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Author: Krause, Beate × End date: 2011 ×

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    Dispersability and particle size distribution of CNTs in an aqueous surfactant dispersion as a function of ultrasonic treatment time
    (New York, NY [u.a.] : Pergamon Press, 2010) Krause, Beate; Mende, Mandy; Pötschke, Petra; Petzold, Gudrun
    The dispersability of carbon nanotubes (CNTs) was assessed by studying the sedimentation of CNTs dispersed in aqueous surfactant solutions at different ultrasonication treatment times using a LUMiSizer® apparatus under centrifugal forces. Different commercially available multiwalled CNTs, namely Baytubes® C150P, Nanocyl™ NC7000, Arkema Graphistrength® C100, and FutureCarbon CNT-MW showing quite different kinetics were compared. In addition, the particle size distributions were analyzed using dynamic light scattering and centrifugal separation analysis. The best dispersabilities were found for Nanocyl™ NC7000 and FutureCarbon CNT-MW; to prepare stable dispersions of Baytubes® C150P or Graphistrength® C100 five times the energy was needed. As a result of the centrifugal separation analysis, it was concluded that Nanocyl™ NC7000 and Baytubes® C150P were dispersed as single nanotubes using ultrasonic treatment whereas small agglomerates or bundles are existing in dispersions containing FutureCarbon CNT-MW and Graphistrength® C100. © 2010 Elsevier Ltd. All rights reserved.
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    A method for determination of length distributions of multiwalled carbon nanotubes before and after melt processing
    (New York, NY [u.a.] : Pergamon Press, 2011) Krause, Beate; Boldt, Regine; Pötschke, Petra
    A relatively simple method to determine the length distribution of carbon nanotubes (CNTs) before and after melt processing was developed. This involves the selection of a suitable solvent for dispersing pristine CNTs as well as to dissolve the matrix of melt mixed composites and the choice of an appropriate nanotube concentration. The length of suitably individualized CNTs was visualized using transmission electron microscopy and length distributions were measured using image analysis. Examples are shown for Baytubes® C150HP and Nanocyl™ NC7000 and their melt mixed composites with polycarbonate where the same procedure was applied to both, measuring the initial length distribution and the distribution after recovering from the composites. These results indicated a significant shortening after melt processing up to 30% of the initial length. © 2010 Elsevier Ltd. All rights reserved.
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    Correlation of carbon nanotube dispersability in aqueous surfactant solutions and polymers
    (New York, NY [u.a.] : Pergamon Press, 2009) Krause, Beate; Petzold, Gudrun; Pegel, Sven; Pötschke, Petra
    In order to assess the dispersability of carbon nanotube materials, tubes produced under different synthesis conditions were dispersed in aqueous surfactant solutions and the sedimentation behaviour under centrifugation forces was investigated using a LUMiFuge stability analyzer. The electrical percolation threshold of the nanotubes after melt mixing in polyamide 6.6 was determined and the state of dispersion was studied. As a general tendency, the nanotubes having better aqueous dispersion stability showed lower electrical percolation threshold and better nanotube dispersion in the composites. This indicates that the investigation of the stability of aqueous dispersions is also able to give information about the nanotubes inherent dispersability in polymer melts, both strongly influenced by the entanglement and agglomerate structure of the tubes within the as-produced nanotube materials. The shape of the nanotubes in the aqueous dispersions was assessed using a SYSMEX flow particle image analyzer and found to correspond to the shape observed from cryofractured surfaces of the polymer composites. © 2008 Elsevier Ltd. All rights reserved.
