Crystallization Elution Fractionation. A New Separation Process for Polyolefin Resins

Summary: New crystallization procedures have been developed for the analysis of the chemical composition distribution in polyolefins by pumping a small flow of solvent during the crystallization cycle. One of the new techniques, crystallization elution fractionation (CEF) combines the separation power of TREF and CRYSTAF and has been shown to provide very fast analysis of the composition distribution.     

Crystallization Elution Fractionation of LLDPEs Made with Metallocene Catalysts

Summary: Temperature rising elution fractionation (TREF) and crystallization analysis fractionation (CRYSTAF) fractionate semicrystalline polymers according to their crystallizabilities from dilute solution and have been widely used to measure the CCD of LLDPE. A new fractionation technique, known as crystallization elution fractionation (CEF), has been developed recently. The main difference between CEF and TREF and CRYSTAF is that the crystallization cycle in CEF is performed dynamically under solvent flow in a long column that contains an inert support material. In this paper, several metallocene-LLDPE resins have been analyzed by CEF to investigate the effect of cooling cycle parameters, comonomer fraction, polymer molecular weight, and blend cocrystallization on the fractionation. This new technique can be used to obtain CCDs with better resolution and in shorter times than TREF and CRYSTAF.     

Characterization of polypropylene�Cpolyethylene blends by temperature rising elution and crystallization analysis fractionation

The introduction of single-site catalysts in the polyolefins industry opens new routes to design resins with improved performance through multicatalyst-multireactor processes. Physical combination of various polyolefin types in a secondary extrusion process is also a common practice to achieve new products with improved properties. The new resins have complex structures, especially in terms of composition distribution, and their characterization is not always an easy task. Techniques like temperature rising elution fractionation (TREF) or crystallization analysis fractionation (CRYSTAF) are currently used to characterize the composition distribution of these resins. It has been shown that certain combinations of polyolefins may result in equivocal results if only TREF or CRYSTAF is used separately for their characterization.     

Simultaneous Deconvolution of the Molecular Weight and Chemical Composition Distribution of Polyolefins Made with Ziegler-Natta Catalysts

Heterogeneous Ziegler-Natta catalysts produce polyolefins that have broad distributions of molecular weight (MWD) and chemical composition (CCD). For such broad distributions, mathematical models are useful to quantify the information provided by polyolefin analytical techniques such as high-temperature gel permeation chromatography (GPC), temperature rising elution fractionation (TREF), and crystallization analysis fractionation (CRYSTAF). In this paper, we developed a mathematical model to deconvolute the MWD and CCD of polyolefins simultaneously, using Flory's most probable distribution and the cumulative CCD component of Stockmayer's distribution. We have applied this procedure to “model” polyolefin resins and to one industrial linear low-density polyethylene (LLDPE) resin. The proposed methodology is able to deconvolute theoretical distributions even when random noise is added to the MWDs and CCDs, and it can be used to calculate the minimum number of active site types on heterogeneous Ziegler-Natta catalysts.     

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     

Crystaf: Crystallization analysis fractionation. A new approach to the composition analysis of semicrystalline polymers

Crystallization Analysis Fractionation is a new technique for the analysis of composition distribution in semicrystalline polymers; more specifically for the analysis of branching distribution in Polyethylene and tacticity in Polypropylene type resins. CRYSTAF as well as TREF (Temperature Rising Elution Fractionation) are separation techniques which fractionate species of differing crystallizability by slow cooling of a polymer solution. TREF however, demands in addition to the crystallization step a second temperature cycle, elution step, to obtain the polymer composition. CRYSTAF, on the other hand, extracts the information in the crystallization cycle by monitoring the solution concentration depression as temperature goes down; thus reducing significantly the analysis time and simplifying the hardware needs. CRYSTAF principles are discussed and various applications are described.     

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.

