Dispersion of Carbon Nanotubes into Thermoplastic Polymers using Melt Mixing

Transcription

1 Dispersion of Carbon Nanotubes into Thermoplastic Polymers using Melt Mixing P. Pötschke 1, A.R. Bhattacharyya 1+, I. Alig 2, S.M. Dudkin 2, A. Leonhardt 3, C. Täschner 3, M. Ritschel 3, S. Roth 4, B. Hornbostel 4 and J. Cech 4 1 Institute of Polymer Research Dresden, Hohe Str. 6, Dresden, Germany 2 Deutsches Kunststoff-Institut Darmstadt, Schlossgartenstr. 6, Darmstadt, Germany 3 Leibniz-Institute for Solid State and Materials Research Dresden, Helmholtzstr. 20, Dresden, Germany 4 Max-Planck-Institute for Solid State Research, Heisenbergstr. 1, Stuttgart, Germany + Present address: Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai , India Abstract. This paper presents melt mixed composites where two ways of introducing nanotubes in polymer matrices were used. In the first case, commercially available masterbatches of nanotube /polymer composites are used as the starting material which are diluted by the pure polymer in a subsequent melt mixing process (masterbatch dilution method) while in the other case nanotubes are directly incorporated into the polymer matrix. As an example of the masterbatch dilution method, composites of polycarbonate with MWNT are presented which are produced using different melt mixing equipments. The lowest percolation threshold was found at about 0.5 wt% MWNT using a Brabender PL-19 single screw extruder. The nanotube dispersion as observed by TEM investigations is quite homogeneous. The direct incorporation method is discussed in composites of polycarbonate with MWNT and SWNT. The nanotube addition significantly changes the stress-strain behavior of the composites: modulus and stress are increased; however, elongation is reduced especially above the percolation concentration. INTRODUCTION Effective utilization of nanotubes in composites with polymers with regard to enhancement of mechanical properties and electrical conductivity depends primarily on the ability to disperse the nanotubes homogeneously in the polymer matrix. However, homogeneous dispersion of nanotubes is difficult due to the intermolecular van der Waals interactions between the nanotubes, thus resulting in the formation of aggregates. This problem presents a major challenge irrespective of the method of composite preparation. In context with industrial applications of nanotube/polymer systems, melt mixing is the preferred method of composite preparation. It is expected that aggregates can be minimized by appropriate application of shear in melt mixing [1-5]. This contribution presents melt mixed composites where in the first case commercially available masterbatches of nanotube/polymer composites are used as the base material which are diluted by the pure polymer in a subsequent melt mixing process (masterbatch dilution method) while in the other case nanotubes are directly incorporated into the polymer. The investigations are focused on the percolation composition as detected by electrical resistivity measurements and the stress-strain behavior of the composites. CP723, Electronic Properties of Synthetic Nanostructures, edited by H. Kuzmany et al American Institute of Physics /04/$

2 EXPERIMENTAL A masterbatch of 15 wt% MWNT1 in polycarbonate (provided by Hyperion Catalysis International Inc, Cambridge, USA) was diluted with different kinds of polycarbonate. MWNT1 are vapor grown and typical diameters range from nm while lengths are between 1 and 10 µm. [6,7]. They are produced as agglomerates and exist as curved intertwined entanglements [8-10]. The polycarbonates used for these studies were PC Iupilon E2000 (PC1, powder, Mitsubishi Engineering Plastics, Japan) with a zero shear viscosity at 260 C (η 0,260 ) of 6800 Pa-s, PC Lexan 121 (PC2, granules, GE Europe, η 0,260 of 3500 Pa-s), and the PC used for the masterbatch production (PC3, powder, supplied by Hyperion Cat., η 0,260 of 1000 Pa-s). Melt compounding was carried out between 240 and 280 C using a Haake twin screw extruder [3], a small scale DACA Micro Compounder [11, 15], or a Brabender PL-19 single screw extruder [16]. The materials were dried at 120 C in vacuum for at least 4 h and fed as granular premixture. Direct incorporation of MWNT and SWNT was performed using the small scale (4.5 cm 3 material) DACA Micro Compounder. After drying of polymer and nanotubes (100 C, 2 h vacuum) the materials were premixed and fed to the running compounder. For direct incorporation of the MWNT2 (purity >95%, diameters nm, length up to more than 100 µm, produced at the Leibniz-Institute for Solid State and Materials Research Dresden by thermal catalytic chemical vapor deposition) were incorporated into PC2 at 260 C, 50 rpm, and 5 min. SWNT (unpurified arc-discharge material, bundled, single tube diameters of nm, prepared at the Max-Planck-Institute for Solid State Research Stuttgart) were incorporated into PC1 powder at 280 C, 50 rpm, and 15 min. Electrical volume resistivity was measured on compression molded sheets (diameter > 60 mm, thickness ~ 0.35 mm). A Keithley electrometer Model 6517 equipped with an 8009 Resistivity Test Fixture was used to measure high resistivity samples. Lower resistivity composites were measured by a four point test fixture combined with a Keithley electrometer Model 2000 using strips (20 mm x 3 mm) cut from the sheets. In addition, dielectric measurements were performed at room temperature in a frequency range from 10-3 to 10 7 Hz using a frequency response analysis system consisting of Solartron SI1260 Impedance/Gain Phase Analyzer and Novocontrol broadband dielectric converter with the BDC active sample cell [12]. Discs (diameter of 20 mm) were cut from the sheets and gold layers were sputtered onto both sides as electrodes. Scanning electron microscopy (SEM) was performed using a LEO VP 435 scanning electron microscope (Leo Elektronenmikroskopie, Germany) on raw materials or fractured samples. Mechanical testing of miniature dogbones (length 20 mm, parallel length 6 mm, gauge width 2 mm) punched from the pressed sheets was performed similar to ISO on a Zwick Z010 tensile tester at an extension rate of 5 mm/min. The values are mean values of 10 tests. RESULTS & DISCUSSION Using the masterbatch dilution method, the mixing task is to expand the already existing percolated MWNT-polymer structure gradually by incorporating the diluting polymer between the individual tubes by maintaining the tube percolation. The PC based masterbatch with 15 wt% MWNT consists of highly interconnected tubes in the 479

