Short Chain Branching Distribution Research Articles

Customizing the comonomer incorporation distribution in Ziegler–Natta‐based LLDPE: Harnessing the influences of the titanation temperature during catalyst synthesis and of polymerization process parameters

AbstractThe microstructure of linear low‐density polyethylene (LLDPE) is strongly influenced by short‐chain branches (SCBs) incorporated into the polymer backbone. Varying the number, distribution, and length of SCBs allows the properties of the resulting polymer to be tailored to meet specific requirements. Using Ziegler–Natta (ZN) catalysts for synthesis has disadvantages in terms of the comonomer incorporation distribution (CID) compared to, for instance, metallocene and post–metallocene catalysts. Nevertheless, ZN catalysts continue to be widely used, as many of the new generations of catalysts are more difficult to handle and cannot match the cheap cost of ZN catalysts. To improve this aspect of ZN catalysts, we investigated the influence of catalyst titanation temperature and polymerization process parameters on the CID. Our results show that it is possible to manipulate the process parameters of the present ZN catalyst system to yield a desired comonomer amount and CID in the polymer. Varying the titanation temperature clearly influenced the titanium content of the catalyst. Molecular‐weight distribution analysis and deconvolution results indicate that changes in the amounts of comonomer incorporated and in the CID are directly related to the catalyst's active site that produces the lowest‐molecular‐weight fraction.

Quantification of Chemical Composition Distribution of Polyolefin Materials for Improved Accuracy and Speed

Crystallization-based separation techniques are widely used for chemical composition distribution (CCD, also often referred as short chain branching distribution SCBD) measurement in the polyolefin industry, in which CCD is among the most critical structural parameters to define their properties and applications. The desire to improve the sample throughput rate by shortening the analysis time in the current CCD techniques would normally sacrifice measurement accuracy due to the cocrystallization phenomena, in which the polymer chains with similar molecular architectures will cocrystallize or coelute enigmatically in the CCD analysis. Herein, we reported the investigation by exploring various separation packing materials on the cocrystallization phenomena. In comparison with the commercially available Crystallization Elution Fractionation (CEF) technique, an improved technique for CCD measurement (iCCD) was developed using the columns packed with gold-coated nickel substrate or spherical gold particles with or without the dynamic cooling process. With iCCD, it was observed with equivalent cocrystallization degree to the industrial gold standard benchmark Wild-TREF method but with over 70+ times improved sample throughput rate (62 vs 4500 min). The experimental CCD chromatogram for a polyolefin blend similarly assembled to the mathematically constructed result further demonstrated the minimized cocrystallization and measurement accuracy in the iCCD method. In addition, the superior mechanical properties of gold-coated nickel particles enabled the long-term robustness and enhanced measurement precision of the iCCD technique during the repeated thermal treatment cycles and pressure variations.

Unraveling the influential mechanism of short-chain branching on the crystallization of trimodal polyethylene by molecular dynamics simulation.

Trimodal polyethylene (PE) has become the focus of research in recent years due to its excellent performance. By means of molecular dynamics simulations, we aim to expound the molecular mechanism of short-chain branching (SCB) in the nucleation process, crystallization process and chain entanglement of trimodal PE. In this study, a series of polyethylene models including different short-chain branching concentrations (SCBCs), short-chain branching lengths (SCBLs), and short-chain branching distributions (SCBDs) were considered. The increase of SCBCs greatly reduces the ability of flipping and movement of PE chains, resulting in more time for nucleation and crystallization and a significant reduction of crystallinity. In contrast, an increase in the SCBL only slightly slows down the diffusion rate of the chain, which leads to a little increase in crystallization time. Most important of all, in the study of SCBD, we find that the distribution of SCBs on a high molecular weight chain, which is the characteristic of trimodal PE, is conducive to the chain entanglement and prevents the occurrence of micro phase separation compared with the case where the SCBs are distributed on a medium molecular weight chain. The mechanism of chain entanglement is proposed to explain the effect of SCBs on tie chain entanglement.

