Rheological and thermomechanical properties of long‐chain‐branched polyethylene prepared by slurry polymerization with metallocene catalysts

A series of polyethylene (PE) samples were prepared in a slurry polymerization with bis(cyclopentadienyl) zirconium dichloride (Cp₂ZrCl₂)/modified methylaluminoxane (MMAO) using a semibatch reactor. The samples had long‐chain branch densities (LCBDs) of a 0.03–1.0 branch per 10,000 carbons and long‐chain branch frequencies (LCBFs) up to a 0.22 branch per polymer molecule. The rheological and dynamic mechanical behaviors of these long‐chain branched PE samples were evaluated. Increasing the LCBF significantly increased the η₀'s and enhanced shear thinning. Long‐chain branching (LCB) also influenced the loss modulus and storage modulus. Increasing the LCBF led to enhanced G′ and G″ values at low shear rates and broader relaxation spectrums. The samples exhibited thermorheologically complex behavior. LCB also played a significant role in the dynamic mechanical behavior. Increasing the LCBF increased the stiffness of the polymer and enhanced the damping or energy dissipation. However, LCB had little influence on the crystalline structure of the PE. The α‐ and γ‐relaxations showed little dependence on the LCBF. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 307–316, 2004     

SEC-MALS method for the determination of long-chain branching and long-chain branching distribution in polyethylene

This paper systematically describes a LCB determination method that can quantify both LCB content and LCB distribution across the molecular weight distribution in polyethylene homopolymers as well as copolymers. Coupling size-exclusion chromatography with multi-angle light scattering (SEC-MALS), this method quantifies molecular weights (MW) and radii of gyration (R_g) simultaneously. The number of LCB per molecule and LCB frequency as a function of MW can be calculated by comparing R_g of a branched polymer with that of a linear control at the same MW using the Zimm-Stockmayer approach. Because the presence of short-chain branching in copolymers results in changes in R_g of the copolymers, their LCB contents cannot be obtained before the short-chain branching (SCB) effect is corrected. Using well-characterized linear PE copolymers as standards, an empirical method is successfully established in this paper to correct the SCB effect. Consequently, this method can be applied to determine LCB in PE copolymers as well. Some practical aspects, such as the selection of formalism for data processing, the LCB detection sensitivity and precision, and long-term reproducibility of this method are also discussed. Finally, examples are given to demonstrate how this method is applied to determine LCB and LCB distribution in practical PE homopolymers and copolymers.     

The preparation and rheology characterization of long chain branching polypropylene

In order to study the rheological behavior of long chain branching (LCB) polypropylene (PP), linear polypropylene was modified by melt grafting reaction in the presence of 2,5-dimethyl-2,5(tert-butylperoxy) hexane peroxide and pentaerythritol triacrylate (PETA) in mixer. The transient torque curves and Fourier transformed infrared spectroscopy (FTIR) results indicated that macroradical recombination reactions took place and PETA had been grafted onto PP backbone. Various rheological plots including viscosity curve, storage modulus, loss angle, Han plot, Cole-Cole plot were used to distinguish LCB PP from linear PP. On the other hand, to quantify the LCB level in modified PPs, a new method was suggested on the basis of macromolecular dynamics models. The results showed that the level of LCB was in the range of 0.025-0.38/10⁴ C . Moreover, the length of the branched chains and the content of the branched component increase with PETA concentration. Furthermore, the LCB efficiency of monomer can also be calculated, less than 20% of grafting monomers was used to form branch structure.     

