RESUMEN

As commodity plastics, polyolefins are in high demand and used in innumerable applications. An important reason for their success-story is their high versatility in terms of applications. The application range of polyolefins was significantly extended through the development of functionalization. A common functionalization for improving the compatibility of polyolefins with more polar polymers and surfaces is grafting with maleic anhydride. While maleic anhydride-grafted polyolefins have found widespread application, methods for their characterization remain rudimentary compared to the developments seen in the structural characterization of polyolefins in general. Herein, we propose two new approaches for determining the degree of functionalization as a function of the molar mass of maleic anhydride grafted polyolefins. On the one hand, the latest generation bandpass filter-based IR detectors are shown to be sensitive to the carbonyl moiety of MAH. After optimization of analysis conditions, the relation between MAH content and molar mass could be unraveled in an easily applicable approach suitable for routine analysis. On the other hand, the high reactivity of MAH was leveraged in a tagging approach. By imidization with a UV chromophore, MAH distribution can be assessed by HT-GPC-UV with significantly higher sensitivity compared to HT-GPC-IR.

Asunto(s)

RESUMEN

Liquid chromatography at critical conditions is of interest as it may unravel molecular information on macromolecular structures not accessible by any other analytical techniques. Yet so far, such conditions have never been experimentally established for copolymers, where a particular need for such information exists. Toward this goal, critical conditions for statistical ethylene propylene copolymers were identified. In the first approach the composition of the binary mobile phase was varied at a constant temperature, and secondly by modulating the adsorption-desorption temperature at constant mobile phase composition. Solvents for both methods were identified by using a novel approach that combines structure retention relationships with Hansen Solubility Parameters. As a result, for the first time, the heterogeneity of an ethylene propylene diene terpolymer sample with regard to the pendant double bond of the diene could be determined. The novel chromatographic approach was validated by measuring the composition of fractions taken over the chromatographic run offline by nuclear magnetic resonance. In summary, this work gave the first experimental evidence for the existence of critical conditions for polyolefin random copolymers, as postulated by Brun. This novel chromatographic approach holds immense potential to engineer complex polymers towards future applications by making use of the now-accessible molecular information.

RESUMEN

The elution behavior of ethylene-norbornene (EN) copolymers prepared with various catalysts was studied in selected binary solvent gradients using porous graphite (HypercarbTM) as stationary phase. It was found that the elution volumes of the EN copolymers correlated with their average norbornene content. For a series with norbornene content lower than 20 mol % the correlation was positive (i.e. increasing elution volumes with increasing norbornene content), whereas for a series with norbornene contents above 20 mol % it was negative (decreasing elution volumes with increasing norbornene content). It is known that EN copolymers have complicated microstructures that depend on norbornene content and the catalyst system used for synthesis. Thus, it is supposed that the opposing trends in the elution behavior of the EN copolymers are caused by differences in their microstructure, ultimately governed by the norbornene content. Our conclusions are supported by results from NMR spectroscopy, which revealed the microstructure, and differential scanning calorimetry (DSC).

Asunto(s)

RESUMEN

The extraction of different stabilizers from a polymer matrix and the subsequent separation of said stabilizers is one of the most important as well as challenging undertakings in polymer chemistry. A multitude of stabilizers exists, each of which may be hard to extract, be difficult if not impossible to separate from other stabilizers or necessitate very selected and time-consuming intermediate stages for separation. Certain polymer matrices even pose additional challenges, such as polyolefins being only soluble at elevated temperatures. One of the most well-established approaches for the extraction of stabilizers is Soxhlet extraction. However, even this highly successful approach shows only limited success with regard to the extraction of the ever more relevant oligomeric stabilizers or the extraction of multiple stabilizers in a one-shot approach. Moreover, performing Soxhlet extractions often necessitates ≥24 h. For these reasons, alternative approaches for the extraction of stabilizers from polymers are highly sought after. An approach with enormous potential is solid-phase extraction, which allows the selective retention and enrichment of stabilizers. Herein, the very first application of high-temperature solid-phase extraction for the extraction of stabilizers from polyolefin matrices is described; as with other extraction techniques, the identification and quantification of the stabilizers is then allowed. At temperatures of 140-160°C, it was possible to adsorb common polyolefin stabilizers selectively on a silica solid phase from their polyolefin matrix. To predict high-temperature solid-phase extraction test conditions, first LC tests are necessary, offering an elegant approach for the separation of polyolefins from oligomeric stabilizers, which was not achievable until now.