Polyolefin analysis by single‐step crystallization fractionation

Temperature rising elution fractionation (TREF) fractionates polymer chains with respect to their crystallizability, independently of molecular weight effects. In order to achieve a good fractionation, TREF requires a time‐consuming polymer deposition step over an inert support before the elution step. A single‐step crystallization fractionation method has been developed recently,^1,2 Crystallization Analysis Fractionation (CRYSTAF), in which the chemical composition (or short chain branching) distribution of olefin copolymers can be measured by monitoring on‐line polymer concentration in solution at decreasing temperatures. For the present experimental investigation, a CRYSTAF‐prototype has been assembled and used to fractionate several linear low‐density polyethylene (LLDPE) samples. These results were compared to the ones measured by the commercial CRYSTAF apparatus from Polymer ChAR. Additionally, CRYSTAF results from Polymer ChAR were compared to analytical TREF results. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 539–552, 1999     

Characterization of polyolefins by comprehensive high-temperature two-dimensional liquid chromatography (HT 2D-LC)

Temperature rising elution fractionation hyphenated to size exclusion chromatography (TREF × SEC) is a routine technique to determine the chemical heterogeneity of semicrystalline olefin copolymers. A serious limitation is its applicability to non crystallizing samples. Comprehensive high temperature two-dimensional liquid chromatography (HT 2D-LC) gives an alternative to characterize the chemical heterogeneity of copolymers irrespective of their crystallizability. We have hyphenated interactive HPLC, which separates polyolefins according to their chemical composition, with high-temperature size exclusion chromatography (SEC), which distinguishes polyolefins with regard to their molar mass at 160 °C. The first separation step was based on a selective adsorption of macromolecules on a Hypercarb® column packed with porous graphite particles and subsequent desorption by a gradient 1-decanol → 1,2,4-trichlorobenzene at 160 °C. The SEC column was calibrated with polypropylene (PP) and polyethylene (PE) standards and it turned out that the injection solvent from the first dimension influenced the elution of PP in the SEC column, while the retention of PE was virtually constant. HT 2D-LC was then used to separate a broad variety of polyolefin blends containing PE, PP with different microstructure, ethylene–propylene (EP) and ethylene–propylene–diene (EP(D)M) rubber and ethylene/1-hexene copolymers. For the first time it has been shown that the elution of iPP in the gradient HPLC is molar mass dependent. The results from the HT 2D-LC separation were compared to those from TREF × SEC-experiments. The particular advantage of HT 2D-LC over TREF × SEC is the fact that HT 2D-LC is also applicable to non crystallizing polyolefin samples. The new technique therefore resolves the problem to analyze the chemical heterogeneity of non crystallizing olefin copolymers like EP and EP(D)M copolymers.     

Updated “Onion Skin” Model for Temperature Rising Elution Fractionation

Recent developments on the temperature rising elution fractionation (TREF) technique, understanding the impact strength of polyethylene blends based on their chemical structure, as well as ongoing discussions on REACH legislation regarding the oligomer fraction of polymers, are all reasons for better comprehension of the separation mechanism in TREF. To achieve this goal, two carefully chosen blends of linear metallocene polyethylene were analyzed by TREF over a large domain of crystallization rates. The results allowed updating the “onion skin” model for the crystallization kinetics during the cooling step of TREF. The advantages and limitations of the TREF technique for different applications are discussed.     

Fractionation process in TREF systems: Validation of thermodynamic model and calculation procedure by Raman LAM studies

In this paper, possible sources for the unexpected distributions of crystalline sequence lengths calculated from temperature rising elution fractionation (TREF) calibration experiments, as reported in a previous work, are investigated. With this aim, chain folding and cocrystalization phenomena were explored in the conditions of crystallization as used for TREF or crystallization analysis fractionation (CRYSTAF). Slow crystallizations were performed from xylene solutions of model low molecular weight ethylene homopolymers with narrow molecular weight distributions. The same experiments were performed with homopolymers having narrow molecular weight distributions and with blends having wide molecular weight distributions. The resulting distributions of the lengths of crystalline methylene sequences were directly studied by Raman in the so‐called longitudinal acoustic mode (LAM) and by DSC. For ethylene homopolymers with molecular weights below 2000 g/mol, the results from Raman LAM indicate that slow crystallization in TREF or CRYSTAF systems occurs in the extended‐chain mode. For higher molecular weights, evidence of chain folding was found. In the case of blends, independent crystallization was observed for each molecular weight when the molecular weight ranges used for the blends are relatively narrow. Cocrystallization was observed when this range was increased. Overall, these results strongly support the inverse technique calculation procedure developed by our group for the calculation of distributions of lengths of crystallizable sequences from TREF spectra. In this context, the results confirm that the unexpected crystallizable sequence lengths found in our previous work really exist and can be associated to chain folding or cocrystallization phenomena. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3083–3092, 2005     