Heterogeneity of active sites of Ziegler‐Natta catalysts: The effect of catalyst composition on the MWD of polyethylene

Experimental data on the molecular weight distribution (MWD) of polyethylene (PE) produced over a broad number of Ziegler‐Natta catalysts differing in composition and preparation procedure are presented. These catalysts include nonsupported TiCl₃ catalyst, four types of supported titanium‐magnesium catalysts (TMC) differing in the content of titanium and the presence of various modifiers in the composition of the support, and a supported catalyst containing VCl₄ as an active component instead of TiCl₄. The studied catalysts produce PE with different molecular weights within a broad range of polydispersity (M_w/M_n = 2.8–16) under the same polymerization conditions. The heterogeneity of active sites of these catalysts was studied by deconvolution of experimental MWD curves into Flory components assuming a correlation between the number of Flory components and the number of active site types. Five Flory components were found for PE produced over nonsupported TiCl₃ catalysts (M_w/M_n = 6.8), and three–four Flory components were found for PE produced over TMC of different composition. A minimal number of Flory components (three) was found for PE samples (M_w/M_n values from 2.8 to 3.3) produced over TMC with a very low titanium content (0.07 wt %) and TMC modified with dibutylphtalate. It was shown that five Flory components are sufficient to fit the experimental MWD curve for bimodal PE (M_w/M_n = 16) produced over VMC. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

制备凝胶色谱在聚氨酯匀泡剂组成分析中的应用

用制备凝胶渗透色谱分离聚氨酯匀泡剂的混合组分,并对分离的组分进行了凝胶色谱、红外光谱等分析。对两个混合组分的聚氨酯匀泡剂的剖析结果进行了讨论。     

Study of polyethylene solution fractionation and resulting fractional crystallization behavior

Solution fractionation for four different polyethylenes including high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and very low‐density polyethylene (VLDPE) are conducted by stepwise controlling both the temperature and the amount of precipitant. The size exclusion chromatograph (SEC) measurements indicate that solution fractionation technique can successfully separate all the polyethylene samples in accordance with their molecular weight and molecular‐weight distributions. In addition, infrared spectroscopy analysis shows that the degree of short‐chain branching for each fraction of each polyethylene varies with the fraction's molecular weight. The effect of the molecular weight with different short‐chain branching on each fraction's crystallinity represents the characteristics of chain components for different polyethylenes. The crystallinities of HDPE, LLDPE, and LDPE decrease with the increase in their molecular weights; however, for VLDPE, its crystallinity increases with the increase in the molecular weight. The research revealed that the degree of short‐chain branching, together with the molecular weight, can greatly affect the crystallinity of polyethylene. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2542–2549, 2004     

Molecular mass characteristics of polyethylene produced with supported vanadium‐magnesium catalysts

Highly active supported vanadium‐magnesium catalysts (VMC) produce polyethylene (PE) with broad and bimodal molecular mass distribution (MMD) in comparison with the famous titanium‐magnesium catalysts (TMC). The effect of hydrogen as an efficient chain‐transfer agent on the MMD of PE has been studied. Increasing hydrogen concentration causes a considerable broadening of MMD of PE due to the shift of the low molecular weight peak on the MMD curve. At the same time, the high molecular weight shoulder stays at the same position even at high hydrogen concentration. This means that VMC contain two types of active centre. One type is very reactive in the chain‐transfer reaction with hydrogen. These centres produce low molecular weight PE in polymerization in the presence of hydrogen. The other type of active centre is not active in chain transfer with hydrogen. These centres produce high molecular weight PE ((1–3) × 10⁶) and hydrogen does not affect the position of the high molecular weight shoulder. MMD data were used to analyze the kinetics of the chain‐transfer reaction with hydrogen and to calculate the rate constants of this reaction. Copyright © 2005 Society of Chemical Industry     

Isothermal crystallization of intercalated and exfoliated polyethylene/montmorillonite nanocomposites prepared by in situ polymerization

