Effects of ultrasonic oscillations on linear viscoelastic behaviors of metallocene‐catalyzed linear low density polyethylene and its blends with low density polyethylene

The effects of ultrasonic oscillations on linear viscoelastic behaviors of metallocene‐catalyzed linear low density polyethylene (mLLDPE) and its blends with low density polyethylene (LDPE) were investigated in this article. The experimental results showed that ultrasonic oscillations can increase the cross modulus, characteristic time, plateau modulus, complex viscosity, zero shear viscosity, and flow activation energy of mLLDPE. Molecular weight of mLLDPE increases but molecular polydispersity index decreases in the presence of ultrasonic oscillations. It has been found for mLLDPE/LDPE blends that the addition of LDPE as well as ultrasonic oscillations can decrease the cross modulus but increase the characteristic time of the blends. The complex viscosity, zero shear viscosity, and flow activation energy of the blends increase by the addition of LDPE, but decrease in the presence of ultrasonic oscillations. Shear thinning effect of the blends is improved because of the addition of LDPE. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3030–3043, 2005     

The compatibilization effect of ethylene/styrene interpolymer on polystyrene/polyethylene blends

In this study, ethylene/styrene interpolymer (ESI) was used as compatibilizer for the blends of polystyrene (PS) and low‐density polyethylene (LDPE). The mechanical properties including impact, tensile properties, and morphology of the blends were investigated by means of uniaxial tension, instrumented falling‐weight impact measurements, and scanning electron microscopy. Impact measurements indicated that the impact strength of the blends increases slowly with LDPE content up to 40 wt %; thereafter, it increases sharply with increasing LDPE content. The impact energy of the LDPE‐rich blends exceeded that of pure LDPE, implying that the LDPE polymer can be further toughened by the incorporation of brittle PS minor phase in the presence of ESI. Tensile tests showed that the yield strength of the PS/LDPE/ESI blends decreases considerably with increasing LDPE content. However, the elongation at break of the blends tended to increase significantly with increasing LDPE content. The compatibilization efficiency of ESI and polystyrene‐hydrogenated butadiene‐polystyrene triblock copolymers (SEBS) for PS/LDPE 50/50 was further compared. Mechanical properties show that ESI is more effective to achieve a combination of LDPE toughness and PS rigidity than SEBS. The correlation between the impact property and morphology of the ESI‐compatibilized PS/LDPE blends is discussed. The excellent tensile ductility of the LDPE‐rich blends resulted from shield yielding of the matrix. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2136–2146, 2007     

Polyethylene‐poly(L‐lactide) diblock copolymers: Synthesis and compatibilization of poly(L‐lactide)/polyethylene blends

A model polyethylene‐poly(L ‐lactide) diblock copolymer (PE‐b‐PLLA) was synthesized using hydroxyl‐terminated PE (PE‐OH) as a macroinitiator for the ring‐opening polymerization of L ‐lactide. Binary blends, which contained poly(L ‐lactide) (PLLA) and very low‐density polyethylene (LDPE), and ternary blends, which contained PLLA, LDPE, and PE‐b‐PLLA, were prepared by solution blending followed by precipitation and compression molding. Particle size analysis and scanning electron microscopy results showed that the particle size and distribution of the LDPE dispersed in the PLLA matrix was sharply decreased upon the addition of PE‐b‐PLLA. The tensile and Izod impact testing results on the ternary blends showed significantly improved toughness as compared to the PLLA homopolymer or the corresponding PLLA/LDPE binary blends. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2755–2766, 2001     

Novel ethylene/norbornene copolymers as nonreleasing antioxidants for food‐contact polyolefinic materials

An efficient procedure for the copolymerization of ethylene (E) with a novel norbornenic comonomer (N_ArOH) bearing a stabilizing moiety analogous to commercial antioxidant 2,6‐di‐tert‐butyl‐4‐methylphenol (BHT) is successfully developed. This study is aimed at: i) tuning the concentration of the stabilizing function along the polymer chain, and ii) preparing “nonreleasing” polymeric additives specifically destined to protect commercial low‐density polyethylene (LDPE). Films obtained from blends of the novel E/N_ArOH copolymers with an antioxidant‐free LDPE matrix are characterized by superior thermal, thermo‐oxidative, and photostability when compared not only with neat LDPE films but also with films stabilized by the commercial BHT additive. Specific migration tests conducted in order to investigate the nonreleasing character of the novel macromolecular additives confirm the reduced risk of migration, from the films into food simulants, of unreacted comonomer or degradation products bearing the antioxidant moiety. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013 , 51, 1007–1016     

Tear anisotropy in films blown from polyethylenes of different macromolecular architectures