3 PC3 matrix which show a good wetting of the tubes with polymer (see discussion in [3]). If the dilution is properly performed, the MWNT should be homogeneously distributed and dispersed in the matrix. Resistivity vs. composition dependencies can be used to detect the percolation composition unequivocally. In addition, the resistivity values can give information about the state of dispersion obtained at a given composition [7, 13]. Figure 1 summarizes the electrical resistivity values obtained by using the different PC and different mixing equipments. Volume resistivity (Ohm cm) PC 2, DACA PC 3, DACA PC 1, Haake PC 2, PL-19 conductive materials, ρ v <10 4 Ohm cm Content of MWNT in PC (wt%) FIGURE 1. Electrical volume resistivity vs. MWNT content in composites prepared by masterbatch dilution method σ' (S/cm) wt%,rpm, min 1.5, 50,15 1.5, 150, 5 1.5, 150,15 1.0,150,15 1.0, 50,15 1.0,150, Frequency (Hz) FIGURE 2. AC conductivity σ vs. frequency for PC3-MWNT1 composites processed in the DACA Micro Compounder at different processing conditions, legend contents amount of MWNT in wt%, rotation speed in rpm, and mixing time in min. Depending on the PC and mixing equipment, percolation was reached between 0.5 and 1.5 wt% MWNT1 content. With PC1 as diluting material and processing in a Haake twin screw extruder, electrical percolation was found between 1 and 2 wt% [3], whereas dilution using the lower viscosity PC3 using the DACA Micro Compounder led to percolation between 1.0 and 1.5 wt% MWNT1 [11]. Using PC2 and processing in the DACA Micro Compounder, percolation was reached between and 1.0 wt% MWNT1 [14]. For PC2 as diluting polymer and processing in the single-screw extruder the percolation was found to occur between 0.25 and 0.5 wt% MWNT1 in PC. In all the cases, composites with contents starting at 1.5 wt% MWNT1 can be regarded as electrically conductive (volume resistivity <10 4 Ohm-cm). For the example of masterbatch dilution with PC3 using the DACA Micro Compounder the influence of processing conditions at compositions near the percolation was investigated more in detail using dielectric spectroscopy. Figure 2 shows the AC conductivity for composites containing 1.0 and 1.5 wt% MWNT1. It is interesting to note that in the case of 1.0 wt% variations in the processing conditions (increasing mixing time) can transform a nonpercolated structure in a percolated one. The relatively low percolation thresholds indicates a quite good distribution and dispersion within the PC matrices as it was shown by TEM investigations in [15]. Using the direct incorporation method, next to a good distribution and dispersion of the tubes also a wetting of the nanotube surface with polymer has to be achieved which depends on the CNT surface characteristics and amount, the interfacial tension between nanotube surface and polymer melt, and the polymer melt viscosity. 480