Modeling of the molecular weight distribution and short chain branching distribution of linear low-density polyethylene from a pilot scale gas phase polymerization process

Molecular weight distribution (MWD) and short chain branching distribution (SCBD) play a decisive role in the performance of linear low-density polyethylene (LLDPE). A model based on a non-standard polymerization reaction scheme is developed for ethylene/α-olefin gas phase copolymerization in a fluidized bed reactor. Experimental MWD and SCBD are simultaneously deconvoluted. The results obtained by this simultaneous deconvolution method are physically more relevant. Reduction of parameters and parameter estimability analysis are conducted to screen parameters to obtain a subset for reliable parameter estimation. The model based on the estimated parameters yields MWD and SCBD which are in good agreement with experimental ones for several LLDPE grades obtained from a pilot-scale gas phase polymerization process. The non-standard polymerization reaction scheme depicts well the “comonomer effect” - activation of dormant ethylene species.

Development of a robust calibration method for improving the long-term precision of polyethylene comonomer content distribution using crystallization elution fractionation

Comonomer content distribution (CCD), also commonly known as chemical composition distribution (CCD) and short chain branching distribution (SCBD), describes the variation of short chain branching composition between individual polymer chains in polyolefin materials. It is of particular importance for controlling polyolefin performance. Crystallization-based separation methods have evolved over the past four decades aiming at resolution, speed, precision, and accuracy. Two of the commonly used techniques are Crystallization Elution Fractionation (CEF) and Temperature Rising Elution Fractionation (TREF), where polymer chains are physically separated along the column or on the surface of the support based on their crystallinity, respectively. CEF analysis takes much less time than TREF. There is a critical need for precise temperature calibrations for data repeatability. This report demonstrates a novel CEF methodology using a two-point technique to consistently and conveniently calibrate the comonomer composition and column temperature. This column temperature calibration methodology was adopted in a study by tracking the reproducibility over a period of 8 years, using multiple instruments located in different laboratories and in different geographies. The results exhibited superior repeatability, with less than 0.3% of the relative error calculated from 3000 data points of the eluting peak temperature, thus demonstrating this as a robust method for industrial labs that require good quality controls.

Organic Versus Inorganic Supports for Metallocenes: The Influence of Rigidity on the Homogeneity of the Polyolefin Microstructure and Properties

A procedure to obtain very uniform polyolefin copolymers by supported metallocenes is presented. Conventional metallocene or Ziegler–Natta catalysts, both immobilized on inorganic supports, yield only copolymers with inhomogeneous comonomer incorporation and broad short-chain branching distribution. The main reasons are diffusional limitations of the monomers or the multisite character of the catalysts. By comparing inorganic and organic supports, we demonstrate that metallocenes immobilized on organic supports solve these problems. In this regard, organic and soft nanosized polystyrene particles (nPS) versus industrially used, hard, and inorganic SiO₂ were used to support [Me₂Si(Ind)₂ZrCl₂/MAO (I) and Me₂Si(Benz[e]-Ind)₂ZrCl₂/MAO (BI)] catalysts for ethylene/1-hexene copolymerization. In the inorganic case, the catalyst systems show a substantial inconsistency in the copolymers’ branching distribution, resulting in phase separation. One phase is hexene-poor with high melting temperature (Tₘ) and high molecular weight (MW). The second, hexene-rich phase, however, shows lower Tₘ and MW. By using organic supports, comonomers are uniformly inserted into the polymer chain and homogeneous microstructured copolymers are obtained. These findings are mainly attributed to diffusion processes of the monomers into the soft organic material. To prove this conclusion and to elucidate the structure of the catalyst system, various characterization techniques such as time-of-flight secondary ion mass spectrometry and scanning electron microscopy–energy-dispersive X-ray were performed.

