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    Synthesis and Self-Assembly Behavior of Double Ullazine-Based Polycyclic Aromatic Hydrocarbons
    (Stuttgart : Georg Thieme, 2021) Richter, Marcus; Borkowski, Michał; Fu, Yubin; Dmitrieva, Evgenia; Popov, Alexey A.; Ma, Ji; Marszalek, Tomasz; Pisula, Wojciech; Feng, Xinliang
    Polycyclic aromatic azomethine ylides (PAMY, 1) are versatile building blocks for the bottom-up synthesis of nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs). Although the chemistry of PAMY was already established few years ago, the cycloaddition of a double PAMY building block has not been reported so far. In this work, we demonstrate the first cycloaddition of a PAMY-dimer (6), which opens the access to three different alkyl ester-substituted N-PAHs with a laterally extended double ullazine scaffold (DU-1, DU-2 and DU-3). Interestingly, the cyclic voltammetry of DU-1-3 exhibited three reversible oxidation waves, which confirmed the electron-rich nature of the double ullazine scaffold. Furthermore, in-situ spectroelectrochemistry study of ethylhexyl ester-substituted DU-3 revealed the formation of different cationic species with new absorption bands up to 1689 nm. Additionally, the influence of the attached substituents on the film formation and supramolecular organization in the thin films were investigated by polarized optical microscopy and grazing incidence wide-angle X-ray scattering.
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    A successful approach to disperse MWCNTs in polyethylene by melt mixing using polyethylene glycol as additive
    (Oxford : Elsevier Science, 2012) Müller, Michael Thomas; Krause, Beate; Pötschke, Petra
    An additive-assisted one-step melt mixing approach was developed to produce nanocomposites based on linear low density polyethylene (LLDPE) with multiwalled carbon nanotube (MWCNT). The polymer granules, nanotube powder (2 wt% Nanocyl™ NC7000) and 1-10 wt% of the non-ionic additives poly(ethylene glycol) (PEG) or poly(ethylene oxide) (PEO) with molar masses between 100 g/mol and 100,000 g/mol were simply fed together in the hopper of a small-scale DSM Xplore 15 twin-screw microcompounder. The produced MWCNT/LLDPE composites showed excellent MWCNT dispersion and highly improved electrical properties as compared to samples without the additive, whereas the effects depend on the amount and molar mass of the additive. When 7 wt% PEG (2000 g/mol) were used, a reduction of the electrical percolation threshold from 2.5 wt% to 1.5 wt% was achieved. © 2012 Elsevier Ltd. All rights reserved.
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    Polypropylene-based melt mixed composites with singlewalled carbon nanotubes for thermoelectric applications: Switching from p-type to n-type by the addition of polyethylene glycol
    (Oxford : Elsevier Science, 2017) Luo, Jinji; Cerretti, Giacomo; Krause, Beate; Zhang, Long; Otto, Thomas; Jenschke, Wolfgang; Ullrich, Mathias; Tremel, Wolfgang; Voit, Brigitte; Pötschke, Petra
    The thermoelectric properties of melt processed conductive nanocomposites consisting of an insulating polypropylene (PP) matrix filled with singlewalled carbon nanotubes (CNTs) and copper oxide (CuO) were evaluated. An easy and cheap route to switch p-type composites into n-type was developed by adding polyethylene glycol (PEG) during melt mixing. At the investigated CNT concentrations of 0.8 wt% and 2 wt% (each above the electrical percolation threshold of ∼0.1 wt%), and a fixed CuO content of 5 wt%, the PEG addition converted p-type composites (positive Seebeck coefficient (S)) into n-type (negative S). PEG was also found to improve the filler dispersion inside the matrix. Two composites were prepared: P-type polymer/CNT composites with high S (up to 45 μV/K), and n-type composites (with S up to −56 μV/K) through the addition of PEG. Two prototypes with 4 and 49 thermocouples of these p- and n-type composites were fabricated, and delivered an output voltage of 21 mV and 110 mV, respectively, at a temperature gradient of 70 K.
