Thermorheological properties of LLDPE/LDPE blends: Effects of production technology of LLDPE

The thermorheological behavior of a number of LLDPE/LDPE blends was studied with emphasis on the effects of the production technology of the linear low‐density polyethylene (LLDPE) and the effects of long chain branching (LCB). Two Ziegler‐Natta LLDPE's (LL3001.32 and Dowlex2045G) and two metallocene LLDPEs (AffinityPL1840 and Exact 3128) were blended with a single low‐density polyethylene (LDPE), with all LLDPEs having distinctly different molecular weight. The weight fractions of the LDPEs used in the blends were 1, 5, 10, 20, 50, and 75%. DSC analysis has shown that the blends with metallocence LLDPEs are miscible in the crystal state, whereas for the Ziegler‐Natta, apart from the two distinct peaks of the individual components, a third peak appears which indicates the existence of a third phase that is created from the cocrystallization of components from the two blended polymers. The linear viscoelastic characterization was performed and mastercurves at 150 °C were constructed for all blends to check miscibility using the time temperature superposition principle. In addition, Van Gurp Palmen and zero‐shear viscosity versus composition were constructed to check the thermorheological behavior of all blends. In general, good agreement is found among these various methods. It was concluded that metallocene LLDPEs are more compatible with LDPE at all LDPE compositions when compared with their Ziegler‐Natta counterparts. Finally, the extensional properties of all blends were studied to examine the effects of different levels of LCB on their extensional rheological properties. It was concluded that extensional rheology is a sensitive tool capable of detecting subtle changes in the polyethylene macrostructure, that is, low levels of LCB. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1669–1683, 2008     

AlR2Cl/MgR2 combinations as universal cocatalysts for Ziegler–Natta,metallocene, and post‐metallocene catalysts

Combinations of dialkylaluminum chlorides and dialkylmagnesium compounds, when used at molar [AlR₂Cl]:[MgR₂] ratios ≥ 2, act as universal cocatalysts for all three presently known types of alkene polymerization catalysts—Ziegler–Natta, metallocene, and post‐metallocene. When these cocatalysts are used with supported Ti‐based Ziegler–Natta catalysts, they produce catalyst systems which are 1.5–2 times more active than the systems utilizing AlR₃ compounds as cocatalysts. Combinations of AlR₂Cl/MgR₂ cocatalysts and various metallocene complexes produce single‐center catalyst systems similar to those formed in the presence of MAO. The same cocatalysts activate numerous post‐metallocene Ti complexes containing bidentate ligands of a different nature and produce multicenter systems of very high activity. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3271–3285, 2009     

Polymerization of propylene by mixed Ziegler–Natta and metallocene catalysts

The polymerization of propylene using a mixture of racemic metallocenes and Ziegler–Natta catalysts was investigated. The polypropylene was obtained as a mixture of a fine powder and beads, with the powder being absorbed partially on the beads. The relative amount of the powder can be controlled by the concentration of the metallocene. The compatibility between the components of the mixed catalytic systems and the effect of the components on the rate of polymerization and on the properties of the new polymers were studied. The metallocene system dramatically affects the melt viscosity, isotacticity and molecular weight of the polymers. The two catalytic systems are able to act jointly, producing different polymers, for which separate melting and crystallization temperatures are obtained. © 1998 John Wiley & Sons, Ltd.     

Melt drawing of ultra‐high molecular weight polyethylene: Comparison of Ziegler‐ and metallocene‐catalyzed reactor powders

