Linear Polyethylene Blends Research Articles

Self-nucleation and heterogeneous nucleation in ethylene/1-octene random copolymer and its linear polyethylene blends

Self-nucleation normally denotes an increase in the crystallization temperature during cooling down from certain preheated (seeding) temperature due to the existence of unmolten crystalline nuclei or some heterogenous structures which can induce melt memory effect. It is one of the crucial parts in the study of polymer crystallization behavior which provides a meaningful guidance in both academia and industry to accelerate crystallization of polymers. In this work, self-nucleation effects of a commercial ethylene/1-octene random copolymer with 12 mol% 1-octene content and its blends with small amount of linear polyethylene (LPE) were investigated. The ethylene/1-octene random copolymer possesses strong self-nucleation effect whereas in the blends the crystallization temperature increases with the seeding temperature and eventually stabilized at certain constant temperature when the seeding temperature is high enough. This result is clearly due to the heterogenous nuclei generated by the crystallization of LPE during cooling. The strength of the heterogenous nucleation effect i.e. the crystallization temperature is positively related to the degree of dispersion of LPE in the mixture.

Self-Nucleation and Hetero-Nucleation in Ethylene/1-Octene Random Copolymer and its Linear Polyethylene Blends

Self-Nucleation and Hetero-Nucleation in Ethylene/1-Octene Random Copolymer and its Linear Polyethylene Blends

Retardation on Blending in the Entangled Binary Blends of Linear Polyethylene: A Molecular Dynamics Simulation Study

We perform atomistic molecular dynamics simulations on the binary blends of entangled polyethylene wherein short probes are dilute in long-chain matrixes as well as on their monodisperse counterparts. Compared to monodisperse systems, the relaxations of the end-to-end vector of short probes are retarded for the suppression of constraint release (CR) on blending, and the suppression degree becomes weaker with increasing of probe length. A parameter reflecting the CR strength is introduced, and its critical value from no CR at all to CR taking effect ranges from 0.071 to 0.46. By subdividing the overall motion into the lateral and longitudinal motions, we find both motions are more restricted in blends, and the time scale for the onset of these differences or of CR effects is smaller than the overall relaxation time. Additionally, retardation on blending behavior is further confirmed by the probability distribution of the chain entanglement lifetime.

Molecular dynamics simulation of linear polyethylene blends: Effect of molar mass bimodality on topological characteristics and mechanical behavior

Blending different molar mass fractions of polyethylene (PE) in order to obtain materials with higher fracture toughness has previously proven beneficial. Our approach has been to use coarse-grained (CG) molecular dynamics (MD) simulations to obtain semicrystalline polyethylene systems on a nanoscale, and then draw them in order to mimic tensile testing. The CG potentials were derived, validated and utilized to simulate melt equilibration, cooling, crystallization and mechanical deformation. Crystallinity, tie chain and entanglement concentrations were continuously monitored. During crystallization, the low molar mass fraction disentangled to a greater degree and ended up with a lower entanglement density than the high molar mass fraction, although the tie chain concentration was higher for the low molar mass fraction. The deformation behavior of semicrystalline PE above its glass transition temperature was then assessed in a uniaxial tensile deformation simulation. The low-strain mechanical properties (i.e. elastic modulus, yield stress and strain) were in accordance with the literature. The high-strain mechanical features and toughness were improved in bimodal systems. The presence of a high molar mass fraction in bimodal systems was shown to affect the crystallinity and tie chain concentration during the strain hardening, leading to tougher model systems. Finally, the bimodal system with equal shares of the molar mass fractions showed the highest toughness and the best ultimate mechanical properties while having a concentration of tie chains and entanglements intermediate between the values for the other systems. This was a clear sign of the non-exclusive role of tie chains and entanglements in the mechanical behavior of bimodal PE and more generally of semicrystalline polymers at high strains.