RESUMEN

Additives are added to polymers in small concentration to achieve desired application properties widely used to tailor the properties. The rapid diversification of their molecular structures, with often only minute differences, necessitates the development of adequate chromatographic techniques. While modified silica so far is the workhorse as stationary phase we have probed the potential of porous graphitic carbon (HypercarbTM) for this purpose. The results show that the multitude of physicochemical interactions between analyte molecules and the graphitic surface enables separations of polyolefin stabilizers with unprecedented selectivity. To support the chromatographic results the adsorption capability of HypercarbTM for selected antioxidants and UV absorbers has been determined by Raman spectroscopy and argon physisorption measurements. The shift of the Graphite-band in the Raman spectra of HypercarbTM upon infusion with additives correlates with the changes in the Adsorption Potential Distributions. The results of argon physisorption measurements go hand in hand with the chronology of desorption of the additives in liquid chromatography experiments. The elution sequence can be explained by van der Waals or London forces, π-π-interactions and electron lone pair donor-acceptor interactions between the graphite surface and analyte functional groups.

Asunto(s)

RESUMEN

Porous graphite as sorbent differs significantly from all other HPLC column packings. It stands out due to its chemically extremely homogeneous surface, which moreover is planar on an atomic level. This sorbent, according to its non-polar but polarizable surface, is able to adsorb polar as well as non-polar small molecules as well as macromolecules. Moreover, it enables their separation induced by minute differences in their molecular architecture, which includes the aspects of planarity, branching or tacticity of macromolecules. Although graphite had already been used many years for the separation of small molecules, the application of porous graphite for separations in the domain of synthetic polymers has been rare. In 2009 it was found that porous graphite enables the separation of polyethylene and polypropylene on the basis of their full adsorption and desorption, when suitable solvents are used. This approach has led to the fast elaboration of HPLC systems for separations of various polar modified as well as non-polar polyolefins. Due to pronounced adsorptive interactions, porous graphite is applicable even at temperatures as high as 160 °C. The results presented in this paper manifest that porous graphite enables to obtain important information about the composition distribution of various synthetic polymers, the architecture of macromolecules (i.e., branching) or their tacticity, and underlines its enormous application potential.

Asunto(s)

RESUMEN

Branching is a molecular metric that strongly influences the application properties of polymers. Consequently, detailed information on the microstructure is required to gain a deeper understanding of structure-property relationships. In the present case, we employ high-performance liquid chromatography to characterize the branching in a poly(bisphenol A carbonate) (PC). To this end, a method was developed based on a mobile phase gradient in a very narrow range (±1.4 vol %) around the point of adsorption (98.9/1.1 vol % chloroform/methyl tert-butyl ether), which we refer to as solvent gradient at near-critical conditions. Application of such gentle gradient enabled separation of PC according to end-groups. The separation mechanism was confirmed by collecting fractions of a separated sample and subsequently analyzing these by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Hyphenating the developed gradient method with size-exclusion chromatography as the second dimension (2D-LC) enabled separation of linear and branched PC chains and determination of the molar mass distribution of the fractions. A reversed elution order was observed for branched species in 2D-LC, meaning that low molar mass chains exhibited higher elution volumes in the first dimension than higher molar masses. This finding was explained by influences of end-groups as well as the architecture of the branched polymer chains.

RESUMEN

The elution behavior of polyethylene (PE) and the three stereoisomers of polypropylene (PP) was studied on porous graphite along with three other carbon-based sorbents, carbon-clad zirconia particles, activated carbon, and exfoliated graphite in a systematic way in this work. Decahydronaphthalene, 1,2,3,4-tetrahydronaphthalene, 1,3,5-trimethylbenzene, tetrachloroethylene, xylene and p-xylene were used as mobile phases. While PE is adsorbed to various extents on all the tested carbonaceous sorbents from the majority of the solvents, PP is fully adsorbed only in selected cases. Testing alcohols (C7-C9) as mobile phase with Hypercarb™ indicates that all stereoisomers of PP are selectively adsorbed and desorbed when a solvent gradient alcohol→1,2,4-trichlorobenzene is used at 160°C. The retention of all stereoisomers of PP increases with the polarity of the alcohol. Linear PE is retained on Hypercarb™ even from 1,2-dichloro- and 1,2,4-trichlorobenzene, when a temperature below 120°C is applied, while it is not retained from these solvents at higher temperatures. All stereoisomeric forms of PP are not adsorbed under the same conditions. Some of the tested new sorbent/solvent systems have potential to be applied in routine analysis of industrially synthesised polyolefins.

