Long-chain Branching In Polyethylene Research Articles

A commercially viable solution process to control long-chain branching in polyethylene.

In polyolefins, long-chain branching is introduced through an energy-intensive, high-pressure radical process to form low-density polyethylene (LDPE). In the current work, we demonstrated a ladder-like polyethylene architecture through solution polymerization of ethylene and less than 1 mole % of α,ω-dienes, using a dual-chain catalyst. The ladder-branching mechanism requires catalysts with two growing polymer chains on the same metal center, thus enchaining the diene without the requirement of a steady-state concentration of pendant vinyl groups. Molecular weight distributions lacking a high-molecular weight tail, distinctive Mark-Houwink signatures, nuclear magnetic resonance characterization, and shear and extensional rheology consistent with highly branched polyethylene architectures are described. This approach represents an industrially viable solution-polymerization process capable of producing controlled long-chain branched polyethylene with rheological properties comparable to those of LDPE or its blends with linear low-density polyethylene (LLDPE).

In Situ Promotion of Long-Chain Branching in Polyethylene from Ziegler–Natta Catalysts

Polyethylene (PE) synthesized by Ziegler–Natta catalysts (MgCl2/TiCl4 catalysts) lack long-chain branching (LCB) structure, which to a great extent limits their end uses. In this paper, we report an in situ methodology of incorporating LCB structure into PE of Ziegler–Natta catalysts. The chemistry centers on a bifunctional reagent, ω-alkenylmethyldichlorosilane, where the reagent is first copolymerized with ethylene to anchor on some polymer chains pendant alkylmethyldichlorosilane groups, then upon treating the polymer with water adjacent polymer chain-borne alkylmethyldichlorosilane groups undergo one-on-one interchain hydrolytic condensation forming a dimeric alkylmethylsiloxane bridge interconnecting two different polymer chains as H-shape LCB structure. Control of ω-alkenylmethyldichlorosilane concentration during copolymerization is key to realize the chemistry. Gel-free LCB-PEs containing controlled amounts of LCB structure were synthesized by accompanying the copolymerization with hydrogen (H2) as chain transfer agent. The effect of alkenyl chain length of ω-alkenylmethyldichlorosilane on the formation efficiency of LCB structure was studied. The chemistry was generally applicable to different Ziegler–Natta catalysts. LCB-PEs containing increasing amounts of LCB structure exhibited proportionally increased extensional flow properties. Moreover, compared to linear plain PE, LCB-PEs were overall found to increase crystallization rates and thus increase crystallinities.

Determination of low degrees of long-chain branching in polybutadiene by double-bond hydrogenation followed by 13C NMR and DEPT spectroscopies

Long-chain branching (LCB) in polybutadiene (PB) was directly and accurately determined for the first time by combining the hydrogenation of PB with 13C NMR and DEPT spectroscopies. LCB in PB was converted into LCB in polyethylene (PE) by the hydrogenation of PB. No LCB in PE was observed in the 13C NMR spectrum. In contrast, LCB in PE was clearly observed in the 13C DEPT 90° spectrum. The methine carbons in LCB and short-chain branching, which is vinyl branching in PB converted into ethyl branching (EB) in PE, were quantified by 13C NMR and DEPT 90° spectroscopies. The numbers of LCB structures of commercially available PB-A and PB-B were 0.27 and 0.17/1000C, respectively. It was proved that these values were accurate with errors of ca. 10% as determined by analyses of control polymers with various ratios of hexyl branching to EB.

Theoretical correlation of linear and non-linear rheological symptoms of long-chain branching in polyethylenes irradiated by electron beam at relatively low doses

Dynamic and transient shear and elongation flow experiments along with gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) analysis are performed on linear low-density polyethylenes (LLDPEs) irradiated at doses below 25 kGy. GPC data indicate no changes in the molar mass distribution, and there are almost no changes in melt and crystallization temperatures, likewise. Contrary, dynamic shear rheological behavior including thermorheological complexity, type of reduced van Gurp-Palmen curves, and zero shear-rate viscosities all disclose growing levels of long-chain branching with irradiation dose. An inverse tube model is developed for binary blend of linear and star chains and used to extract the fraction of the branched components. Modeling results reveal progressive increase in the length and fraction of star chains, as evidenced by appearance of an anomalous double overshoot in the transient shear viscosities. Detection of strain hardening in extensional stress growth coefficient data, well-quantified by molecular stress function model, is also in agreement with the predictions of tube model.