Time-resolved SAXS studies of morphological changes in a blend of linear polyethylene with homogeneous ethylene-1-octene copolymer

Isothermal melt crystallization in the 15/85 (m/m) blend of a high density polyethylene (HDPE) and a homogeneous ethylene copolymer with 5.5mol% 1-octene was studied by time-resolved SAXS method with synchrotron radiation over a wide-range of crystallization temperatures. The SAXS profile was analyzed by means of the correlation function which allows to elucidate the evolution of the morphological parameters of polyethylene lamellar structure (long period (LP), thicknesses of crystalline (LC) and amorphous (LA) layers) during a crystallization process. It was found that for the samples crystallized at 100°C, 120°C and 122°C LC increases with time. The lamellar thickening rate strongly depends on crystallization temperature. At 40°C thickening of the crystalline layers does not occur.The time evolution of the lamellar structure in the blend studied confirms the role of hexyl branches of homogeneous copolymer in the crystallization process of polyethylene. The branches introduce steric constraints which hinder the crystallization of HDPE, thus decreasing the size of the HDPE lamellar crystals.

Flow instability for binary blends of linear polyethylene and long-chain branched polyethylene

The rheological properties and flow instability are studied for binary blends composed of a long-chain branched polyethylene and a linear polyethylene. It is found that the blends containing a linear-polyethylene with high shear viscosity exhibit higher oscillatory moduli, drawdown force, and strain-hardening behavior. The blends showing the anomalous rheological phenomena show sharkskin failure in low shear rate region as compared with a pure linear polyethylene. Moreover, the blends exhibit severe gross melt fracture at low output rate. Enhanced strain-hardening in elongational viscosity and large entrance angle at a die entry will be responsible for the severe gross melt fracture for the blends.

Ethylene polymerization with hybrid nickel diimine/Cp2TiCl2 catalyst: a new method to prepare blends of linear and branched polyethylene

AbstractBlends of linear polyethylene (LPE) and branched polyethylene (BPE) display very good mechanic properties that can be beneficial for various applications such as shear thinning and melt elasticity. LPE, BPE and amorphous polyethylene can be produced using nickel diimine (DMN) catalyst under various polymerization conditions, while LPE can be obtained using metallocene catalyst. Thus, LPE/BPE blends can be achieved by in situ polymerization using a hybrid DMN/metallocene catalyst. A novel hybrid catalyst made of DMN and Cp2TiCl2 was designed and used for ethylene polymerization. A synergistic effect of the two active sites in the hybrid DMN/metallocene catalyst was observed. Blends of linear and low branched polyethylene were synthesized when polymerization was conducted at low temperature (0 °C), while blends of linear and highly branched polyethylene were obtained at high temperature (50 °C). However, the miscibility of the polymers obtained at 50 °C was dramatically reduced as compared to those obtained at 0 °C. Mesoporous particles (MCM‐41) consisting of aluminosilicate with cylindrical pores were used to support the hybrid catalyst, in which MCM‐41 provides sufficient nanoscale pores to facilitate the polymerization in well‐controlled confined spaces. Blends of LPE and BPE were synthesized by in situ polymerization without adding comonomer and characterized. The miscibility of the polymer blends can be improved by supporting the hybrid catalyst on MCM‐41. Copyright © 2009 Society of Chemical Industry

Probing the Melt Miscibility of a Commercial Polyethylene Blend by Small-Angle Light Scattering

Small-angle light scattering (SALS) was used to determine the binary interaction parameter χ in a molten blend of linear polyethylene (LPE, M w = 52 kg/mol, PDI = 2.9) and linear low-density polyethylenes (LLDPEs) based on homogeneous ethylene-1-butene copolymers (LLDPE-1, 18.7 mol% butane branches, M w = 58.1 kg/mol, and LLDPE-2, 5.9 mol% butene branches, M w = 70 kg/mol). Traditional cloud point measurements (cooling at 0.5–3°C/min) and melt annealing/isothermal measurements under V V (parallel-polarized) optical alignment were used to evaluate the melt miscibility and determine the binary interaction parameter χ (7.74 × 10−4 at 423 K). This value of χ is consistent with the value (7.69 × 10−4 at 423 K) determined by earlier small-angle neutron scattering (SANS) measurements obtained by Crist and Rhee on polyethylene blends with the same branching-density difference (Δ y = 0.187). In addition, it was shown that SALS can be used to determine directly the melt miscibility of undeuterated polyolefin blends over branching-density differences (Δ y > 0.06), with results that are similar to the range obtained by neutron scattering. Our results are significant because they show that the low optical contrast between coexisting phases in polyolefin blends does not limit the determination of phase boundaries by SALS as was previously assumed. The blends studied exhibit upper critical solution temperature behavior.