High‐density polyethylene fractionation with supercritical propane

During the development of column extraction techniques, two methods of separation were identified. The first method is based on altering polymer solubility by varying the ratio of solvent in a solvent/nonsolvent mixture at a constant temperature above the polymer melting point (gradient solvent elution fractionation). This method fractionates polymers according to molecular weight. The second method is based on altering polymer solubility by varying solvent temperature (temperature rising elution fractionation—TREF). TREF fractionates semicrystalline polymers with respect to their crystallizability, independently of molecular weight effects. In the present article, supercritical propane will be used to fractionate a high‐density polyethylene sample by molecular weight and short chain branching. The main advantage of supercritical fluid fractionation is that large polymer fractions with narrow molecular weight distributions (isothermal fractionation) or narrow short chain branching distributions (isobaric fractionation) can be obtained without using hazardous organic chlorinated solvents. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 553–560, 1999     

Cocrystallization of ethylene/1‐octene copolymer blends during crystallization analysis fractionation and crystallization elution fractionation

Blending of ethylene/1‐octene copolymers can be used to achieve a well‐controlled broad chemical composition distribution (CCD) required in several polyolefin applications. The CCD of copolymer blends can be estimated using crystallization analysis fractionation (CRYSTAF) or crystallization elution fractionation (CEF). Unfortunately, both techniques may be affected by the cocrystallization of chains with different compositions, leading to profiles that do not truly reflect the actual CCD of the polymer. Therefore, understanding how the polymer microstructure and the analytical conditions influence copolymer cocrystallization is critical for the proper interpretation of CRYSTAF and CEF curves. In this investigation, we studied the effect of chain crystallizabilities, blend compositions, and cooling rates on cocrystallization during CEF and CRYSTAF analysis. Cocrystallization is more prevalent when the copolymer blend has components with similar crystallizabilities, one of the components is present in much higher amount, and fast cooling rates are used. CEF was found to provide better CCD estimates than CRYSTAF in a much shorter analysis time. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Analyzing TREF data by stockmayer's bivariate distribution

The Stockmayer bivariate distribution is used to qualitatively and quantitatively interpret TREF (temperature rising elution fractionation) spectra of polyolefins made by multiple site type catalysts. TREF spectra simulated by the proposed method can adequately fit experimental TREF data presented in the literature and can be used to help understand the mechanism of TREF separation and the nature of multiple site types catalysts.     

Advanced Characterization of Propylene-Based Copolymers and Olefin Block Copolymers

Summary: The newly developed interactive separation of polyolefins by high temperature liquid chromatography (HTLC) provides new information about the chemical composition distribution of polyolefin elastomers. The technique has the advantage of being quantitative in its separation, and has high resolution for the separation of polyolefins by their chemical composition without influence by cocrystallization. Chemical composition distributions can be determined for individual polymers and blends which contain the full range of comonomer typically present in polyethylene and poylypropylene homopolymers, semi-crystalline copolymers, and amorphous copolymers. HTLC analysis in combination with the other fractionation techniques, such as DSC, TREF, NMR, and xylene fractionation, can be used to estimate the amount of olefin block copolymer present in a block composite produced by chain shuttling catalysis.     

