomer type and content of copolymers. In the first part of this paper, well-established analytical and rheological methods are applied to distinguish linear and branched samples. The resolution limit of these methods is demonstrated by the investigation of a blend from a linear-low density PE (LLDPE) and a branched low density PE (LDPE). Special attention is paid to nuclear-magnetic resonance (NMR) spectroscopy in order to reliably determine the comonomer type and content of the samples chosen. The main focus of this paper lies on thermorheological investigations and correlations with the molecular structure. The comparatively high sensitivity of such investigations is highlighted: even small amounts of long-chain branches (LCB) are revealed. The activation energy (E a) of linear samples increases with growing comonomer length and content, respectively. As such, thermorheology is demonstrated to be an interesting rheological "tool" to get an insight into branching structures. Moreover, the experimental effort is relatively small as solely usual dynamic-mechanical experiments at different temperatures are required.
Polymer Science Series B, 2014
In this research, the effect of long chain branching (LCB) and polymerization conditions on ther mal, mechanical, and rheological properties of polyethylene synthesized via a metallocene polymerization was studied. The LCB was varied in the range of 0.64-1.14 per 10 4 atoms of C. 13 C NMR spectra showed that the distributions of both short as well as long chain branches in the polymer backbone chain are influenced by polymerization conditions. The increase in ethylene pressure leads to rise in polymer yield, catalyst activ ity, molecular weight, and narrowing of molecular weight distribution. In contrast, the increase of polymer ization duration results in broadening of MWD and a decrease in catalyst activity. In addition, the influence of frequency and LCB on dynamic shear and extensional melt rheology has been reported. The polymer crys tallization was discussed in light of Avrami model and modified Hoffman Lauritzen theory. LCB promoted the transport of chain segments but retarded the nucleation in polyethylene crystallization. The tensile strength decreased with the increase in LCB content. The different macroscopic properties were correlated to LCB content.
Journal of Applied Polymer Science, 1990
The dynamic mechanical properties of different low-density and linear low-density polyethylenes have been measured as a function of crystallization conditions. The mechanical results have been explained accounting for molecular segregation as well as reorganization capacity of the crystallizable segments during isothermal thickening. The presence of a high amount of segregated material does not allow detection of any relationship between crystallization conditions and mechanical relaxations for three of the four samples. The average values of the resonance relaxation temperatures and the relaxation strengths are dependent on branch content and branch type.
Journal of Polymer Science Part B: Polymer Physics, 2015
The influence of short-chain branching on the formation of single crystals at constant supercooling is systematically studied in a series of metallocene catalyzed highmolecular-weight polyethylene samples. A strong effect of short-chain branching on the morphology and structure of single crystals is reported. An increase of the axial ratio with short-chain branching content, together with a characteristic curvature of the (110) crystal faces are observed. To the best of our knowledge, this is the first time that this observation is reported in high-molecular-weight polyethylene. The curvature can be explained by a continuous increase in the step initiation-step propagation rates ratio with short-chain branching, that is, nucleation events are favored against stem propagation by the presence of chain defects. Micro-diffraction and WAXS results clearly indicate that all samples crystallize in the ortho-rhombic form. An increase of the unit cell parameter a 0 is detected, an effect that is more pronounced than in the case of single crystals with ethyl and propyl branches. The changes observed are compatible with an expanded lattice due to the presence of branches at the surface folding. A decrease in crystal thickness with branching content is observed as determined from shadow measurements by TEM. The results are in agreement with additional SAXS results performed in single crystal mats and with indirect calorimetry measurements.