Dispersion of Carbon Nanotubes Into Thermoplastic Polymers Using Melt Mixing

8/13/2019 Dispersion of Carbon Nanotubes Into Thermoplastic Polymers Using Melt Mixing

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Dispersion of Carbon Nanotubes intoThermoplastic Polymers using Melt Mixing

P. Ptschke 1, A.R. Bhattacharyya 1+, I. Alig 2, S.M. Dudkin 2, A. Leonhardt 3,C. Tschner 3, M. Ritschel 3, S. Roth 4, B. Hornbostel 4 and J. Cech 4

1 Institute of Polymer Research Dresden, Hohe Str. 6, 01069 Dresden, Germany2 Deutsches Kunststoff-Institut Darmstadt, Schlossgartenstr. 6, 64289 Darmstadt, Germany

3 Leibniz-Institute for Solid State and Materials Research Dresden, Helmholtzstr . 20, 01069 Dresden,Germany

4 Max-Planck-Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany+ Present address: Department of Metallurgical Engineering & Materials Science, Indian Institute of

Technology Bombay, Powai, Mumbai 400076, 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 asubsequent melt mixing process (masterbatch dilution method) while in the other case nanotubes aredirectly 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 meltmixing equipments. The lowest percolation threshold was found at about 0.5 wt% MWNT using aBrabender PL-19 single screw extruder. The nanotube dispersion as observed by TEM investigationsis quite homogeneous. The direct incorporation method is discussed in composites of polycarbonatewith MWNT and SWNT. The nanotube addition significantly changes the stress-strain behavior of thecomposites: 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 toenhancement of mechanical properties and electrical conductivity depends primarily onthe ability to disperse the nanotubes homogeneously in the polymer matrix. However,homogeneous dispersion of nanotubes is difficult due to the intermolecular van der Waalsinteractions 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 beminimized by appropriate application of shear in melt mixing [1-5].

This contribution presents melt mixed composites where in the first case commerciallyavailable masterbatches of nanotube/polymer composites are used as the base materialwhich are diluted by the pure polymer in a subsequent melt mixing process (masterbatchdilution method) while in the other case nanotubes are directly incorporated into the

polymer. The investigations are focused on the percolation composition as detected byelectrical resistivity measurements and the stress-strain behavior of the composites.

CP723, Electronic Properties of Synthetic Nanostructures, edited by H. Kuzmany et al. 2004 American Institute of Physics 0-7354-0204-3/04/$22.00

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EXPERIMENTAL

A masterbatch of 15 wt% MWNT1 in polycarbonate (provided by Hyperion CatalysisInternational Inc, Cambridge, USA) was diluted with different kinds of polycarbonate.

MWNT1 are vapor grown and typical diameters range from 10-15 nm while lengths are between 1 and 10 m. [6,7]. They are produced as agglomerates and exist as curvedintertwined entanglements [8-10]. The polycarbonates used for these studies were PCIupilon E2000 (PC1, powder, Mitsubishi Engineering Plastics, Japan) with a zero shearviscosity at 260C ( 0,260 ) of 6800 Pa-s, PC Lexan 121 (PC2, granules, GE Europe, 0,260of 3500 Pa-s), and the PC used for the masterbatch production (PC3, powder, supplied byHyperion Cat., 0,260 of 1000 Pa-s). Melt compounding was carried out between 240 and280C 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 at120C 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(100C, 2 h vacuum) the materials were premixed and fed to the running compounder.For direct incorporation of the MWNT2 (purity >95%, diameters 20-50 nm, length up tomore than 100 m, produced at the Leibniz-Institute for Solid State and MaterialsResearch Dresden by thermal catalytic chemical vapor deposition) were incorporated intoPC2 at 260C, 50 rpm, and 5 min. SWNT (unpurified arc-discharge material, bundled,single tube diameters of 1.0-1.3 nm, prepared at the Max-Planck-Institute for Solid StateResearch Stuttgart) were incorporated into PC1 powder at 280C, 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 an8009 Resistivity Test Fixture was used to measure high resistivity samples. Lowerresistivity composites were measured by a four point test fixture combined with aKeithley electrometer Model 2000 using strips (20 mm x 3 mm) cut from the sheets. Inaddition, dielectric measurements were performed at room temperature in a frequencyrange from 10 -3 to 10 7 Hz using a frequency response analysis system consisting ofSolartron SI1260 Impedance/Gain Phase Analyzer and Novocontrol broadband dielectricconverter with the BDC active sample cell [12]. Discs (diameter of 20 mm) were cutfrom the sheets and gold layers were sputtered onto both sides as electrodes.

