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Landsberg am Lech, Germany

Makan A.C.,Stellenbosch University | Otte T.,Postnova Analytics GmbH | Otte T.,Hamilton Bonaduz AG | Pasch H.,Stellenbosch University

Industrial polybutadienes synthesized by Ziegler-Natta catalysts may exhibit very high molar masses, significant degrees of branching, and gel species. Such samples were analyzed by size exclusion chromatography (SEC) and asymmetric flow field-flow fractionation (AF4). Both fractionation techniques were coupled to a multiangle laser light scattering (MALLS) detector as well as a refractive index (RI) detector. The initial results obtained from SEC measurements completely contradicted the processing behavior of the materials. SEC seemed not to be suitable for accurate characterization of these materials. SEC separation was strongly influenced by coelution of linear (low molar mass) and branched (high molar mass) macromolecules, shear degradation of the larger macromolecules, and filtration of the gel fraction. For this reason the polybutadienes were subsequently analyzed by AF4 in order to compare the results and identify the limitations of SEC. The results showed that, as a specialized tool, AF4 can be used for the extended and more accurate analysis of synthetic rubber materials, especially in the case of very high molar masses. SEC is not suitable for such samples because it does not properly address the high molar mass fraction and the branched macromolecules. © 2012 American Chemical Society. Source

Otte T.,German Institute for Polymers | Brull R.,German Institute for Polymers | Macko T.,German Institute for Polymers | Pasch H.,German Institute for Polymers | Klein T.,Postnova Analytics GmbH
Journal of Chromatography A

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 °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. © 2009. Source

Otte T.,Postnova Analytics GmbH | Pasch H.,Stellenbosch University | Brull R.,German Institute for Polymers | MacKo T.,German Institute for Polymers
Macromolecular Chemistry and Physics

The molar mass distributions of high-molar-mass PE and PP were characterized using HT-AF4 and HT-SEC in combination with IR and MALS detection. Calculated molar mass distributions, average molar masses and radii of gyration were compared. It was found that HT-AF4 can detect the real extent of thermo-oxidative degradation during dissolution. HT-SEC is affected by shear and thermo-oxidative degradation while in HT-AF4 no shear stress exists. As a consequence, the molar mass averages obtained from HT-SEC are pronouncedly lower as obtained from HT-AF4. In addition, branched polyolefins cannot be correctly characterized by SEC-MALS due to abnormal co-elution effects. HT-AF4 is therefore the method of choice for studying the thermo-oxidative degradation of macromolecules with high molar masses. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Otte T.,Postnova Analytics GmbH | Otte T.,German Institute for Polymers | Pasch H.,Stellenbosch University | Macko T.,German Institute for Polymers | And 5 more authors.
Journal of Chromatography A

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 (Rg) 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. © 2011 Elsevier B.V. Source

Postnova Analytics GmbH | Date: 2012-10-18

An apparatus for measuring the light scattering properties of a sample in a liquid medium, wherein the liquid medium with the sample is illuminated by a laser beam in a measuring cell transversely to the direction of filling the liquid medium in the measuring cell or transversely to the flow direction of the liquid medium within the measuring cell, comprising a laser, a cylindrical measuring cell, a first inner aperture system, a second outer aperture system and at least two detectors, wherein the detectors are arranged outside of the second outer aperture system so that they collect the light scattered on the sample within set, different angle ranges, wherein the first inner aperture system and the second outer aperture system are formed and arranged circularly and concentrically around the axis of the measuring cell. Use of the apparatus and a method that makes use of the apparatus are also disclosed.

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