Temperature Gradient Interaction Chromatography Research Articles

Advances in High-Temperature Interaction Chromatography of Polyolefins: A Tutorial on Solvent and Temperature Gradient.

The advancement of high-temperature interaction chromatography (HT-IC) is a key milestone in polyolefin characterization. Temperature gradient interaction chromatography (HT-TGIC) and solvent gradient interaction chromatography (HT-SGIC) are the main separation methods. This Tutorial explains these methods, their historical uses, recent improvements, and suitable detectors for each technique.

Reverse Engineering of Chemically Similar Bimodal High Density Polyethylenes: A Comprehensive Study Using Advanced Chromatographic Techniques

AbstractBimodal 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.

Temperature gradient interaction chromatography of linear polyethylene and isotactic polypropylene

Samples of linear polyethylene and isotactic polypropylene were separated according to their average molar mass using temperature gradient interaction chromatography (TGIC) with mobile phases at near critical composition. It is known that at the critical mobile phase composition, the adsorption of polymers is very sensitive to variations of the temperature. This effect was utilized here. The temperature of the column (Hypercarb™) was changed from 80 °C to 170 °C according to a programmed temperature gradient. Polymers were first adsorbed in the column at 80 °C and desorbed by the temperature increase. In effect, the polymers eluted at different temperatures. The higher the molar mass, the higher both the elution temperature and the elution volume. This suggests that TGIC has the potential to improve separations of polyolefins according to their molar mass and will allow to separate polyolefins according to their long-chain branching content.

Improving temperature gradient interaction chromatography of polyolefins by simultaneous use of different stationary phases

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 C30C130 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.

Temperature Gradient Interaction Chromatography: A Perspective

The application of temperature gradient interaction chromatography (TGIC) as an advanced technique for the characterisation of polymers is discussed, in comparison to other liquid chromatography techniques and in particular the ubiquitous size exclusion chromatography. Specifically, the use of reversed-phase TGIC for the interrogation of complex branched polymers and normal-phase TGIC for characterisation of high-molar mass end-functionalised polymers is highlighted.

Advanced Liquid Chromatography of Polyolefins Using Simultaneous Solvent and Temperature Gradients.

Olefin copolymers are complex polymer materials that exhibit multiple distributions in molecular properties such as molar mass, chemical composition, and branching. To address the multivariate molecular compositions, chromatographic protocols have been developed that synergistically combine solvent and temperature gradients. As representative examples, blends of olefin copolymers have been fractionated on porous graphitic carbon stationary phases. This is the first study that makes complementary use of solvent and temperature gradient interaction chromatography (SGIC and TGIC, respectively) to capitalize on the advantages of both techniques. In a first experimental setup, solvent and temperature gradients were used simultaneously and complex blends of low molar mass polyethylene and ethylene-co-1-octene copolymers were separated with high efficiency. The separation of oligomers was observed to be significantly better in SGIC as compared to TGIC, while comonomer blends could be separated in either TGIC or SGIC mode. In another innovation, a two-column setup was employed where the columns were placed in different temperature zones. It was demonstrated that the separation of both low and high comonomer content blends was improved significantly when the temperatures of the two zones were manipulated reasonably.

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.

Branching Analysis of Comb‐Shaped Polystyrene with Long Chain Branches

Model comb‐shaped polystyrene (PS) samples are synthesized by anionic polymerization and the as‐prepared PS heterogeneous combs are further fractionated by temperature gradient interaction chromatography (TGIC) to obtain well‐defined random combs with 1–4 branches at random positions on a backbone. Size exclusion chromatography‐triple detection and TGIC‐triple detection analyses are carried out with the fractionated PS random combs in a good and a theta solvent, respectively. The experimentally observed geometric contraction factor (g), the ratio of the mean square radius of gyration and the hydrodynamic contraction factor (g′), and the ratio of the intrinsic viscosity are compared. It is observed that the two contraction factors follow the relationship of g′ = gε reasonably well and the exponent ε of the PS combs is found as ∼1 in a good solvent and ∼0.66 in a theta solvent. Furthermore, it is found that the theoretical grw is not in good agreement with the experimental results. image

