Crystallization Analysis Fractionation Research Articles

CRYSTAF, DSC and SAXS Study of the Co-Crystallization, Phase Separation and Lamellar Packing of the Blends with Different Polyethylenes.

The crystallization of polyethylene (PE) blends is a highly complex process, owing to the significant differences in crystallizability of the various PE components and the varying PE sequence distributions resulting from short- or long-chain branching. In this study, we examined both the resins and their blends through crystallization analysis fractionation (CRYSTAF) to understand the PE sequence distribution and differential scanning calorimetry (DSC) to investigate the non-isothermal crystallization behavior of the bulk materials. Small-angle X-ray scattering (SAXS) was utilized to study the crystal packing structure. The results showed that the PE molecules in the blends crystallize at different rates during cooling, resulting in a complicated crystallization behavior characterized by nucleation, co-crystallization, and fractionation. We compared these behaviors to those of reference immiscible blends and found that the extent of the differences is related to the disparity in crystallizability between components. Furthermore, the lamellar packing of the blends is closely associated with their crystallization behaviors, and the crystalline structure varies significantly depending on the components' compositions. Specifically, the lamellar packing of the HDPE/LLDPE and HDPE/LDPE blends is similar to that of the HDPE component owing to its strong crystallizability, while the lamellar packing of the LLDPE/LDPE blend is approximately an average of the two neat components.

Graphite-LDH hybrid supported zirconocene for ethylene polymerization: Influence of the support on the crystallization kinetics and thermal stability of polyethylene

A hybrid nanomaterial, low crystallinity graphite-layered double hydroxide, was used as a support for zirconocene catalyst. The hybrid nanomaterial graphite-NiAl layered double hydroxides (G/LDH) were synthesized by the co-precipitation method. The synthesized nanomaterial was used as a support for zirconocene catalyst. The polymerization reactions were carried out for ethylene polymerization. The synthesized polyethylene (PE) was analyzed by using differential scanning calorimetry (DSC), Crystallization analysis & fractionation (CRYSTAF), and thermogravimetric analysis for its thermal and microstructural characteristics. The crystallization kinetics were studied by the Ozawa and combined Ozawa and Avrami models. It was found that the presence of G/LDH from the catalyst support nucleated the PE crystallization and shifted the crystallization onset temperature to a higher value. However, the overall crystallization rate was slowed by the presence of the nanomaterial due to growth impingement. Moreover, the PE synthesized by G/LDH supported catalyst possessed higher thermal stability than PE synthesized by unsupported zirconocene catalyst. The integral isoconversional method was used to evaluate the activation energy of thermal degradation and crystallization kinetics. The degradation mechanism was validated by the application of the integral master plot technique. The degradation mechanism of neat PE resembled phase boundary controlled mechanism second and third-order, i.e. (R2, R3), while PE synthesized by G/LDH supported catalyst had a shift in degradation mechanism from (R2, R3) to a diffusion-limited mechanism at the later stages of degradation.

Microstructure Characterization of Amorphous Phase of Polypropylene Impact Copolymers

AbstractIn this work, three polypropylene impact copolymers (ICPs) with different chemical compositions are characterized using gel permeation chromatography (GPC) with triple detection: infrared (IR), viscometer (VIS) and multi angle light scattering (MALS), crystallization analysis fractionation (CRYSTAF), thermal gradient interaction chromatography (TGIC), nuclear magnetic resonance (13C‐NMR) and atomic force microscopy (AFM). The average rubber composition is determined and correlated with the ICP morphology and final mechanical properties. GPC‐IR is used to analyze the ethylene content and distribution along the molecular weight distribution (MWD). The amorphous phase composition distribution is analyzed as a function of temperature by TGIC. The microstructure and morphology of ICP amorphous phase are studied in detail by 13C‐NMR, GPC 3D, TGIC, SEM and AFM allowing to differentiate the samples.

