Thermal Gradient Interaction Chromatography Research Articles

Use graphene coated silica core shell particles to provide rapid, precise, and equivalent chemical composition distribution analysis for polyolefin materials by high temperature thermal gradient interaction chromatography

High temperature thermal gradient interaction chromatography (HT-TGIC) has been widely used to measure chemical composition distribution due to its applicability to separate crystalline and non-crystalline amorphous polyolefin materials. The compatibility of HT-TGIC with various detectors (infrared (IR), light scattering (LS), and viscometer) has also allowed a comprehensive analysis of molecular architecture of polyolefin and recycled plastics. The introduction of an easy-to-fabricate graphene coated onto non-porous silica particles as HT-TGIC column in 2020 showed a superior chromatographic performance over the traditional graphite column. A reduction in peak broadness (∼47 %) under identical experimental conditions was demonstrated in that research. This paper similarly uses a graphene column but with the focus on optimization of experimental parameters (concentration, and thermal cooling and heating rates etc.). Equivalent chemical composition distribution (CCD) data to that obtained by the incumbent graphite column over a wide range of polyolefins products was achieved, in addition to a shortened analysis time from 120 min down to 88 min per sample. The materials studied included semicrystalline linear low-density polyethylene (LLDPE), elastomers, terpolymers, model blends to mimic recycled plastics. The results also suggest that the elimination of substrate pores enable a better HT-TGIC separation. Coupling the ease and reproducibility of the graphene column fabrication process enables long term chromatographic robustness. This not only results in equivalent CCD data compared to the traditional graphite column but also a 27 % reduction in analysis time. These results demonstrate a substantial advancement of technology in the high throughput industrial laboratory setting where fast testing turnaround time is critical. In addition, simple fabrication with commercially available silica particles and graphene nanopowder provides a cost-effective approach to make HT-TGIC columns reproducibly.

Quantifying the Effect of Polyethylene Molecular Weight, Comonomer Fraction, and Comonomer Type on High-Temperature Thermal Gradient Interaction Chromatography

High-temperature thermal gradient interaction chromatography (HT-TGIC) can be used to measure the chemical composition distribution of ethylene/α-olefin copolymers using a column packed with porous...

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.

Analyses of Short Chain Branches in Polyolefins with Improved 1H NMR Spectroscopy.

Polyolefin microstructures, for example, short chain branching (SCB) and short chain branch distribution (SCBD), have a direct impact on properties and thus ultimately influence end-use applications. The 1H NMR approach to analyze SCB and SCBD is particularly useful when only a limited amount of sample is available, for example, polyolefin film layers or the fractions from polyolefin separation techniques, such as gel permeation chromatography (GPC), crystallization elution fractionation (CEF), high temperature liquid chromatography (HTLC), and thermal gradient interaction chromatography (TGIC). In this paper, we discuss the best approach to find a good decoupling frequency and propose an improved 1H pulse sequence with homonuclear decoupling for better measuring SCB. With this new pulse it is possible to reach a S/N of 10 (level of quantification) for the methyl signal from SCB in an ethylene-hexene copolymer (EH, 3.6 mol % H) in 3.5 min with 0.5 μg of sample. We also show an easy method to calculate SCB/1000C and demonstrate the proper use of heteronuclear single quantum coherence (HSQC) to measure SCB in a complicated system. A very quick approach to examine the presence of a small amount of LDPE in a polyolefin sample is also suggested, which can reduce NMR acquisition time from a couple of days to a few minutes.

Comprehensive branching analysis of star-shaped polystyrenes using a liquid chromatography-based approach.

