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- ItemA 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, PetraAn 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.
- ItemPolypropylene-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, PetraThe 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.
- ItemDispersability of multiwalled carbon nanotubes in polycarbonate-chloroform solutions(Oxford : Elsevier Science, 2014) Staudinger, Ulrike; Krause, Beate; Steinbach, Christine; Pötschke, Petra; Voit, BrigitteThe dispersion of commercial multiwalled carbon nanotubes (MWCNTs, Nanocyl™ NC7000) in chloroform and in polycarbonate (PC)-chloroform solutions was investigated by variation of the polymer concentration, MWCNT amount and sonication time and compared with PC/MWCNT composites, which were processed by melt mixing, subsequently dissolved in chloroform and dispersed via sonication under the same conditions. The sedimentation behaviour was characterised under centrifugal forces using a LUMiSizer® separation analyser. The space and time resolved extinction profiles as a measure of the stability of the dispersion and the particle size distribution were evaluated. Sonication up to 5 min gradually increases the amount of dispersed particles in the solutions. A significant improvement of the MWCNT dispersion in chloroform was achieved by the addition of PC indicating the mechanism of polymer chain wrapping around the MWCNTs. In dispersions of melt mixed PC/MWCNT composites the dispersion of MWCNTs is significantly enhanced already at a low sonication time of only 0.5 min due to very efficient polymer wrapping during the melt mixing process. However, the best dispersion quality does not lead to the highest electrical conductivity of thin composite films made of these PC/MWCNT dispersions.
- ItemLocalization of carbon nanotubes in polyamide 6 blends with non-reactive and reactive rubber(Oxford : Elsevier Science, 2014) Krause, Beate; Schneider, Cecile; Boldt, Regine; Weber, Martin; Park, Hye Jin; Pötschke, PetraBlending of two immiscible polymer matrices can be an effective way to combine favourable properties of both blend partners. The additional incorporation of multiwalled carbon nanotubes (MWCNTs) in such thermoplastic blends may further enhance the blend properties and especially generate electrical conductivity. In the present study, 20 wt.% of non-reactive rubber and maleic anhydride functionalized rubber were melt blended with polyamide 6 and 3 wt.% MWCNTs by using different incorporation strategies. For the blends containing non-reactive rubber, the MWCNTs were always localized selectively in the thermodynamically preferred polyamide phase as shown by TEM images and electrical measurements. Interestingly, the different strategies resulted in different localization behaviours of the MWCNTs in case of the reactive rubber. These findings demonstrate the significant influence of maleic anhydride groups of the rubber component on localization of MWCNTs in the different blend phases which results in different values of electrical volume resistivity of the blends. © 2014 The Authors. Published by Elsevier Ltd.
- ItemMelt mixed SWCNT-polypropylene composites with very low electrical percolation(Oxford : Elsevier Science, 2016) Krause, Beate; Pötschke, Petra; Ilin, Evgeniy; Predtechenskiy, MikhailSinglewalled carbon nanotube material of the type TUBALL™ (OCSiAl) was used to prepare composites with polypropylene by melt mixing using a conical twin screw micro-compounder. The compression moulded composites showed electrical percolation between 0.075 and 0.1 wt % and achieved volume resistivity values lower than 1 kOhm-cm already at 0.8 wt % loading. Light microscopy and scanning electron microscopy revealed good distribution and dispersion into small diameter bundles as well as retained high nanotube length. In connection with the very low percolation threshold this indicates that the SWCNT material shows an exceptionally good dispersibility which may be due to relatively high nanotube diameters with a mean value of 1.6 nm. In tensile tests already 0.1 wt % nanotube additions resulted in slight increase in Young's modulus and maximum stress. Tuball™ SWCNT material seems to be very promising for conductivity enhancement.
- ItemThe influence of matrix viscosity on MWCNT dispersion and electrical properties in different thermoplastic nanocomposites(Oxford : Elsevier Science, 2012) Socher, Robert; Krause, Beate; Müller, Michael T.; Boldt, Regine; Pötschke, PetraComposites of MWCNTs having each three different levels of matrix viscosity with five different polymers (polyamide 12, polybutylene terephthalate, polycarbonate, polyetheretherketone and low density polyethylene) were melt mixed to identify the general influence of matrix viscosity on the electrical properties and the state of MWCNT dispersion. Huge differences in the electrical percolation thresholds were found using the same polymer matrix with different viscosity grades. The lowest percolation thresholds were always found in the composites based on the low viscosity matrix. The state of primary MWCNT agglomerate dispersion increased with increasing matrix viscosity due to the higher input of mixing energy. TEM investigations showed nanoagglomerated structures in the low viscosity samples which are obviously needed to achieve low resistivity values. The effect of nanotube shortening was quantified using two different viscosity grades of polycarbonate. Due to the higher mixing energy input the nanotube shortening was more pronounced in the high viscosity matrix which partially explains the higher percolation threshold. © 2011 Elsevier Ltd. All rights reserved.