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    Characterization of the state of dispersion of carbon nanotubes in polymer nanocomposites
    (Weinheim : Wiley-VCH Verl., 2011) Buschhorn, Samuel T.; Wichmann, Malte H. G.; Sumfleth, Jan; Schulte, Karl; Pegel, Sven; Kasaliwal, Gaurav R.; Villmow, Tobias; Krause, Beate; Göldel, Andreas; Pötschke, Petra
    A practical overview of possibilities and limits to characterize the state of dispersion of carbon nanotubes (CNT) in polymer based nanocomposites is given. The most important and widely available methods are discussed with practical employment in mind. One focus is the quantitative characterization of the state of dispersion in solid samples using microscopy techniques such as optical microscopy or transmission electron microscopy. For dispersions of CNTs in aqueous media, solvents or monomers a sedimentation analysis is presented. This way dispersability and dispersion state of CNTs can be assessed. Indirect methods such as electrical conductivity measurements and rheological tests, dynamic differential scanning calorimetry and mechanical test are discussed. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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    Comparison of the molecular properties and morphology of polypropylenes irradiated under different atmospheres and after annealing
    (Hoboken, NJ [u.a.] : Wiley InterScience, 2006) Krause, Beate; Häußler, Liane; Voigt, Dieter
    Electron-beam irradiation, a well-known way of generating long-chain branching, was used to modify polypropylene. Samples were investigated with differential scanning calorimetry, polarized light microscopy, and size exclusion chromatography. Independently of the atmosphere, postannealing led to the deactivation of residual radicals and to the reduction of the nucleus density. In comparison with the initial polypropylene, the crystallization temperatures increased for nonannealed samples but decreased for annealed samples. Stable products were obtained only by irradiation in nitrogen followed by annealing. A reaction including free radicals with oxygen in the ambient atmosphere led to increasing molar mass degradation and the formation of long-chain branching after Storage. © 2006 Wiley Periodicals, Inc.
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    Characterization of electron beam irradiated polypropylene: Influence of irradiation temperature on molecular and rheological properties
    (Hoboken, NJ [u.a.] : Wiley InterScience, 2006) Krause, Beate; Voigt, Dieter; Häuβler, Liane; Auhl, Dietmar; Münstedt, Helmut
    The aim of the investigations was to analyze the influence of the temperature during the irradiation process of polypropylene on the molar mass, the formation of long chain branching and the final branching topology. A linear isotactic polypropylene homopolymer was modified by electron beam irradiation at different temperatures, with two irradiation doses to insert long chain branching. The Samples were analyzed by size exclusion chromatography coupled with a multiangle laser light scattering detector, by differential scanning calorimetry, and by shear and elongational rheology. The shear and elongational flow behavior isdiscussed in terms of the influence of molecular parameters and used to analyze the topology of the irradiated samples. With increasing temperature, a slight reduction of the molar mass, an increase of long chain branching and an increase of crystallization temperature were found. © 2006 Wiley Periodicals, Inc.
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    Long-chain branching of polypropylene by electron-beam irradiation in the molten state
    (Hoboken, NJ [u.a.] : Wiley InterScience, 2006) Krause, Beate; Stephan, M.; Volkland, S.; Voigt, D.; Häußler, L.; Dorschner, H.
    The electron-beam irradiation of polymers generates modification effects in the macromolecular structure and material properties. Therefore, irradiation processing is mostly realized in the polymer solid state. In this way, the modification of linear polypropylene may result in long-chain branching of polypropylene macromolecules. The objective of this article is to investigate the effect of a polymer in the molten state during electron-beam irradiation on the macromolecular structure and material properties of polypropylene. For this procedure, a special irradiation vessel (BG3) has been developed in which a rapid transfer of polymer films from the solid state to the molten state and a defined temperature during electron-beam irradiation are realizable. The irradiated samples have been analyzed by high-temperature size exclusion chromatography coupled with a multi-angle laser light scattering detector and differential scanning calorimetry (DSC) measurements. With an increasing irradiation dose, a high reduction of the molar mass and an increasing amount of long-chain branching are found. Compared with irradiation in the solid state, the modification in the molten state leads to a higher degree of branching. The Theological experiments in elongation flow clearly exhibit the existence of long-chain branching. Furthermore, DSC measurements show that the glass-transition temperature and peak temperatures of melting and crystallization decrease. © 2005 Wiley Periodicals, Inc.