The comprehensive branching analysis of complex polymers is still a challenge in advanced polymer analysis. Average branching information (average number and length of branches) can be obtained by spectroscopic methods, mainly NMR spectroscopy. The determination of the branching distribution, i.e., the concentration of macromolecules with a given number of branches, however, requires fractionation. Typically, size exclusion chromatography is used that separates the complex mixture with regard to molecular size in solution and not strictly with regard to the number of branches. In the present approach, model star-shaped polystyrenes were synthesized with a pre-determined architecture to give theoretical three-arm, four-arm, and six-arm structures. The branched samples were compared with a linear analogue of comparable molar mass known not to contain branching. Triple detector size exclusion chromatography with refractive index, multiangle light scattering, and online viscometer detection was used to determine absolute molar masses, radii of gyration, and branching distributions of the star-shaped polymers. 1H-NMR was used to calculate the average functionality and a reasonable agreement between the results of the two methods was obtained. Thermal gradient interaction chromatography and solvent gradient interaction chromatography were employed to separate the complex reaction products according to chemical composition (number of branches) and to resolve by-products. The separation capabilities of the two chromatographic techniques were compared and evaluated. Comprehensive two-dimensional liquid chromatography was used to separate the polydisperse star-shaped polystyrenes with regard to both branching and molar mass. Graphical abstract.

Molecular Dynamics Simulation of Ethylene/Hexene Copolymer Adsorption onto Graphene: New Insight into Thermal Gradient Interaction Chromatography

Molecular Dynamics Simulation of Ethylene/Hexene Copolymer Adsorption onto Graphene: New Insight into Thermal Gradient Interaction Chromatography

Molecular Dynamics Simulation of Ethylene/Hexene Copolymer Adsorption onto Graphene: New Insight into Thermal Gradient Interaction Chromatography

AbstractCrystallization‐based fractionation techniques are powerful methods for the analysis of short‐chain branching (SCB) in linear low‐density polyethylene. Recently, thermal gradient interaction chromatography (TGIC) has been developed for SCB determination of semi‐crystalline and amorphous polyolefins. In TGIC, the fractionation mechanism relies on the interaction of polyolefin chains with a graphite surface upon temperature change for a given solvent strength. Using molecular dynamics simulations, both the enthalpic and entropic contributions responsible for the separation of short‐chain branched polyethylene are estimated. A new thermodynamic framework for understanding the fractionation mechanism of TGIC is proposed. It is confirmed that at high SCB content, the decrease of the adsorption enthalpy is mostly due to the increased steric hindrance of short branches. However, for low chain branching density, strong increase of the persistence length observed during adsorption decreases the conformational entropy of the polymer and thus counter‐balances the favorable adsorption enthalpy. This entropic contribution may also explain the narrowing of the peak observed experimentally with low short‐chain branching density at high elution temperature.

Analytical temperature rising elution fractionation of different polyethylene types using a column filled with carbon nanotubes

A high temperature gel permeation chromatograph (GPC) was coupled with a gas chromatograph (GC) oven to obtain an analytical temperature rising elution fractionation (TREF) system with evaporative light scattering detection. On this instrument, a new column partially filled with pristine carbon nanotubes (CNT) was tested by evaluating the elution profiles in function of temperature (thermograms) of different polyethylene (PE) types. By comparing these thermograms with those obtained with a traditional TREF column filled with metallic wires, the adsorption of polymer chains on the pristine CNT was clearly evidenced. The thermograms given by the column filled with CNT are similar with those provided by literature when chromatographic columns filled with porous graphitic carbon are used for this application, usually described as high temperature thermal gradient interaction chromatography (HT-TGIC).

Comprehensive Analysis of Oxidized Waxes by Solvent and Thermal Gradient Interaction Chromatography and Two-Dimensional Liquid Chromatography

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 high-temperature 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.

Mathematical Modeling of Multiple High Temperature Thermal Gradient Interaction Chromatography (m‐HT‐TGIC) for Ethylene/1‐Olefin Copolymer Blends

High temperature thermal gradient interaction chromatography (HT‐TGIC) have been developed to measure the chemical composition distribution (CCD) of ethylene/1‐olefin copolymers over a wide range of compositions. Multiple high temperature thermal gradient interaction chromatography (m‐HT‐TGIC) is a concept developed to further enhance the physical separation of copolymer components by performing multiple adsorption/desorption cycles, similarly to the operation of multiple crystallization elution fractionation (m‐CEF) developed earlier. In this work, a mathematical model for describing m‐HT‐TGIC is developed based on population balances in multiple non‐equilibrium adsorption/desorption stages for ethylene/1‐olefin copolymer blends. The effects of number of m‐HT‐TGIC cycles, section length, and column length are also reported and discussed.

