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Bimodal high‐density polyethylene (bHDPE) is a complex, multicomponent polyethylene (PE) material whose synthesis in a multistage process can be challenging. Three bHDPEs with good and bad end‐use properties are reverse engineered using advanced analytical techniques. Average chemical composition is determined using 13C NMR and 1‐butene is identified as the comonomer for the good resin (bHDPE 1) while 1‐hexene is the comonomer in bHDPE 2 and 3. The presence of comonomer in the high molar mass fractions of the samples is shown using high‐temperature triple‐detection size exclusion chromatography (HT‐SEC‐d3). Chemical composition separation using high‐temperature interaction chromatography (HT‐IC) is achieved using porous graphitic carbon (PGC) and silica stationary phases. Some problems in temperature gradient interaction chromatography (TGIC) on PGC are overcome by using a non‐adsorptive stationary phase, enabling better separation and visualization of homopolymer and copolymer components. Coupling HT‐SEC in 2D liquid chromatography (2D‐LC) analyses at high temperatures reveals the presence of a larger copolymer component in bHDPE 1 at high elution volume. In contrast, bHDPE 2 and bHDPE 3 have copolymer components at low elution volumes, indicating poor comonomer distribution in the copolymer component which ultimately explains the poor mechanical properties at similar comonomer contents. [ABSTRACT FROM AUTHOR]

State-of-the-art liquid interaction chromatography (IC) allows separation of macromolecules according to chemical composition (CC), functionality or molecular topology in contrast to size-based approaches such as size exclusion chromatography (SEC), which has its limitations in distinguishing chemical structures. Yet, for macromolecules with multivariate molecular distributions, online identification and characterization of distinct components in bulk samples is a considerable challenge. A useful approach to narrow down the complexity of a given sample is preparative molar mass fractionation (pMMF) that provides fractions in mg or g scale, which can be subjected to further advanced analysis to elucidate the physical and structural properties of the sample. In this study, novel chain-walking polyethylenes (CWPE), which differ substantially in macromolecular architecture, are fractionated using pMMF. Using IC in temperature and solvent gradient modes (TGIC, SGIC), the pMMF fractions are separated according to branching topology. Quadruple-detector high temperature SEC is carried out to determine the physical properties and study the MM-dependent structural transformation of CWPE. A detailed branching analysis was carried out by means of nuclear magnetic resonance. Two-dimensional liquid chromatography (LC) hyphenating IC and SEC provides comprehensive bivariate correlations between MM and molecular topology. [ABSTRACT FROM AUTHOR]

Linear low density polyethylene (LLDPE) exhibits a complex molecular structure that is characterized by molar mass and chemical composition distributions. Both molecular parameters complementarily influence the final application properties. Typically, the molecular structure of commercial polyolefins is characterized by a set of technical parameters including the melt flow index, the crystallization and melting temperatures and the comonomer content as obtained using Fourier transform infrared or NMR spectroscopy. LLDPEs with high comonomer contents are typically regarded as plastomers or elastomers. Due to their low crystallinities, characterization of these materials using crystallization‐based analytical techniques is of limited use since the majority of the material is rather amorphous. Such materials need specific alternative analytical methods that may be based on molar mass and/or chemical composition fractionation. Here it is shown that for a comprehensive analysis of LLDPEs with similar bulk properties, preparative molar mass fractionation (pMMF) and advanced analysis of the fractions are required. The pMMF fractions are comprehensively analyzed using size exclusion chromatography, differential scanning calorimetry and high‐temperature high‐performance liquid chromatography to provide detailed information on molar mass and copolymer composition. Correlated information of these molecular parameters is obtained by comprehensive two‐dimensional liquid chromatography. © 2019 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Oxidized waxes possess far superior properties as compared to the alkanes they are derived from. The separation of alkane oligomers via gas chromatography (GC) becomes a challenge when polar oxygen-containing functional groups are introduced or when higher molar masses are targeted. In the present study, the separation and analysis of oligomers in oxidized and non-oxidized waxes using different liquid chromatographic techniques are investigated. Oligomers in two oxidized waxes and a non-oxidized wax from which they are derived, are separated using high-temperature solvent gradient interaction chromatography (HT-SGIC) and high-temperature two-dimensional liquid chromatography (HT-2D-LC). Evaporative light scattering detector conditions are tailored to provide the best detection with the solvent system at use. It is shown that oligomers in oxidized and non-oxidized waxes can be separated and identified using the mentioned techniques. It has been found that the ELSD detector response systematically decreases as the oxidation levels of the waxes increase. Coupling of HT-HPLC and high-temperature size exclusion chromatography (HT-SEC) in a comprehensive 2D-LC setup shows a broadening of the molar mass distributions of the lower oligomer fractions as a consequence of the modification indicating changes in the oligomer chain microstructures. A preparative fractionation technique is utilized to collect specific oligomer fractions from the bulk waxes followed by hyphenation to HT-HPLC and other techniques. HPLC is shown to provide more detailed information on the oligomer composition of waxes when coupled to a pre-fractionation technique. [ABSTRACT FROM AUTHOR]

