Robert Brüll

Investigation of the Melting and Crystallization Behavior of Random Propene/α-Olefin Copolymers by DSC and CRYSTAF

The melting and crystallization behavior of random propene/higher linear α-olefin copolymers synthesized with the catalyst system (CH3)2Si(2-methylbenz-[e]indenyl)2ZrCl2/MAO were investigated. According to Florys theory, the melting point depression is linearly related to the amount of comonomer incorporated irrespective of the nature thereof. The crystallization temperature decreased as well linearly with increasing comonomer content, but was independent of the comonomer type. The comonomer amount had an equally depressant effect on the melting temperature and on the crystallization temperature regardless of whether the crystallization occurred from melt or from dilute solution.

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.

Characterization of branched ultrahigh molar mass polymers by asymmetrical flow field-flow fractionation and size exclusion chromatography

The molar mass distribution (MMD) of synthetic polymers is frequently analyzed by size exclusion chromatography (SEC) coupled to multi angle light scattering (MALS) detection. For ultrahigh molar mass (UHM) or branched polymers this method is not sufficient, because shear degradation and abnormal elution effects falsify the calculated molar mass distribution and information on branching. High temperatures above 130 °C have to be applied for dissolution and separation of semi-crystalline materials like polyolefins which requires special hardware setups. Asymmetrical flow field-flow fractionation (AF4) offers the possibility to overcome some of the main problems of SEC due to the absence of an obstructing porous stationary phase. The SEC-separation mainly depends on the pore size distribution of the used column set. The analyte molecules can enter the pores of the stationary phase in dependence on their hydrodynamic volume. The archived separation is a result of the retention time of the analyte species inside SEC-column which depends on the accessibility of the pores, the residence time inside the pores and the diffusion ability of the analyte molecules. The elution order in SEC is typically from low to high hydrodynamic volume. On the contrary AF4 separates according to the diffusion coefficient of the analyte molecules as long as the chosen conditions support the normal FFF-separation mechanism. The separation takes place in an empty channel and is caused by a cross-flow field perpendicular to the solvent flow. The analyte molecules will arrange in different channel heights depending on the diffusion coefficients. The parabolic-shaped flow profile inside the channel leads to different elution velocities. The species with low hydrodynamic volume will elute first while the species with high hydrodynamic volume elute later. The AF4 can be performed at ambient or high temperature (AT-/HT-AF4). We have analyzed one low molar mass polyethylene sample and a number of narrow distributed polystyrene standards as reference materials with known structure by AT/HT-SEC and AT/HT-AF4. Low density polyethylenes as well as polypropylene and polybutadiene, containing high degrees of branching and high molar masses, have been analyzed with both methods. As in SEC the relationship between the radius of gyration (R(g)) or the molar mass and the elution volume is curved up towards high elution volumes, a correct calculation of the MMD and the molar mass average or branching ratio is not possible using the data from the SEC measurements. In contrast to SEC, AF4 allows the precise determination of the MMD, the molar mass averages as well as the degree of branching because the molar mass vs. elution volume curve and the conformation plot is not falsified in this technique. In addition, higher molar masses can be detected using HT-AF4 due to the absence of significant shear degradation in the channel. As a result the average molar masses obtained from AF4 are higher compared to SEC. The analysis time in AF4 is comparable to that of SEC but the adjustable cross-flow program allows the user to influence the separation efficiency which is not possible in SEC without a costly change of the whole column combination.

A review on the development of liquid chromatography systems for polyolefins

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.

The copolymerization of Propylene with higher, linear α-olefins

Propylene was copolymerized with the linear α-olefins 1-octene, 1-decene, 1-tetradecene, and 1-octadecene. The metallocene catalyst Me2Si(2-MeBenz[e]Ind)2ZrCl2, in conjunction with methylalumoxane as a cocatalyst, was used to synthesize the copolymers. The copolymers were characterized by 13C and 1H NMR with a solvent mixture of 1,2,4-trichlorobenzene (TCB) and benzene-d6 (9/1) at 100 °C. Thermal analyses were carried out to determine the melting and crystallization temperatures, whereas the molecular weights and molecular weight distributions were determined by gel permeation chromatography with TCB at 140 °C. Glass-transition temperatures were determined with dynamic mechanical analysis. Relationships among the comonomer type and amount of incorporation and the melting/crystallization temperatures, glass-transition temperature, crystallinity, and molecular weight were established. Moreover, up to 3.5% of the comonomer was incorporated, and there was a decrease in the molecular weight with increased comonomer content. Also, the melting and crystallization temperatures decreased as the comonomer content increased, but this relationship was independent of the comonomer type. In contrast, the values for the glass-transition temperature also decreased with increased comonomer content, but the extent of the decrease was dependent on the comonomer type.

