Ethylene-hexene Copolymer Research Articles

Short-term mechanical and structural approaches for the evaluation of polyethylene stress crack resistance

Short-term mechanical behavior together with structural habits at various scale levels of polyethylene are studied in relation to molecular architecture in order to contribute to the understanding of the environmental stress crack resistance (ESCR). Three series of homopolyethylene and ethylene–hexene copolymers issued from 1st and 2nd generation chromium oxide catalysis as well as tandem-reactor Ziegler-Natta catalysis are investigated. Properties are discussed via the incidence on the chain topology of the co-unit concentration and distribution as well as the molar weight distribution. The creep compliance is mainly governed by density, i.e. stiffness. However, at similar density, the three polymers series displays a clear compliance drop that follows the increasing ESCR hierarchy. The natural draw ratio displays monotonous decrease with decreasing density that perfectly parallels the ESCR increase. This property also holds for materials of similar density. The creep compliance and the natural draw ratio are indicators of the macromolecular network strength via the intercrystalline tie molecules. Stepwise isothermal segregation (SIS) allows indirect assessment of the chain fraction that generates a high rate of tie molecules. Correlation is made between SIS and ESCR via the tie molecules that provide strength to the macromolecular network. SIS also enables discriminating polymers of similar density but different molecular architecture. Small-angle X-ray scattering affords quantitative evaluation of the stacking disorder of the semi-crystalline microstructure for which a correlation is established with the crystallization kinetics and the resulting chain topology.

Behavior of poly(ethylene‐co‐olefin) polymers as elastomeric materials

AbstractThe properties of two new ethylene‐α‐olefin copolymers, namely, ethylene–1‐hexene copolymer (EHC) and ethylene–1‐octadecene copolymers (EOC), synthesized via metallocene catalysts were evaluated. The copolymerization was carried out in an autoclave reactor with Et(Indenyl)2ZrCl2/methylaluminoxane as a catalyst system. These single‐site catalysts (metallocene type) allow one to obtain very homogeneous copolymers with excellent control of the molecular weight distribution and proportion of comonomer incorporation. So, copolymers with 18 mol % comonomer in the case of EHC and 12 mol % for EOC were shaped, and activities around 100,000 kg of polymer mol−1 of Zr bar−1 h−1 were reached. The properties of these copolymers were compared with other commercial elastomers, such as ethylene–propylene copolymers synthesized by Ziegler–Natta catalysts and an ethylene–octene copolymer obtained via metallocene catalysts. The results show that these new copolymers, in particular, EOC, had excellent elastomeric properties. Furthermore, they had a relatively low viscosity, which implied a good response during processing. Moreover, the effectiveness of these copolymers as impact modifiers for polyolefins was also studied. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3008–3015, 2004

Physical gelation of crystallizing metallocene and Ziegler-Natta ethylene–hexene copolymers

The effect of molecular architecture on the evolution of viscoelastic properties during crystallization was investigated using ethylene–hexene copolymers manufactured via metallocene (M-LLDPE) and Ziegler-Natta (ZN-LLDPE) processes. Differences in branching distribution were shown to have a drastic effect on the viscoelastic properties near the gel point. It is shown that the branching distribution rather than branch content is the determining parameter for the evolution of the rheological properties during isothermal and non-isothermal crystallization, and for the width of the solidification interval. We developed a partial melting technique for the preparation of stable critical gels of LLDPE whose viscoelastic properties correspond to the intermediate state between melt and solid. Local molecular conformation and crystallinity in these gels were characterized by Raman spectroscopy, which shows that the transition from melt-like to solid-like rheological behavior (physical gelation) in LLDPE occurs at a very low overall crystallinity of less than 5%.

Influence of the molecular architecture of low‐density polyethylene on the texture and mechanical properties of blown films

AbstractThree types of low‐density polyethylene materials were investigated with respect to the influence of the molecular architecture on the mechanical and use properties of blown films. The materials were a branched polyethylene synthesized by free‐radical polymerization under high‐pressure (HP‐LDPE), a linear ethylene–hexene copolymer (ZN‐LLDPE) produced by low‐pressure Ziegler–Natta catalysis, and an ethylene–hexene copolymer (M‐LLDPE) from metallocene catalysis. The extrusion and blowing conditions were identical for the three materials, with a take‐up ratio of 12 and a blow‐up ratio of 2.5. The blown films displayed a decreasing puncture resistance in the order M‐LLDPE, ZN‐LLDPE, and HP‐LDPE. In parallel, the tear resistance of the films became increasingly unbalanced in the same order of the polymers. The morphological study showed an increased anisotropy of the films in the same polymer order, the crystalline lamellae being increasingly oriented normal to the take‐up direction. This texturing caused a detrimental effect on the mechanical properties of the films, notably increasing the capacity for crack propagation. The phenomenon was ascribed to the kinetics of chain relaxation in the melt that governed the ability of the chains to recover an isotropic state from the flow‐induced stretching before crystallization. The puncture resistance was examined in terms of both texture and strain‐hardening capabilities. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 327–340, 2003