This report addresses the comprehensive analysis of oxidized/functionalized polyethylene waxes according to chemical composition and molar mass by selective chromatographic methods. For the first time, tailored hightemperature interaction chromatography in solvent gradient (HT-SGIC) and thermal gradient (HT-TGIC) modes are used for the chemical composition separation of these materials. Separation protocols are developed using three model wax samples with different degrees of oxidation. For the chromatographic separations polar silica gel is used as the stationary phase. Solvent gradients of decane and cyclohexanone are used in HT-SGIC at 110 °C to separate the bulk waxes into several heterogeneous fractions according to polarity and the type of functionality. Column temperature and gradient manipulation are shown to influence chromatographic resolution and retention. The HT-SGIC investigations are complemented by HT-TGIC separations where a solvent mixture of decane and cyclohexanone is used as the mobile phase in isocratic mode. It is shown that HT-SGIC and HT-TGIC provide different types of separation, however, both are predominantly based on differences in functionality. To provide comprehensive information on chemical composition (functionality) and molar mass, HT-SGIC and HT-TGIC are coupled to HT-SEC, using ortho-dichlorobenzene as the second dimension mobile phase. Clear differences between oxidized and nonoxidized waxes are detected in HT-2D-LC providing comprehensive information on the molecular heterogeneity of these materials. [ABSTRACT FROM AUTHOR]

• A two-column set-up is used for HT-TGIC of polyethylene oligomers. • Silica acts as non-interactive phase for crystallization and redissolution. • Adsorptive porous graphitic carbon controls migration of eluting oligomers. • Shorter adsorptive columns are more efficient than standard TGIC experiments. • Longer columns reverse separation efficiency and promote remixing of sample. Temperature gradient interaction chromatography (TGIC) at high temperatures is a powerful method for the chemical composition separation of polyolefins. TGIC is a two-step process where the sample is crystallized on the stationary phase at low temperature followed by the elution of the sample components using a temperature gradient towards high temperatures. For TGIC typically a porous graphitic carbon (PGC) stationary phase is used. The separation mechanism is based on crystallization and adsorption/desorption phenomena and it has been shown that co-crystallization and co-adsorption may affect the separation. The present study reports on the simultaneous use of a non-adsorptive and an adsorptive stationary phase (column) in series to utilize both crystallization and adsorption for improved separation in TGIC. A silica column is used as the non-adsorptive support to allow for the crystallization of the polyolefin sample in the absence of an adsorptive force followed by the typical PGC column for adsorption/desorption. Accordingly, the loci of crystallization and adsorption/desorption are well separated from each other and can be adjusted independently. This novel column setup allows the sample to be introduced slowly onto the second (adsorptive) column eliminating possible co-adsorption and poor selectivity. Low molar mass polyethylene comprising of oligomers with approximately C 30 C 130 was used to illustrate the importance of a non-adsorptive column for improved separation. Utilizing a non-adsorptive silica column allows for higher dynamic flow rates during crystallization, which improves separation. Shorter adsorptive columns are found to be more efficient in this experimental protocol as compared to standard TGIC experiments. Smaller PGC column sizes result in reduced longitudinal and Eddy diffusion and, hence, higher resolution of low and high molar mass polyolefins. [ABSTRACT FROM AUTHOR]

The present study investigates the changes in the microstructure of heterophasic ethylene propylene copolymers (HEPCs) with increasing ethylene content, before and after visbreaking. Three samples obtained from commercial gas process reactors at Sasol Polymers at varying times after the addition of the ethylene comonomer are further visbroken to produce three more samples. Bulk sample analyses indicate that the ethylene-rich rubber phase which is essential in aiding peroxide mobility during visbreaking as observed in narrower dispersities and lower peak molar masses with increasing ethylene content. Further investigations of the microstructure via offline coupling of preparative molar mass fractionation (pMMF) to advanced analytical techniques such as solvent gradient interaction chromatography (SGIC) reveal that ethylene-rich copolymer chains are not significantly affected by the peroxide as the polypropylene homopolymer. Consequently, the polypropylene (PP) homopolymer and polypropylene-rich fractions with high molar mass diminish after visbreaking. Furthermore, the increase in ethylene content was observed to reduce the impact of the peroxide on the fraction quantities before and after visbreaking implying that visbreaking affects more the polyolefin chains with more PP segments and less those with more ethylene comonomer. • High temperature interaction chromatography is used to separate components of heterophasic ethylene-propylene copolymers. • Two-dimensional liquid chromatography is used to track the changes in the alloy components before and after visbreaking. • There is evidence that the rubber phase may promote mobility of peroxide in the polypropylene matrix during degradation. • Fractionation and analysis of fractions confirms peroxide impact on polypropylene rather than ethylene-rich components. [ABSTRACT FROM AUTHOR]