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    The force of MOFs: The potential of switchable metal-organic frameworks as solvent stimulated actuators
    (Cambridge : RSC, 2020) Freund, Pascal; Senkovska, Irena; Zheng, Bin; Bon, Volodymyr; Krause, Beate; Maurin, Guillaume; Kaskel, Stefan
    We evaluate experimentally the force exerted by flexible metal-organic frameworks through expansion for a representative model system, namely MIL-53(Al). The results obtained are compared with data collected from intrusion experiments while molecular simulations are performed to shed light on the re-opening of the guest-loaded structure. The critical impact of the transition stimulating medium on the magnitude of the expansion force is demonstrated.
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    Impact of synthesis temperature on morphology, rheology and electromagnetic interference shielding of CVD-grown carbon nanotube/polyvinylidene fluoride nanocomposites
    (Lausanne : Elsevier Sequoia, 2017) Mirkhani, Seyyed Alireza; Arjmand, Mohammad; Sadeghi, Soheil; Krause, Beate; Pötschke, Petra; Sundararaj, Uttandaraman
    Employing chemical vapor deposition technique, multi-walled carbon nanotubes (CNTs) were synthesized over Fe catalyst at a broad range of temperatures, i.e. 550° C to 950° C (at 100° C intervals). CNTs were melt-mixed into a polyvinylidene fluoride (PVDF) matrix at various loadings, and then compression molded. Surprisingly, despite the ascending trend of CNT powder conductivity with the synthesis temperature, the nanocomposites made with CNT synthesized at 650° C had significantly lower percolation threshold (around 0.4 wt%) and higher electromagnetic interference shielding effectiveness (EMI SE) (20.3 dB over the X-band for 3.5 wt% CNT and 1.1 mm thickness) than the other temperatures. Exhaustive characterization studies were conducted on both CNTs and composites to unveil their morphological and electrical characteristics. Superior EMI shielding of CNT650° C was attributed to a combination of high carbon purity, aspect ratio, crystallinity, and moderate powder conductivity along with decent state of dispersion within the PVDF matrix.
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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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    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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    Effect of synthesis catalyst on structure of nitrogen-doped carbon nanotubes and electrical conductivity and electromagnetic interference shielding of their polymeric nanocomposites
    (New York, NY [u.a.] : Pergamon Press, 2016) Arjmand, Mohammad; Chizari, Kambiz; Krause, Beate; Pötschke, Petra; Sundararaj, Uttandaraman
    Different catalysts including Co, Fe, and Ni were used to synthesize nitrogen-doped carbon nanotubes (N-CNTs) by chemical vapor deposition technique. Synthesized N-CNTs were melt mixed with a polyvinylidene fluoride (PVDF) matrix using a small scale mixer at different concentrations ranging from 0.3 to 3.5 wt%, and then compression molded. The characterization techniques revealed significant differences in the synthesis yield and the morphological and electrical properties of both N-CNTs and nanocomposites depending on the catalyst type. Whereas Co and Fe resulted in yields comparable to industrial multiwalled CNTs, Ni was much less effective. The N-CNT aspect ratio was the highest for Co catalyst, followed by Ni and Fe, whereas nitrogen content was the highest for Ni. Raman spectroscopy revealed lowest defect number and highest N-CNT crystallinity for Fe catalyst. Characterization of N-CNT/PVDF nanocomposites showed better dispersion for N-CNTs based on Co and Fe as compared to Ni, and the following order of electrical conductivity and electromagnetic interference shielding (from high to low): Co > Fe > Ni. The superior electrical properties of (N-CNT)Co nanocomposites were ascribed to a combination of high synthesis yield, high aspect ratio, low nitrogen content and high crystallinity of N-CNTs combined with a good state of N-CNT dispersion.
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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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    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.