Temperature Rising Elution Fractionation Fractions Research Articles

Chemical composition separation of a propylene–ethylene random copolymer by high temperature solvent gradient interaction chromatography

A propylene–ethylene random copolymer was fractionated by preparative temperature rising elution fractionation (TREF). The structural heterogeneity of the bulk sample and its TREF fractions was studied by high temperature liquid chromatography with a solvent gradient elution from 1-decanol to 1,2,4-trichlorobenzene. HPLC alone cannot resolve those propylene-ethylene copolymers with high ethylene content in the bulk sample, due to their low weight fractions in the bulk sample and a small response factor of these components in the ELSD detector, as well as their broad chemical composition distribution. These components can only be detected after being separated and enriched by TREF followed by HPLC analysis. Chemical composition separations were achieved for TREF fractions with average ethylene contents between 2.1 and 22.0mol%, showing that copolymers with higher ethylene contents were adsorbed stronger in the Hypercarb column and eluted later. All TREF fractions, except the 40°C fraction, were relatively homogeneous in both molar mass and chemical composition. The 40°C fraction was rather broad in both molar mass and chemical composition distributions. 2D HPLC showed that the molar masses of the components containing more ethylene units were getting lower for the 40°C fraction. HPLC revealed and confirmed that co-crystallization influences the separation in TREF of the studied propylene-ethylene copolymer.

Molecular chain heterogeneity of a branched polyethylene resin using cross-fractionation techniques

The typical standard long chain branching (LCB) polyethylene (PE) SRM1476 is issued by the American National Bureau of Standards, which is widely used as a standard reference material in chromatographic experiments. Although some works have been reported in literatures, they are not consistent, such as the data about its molecular weight and molecular weight distribution. Therefore, it is necessary to further investigate the microstructure features of SRM1476 in order to better serve as a reference object in comparison with other unknown resins. In this study, the molecular chain heterogeneity of SRM1476 is investigated extensively and compared with the linear PE SRM1475. Based on the characterization of the original sample, SRM1476 actually has both LCB and short chain branching (SCB) structures, as well as the SCB content is more than LCB content in 13C-NMR results. After successive self-nucleation and annealing (SSA) thermal fractionation, SRM1476 shows a broad-range endotherm with multiple melting peaks (more than eight peaks), which proves the molecular chain heterogeneity in SRM1476. SRM1476 is fractionated into nine fractions by preparative temperature rising elution fractionation (TREF). At the high elution temperature region, the amounts of fractions are more than 90 %, and their molecular weights are higher. At the low elution temperature region below 50 °C, molecular weights of fractions are lower and their amounts are less than 5 %. Differential scanning calorimetry (DSC) melting curves of TREF fractions with higher elution temperatures shift toward higher temperature with sharper melting peaks. In the results of TREF-SSA cross-fractionation, each fraction shows a broad-range endotherm with multiple melting peaks that shift toward the high elution temperature region with the elution temperature. The arithmetic mean methylene sequence lengths of TREF fractions gradually increase from 33 to 105 as elution temperature increases. Results from TREF-DSC and TREF-SSA cross-fractionations indicate that SRM1476 and its TREF fractions have both intramolecular and intermolecular heterogeneity.

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

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

Comprehensive high temperature two-dimensional liquid chromatography combined with high temperature gradient chromatography-infrared spectroscopy for the analysis of impact polypropylene copolymers