Blown films of different types of polyethylenes, such as branched low‐density polyethylene (LDPE) and linear high‐density polyethylene (HDPE), are well known to tear easily along particular directions: along the film bubble's transverse direction for LDPE and along the machine direction (MD) for HDPE. Depending on the resin characteristics and processing conditions, different structures can form within the film; it is therefore difficult to separate the effects of the crystal structure and orientation on the film tear behavior from the effects of the macromolecular architecture, such as the molecular weight distribution and long‐chain branching. Here we examine LDPE, HDPE, and linear low‐density polyethylene (LLDPE) blown films with similar crystal orientations, as verified by through‐film X‐ray scattering measurements. With these common orientations, LDPE and HDPE films still follow the usual preferred tear directions, whereas LLDPE tears isotropically despite an oriented crystal structure. These differences are attributed to the number densities of the tie molecules, especially along MD, which are considerably greater for linear‐architecture polymers with a substantial fraction of long chains, capable of significant extension in flow. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 413–420, 2005     

A study on melt grafting of maleic anhydride onto low‐density polyethylene and its blend with polyamide 6

Graft copolymerization of low‐density polyethylene (LDPE) with a maleic anhydride (MAH) was performed using intermeshing corotating twin‐screw extruder in the presence of benzoyl peroxide (BPO). The LDPE/polyamide 6 (PA6) and LDPE‐g‐MAH/PA6 blends were prepared in a corotating twin‐screw extruder. The melt viscosity of the grafted LDPE was measured by a capillary rheometer. The grafted copolymer was characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microcopy (SEM). The influence of the variation in temperature, BPO and MAH concentration, and temperature on the grafting degree and on the melt viscosity was studied. The grafting degree increased appreciably up to about 0.45 phr and then decreased continuously with an increasing BPO concentration. According to the FTIR analysis, it was found that the amount of grafted MAH on the LDPE chains was ～5.1%. Thermal analysis showed that melting temperature of the graft copolymers decreases with increasing grafting degree. In addition to this, loss modulus (E″) of the copolymers first increased little with increasing grafting and then obviously decreased with increasing grafting degree. Furthermore, the results revealed that the tensile strength of the blends increased linearly with increasing PA6 content. The results of SEM and mechanical test showed that the blends have good interfacial adhesion and good stability of the phase structure, which is reflected in the mechanical properties. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 267–275, 2010     

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     

Additive‐like amounts of HDPE prevent creep of molten LDPE: Phase‐behavior and thermo‐mechanical properties of a melt‐miscible blend

Low‐density polyethylene (LDPE) is the preferred type of polyolefin for many medical and electrical applications because of its superior purity and cleanliness. However, the inferior thermo‐mechanical properties as compared to, for example, high‐density polyethylene (HDPE), which arise because of the lower melting temperature of LDPE, constitute a significant drawback. Here, we demonstrate that the addition of minute amounts of HDPE to a LDPE resin considerably improves the mechanical integrity above the melting temperature of LDPE. A combination of dynamic mechanical analysis and creep experiments reveals that the addition of as little as 1 to 2 wt% HDPE leads to complete form stability above the melting temperature of LDPE. The investigated LDPE/HDPE blend is found to be miscible in the melt, which facilitates the formation of a solid‐state microstructure that features a fine distribution of HDPE‐rich lamellae. The absence of creep above the melting temperature of LDPE is rationalized with the presence of tie chains and trapped entanglements that connect the few remaining crystallites. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 146–156     

Effects of poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] on the charge distributions of low‐density polyethylene/polystyrene blends

The effect of the triblock copolymer poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] (SEBS) on the formation of the space charge of immiscible low‐density polyethylene (LDPE)/polystyrene (PS) blends was investigated. Blends of 70/30 (wt %) LDPE/PS were prepared through melt blending in an internal mixer at a blend temperature of 220 °C. The amount of charge that accumulated in the 70% LDPE/30% PS blends decreased when the SEBS content increased up to 10 wt %. For compatibilized and uncompatibilized blends, no significant change in the degree of crystallinity of LDPE in the blends was observed, and so the effect of crystallization on the space charge distribution could be excluded. Morphological observations showed that the addition of SEBS resulted in a domain size reduction of the dispersed PS phase and better interfacial adhesion between the LDPE and PS phases. The location of SEBS at a domain interface enabled charges to migrate from one phase to the other via the domain interface and, therefore, resulted in a significant decrease in the amount of space charge for the LDPE/PS blends with SEBS. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2813–2820, 2004     

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.     