4 MWNT2 used for this study (Fig. 3) are relatively long, less tangled and quite straight. It is assumed that some of the very long tubes break during melt mixing with high viscous polymers. The cryofracture shown in Fig. 4 illustrates a homogeneous distribution and dispersion without agglomeration of the nanotubes. In this system, percolation started at 3 wt% MWNT2 addition. 2 µm 2 µm FIGURE 3. SEM micrographs of MWNT2(left) and a composite of PC2 with 1.5 wt% MWNT2 The incorporation of nanotubes significantly changes the stress-strain behavior of the composites as it is shown in Fig. 4. PC2 exhibits a pronounced yielding behavior followed by cold drawing and shows elongation at break of about 30% [14]. After adding MWNT2, next to an increase in Young s modulus (9% at 1wt% and 16% at 5wt% MWNT2) the elongation at break is reduced dramatically starting at 1.5 wt% MWNT2 since the nanotubes start to form a network. Starting at 2 wt% MWNT2, the pronounced yielding and cold drawing behavior is no longer visible and the break occurs before reaching the elongation (and stress) typical for the yield point of the pure PC. Thus, tensile strength as the maximum stress in the stress-strain curves is only increased at low MWNT contents (9% at 1.5 wt% MWNT2). In the composites of PC1 with SWNT, percolation was reached between 2 and 3 wt% SWNT addition [16]. The changes in the stress-strain behavior (Fig. 4) are similar to those ones described before. Young's modulus (MPa) SWNT in PC1 MWNT in PC2 Stress (MPa) σ yield σ break Elongation at break (%) Content of CNT in PC (wt%) Content of CNT in PC (wt%) 0 FIGURE 4. Properties of PC2/MWNT2 (full symbols) and PC1/SWNT (open symbols) composites; left: Young s modulus; right: stress at yield point (σ yield ), stress at break (σ break ), and elongation at break 481

5 The modulus shows an increase with SWNT addition (46% at 7.5 wt% SWNT) which is higher than that one achieved for PC2/MWNT2 composites. The pronounced cold drawing behavior after yielding is observable up to 3 wt% SWNT. The stress at yield is increased by about 7 MPa as well as the stress level beyond the yield point [16]. At higher SWNT contents the break occurs just after the yield point leading to significantly decreased elongations at break. This behavior can be understood by the percolated network of the SWNT which hinders the polycarbonate in its typical deformation behavior especially in developing a cold drawing process [14]. CONCLUSIONS Melt mixing using the masterbatch technique is a method which is applicable and easily accessible for small and medium-sized enterprises. The results obtained for polycarbonate based materials show that the percolation only slightly depends on the PC used for dilution, the mixing equipment, and the processing conditions. In composites based on PC percolation was always reached at 1.5wt% MWNT using a masterbatch provided by Hyperion Catalysis International. Using a suitable equipment, in our case a Brabender PL-19 extruder, percolation could be reached already at 0.5 wt% MWNT. It could be shown that also the direct incorporation of CNT using melt mixing leads to a good dispersion of CNT in PC. For the selected materials used, percolation occurred in the range between 2 and 3 wt% CNT. The incorporation of nanotubes significantly changes the stress-strain behavior of the composites: modulus and stress are enhanced; however, the elongation is reduced especially above the percolation concentration. Further optimization of melt processing conditions and nanotube pretreatment (purification, predispersion in solvents, functionalization) seem to be promising tasks. REFERENCES [1] R. Haggenmueller. H.H. Gommans, A. Rinzler, J.E. Fischer and K.I. Winey, Chem. Phys. Lett. 330, (2000). [2] J.R. Hagerstrom and S.L. Greene. Electrostatic dissipating composites containing Hyperion fibril nanotubes. Commercialization ofnanostructured Materials, Miami, USA, April 7, [3] P. Pötschke, T.D. Fornes, D.R. Paul Polymer 43, (2002). [4] R. Andrews, D. Jacques, M. Minot, T. Rantell Macromol. Mat. Eng. 287, (2002). [5] M. Sennett, E. Welsh, J.B. Wright, W.Z. Li, J.G. Wen, Z.F. Ren. Appl. Phys. A 76, (2003). [6] Plastics Additives & Compounding 3(2001) 9, [7] D.W. Ferguson, E.W.S. Bryant, H.C. Fowler, ESD thermoplastic product offers advantage for demanding electronic applications, ANTEC 98, p [8] M.S.P. Shaffer, X. Fan, A.H. Windle, Carbon, 36, (1998). [9] J. Sandler, M.S.P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte, A.H. Windle, Polymer 40, 5967 (1999). [10] M.S.P. Shaffer, A.H. Windle. Adv Mat. 11, (1999). [11] P. Pötschke, A.R. Bhattacharyya, A. Janke, H. Goering, Comp. Interf. 10, (2003). [12] P. Pötschke, S.M. Dudkin, I. Alig, Polymer 44, (2003). [13] P. Pötschke, A.R. Bhattacharyya, A. Janke, Carbon 42, (2004). [14] P. Pötschke, Percolation and stress strain behaviour of polycarbonate- multiwalled carbon nanotube composites, Polymer, 2004 (submitted). [15] P. Pötschke, A.R. Bhattacharyya, A. Janke, European Polymer Journal 40, (2004). [16] P. Pötschke., A.R. Bhattacharyya, A. Janke, S. Pegel, A. Leonhardt, C. Täschner, M. Ritschel, S. Roth, B. Hornbostel, J. Cech, Melt Mixing as Method to Disperse Carbon Nanotubes into Thermoplastic Polymers, Fullerenes, Nanotubes, and Carbon Nanostructures, 2004 (submitted) 482

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