Rheological characterization of melt processed polycarbonate-multiwalled carbon nanotube composites
Introduction
Since the first patent on multiwalled carbon nanotubes was filed in 1987 [1] and Iijima announced his discovery of carbon nanotubes (CNT) using high resolution TEM in 1991 [2], considerable effort has been focused on using this special kind of filler in polymer based composites [3], [4]. Carbon nanotubes are termed as either single-walled carbon nanotubes (SWNT) or multiwalled carbon nanotubes (MWNT) according to the number of graphite layers forming the tubes. MWNT are electrically conductive due to the graphite lattice whereas SWNT behave as conductors or semiconductors depending on the chirality of the graphite sheets [4]. CNT possess a very high aspect ratio up to 100–1000 and higher [5], [6]. This high aspect ratio makes it possible to reach the electrical percolation of polymer based composites at very low content of CNT. In addition, CNT have exceptional mechanical properties, as high as 20 GPa in tensile strength and moduli of approximately 1 TPa [3], [7]. Next to other techniques, like solution processing and in-situ polymerization of polymers in the presence of CNT, also melt compounding was shown to be a suitable way in order to disperse CNT quite homogenously in a polymer matrix [8], [9], [10], [11]. Within the melt mixing processing, there are two ways: (i) CNT can be directly added to polymers during melt mixing or (ii) commercially available masterbatches of nanotube/polymer composites can be used as a starting material and diluted by adding an appropriate amount of pure polymer in the subsequent melt mixing process.
The task of this work is to characterize rheologically composites of polycarbonate (PC) with MWNT over a wide range of the compositions. The work is based on a previous paper from one of the authors [10] in which the masterbatch dilution method was used to produce composites of polycarbonate with MWNT using a twin-screw extruder in kilogram scale. There we observed a qualitative change in the rheological behavior between 1 and 2 wt.% MWNT. In this new set, we produced composites by the same masterbatch dilution technique but with smaller concentration steps especially in this composition range. Because of material limitations we used another mixing equipment, namely a small-scale conical twin-screw compounder, and another polycarbonate having a lower melt viscosity as compared to the experiments described in [10].
Section snippets
Materials and methods
A masterbatch consisting of 15 wt.% multiwalled carbon nanotube (MWNT) in polycarbonate (PC) was obtained from Hyperion Catalysis International, Inc. (Cambridge, MA, USA) [6], [8]. In this masterbatch the diameter of the MWNT is between 10 and 15 nm [11]. The masterbatch was diluted by a commercial polycarbonate with the designation Lexan 121 (GE Plastics Europe) using melt mixing in a DACA-Micro-Compounder (DACA Instruments, Goleta, CA, USA) at 265 °C. This compounder with a capacity of 4.5 cm3
Results
The storage modulus G′, loss modulus G″, and complex modulus G* of neat PC and PC composites measured at 280 °C are logarithmically plotted as a function of angular frequency in Fig. 1, Fig. 2, Fig. 3, respectively. At the measuring temperature of 280 °C the neat polycarbonate which was processed in the same way like the composites shows a slope of G’ versus frequency lower than expected for the terminal region. However, a master curve of this material measured in a temperature range between 170
Discussion
The rheological results clearly indicate a transition from a liquid like behavior to a solid like behavior between loadings of 0.25 and 0.5 wt.% MWNT in polycarbonate. Surprisingly, this observed MWNT concentration is much lower than it was expected from the previous paper [10] where such a qualitative change in the rheological behavior was found between 1 and 2 wt.% MWNT loading. We attribute this difference to the lower molecular weight of the used diluting polycarbonate, the different mixing
Summary
In this study, polycarbonate (PC)/multiwalled carbon nanotubes (MWNT) composites were produced by masterbatch dilution technique. The composites were rheologically characterized by frequency sweeps at 280 °C using an ARES-rheometer. The results showed that the dynamic moduli and the viscosity were increased with the incorporation of MWNT into PC. At 0.5 wt.% MWNT a significant change in the qualitative rheological behavior was detected which indicates a transition from liquid like to solid like
Acknowledgments
We thank the German Federation of Industrial Cooperative Research Associations “Otto von Guericke” (AIF) for financial support of this work within the project 122 ZBG. We are grateful for providing of the PC masterbatch to Hyperion Catalysis International, Inc. In addition, we thank Ingo Alig, Sergej Dudkin, and Dirk Lellinger (from DKI Darmstadt) and Gert Heinrich (from IPF Dresden) for useful discussion in order to interpret the measurement results.
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