Atypical Multiple Site Behavior of Hafnocene Catalysts in Ethylene/1-Hexene Copolymerization Using Trioctylaluminum and Borate

Solution copolymerizations of ethylene and 1-hexene were performed using the dichloride (HfCl2) and dimethyl (HfMe2) analogues of the bis(n-propylcyclopentadienyl)hafnium precatalyst. Tetrakis(pentafluorophenyl) borate dimethylanilinium salt ([B(C6F5)4]−[Me2NHPh]+) (B) or methylaluminoxane (MAO) was employed as activator, and tri-n-octylaluminum (TOA) was used as scavenger. Crystallization analysis fractionation (CRYSTAF) or crystallization elution fractionation (CEF) was used to measure the chemical composition distributions (CCD). Above a minimum 1-hexene threshold concentration, the CCD profiles of all copolymers were bimodal, and the areas under the peaks depended on the B/HfCl2 and B/Al ratios. Decreasing the TOA concentration reduced the weight fraction of the higher crystallinity copolymer. Copolymerizations using MAO also produced copolymers with bimodal CCDs. Copolymerizations using HfMe2/B without TOA produced copolymers with unimodal CCD. These results suggest that TOA reacts with either the catalyst or the activator, generating a new active site with a lower propensity for incorporating comonomer than the original catalyst system.

Photooxidative degradation of graphene‐reinforced high‐density polyethylene nanocomposites

ABSTRACTNatural weathering of high‐density polyethylene (HDPE) and its graphene nanocomposites was studied. The nanocomposites, prepared by in situ polymerization of ethylene, were exposed to the natural environment for 30, 60, and 90 days. The effects of weathering on the crystallinity, melting temperature, and lamellar thickness distribution were studied using differential scanning calorimetry. The changes in chain microstructure and molecular weight were determined by crystallization analysis fractionation and gel permeation chromatography, respectively. The effects of weathering on the dynamic mechanical properties and the morphology of the nanocomposite were also determined. The parent HDPE suffered severe degradation as expected. However, graphene‐reinforced HDPE nanocomposites show resistance to degradation. The changes in the crystallinity, lamellar thickness, molecular weight, and dynamic mechanical properties of graphene nanocomposites were less pronounced as compared to the parent HDPE. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47030.

Comprehensive branching analysis of polyethylene by combined fractionation and thermal analysis

AbstractLinear low density polyethylene (LLDPE) and low density polyethylene (LDPE) differ significantly in their branching types and branching distributions. For a comprehensive analysis, preparative temperature rising elution fractionation and/or preparative molar mass fractionation are used to fractionate typical LLDPE and LDPE bulk resins into narrowly distributed fractions. The chain structures of the bulk resins and their fractions are further analysed using SEC, crystallization analysis fractionation, DSC and high‐temperature HPLC to provide detailed information on short chain branching in LLDPE and long chain branching in LDPE. For LDPE it is shown that the multiple fractionation approach is a powerful source of sample libraries that may have similar molar masses and different branching structures or alternatively similar branching but different molar masses. The analysis of these library samples by thermal analysis provides a much deeper insight into the molecular heterogeneity of the samples compared to bulk sample analysis. © 2018 Society of Chemical Industry

Characterizing Long Chain Branching in Polypropylene

In order to improve elongational viscosity of polypropylene, allowing its use in a wide range of applications, different samples of branched polypropylene have been produced by reactive extrusion and evaluated by different techniques. It is known that the long chain branching (LCB) can act as a nucleating agent in the crystallization process. This is confirmed by differential scanning calorimetry (DSC), optical microscopy, and crystallization analysis fractionation (CRYSTAF), where an increase in the crystallization temperature can be observed when comparing linear and branched samples. In the chemical composition distribution profile of the analyzed samples, it is possible to see differences in the amount of crystallized material among the branched PP samples and it is possible to correlate it with the long chain branching content and distribution, measured by Triple Detector Gel Permeation Chromatography (3D‐GPC), using the Mark‐Houwink plot (MH Plot), conformation plot, and gpcBR methodology. A decrease in dissolution temperature, and an increase in soluble fraction is observed by temperature rising elution fractionation (TREF) at increasing LCB content in the polymer samples. These results indicate that the presence, amount, or distribution of long chain branching affects the crystallization kinetics and the crystal morphology. CRYSTAF and TREF show a good correlation with LCB content measured by GPC.