Impact polypropylene copolymers (IPC) are extremely complex materials that can only be effectively analysed by multidimensional analytical approaches. IPC consists of isotactic polypropylene (iPP) as the major phase, ethylene–propylene (EP) copolymers of various compositions and small amounts of polyethylene. The molecular heterogeneity of two IPC samples having different ethylene contents was studied by using a novel cross-fractionation technique, developed from a combination of various analytical separation methods into an effective characterisation tool for complex polyolefins. The initial step involves the fractionation of the sample into EP rubber, EP segmented copolymer, and iPP, by preparative temperature rising elution fractionation (TREF). The resulting fractions are still distributed with regards to chemical composition and molar mass. The separation with respect to these parameters is conducted by comprehensive HT 2D-LC. This is the first time that the individual components in all TREF fractions of an IPC are separated and analysed mutidimensionally, by both SEC-FTIR, high-temperature (HT) HPLC-FTIR, and HT 2D-LC. Molar mass analysis of the chemically homogeneous fractions from HT HPLC is accomplished by HT SEC in the second dimension of HT 2D-LC. The chemical composition of all species is determined by coupling FTIR spectroscopy to HT HPLC via an LC-transform interface. This novel approach reveals the capability of this hyphenated technique to determine the exact chemical composition of the individual components in the complex TREF fractions of IPCs. The HT HPLC-FTIR results confirm the separation mechanism in the given chromatographic system using a 1-decanol to TCB solvent gradient and a Hypercarb stationary phase. The components of differing chemical composition are separated according to the nature and length of the propylene/ethylene segments, with their arrangement in the chains strongly affecting their adsorption/desorption on the stationary phase. FTIR analysis provides information on the ethylene and propylene contents of the fractions as well as on the ethylene and propylene crystallinities.

Use of FT‐IR Spectrometry for On‐Line Detection in Temperature Rising Elution Fractionation

AbstractSummary: Temperature rising elution fractionation (TREF) has become a popular analytical technique that is able to determine the chemical composition distribution (CCD) of an ethylene/α‐olefin copolymer. An infrared (IR) detector is commonly used in TREF detection to measure the concentration of the polymer solution exiting the column as a function of elution temperature. The chemical composition of the eluting polymer at a given elution temperature can be predicted from the relationship between comonomer content and TREF elution temperature pre‐established through 13C nuclear magnetic resonance (NMR) analysis of TREF fractions. In this article, a Fourier transform infrared (FT‐IR) spectrometer has been coupled with a TREF instrument to provide a more powerful tool for characterizing complex olefin copolymers. The Partial Least Squares (PLS) technique is used when analyzing the FT‐IR spectra of the eluting polymer solutions. The power of on‐line FT‐IR detection in TREF is demonstrated using a few complex copolymer systems, such as ethylene‐octene copolymer, polystyrene grafted ethylene‐vinyl acetate copolymer and ethylene‐methyl acrylate copolymer.

Successive solution fractionation mechanisms for high‐density polyethylene

AbstractBACKGROUND: Preparative fractionation techniques are currently used in order to obtain large amounts of polyethylene fractions. Preparative successive solution fractionation (SSF) and temperature rising elution fractionation (TREF) are compared as regards obtaining, at a multi‐gram scale, low‐dispersity fractions of high‐density polyethylene (HDPE). The operative separation mechanisms during a SSF of a broad HDPE, which are not yet totally elucidated, are also studied in this work.RESULTS: SSF and TREF approaches lead to the separation of HDPE macromolecules according to their molar masses. If very homogeneous fractions (dispersities from 1.1 to 1.3) are isolated in TREF at the lowest elution temperature, the collected mass is too low. At higher elution temperatures, the fractions have too broad a molar mass distribution (dispersities from 2.7 to 3.7). With the SSF procedure, dispersities are not as low as for the first TREF fractions. But, the relative weight fraction is better distributed between the different extraction temperatures. The molar mass distribution exhibits a dispersity of around 1.9.CONCLUSION: The SSF method is the most suitable way to obtain large gram amounts of low‐dispersity (ca 2) HDPE fractions over a wide molar mass range. Complementary gram‐scale rheological characterization is thus possible enabling a better comprehension of the SSF mechanism. Liquid–liquid demixing is the main mechanism in SSF, but its relative importance depends on polymer characteristics and solvent quality. Copyright © 2008 Society of Chemical Industry

Study on molecular chain heterogeneity of linear low‐density polyethylene by cross‐fractionation of temperature rising elution fractionation and successive self‐nucleation/annealing thermal fractionation

