High-temperature two-dimensional liquid chromatography of ethylene-vinylacetate copolymers

Temperature rising elution fractionation hyphenated to size exclusion chromatography (TREF × SEC) is a routine technique to determine the chemical heterogeneity of semicrystalline olefin copolymers. Its applicability is limited to well crystallizing samples. High-temperature two-dimensional liquid chromatography, HT 2D-LC, where the chromatographic separation by HPLC is hyphenated to SEC (HPLC × SEC) holds the promise to separate such materials irrespective of their crystallizability. A model blend consisting of ethylene-vinyl acetate (EVA) copolymers covering a broad range of chemical composition distribution including amorphous and semicrystalline copolymers and a polyethylene standard was separated by HT 2D-LC at 140 °C. Both axes of the contour plot, i.e. the compositional axis from the HPLC and the molar mass axis from the SEC separation were calibrated for the first time. Therefore, a new approach to determine the void and dwell volume of the developed HT 2D-LC instrument was applied. The results from the HT 2D-LC separation are compared to those from a cross-fractionation (TREF × SEC) experiment.     

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     

Crystallization Elution Fractionation and Thermal Gradient Interaction Chromatography. Techniques Comparison

Summary: The chemical composition distribution has been shown to be the most critical and discriminating parameter in understanding the performance of industrial polyolefins with non homogeneous comonomer incorporation. The chemical composition distribution is being analyzed by well known techniques such as temperature rising elution fractionation, TREF, crystallization analysis fractionation, CRYSTAF and crystallization elution fractionation, CEF. These techniques separate according to crystallizability and provide a powerful and predictable separation of components based on the presence of branches, irregularities or tacticity differences, independently of the molar mass. TREF, CRYSTAF and CEF can not be used, however, for the separation of more amorphous resins, and may not always provide the best solution for complex multi-component resins due to the existence of some co-crystallization. The application of high temperature interactive HPLC to polyolefins opened a new route to characterize these types of polymers. The use of solvent gradient HPLC for separation of polyethylene and polypropylene and the developments in HPLC on carbon based columns extended further the application of high temperature HPLC in polyolefins. A new approach has been developed recently using the carbon based column but replacing solvent gradient by a thermal gradient which facilitates the analysis of polyethylene copolymers and provides a powerful tool for the analysis of elastomers. Thermal gradient interaction chromatography (TGIC) is being compared with TREF and CEF with the analysis of model samples. The advantages/disadvantages of each technique are being investigated and discussed. The combination of TGIC and TREF/CEF provides an extended range of separation of polyolefins.     

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.     

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.     

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.     

Surface and bulk modification of polyolefins by functional aryl nitrenes as highly reactive intermediates

Aromatic azides with hindered amine light stabilizer (HALS) residues or hydrophilic groups such as glucose, sucrose and dextrine residues were synthesized and used for surface modification of polyolefins. By UV‐irradiation nitrenes were formed, which are able to react with polyolefin surfaces. By photochemical immobilization of the carbohydrates hydrophilicity of PE and PP was strongly increased (surface tensions > 44mN/m). Light stability of PP surfaces modified with HALS azides was comparable with PP, stabilised with Tinuvin 770. Bulk modification of ethylene‐propylene and ethylene‐octene copolymers was achieved by grafting nitrenes formed by thermal decomposition of azido benzoic acid. In a circulating air oven up to 1.55 wt% amino benzoic acid residues could be bonded covalently to ethylene‐propylene‐copolymers, less than half of it to ethylene‐octene‐copolymers. Reactive extrusion resulted in grafting yields of more than 50% for both types of copolymers.     

Characterization of Ethylene methyl methacrylate and Ethylene butylacrylate Copolymers with Interactive Liquid Chromatography

Summary: Copolymers of ethylene with methyl methacrylate (EMMA) and butyl acrylate (EBA), which are of different average chemical composition and block lengths according to NMR analysis, were analyzed by size exclusion chromatography (SEC), differential scanning calorimetry (DSC), Crystallization Analysis Fractionation (CRYSTAF), and high performance liquid chromatography at high temperature (HT-HPLC). With CRYSTAF and DSC crystallizing fractions were detected only in some samples. HT-HPLC fractionated all the samples irrespective of their crystallinity. Homopolymers, PMMA and PE were also found in the copolymer samples of EMMA. EMMA and EBA were separated in HPLC according to the content of polar comonomer. A linear correlation between the MMA content and elution volume could not be established due to the presence of homopolymers as admixtures. In such a case the average chemical composition obtained by NMR does not correspond to the real chemical composition of the copolymers. Unlike EMMA the EBA samples eluted in single peaks, which was used for evaluation of their chemical composition distribution. The comparison of results obtained by fractionation via CRYSTAF and HT-HPLC clearly demonstrates the advantages of the chromatographic approach to study the chemical heterogeneity of olefin based copolymers.     