Copolymerization of Ethylene with 1,9‐Decadiene: Part I – Prediction of Average Molecular Weights and Long‐Chain Branching Frequencies

Long chain branches improve the viscoelastic properties of polyolefins and make them easier to process. The frequency of long chain branching in polyethylenes made with coordination catalysts can be substantially increased by copolymerizing ethylene with small amounts of nonconjugated dienes using an adequate catalyst. In this work, the kinetics of copolymerization of ethylene and 1,9‐decadiene is investigated using a constrained geometry catalyst in a solution polymerization semi‐batch reactor, and a novel mathematical model is proposed to describe the resulting molecular weight distributions. A hybrid approach, combining particle swarm optimization, parameter identifiability procedures, and the Gauss–Newton method is applied to estimate the model parameters. The proposed mechanism includes macromonomer reincorporation through pendant double bonds that result from diene incorporation. The predicted long chain branching frequencies are validated by Monte Carlo simulation. Experimental average molecular weights and ethylene feed flow rates are successfully predicted by both methods. image

Long-chain branching in PE from Cr/aluminophosphate catalysts

Phillips Cr/silica catalysts have long been known to produce long-chain branching (LCB) in polyethylene (PE) through macromer incorporation. In fact, the manipulation of LCB levels through catalyst variables is a very useful way of controlling properties and molding behavior in commercial PE resins. Using alumina as the support, instead of silica, is one way of achieving very low levels of LCB. Adding small amounts of phosphate to the alumina base as a modifier causes a large rise in LCB levels. Phosphate also promotes the activity and melt index potential of the catalyst by contributing a large new low-MW population of PE chains. In this study LCB levels were measured and found to correlate nicely with the amount of phosphate added. The polymer from such a catalyst was then solvent-fractionated to determine the location of the LCB. It was found to be concentrated in this same low-MW polymer, to which the phosphate contributes, which suggests that the phosphate-associated Cr sites have a tendency to produce LCB. However, the incorporation of 1-hexene resulted in a flat short-chain branch (SCB) profile, indicating that there was no parallel behavior among the different sites between SCB and LCB incorporation. These results indicate a slightly different mechanistic pathway between the two monomers (macromer and 1-hexene). Furthermore the Cr sites must have incorporated macromers that were produced only at the same site, and not (as is usually portrayed) randomly from a general pool of vinyls contributed by all sites. To explain this finding, alternatives to the widely accepted random-incorporation mechanism are proposed, including a “tethered” vinyl end-group, and intra-, rather than the traditional inter-, molecular insertion of macromer.

Viscoelasticity and macromolecular topology in single-site catalyzed polyethylene

The relationship between polymer chain topology, i.e. long chain branching (LCB), and rheological properties in polyolefins obtained using single-site catalysts has been the topic of major scientific activity since the late 1990s [1–14]. Indeed, melt-state rheological measurements are being considered as one of the most sensitive methods for detecting sparse LCB in polyethylene (PE) [1, 15]. For example the flow activation energy, Ea, related to the temperature dependence of viscoelastic properties, is altered by the presence of LCB, and the cross-point frequency between storage and loss moduli, xx, and the crosspoint modulus, Gx, are also very sensitive features to the molecular architecture [16–18]. In this context, the study of these properties can give us information about the relationship between polymer chain topology and catalyst structure. In this work, a set of PE samples was synthesized using different metallocene catalysts. The main objective is to find relationships between chemical structure of the catalysts and polymer chain architecture by rheological testing. This is an interesting task as there is still a lack of information in the literature concerning how the catalyst structure affects the creation of LCB during polymerization [15]. The PE samples were synthesized using the catalysts shown in Table 1, together with MAO as co-catalyst. The synthesis of the samples was carried out under the same experimental conditions: a reactor temperature of 20 C, an ethylene pressure of 2 bar, 3 9 10 mol L of metallocene concentration, and a molar ratio MAO (methyl aluminoxane)/metallocene of 3,000 [17–21]. The metallocene catalysts were prepared using synthetic protocols that we have previously reported [20, 21]. The molecular properties of the polymer samples obtained by size exclusion chromatography analysis are listed in Table 1. The details about the experimental procedures are described elsewhere [18, 19]. It has been very difficult to estimate the molecular features in the case of C4 sample, probably due to extremely high molecular weight. In this case we have estimated molecular weight and polydispersity index by rheological methods. The materials were stabilized with 1.0 wt.% Irganox 1010 to avoid degradation at high temperatures. The rheological measurements were carried out using parallel plate geometry (15 mm diameter). Frequency sweeps in the linear viscoelastic regime were performed with a Bohlin CVO rheometer in the angular frequency range between 0.006 and 60 rad s. The linear viscoelastic regime has been located by previous strain sweeps. The measurements have been performed in the temperature range between 145 and 190 C and the time-temperature superposition principle (TTSP) applied to a reference temperature of T0 = 190 C. From these results, the characteristic features described above (Ea, xx, and Gx) have been obtained for the materials under study (see Table 1). The material labeled as A0 has been synthesized by using the [Zr(g-C5H5)2Cl2] catalyst (A), which is the simplest structural fragment used in this study. This catalyst has been previously reported to produce LCB [2, 14]. The sample shows a very high Ea value and a significant elastic character, in spite of the very narrow molecular J. F. Vega (&) J. Martinez-Salazar Departamento de Fisica Macromolecular, Instituto de Estructura de la Materia, CSIC, Serrano 113 bis, 28006 Madrid, Spain e-mail: imtv477@iem.cfmac.csic.es