RESUMEN

High-temperature two-dimensional liquid chromatography (HT 2D-LC) was developed for the separation and characterization of functional polyolefins. Therefore, the key experimental parameters, namely the injection volume, the mobile phase composition, the flow rate in SEC and the time and phase of sampling into the second dimension, were systematically varied and their influence on the resolution of separation were studied. The HPLC separation of ethylene-vinylacetate waxes was realized using silica gel as stationary phase and a solvent gradient decalin→cyclohexanone, while SEC separations were realized in the chromatographic system polystyrene divinyl benzene column/1,2,4-trichlorobenzene. By choosing suitable experimental parameters, the run time needed for one complete 2D-LC analysis of a polymer sample was shortened from about 200 min to 100 min. However, the developed method failed to adsorb polypropylene and ethylene/1-butene copolymers grafted with 13 or 3 mol.% of methyl methacrylate respectively. Using porous graphite as a stationary phase and a solvent gradient 1-decanol→1,2,4-trichlorobenzene as mobile phase 2D-LC separations of both grafted polyolefins were realized.

Asunto(s)

RESUMEN

A new separation principle was recently introduced into the analytical characterization of polyolefins by researchers from the German Institute for Polymers in Darmstadt. It was demonstrated that polyolefins can be selectively separated via high-performance liquid chromatography on the basis of their adsorption/desorption behaviours at temperatures as high as 160 °C. A Hypercarb® column packed with porous graphite gave the best results. The mobile phase consisted of a mixture of 1-decanol and 1,2,4-trichlorobenzene. In this work, the same chromatographic system is applied to the separation of ethylene/alkene and ethylene/norbornene copolymers. It was found that the elution volumes of the samples correlate linearly with the average chemical composition of samples. The elution volume is indirectly proportional to the concentration of branches in the ethylene/alkene copolymer. Branching shortens the length of continuous methylene sequences of the polymer backbone, thus decreasing the probability of orientation of a methylene sequence in a flat conformation on the graphite surface, which enables the most intensive van der Waals interactions between the methylene backbone and the carbon surface. An opposite trend in the elution order has been found for ethylene/norbornene copolymers. The elution volume of the ethylene/norbornene copolymers increased with the concentration of norbornene. It indicates pronounced attractive interactions between graphite and the cyclic comonomer.

Asunto(s)

RESUMEN

The elution behavior of linear polyethylene and isotactic, atactic and syndiotactic polypropylene was tested using three different carbon column packings: porous graphite (Hypercarb), porous zirconium oxide covered with carbon (ZirChrom-CARB), and activated carbon TA 95. Several polar solvents with boiling points above 150°C were selected as mobile phases: 2-ethyl-1-hexanol, n-decanol, cyclohexylacetate, hexylacetate, cyclohexanone, ethylene glycol monobutyl ether and one non-polar solvent, n-decane. Polyethylene standards were completely or partially adsorbed in all tested sorbent/solvent systems. Polypropylene standards were partially adsorbed on Hypercarb and carbon TA95, but did not adsorb on ZirChrom-CARB. ZirChrom-CARB retained polyethylene pronouncedly when 2-ethyl-1-hexanol, cyclohexylacetate or hexylacetate were used as mobile phases at temperature 150 or 160°C, while all three basic stereoisomers of polypropylene eluted in size exclusion mode in these sorbent/solvent pairs. This is very different from the system Hypercarb/1-decanol, which separated polypropylene according to its tacticity. The opposite elution behavior of polyethylene and polypropylene in system ZirChrom-CARB/2-ethyl-1-hexanol (polypropylene eluted, polyethylene fully adsorbed) enabled to realize separation of blends of polyethylene and polypropylene. Ethylene/1-hexene copolymers were separated according to their chemical composition using system Hypercarb/2-ethyl-1-hexanol/1,2,4-trichlorobenzene.

Asunto(s)

RESUMEN

Polyolefins are the most widely produced synthetic polymer commodity and are found in countless applications ranging from bottles, packaging films to bullet-proof jackets, etc. Such widely different applications rely on high variability in the physical properties of polyolefins, which is a result of variations in microstructure, chemical composition and molar mass. Though polyolefins contain only carbon (C) and hydrogen (H) atoms, the microstructures of polyolefins are extremely variable, differing in the nature of the monomers (e.g. ethylene versus propylene), the degree of branching, chemical composition in the case of copolymers and finally their molar masses. Production, research and development of polyolefins require the analysis of polyolefin samples in terms of all these parameters. Development of efficient and robust analytical techniques based on the interactive LC is reviewed. The needed computational/theoretical studies to understand the retention mechanism in the newly developed chromatography systems are discussed.