Monitoring polyolefin modification along the axis of a twin‐screw extruder. II. Maleic anhydride grafting

The physico‐chemical phenomena developing along the screw axis of a twin‐screw extruder during the grafting of maleic anhydride (MA) onto polyolefins [polyethylene (PE), ethylene–propylene rubber (EPM), and polypropylene (PP)] were investigated. For this purpose, sampling devices located along the extruder barrel were used to collect polymer samples that were subsequently characterized to follow the degrees of grafting and crosslinking or degradation. A similar evolution of MA grafting was observed regardless of the polyolefin type or MA and peroxide concentration when grafting was performed under identical conditions, that is, the same peroxide type and set temperature. A correlation between the MA grafting and the calculated peroxide decomposition was established. Chemical reactions occurred along the extruder axis until the peroxide was fully converted. More detailed quantitative measurements of the peroxide decomposition and MA grafting would allow the development of accurate process models. The final MA content depended on the polyolefin composition (PE > EPM ≫ PP). As expected for PE, crosslinking occurred in addition to grafting, but after a certain residence time, the PE network degraded. The PP viscosity reduction after MA grafting was due to the conversion of tertiary PP radicals into primary PP radicals after grafting. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3919–3932, 2000     

High Temperature Interaction Chromatography of Olefin Copolymers

Summary: The synthesis and characterization of polyolefins continues to be one of the most important areas for academic and industrial polymer research. One consequence of the development of new “tailor-made” polyolefins is the need for new and improved analytical techniques for the analysis of polyolefins with respect to molar mass and chemical composition distribution. The present article briefly reviews different new and relevant chromatographic techniques for polyolefin analysis. For the fast analysis of the chemical composition distribution of polyolefins a new high-temperature gradient high-performance liquid chromatography (HPLC) system has been introduced. The efficiency of this system for the separation of various olefin copolymers is demonstrated. The correlation between elution volume and chemical composition can be accessed by on-line coupling of high temperature HPLC with FTIR spectroscopy. For the elucidation of the chemical composition as a function of molar mass high-temperature size exclusion chromatography and ¹H-NMR spectroscopy can be coupled. It is shown that the on-line NMR analysis of chromatographic fractions yields information on microstructure and chemical composition in addition to molar mass distribution.     

Characterization of the microstructure of impact polypropylene alloys by preparative temperature rising elution fractionation

Two polypropylene alloys (Samples A and B), as impact polypropylene (PP) with similar ethylene contents and melt indices but different impact properties at low temperatures, are fractionated into eight fractions using preparative temperature rising elution fractionation. The microstructure of the original samples and their fractions are studied using high-temperature gel permeation chromatography, Fourier transform infrared spectroscopy, ¹³C nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. The results indicate that the two alloys are mainly composed of four portions: ethylene–propylene random copolymer (EPR), ethylene–propylene segmented copolymer, ethylene–propylene block copolymer, and propylene homopolymer. Sample A contains more EPR and more fractions with higher isotacticity eluted at 120 and 140 °C than Sample B. The difference in the microstructure distributions of both PP alloys results in observable differences in their mechanical properties: Sample A has better impact toughness and possesses higher rigidity than Sample B. Sample A also exhibits better balance between toughness and stiffness.     

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

Summary: 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.     

Characterization of impact polypropylene copolymers by solvent fractionation

The compositional heterogeneity of two impact polypropylene copolymers(IPCs) was studied by a combinatory investigation of temperature rising elution fractionation(TREF) and solvent fractionation.The chain structures and composition of fractions obtained from solvent fractionation were examined in detail.The TREF results shows that there are much more E-P segmented copolymer and more uniform distribution of ethylene sequence in IPC-1,which is responsible for its better comprehensive mechanical performance.The fractions from hexane and heptane are ethylene-propylene rubber phase and E-P block copolymers respectively.The result of solvent fractionation method also shows that custom hexane or heptane extractions can not extract the E-P copolymer completely.     