Long-term mechanical performance of polyethylene pipe materials in presence of carbon black masterbatch with different carriers

In this paper, two types of carbon black (CB) masterbatch with different carriers, i.e. HDPE and LDPE, are used to produce black compounds using three PE100 resins with various short chain branching distributions. Due to difference in short chain branch (SCB) distribution, the used polyethylene resins behave differently in microstructure development and long-term creep behavior. The microstructural analysis using different DSC techniques and rheological measurements revealed more sensitivity of the polyethylene resin with uniform comonomer distribution to the carbon black aggregates and their polymeric carriers. The Full Notched Creep Test (FNCT) was performed to determine the long-term creep performance of the black compounds; it is shown that the sample having more uniform comonomer distribution is more resistant compared to other samples. On the other hand, by addition of carbon black masterbatch, resistance to slow crack growth in samples decreases since carbon black aggregates can act as stress concentration spots in the structure. However, with addition of the masterbatch with LDPE carrier polymer, the reduction of this value in samples is lower compared to one with HDPE carrier. The reason for this observation is that long branches of LDPE polymer enter the structure of lamellae in the PE100 resins, making them more coherent and increasing the number of tie molecules. The samples that are blended with LDPE polymer have a rougher surface, which means linkage between two sides of crack was stronger due to higher entanglement density in these samples. The impact test confirms the same trend as FNCT test, with the sample containing LDPE carrier having higher impact strength.

Using Artificial Intelligence Techniques to Design Ethylene/1‐Olefin Copolymers

AbstractFour global optimization techniques, genetic algorithm, particle swarm, improved ant colony, and modified artificial bee colony, are compared to find alternative polymerization conditions to make ethylene/1‐olefin copolymers with targeted microstructures and polymerization yields. The polymer microstructure targets are divided in three groups: 1) molecular weight distribution, chemical composition distribution, and polymer yield; 2) number and weight average molecular weights, average comonomer content, and polymer yield; and 3) molecular weight distribution, short chain branching distribution, and polymer yield. The modified artificial bee colony optimization generated the fewest number of incorrect solutions, while the polymer microstructure target group 1 generated the most successful solutions.

Analyses of Short Chain Branches in Polyolefins with Improved 1H NMR Spectroscopy.

Polyolefin microstructures, for example, short chain branching (SCB) and short chain branch distribution (SCBD), have a direct impact on properties and thus ultimately influence end-use applications. The 1H NMR approach to analyze SCB and SCBD is particularly useful when only a limited amount of sample is available, for example, polyolefin film layers or the fractions from polyolefin separation techniques, such as gel permeation chromatography (GPC), crystallization elution fractionation (CEF), high temperature liquid chromatography (HTLC), and thermal gradient interaction chromatography (TGIC). In this paper, we discuss the best approach to find a good decoupling frequency and propose an improved 1H pulse sequence with homonuclear decoupling for better measuring SCB. With this new pulse it is possible to reach a S/N of 10 (level of quantification) for the methyl signal from SCB in an ethylene-hexene copolymer (EH, 3.6 mol % H) in 3.5 min with 0.5 μg of sample. We also show an easy method to calculate SCB/1000C and demonstrate the proper use of heteronuclear single quantum coherence (HSQC) to measure SCB in a complicated system. A very quick approach to examine the presence of a small amount of LDPE in a polyolefin sample is also suggested, which can reduce NMR acquisition time from a couple of days to a few minutes.

A Short‐Chain Branching Distribution Determination Technique for Polyethylene Using IR5‐Detected GPC

AbstractIn this paper, a robust short‐chain branching (SCB) determination technique recently developed using IR5‐detected GPC (GPC‐IR5) for polyethylene resins is reported, wherein the IR5 detector is an infrared detector equipped with five filters set at different frequency ranges and a mercury‐cadmium‐telluride detector with thermoelectric cooling capability. Both intensity ratios of methyl/methylene and methyl/all C─H groups have very good linear relationship with the SCB content. Based on these relationships, an excel spreadsheet method is developed to deduce the SCB distribution across the molecular weight distribution using in‐house developed algorithms. The uncertainty of SCB determination using the GPC‐IR5 technique is found to be chromatographic signal‐to‐noise ratio dependent and can be explicitly expressed and calculated slice by slice. The SCB uncertainty is also found to be a function of experimental conditions and environment, which can affect not only the signals but also the noise levels at different IR bands. At a given polymer concentration, the higher the SCB content, the smaller the relative error bars. The SCB detection limit of this GPC‐IR5 technique is found to be at ≈0.5–1.0 SCB per 1000 TC for all samples tested. Some practical aspects for application of this technique are also discussed in this paper.

