Rheological characterization of melt processed polycarbonate-multiwalled carbon nanotube composites

J. Non-Newtonian Fluid Mech. 128 (2005) 2–6

Rheological characterization of melt processed polycarbonatemultiwalled carbon nanotube composites Mahmoud Abdel-Goad ∗ , Petra P¨otschke Leibniz Institute of Polymer Research Dresden, Hohe Str. 6, 01069 Dresden, Germany Received 28 September 2004; received in revised form 24 December 2004; accepted 19 January 2005

Abstract In this study polycarbonate/multiwalled carbon nanotube composites were produced with different compositions by diluting a masterbatch using melt mixing in a DACA-Micro-Compounder. The composites were rheologically characterized using an ARES-rheometer in the dynamic mode under nitrogen atmosphere at 280 ◦ C and frequency varying from 100 to 0.056 rad/s. The results showed that the dynamic moduli and the viscosity increased with increasing MWNT content. At a concentration of 0.5 wt.% MWNT, a significant change in the frequency dependence of the moduli was observed which indicates a transition from a liquid like to a solid like behavior of the nanocomposites. This transition can be related to the formation of a combined network between the nanotubes and the polymer chains. © 2005 Elsevier B.V. All rights reserved. Keywords: Polycarbonate; Multiwalled carbon nanotube; Nanocomposite; Melt rheology

1. 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 ∗

Corresponding author. E-mail addresses: [email protected] (M. Abdel-Goad), [email protected] (P. P¨otschke). 0377-0257/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnnfm.2005.01.008

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 [8–11]. 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

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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].

2. Experimental 2.1. 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 has two conical co-rotating screws with a bypass allowing the material to circulate for defined periods. Prior to mixing the materials were dried for a minimum of 16 h at 80 ◦ C in a vacuum oven and added to the running compounder as granular premixtures. The masterbatch was diluted by PC Lexan 121 in order to obtain 23 concentrations between 0.1 and 12.5 wt.% MWNT in polycarbonate [12], which corresponds to volume concentrations between 0.07 and 8.62 vol.% assuming a MWNT density of 1.75 g/cm3 [9]. A rotational speed of 50 rpm was applied for 5 min. Plates with a diameter of about 30 mm and a thickness of about 0.5 mm were pressed from extruded strands using a Vogt press at 260 ◦ C. From these plates, samples with the diameter of 25 mm were punched for the rheological measurements.

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3. 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 Figs. 1–3, 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 and 280 ◦ C as shown in [12] revealed that the slopes of G and G nearly follow the theoretical expectations. We have to consider that this is a commercial material with a broad molecular weight distribution and possibly containing some processing additives. In addition, this master curve indicates that there is no significant thermal degradation of pure PC during the rheological measurements.

Fig. 1. Storage modulus, G , of PC and PC/MWNT composites as a function of frequency.

2.2. Rheological measurements The melt rheological properties of the material were determined using an ARES-rheometer (Rheometrics Scientific). The measurements were performed in the dynamic mode using 25 mm parallel plates geometry with gap settings according to the disk thickness under liquid nitrogen atmosphere. The strain amplitude was kept to be 1% in the whole frequency range which was checked to be in the linearviscoelastic range by strain sweeps for the compositions presented in this paper. The samples were measured at different temperatures [12]; however, in this paper only the results obtained for a measurement temperature of 280 ◦ C as a function of the angular frequency (ω) are discussed. The frequency was varied between 100 and 0.056 rad/s. For presentation of the rheological results, in this paper only the compositions up to 3 wt.% MWNT are plotted in the graphs. At higher MWNT concentrations, the curves leveled off and did not gave additional information.

Fig. 2. Loss modulus, G , of PC and PC/MWNT composites as a function of frequency.

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Fig. 3. Complex modulus, G∗ , of PC and PC/MWNT composites as a function of frequency.