Ternary Solid–Liquid Equilibria of Semicrystalline, Branched Polymer Solvent Mixtures—A Theoretical Study by Means of Lattice Cluster Theory

Analytical polymer separation techniques, like CRYSTAF (crystallization analysis fractionation) and TREF (temperature rising elution fractionation), play a crucial role in gaining insight into the interrelation between molecular architecture and properties. Often polymer samples are semicrystalline, as it consists of amorphous regions which exhibit a liquid-like structure, and ordered crystallized structures called crystallites. Additionally, polymers exhibit a certain degree of short-chain branching, and they are polydisperse with respect to molecular weight and chemical composition. To explain the theoretical background of common polymer separation techniques, a fair prediction of solid–liquid equilibria (SLE) of polymer solvent systems is required. By application of Lattice Cluster Theory (LCT), developed by Freed and co-workers, the molecular architecture can be considered. Recently, LCT was applied to calculate the SLE of binary semicrystalline, high-polymer solvent mixtures having an arbitrary molec...

Polyethylenes in blown films: Effect of molecular structure on sealability and crystallization kinetics

Three types of polyethylene (PE), with different molecular structure, were selected and used in the production of films using a semi‐industrial blown film extrusion line. Carbon nuclear magnetic resonance and crystallization analysis fractionation tests were performed on the PE polymers to assess their molecular structure. Sealability properties of the films, as well as crystallization kinetics, were evaluated and the results are discussed according to the molecular structures of the polymers. It was found that distribution of short chain branching (average comonomer sequence nH or nO) on the backbone of the PE chains are the main factor controlling the beginning of sealability. The completely random placement of the comononer on the polymer backbone (monomer dispersity 100%) generated crystals with smaller size and a lower melting temperature. Sealing was controlled by crystal distribution, chain diffusion, and entanglement formation at the interface. PE with lower melting point and linear molecular structure showed greater hot tack strength. PE with long chain branching showed a delay on seal process leading to lower sealing speed. POLYM. ENG. SCI., 57:52–59, 2017. © 2016 Society of Plastics Engineers

Separation of maleic anhydride grafted polypropylene using multidimensional high-temperature liquid chromatography

Functionalization addresses a property gap of polyolefins and opens new perspectives due to improved surface properties in applications like composites (e.g., glass fiber reinforced polypropylene) and anti-corrosive coatings for metals. Various techniques have been developed to characterize functionalized polyolefins, yet no analytical approach addressing their chemical heterogeneity exists. Using High Temperature Size Exclusion Chromatography (HT-SEC) coupled to infrared spectroscopy we could show for two model samples of polypropylene grafted maleic anhydride (PP-g-MA), differing in their nominal MA content, that the grafting density increases with decreasing molar mass. Crystallization Analysis Fractionation (CRYSTAF) does not enable to separate these samples according to their composition to the extent required. Yet, when using High Temperature High Performance Liquid Chromatography (HT-HPLC), with either silica gel or Mica as stationary phase and a gradient mobile phase, a deformulation into a grafted and a non-grafted fraction could be achieved. This was confirmed by analyzing the eluted fractions by infrared spectroscopy. Hyphenating the separation according to composition with a separation according to molar mass (HT-HPLC x HT-SEC) enabled for the first time to reveal the bivariate distribution of PP-g-MA with regard to the molar mass and composition. Using on-line infrared detection quantitative information on the compositional and molar mass parameters of the individual fractions could be obtained.

Exploring the Compositional Heterogeneity of Vis‐Broken Impact Poly(propylene) Copolymers by Advanced Fractionation Methods

Vis‐breaking or rheology control is an important technical process to improve the processability of impact poly(propylene) copolymers (IPCs). In the vis‐breaking process the molar mass and its dispersity are reduced and the crystallinity changes and polar carbonyl functionalities are introduced due to the reaction of the polymer with peroxide. Although the fundamental principles of vis‐breaking are well understood, not much research has been devoted to the investigation of the molecular changes brought about by the vis‐breaking process. In the present study, the effect of vis‐breaking on the molecular parameters of IPCs is elucidated using advanced fractionation methods. After the analysis of the bulk sample properties, the samples are fractionated using preparative temperature rising elution fractionation and the fractions are analyzed by Fourier transform infrared spectroscopy, differential scanning calorimetry, crystallization analysis fractionation, and high temperature high performance liquid chromatography techniques. For the first time, an effective multidimensional analytical approach is established to study compositional heterogeneities in vis‐broken IPC materials. image