Scanning electron microscopy (SEM) was performed using a LEO VP 435 scanningelectron microscope (Leo Elektronenmikroskopie, Germany) on raw materials orfractured samples. Mechanical testing of miniature dogbones (length 20 mm, parallellength 6 mm, gauge width 2 mm) punched from the pressed sheets was performed similarto ISO 527-2 on a Zwick Z010 tensile tester at an extension rate of 5 mm/min. The valuesare mean values of 10 tests.

RESULTS & DISCUSSION

Using the masterbatch dilution method, the mixing task is to expand the alreadyexisting 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

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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 distributedand dispersed in the matrix. Resistivity vs. composition dependencies can be used todetect 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 PCand different mixing equipments.

0 2 4 6 8 10 12 14 1610 -110 110 310 510 710 9

10 1110 1310 1510 17

conductive materials, v


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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 highviscous polymers. The cryofracture shown in Fig. 4 illustrates a homogeneousdistribution and dispersion without agglomeration of the nanotubes. In this system,

percolation started at 3 wt% MWNT2 addition.

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 thecomposites as it is shown in Fig. 4. PC2 exhibits a pronounced yielding behaviorfollowed by cold drawing and shows elongation at break of about 30% [14]. After addingMWNT2, next to an increase in Youngs modulus (9% at 1wt% and 16% at 5wt%MWNT2) the elongation at break is reduced dramatically starting at 1.5 wt% MWNT2since the nanotubes start to form a network. Starting at 2 wt% MWNT2, the pronouncedyielding and cold drawing behavior is no longer visible and the break occurs beforereaching 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 lowMWNT 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 tothose ones described before.

0 1 2 3 4 5 6 7 8550

600

650

700

750

800

850

900

950SWNT in PC1

MWNT in PC2

Y o u n g

' s m o

d u

l u s

( M P a

)

Content of CNT in PC (wt%)0 1 2 3 4 5 6 7 8

0

10

20

30

40

50

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break

yield

E l o n g a

t i o n a

t b r e a

k ( % )

S t r e s s

( M P a

)

Content of CNT in PC (wt%)

025

50

75

100

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FIGURE 4. Properties of PC2/MWNT2 (full symbols) and PC1/SWNT (open symbols) composites; left:Youngs modulus; right: stress at yield point ( yield ), stress at break ( break ), and elongation at break

m m

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The modulus shows an increase with SWNT addition (46% at 7.5 wt% SWNT) whichis higher than that one achieved for PC2/MWNT2 composites. The pronounced colddrawing behavior after yielding is observable up to 3 wt% SWNT. The stress at yield isincreased 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 significantlydecreased elongations at break. This behavior can be understood by the percolatednetwork 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 easilyaccessible for small and medium-sized enterprises. The results obtained for

polycarbonate based materials show that the percolation only slightly depends on the PCused 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 aBrabender 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 agood dispersion of CNT in PC. For the selected materials used, percolation occurred inthe range between 2 and 3 wt% CNT. The incorporation of nanotubes significantlychanges 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 ,219-225 (2000).

[2] J.R. Hagerstrom and S.L. Greene. Electrostatic dissipating composites containing Hyperion fibrilnanotubes. Commercialization ofNanostructured Materials, Miami, USA, April 7, 2000.

[3] P. Ptschke, T.D. Fornes, D.R. Paul Polymer 43, 3247-3255 (2002).[4] R. Andrews, D. Jacques, M. Minot, T. Rantell Macromol. Mat. Eng. 287, 395-403 (2002).[5] M. Sennett, E. Welsh, J.B. Wright, W.Z. Li, J.G. Wen, Z.F. Ren. Appl. Phys . A 76 , 111-113 (2003).[6] Plastics Additives & Compounding 3(2001) 9, 20-22.[7] D.W. Ferguson, E.W.S. Bryant, H.C. Fowler, ESD thermoplastic product offers advantage for

demanding electronic applications, ANTEC98, 1998. p.1219-1222.[8] M.S.P. Shaffer, X. Fan, A.H. Windle, Carbon, 36 , 1603-1612 (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 , 937-941 (1999).[11] P. Ptschke, A.R. Bhattacharyya, A. Janke, H. Goering, Comp. Interf. 10, 389-404 (2003).[12] P. Ptschke, S.M. Dudkin, I. Alig, Polymer 44, 5023-5030 (2003).[13] P. Ptschke, A.R. Bhattacharyya, A. Janke, Carbon 42 , 965-969 (2004).[14] P. Ptschke, Percolation and stress strain behaviour of polycarbonate- multiwalled carbon nanotube

composites, Polymer , 2004 (submitted).[15] P. Ptschke, A.R. Bhattacharyya, A. Janke, European Polymer Journal 40 , 137-148 (2004).[16] P. Ptschke., A.R. Bhattacharyya, A. Janke, S. Pegel, A. Leonhardt, C. Tschner, 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)

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Dispersion of Carbon Nanotubes Into Thermoplastic Polymers Using Melt Mixing

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