Study on single and binary catalytic systems of pyridine-imine catalysts based on nickel and iron in synthesis of reactor blends and low-density polyethylene nanocomposites

In the presence of modified methylaluminoxane as cocatalyst, the behavior of a binary catalytic system based on pyridine-imine nickel ( N ) and iron ( F ) catalysts was evaluated in order to reach a proper mixture of polyethylene (PE). A computational study along with kinetic profile suggested that the catalyst F with higher electron affinity (A) and electrophilicity (ω) in the methyl cationic active center and stronger interaction with the monomer led to high integrated monomer consumption and higher activity. In addition, the samples produced by the mixture of catalysts showed a higher value of [19.4 × 10⁴ g (PE) mol (Fe+Ni)⁻¹ h⁻¹)], melting point (127.8 °C), and crystallinity extent (41.29%) than the samples produced by the single catalysts. The addition of multiwalled carbon nanotubes (MWCNT) into the polymerization media reduced the activity of catalysts [from 7.50 × 10⁴ to 0.66 × 10⁴ g (PE) mol (Fe+Ni)⁻¹ h⁻¹] and the thermal properties of the low-density polyethylene nanocomposite samples. However, the sample containing 2.33% MWCNT_20-30 improved the total thermal stability of the neat polyethylene blend up to 400 °C. Scanning electron microscope images of the samples demonstrated irregular to virtually uniform morphologies were obtained through the in situ and solution-mixing techniques. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47376.     

Variation of internal structure and performance of polyethylene- and polypropylene-modified bitumen during blending process

Linear-viscoelastic characteristics and performance are evaluated throughout the blending process of polyethylene and polypropylene with bitumen. Results indicate that type, form and percentage of polyolefin play a significant role in the time evolution of the composite's mechanical response. Toluene extraction of modified bitumen revealed, for the first time, the formation of a sponge-like polymer network. Visual inspection and Fourier transform infrared analysis of the polyolefins recovered after extraction indicates higher affinity of the polyethylene with bitumen in agreement with the rheological test results. The use of polypropylene is discouraged if rutting performance is a concern, and polyethylene in both pellets and powder form at 4%, and after 210 min of blending produces a modified bitumen with acceptable performance.     

1H T2‐NMR monitoring of crosslinked polyolefin aging

This work examines the correlation between the ¹H‐NMR T₂ relaxation constant and the mechanical properties of aged crosslinked polyolefin cable insulation. T₂ experiments on unswollen samples could not differentiate between unaged and highly aged materials; all exhibited ¹H T₂ constants of approximately 0.5 ms. To accentuate the effects of aging, samples were swollen in various solvents. Unaged samples had T₂ values of approximately 15 ms in good solvents. With thermal aging, T₂ values decreased as the ultimate tensile elongation decreased. However, the correlation between T₂ and elongation differed for samples irradiated with high‐energy radiation and for materials aged above versus below the crystalline melting temperature. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2578–2582, 2003     

Entanglement network and relaxation temperature dependence of single‐site catalyzed ethylene/1‐hexene copolymers

In this work, we investigate the linear viscoelastic response of high molecular weight ethylene/1‐hexene copolymers, characterized by a narrow molecular weight distribution and comonomer content in the range from 0 to 10 mol %. A variation in the entanglement plateau modulus has been found in agreement with the recently developed packing length model. The packing model applied to viscoelastic data suggests decreased values of the characteristic ratio, accordingly with recent computer simulation results. The flow activation energy increases as the side chain content increases. This feature is thought to be related to the mobility of the molecules. The presence of side branches due to the comonomer hinders the mobility of the molecules, and increases the thermal barrier for the segmental motion. Then in the comonomer content range studied, the increase of the flow activation energy goes parallel with a decrease in the characteristic ratio. This result suggests that more parameters than only the stiffness of the chain modulate the thermal dependence of viscoelastic properties. A more refined study is necessary combining experiments with computer simulations in order to elucidate these aspects. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Metallocene‐based functionalized polyolefins as compatibilizers in polyolefin nanocomposites

This article reports a study of some functionalized polyolefins evaluated as compatibilizers in polyethylene nanocomposites. The functionalized polymers were prepared by direct metallocene‐mediated copolymerizations of ethylene and a functional comonomer. The prepared nanocomposites were evaluated for mechanical and barrier property enhancement. A good combination of mechanical and barrier properties was obtained with the metallocene‐based functionalized polyethylene. The toughness–stiffness balance was better than or comparable to that achieved with conventional functionalized polymers such as maleic anhydride grafted polyethylene. The results also indicated that these metallocene‐based functionalized polyolefins, when used as compatibilizers, could have relatively higher molar masses and lower functionality than those of conventional post‐reactor‐modified compatibilizers, and so the drawbacks associated with the latter could be avoided. Their inherent properties could also further improve the final nanocomposite properties. This was attributed to the more homogeneous nature of metallocene‐catalyzed polymers in comparison with post‐reactor‐modified products. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1094–1100, 2004     