Combination of TREF, high-temperature HPLC, FTIR and HPer DSC for the comprehensive analysis of complex polypropylene copolymers

A novel, powerful analytical technique, preparative temperature rising elution fractionation (prep TREF)/high-temperature (HT)-HPLC/Fourier transform infrared spectroscopy (FTIR)/high-performance differential scanning calorimetry (HPer DSC)), has been introduced to study the correlation between the polymer chain microstructure and the thermal behaviour of various components in a complex impact polypropylene copolymer (IPC). For the comprehensive analysis of this complex material, in a first step, prep TREF is used to produce less complex but still heterogeneous fractions. These chemically heterogeneous fractions are completely separated by using a highly selective chromatographic separation method—high-temperature solvent gradient HPLC. The detailed structural and thermal analysis of the HPLC fractions was conducted by offline coupling of HT-HPLC with FTIR spectroscopy and a novel DSC method—HPer DSC. Three chemically different components were identified in the mid-elution temperature TREF fractions. For the first component, identified as isotactic polypropylene homopolymer by FTIR, the macromolecular chain length is found to be an important factor affecting the melting and crystallisation behaviour. The second component relates to ethylene–propylene copolymer molecules with varying ethylene monomer distributions and propylene tacticity distributions. For the polyethylene component (last eluting component in all semi-crystalline TREF fractions), it was found that branching produced defects in the long crystallisable ethylene sequences that affected the thermal properties. The different species exhibit distinctively different melting and crystallisation behaviour, as documented by HPer DSC. Using this novel approach of hyphenated techniques, the chain structure and melting and crystallisation behaviour of different components in a complex copolymer were investigated systematically.

Analysis of polyolefin blends by CRYSTAF

Crystallization analysis fractionation (CRYSTAF) has been introduced for the analysis of the composition of polyolefin blends and the chemical composition distribution of polyolefins. Blends of syndiotactic and isotactic polypropylene (sPP and iPP) and of sPP/High density polyethylene (HDPE) have been fractionated by CRYSTAF and the results been compared to those from DSC. While the blends of sPP and HDPE cannot be separated by DSC a quantitative determination of both components is possible by CRYSTAF over the whole range with the detection limit being 1% on both ends. Furthermore it is demonstrated that the separation of ternary blends of sPP, iPP and HDPE is possible by CRYSTAF.     

High Temperature Thermal Gradient Interaction Chromatography (HT-TGIC) for Microstructure Analysis of Polyolefins

Summary: High temperature thermal gradient interaction chromatography (HT-TGIC) is a newly developed technique to analyze comonomer distributions in polyolefins. This paper documents the key differences between crystallization elution fractionation (CEF) and HT-TGIC, and the advantages of using multiple detectors in HT-TGIC to provide comprehensive microstructure characterization. A demonstration of the technique using a specifically designed blend, provides comprehensive data in less than 1.5 hours of analysis time by HT-TGIC. This paper also reports that HT-TGIC has excellent short term repeatability.     

结晶分级技术在支化聚乙烯研究中的应用

介绍了近年发展起来的几种结晶分级技术及其在支化聚乙烯结构表征及性能研究方面的应用。利用升温淋洗分级技术(TREF),可根据结晶特性的不同将高分子分离成多个分布较窄的级份,通过分别表征各级份的链结构,从而可获得高分子链结构方面较为准确的信息。基于差示扫描量热技术(DSC)发展起来的两类热分级技术,主要包括逐步结晶热分级(SC)和连续自成核退火分级(SSA)技术,虽然不能从物理上对高分子进行分级,但通过选择适当的操作参数,也能得到一系列与升温淋洗分级实验类似的链结构信息,并且具有设备简单、操作方便、样品用量少、耗时短等优点。本文结合我们自己的工作,对各种分级技术的原理、实验操作及应用进行了系统综述,并展望了结晶分级技术发展的某些可能趋势。     

Correlation between GPC,DSC, and Analytical TREF for Linear Polyethylene

Analytical temperature rising elution fractionation (TREF) of linear polyethylene (PE) samples with different densities was done in 1-chloronaphthalene using a gel permeation chromatograph (GPC) coupled with a gas chromatograph. The corrected peak elution temperatures completed the previously obtained data in trichlorobenzene, xylene, and dibutoxymethane. A mathematical correlation was found for diluted linear PE samples between the α parameter of the Mark-Houwink-Sakurada equation governing the retention time in GPC, the bulk melting temperature measured by differential scanning calorimetry (DSC), and the TREF peak elution temperature. The extrapolation to the melting temperature measured by DSC gives α = 0.5, thus confirming the hypothesis that polymer conformations in the melt are similar to those in a theta solvent.