Relation between molecular structure and flow instability for ethylene/α-olefin copolymers

Surface instabilities in a capillary extrusion have been studied for various ethylene/α-olefin copolymers. It is found that the onset stress of shark-skin failure for ethylene/1-hexene copolymer (EHR) decreases rapidly with increasing 1-hexene content, whereas that of ethylene/propylene copolymer (EPR) is independent of propylene content in the experimental region. Consequently, EHR with high 1-hexene content exhibits shark-skin at low stress level compared to EPR. Lower rubbery plateau modulus, leading to higher Deborah number at the same stress level, is attributed to the lower onset stress. Further, the low entanglement density will cause cracks at lower stress level like glassy polymers, which is also responsible for the low onset stress for shark-skin.

Blends of single-site linear and branched polyethylene. II. Morphology characterisation

In this work, the morphology of a low-molecular weight single-site based linear polyethylene and several high molecular weight single-site ethylene–hexene copolymers, as well as their blends are investigated in the solid state using differential scanning calorimetry, transmission electron microscopy, atomic force microscopy and optical microscopy. Such model blends are believed to be important to elucidate the structure of polyethylene with a bimodal molecular weight distribution. The lamellae- and spherulite structure in the blend components and in the blends, and the ability to cocrystallise from the melt are found to be highly dependent on the amount of comonomer incorporation in the branched blend component.

Blends of single-site linear and branched polyethylene. I. Thermal characterisation

Blends of a low-molecular-weight linear polyethylene and several different ethylene–hexene copolymers are investigated using differential scanning calorimetry and transmission electron microscopy. All blend components are based on single-site catalysts. For cooling rates higher than 1°C/min, reorganisation effects are generally present, making the interpretation of the blends less clear. The reorganisation effects are found to be dependent of the amount of comonomer in the branched blend component, the compositions of the blends and the cooling and heating rates applied. For a low content of the linear blend component, even 1.8 mol% comonomer content in the branched blend component is sufficient to create two crystal populations during crystallisation. For higher amounts of the linear blend component 2.5 mol% comonomer will result in two crystal populations.

Thermal analysis of single-site polymers in binary blends of low-molecular-weight linear polyethylene and high-molecular-weight branched polyethylene

The cocrystallization phenomenon has been studied by blending a low-molecular-weight linear polyethylene (LPE) with a high-molecular-weight lightly branched ethylene–hexene copolymer, as a function of blend composition and cooling rate. The materials are based on single-site catalysts. Differential scanning calorimetry indicates that the upper limit on the degree of branches for cocrystallization with LPE is lower than observed in blends with Ziegler–Natta based materials, in accordance with results obtained by other authors on different single-site blend systems. The effect of the cooling rate on blends with low LPE content is probed, and the observed behaviour is discussed in light of earlier observations on similar blends.

Relationships between microstructure, fracture-surface morphology, and mechanical properties in ethylene and propylene polymers and copolymers

The fracturing of four different polyolefin materials (a polypropylene homopolymer, a propylene–ethylene copolymer, a polyethylene homopolymer, and an ethylene–hexene copolymer) was studied with the objective of developing a better understanding of the relationships between the morphology of semicrystalline polymers, the morphology and growth kinetics of their fracture surfaces, and their mechanical properties. A scanning electron microscope and an optical microscope were used to obtain images of the fracture surfaces. The samples were injection-molded or hot-pressed to generate different microstructures. Fracture experiments were performed at 23, 0, and −20°C to generate fracture surfaces with different morphologies from the same supermolecular structure. It appears that the fracture propagates through the spherulites in a brittle manner. The macroscopic aspect of the fracture surfaces is temperature-independent and changes are visible only at the microscopic scale. Over the range of temperatures studied, the rms roughness [root mean square roughness decreased by only about 20%, while the fracture energy of all but one of the materials (a high-density ethylene–hexene copolymer) decreased by about 60% as the temperature was reduced. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2370–2382, 2000

Characterization of molecular heterogeneities of LLDPE by multiple crystallization–dissolution steps

This article demonstrates that a fractionation technique, based on molecular segregation during the crystallization from the melt and dissolution of the crystals, allows a complete characterization of the molecular heterogeneity in the comonomer distribution of linear low density polyethylene. An ethylene–1-hexene copolymer with a bimodal short chain branching distribution has been fractionated by multiple step crystallizations followed by selective dissolutions in tetrachloroethylene. The relative amount of each component of the distribution in each fraction was determined from the melting curves of isothermally crystallized fractions.