Solid-state relaxations in linear low-density (1-octene comonomer), low-density,and high-density polyethylene blends

Extensive thermal and relaxational behavior in the blends of linear low-density polyethylene (LLDPE) (1-octene comonomer) with low-density polyethylene (LDPE) and high-density polyethylene (HDPE) have been investigated to elucidate miscibility and molecular relaxations in the crystalline and amorphous phases by using a differential scanning calorimeter (DSC) and a dynamic mechanical thermal analyzer (DMTA). In the LLDPE/LDPE blends, two distinct endotherms during melting and crystallization by DSC were observed supporting the belief that LLDPE and LDPE exclude one another during crystallization. However, the dynamic mechanical β and γ relaxations of the blends indicate that the two constituents are miscible in the amorphous phase, while LLDPE dominates α relaxation. In the LLDPE/HDPE system, there was a single composition-dependent peak during melting and crystallization, and the heat of fusion varied linearly with composition supporting the incorporation of HDPE into the LLDPE crystals. The dynamic mechanical α, β, and γ relaxations of the blends display an intermediate behavior that indicates miscibility in both the crystalline and amorphous phases. In the LDPE/HDPE blend, the melting or crystallization peaks of LDPE were strongly influenced by HDPE. The behavior of the α relaxation was dominated by HDPE, while those of β and γ relaxations were intermediate of the constituents, which were similar to those of the LLDPE/HDPE blends. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1633–1642, 1997     

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     

Compatibilizing effect of isocyanate functional group on polyethylene terephthalate/low density polyethylene blends

To evaluate the compatibilizing effects of isocyanate (NCO) functional group on the polyethylene terephthalate/low density polyethylene (PET/LDPE) blends, LDPE grafted with 2-hydroxyethyl methacrylate-isophorone diisocyanate (LDPE-g-HI) was prepared and blended with PET. The chemical reaction occurred during the melt blending in the PET/LDPE-g-HI blends was confirmed by the result of IR spectra. In the light of the blend morphology, the dispersions of the PET/LDPE-g-HI blends were very fine over the PET/LDPE blends. DSC thermograms indicated that PET microdispersions produced by the slow cooling of the PET/LDPE-g-HI blends were largely amorphous, with low crystallinity, due to the chemical bonding. The tensile strengths of the PET/LDPE-g-HI blends were higher than those of the PET/LDPE blends having a poor adhesion. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 447–453, 1998     

Self‐initiating performance of maleic anhydride on surface photografting polymerization

Maleic anhydride (MAH) was photografted onto low‐density polyethylene (LDPE) films with a grafting efficiency of about 70% in the absence of a photoinitiator. The self‐initiating performance was attributed to a mechanism of abstracting hydrogen atoms from LDPE chains by excited MAH dimers. The supporting experimental results were as follows: (1) the far‐UV radiation (200–300 nm) was indispensable for the graft polymerization and 2) the crosslinking reaction of LDPE inevitably accompanied the grafting of MAH. In addition, the initiation performance of MAH was further confirmed by surface photografting of acrylic acid in the presence of MAH, where MAH was used as the photoinitiator. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3246–3249, 2001     

Morphology and mechanical properties of blown films of a low‐density polyethylene/linear low‐density polyethylene blend

The morphologies of films blown from a low‐density polyethylene (LDPE), a linear low‐density polyethylene (LLDPE), and their blend have been characterized and compared using transmission electron microscopy, small‐angle X‐ray scattering, infrared dichroism, and thermal shrinkage techniques. The blending has a significant effect on film morphology. Under similar processing conditions, the LLDPE film has a relatively random crystal orientation. The film made from the LDPE/LLDPE blend possesses the highest degree of crystal orientation. However, the LDPE film has the greatest amorphous phase orientation. A mechanism is proposed to account for this unusual phenomenon. Cocrystallization between LDPE and LLDPE occurs in the blowing process of the LDPE and LLDPE blend. The structure–property relationship is also discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 507–518, 2002; DOI 10.1002/polb.10115     

Crossover behavior in crystal growth rate from n‐alkane to polyethylene

A crystal growth rate equation, parameterized from molecular dynamics simulations of n‐alkanes, is compared to recent experiments on growth rates for polyethylene at high undercooling. The analysis reveals that the growth rate of alkanes and polyethylene can both be described by the same relationship. The appropriate relaxation time is used to describe the kinetic barrier to crystallization. For chains shorter than the entanglement length, this is the Rouse time. For chains longer than the entanglement molecular weight, kinetic limitations are modeled by the local relaxation of an entangled segment at the interface. This model supports a different mechanism for fast crystal growth at high undercooling than that usually inferred from slow growth data near the melting temperature. Use of the crystal growth rate model is illustrated for polyethylene crystallizing under conditions of slow cooling and fast cooling. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2468–2473, 2005     