RESUMEN

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.

Asunto(s)

RESUMEN

The automated procedure for the monitoring of the adsorption process in the solute-sorbent-solvent system has been elaborated. It uses commercially available instrument CRYSTAF model 200. The application of CRYSTAF enabled monitoring of adsorption of linear polyethylene with weight average molar masses of 2, 14, and 53 kg/mol from 1,2,4-trichlorobenzene onto zeolite SH-300 at temperature as high as 140 degrees C. It is the authors' understanding that this is the first demonstration of an adsorption isotherms for polyethylene. The measurement with the CRYSTAF instrument reduces manual manipulations with dangerous solvents at high temperature and enables automated long-time monitoring of the concentration of the solute in an adsorption system.

RESUMEN

The on-line coupling of gel permeation chromatography (GPC) and 1H NMR operating at temperatures up to 130 degrees C is presented. A NMR flow probe with a cell volume of 120 microL and a stop-flow valve are developed for on-flow and stop-flow NMR measurements at high temperatures. To maintain high and constant temperatures through the whole probe, the flow probe contains two separate heating circuits. A modified stop-flow valve is developed as a control device for enabling on-flow and stop-flow experiments at high temperature conditions. Heated transfer lines connect the flow probe with the high temperature GPC system. Due to their semicrystalline nature, polyolefins can be studied by liquid chromatography only at temperatures above 100 degrees C. The novel high temperature GPC-NMR system is used for the separation of complex polyolefins regarding their molar mass and for the analysis of different chemical structures. Blends of polyethylene, poly(methyl methacrylate), and ethylene-methyl methacrylate copolymers are separated according to the molar masses of the components. The compositions of the components are directly studied by on-line NMR. Moreover, the chemical composition distribution of an ethylene-methyl methacrylate copolymer sample is analysed. Differences between results of on-flow and stop-flow measurements are discussed.

Asunto(s)

RESUMEN

The adsorption of polyethylene and polypropylene on zeolites depends on the nature of zeolite, the solvent as well as the molar mass of the polymer sample. For example, linear polyethylene is strongly retained on zeolite SH-300 from decalin, while isotactic, syndiotactic or atactic polypropylene is fully eluted in this system. On the other hand, polypropylene is retained on zeolite CBV-780 from diphenylether, while linear polyethylene is eluted. These differences in the elution behaviour have been utilised for selective removal of either linear polyethylene or polypropylene from blends of both polymers. The desorption of the retained polymer is difficult, or at times impossible. However, the selected adsorption systems have complimentary character, i.e. either one or second component is eluted or fully retained. Thus these sorbent/solvent systems, identified herein, are the first isocratic chromatographic systems, which enable selectively to remove polyethylene or polypropylene from their mixture. Moreover, decalin/SH-300 enables the removal of both linear and branched polyethylene from mixtures with random ethylene/propylene copolymers (polyethylene fully retained, ethylene/propylene copolymers eluted).

Asunto(s)

RESUMEN

Faujasite type zeolite CBV-780 was tested as adsorbent for isotactic polypropylene by liquid chromatography. When cyclohexane, cyclohexanol, n-decanol, n-dodecanol, diphenylmethane, or methylcyclohexane was used as mobile phase, polypropylene was fully or partially retained within the column packing. This is the first series of sorbent-solvent systems to show a pronounced retention of isotactic polypropylene. According to the hydrodynamic volumes of polypropylene in solution, macromolecules of polypropylene should be fully excluded from the pore volume of the sorbent. Sizes of polypropylene macromolecules in linear conformations, however, correlate with the pore size of the column packing used. It is presumed that the polypropylene chains partially penetrate into the pores and are retained due to the high adsorption potential in the narrow pores.

RESUMEN

Linear polyethylene and isotactic polypropylene standards were injected into columns which contained MFI (SH-300 and silicalite) or faujasite (CBV-780) type zeolites. 1,2,4-Trichlorobenzene, cyclohexanone, 2-ethyl-hexanol, decalin and tetralin were used as mobile phases at 140 degrees C. It was found that polyethylene is fully retained on zeolite SH-300 when decalin is used as a mobile phase. Moreover, polyethylene is partially retained on zeolite SH-300 from tetralin and from 1,2,4-tichlorobenzene, on silicalite from decalin and in a very small extent on zeolite CBV-780 from decalin. Using all other solvents, polyethylene and polypropylene were not retained in any of the columns tested. This is the first experimental observation of polyethylene adsorption from a solvent on a chromatographic stationary phase.

Asunto(s)