Morphology maps of binary blends of copolymers produced using the metallocene catalyst process

Three lightly branched commercial polyethylene copolymers, produced using the metallocene catalyst process, were blended together in binary pairs. Two of the materials were ethylene–butene copolymers of differing butene content, the third was an ethylene–hexene copolymer with the same branch content as the more lightly branched ethylene–butene material. Note that the components of one system differ in both branch type and branch content; those of another, in branch content but not branch type; and those of the third in branch type only. Morphology maps were produced for all three systems. Where the two components had the same branch content, only one crystal type was seen on quenching melts of all compositions from all temperatures. Where the branch contents varied, two crystal types were seen when blends of some compositions were quenched from some temperatures, and the morphology maps resembled those found when copolymers produced by traditional methods are blended—or when a linear polyethylene is blended with a lightly branched copolymer. These findings confirm the view that it is the branch content of the copolymers that effects the morphology most strongly, the branch type is of secondary importance.

Influence of Stereoregularity of Polypropylene on Miscibility with Ethylene−1-Hexene Copolymer

The influence of stereoregularity of polypropylene on the miscibility with the ethylene−1-hexene copolymer (EHR) whose 1-hexene content is 57 mol % was studied by means of viscoelastic measurements, electron microscopic observation, and so on. In this study, two types of polypropylenes were used; one is an isotactic polypropylene (iPP), and the other is a syndiotactic polypropylene (sPP). The sPP is immiscible with the EHR in both molten and solid states, whereas the iPP is miscible with the EHR in the amorphous region. The difference in the miscibility with the EHR between sPP and iPP depends on the chain stiffness.

Structure and mechanical properties for binary blends of polypropylene and ethylene-1-hexene copolymer

The structure and mechanical properties of the injection-molded products for the binary blends composed of an isotactic polypropylene (PP) and a rubbery ethylene-1-hexene copolymer (EHR) were studied. The following two types of blends were employed: one is the incompatible blend of PP and ethylene-rich EHR; the other is the compatible blend of PP and 1-hexene-rich EHR. The incompatible blend shows a phase-separated morphology, in which EHR domains in the skin layer highly orient to the flow direction. On the other hand, the compatible blend shows fairly homogeneous morphology in the skin and core regions, in which EHR molecules are dissolved into the amorphous PP region. The measurements of birefringence and infrared dichroism revealed that the magnitude of molecular orientation along the flow direction for the compatible blend is larger than that for the incompatible blend. Nevertheless, it was also found that anisotropy of the mechanical properties for the compatible blend is less prominent, which is attributed to lack of the mechanical connection between neighbor crystalline fragments aligned perpendicular to the flow direction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 701–713, 1999

Short Chain Branching Effect on the Cloud-Point Pressures of Ethylene Copolymers in Subcritical and Supercritical Propane

The cloud-point pressures of copolymers of ethylene with propylene, butene, hexene, and octene in propane were measured with a variable-volume optical batch cell to investigate the effects of the number and length of branches on the phase behavior in the temperature range from 25 to 200 °C and at pressures up to 700 bar. As the degree of branching increased in the ethylene-propylene copolymers, ethylene-butene copolymers, and ethylene-hexene copolymers, the cloud-point pressures decreased. At the same degree of branching, the cloud-point pressures decreased slightly upon increasing branch length. The copolymer SAFT (statistical association fluid theory) equation of state was found to correlate the experimental cloud-point data by adjusting the branch segment energy.

Analysis of microstructure of ethylene-1-hexene copolymer prepared over thermally pretreated MgCl2/THF/TiCl4bimetallic catalyst

Thermally pretreated catalysts were prepared by heating MgCl2/THF/TiCl4 (TT-0) at 80°C for 5 min (TT-1) and 60 min (TT-2), and at 108°C for 5 min (TT-3) and 60 min (TT-4). Ethylene–1-hexene copolymers were prepared with these catalysts. The TT-1 catalyst produced more blocky and higher 1-hexene content polymer than TT-0, 2, 3, and 4. Temperature rising elution fractionation (TREF) analysis was used to investigate the chemical composition distribution of the ethylene–1-hexene copolymer, exhibiting bimodal distribution for TT-0 and trimodal for TT-1, 2, 3, and 4. A portion of higher hexene content of the copolymer markedly increased when the copolymerization was performed with TT-1, indicating that copolymerization active sites were newly generated. Portion of homopolyethylene increased drastically when the copolymerization was performed with TT-4, indicating that ethylene homopolymerization active sites were increased. Gel permeation chromatography (GPC) also revealed that three kinds of active sites existed on the catalyst. 13C-NMR spectrum of each fraction after TREF analysis suggested that the isospecific active site could polymerize 1-hexene well, resulting in random and alternating copolymers. A scheme for generation of the active site and change of its nature during thermal treatment of bimetallic complex catalyst is proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 291–300, 1998