Synthesis and Characterization of an Exact Polystyrene-graft-polyisoprene: A Failure of Size Exclusion Chromatography Analysis

An exact polystyrene-graft-polyisoprene (PS-g-PI) synthesized by iterative anionic polymerization and graft reaction using diphenylethylene functional groups was characterized by liquid chromatography. As-prepared graft copolymer contains various byproducts other than the target PS-g-PI. After the fractionation by size exclusion chromatography (SEC), the PS-g-PI appears quite homogeneous by SEC analysis, but the temperature gradient interaction chromatography (TGIC) separation with respect to the PI branch number showed a significantly wide distribution in the number of PI grafts. The resolved TGIC peaks were fractionated, and the molecular weight of each block (backbone or branches) was estimated by liquid chromatography at critical condition (LCCC) analysis at the critical adsorption condition of the opposite block. The composition determined by LCCC analysis and 1H NMR analysis yielded a reasonably self-consistent result. Through this study, we demonstrated that SEC analysis of this type of branched co...

Branching analysis of star-shaped polybutadienes by temperature gradient interaction chromatography-triple detection

Size exclusion chromatography (SEC) coupled with triple detection (TD) method is widely used for branching analysis of polymers. However, there is an inherent limitation of SEC-TD method that SEC separates polymers according their hydrodynamic size. The SEC resolution of branched polymers is not high since the hydrodynamic size of branched polymers does not change as much as linear polymers with molecular weight (MW), let alone the serious band-broadening in SEC. As a solution of the problem, we propose temperature gradient interaction chromatography (TGIC)-TD method taking advantage of TGIC's characteristics of separating branched polymers according to their MW with much higher resolution than SEC. The potential of the method was demonstrated using regular star-shaped polybutadienes prepared by anionic polymerization. They were characterized by both SEC-TD and TGIC-TD methods. Detailed analysis of branch distribution was possible for narrow MW fractions in TGIC-TD while SEC-TD only provides with average branch number for SEC fractions.

Challenging Tube and Slip-Link Models: Predicting the Linear Rheology of Blends of Well-Characterized Star and Linear 1,4-Polybutadienes

We compare predictions of two of the most advanced versions of the tube model, namely the “Hierarchical model” by Wang et al. [J. Rheol. 2010, 54, 223] and the BoB (branch-on-branch) model by Das et al. [J. Rheol. 2006, 50, 207], against linear viscoelastic G′ and G″ data of binary blends of nearly monodisperse 1,4-polybutadiene 4-arm star polymer of arm molar mass 24 000 g/mol with a monodisperse linear 1,4-polybutadiene of molar mass 58 000 g/mol. The star was carefully synthesized and characterized by temperature gradient interaction chromatography and by linear rheology over a wide frequency region through time–temperature superposition. We found large failures of both the Hierarchical and BoB models to predict the terminal relaxation behavior of the star/linear blends, despite their success in predicting the rheology of the pure star and pure linear polymers. This failure occurred regardless of the choices made concerning constraint release, such as assuming arm retraction in “fat” or “skinny” tubes. Allowing for “disentanglement relaxation” to cut off the constraint release Rouse process at long times does lead to improved predictions for our blends, but leads to much worse predictions for other star/linear blends described in the literature, especially those of Shivokhin et al. [Macromolecules 2014, 47, 2451]. In addition, our blends and those of Shivokhin et al. were also tested against a coarse-grained slip-link model, the “clustered fixed slip-link model (CFSM)” of Schieber and co-workers [J. Rheol. 2014, 58, 723], in which several Kuhn steps are clustered together for computational efficiency. The CFSM with only two molecular-weight- and chain-architecture-independent parameters was able to give very good agreement with all experimental data for both of these sets of blends. In light of its success, the CFSM slip-link model may be used to address the constraint release issue more rigorously and potentially help develop improved tube models.