Analysis of polyolefin blends by crystallization analysis fractionation

Crystallization analysis fractionation (CRYSTAF) is a new technique for the analysis of the composition of polyolefin blends. CRYSTAF fractionates blend components of different crystallizability by slow cooling of a polymer solution. During the crystallization step the concentration of the polymer solution is monitored as a function of temperature. Different from DSC, blends of HDPE, LDPE and PP are separated into the components and quantitative information can be obtained directly from the crystallization curves. Even very low amounts of one component in PE/PP and HDPE/LDPE blends can be quantified with good accuracy. The applicability of the technique for the analysis of Ziegler-Natta, and metallocene-catalyzed polyolefins is demonstrated and the analysis of waste plastics fractions is discussed.

Optimisation of ambient and high temperature asymmetric flow field-flow fractionation with dual/multi-angle light scattering and infrared/refractive index detection

Asymmetric flow field-flow fractionation (AF4) enables to analyse polymers with very high molar masses under mild conditions in comparison to size exclusion chromatography (SEC). Conventionally, membranes for AF4 are made from cellulose. Recently, a novel ceramic membrane has been developed which can withstand high temperatures above 130 degrees C and chlorinated organic solvents, thus making it possible to characterise semicrystalline polyolefins by HT-AF4. Two ceramic membranes and one cellulose membrane were compared with regard to their quality of molar mass separation and the loss of the polymer material through the pores. Separating polystyrene standards as model compounds at different cross-flow gradients the complex relationship between cross-flow velocity, separation efficiency, the molar mass and peak broadening could be elucidated in detail. Moreover, the dependence of signal quality and reproducibility on sample concentration and mass loading was investigated because the evaluation of the obtained fractograms substantially depends on the signal intensities. Finally, the performance of the whole system was tested at high temperature by separating PE reference materials of high molar mass.

Application of the evaporative light scattering detector to analytical problems in polymer science

Over the last two decades the evaporative light scattering detector (ELSD) has found more and more use in liquid chromatography (LC) of synthetic polymers. The reason behind this is that it can be used for a significantly wider variety of analyte/solvent combinations. Although in many of the applications the ELSD has been used in a qualitative manner, it can also be used quantitatively. For quantitative interpretation of analyses it is, in the case of synthetic polymers, essential to know how parameters, which characterize a polymer sample (i.e., molar mass and chemical composition), as well as parameters, which are a consequence of the LC separation (i.e., composition and flow rate of the mobile phase, its composition), influence the response of the ELSD. This review gives a tabulated overview over applications of ELS detectors in polymer analysis. The influence of parameters arising from either the polymer side or the chromatographic separation is discussed in detail and, in addition, the influence of the ELS detectors running conditions, i.e. type and flow rate of gas and temperature of nebulizer and evaporator), will be reviewed. This information will prove valuable whenever the calibration of an ELSD for the quantitative analysis of synthetic polymers is attempted.

Elution behavior of polyethylene and polypropylene standards on carbon sorbents

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.

Polymerization of higher linear α-olefins with (CH3)2Si(2-methylbenz[e]indenyl)2ZrCl2

Poly-α-olefins ranging from poly-1-pentene to poly-1-octadecene with narrow polydispersities were synthesized with (CH3)2Si(2-methylbenz[e]indenyl)2ZrCl2 and methylaluminoxane at polymerization temperatures (Tp s) ranging from −15 to 180 °C and were characterized by gel permeation chromatography, NMR spectroscopy, and differential scanning calorimetry. The molar masses of the homopolymers obtained with (CH3)2Si(2-methylbenz[e]indenyl)2ZrCl2 were notably higher than those of poly-α-olefins synthesized with other zirconium-based metallocenes under similar conditions. The temperature dependence of the molar mass distribution of the poly-α-olefins can be described by a common exponential decay function regardless of the investigated monomer. At Tp s ranging from 20 to 100 °C, moderate isotacticity prevailed, but outside this temperature range, the polymers were less stereoregular.