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    Electrical, mechanical, and glass transition behavior of polycarbonate-based nanocomposites with different multi-walled carbon nanotubes
    (Oxford : Elsevier Science, 2011) Castillo, Frank Yepez; Socher, Robert; Krause, Beate; Headrick, Robert; Grady, Brian P.; Prada-Silvy, Ricardo; Pötschke, Petra
    Five commercially available multi-walled carbon nanotubes (MWNTs), with different characteristics, were melt mixed with polycarbonate (PC) in a twin-screw micro compounder to obtain nanocomposites containing 0.25-3.0 wt.% MWNT. The electrical properties of the composites were assessed using bulk electrical conductivity measurements, the mechanical properties of the composites were evaluated using tensile tests and dynamic mechanical analysis (DMA), and the thermal properties of the composites were investigated using differential scanning calorimetry (DSC). Electrical percolation thresholds (pcs) were observed between 0.28 wt.% and 0.60 wt.%, which are comparable with other well-dispersed melt mixed materials. Based on measurements of diameter and length distributions of unprocessed tubes it was found that nanotubes with high aspect ratios exhibited lower pcs, although one sample did show higher pc than expected (based on aspect ratio) which was attributed to poorer dispersion achieved during mixing. The stress-strain behavior of the composites is only slightly altered with CNT addition; however, the strain at break is decreased even at low loadings. DMA tests suggest the formation of a combined polymer-CNT continuous network evidenced by measurable storage moduli at temperatures above the glass transition temperature (T g), consistent with a mild reinforcement effect. The composites showed lower glass transition temperatures than that of pure PC. Lowering of the height of the tanδ peak from DMA and reductions in the heat capacity change at the glass transition from DSC indicate that MWNTs reduced the amount of polymer material that participates in the glass transition of the composites, consistent with immobilization of polymer at the nanotube interface. © 2011 Elsevier Ltd. All rights reserved.
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    Comparison of nanotubes produced by fixed bed and aerosol-CVD methods and their electrical percolation behaviour in melt mixed polyamide 6.6 composites
    (Barking : Elsevier, 2010) Krause, Beate; Ritschel, M.; Täschner, C.; Oswald, S.; Gruner, W.; Leonhardt, A.; Pötschke, Petra
    The electrical percolation behaviour of five different kinds of carbon nanotubes (CNTs) synthesised by two CVD techniques was investigated on melt mixed composites based on an insulating polyamide 6.6 matrix. The electrical percolation behaviour was found to be strongly dependent on the properties of CNTs which varied with the synthesis conditions. The lowest electrical percolation threshold (0.04 wt.%) was determined for as grown multi-walled carbon nanotubes without any purification or chemical treatment. Such carbon nanotubes were synthesised by the aerosol method using acetonitrile as ferrocene containing solvent and show relatively low oxygen content near the surface, high aspect ratio, and good dispersability. Similar properties could be found for nanotubes produced by the aerosol method using cyclohexane, whereas CNTs produced by the fixed bed method using different iron contents in the catalyst material showed much higher electrical percolation thresholds between 0.35 and 1.02 wt.%. © 2009 Elsevier Ltd. All rights reserved.
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    Electrical and thermal properties of polyamide 12 composites with hybrid fillers systems of multiwalled carbon nanotubes and carbon black
    (Barking : Elsevier, 2011) Socher, Robert; Krause, Beate; Hermasch, Sylvia; Wursche, Roland; Pötschke, Petra
    Hybrid filler systems of multiwalled carbon nanotubes (MWCNTs) and carbon black (CB) were incorporated into two types of polyamide 12 (PA12) using small-scale melt mixing in order to identify potential synergistic effects on the interaction of these two electrical conductive fillers. Although no synergistic effects were observed regarding the electrical percolation threshold, at loadings well above the percolation threshold higher volume conductivities were obtained for samples containing both, MWCNT and CB, as compared to single fillers. This effect was more pronounced when using a higher viscous PA12 matrix. The formation of a co-supporting network can be assumed. The combined use of CB and MWCNTs improved the macrodispersion of MWCNT agglomerates, which can be assigned as a synergistic effect. DSC measurements indicated an effect of the nanofiller on crystallisation temperatures of PA12; however this was independent of the kind or amount of the carbon nanofiller. © 2011 Elsevier Ltd.