Structural and Mechanical Properties Changes of Ethylene‐α‐olefin Copolymer Blends Induced by Thermal Treatments and Composition

Summary: Blends of single‐site catalysed ethylene‐α‐butene (C₄VLDPE) and ethylene‐α‐octene (C₈VLDPE) copolymers were prepared by melt extrusion. The phase morphology, thermal and mechanical properties of the blends have been investigated by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), tensile test and dynamic mechanical analysis (DMA). Depending on the composition and thermal history, significant differences in structure and behaviour were found. It was also found that some degree of co‐crystallization occurred for quenched blends; whereas most of the oven slowly cooled blends showed two well‐defined melting peaks, indicating that the slow cooling favoured partial segregation of the fractions with different degrees of branching to form two morphologies. Moreover, SEM revealed morphology of the thinner crystals distributed in‐between the thicker sheaf‐like crystals for the slowly cooled blends with 20–50% C₈VLDPE. Therefore, the synergism in mechanical properties for the blends with 20–50% C₈VLDPE is due to a combination of larger crystal size, more complete phase separation and interfacial interaction produced by the segregation effect of the slow cooling treatment. DMA studies showed that the storage modulus increased as the addition of C₈VLDPE and modulus for the slowly cooled blends are about twice those measured for the quenched ones, indicating higher stiffness of the blends. The smooth shift of β relaxation temperature with addition of C₈VLDPE for both sets of blends confirmed the miscibility in the amorphous phase.

SEM image of the C₄VLDPE‐C₈VLDPE (50/50) blend after oven slow cooling treatment.     

Compatibilization of PP/elastomer/microsilica composites with functionalized polyolefins: Effect on microstructure and mechanical properties

The morphology and mechanical properties of polypropylene/elastomer/silica composites were investigated with the aim of improving stiffness and impact resistance. Two different types of silica were tested: Precipitated silica and polymer grade microsilica (silica fume). The composites were compatibilized with commercial polypropylene and polyethylene containing maleic anhydride functionality as a means of controlling their microstructure and ultimately their mechanical properties. Comparisons were made with surface coated silica and hydroxyl-functionalized copolymers prepared with metallocene catalysts. The effect of adding the polymeric compatibilizers was assessed by morphology studies, thermal analysis and mechanical testing. Significant improvements in impact strength were obtained by tailoring the microstructure of polypropylene/elastomer/microsilica composites. With introduction of PP-g-MAH as compatibilizer, stiffness was enhanced simultaneously with impact strength. DSC curves of crystallization provided evidence to support the formation of different microstructures.     

Rheological and morphological properties of high‐density polyethylene and poly(ethylene–octene) blends

The rheological and morphological properties of blends based on high‐density polyethylene (HDPE) and a commercial ethylene–octene copolymer (EOC) produced by metallocene technology were investigated. The rheological properties were evaluated in steady and dynamic shear experiments at 190°C in shear rates ranging from 90 s^?1 to 1500 s^?1 and frequency range between 10^?1 rad/s and 10² rad/s, respectively. These blends presented a high level of homogeneity in the molten state and rheological behavior was generally intermediate to those of the pure components. Scanning electron microscopy (SEM) showed that the blends exhibit dispersed morphologies with EOC domains distributed homogeneously and with particle size inferior to 2 μm. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2240–2246, 2002     

Morphology and mechanical properties of blends of thermoplastic polyurethane and polyolefins

Polyolefins (PO) were melt mixed with thermoplastic polyurethane (TPU) in a 20 : 80 weight ratio with or without compatibilizer containing 0.5 wt % of maleic anhydride. Effects of component viscosities on morphology and on mechanical properties of the blend were studied by scanning electron microscope (SEM), tensile property analysis, and dynamic mechanical analysis (DMA). It was found that the disperse particle size of compatibilizer‐free blends decreased with the decreasing viscosity ratio of the disperse phase to TPU. The efficiency of the compatibilizer in reducing the particle size varied with viscosity ratios of the disperse phase to compatibilizer. However, the particle size did not decrease with the decreasing viscosity ratio monotonically. With lower viscosity ratio, addition of 5 wt % compatibilizer resulted in a greater reduction of particle size and less loss in the tensile properties as compared to the TPU matrix. For the polyethene (PE) that has the lowest viscosity value among all the POs, its size in the blend was stabilized with the addition of compatibilizer and no compatibilization was detected by DMA and by tensile property analysis. The mobility of the disperse phase and compatibilizer and the dispersion competition between them seemed important. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 875–883, 2006