NMR Study of the Separation Mechanism of Polyethylene–Octene Block Copolymer by HT-LC with Graphite

High temperature liquid chromatography (HT-LC) and temperature gradient interaction chromatography (TGIC) using a graphite substrate were recently invented to characterize polyolefin microstructures and composition distributions. Their separation mechanisms have been explored by using a series of random copolymers. In addition, the separation mechanism of TGIC for random copolymers was also studied with a high temperature NMR cryoprobe. The proposed separation mechanism for a random copolymer is that the separation is likely based on comonomer content, in other words, the number-averaged ethylene sequence length. Further understanding of HT-LC separation mechanism with polyethylene–octene block copolymers is beneficial in order to extend the use of HT-LC and TGIC to a wider variety of polyolefin materials, and to better characterize more challenging microstructures of new materials. With the high temperature NMR cryoprobe technology, it is possible to perform 13C NMR of polyolefins with a few milligrams o...

Normal-phase (temperature gradient) interaction chromatography – A powerful tool for the characterisation of high molecular weight chain-end functionalised polymers

We report here, for the first time, quantitative analysis of end-group functionalisation and the extent of end-group modification of polymers with molar mass up to 200,000gmol−1, using a combination of isothermal and temperature gradient interaction chromatography. At such high molecular weights, other common analytical techniques such as MALDI-ToF-MS and NMR spectroscopy are simply unable to offer any quantitative insight into the end-group functionality of polymers. Thus, normal phase isothermal interaction chromatography (NP-IIC) has been used to characterise a series of polystyrene samples, with identical molar mass (c. 90,000gmol−1), each carrying a single chain-end functionality of varying polarity, introduced via a series of end-group transformation reactions – from silyl-protected alcohol→primary alcohol→alkyl bromide. It is shown that the polarity of the functional group dictates the elution volume of the polymers and that NP-IIC could be used to monitor the quantitative end-group transformation. Thus, rather than estimating the success of the transformation reaction from the very small peaks observed in 1H NMR spectra, NP-IIC gives clear chromatographic peaks, whose retention volumes change dramatically only as a consequence of each new chain-end functional group. We also describe the analysis, by a combination of normal phase isothermal and temperature gradient interaction chromatography, of a series of linear polybutadiene samples of varying molar mass (28,000–200,000gmol−1) each carrying a single primary alcohol functionality at the chain-end. The primary alcohol functionality enabled the separation of functionalised and non-functionalised polymers, which eluted at different retention volumes despite identical molecular weights. It is all the more remarkable that complete (baseline) resolution can be achieved for two polymers with an identical molar mass (<200,000gmol−1), which differ only in that one sample is functionalised with a single primary alcohol group.

Molecular rheology of branched polymers: decoding and exploring the role of architectural dispersity through a synergy of anionic synthesis, interaction chromatography, rheometry and modeling