Anomalous rheological response for binary blends of linear polyethylene and long‐chain branched polyethylene

AbstractThe rheological properties are studied for binary blends composed of a low‐density polyethylene (LDPE) and a linear polyethylene. It is found that some blends exhibit higher oscillatory shear moduli and drawdown force than the individual pure components, demonstrating that relaxation mechanism with longer characteristic time appears in the blend. The anomalous rheological behavior is detected more clearly for the blends with autoclave‐LDPE than those with tubular‐LDPE. Furthermore, the number of short‐chain branches in a linear polyethylene has no influence on the rheological properties of the blends, suggesting that the phase separation will not be responsible for the anomalous behavior. It is also found that blends of autoclave‐LDPE and tubular‐LDPE show no synergetic effect. © 2008 Wiley Periodicals, Inc. Adv Polym Techn 26:173–181, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20100

Influence of the Phase Morphology on the Viscoelastic Behaviour of Polymer Blends (PP-EPR)

Melts of polyethylene-propylene-rubber (EPR, blends of linear polyethylene, linear polypropylene, and PE-PP copolymer) show phase separation. The influence of the phase morphology on the viscoelastic behaviour is studied with the aid of the dynamical moduli. The measured moduli of the fraction of the linear homopolymer of an EPR agree with the moduli calculated from the molar mass distribution of this fraction. In contrast, the copolymer fraction shows a strong rubber-like behaviour (represented by a characteristic relaxation time spectrum). Mixtures of 80 wt% linear PP and of EPRs with low copolymer content are not rubber-like liquids. In addition, the moduli from them can be calculated from the moduli of the components with the aid of Palierne’s emulsion model. This clearly reveals that the linear PP forms the matrix in this case. Mixtures of 80 wt% linear PP and of EPRs with high copolymer content show rubber-like behaviour and Palierne’s emulsion model is not applicable on them. Obviously, the copolymer fraction forms a physical network.

Molecular changes on drawing isotopic blends of polyethylene and ethylene copolymers. Part 2. MCIRS studies

The technique of mixed crystal FTIR spectroscopy has been applied to uniaxially drawn isotopic blends of linear polyethylene and ethylene copolymers. Following the drawing of melt-quenched samples at 1mmmin−1, clamped samples were cooled with liquid nitrogen. Analysis of the CD2 bending vibration of the ‘guest’ perdeuterated species enabled the arrangement of crystal stems to be characterised. Changes in band components observed were identified with the processes of polymer crystallisation, the untwisting of lamellar ribbons perpendicular to the draw direction and coarse slip. For example, the appearance of new doublets or increased splittings is related to crystallisation, while an increase in singlet band area, at least at higher draw ratios, is indicative of coarse slip. Crystallisation appears to be complete at a draw ratio of 4 for a linear polyethylene sample with low guest molecular weight. For similar copolymer ‘guest’ molecular weights, the processes identified above are delayed to higher levels of deformation. This was attributed to copolymer branches hindering both crystallisation and chain translation through lamellae. Estimates of the crystallite block size present after drawing enable a quantitative description of coarse slip to be obtained. A progressive increase in width of the central singlet for two of these samples is tentatively ascribed to variations in molecular strain.