Elution Temperatures of Polyethylene Copolymers in Temperature Rising Elution Fractionation (TREF): Comparison in Xylene and Trichlorobenzene Diluents

Temperature rising elution fractionation (TREF) became the preferred technique to characterize the short chain branching distribution of polyethylene copolymers. Due to technical limitations, preparative TREF (PTREF) is usually done in xylene, while trichlorobenzene is used in analytical TREF (ATREF). Attempts to correlate the TREF elution temperatures based on data published by different authors erroneously showed higher elution temperatures for xylene than for trichlorobenzene. Our study rectifies this error. The experiments were done in both solvents on the same analytical TREF instrument. For the analyzed polyethylene copolymers, we found that the average elution temperature in xylene is 3.7° ± 1°C lower than in trichlorobenzene.     

13C NMR Study of the grafting of 13C labeled maleic anhydride onto PE,PP and EPM

The chemical structure of polyolefins grafted with maleic anhydride (MA) has been the subject of much speculation, but thorough experimental studies are rare. MA with 99% ¹³C in the double bond was synthesized and grafted onto PE, EPM and PP in the melt and solution. 1D INADEQUATE ¹³C NMR spectroscopy was used to characterize the products. Saturated, monomeric MA graft structures are formed. Only for grafted PE short MA oligomers are demonstrated. Grafting occurs on secondary and tertiary carbons depending on the composition of the polyolefin. For PP a new, unsaturated MA graft structure on the polymer chain terminus is identified. All graft structures are rationalized using a simple grafting mechanism.     

Structural characterization of polyolefins by pyrolysis—hydrogenation glass capillary gas chromatography

In order to obtain quantitatively specific and high-resolution pyrograms of high polymers, fundamental splitting conditions for pyrolysis—gas chromatography were studied using a vertical micro-furnace type of pyrolyzer attached to a glass capiliary separation column. High-resolution pyrograms were measured for polyolefins such as polyethylenes (PE), polypropylenes (PP) and various kinds of ethylen—propylene copolymers [P(E-co-P)] using a pyrolysis—hydrogenation device. Microstructures such as short branchings for PE, stereoregularities and chemical inversions for PP and sequence distributions for P(E-co-P) are discussed on the basis of the pyrograms.     

Correlation of the elution behavior in temperature rising elution fractionation and melting in the solid‐state and in the presence of a diluent of polyethylene copolymers

Compositionally homogeneous poly(ethylene‐α‐olefin) random copolymers with 1‐butene and 1‐hexene comonomers have been studied. The melting of solution‐crystallized specimens of these copolymers in the presence of trichlorobenzene as a diluent with differential scanning calorimetry (DSC) is well correlated with analytical temperature rising elution fractionation (A‐TREF) elution temperature profiles. This indicates that the A‐TREF experiment is essentially a diluent melting experiment. Furthermore, the correction of the corresponding solid‐state melting endotherms of these copolymers with Flory's diluent melting equation yields curves that also correlate very well with the DSC diluent melting curves and the A‐TREF elution temperature profiles. Values of χ, the Flory–Huggins interaction parameter, are determined for these copolymers in trichlorobenzene. χ decreases as short‐chain branching increases. The A‐TREF elution temperature profiles of one of these copolymers are the same, within experimental error, for dilute‐solution crystallizations of the copolymer performed over an extremely broad time schedule (10 s to 3 days). This indicates the profound effect of the branches, as limiting points of the ethylene sequences, in controlling the crystal thickness distribution, which in turn controls the melting point in the presence of the diluent, or the elution temperature from the A‐TREF. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2819–2832, 2001     

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.     

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.     

Novel polyolefin materials via catalysis and reactive processing

Recent advances in transition metal catalyzed olefin polymerization and melt processing stimulate the production of new polymers derived from old monomers. Modern polyolefin processes do not require polymer purification and give excellent control of molecular and supermolecular polyolefin architectures. Progress in catalyst design and preparation of tailor-made homo-and copolymers is highlighted for isotactic, syndiotactic, atactic and stereo-block polypropylene (PP), novel 1-olefin copolymers, and ethylene copolymers with polar monomers, e.g., CO and acrylics. Today polyethylene short-and long-chain-branching is controlled either by uniform ethylene copolymerization with 1-olefins using single-site” metallocene catalysts, or by migratory polyinsertion of ethylene, respectively. Stiff cycloaliphatic polymers expand the frontiers of polyolefins into engineering applications. New families of polyethylenes and EPM with pendent polypropylene chains are obtained via copolymerization of PP macromonomers or polymer-analoguous coupling of functionalized PP during melt processing.