Thermal expansion coefficient and bulk modulus of polyethylene closed‐cell foams

A regular Kelvin foam model was used to predict the linear thermal expansion coefficient and bulk modulus of crosslinked, closed‐cell, low‐density polyethylene (LDPE) foams from the polymer and gas properties. The materials used for the experimental measurements were crosslinked, had a uniform cell size, and were nearly isotropic. Young's modulus of biaxially oriented polyethylene was used for modeling the cell faces. The model underestimated the foam linear thermal expansion coefficient because it assumed that the cell faces were flat. However, scanning electron microscopy showed that some cell faces were crumpled as a result of foam processing. The measured bulk modulus, which was considerably smaller than the theoretical value, was used to estimate the linear thermal expansion coefficient of the LDPE foams. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3741–3749, 2004     

Nonlinear shear of entangled polymers from nonequilibrium molecular dynamics

This study aims to use molecular dynamics (MD) simulations of Kremer–Grest (KG) chains to inform future developments of models of entangled polymer dynamics. We perform nonequilibrium MD simulations, under shear flow, for well‐entangled KG chains. We study chains of 512 and 1000 KG beads, corresponding to 8 and 15 entanglements, respectively. We compute the linear rheological properties from equilibrium simulations of the stress autocorrelation and obtain from these data the tube model parameters. Under nonlinear shear flow, we compute the shear viscosity, the first and second normal stress differences, and chain contour length. For chains of 512 monomers, we obtain agreement with the results of Cao and Likhtman (ACS Macro. Lett. 2015, 4, 1376). We also compare our nonlinear results with the Graham, Likhtman and Milner‐McLeish (GLaMM) model. We identify some systematic disagreement that becomes larger for the longer chains. We made a comparison of the transient shear stress maximum from our simulations, two nonlinear models and experiments on a wide range of melts and solutions, including polystyrene (PS), polybutadiene, and styrene–butadiene rubber. This comparison establishes that the PS melt data show markedly different behavior to all other melts and solutions and KG simulations reproduce the PS data more closely than either the GLaMM or Xie and Schweizer models. We discuss the performance of these models against the data and simulations. Finally, by imposing a rapid reversing flow, we produce a method to extract the recoverable strain from MD simulations, valid for sufficiently entangled monodisperse polymers. We explore how the resulting data can probe the melt state just before the reversing flow. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1692–1704     

Tailored side‐chain architecture of benzil voltage stabilizers for enhanced dielectric strength of cross‐linked polyethylene

The synthesis and physico‐chemical properties of seven benzil‐type voltage stabilizers are reported. The benzil core is substituted with alkyl chains of different length that are linked to the benzil core via an ester, ether, or tertiary amine group. All additives can be melt‐processed with low‐density polyethylene (LDPE). Fourier‐transform infrared spectroscopy confirms that benzil compounds are not affected by the LDPE cross‐linking reaction induced by dicumyl peroxide. Moreover, a combination of gel content measurements, thermal analysis, and small‐angle X‐ray scattering indicates that the presence of benzil voltage stabilizers does not significantly alter the microstructure of cross‐linked polyethylene (XLPE). Electrical tree inhibition experiments under high‐voltage alternating current conditions show that all investigated additives substantially enhance the dielectric strength of the insulating material at a concentration of only 10 mmol kg^?1. The highest improvement in dielectric strength, of more than 70% with respect to reference XLPE, is obtained with voltage stabilizers, which carry short (methyl) side chains that are linked to the benzil core via an ester or tertiary amine group. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1047–1054     

Stress–strain behavior of low‐density polyethylene/poly(methyl methacrylate) blends with modulated interfaces with a hydrogenated polybutadiene‐block‐poly(methyl methacrylate) diblock copolymer

The stress–strain diagrams and ultimate tensile properties of uncompatibilized and compatibilized hydrogenated polybutadiene‐block‐poly(methyl methacrylate) (HPB‐b‐PMMA) blends with 20 wt % poly(methyl methacrylate) (PMMA) droplets dispersed in a low‐density polyethylene (LDPE) matrix were studied. The HPB‐b‐PMMA pure diblock copolymer was prepared via controlled living anionic polymerization. Four copolymers, in terms of the molecular weights of the hydrogenated polybutadiene (HPB) and PMMA sequences (22,000–12,000, 63,300–31,700, 49,500–53,500, and 27,700–67,800), were used. We demonstrated with the stress–strain diagrams, in combination with scanning electron microscopy observations of deformed specimens, that the interfacial adhesion had a predominant role in determining the mechanism and extent of blend deformation. The debonding of PMMA particles from the LDPE matrix was clearly observed in the compatibilized blends in which the copolymer was not efficiently located at the interface. The best HPB‐b‐PMMA copolymer, resulting in the maximum improvement of the tensile properties of the compatibilized blend, had a PMMA sequence that was approximately half that of the HPB block. Because of the much higher interactions encountered in the PMMA phase in comparison with those in HPB (LDPE), a shorter sequence of PMMA (with respect to HPB but longer than the critical molecular weight for entanglement) was sufficient to favor a quantitative location of the copolymer at the LDPE/PMMA interface. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 22–34, 2005