Molecular changes on drawing isotopic blends of polyethylene and ethylene copolymers: 1. Static and time-resolved sans studies

Small angle neutron scattering (sans) has been used to observe changes in molecular orientation on deformation of isotopic, melt quenched blends of linear polyethylene and copolymers with butyl and hexyl branches, using uniaxial drawing. In no case was there evidence of significant isotopic fractionation either before or during deformation. Samples left clamped in the neutron beam following deformation showed no evidence of molecular relaxation, while significant changes in molecular orientation with time were observed for samples unclamped after deformation. This latter behaviour could be fitted adequately by a single exponential decay and the relaxation time was found to be of the order of tens of minutes. This relaxation time increased with both increasing molecular weight and the presence of branching. For samples drawn to draw ratios of up to 3.0, the anisotropy in the radius of gyration was compared with predictions for affine deformation. Only in the cases of linear labelled guest molecules in either a linear or copolymer host were significant departures from the affine model observed: no such departures were found for copolymer guest molecules. This is interpreted in terms of a delay in crystallite disruption until higher draw ratios for copolymer guest molecules with respect to linear guest.

Morphology maps of blends of a linear polyethylene with ethylene-ce4-methyl pentene-i copolymers

A linear polyethylene (LPE) has been blended with each of three ethylene-co-4-methyl pentene-1 (E4MP1) copolymer fractions of differing branch content (2.0, 4.3, and 6.1 mol% 4-methyl pentene-1). These copolymers have Y-shaped branches, each containing four carbon atoms. The blends were studied using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) following experimental procedures that have been extensively used to investigate blends of LPEs with copolymers with alkane (linear) branches. When the copolymer is lightly branched, LPE/E-4MPl copolymer blend systems give, to a first approximation, morphology maps similar to those found for linear/branched polyethylene systems in which the branched polyethylene has linear alkane branches. Single morphologies are found for high LPE contents, but there are double morphologies (regions of thick crystals well separated in a matrix of thin crystals) at low LPE contents after quenching from within a restricted temperature range. As the branch density of the copolymer increases, the same changes in morphology map are seen for the LPE/E4MPl copolymer blend systems as for lineadalkane branched copolymer systems. The final morphology map, characteristic of a reasonably heavily branched copolymer, is reached at a lower branch density (near 6.1 mol%) for the LPE/E4MPl copolymer blend systems than for LPE/linear alkane branched copolymer systems. For LPE/E-4MP1 copolymer blend systems, as for linearlbranched polyethylene systems in which there are alkane branches, TEM is more sensitive than DSC detecting changes from single to double morphology.

Morphology, melting behaviour and co-crystallization in polyethylene blends: the effect of cooling rate on two homogeneously mixed blends

The crystalline textures of a blend of linear polyethylene with low density polyethylene and a blend of deuterated linear polyethylene with low density polyethylene have been investigated as a function of rate of cooling from the melt. Differential scanning calorimetry, transmission electron microscopy and Fourier transform infrared spectroscopy have been used to study co-crystallization and phase segregation. On the basis of previous studies, the blend compositions were chosen so that the melt contained a single phase. Segregation between components was found to occur, as a result of crystallization, during cooling at all rates other than rapid quenching. The degree of segregation was found to increase with decreasing cooling rate. The morphologies resulting from this phase segregation during crystallization, have been compared with morphologies, previously determined, of blends of other compositions rapidly quenched from what was believed to be a phase separated melt. The scale of phase segregation is found to differ by more than an order of magnitude between the two types of morphology. The results support the argument that those rapidly quenched samples, which contain two crystal types, do so because of phase separation in the melt and not because of phase separation on crystallization. A comparison is made between two methods of sample preparation for transmission electron microscopy: permanganic etching/replication and chlorosulfonic fixation/sectioning. We have found the latter method better for revealing detailed morphological features between large lamellae.

Liquid-liquid phase separation in ternary blends of linear polyethylene with two ethylene-butene copolymers

A commercial ethylene-butene linear low density polyethylene has been fractionated by preparative temperature rising elution fractionation. Each fraction was characterized by Fourier transform infra-red spectrometry to determine the branch content and by gel permeation chromatography to determine the molecular weight. Two fractions were then blended, in various proportions, with each other and with a linear polyethylene. Using differential scanning calorimetry and transmission electron microscopy to examine quenched blends, the phase behaviour of this ternary system was investigated at temperatures above the melting point. The phase behaviour is very similar to that previously found for a ternary system of a linear polyethylene with two ethylene-octene copolymers with branch contents close to those of the ethylene-butene copolymers used in the present work. We deduce that the phase behaviour of blends containing ethylene-butene copolymers is essentially the same as that of blends containing ethylene octene copolymers. We further deduce that the length of the branches must be of secondary importance in determining phase behaviour. (Previously we have shown that the variation of molecular weight has a secondary effect on phase behaviour). Thus we conclude that in blends of linear polyethylene with lightly branched ethylene copolymers it is the number of branches that is the most important factor influencing the extent of phase separation in the melt.