Polymerization Kinetics and Microstructure of Ethylene/1‐Hexene Copolymers Made with Dual Metallocenes

AbstractMathematical models are developed to describe the polymerization of ethylene and 1‐hexene with a constrained geometry catalyst (CGC‐Ti) and with bis(cyclopentadienyl)‐zirconium (IV) dichloride (Cp2ZrCl2). Particle swarm optimization is used to fit these models to homo‐ and copolymerization data. The models are also used to describe copolymerizations with mixtures of CGC‐Ti and Cp2ZrCl2 to make copolymers with inverse short chain branching distribution. Copolymer molecular weight and short chain branch distributions, as well as polymerization rates with the dual metallocene system, are measured to test whether they agreed with model predictions. The results show that the two metallocenes do not interact strongly when used as a mixture to make ethylene/1‐hexene copolymers.

Dominant Effects of Short-Chain Branching on the Initial Stage of Nucleation and Formation of Tie Chains for Bimodal Polyethylene as Revealed by Molecular Dynamics Simulation.

The molecular mechanism of short-chain branching (SCB), especially the effects of methylene sequence length (MSL) and short-chain branching distribution (SCBD) on the initial stage of nucleation, the crystallization process, and particularly the tie chain formation process of bimodal polyethylene (BPE), were explored using molecular dynamics simulation. This work constructed two kinds of BPE models in accordance with commercial BPE pipe resins: SCB incorporated in the long chain or in the short chains. The initial stage of nucleation was determined by the MSL of the system, as the critical MSL for a branched chain to nucleate is about 60 CH2. SCB incorporated in the long chain led to a delay of the initial stage of nucleation relative to the case of SCB incorporated in the short chains. The increase of branch length could accelerate the delay to nucleation. The location of short chain relative to the long chain depended on the MSL of the short chain. As the MSL of the system decreased, the crystallinity decreased, while the tie chains concentration increased. The tie chains concentration of the BPE model with branches incorporated in the long chain was higher than that with branches incorporated in the short chain.

Studies on the use of filter-based IR detector for short-chain branching characterization of polyolefin copolymers with high temperature size exclusion chromatography

Even though polyolefins are simple polymers from chemical structure point of view, their full characterization in practice is still an intriguing task. Basic macromolecular characteristics of polyethylene or polypropylene like molar mass moments and their distributions (MMD) but also chain conformation and thus information on long-chain branching, can be addressed with high-temperature size-exclusion chromatography (HT-SEC). Hyphenation of infrared detection to size-exclusion chromatography expands possibilities of SEC even more and allows to reveal comonomer incorporation across molecular weight and thus help in the study of catalytic systems used in polyolefin synthesis.Multiband filter-based infrared detector gives an easy and fast access to so-called short chain branching distribution (SCB) vs MMD by coupling to HT-SEC. In our previous work [31], we summarize recent findings on application of a filter-based detector towards characterization of polyolefins synthesized with different catalytic systems and varying comonomer types. In this work, an influence of the comonomer type on the detector calibration line is elaborated. Statistical evaluation of different calibration lines is discussed in detail. Based on this evaluation practical aspects of filter-based IR detectors calibration are mentioned. Finally, numerical computer simulations can predict how much calibration lines deviate from linearity based on the spectra of the polymer and on the bandwidth of the detector filters.

LDPE Production in Tubular Reactors: Comprehensive Model for the Prediction of the Joint Molecular Weight-Short (Long) Chain Branching Distributions

Good control on molecular properties, and as a consequence on end-use properties, is very important for low-density polyethylene (LDPE) manufacturers. However, the connection between the architecture of polymer chains and the kinetic mechanism and polymerization conditions is still a subject of study. In this work, we present a comprehensive model of the polymerization of ethylene in high-pressure tubular reactors. In addition to the usual predictions of conversion, temperature profiles, and average molecular properties, this model also provides bivariate distributions such as molecular-weight–long-chain branching distribution and molecular-weight–short-chain branching distribution of LDPE produced under different operating conditions. The 2D probability-generating function technique is applied to obtain the bivariate distributions. This is a deterministic technique that allows the calculation of the distributions without any prior assumption of their shape.