Describing and quantifying the chemical composition distribution in unimodal and multimodal ZN‐polyethylene using CRYSTAF

ABSTRACTA new approach in describing and quantifying the chemical composition distribution (CCD) in Ziegler–Natta‐polyethylene copolymers was developed by using crystallization analysis fractionation (Crystaf). Copolymers of ethylene and α‐olefins (1‐butene and 1‐hexene) polymerized with different ZN catalyst systems were analyzed. Distinct differences in the CCD between the different polymer types (catalyst‐cocatalyst system and comonomer type) were observed and could be quantified. Same approach was applied to 2‐dimensional fractionation technique, cross fractionation chromatography, to describe and quantify the CCD of multimodal polyethylene copolymers. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43089.

On the Homogeneity of Metallocene Ethylene–Propylene Copolymers as Investigated by Multiple Fractionation Techniques

The chemical heterogeneity of ethylene–propylene copolymers by multiple fractionation techniques is addressed. Three metallocene copolymer samples with different ethylene contents, ranging from 30 to 50 mol% are analyzed using bulk methods to confirm their molecular heterogeneity. In a second step, the samples are fractionated by temperature rising elution fractionation (TREF) to obtain fractions at 30, 60, 90, and 130 °C. These fractions are subsequently analyzed regarding their thermal and molecular properties. Differential scanning calorimetry, crystallization analysis fractionation, and high‐temperature high performance liquid chromatography (HT‐HPLC) results reveal that the TREF fractions collected at 130 °C are mainly due to polypropylene homopolymer, which is rather unexpected considering the high ethylene contents of the bulk samples. Most importantly, HT‐HPLC reveals a remarkably high chemical heterogeneity of the fractions and thus the bulk samples. Solution 13C NMR provides the comonomer contents and sequence distributions of the fractions. These indicate that the same TREF fractions from different samples have distinctively different chemical compositions. image

A Monte Carlo-based feeding policy for tailoring microstructure of copolymer chains: Reconsidering the conventional metallocene catalyzed polymerization of α-olefins

A standard Monte Carlo-based program with innovative data storage structure was developed and put into practice to tailor ethylene/1-hexene copolymers through semibatch single site metallocene catalyzed copolymerization. The distribution of copolymer composition, ethylene sequence length, longest ethylene sequence length, as well as the number-average degree of polymerization, and sequential placement of 1-hexene comonomer segments along copolymer chains were monitored and evaluated applying a computerized feeding to metallocene catalyzed ethylene/1-hexene copolymerization. In particular, bivariate copolymer composition–chain length (CC–CL) distribution was compared for two feeding recipes with uncontrolled and well-controlled comonomer insertion. The advantages of controlled feeding in comparison to uncontrolled feeding were discussed in view of aforementioned architectural features. To obtain macromolecules with tailored comonomer distributions, special feeding strategies were developed by training and examining the developed model to capture crystallization analysis fractionation (CRYSTAF) of ethylene/1-hexene chains, as a unique signature of tailored copolymers with rather narrow bivariate CC–CL distribution. The simulation results appropriately highlight the critical importance of computerized feeding with respect to uncontrolled feeding.

Characterization of the Chemical Composition Distribution of Ethylene/1‐Alkene Copolymers with HPLC and CRYSTAF—Comparison of Results

A series of copolymers of ethylene with propene, 1‐hexene, 1‐octene, and 1‐octadecene is characterized by size‐exclusion chromatography (SEC), nuclear magnetic resonance spectroscopy (NMR), crystallization analysis fractionation (CRYSTAF), and high‐temperature interactive liquid chromatography. Four different solvent pairs are used as the mobile phase, while porous graphite is used as the column packing. The elution volumes of the polymer samples do not correlate with their average molar mass (SEC); however, they correlate with the average chemical composition (NMR). High performance liquid chromatography (HPLC) enables separation of the copolymers over the full range of their composition and independent of their crystallinity. Dependence between the elution volume and the average chemical composition distribution (CCD) of the copolymer is linear and it is a function of the length of branches as well as the type of the mobile phase. The CCDs of copolymers derived from HPLC profiles are similar to, yet broader than the CCDs obtained with CRYSTAF. image