Absence of phase separation effects in blends of linear polyethylene fractions of differing molecular weight

Blends of linear polyethylenes of differing molecular weights ( MWs) have been studied to try to determine whether there is any observable phase separation in the melt. Using four linear polyethylene fractions, of MWs between 2 × 10 6 and 2.5 × 10 3, blends were prepared and four blend systems investigated, the two high MW polymers being blended with each of the two lower MW materials in turn. The blends were studied by transmission electron microscopy and differential scanning calorimetry, techniques previously used to study blends of linear with branched polyethylenes. In these systems, for some compositions of blends of linear with branched polyethylene, quenched from some temperatures, phase separation on a scale of micrometres has been recorded. In contrast, no indications of phase separation were observed on examination of the blends of linear with linear polyethylenes. From this it follows that if there is any liquid-liquid phase separation in linear with linear polyethylene blends, taking place as a result of molecular weight differences, it cannot be detected by the methods used to detect liquid—liquid phase separation in linear with branched polyethylene blends; and, conversely, the phase separation observed in linear with branched polyethylene blend systems does not take place as a result of molecular weight difference alone.

Water treeing in binary linear polyethylene blends: the mechanical aspect

Water treeing tests were performed on low density polyethylene (LDPE) and four different binary blends of sharp linear polyethylene (LPE) fractions (M/sub w/=2500 and 76000), which were either quenched in air from the melt or isothermally crystallised at 123/spl deg/C. Although the morphology and initial mechanical properties of the materials tested were significantly different, the vented tree growth characteristics were similar for all of them. This is in disagreement with the electromechanical models of water treeing, which correlate water tree growth with the fracture toughness of the material. Time to breakdown distributions were also similar for both LDPE and the binary LPE blends, which indicates that, regardless of the initial material morphology and the actual structure of water trees, the length of water trees is one of the controlling factors in insulation failure. The visible light image of water trees in LPE blends did not disappear upon drying as it usually does in LDPE and crosslinked polyethylene insulation. >

Liquid-liquid phase separation in a ternary blend of polyethylenes

Blends of linear polyethylene with two ethylene—octene copolymers of 2 and 8 mol% octene content have been prepared and their melt-phase behaviour determined by examination of quenched samples by differential scanning calorimetry and transmission electron microscopy using surface replicas. The ternary phase diagram for the system has been determined. It shows three distinct regions of liquid—liquid phase separation, all at high branch content.

Liquid-liquid phase segregation in blends of a linear polyethylene with a series of octene copolymers of differing branch content

A linear polyethylene (LPE) has been blended with a series of random ethylene-octene copolymers of differing octene content. Melts of blends containing the lowest octene contents (2, 3, 5 and 8 mol%) showed liquid-liquid phase separation at some temperatures and for some compositions. The phase-separated regions were in the shape of closed loops and asymmetrically placed at low LPE content. This behaviour has been found previously for LPE/branched PE blends. However, unexpectedly, the phase-separated loops became smaller as the octene content of the copolymer increased. Blends of LPE with a 12 mol% octene copolymer showed extensive separation and rather different morphologies.

Morphology of binary linear polyethylene blends

The morphology of binary mixtures of sharp fractions of linear polyethylene (molecular weights: 2500, 11000, 22000 and 66000) has been studied by polarized microscopy and small-angle light scattering. The samples were crystallized under isothermal conditions over a wide temperature range (380–403 K). Three types of supermolecular structure have been observed: axialites, non-banded spherulites and banded spherulites. Data are presented displaying the morphology as a function of both composition and crystallization temperature. The results are discussed with reference to current theories for polymer crystallization.