AbstractThe molecular chain heterogeneity of commercial linear low‐density polyethylene (LLDPE) was investigated by cross‐fractionation of temperature rising elution fractionation (TREF) and successive self‐nucleation/annealing (SSA) thermal fractionation by use of DSC. The results indicate that the linear relationships between crystallinity or melting temperature and the elution temperature are confirmed by TREF fractions. Intermolecular heterogeneity exists in the original LLDPE, whereas there is less intermolecular heterogeneity in the TREF fractions. After SSA thermal fractionation, the multiple endothermic peaks for both LLDPE and their TREF fractions are mainly attributed to the heterogeneities of ethylene sequence length (ESL) and lamellar thickness. The statistical terms, including weighted mean Lw, arithmetic mean Ln, and broad index Lw/Ln, were introduced to evaluate the heterogeneities of ESL and lamellar thickness of polyethylene. The difference of broadness index indicates that TREF fractions of LLDPE have less inter‐ and intramolecular heterogeneities of both ESL and lamellar thickness than those of the original LLDPE. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1710–1718, 2004

Characterization study of gamma-irradiated, high melt-strength polypropylene

Polypropylene undergoes simultaneous crosslinking and degradation when irradiated. However, there is some speculation regarding the occurrence of branching under special conditions. It is also well known that the melt-strength property of a polymer increases with molar mass and with long chain branching. The aim of this study was to examine the amount of branching, as well as the formation and molar mass of branched molecules. The hypothesis that PP fragments are grafted with high molar mass PP molecules to form the branched molecules was also investigated. Temperature rising elution fractionation (TREF) was performed to fractionate the sample and allow the study of molecular configuration and molar mass. To verify crystal parameters, TREF fractions were analyzed with DSC. Molar mass distribution was determined by SEC. The results showed that the primary reaction in the initial step of irradiation was degradation. The highly modified molecules were present in both the high and the low molecular weight fraction, but the high molecular weight fraction appeared to have higher concentrations. The results also confirm the presence of branching.

Characterization of the Microstructure of Biaxially Oriented Polypropylene Using Preparative Temperature-Rising Elution Fractionation

Two commercial biaxially oriented polypropylene (BOPP) resins, resin A and resin B , having different processing properties, were fractionated by preparative temperature-rising elution fractionation (TREF). The TREF fractions were further characterized by gel permeation chromatography (GPC), gel permeation chromatography coupled with light scattering (GPC-LS), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). GPC-LS did not find visible long-chain branching in either resin A or B . The results from TREF and DSC indicate that the fractional melting parameter f ( T ) may be used to predict the profile of the TREF cumulative weight distribution curve. GPC results show that the molecular weights of the fractions tend to increase with elution temperature. WAXD and DSC data show that the crystallinity of fractions does not increase monotonically with increase of elution temperature. There appears to be a maximum in the plot of crystallinity versus elution temperature. The high-speed BOPP resin A has a lower isotacticity but a homogeneous isotacticity distribution and a higher molecular weight but a broader molecular weight distribution than resin B .

Characterization of the molecular structure of two highly isotactic polypropylenes

Two polypropylenes, PP 1 and PP 2, produced with different heterogeneous Ziegler–Natta catalytic systems were studied in this work. Preliminary characterization of the non-fractionated materials showed that a low difference in their average tacticity (PP 2>PP 1) leads to an important modification of their rigidity properties. In order to establish correlation between the molecular structure parameters and the rigidity properties of these polymers, fractionation of the materials according to crystallizability was performed by means of temperature rising elution fractionation (TREF). Analysis of the fractions of both PP 1 and PP 2 was carried out by means of 13C NMR, size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The results first showed that TREF does not strictly fractionate PP according to tacticity, but according to the longest crystallizable sequence in a chain. 13C NMR, SEC and DSC analysis of the fractions demonstrated that the inter-chain tacticity distributions of the polypropylenes is affected by the change of the polymerization conditions, which, in turn, modifies the rigidity properties of the materials. Some results also seem to indicate that the intra-chain tacticity distributions are different for the two PP. An AFM study of the elastic modulus was carried out for the first time on the TREF fractions. It showed that the rigidity of the fractions strongly increases as the TREF elution temperature increases in accordance with a concomitant increase of isotacticity and the crytallinity of the fractions. PP 2 TREF fractions were, moreover, found to exhibit a higher elastic modulus than PP 1 TREF fractions at all elution temperatures. This study allowed us to further identify the TREF fractions that were responsible for differences in rigidity. To summarize, it is shown how the experimentally observed increase of the average rigidity of one of these two polypropylenes can be rationalized via information collected from a TREF fractionation.