The drawing behavior of the ultra‐high molecular weight polyethylene (UHMW‐PE) melts has been studied by comparing the stress/strain curves for two types of samples as polymerized using conventional Ziegler and newer metallocene catalyst systems. Two UHMW‐PE samples, having the same viscosity average molecular weight of 3.3 × 10⁶, but different molecular weight distribution, have been drawn from melt at special conditions. The sample films for drawing were prepared by compression molding of reactor powders at 180°C in the melt. Differences in the structural changes during drawing and resultant properties, ascribable to their broad or narrow molecular weight distribution, were estimated from tensile tests, SEM observations, X‐ray measurements and thermal analyses. The metallocene‐catalyzed sample having narrower molecular weight distribution, could be effectively drawn from the melt up to a maximum draw ratio (DR) of 20, significantly lower than that obtained for the Ziegler‐catalyzed sample, ∼ 50. The stress/strain curves on drawing were remarkably influenced by draw conditions, including draw temperature and rate. However, the most effective draw for both was achieved at 150°C and a strain rate of 5 min⁻¹, independent of sample molecular weight distribution. The efficiency of drawing, as evaluated by the resultant tensile properties as a function of DR, was higher for the metallocene‐catalyzed sample having narrower molecular weight distribution. Nevertheless, the maximum achieved tensile modulus and strength for the Ziegler sample, 50–55 and 0.90 GPa, respectively, were significantly higher than those for the metallocene sample, 20 and 0.65 GPa, respectively, reflecting the markedly higher drawability for the former than the latter. The stress/strain behavior indicated that the origin of differences during drawing from the melt could be attributed to the ease of chain relaxation for the lower molecular weight chains in the melt. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1921–1930, 1999     

A WAXD/SAXS/DSC study on the melting behavior of Ziegler–Natta and metallocene catalyzed isotactic polypropylene

A small- and wide-angle X-ray scattering study was performed on two metallocene catalyzed isotactic polypropylene (miPP) resins. The results were compared with two similar molecular weight Ziegler–Natta catalyzed isotactic polypropylene (zniPP) materials. Wide-angle X-ray diffraction (WAXD) results showed the existence of two crystalline structures in the metallocene samples, the α-monoclinic and γ-orthorhombic crystal structure, with increasing relative amounts of γ-orthorhombic phase as the lamellae thickness increased. Differential scanning calorimetry (DSC) scans exhibited a melting peak for each crystal structure. The metallocene resins had the same equilibrium melting temperature (186 ± 2 °C) as the high tacticity Ziegler–Natta (ZNHT) resin, whereas a second Ziegler–Natta resin had a lower equilibrium melting temperature (178 ± 2 °C). The equilibrium melting temperature for the γ-orthorhombic crystal structure in the metallocene resins was found to be 178 ± 4 °C. The results were explained by the distribution of defects within the miPP chains, generating higher fold surface free energies for the miPP resins. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3050–3064, 1999     

Oxygen permeability in blends of a vinyl alcohol/ethylene copolymer and a metallocenic ethylene/1‐octene copolymer

This work describes the preparation, characterization, and evaluation of the oxygen permeability of blends based on a polyolefin synthesized with a metallocene catalyst (ethylene/1‐octene copolymer) and a vinyl alcohol/ethylene copolymer in an attempt to establish the corresponding relationships between the composition, morphology, and transport properties to design materials with optimized and enhanced agricultural and food packaging performances. Moreover, microhardness measurements have been used to analyze the mechanical response of these blends and to obtain information about the dispersion of the two immiscible components within the blends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3766–3774, 2004     

Polymerization of ethylene using high-activity Ziegler-type catalysts: Active center determination

The concentration of active centers of a high-activity magnesium chloride-supported Ziegler–Natta catalyst has been determined using three different methods. The initial active center concentration has been determined by quenching the reaction slurry with MeOT. To determine the concentration of the propagation species along the course of the polymerization the radio-tagging agent, ¹⁴CO, and the tagging agent, CS₂, were used. CS₂, was also investigated as a tagging agent of the growing chains, in a metallocene catalyst system. The results obtained were compared to obtain some insight about the reliability of each method and the kind of information each method can provide. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 573–585, 1998     

Processability of a metallocene‐catalyzed linear PE improved by blending with a small amount of UHMWPE

The processability of a metallocene‐catalyzed polyethylene blended in the melt with low amounts of ultra‐high molecular weight polyethylene was evaluated. Our morphological and thermal studies revealed that the blends were structurally heterogeneous, formed by a matrix of metallocene polyethylene and homogeneously distributed particles of ultra‐high molecular weight material. The visible particles were smaller than those of the reactor powder. Also observed was some degree of interaction between both phases. Rheological data indicated an intense effect of morphology on viscoelastic functions, beyond that expected for a system composed of noninteractive phases. Collectively, our findings suggest the existence, to some extent, of a homogeneous phase at the molecular level composed of a fraction of ultra‐high molecular weight species and metallocene polyethylene. This gives rise to a striking behavior during processing in that the blends show improved extrusion performance the higher their ultra‐high molecular weight polyethylene content. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2963–2971, 2005     