Chemical Composition Fractionation of Olefin Plastomers/Elastomers by Solvent and Thermal Gradient Interaction Chromatography.

Olefin plastomers/elastomers are typically copolymers with high comonomer contents and low crystallinities. Therefore, the fractionation of these materials with crystallization-based methods is not feasible. On the other hand, solvent and temperature gradient interaction chromatography (SGIC and TGIC, respectively) are suitable techniques for the separation of olefin copolymers with regard to their chemical composition. In this study, the application ranges of both techniques are investigated and compared for ethylene-propylene (EP) copolymers. A linear dependency of ethylene content versus elution volume is obtained with SGIC in practically the whole ethylene range. In the case of TGIC, a linear dependency is obtained within certain ethylene content limits. The accessible ethylene content separation range for TGIC is 50-100 mol% ethylene, and a broader 26-100 mol% ethylene range is accessible for SGIC, the latter being the technique of choice in the analysis of EP rubbers.

Joint Effect of Poly(ethyhlene‐co‐1‐octene) Chain Length and 1‐Octene Fraction on High‐Temperature Thermal Gradient Interaction Chromatography

High‐temperature thermal gradient interaction chromatography (HT‐TGIC) fractionates polyolefins based on an adsorption–desorption mechanism. Several factors influence the shape and position of HT‐TGIC chromatograms, notably polymer microstructure, analytical conditions, and, to a lesser extent, solvent type. This article investigates the joint influence of chain length and comonomer content of a series of polyethylene and ethylene/1‐octene copolymers having similar 1‐octene fractions (0–13 mol%) and a wide range of molecular weights on HT‐TGIC fractionation. For each series of copolymers having similar 1‐octene fraction, the elution peak temperature decreases exponentially and the profiles become increasingly broader below a critical number average chain length value. The authors use Monte Carlo simulation and Stockmayer distribution to explain the observed behavior, finding that no simple correlation exists between ethylene sequences in the copolymers and peak elution temperature, but that there is strong evidence that axial dispersion is responsible for symmetrical broadening of the HT‐TGIC profiles. The authors also study the HT‐TGIC of binary blends, finding that components with similar 1‐octene contents and dissimilar chain lengths tend to increase co‐adsorption/co‐desorption effects. image

Effect of Column Type on Polyolefin Fractionation by High‐Temperature Thermal Gradient Interaction Chromatography

SummaryHigh‐temperature thermal gradient interaction chromatography (HT‐TGIC) can be used to measure the chemical composition distribution (CCD) of semi‐crystalline and amorphous polyolefins. HT‐TGIC extends the range of polyolefin chemical compositions that can be measured today with crystallization‐based techniques. Hypercarb® columns packed with porous graphitic carbon are commonly used as the stationary phase in HT‐TGIC fractionation. We used a set of ethylene/1‐octene copolymers having different comonomer fractions (up to 25% of 1‐octene) and approximately the same molecular weight average to investigate how commercial Hypercarb® columns with distinct lengths and particle sizes affected HT‐TGIC fractionation. Binary copolymer blends were also analyzed to study how different blend compositions affected HT‐TGIC peak positions and shapes.

High Temperature Thermal Gradient Interaction Chromatography (HT‐TGIC) of Ethylene/1‐Octene Copolymers: Model Development and Validation

SummaryHigh temperature thermal gradient interaction chromatography (HT‐TGIC) is a new polymer characterization technique developed to characterize the chemical composition distribution (CCD) of ethylene/1‐olefin copolymers based on interactions between polymer chains and the graphitic surface. In this work, a mathematical model was developed to describe HT‐TGIC fractionation using population balances and a non‐equilibrium, multi‐stage adsorption/desorption model. The proposed model was validated with HT‐TGIC experimental profiles of ethylene/1‐octene copolymers with various comonomer contents. Good agreement between experiments and model results was observed.