Long Chain Branching in Polyethylene from the Phillips Chromium Catalyst

Supported chromium oxide polymerization catalysts can impart significant levels of long chain branching (LCB) to polyethylene made in a slurry process. LCB often dominates the rheological behavior of the polymer, and is thus responsible for the performance of many HDPE grades during commercial molding operations. LCB is difficult to accurately measure but its presence can be inferred from the visco‐elastic character of the polymer, relative to its molecular weight (MW) and MW distribution. Depending on catalyst choice, LCB levels from chromium oxide catalysts can range from near zero, to very high, rivaling the tightly bridged metallocenes. The most common catalyst and reactor variables were investigated with respect to their effect on LCB formation. The simple “macromonomer insertion” view does not in itself account for many of these LCB responses. In fact, no overall correlation was found between LCB and the catalyst's ability to incorporate large comonomers. Instead, variables that influenced LCB most also govern the active site density, including activation temperature, chromium loading, and the presence of cocatalysts or poisons in the reactor. The physical structure of the catalyst support also had a strong influence on the elastic behavior of the polymer, independent of any effect on MW or MW distribution. These and other variables are discussed.

Using Rheological Data To Determine the Branching Level in Metallocene Polyethylenes

A technique for using rheological information to determine the level of long chain branching in polyethylenes produced using a constrained geometry catalyst and having low levels of branching is described. The complex viscosity as a function of frequency is used, along with a molecular weight distribution (MWD) determined by gel permeation chromatography (GPC). A procedure previously proposed to infer the MWD of linear polymers from complex viscosity data is used to determine a “viscosity MWD” for the branched materials, and this is compared with the GPC MWD. The difference in the location of the peaks in the two curves is then correlated with the level of branching. Advice is provided regarding the range and density of experimental viscosity data needed for a reliable determination of long chain branching.

Diagnosing long-chain branching in polyethylenes

We propose a novel method for assessing sparse long-chain branching in synthetic polymers such as high-density polyethylene at levels far below the limits of detectability by the usual methods of solution viscometry, size-exclusion chromatography, and NMR spectrometry on solutions. The new method exploits the extreme sensitivity of melt Newtonian viscosity to random branching architecture, along with the systematic phenomenological description thereof developed recently in fundamental studies by Lusignan et al. The method satisfies the only validation criterion presently available to us: it finds long-branch contents in quantitative agreement with stoichiometric yields calculated for several series of linear precursor polyethylenes treated with very low levels of peroxide.

Resolution of Signals from Long-Chain Branching in Polyethylene by 13C NMR at 188.6 MHz

Resolution of Signals from Long-Chain Branching in Polyethylene by ¹³C NMR at 188.6 MHz

Novel mechanism for the formation of long-chain branching in polyethylene

Polyethylene produced by a vanadium-based polymerization catalyst contains long-chain branching as determined by NMR and rheology, even though the polymer has very low levels of vinyl unsaturation. A new mechanism is proposed for the formation of the long-chain branching, which involves CH bond activation of the polyethylene backbone through a σ-bond metathesis reaction, followed by ethylene insertion at the new VC bond. Consistent with the proposed CH bond activation mechanism, the polymerization catalyst was also found to insert ethylene into the CH bonds of alkanes such as heptane. A bridged metallocene catalyst was also found to activate CH bonds of alkanes suggesting this new mechanism may explain the formation of long-chain branching in some metallocene-produced polyethylene. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2889–2898, 1998

Small-Amplitude Oscillatory Shear Flow Measurements as a Tool To Detect Very Low Amounts of Long Chain Branching in Polyethylenes