Polyethylene/montmorillonite (PE/MMT) nanocomposites with different dispersion states of MMT were prepared by in situ polymerization. Isothermal crystallization of the intercalated nanocomposite, in which the PE chains were confined in the MMT layers, was studied and was compared with that of the exfoliated nanocomposite. It is observed that the intercalated sample has longer induction period, longer crystallization half time and larger crystallization activation energy than the exfoliated sample, showing that crystallization of PE is retarded due to confinement of the MMT layers. Analysis of crystallization kinetics shows that Avrami exponent (n) increases gradually with crystallization temperature. However, the maximal value of n is 2.0 for the intercalated sample, but it can reach 3.0 for the exfoliated sample. It is inferred that the stems of the PE crystals confined in the MMT layers are parallel to the MMT layers. The Hoffman-Weeks extrapolation method cannot be applied in the intercalated sample because of the small lateral surface of the PE crystals. Based on the depression of the melting temperature, the specific free energy of the PE/MMT interface was estimated, which is about 1.0 mJ/cm², much smaller than the free energy of the lateral surface of PE crystals. This is attributed to the origin of the strong nucleation effect of MMT.     

溶液法聚乙烯蜡接枝马来酸酐的表征

以溶液法合成了聚乙烯蜡接枝马来酸酐(PEW-g-MAH)。利用酸碱中和滴定法测定 PEW-g-MAH 中酸酐含量,借助红外光谱分析证明接枝物的存在,探讨了取样量、滴定温度对测试结果的影响,得到表征 PEW-g-MAH 的优化实验方法。结果表明,在温度70℃下取样量达到1.5g-2g 时,能得到较稳定的接枝率测试数据。     

Synthesis of low molecular weight polyethylenes and polyethylene mimics with controlled chain structures

The controlled synthesis of narrowly distributed low molecular weight polymers with functionalization possibilities is of great industrial interests. Although living polymerization allows for control over polymer architecture, the production of low molecular weight polymers with low polydispersities via living polymerization systems is challenged by the use of large amounts of catalysts and broadening in molecular weight distribution. This review addresses the synthesis of narrowly distributed, functional, low molecular weight polyethylene and polyethylene mimics. The review is structured for quick identification of relevant systems for the production of specific polymer architectures with specific cost, efficiency, and safety concerns.     

Improvement in processability of metallocene polyethylene by ultrasound and binary processing aid

The effect of ultrasonic vibration and binary processing aid in improving the processability of metallocene linear low‐density polyethylene (mLLDPE) was investigated. During extrusion, ultrasonic vibration clearly reduced the die pressure and apparent viscosity of mLLDPE but had only a slight effect on its melt fracture. The effect of diatomite/PEG binary processing aid (BPA) was excellent in reducing the viscosity and eliminating the sharkskin fracture of mLLDPE. The effect of ultrasonic vibration and binary processing aid in improving the processability of mLLDPE was synergetic. With a combination of ultrasonic vibration and a small amount of processing aid, the flowability of mLLDPE was further improved, and the critical shear rate for the onset of sharkskin fracture was increased. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1927–1935, 2007     

Temperature‐sensitive membranes prepared by the plasma‐induced graft polymerization of N‐Isopropylacrylamide into porous polyethylene membranes

A temperature‐responsive polymer, poly(N‐isopropylacrylamide) (PNIPAAm), was grafted onto porous polyethylene membranes by a plasma‐induced graft polymerization technique. A wide range of grafting was achieved through variations in the grafting conditions, including the postpolymerization temperature, time, monomer concentration, and graft‐reaction medium. The active species induced by plasma treatment was proven to be long‐living via a postpolymerization time of 95 h. Different solvent compositions, that is, water, methanol, benzene, and water/methanol, were used as reaction media, and water showed a much higher polymerization rate than the organic solvents. Based on the hydrophilicity of the active species, a mechanism explaining the solvent effect in plasma‐induced graft polymerization was examined. Characterizations by scanning electron microscopy, X‐ray photoelectron spectroscopy (XPS), and micro Fourier transform infrared showed that the grafted polymers were located on both the outer surface and inside pores of the membranes. The XPS analysis also confirmed that the polar amide groups tended to distribute more outward when grafted PNIPAAm was in its expanding state than when it was in its shrinking state. Water permeation experiments showed that the permeability of the grafted membranes varied dramatically with a slight temperature change in the vicinity of the lower critical solution temperature (LCST) of PNIPAAm. The effective pore radii of the grafted membranes above and below the LCST could be depicted by Hagen‐Poiseuille's law. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3180–3187, 2003     