High Temperature Thermal Gradient Interaction Chromatography (HT‐TGIC) for Blends of Ethylene/1‐Octene Copolymers: A Mathematical Model

SummaryHigh temperature thermal gradient interaction chromatography (HT‐TGIC) was recently developed to characterize chemical composition distribution (CCD) of ethylene/1‐olefin copolymers based on the interaction between polymer chains and graphitic surface. In this work, a mathematical model of HT‐TGIC was developed based on the population balance in a non‐equilibrium, multi‐stage adsorption/desorption process. Results from the proposed model were found to be in a good agreement with the experimental results of blends over various operating conditions.

Effect of Solvent Type on High‐Temperature Thermal Gradient Interaction Chromatography of Polyethylene and Ethylene–1‐Octene Copolymers

The effects of the solvent type and operation conditions on the high‐temperature thermal gradient interaction chromatography (HT‐TGIC) of ethylene homopolymers, ethylene–1‐octene copolymers, and their blends are investigated. While the HT‐TGIC profiles of single polymers measured with 1,2,4‐trichlorobenzene (TCB) and chloronaphthalene (CN) are similar, they are always narrower when o‐dichlorobenzene (ODCB) is used, particularly for samples with lower 1‐octene fractions. Significant differences between the experimental and the calculated profiles of binary blends are observed with all three solvents, but better peak separation is seen when the ODCB is used. Having higher fractions of a 1‐octene‐poor component in the blend causes a more significant distortion of the shape expected for the peak from the component with the higher 1‐octene fraction. The effect of the molecular weight on HT‐TGIC profiles is also studied using samples with the same comonomer content and different molecular weights. Samples with low molecular weight have broader distributions and significant lower‐temperature tails, particularly when TCB is used. Chain crystallization after adsorption effects may also play a minor role for low‐comonomer samples. Finally, HT‐TGIC profiles are compared with their equivalent crystallization elution fractionation (CEF) profiles. The HT‐TGIC curves are broader than the equivalent CEF profiles, but these differences decrease as the comonomer content increases. image

Characterization of Ethylene/α‐Olefin Copolymers Using High‐Temperature Thermal Gradient Interaction Chromatography

Several crystallization‐based techniques are used to measure the chemical‐composition distribution of polyolefins, but they are limited to semicrystalline polyolefins. Recently, high‐temperature thermal gradient interaction chromatography (HT‐TGIC) has been developed to quantify the chemical‐composition distribution of semicrystalline and amorphous polyolefins, thus broadening the range of techniques available for the analysis of polyolefin chemical‐composition distribution. In HT‐TGIC, the fractionation mechanism relies on the interaction of polyolefin chains with a graphite surface upon temperature change in an isocratic solvent. In the present investigation, a series of ethylene/1‐octene copolymers having approximately the same molecular weight average and different comonomer fractions (up to 25% of 1‐octene) is synthesized using a metallocene catalyst to investigate the fractionation mechanism of HT‐TGIC. Three copolymer samples and their blends are also studied to determine which operation parameters influence the HT‐TGIC peak shape and position. The cooling rate has no significant effect on the desorption temperature and the broadness of the HT‐TGIC chromatograms. On the other hand, the heating rate and the elution flow rate substantially influence the peak temperature and breadth. image

Fractionation of Ethylene/1-Octene Copolymers by High-Temperature Thermal Gradient Interaction Chromatography

High-temperature thermal gradient interaction chromatography (HT-TGIC) was recently introduced into the area of polyolefin analysis. This new technique uses commercially available Hybercarb columns to separate polyolefin chains based on their interaction with the porous graphitic carbon stationary phase in a temperature gradient mode. In this paper, a systematic study of HT-TGIC operation conditions is presented for ethylene/1-octene copolymers and their blends. One of the objectives of this investigation is to identify the best set of conditions that maximize peak separation of copolymers in a binary blend. Although the chromatograms of individual resins and their blends are independent of the cooling rate, it will be shown that sample volume (column loading) and heating rate during the elution cycle affect the peak separation of the components in binary blends.

Crystallization Elution Fractionation and Thermal Gradient Interaction Chromatography. Techniques Comparison

AbstractSummary: 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.

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

AbstractSummary: 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.