Unusual ethylene polymerization results with metallocene catalysts supported on silica

With the development of methods to support metallocenes and methylaluminoxane cocatalysts on suitable carriers, it became possible to combine the specific advantages of homogeneous metallocene catalysis with those of heterogeneous Ziegler catalysts in olefin polymerization. By means of ethylene polymerization it could be shown that the method of supporting methylaluminoxane and metallocene on porous silica has a substantial influence on the progress of polymerization. In particular, fragmentation of catalyst particles during polymerization can be circumvented, maintaining the catalyst activity, if active catalyst sites are being generated on the particle surface only. A method of preparation for such newly designed supported metallocene catalysts is presented, where the active catalyst sites are located exclusively on the particle surface. Furthermore, the kinetics of ethylene polymerization and morphology properties prior to and after polymerization are discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 677–682, 1999     

Melting behavior of polyethylene/α‐olefin copolymers: Narrow composition distribution copolymers and fractions from Ziegler–Natta and single‐site catalyst products

The melting temperature and heat of fusion were measured for an extensive series of compositionally uniform copolymers of ethylene with butene‐1, hexene‐1, and octene‐1. Fractions and whole polymers that exhibited minimal interchain compositional heterogeneity were from commercial copolymers made with either Ziegler–Natta (ZN) or single‐site metallocene catalysts. The present results do not support recent claims that ZN and corresponding metallocene catalyst copolymers melt at significantly different temperatures, nor the implication that comonomer incorporation is “blocky” in ZN copolymers. In five of the six comonomer/catalyst systems the dependencies of the melting temperature on comonomer type and amount were scarcely distinguishable. This common behavior is the same as that for a model random copolymer, so we conclude that most ethylene/α‐olefin copolymers have random distributions of ethylene sequences. The exception in the present study is a metallocene ethylene/butene‐1 copolymer that melts at lower temperatures and apparently has perceptibly alternating sequence distributions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3416–3427, 2004     

Semicrystalline blends of polyethylene and isotactic polypropylene: Improving mechanical performance by enhancing the interfacial structure

The low‐temperature mechanical behavior of semicrystalline polymer blends is investigated. Isotactic polypropylene (iPP) is blended with both Zeigler–Natta polyethylene (PE) and metallocene PE. Transmission electron microscopy (TEM) on failed tensile bars reveals that the predominate failure mode in the Zeigler–Natta blend is interfacial, while that in the metallocene blend is failure of the iPP matrix. The observed change in failure mode is accompanied by a 40% increase in both tensile toughness and elongation at −10 °C. We argue that crystallite anchoring of interfacially entangled chains is responsible for this dramatic property improvement in the metallocene blend. The interfacial width between PE and iPP melts is approximately 40 Å, allowing significant interfacial entanglement in both blends. TEM micrographs illustrate that the segregation of low molecular weight amorphous material in the Zeigler–Natta blend reduces the number and quality of crystallite anchors as compared with the metallocene blend. The contribution of anchored interfacial structure was further explored by introducing a block copolymer at the PE/iPP interface in the metallocene blend. Small‐angle X‐ray scattering (SAXS) experiments show the block copolymer dilutes the number of crystalline anchors, decoupling the interface. Increasing the interfacial coverage of the block copolymer reduces the number of anchored interfacial chains. At 2% block copolymer loading, the low‐temperature failure mode of the metallocene blend changes from iPP failure to interfacial failure, reducing the blend toughness and elongation to that of the Zeigler–Natta blend. This work demonstrates that anchored interfacial entanglements are a critical factor in designing semicrystalline blends with improved low‐temperature properties. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 108–121, 2000     

Blends of poly(methyl methacrylate) (PMMA) with PMMA ionomers: Mechanical properties