With increasing content of MWNT the moduli increase as it was expected and found in previous investigations (compare [10]). This increase is significant particularly at low frequency and is more pronounced in the storage modulus as compared to the loss modulus. However, in the G versus frequency behavior some small uncertainties between pure PC and composites with 0.1 wt.% MWNT and 0.25 wt.% MWNT are visible indicating the possible error range in weighting the premixtures, mixing homogeneity, and measurement exactness. All other compositions exactly follow the order of MWNT amount. A noticeable qualitative change in the moduli versus frequency curves was observed when increasing the loading from 0.25 to 0.5 wt.% MWNT. At low frequencies where the flow regime is located, a change from liquid like behavior to solid like behavior is visible starting at 0.5 wt.% MWNT. As shown in Figs. 1–3, this change is especially clearly noticeable in G (Fig. 1) where at low frequency a plateau (G is independent on frequency) starts to develop. Here, the relaxation exponent n (G ∼ ωn ) decreases with increasing the MWNT loading. Similar tendencies are also observable in the developments of the loss factor (Fig. 4) and the complex viscosity |η∗ | (Fig. 5) versus frequency with the MWNT loading. The loss factor (tan δ = G /G ) decreases with increasing MWNT content and this decrease again is more significant at low frequency. As seen in this Fig. 5, the complex viscosity |η∗ | increases with increasing MWNT loading whereas again the changes are especially significant at low frequency. At 0.5 wt.% MWNT a significant change in the frequency dependence of the viscosity occurs. Below this concentration, |η∗ | is nearly flat (Newtonian-plateau) at very low frequency as shown for neat PC and nanocomposites with 0.1 and 0.25 wt.% MWNT. The shear thinning exponent n (|η∗ | ∼ ω−n ) remarkably decreases with MWNT content. This exponent, determined in the frequency range between 0.56 and 0.056 rad/s is plotted in Fig. 6 versus the MWNT content. This figure shows two

Fig. 4. Loss factor, tan δ, of PC and PC/MWNT composites as a function of frequency.

distinct regions. At low MWNT concentrations, the exponent is slightly increased as the MWNT content increases. At 0.5 wt.% MWNT, the exponent increases significantly. Starting from about 1.25 wt.% MWNT, the exponent reaches a steady-state region, where the value of the exponent is higher than 0.96 (nearly 1).

4. 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 conditions (higher mixing

Fig. 5. Complex viscosity, |η∗ |, of PC and PC/MWNT composites as a function of frequency.

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indication without the existence of an electrically percolated network. Nevertheless, rheological measurements are meaningful in order to detect differences in the state of dispersion of carbon nanotubes when using comparable polymer matrix materials and evaluating composites with the same loading. An example is shown by Du et al. [13] comparing composites of PMMA with 1 wt.% SWNT having different state of dispersion.

5. Summary

Fig. 6. Shear thinning exponent, n, vs. MWNT content. The shear thinning exponent n was calculated as the absolute value of the slope in the viscosity vs. frequency curves in the frequency range between 0.56 and 0.056 rad/s.

temperature and longer mixing time) and possibly also to the higher measuring temperature used in this study (see discussion in [12]). It seems that the system under investigation here leads to a more homogeneous incorporation of the diluting polycarbonate chains in between the already existing highly dense MWNT network of the masterbatch under the processing conditions used. The lower molecular weight of the PC and the longer mixing time at higher temperature (despite of possibly lower shear forces in the micro-compounder) favor the MWNT dispersion, thus, also leading to a lower electrical percolation threshold. In the system under investigation here, electrical percolation was found to occur between 0.875 and 1 wt.% MWNT loading [12]. This indicates that the MWNT form at this concentration a network-like structure with tube-tube distances lower than about 5 nm. This distance is reported to be the limit for electron hopping mechanism assuming that this mechanism applies in order to get conductivity [13]. In the previous paper [10], where the concentration steps were very big, the qualitative change in rheological properties and the electrical percolation coincidences and were observed between 1 and 2 wt.% MWNT. However, as it was found in the investigations reported here, in previous investigations by our group [14], and as well by Du et al. using direct incorporation of SWNT in PMMA [13], rheological measurements can show the transition from a liquid like to a solid like behavior already at lower nanotube loadings than necessary for electrical percolation. Therefore, it was assumed that it is not the geometrical percolation of the nanotubes which leads to these changes in rheological behavior but the formation of a combined nanotube-polymer chain network. Du et al. [13] attribute the onset of solid like behavior to the composition at which the size of the polymer chain (average diameter of random coils) is somewhat larger than the separation between the nanotubes. P¨otschke et al. [12] assumed the formation of some kind of ‘entanglements’ between the polymer chains and the nanotubes which can lead to a rheological network

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 ARESrheometer. 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 behavior. This rheological indication of a network formation occurred at lower MWNT loadings than in previous investigations on a similar system. The transition from liquid like to solid like behavior was discussed in context with the formation of a combined nanotube–polymer chain network.

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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