A Dynamic Simulator for Slurry‐Phase Catalytic Olefin Copolymerization in a Series of CSTRs: Prediction of Distributed Molecular and Rheological Properties

AbstractA comprehensive mathematical model is developed to simulate the dynamic Ziegler–Natta ethylene‐α‐olefins copolymerization in a series of slurry‐phase continuous stirred tank reactors. A generalized multisite kinetic mechanism is considered to describe the molecular and compositional developments (joint molecular weight‐copolymer composition distribution) in a series of continuous stirred tank reactors (CSTRs). Dynamic macroscopic mole balances are derived to calculate the dynamic evolution of all species concentrations in three phases (i.e., gas, liquid, and polymer) of the multiphase system. The polymer molecular properties (i.e., molecular weight distribution, short chain branching distribution, etc.) are determined by two different approaches, namely, the method of double moments and a Monte Carlo algorithm. Detailed thermodynamic calculations are carried out to calculate the solubility of all species in the various phases. Particular emphasis is given to the calculation of mass transfer rates from the gas to the continuous liquid phase and from the liquid to the polymer swollen phase. Comparison of model predictions with experimental measurements has shown that the present plant simulator is capable of simulating the dynamic operation of ethylene with α‐olefins copolymerization over a broad range of operating conditions and, thus, can be used in future process and product optimization and control of lab‐, pilot‐, and industrial‐scale plants.

Correlating polyethylene microstructure to stress cracking: Development of primary structure parameters

New primary structure parameters (PSP3 and PSP3*) were developed to help better understand the effects of polymer microstructure on polyethylene stress cracking as determined by post yield tensile tests. Previously, the structure parameter, PSP2, was obtained using only molecular weight distributions (MWD) and short chain branching distributions (SCBD) obtained from size exclusion chromatography coupled to a Fourier transform inferred detector (SEC-FTIR). By augmenting the MWD and SCBD with differential scanning colorimetry (DSC) analysis to better estimate crystalline lamella distributions, a more robust process was developed to estimate the critical lamella distance, L, that a molecule must span to form a tie molecule. Improvements were also made to estimate the end to end distance, R, by including SCB effects on persistence length, and consideration for the probability of more than one tie molecule per polymer chain was included in the calculation. By generating a three dimensional, digital, crystalline-lattice and using random walk statistics, PSP3 and PSP3* values were estimated for various monomodal and bimodal copolymers as well as homopolymer resin sets. Furthermore, a new parameter; the average number of crystalline lamella per polymer chain (ANCC) was developed. The entire method required low computational time, while allowing us to investigate very large molecular weights. These latter parameters (PSP3* and ANCC) capture for the first-time, ties formed between multiple lamella by a single polymer chain. Good correlations between the strain hardening modulus (ISO, 18488 test standard) and PSP3* were found as well between ANCC−1 values and the % natural draw ratio (NDR) at 80 °C.

Effect of short-chain branching on the tie chains and dynamics of bimodal polyethylene: Molecular dynamics simulation

Molecular dynamics simulations were performed on bimodal polyethylene (BPE) in order to shed light on the molecular mechanism of short-chain branching on BPE nucleation process, crystallization process and especially the formation process of tie chains. The study contemplates two kinds of BPE model chains with the same branch content (BC), branch length (BL) but with different short-chain branching distribution (SCBD): short chain branches incorporated in either the long chain or the short chain. For the nucleation process, nucleation time of all models increased with the increase of BC. Short branches distributed on the long chain provoke a delay of the nucleation as compared to the case of short branches distributed on the short chain. Additionally the nucleation process is further delayed as the BL increases. Crystallization rate and crystallinity decrease as BC increases. The crystallinity of the BPE mode chains with branches placed on the long chain were lower than the case of branches placed on the short chain above a critical BC (5/1000C), and this critical BC was independent with BL. When branches distribute on the long chain, short chain is more likely to locate inside the long chain, while for the case of branches distribute on the short chain, the results are opposite. The concentration of tie chains increased with the increase of BC. For BPE model chains with branches placed on the long chains are more likely to form tie chains when compared with the case of branches placed on the short chain. The concentration of tie chains is insensitive to BL. A modified tie chains formation process mechanism is proposed to explain the SCBD effect on tie chains.