Influência da estrutura de diferentes copolímeros de etileno e a-olefinas na funcionalização com anidrido maleico

A funcionalização de copolímeros de etileno e a-olefinas com anidrido maleico (AM) foi realizada em solução de xileno com peróxido de dibenzoíla (DBP) como iniciador. Foi estudado o efeito das diferentes estruturas dos copolímeros, como número e comprimento de ramificação, na incorporação do AM. A funcionalização também foi realizada em estado fundido utilizando-se um misturador Rheomix 600 e uma extrusora Rheocord 9000 da Haake. A funcionalidade foi determinada por titulometria de neutralização e os produtos foram caracterizados por espectroscopia na região do infravermelho (FT-IR) e por cromatografia de permeação em gel (GPC). A funcionalidade dos copolímeros de etileno com 1-hexeno aumentou com o aumento do teor de comonômero e dos copolímeros com 1-octeno e 1-deceno aumentou com o aumento do teor de a-olefina até um máximo, decrescendo e mantendo-se constante.

The effect of molecular weight and crystallite structure on yielding in ethylene copolymers

Nominal stress-strain curves of a series of random ethylene-hexene copolymers having narrow composition and most probable molecular weight distributions were investigated. A series of such molecular weight copolymers with a constant concentration of branches were crystallized under a variety of conditions. In each molecular weight series the level of crystallinity was approximately constant. Particular attention was focused on the yield region and the nature of the yielding process. It was found, quite surprisingly, that the yield stress was not solely dependent on the crystallinity level. Moreover, the shape of the force-elongation curve in the yield region was very dependent on the molecular weight and the crystallization mode. These changes in yielding correlated quite well with the overall crystallite structure that was characterized by thin section transmission electron microscopy. The orthorhombic unit cell of polyethylene was maintained in all the samples despite the changes that occurred in the overall crystallite structure. © 1997 John Wiley & Sons, Inc.

Rheological properties for binary blends of i-PP and ethylene-1-hexene copolymer

The dynamic viscoelastic properties for binary blends consisting of an isotactic polypropylene (i-PP) and an ethylene-1-hexene copolymer (EHR) were investigated in both solid and molten states to reveal the relation between miscibility in the molten state and the morphology in the solid state. In this study, two types of EHRs were employed: “ethylene-rich” EHR and “1-hexene-rich” EHR. The blend of i-PP and EHR of 30 mol % 1-hexene content shows a very long time relaxation due to the phase separation in the molten state. The blend film shows two separate glass-relaxation processes associated with those of the pure components. These indicate that the blend shows phase-separated morphology in the solid state as well as in the molten state. On the other hand, dynamic moduli in the molten state of the blends of i-PP and EHR of 57 mol % 1-hexene were found to be intermediate between those of individual pure components. Furthermore, the apparent activation energy of the blends is constant and is identical with those of i-PP and the EHR. The blend films show a single glass-relaxation process at the temperatures between those of the pure components, indicating that the EHR molecules are incorporated in the amorphous region of i-PP in the solid state. Accordingly, it was found that the polymer miscibility in the molten state for the i-PP/EHR blends directly affects the morphology in the solid state. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 467–474, 1997

The critical molecular weight for resisting slow crack growth in a polyethylene

An ethylene-hexene copolymer was fractionated into five fractions and the density of short-chain branches was measured for each fraction. The slow crack growth behavior was measured on each fraction by sandwiching the small amount of fractionated resin of about 0.2 g between polyethylene grips. The resistance to slow crack growth was negligible for the three fractions whose Mw was less than 1.5 × 105. For the fourth fraction with Mw greater than 1.5 × 105, the resistance to slow crack growth was very high, being greater than that for the whole resin even though its density of short-chain branches was less than that of the whole resin. It is concluded that a molecular weight greater than 1.5 × 105 is required to create the number of tie molecules that is necessary to produce a high resistance to slow crack growth in this particular copolymer. © 1996 John Wiley & Sons, Inc.

Slow crack growth of blends of high density and linear low density polyethylenes as influenced by morphology

A high density polyethylene homopolymer was blended with a linear low density ethylene hexene copolymer over the composition range of 0–100%. The resistance to slow crack growth was measured by a notched tensile test under constant load at 42, 60 and 80°C. The morphology was varied by slow cooling, quenching and annealing at 129°C for 24 h. Whereas the resistance to slow crack growth increased by a factor of 10 6 over the composition range, the differences in morphology could only change the resistance to slow crack growth at the most by a factor of ∼3. The effect of temperature was independent of the effect of composition on slow crack growth. The effects of molecular and morphological structure are discussed.