Rigid–rigid blends made of ionomer and ionomer precursor polymer, based on poly(methyl methacrylate) (PMMA), have been investigated. Two series of blends have been prepared for studying mechanical properties. In one series, dynamic mechanical properties were determined over a wide range of temperatures. As the weight fraction of the ionomer was increased, there was a modest increase of modulus at ambient temperature and a very large increase in the rubbery modulus at elevated temperatures above the glass transition temperature of PMMA. In a second series of tests, tensile stress–strain measurements, made at an ambient temperature, were carried out over a wide range of blend compositions. For all blends tested, the mechanical properties exhibited a synergistic enhancement, i.e., average values of modulus, strength and fracture energy were all higher than expected based on the rule of mixtures. Measurements of fracture toughness also exhibited synergy, with a maximum value, higher than the value of either blend component, being attained in blends containing about 30 wt % of the PMMA ionomer. These results are interpreted in terms of a higher resistance to fracture of the more chain-entangled ionomer phase and good interfacial adhesion between the two components of the blend. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1235–1245, 1998     

Copolymerization of ethylene and α‐olefins with combined metallocene catalysts. I. A formal criterion for molecular weight bimodality

Metallocene and other single‐site catalysts can be combined to produce polyolefins with broadened distributions of molecular weight, chemical composition, and long‐chain branching. These resins are finding increasing applications because of their enhanced properties compared to ones made with conventional Ziegler–Natta catalysts. Resins with bimodal molecular weight distributions (MWDs) have especially attractive mechanical and rheological properties. Although the use of these resins is expected to increase, there are very few studies available to quantify MWD bimodality or to decide a priori which combinations of metallocene catalysts will lead to the formation of polyolefins with bimodal MWDs. In this article, a necessary condition for the production of polymer with bimodal MWD using two single‐site‐type catalysts is derived. Additionally, a bimodality index is defined to quantify MWD bimodality. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1408–1416, 2000     

Mechanical properties of blends of PAMAM dendrimers with poly(vinyl chloride) and poly(vinyl acetate)

Hybrid blends of poly(amidoamine) PAMAM dendrimers with two linear high polymers, poly(vinyl chloride), PVC, and poly(vinyl acetate), PVAc, are reported. The interaction between the blend components was studied using dynamic mechanical analysis, xenon nuclear magnetic resonance (NMR) spectroscopy, and tensile property measurements. The data suggest a much higher degree of interaction between components of PVAc-containing blends compared to those containing PVC. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2111–2117, 1998     

Compatibilization of polyethylene/polyamide 6 blends with functionalized polyethylenes prepared with metallocene catalyst

Metallocene catalyst technology was utilized to prepare functionalized polyethylenes, which were used as compatibilizers in polyethylene/polyamide 6 (40/60) blends. Polymerization of ethylene with 10-undecen-1-ol, 10-undecenoic acid, or N-methyl-10-undecenylamine resulted in ethylene copolymers with a small amount (0.2–1.2 mol %) of functionalized side chains. The blends were prepared in a twin-screw midiextruder, and injection molded with a mini-injection molding machine. The effect of the new compatibilizers on morphology and mechanical and thermal properties was studied. Toughness as well as stiffness and strength increased significantly with an addition of 10 wt % compatibilizer. Morphology became much more uniform, and crystallization and melting behavior changed. The Molau test with FTIR analysis was used to determine that the desired reactions between the compatibilizer and polyamide had actually taken place. The results showed functionalized polyethylenes prepared with metallocene catalysts to act as effective compatibilizers in polyethylene/polyamide 6 blends. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3099–3108, 1999     

Characterization of electron beam irradiation blends based on metallocene ethylene‐1‐octene copolymer

Binary blends using metallocene ethylene‐1‐octene copolymer as matrix were prepared and subjected to electron beam (EB) irradiation (50, 100, and 200 kGy). Gel content and melt flow index values indicated that the blends were crosslinking. Fourier transform infrared‐ATR spectroscopy was used to study the crosslinking and oxidative degradation of the blends via tertiary carbon and carboxyl group formation, respectively. Thermal and mechanical properties were studied showing that the crystallinity of both matrix and dispersed phase decreased with irradiation dose, and that the thermoplastic elastomers with good mechanical properties may be obtained by EB irradiation. Chain branching and scission were also detected at all irradiation doses, although at the highest doses (200 kGy) a crosslinking reaction was the most predominantly observed effect. The successive self‐nucleation annealing technique was used to determine the EB irradiation effects on crystallization of some blends in which crosslinking and chain branching take place, modifying the chain's structure and therefore crystalline regions in the matrix and the dispersed phase. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2432–2440, 2007     