Dynamic viscoelastic results of 23 noncommercial metallocene-catalyzed polyethylenes and poly(ethylene/1-hexene) copolymers, in the range 130−190 °C, are presented. The effects of well-determined structural parameters such as molecular weight, polydispersity, and degree of short chain branching (SCB), are analyzed. The molecular weight varies between 60 000 and 325 000, the polydispersity between 1.8 and 7.3, and SCB between 0 and 48.5 branches/1000 C atoms. It is observed that a group of 11 polymers displays rheological specific features which can be summarized as follows: (a) higher dynamic viscosities at low frequencies than other polyethylenes and ethylene/1-hexene copolymers of similar molecular weight, polydispersity and SCB degree; (b) higher relaxation times than narrow molecular weight distribution polyethylenes of similar dynamic viscosities at low frequencies but similar relaxation times to those of broad molecular weight distribution; (c) higher values of elastic modulus, in comparison with polyethylenes of similar molecular weight, polydispersity, and SCB but of the same order of magnitude as those of broader molecular weight distribution; (d) higher activation energy of flow than linear polyethylenes of the same molecular weight, polydispersity, and SCB level. An analysis of the literature results leads us to suspect that the polymers which show a “dissident” behavior possess a certain very low degree of long chain branching (LCB). The analysis of the samples by SEC coupled with intrinsic viscosimetry reveals that some of these 11 polymers are long chain branched. However, this technique does not appear to be enough sensitive to detect very small amounts of LCB, and an alternative single rheological method, based on the effect of temperature on dynamic viscosity, is proposed to evaluate the possible presence of LCB.

Real Time Video Techniques in the Analysis of Blown Film Instability

Abstract This paper describes a device which was developed to measure the degree of instability during blown film processing. Two types of data analysis are developed which can be used to analyze the instability data and the relative merits of each are discussed. The paper also describes how the device was used to measure the effect of adding two stabilizers (2 MI LDPE and 0.25 MI LDPE) at different concentrations to a 1 MI LLDPE during blown film processing. The process stability improved with higher concentrations of either LDPE with the 0.25 MI LDPE having the stronger effect. These results are consistent with the belief that long chain branching in polyethylenes plays a major role in stabilizing the blown film processing of polyethylenes.

The structure of long-chain branching in polyethylene

It is shown that analysis of the way in which the intrinsic viscosity of fractions of a branched polydisperse polymer depends on the molecular mass of the fractions can give information about the macromolecule's type of branching. It is concluded on the basis of an experimental study of PE that the long-chain branching in PE is random in structure.

Long Chain Branching in Polyethylene

Abstract Long chain branching frequency in polyethylene has been measured. Molecular weights determined directly by low angle laser light scattering of eluting species in gel permeation chromatography were compared with those estimated by universal calibration. Erroneous values for long chain branching frequency are produced if care is not taken to disrupt polyethylene aggregates in the GPC solvent. Ethyl and hexyl aide chains do not register as long branches in this analysis but sixteen carbon sidechains are counted. In low density polyethylene the long chain branching frequency is generally highest at low molecular weights. This is because these polymers are produced in a non-isothermal free radical polymerization. Chain transfer to dead polymer, which produces long branches, occurs most frequently under the reaction conditions that also yield low molecular weight polyethylene.

High‐temperature coupling of high‐speed GPC with continuous viscometry. I. Long‐chain branching in polyethylene

AbstractThe coupling of a high‐temperature liquid chromatograph (Waters 150C) with a home‐made continuous capillary viscometer is described. This detector is the only one suitable for high‐speed GPC when the small volume of the mobile phase prohibits the coupling with a classical viscometer. The pressure drop of the GPC effluent through the capillary is continuously measured along with the refractive index change. This dual detection leads to the determination of the intrinsic viscosity as a function of the elution volume, thus allowing a precise use of Benoit's universal calibration. The accuracy of our system is demonstrated in the case of the characterization of linear and branched polyethylene samples. The results concerning the average molecular weights as well as the branching factors (structure parameter g′ and long‐chain branching frequencyλ) are in close agreement with those obtained by the classical way (coupling traditional GPC and discontinuous viscometry). It is well known that an estimate of the λ coefficient is extremely dependent on several hypotheses. However, for a set of commercial low‐density polyethylenes, we obtained λ values about 0.5 × 10−4, with no marked change along the molecular weight range.

An Analytical Method for Long-Chain Branching in Free-Radical Polymerization of Ethylene

Abstract Quantitative measurements on long-chain branching in polyethylene have not been established. This paper presents a method available for calculating the degree of long-chain branching and the exponent α in the hydrodynamic relation g = Gα (where g is the ratio of the intrinsic viscosities of branched and linear molecules at the same molecular weight, and G is the corresponding ratio of mean square radii of gyration), by means of polymerization parameters, size exclusion chromatography, viscosity measurement, and amount of total unsaturation. The calculation based on our experimental data shows the following, in radical polymerization of ethylene. (1) With reaction pressure and temperature varied, the degree of long-chain branching was 0.5 to 3.7 per 1000 carbons. (2) As for the rate constant for long-chain branching, the activation energy Etrp - Ep and the activation volume Δ Vtrp ∗ - ΔVp ∗ were 4.1 kcal/mol and 20 cc/mol, respectively. (3) The value of the exponent α, which is considered to repre...