Does the length of the short chain branch affect the mechanical properties of linear low density polyethylenes? An investigation based on films of copolymers of ethylene/1-butene, ethylene/1-hexene and ethylene/1-octene synthesized by a single site metallocene catalyst

Three nearly identical linear low density polyethylene resins based on copolymers of ethylene with 1-butene (B), 1-hexene (H) and 1-octene (O) were utilized to investigate the effect of short chain branch length on the mechanical properties of blown and compression molded (quenched and slow cooled) films. The content of short chain comononer in the three copolymers was ca. 2.5-2.9 mol% that corresponded to a density of 0.917-0.918 g/cm³. Within a given series, the tensile properties of these films do not show any significant difference at slow deformation rates (up to 510 mm/min), even though the DSC and TREF profiles of ‘H’ and ‘O’ differed slightly in comparison to ‘B’. However, at higher deformation rates (ca. 1 m/s), the breaking strength of these films was found to increase with increasing short chain branch length. In addition, the Spencer impact and Elmendorf tear strength of the blown films were also observed to increase with increasing short chain branch length. Further, dart impact strength and high-speed puncture resistance (5.1 m/s) of 1-octene and 1-hexene based samples was also observed to be higher than that based on 1-butene. The blown films displayed low and comparable levels of equivalent in-plane birefringence and crystalline orientation by wide angle X-ray scattering. This confirms that the differences in mechanical properties in the blown film series are not attributable to differences in molecular orientation. The deformation behavior of both the compression molded and blown films were also investigated in a well-defined controlled regime by analyzing their essential work of fracture. It was found that the essential work of fracture of films based on 1-hexene and 1-octene was higher than that of films based on 1-butene. While the origin of these differences in mechanical properties with increasing short chain branch length is not fully understood, the present investigation confirms this effect to be pronounced at high deformation rates for both the blown and compression molded quenched films.     

Supported titanium–magnesium catalysts with different titanium content: Kinetic peculiarities at ethylene homopolymerization and copolymerization and molecular weight characteristics of polyethylene

The effect of Ti content on the activity of titanium–magnesium catalysts (TMC) and molecular weight distribution (MWD) of polyethylene (PE) produced has been studied. It was found that the activity enhances sharply as Ti content decreases from 0.6 to 0.07 wt %, and shows no significant changes in the Ti content range of 0.6–5.0 wt %. The maximum activity (36 kg PE/mmol Ti × h × bar C₂H₄) was observed for TMC with the lowest Ti content. The catalyst with low titanium content (～ 0.1 wt % of Ti) produced PE with narrower MWD (M_w/M_n = 3.1–3.5) as compared to catalysts with higher titanium content (3–5 wt % of Ti; M_w/M_n = 4.8–5.0). New data on the effect of hydrogen on MWD of PE have been found. Increasing hydrogen concentration results in broadening the MWD of PE, especially in the case of TMC with high titanium content. The data presented indicate the heterogeneity of active centers of TMC in the reaction of chain transfer with hydrogen. The data on the ethylene–hexene‐1 copolymerization over TMC with different titanium content are presented. Comonomer reactivity ratios were shown to be independent of the Ti content in TMC. Presumably the difference in activity of these catalysts is mainly caused by the difference in the number of active centers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:5436–5442, 2006     

Correlations between the processing variables and morphology of crosslinked polyethylene

Crosslinked polyethylene (XLPE) is progressively replacing other types of polyethylene in electric cable insulation because of its combination of better mechanical and thermal performance and good dielectric properties. Besides its influence on the mechanical and thermal properties, the morphology plays a very important role in the electrical behavior of XLPE. The morphology has been studied in terms of the crystallinity and crosslinking density. These two apparently independent parameters govern the morphology of XLPE and have been investigated in connection with the systematic variation of the processing parameters (crosslinker concentration, cure time, and temperature). The materials have been characterized in terms of the extent of crosslinking (the gel content), the crosslinking density (the average molecular weight between crosslinks), and the crystallinity. A morphological model is proposed for these materials in terms of the mechanisms and rates of the processes involved. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 222–230, 2004     