- ItemMelt mixed PCL/MWCNT composites prepared at different rotation speeds: Characterization of rheological, thermal, and electrical properties, molecular weight, MWCNT macrodispersion, and MWCNT length distribution(Oxford : Elsevier Science, 2013) Pötschke, Petra; Villmow, Tobias; Krause, BeateComposites of poly(caprolactone) (PCL) and 0.5 wt.% multiwalled carbon nanotubes (MWCNT) were prepared by melt-mixing in a conical twin-screw micro-compounder by varying the rotation speed between 25 and 400 rpm at constant mixing time and temperature. The state of dispersion analyzed by light microscopy was improved with increasing rotation speed but levels off starting at about 100 rpm. PCL molecular weight as well as crystallization and melting behavior did show only insignificant difference when varying the rotation speed. Concerning melt rheological properties, storage modulus G′ and complex viscosity η* at 0.1 rad/s increased up to a rotation speed of about 75 rpm illustrating improved dispersion. When further increasing the speed G′ and η* decreased which was attributed to more pronounced nanotube shortening as quantified by TEM measurements. Both effects - improved dispersion and nanotube shortening - are also reflected in the electrical resistivity values of compression molded samples which show a minimum of resistivity at the rotation speed of 75 rpm corresponding to a specific mechanical energy input of 0.47 kWh/kg. © 2013 Elsevier Ltd. All rights reserved.
- ItemComparative study of singlewalled, multiwalled, and branched carbon nanotubes melt mixed in different thermoplastic matrices(Oxford : Elsevier Science, 2018) Krause, Beate; Barbier, Carine; Kunz, Karina; Pötschke, PetraIn this contribution, three different types of CNTs, namely single-walled (SWCNT), multi-walled (MWCNT) and branched MWCNTs were melt mixed in amounts of 0.1–10 wt.-% in polypropylene (PP), polycarbonate (PC) and poly(vinylidene fluoride) (PVDF) using a small-scale microcompounder. The filler dispersion of compression-moulded samples was characterized using light and electron microscopy, and the electrical and thermal properties were measured. The lowest electrical percolation thresholds were found for composites of PP/SWCNT, PP/branched MWCNT and PC/branched MWCNT, which percolated already at <0.1 wt.-% CNT loading. Low values of electrical volume resistivity of about 3 Ohm·cm (PVDF), 7 Ohm·cm (PP) and 2 Ohm·cm (PC) could be reached when loading with 2 wt.-% branched MWCNT. A homogeneous dispersion in the macro- and microlevel was observed especially for composites containing branched MWCNTs. For all CNT types, a matrix nucleation effect was found in PP and PVDF using differential scanning calorimetry.
- ItemInterfacial chemistry using a bifunctional coupling agent for enhanced electrical properties of carbon nanotube based composites(Oxford : Elsevier Science, 2013) Socher, Robert; Jakisch, Lothar; Krause, Beate; Oertel, Ulrich; Voit, Brigitte; Pötschke, PetraA bifunctional coupling agent (BCA) containing one oxazoline and one benzoxazinone group was applied to promote a reaction between polyamide 12 (PA12) and multiwalled carbon nanotubes (MWCNTs) during melt mixing. With this modification, the MWCNT content needed for the electrical percolation was significantly reduced by more than a factor of three. For amino functionalized MWCNT-PA12 composites adding 1 wt.% BCA electrical percolation was reached at only 0.37 wt.% MWCNTs compared to 1.0 wt.% without BCA. With the help of a model reaction, the covalent attachment of the BCA to the MWCNTs could be shown by thermogravimetric analysis (TGA) and via fluorescence spectroscopy. Model compounds were applied containing either only the oxazoline or the benzoxazinone group to show that the better electrical properties in the PA12-MWCNT composites were a result of a covalent bond between the polymer and the nanotube which only takes place when the BCA was used. In addition, significantly higher electrical conductivity values were obtained by the addition of BCA as well with amino functionalized as with nonmodified commercial MWCNTs. This surprising result was attributed to the significant hydroxy group content on the surface of those commercial MWCNTs. © 2013 Elsevier Ltd. All rights reserved.
- ItemA promising approach to low electrical percolation threshold in PMMA nanocomposites by using MWCNT-PEO predispersions(Oxford : Elsevier Science, 2016) Mir, Seyed Mohammad; Jafari, Seyed Hassan; Khonakdar, Hossein Ali; Krause, Beate; Pötschke, Petra; Taheri Qazvini, NaderElectrical conductive poly(methyl methacrylate) (PMMA) nanocomposites with low percolation threshold are very challenging to be prepared. Here, we show that the miscibility between poly(ethylene oxide) (PEO) as matrix for predispersions of multi-walled carbon nanotubes (MWCNTs) and PMMA represents an efficient approach to achieve very low electrical percolation threshold. PMMA/PEO-MWCNTs nanocomposites were prepared by a two-step solution casting method involving pre-mixing of MWCNTs with PEO and then mixing of PEO-MWCNTs with PMMA, resulting in a PMMA/PEO ratio of 80/20 wt%. The electrical percolation threshold (EPT) value was determined to be ~ 0.07 wt% which is significantly lower than most of the reported EPT values in the literature for PMMA/CNT composites. The very low electrical percolation threshold was attributed to the effectual role of PEO in self-assembly of secondary structures of nanotubes into an electrically conductive network. This was further confirmed by transmission electron microscopy and by comparing the obtained EPT value with the prediction of the excluded volume model in which statistical percolation threshold is defined based on uniform distribution of high-aspect ratio sticks in a matrix. Moreover, based on UV–Vis measurements and linear viscoelastic rheological measurements, optical and rheological percolation thresholds were obtained at nearly 0.01 wt% and 0.5 wt%, respectively.