A series of samples with different content of multi‐walled carbon nanotubes (0.5‐7.5 wt%) in polycarbonate was investigated by electrical conductivity measurements. During isothermal annealing of the as‐produced samples in the melt state an increase of the conductivity with time was observed. A similar conductivity recovery was found after a short shear deformation of the melt. The time‐dependent transition from insulating to conductive state is explained by dynamic formation of a conductive network of interconnected carbon nanotube agglomerates. The conductive networks were found to be stable after cooling the samples below their glass transition temperature. Comparison of the room temperature conductivity plotted vs. nanotube content for as‐produced and annealed samples shows a tremendous shift of the insulator‐conductor transition from above 5 wt% to about 0.75 wt%, respectively. For the description of the insulator‐conductor transition three different approaches were tested: classical percolation, general effective medium model (GEM), and Fournier equation. All three models fit the data satisfactory. Dynamic percolation was described by the GEM model coupled to a kinetic equation for CNT agglomeration. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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Abstract. In this study we report on morphological and rheological characterization of multi-walled carbon nanotube (MWNT)-polycarbonate composites produced by injection molding. The main focus is to carry out nonlinear viscoelastic experiments that allow following the structural rearrangements of carbon nanotubes in the polycarbonate melt. Small angle X-ray scattering reveals only a slight orientation of MWNTs in the as-received samples, i.e. after application of extremely high shear rates. Thus, the main structural effect observed during the stress growth experiment is the breakage of MWNT agglomerates. To study this effect in detail a flocculation experiment, in which the sample undergoes oscillatory deformation first at a small strain amplitude in the linear regime succeeded by higher amplitudes in the nonlinear regime, has been carried out. The agglomeration process manifests itself in an increase of the storage and loss moduli in the linear regime, whereas the deagglomeration process does vice versa. The corresponding effects can be described in the frame of a superposition approach that takes into account the stress contribution of the polycarbonate matrix, the hydrodynamic reinforcement due to embedded nanotubes and the viscoelastic stress due to the presence of a MWNT-network.

Carbon Nanotubes are currently being synthesized by different methods. The methods differ largely by the achievable space-time yield, amorphous carbon content and carbon purity. High space-time yields can only be achieved by CVD processes. Though usually low contents of amorphous carbon are observed in CVD processes, C-purity is generally decreased due to catalyst residues. Bayer Technology Services has developed a high-yield process for multi-walled carbon nanotubes (MWNT, trade name Baytubes ® ) based on a CCVD process starting from light hydrocarbons. The produced Baytubes ® are characterized by a very high graphite content and an excellent purity (carbon content > 95 wt%). Due to their high purity and quality, Baytubes ® can be employed directly in further processing. The Baytubes ® were incorporated into different polymers and also into liquids. The primary feature for good mechanical and electrical properties of the polymer composites is a full dispersion of the CNT in the polymer matrix. TEM analysis is an important method to investigate the CNT dispersion on the nanometer scale. But it is also necessary to combine different methods to ensure a good and constant dispersion quality. Thus, different analysis methods were developed by BTS during product development. Process scale-up is under way and a nominal production capacity of 60 tons per year has already been commissioned.

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