2016, Mir, Seyed Mohammad, Jafari, Seyed Hassan, Khonakdar, Hossein Ali, Krause, Beate, Pötschke, Petra, Taheri Qazvini, Nader

Electrical 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.

2016, Krause, Beate, Pötschke, Petra, Ilin, Evgeniy, Predtechenskiy, Mikhail

Singlewalled 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.

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.

2018, Krause, Beate, Barbier, Carine, Kunz, Karina, Pötschke, Petra

In 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.