Molecular heterogeneity of ethylene-propylene rubbers: New insights through advanced crystallization-based and chromatographic techniques

For a long time ethylene-propylene rubber (EPR) copolymers with high comonomer contents were believed to be amorphous materials with a random copolymer composition. This is not completely correct as has been shown by temperature rising elution fractionation (TREF) combined with differential scanning calorimetry (DSC), crystallization analysis fractionation (CRYSTAF), and high temperature–high-performance liquid chromatography (HT-HPLC). When using only conventional crystallization-based fractionation methods, the comprehensive compositional analysis of EPR copolymers was impossible due to the fact that large fractions of these copolymers do not crystallize under CRYSTAF conditions. In the present work, HT-HPLC was used for the separation of the EPR copolymers according to their ethylene and propylene distributions along the polymer chains. These investigations showed the existence of long ethylene sequences in the bulk samples which was further confirmed by DSC. The results on the bulk samples prompted us to conduct preparative fractionations of EPR copolymers having varying ethylene contents using TREF. Surprisingly, significant amounts of crystallizing materials were obtained that were analyzed using a multistep protocol. CRYSTAF and DSC analyses of the TREF fractions revealed the presence of components with large crystallizable sequences that had not been detected by the bulk samples analyses. HT-HPLC provided a comprehensive separation and characterization of both the amorphous and the crystalline TREF fractions. The TREF fractions eluting at higher temperatures showed the presence of ethylene-rich copolymers and PE homopolymer. In order to obtain additional structural information on the separated fractions, HT-HPLC was coupled to Fourier transform-infrared (FT-IR) spectroscopy. The FT-IR data confirmed that the TREF fractions were separated according to the ethylene contents of the eluted samples. Preparative TREF analysis together with a combination of various analytical methods proved to be useful tools in understanding the complex molecular composition of these rubber samples. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 863–874

Use of solution crystallization analysis by laser light scattering for studying the solution crystallization of various Ziegler–Natta‐catalysed polypropylenes

AbstractSolution crystallization analysis by laser light scattering (SCALLS) involves the observation of the scattering of diode mercury laser lamp light after it passes through a polymer solution. An increase in turbidity occurs when the hot polymer solution is cooled and the polymer starts to crystallize out of solution. This causes a decrease in the amount of laser light that can pass through the solution and an increase in the amount of scattered light. The reverse of this process leads to the turbidity decreasing with an increase in temperature. According to this concept, it is possible to follow the solution crystallization of various polypropylenes under controlled cooling. In this study, SCALLS was able to differentiate between different isotactic and syndiotactic polypropylenes with similar chemical structures, but different tacticity and molecular weights. Furthermore, SCALLS provided good crystallization information that is similar to that from crystallization analysis fractionation and temperature rising elution fractionation. In addition, SCALLS can be used as a quantitative tool for the measurement of weight fractions during dissolution. © 2014 Society of Chemical Industry

Effect of calcium zirconate nanofiller on Polyethylene productivity and properties

Polyethylene nanocomposites were prepared by in-situ polymerization technique by using highly active metallocene catalyst zirconocene (Cp2ZrCl2) in combination with methylaluminoxane (MAO). The influence of different concentrations (0, 0.6,0.83,1.6 and 2.5 wt.%) of nano calcium zirconate on catalytic activity and polymer properties was investigated. There was a steep increase in the activity of zirconocene catalyst (from 137 to 185 × 10−3 g-PE/mol h bar,) by addition 0.6 wt.% of the nanofiller The results of Differential Scanning Calorimetry (DSC), Crystallization Analysis Fractionation (CRYSTAF) and Wide Angle X-ray Diffraction (WAXD) show that the percentage crystallinity was influenced with respect to the nanofiller concentration. Microcalorimeter was used to study the effect of nanofiller on the pyrolysis of polyethylene. The fibrils morphology appearing on the fractured surface was reduced significantly with the addition of nanofiller as is evident from SEM morphology.