Polyolefins Made with Dual Metallocene Catalysts: How Microstructure Affects Polymer Properties

AbstractSlow crack growth resistance correlates with the lifetime of polyethylene pipes. Since this measurement may take considerable time, alternative ways to estimate it help expedite the development of new polyethylene resins. The primary structural parameter (PSP2), calculated from the distribution of molecular weight and short chain branching of polyethylenes, can predict the stress crack growth resistance of polyethylene resins without needing to measure it over long periods of time. In this investigation, a model is developed to calculate PSP2 of ethylene homopolymers and ethylene/1‐hexene copolymers made with two metallocene catalysts. For each metallocene, the effect of the following parameters on PSP2 is investigated: (1) polymer mass fraction, (2) polymer average molecular weight, and (3) average comonomer fraction. The simulation trends agree with previously published experimental results. The generated 3D surface response plots are useful guides for the design of polyethylenes having optimized slow crack growth resistance.

A Novel SiO2‐Supported Fluorine Modified Chromium–Vanadium Bimetallic Catalyst for Ethylene Polymerization and Ethylene/1‐Hexene Copolymerization

AbstractPhillips catalyst is one of the most significant industrial ethylene polymerization catalysts. Chemical modifications have been carried out to tune the Phillips catalyst performance and improve the polyethylene properties. After the modification of the catalyst by fluorine, the polyethylene product with higher molecular weight (MW) and narrower molecular weight distribution (MWD) is suitable for producing automobile fuel tanks. Vanadium containing Phillips catalyst enhances α‐olefin incorporation and MW regulation. In present work, fluorine modified and unmodified chromium–vanadium (Cr–V) bimetallic catalysts are prepared and explored. Compared with the fluorine‐free catalyst, the activities of F‐modified bimetallic catalysts slightly decrease with the increasing MW of the product and the hydrogen response increases slightly. Due to the synergistic effect of the chromium, vanadium and fluorine on the silica gel support, the short‐chain branch distribution (SCBD) of copolymers from F‐modified Cr–V bimetallic catalyst (Cr–V–F)600 is more beneficial than that of Cr–V bimetallic catalyst (Cr–V)600 and F‐modified Cr–V bimetallic catalyst (Cr–V–F)500. The fluorination of Cr–V bimetallic catalysts has not only preserved the high polyethylene activity of bimetallic active sites but also produced the advantage of the high MW ability from fluorine. image

Dynamic mechanical analysis of ethylene/1‐hexene copolymers: The effect of the catalyst type on the short‐chain branching distribution and properties of the amorphous and crystalline phases

ABSTRACTDynamic mechanical analysis was used to study ethylene/1‐hexene copolymers with different compositions, molecular weight distributions, and profiles of short‐chain branching (SCB) versus molecular weight. These copolymers were produced over a highly active supported titanium–magnesium catalyst (TMC), a highly active supported vanadium–magnesium catalysts (VMC), and a supported zirconocene catalyst. A higher fraction of the crystalline phase in the copolymers prepared with VMC was shown to result in higher elastic modulus values. β relaxation was found to be sensitive to the SCB distribution versus the molecular weight. The copolymers prepared with the zirconocene catalyst and VMC were characterized by more uniform SCB distributions and higher temperatures of β relaxation compared to the copolymers prepared with TMC. The mobility of the polymer chains at room temperature in the amorphous phase obtained by the spin‐probe method rose with increasing branch content in the copolymers and was not sensitive to different SCB distribution profiles. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44638.