Cationic polymerization of ethylene oxide with Maghnite‐H as a clay catalyst in the presence of ethylene glycol

The activated‐monomer cationic ring‐opening polymerization of ethylene oxide, initiated with ethylene glycol and using an acid‐exchanged montmorillonite clay called Maghnite‐H⁺ as an effective catalyst, was carried out to obtain the corresponding homopolymers with narrow polydispersity ratios. The molecular weights of the obtained polymers were controlled with the feed ratio of the monomer to the initiator. The effects of the amount of the catalyst and time on the polymerization yield and viscosity of the polymers were studied. The structure was confirmed with proton nuclear magnetic resonance and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Experimental study on the effect of molecular weight and chemical composition distribution on the mechanical response of high-density polyethylene

This article aims to appraise the effect of microstructure comprising molecular weight distribution and chemical composition distribution on the mechanical properties of high-density polyethylene (HDPE). HDPE resins were synthesized using several titanium–magnesium-supported Ziegler–Natta catalysts in the industrial gas phase reactor under the same polymerization condition. Gel permeation chromatography and crystallization elution fractionation (CEF) were conducted on the resins to characterize the molecular weight and comonomer distribution. Crystallization, thermal and rheological behavior were evaluated following differential scanning calorimetry, polarization light microscopy, and rheometric mechanical spectrometry. The resins with higher soluble fraction in trichlorobenzene below 80°C (highly branched low molecular weight chains) exhibited longer crystallization time based on the crystallization kinetic obtained from the Avrami model. Rheological determination of the molecular weight between entanglements (M_e) and the average lamella thickness based on the Gibbs–Thomson equation revealed that the entanglement density and impact strength decreased, and the average lamella thickness increased with an increase in the ratio of CEF eluted fraction below 80°C to the crystallizable fraction in the range of 80–90°C.     

限制几何构型茂金属/硼化物催化1-癸烯齐聚及其产物表征

用限制几何构型的茂金属催化体系2-Me4Cp-4,6-tBu2-PhOTiCl2/Al(iBu)3/Ph3C+B(C6F5)4-对1-癸烯的齐聚进行了研究。探讨了Al与Ti摩尔比和反应温度对聚合反应的影响。用GC和13CNMR表征了聚合物组成和结构。结果表明,聚合物主要是二聚物、三聚物、四聚物、五聚物。该齐聚物具有高黏度指数(VI=238)、低凝点(≤-62℃)的性质,是理想的润滑油基础油组分。     

Ethylene polymerization over supported titanium‐magnesium catalysts: Effect of polymerization parameters on the molecular weight distribution of polyethylene

The data on the effects of polymerization duration, cocatalyst, and monomer concentrations upon ethylene polymerization in the absence of hydrogen, and the effect of an additional chain transfer agent (hydrogen) on the molecular weight (MW), molecular weight distribution (MWD), and content of vinyl terminal groups for polyethylene (PE) produced over the supported titanium‐magnesium catalyst (TMC) are obtained. The effects of these parameters on nonuniformity of active sites for different chain transfer reactions are analyzed by deconvolution of the experimental MWD curves into Flory components. It has been shown that the polymer MW grows, the MWD becomes narrower and the content of vinyl terminal groups in PE increases with increasing polymerization duration. It is assumed to occur due to the reduction of the rate of chain transfer with AlEt₃ with increasing polymerization duration. The polydispersity of PE is found to rise with increasing AlEt₃ concentration and decreasing monomer concentration due to the emergence of additional low molecular weight Flory components. The ratios of the individual rate constants of chain transfer with AlEt₃, monomer and hydrogen to the propagation rate constant have been calculated. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011