Synthesis and Characterization of Imino‐Phenolate Titanium Complexes and Their Use in Homo‐ and Copolymerization of Ethylene

A set of titanium precatalysts [(L)TiCl3] based on imino‐phenolate tridentate ligands (Ti1, L = 3,5‐tBu‐2‐(OH)C6H2CH‐N‐8‐C9H6N; Ti2, L = 3,5‐tBu‐2‐(OH)C6H2CH‐N‐C2H4‐O‐C6H5; Ti3, L = 3,5‐tBu‐2‐(OH)C6H2CH‐N‐CH2‐C6H4OMe; Ti4, L = 3,5‐tBu‐2‐(OH)C6H2CH‐N‐C6H4‐C6H5O) is prepared and characterized by elemental analysis, IR, and NMR spectroscopy. Upon activation with methylaluminoxane (MAO), these precatalysts show good activity (200–660 kg of PE mol[Ti]−1 h−1 at 60 °C) in ethylene polymerization yielding high‐density poly­ethylenes (HDPEs) with viscosity‐average molecular weight (Mv) in the range between 184 and 355 × 103 g mol−1. The Mv values of the resulting polyethylenes indicate strong influence of the aluminum concentration on the molecular weight, which can be associated to the probability of chain transfer from titanium to aluminum at high [Al]/[Ti] molar ratios. The polydispersity index (PDI) calculated from Gc(ωc) values are broad (24 to >40) indicating the multisite nature of this class of titanium precatalysts. Ti1–Ti4 are also tested in the copolymerization of ethylene with 1‐hexene affording high‐molecular‐weight poly(ethylene‐co‐1‐hexenes) with comonomer incorporation varying from 1.0 to 4.1 mol%. The crystallization analysis fractionation (CRYSTAF) profiles of copolymers produced by Ti2–Ti3 indicate the production of HDPE with very low efficiency (quantity and distribution) on incorporating 1‐hexene. On the other hand, the polymer produced by Ti1 shows a broad peak of branching fraction with different amount and comonomer distribution, suggesting a good 1‐hexene distribution along of main polymer chain. image

Preparative solution crystallization fractionation: a simple and rapid fractionation method for the chemical composition separation of complex ethylene-propylene copolymers

The molecular structure elucidation of complex ethylene-propylene copolymers (EPCs) has benefited tremendously from the ability to combine preparative temperature rising elution fractionation (prep TREF) with various conventional analytical techniques. Recently reported, prep TREF-high-temperature solvent gradient interaction chromatography (HT-SGIC) (Cheruthazhekatt et. al, Macromolecules 45:2025-2034, 2012) is one of the most effective and highly useful coupled methods that allow for the exact measurement of the chemical composition distribution (CCD) present in various components of EPCs. The major drawback of prep TREF involving slow crystallization and elution steps is the long time per experiment. Here, we present a new and by far the simplest and fastest preparative fractionation method for complex polyolefins-preparative solution crystallization fractionation (prep SCF). The scope of the present study was to achieve a fast fractionation of complex bulk samples into an amorphous, semicrystalline and highly crystalline fraction, in sufficient amounts for the subsequent detailed compositional analysis. The effects of two different solvents, xylene and trichlorobenzene (TCB), and their influence on the solution crystallization of chemically different components of EPC were systematically investigated by combining prep SCF with crystallization analysis fractionation (CRYSTAF), FTIR, differential scanning calorimetry (DSC) and HT-SGIC analyses. Significant differences in the chemical composition of similar SCF fractions obtained from xylene and TCB were observed indicating the strong influence of the solvent on solution crystallization. Prep SCF-HT-SGIC results showed that, under similar experimental conditions, TCB as the fractionation solvent provides superior separation of complex semicrystalline ethylene-propylene (EP) components. Very interestingly, for the first time, separation of soluble fractions (30°C) of iPP, EPC and PE homopolymer components in complex EPC was achieved by prep SCF in TCB. On the other hand, SCF fractionation in xylene provides a soluble fraction that is perfectly amorphous as has been shown by DSC and CRYSTAF. Based on these results, the present SCF approach and an updated method of the combination of prep SCF-HT-SGIC hold significant promise for the fractionation and characterization of similar complex EPCs in a simple way within a short analysis time, by using significantly smaller amounts of solvent compared to the previously reported, rather time-consuming, prep TREF-HT-SGIC combination. No similarly selective solution crystallization fractionations in preparative scale have been reported before.