An emerging challenge in polymer physics is the quantitative understanding of the influence of a macromolecular architecture (i.e., branching) on the rheological response of entangled complex polymers. Recent investigations of the rheology of well-defined architecturally complex polymers have determined the composition in the molecular structure and identified the role of side-products in the measured samples. The combination of different characterization techniques, experimental and/or theoretical, represents the current state-of-the-art. Here we review this interdisciplinary approach to molecular rheology of complex polymers, and show the importance of confronting these different tools for ensuring an accurate characterization of a given polymeric sample. We use statistical tools in order to relate the information available from the synthesis protocols of a sample and its experimental molar mass distribution (typically obtained from size exclusion chromatography), and hence obtain precise information about its structural composition, i.e. enhance the existing sensitivity limit. We critically discuss the use of linear rheology as a reliable quantitative characterization tool, along with the recently developed temperature gradient interaction chromatography. The latter, which has emerged as an indispensable characterization tool for branched architectures, offers unprecedented sensitivity in detecting the presence of different molecular structures in a sample. Combining these techniques is imperative in order to quantify the molecular composition of a polymer and its consequences on the macroscopic properties. We validate this approach by means of a new model asymmetric comb polymer which was synthesized anionically. It was thoroughly characterized and its rheology was carefully analyzed. The main result is that the rheological signal reveals fine molecular details, which must be taken into account to fully elucidate the viscoelastic response of entangled branched polymers. It is important to appreciate that, even optimal model systems, i.e., those synthesized with high-vacuum anionic methods, need thorough characterization via a combination of techniques. Besides helping to improve synthetic techniques, this methodology will be significant in fine-tuning mesoscopic tube-based models and addressing outstanding issues such as the quantitative description of the constraint release mechanism.

Molecular Heterogeneity of Polystyrene-Modified Fullerene Core Stars

Structure–property relationships of polymeric materials require careful characterization of macromolecular properties. In this work we use 2-dimensional chromatography, coupling temperature gradient interaction and size exclusion chromatography, as well as density functional theory calculations to clarify that when poly(styryl)lithium anions are reacted with stringently purified C60, stars with greater than six arms can be attained, in contrast to prior reports. We attribute this apparent contradiction to the limited ability of size exclusion chromatography to separate polymers having branched architectures. These results clarify the chemistry of anionically modified fullerenes and also show that more sophisticated separation techniques are necessary when characterizing architecturally complex polymers.

There’s plenty of gloom at the bottom: the many challenges of accurate quantitation in size-based oligomeric separations

There is a variety of small-molecule species (e.g., tackifiers, plasticizers, oligosaccharides) the size-based characterization of which is of considerable scientific and industrial importance. Likewise, quantitation of the amount of oligomers in a polymer sample is crucial for the import and export of substances into the USA and European Union (EU). While the characterization of ultra-high molar mass macromolecules by size-based separation techniques is generally considered a challenge, it is this author's contention that a greater challenge is encountered when trying to perform, for quantitation purposes, separations in and of the oligomeric region. The latter thesis is expounded herein, by detailing the various obstacles encountered en route to accurate, quantitative oligomeric separations by entropically dominated techniques such as size-exclusion chromatography, hydrodynamic chromatography, and asymmetric flow field-flow fractionation, as well as by methods which are, principally, enthalpically driven such as liquid adsorption and temperature gradient interaction chromatography. These obstacles include, among others, the diminished sensitivity of static light scattering (SLS) detection at low molar masses, the non-constancy of the response of SLS and of commonly employed concentration-sensitive detectors across the oligomeric region, and the loss of oligomers through the accumulation wall membrane in asymmetric flow field-flow fractionation. The battle is not lost, however, because, with some care and given a sufficient supply of sample, the quantitation of both individual oligomeric species and of the total oligomeric region is often possible.

Synthesis and temperature gradient interaction chromatography of model asymmetric star polymers by the “macromonomer” approach

We describe herein the synthesis and characterisation of a series of asymmetric three arm polystyrene stars via the “macromonomer” approach. The stars have been designed as model polymers to probe branched polymer dynamics and in particular to establish the chain-length of side-arm which precipitates a change in the rheological properties of the resulting polymers from “linear-like” to “star-like”. Thus, a homologous series of three arm stars have been prepared in which the molar mass of two (long) arms are fixed at 90,000gmol−1 and the molar mass of the remaining (short) arm is varied from below the entanglement molecular weight (Me) to above Me. The arms were prepared by living anionic polymerisation, resulting in well-defined chain lengths with narrow molecular weight distribution. In contrast to the usual chlorosilane coupling approach, the macromonomer approach involves the introduction of reactive chain-end functionalities on each of the arms, either through the use of a functionalised (protected) initiator or a functional end-capping agent, which allows the stars to be constructed by a simple condensation coupling reaction. In this study we will compare the relative efficiency of a Williamson and ‘click’ coupling reaction in producing the stars. Most significantly, although this approach maybe a little more time-consuming than the more common silane coupling reaction, in the present study the “long” arm may be produced in sufficient quantity such that all of the asymmetric stars are produced with long arms of identical molecular weight – the only remaining variable being the molecular weight of the short arm. This will allow for a far more robust interpretation of the resulting characterisation of the dynamic properties. Temperature gradient interaction chromatography was used alongside size exclusion chromatography to characterise the structural dispersity of the resulting stars and establish the degree of structural homogeneity.