Organic nanoparticles as fragmentable support for Ziegler–Natta catalysts

A fragmentable support material for Ziegler–Natta catalysts is presented based on micrometer‐sized aggregates of polystyrene nanoparticles. Hydroxyl anchoring groups are introduced by copolymerization of hydroxymethylstyrene in emulsion process to immobilize the catalysts. The catalytic activity in ethylene slurry polymerizations is found to be directly correlated to the hydroxyl group content of the supports. Furthermore, the fragmentation behavior of dye‐labeled support aggregates into the initial nanoparticles is demonstrated using laser scanning confocal fluorescence microscopy as a nondestructive method. These supported catalysts fulfill two important design criteria, high fragmentability and high catalyst loading, and produce high‐density polyethylene with medium molecular weight distributions (MWDs = 3–4). These values lie between those obtained using single‐site metallocene‐based (narrow MWD < 3) or inorganic supported multi‐site Ziegler–Natta‐based (broad MWD = 4–12) polymerizations without the need of blending. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 15–22     

Synthesis of polypropylene-co-p-methylstyrene copolymers by metallocene and Ziegler–Natta catalysts

This paper discusses the copolymerization reaction of propylene and p-methylstyrene (p-MS) via four of the best-known isospecific catalysts, including two homogeneous metallocene catalysts, namely, {SiMe₂[2-Me-4-Ph(Ind)]₂}ZrCl₂ and Et(Ind)₂ZrCl₂, and two heterogeneous Ziegler–Natta catalysts, namely, MgCl₂/TiCl₄/electron donor (ED)/AlEt₃ and TiCl₃. AA/Et₂AlCl. By comparing the experimental results, metallocene catalysts show no advantage over Ziegler–Natta catalysts. The combination of steric jamming during the consective insertion of 2,1-inserted p-MS and 1,2-inserted propylene (k₂₁ reaction) and the lack of p-MS homopolymerization (k₂₂ reaction) in the metallocene coordination mechanism drastically reduces catalyst activity and polymer molecular weight. On the other hand, the Ziegler–Natta heterogeneous catalyst proceeding with 1,2-specific insertion manner for both monomers shows no retardation because of the p-MS comonomer. Specifically, the supported MgCl₂/TiCl₄/ED/AlEt₃ catalyst, which contains an internal ED, produces copolymers with high molecular weight, high melting point, and no p-MS homopolymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2795–2802, 1999     

Novel characterization of the crystalline segment distribution and its effect on the crystallization of branched polyethylene by differential scanning calorimetry

Based on a thermal segregation treatment, a novel semiquantitative method for the characterization of the crystalline segment distribution in branched polyethylene copolymers was established by the results of differential scanning calorimetry being treated with the Gibbs–Thomson equation. The method was used to describe the segment distribution of Ziegler–Natta‐catalyzed linear low‐density polyethylene (Z–N LLDPE), metallocene‐catalyzed linear low‐density polyethylene (m‐LLDPE), and a commercial linear low‐density polyethylene with a wide molecular weight distribution. The isothermal crystallization kinetics of Z–N LLDPE and m‐LLDPE were studied to assess the effect of different segment distributions. According to their molecular characteristics, the crystallization behaviors were analyzed. They indicated that the different segment distributions of the two polymers resulted in different crystallization processes, including the nucleation and growth of crystals under various crystallization conditions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2107–2118, 2002     

Metallocene polyolefins and their blends#

Commercial copolymers of 1‐octene and ethylene: metallocene catalyzed (mLLDPE) and Ziegler‐Natta catalyzed (znLLDPE), a low density polyethylene (LDPE), and high density polyethylene (HDPE), were characterized with respect to branching, crystallization behaviour and dynamic‐mechanical properties. It was found that the crystallinity of the polymers is more influenced by the homogeneity of the short‐chain branching than by its content. The study of blends of mLLDPE and znLLDPE with LDPE and HDPE showed that the interaction between mLLDPE and LDPE is stronger than between znLLDPE and LDPE. Blends containing mLLDPE showed a composition depending improvement of the storage modulus G' which was not observed in znLLDPE/LDPE blends. The HPDE blends followed a linear mixing rule. Co‐crystallization was found mLLDPE/LDPE and partially in znLLDPE/LDPE and znLLDPE/HDPE blends, respectively.