Characterization of commercial linear low-density polyethylene by TREF-DSC and TREF-SEC cross-fractionation

A new technique for characterization of linear low-density polyethylene (LLDPE) is presented in this report. The molecular structure of two commercial LLDPEs, produced by copolymerization of ethylene with 1-butene over a Ziegler-Natta and a metallocene catalyst, was investigated. The LLDPE resins were fractionated by temperature rising elution fractionation (TREF), and the TREF fractions were further analyzed by size exclusion chromatography and differential scanning calorimetry (DSC) coupled with successive nucleation/annealing (SNA). The cross-fractionation techniques provided detailed information about the molecular structure of different types of LLDPEs; of particular interest is the TREF-SNA-DSC cross-fractionation which allowed a direct observation of methylene sequence distribution and thus short chain branch (SCB) distribution. TREF-size exclusion chromatography cross-fractionation showed that the molar mass of the Ziegler-Natta LLDPE increased monotonically with decreasing SCB, whereas the plot of Mw vs SCB for the metallocene LLDPE showed a maximum. TREF-SNA-DSC cross-fractionation clearly showed that the metallocene LLDPE only had intramolecular heterogeneity in SCB distribution, whereas the Ziegler-Natta LLDPEs exhibited both intermolecular and intramolecular heterogeneity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 960–967, 2000

Sequences distribution of poly(ethylene terephthalate‐isophthalate) copolymers: Experimental TREF study and numerical simulation

AbstractThe fractionation of a statistical copolyester made of poly(ethylene terephthalate‐ isophthalate) has been performed using Temperature Rising Elution Fractionation (TREF). TREF fractions were analyzed by size exclusion chromatography, thermal analysis, 1H‐nuclear magnetic resonance and small angle X‐Ray diffractometry. Results show that the fractions exhibit both interchain and intrachain defect distribution. As the intrachain defect distribution can not be determined by experimental techniques, a computer simulation based on the determination of terephthalic sequences length in a virtual polymeric distribution was performed. The simulated chains were constructed using a stochastic process. The analysis of the results obtained on the distribution of the simulated sequences in the polymer correlates quantitatively well between the new fractionation mechanism proposed for copolyesters and the experimental results.

Modeling the Fractionation Process in TREF Systems. III. Model Validation With Low Molecular Weight Homopolymers

Low molecular weight semicrystalline homopolymers are used as a model system for temperature rising elution fractionation (TREF) analysis. An already proposed thermodynamic model for TREF analysis is used to characterize TREF fractions from low molecular weight polyethylenes Mn = 500 to 3000 and some of their mixtures. In this molecular weight range it is possible, under appropriate crystallization, conditions, to form extended-chain crystals, and therefore lamellar thicknesses become comparable to extended-chain lengths. Lamellar thicknesses calculated from TREF spectra permit calculations of the molecular weights of the fractions, up to a limit of about 142 CH2, where partially folded-chain crystallites appear under these operating conditions. Also homopolymers blends are fractionated and the TREF spectra analyzed to test model predictions. It is shown that appearance of chain folding may set a resolution limit to the analysis of commercial copolymers by TREF. © 1996 John Wiley & Sons, Inc.