Rheological and dielectrical characterization of melt mixed polycarbonate-multiwalled carbon nanotube composites
Introduction
The reinforcing influence of active fillers is known to play an important role for improvement of mechanical properties of polymer composites. In case of conductive additives the electrical properties of the material are also strongly influenced. New opportunities arise by using nanoscaled fillers, especially carbon nanotubes (CNT) which exhibit excellent electrical properties and distinct fibrous shape with very large aspect ratios as high as 1000–10,000. This high aspect ratio allows the formation of a percolated structure at very low volume contents. According to theoretical prediction on idealized cylindrical shaped fillers at an aspect ratio of 1000 only 0.05 vol.% of filler is necessary for percolation [1].
Electrical measurements are an unambiguous criterion of the existence of a percolated network in case of conductive fillers in an isolating matrix. Dielectric measurements performed with varying frequency can lead to additional information about the percolation network as it was shown for percolation structures of other conductive fillers, e.g. carbon black in polymeric matrices [2], [3], [4]. Recently, also results on percolated structures of carbon nanotubes in polymers were presented (see e.g. [5], [6], [7], [8]). Our group recently presented first results on melt processed polycarbonate–MWNT composites in the frequency range of 10−4 to 107 Hz with the special focus on the influence of melt processing conditions on the dispersion below and above the percolation composition [9].
Oscillatory melt rheology is known to be a very sensitive method to characterize the structure of polymer melts. It is described in literature that interconnected structures of anisometric fillers lead to qualitative changes in the spectra of dynamic moduli and viscosity [10], [11]. The complex viscosity changes from a Newtonian plateau to a continuous decrease with increasing frequency, whereas storage modulus G′ and loss modulus G″ flatten significantly and merge into a secondary plateau at low frequencies.
For carbon nanotube filled polymer composites, as polycarbonate/MWNT composites [12] and polyamide 6/MWNT composites [13], similar effects have been reported. Characteristic changes in the rheological behaviour with increasing nanotube content were found for temperatures well above the glass transition or melting temperature. The rheologically determined threshold was related to the electrically measured percolation composition. The changes in frequency dependences of G′, G″, and/or |η*| with increasing nanotube content were discussed in the frame of liquid-to-solid or liquid-to-gel transitions as introduced by Winter et al. [14]. Although the rheological data for CNT-polymer composites represent the general feature of a liquid-to-solid transition the situation seems to be somewhat more complicated, and the specific interactions between the percolation structures formed by the CNT and the temporary entanglement network of the polymer matrix should be taken into consideration.
The aim of this paper is to give some experimental evidences for the interaction between CNT and the ‘physical network’ formed by entangled polymer chains. Furthermore, the relation between the electrical percolation threshold, which is assumed to be mainly related to the geometrical percolation of the nanotubes, and the changes in the frequency dependent rheological properties with increasing CNT content will be discussed.
For this purpose, the rheological measurements were performed in a quite broad temperature range with composites in which the CNT content was varied in very small steps around the electrical percolation threshold. Due to the temperature variation it was possible to change the viscoelastic behaviour of the polymer matrix systematically whereas the CNT network was expected to remain almost unchanged.
Section snippets
Composite preparation
The composites of PC with MWNT were produced by melt mixing dilution of a masterbatch of 15 wt% multiwalled carbon nanotube (MWNT) in polycarbonate which was obtained from Hyperion Catalysis International, Inc. (Cambridge, MA, USA) with pure PC (Lexan 121, GE Europe). The nanotubes are vapor grown and according to Hyperion typically consist of 8–15 graphitic layers wrapped around a hollow 5 nm core [15] with lengths over 10 μm. TEM investigations on PC-MWNT composites based on the same kind of
Dielectric measurements
Fig. 1 shows real part of the permittivity (ε′) and real part of the conductivity (σ′) as function of frequency for different MWNT content. Due to the high DC conductivity of the samples with contents higher than 1.375 wt% MWNT, it was not possible to measure reasonable values of ε′ for these composites. According to Fig. 1, the samples can be separated into two groups. The composites with MWNT content lower than 1.0 wt% have a nearly constant ε′ value, whereas composites with MWNT loading ≥1.0
Discussion
It is usually assumed that the change in rheological properties near the percolation threshold of a filler network embedded in a viscoelastic liquid is equivalent to the so-called ‘liquid-solid transition’. The ‘gelation’ feature is a typical example for such a ‘liquid-solid transition’ and has been extensively described by Winter et al. (see e.g. [14]).
Discussion of the rheological data presented in Fig. 3, Fig. 4, Fig. 5 in the frame of such a simple percolation picture or a liquid–solid
Conclusion
The dielectric measurements at room temperature clearly indicate an electrical percolation threshold at about 1 wt% MWNT at which the DC conductivity values increase over more than 7 decades. This percolation threshold seems to reflect the MWNT network embedded in the polymer matrix.
In melt rheology, the dynamic mechanical moduli and viscosity are found to be increased with the incorporation of MWNT into PC. In addition, a strong temperature dependence of characteristic changes in the
Acknowledgements
We thank the Bundesministerium für Wirtschaft (German Federal Ministry for Economic Affairs) via the Arbeitsgemeinschaft Industrielle Forschungsgesellschaften (AiF project No. 122Z). Furthermore we thank the Forschungsgesellschaft Kunststoffe e.V. and the European Science Foundation (SUPERNET). We also are grateful to Hyperion Catalysis International, Inc., Cambridge, USA for supplying the masterbatch of PC with MWNT. Prof. Gert Heinrich (IPF Dresden) is thanked for useful discussion of the
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