Synthesis and characterization of an exact comb polyisoprene with three branches having the middle branch twice the molecular weight of the other two identical external branches

An exact comb polyisoprene (PI) with three branches, with the middle branch having twice the molecular weight of the two other identical external branches, was synthesized by using anionic polymerization high vacuum techniques and appropriate chlorosilane chemistry. The synthetic approach involves (a) the selective replacement of the two chlorines of 4-(dichloromethylsilyl)diphenylethylene (DCMSDPE, key molecule) with identical PI chains by titration with PILi, (b) the addition of sec-BuLi to the double bond of DPE followed by the polymerization of isoprene from the newly created anionic site to form a 3-arm living star PI, (c) the selective replacement of the two chlorines of trichloromethylsilane with 3-arm star PI to form an H-shape intermediate, and (d) the replacement of the remaining chlorine of trichloromethylsilane by linear PI chains with double the molecular weight. All intermediate and final products were characterized via size exclusion chromatography, temperature gradient interaction chromatography and 1H-NMR spectroscopy. As expected, due to the inability to control the exact stoichiometry of the linking reactants, the main product (exact comb PI) is contaminated by a few by-products, despite the fact that anionic polymerization is the most efficient way to produce well-defined polymers.

Analytical Rheology of Asymmetric H-Shaped Model Polybutadiene Melts

An asymmetric H-shaped model polybutadiene (PBd) melt, H(SS13LL20)B58, with a backbone of molar mass 58 kg/mol and with two short arms of molar mass 13 kg/mol and two long arms with molar masses around 20 kg/mol, was designed and synthesized by anionic polymerization and purified by fractional precipitation. To obtain this novel structure, two short arms were synthesized and linked together using 4-(dichloromethylsilyl)diphenylethylene (DCMSDPE). The backbone was then grown from this linkage point to produce an asymmetric star intermediate terminating in a reactive anion at the end of the backbone. The two long arms were similarly grown in a separate reaction vessel and then linked by DCMSDPE, and these linked long arms were then attached to the backbone of the asymmetric star. The multiple steps of this reaction scheme led to multiple possible byproducts. The most likely of these were identified and quantified by temperature gradient interaction chromatography (TGIC) performed on both purified and unpuri...

Complex Branched Polymers for Structure–Property Correlation Studies: The Case for Temperature Gradient Interaction Chromatography Analysis

The idiom “can't see the wood for the trees” would appear uniquely apt in describing attempts to characterize structural dispersity in complex branched polymers, especially dendritically branched polymers, by size exclusion chromatography (SEC) alone. SEC is incapable of resolving polymers with nearly identical hydrodynamic volumes which may differ in molecular weight—a particular problem in the characterization of branched polymers. This paper discusses in detail the variety of synthetic methodologies used to synthesize complex branched polymers including H-shaped polymers, comb-branched polymers, and with particular emphasis on dendritically branched polymers, also known as Cayley trees and DendriMacs. The advantages and limitations of each method are discussed, supported by a review of recent reports of temperature gradient interaction chromatography (TGIC) analysis of such polymers. With one eye to the future, recent results should provoke the polymer chemists to devise improved synthetic strategies, ...