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MWCNT composites prepared with MWCNT masterbatch chips
European Polymer Journal 44 (2008) 1620–1630
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Rheological and electrical properties of polypropylene/MWCNT composites prepared with MWCNT masterbatch chips Seung Hwan Lee, Myung Wook Kim, Sung Ho Kim, Jae Ryoun Youn * Research Institute of Advanced Materials (RIAM), Department of Materials Science and Engineering, Seoul National University, 56-1, Shillim-dong, Gwanak-gu, Seoul 151-744, Republic of Korea
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a r t i c l e
i n f o
Article history: Received 17 October 2007 Received in revised form 15 February 2008 Accepted 24 March 2008 Available online 29 March 2008
Keywords: Carbon nanotubes Masterbatch Compatibilizer Rheology Electrical percolation
a b s t r a c t Melt compounded PP/MWCNT (polypropylene/multi-walled carbon nanotube) composites were prepared by diluting highly concentrated masterbatch chips. Maleic anhydride grafted polypropylene (PP-g-MAH) was used as a compatibilizer to promote dispersion and interaction of MWCNTs. Rheological properties were investigated with respect to the MWCNT and compatibilizer loadings, and related to morphological and electrical properties. As the MWCNT loading was increased, shear viscosity and yield stress were increased at low shear rate region because of increased interaction between MWCNT particles. When the MWCNT loading was low, MWCNT dispersion was improved by the PP-gMAH compatibilizer because MWCNTs were wetted sufficiently due to the presence of the compatibilizer. However, rheological and electrical properties of highly concentrated MWCNT composites with the compatibilizer were not improved compared with PP/ MWCNT composites without the compatibilizer because the compatibilizer did not provide sufficient wrapping of MWCNT particles. Electrical and morphological properties of PP/ MWCNT composites were correlated with the rheological properties in steady and dynamic oscillatory shear flows. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Polymer/carbon nanotube (CNT) composites have attracted considerable interest due to their good electrical conductivity and superior reinforcing capability. However, it is difficult to dissolve and disperse CNTs in solvent or polymer melt because of their stable structure and strong resistance to wetting. A suitable functionalization method of CNTs has become an interest subject for developing new composites because it can improve solubility and processability [1]. Many studies have been reported on effective methods for improving the dispersion and interfacial interaction between the CNTs and polymer matrix. Song et al. [2] investigated the influence of solvent treatment
* Corresponding author. Tel.: +82 2 880 8326; fax: +82 2 885 1748. E-mail address: [email protected] (J.R. Youn). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.03.017
and CNT loadings on dispersion of CNTs through rheological measurements and reported that the CNT suspension in epoxy resin with good dispersion exhibited lower storage modulus and complex viscosity than that with poor dispersion because interaction between CNTs and epoxy matrix is decreased. Compatibilization is one of the most effective noncovalent functionalization methods to improve CNT dispersion and interfacial interaction between CNTs and polymer matrix. The interaction between functional groups of the compatibilizer and carboxyl or amine groups of CNTs stabilized the morphology and improved the interfacial adhesion between CNTs and the polymer matrix [3,4]. Bellayer et al. [5] prepared well-dispersed PS/CNT nanocomposites via melt extrusion using various compatibilizers containing trialkylimidazolium salt and examined effects of the compatibilizers on internal structure of PS/CNT composites by using FE-SEM, Fourier-transform infrared spectrometer,
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tained in the masterbatch chips were produced by the chemical vapor deposition (CVD) method and were typically consisted of 8–15 graphitic layers wrapped around a hollow core of several nanometer scales. The masterbatch chip was melt compounded using a MDK/E 46-11D Buss-Kneader (L/D = 11/1) and was supplied in pellet form. PP-g-MAH (PolybondÒ3150, CROMPTON, USA) was used as a compatibilizer to promote the interaction between PP matrix and MWCNTs and to improve the wetting of MWCNT particles. Its MFR was 50.0 g/10 min and maleic anhydride content was 0.5 wt%. Materials used in this study are listed in Table 1. 2.2. Preparation of PP/MWCNT composites Two types of melt compounded PP/MWCNT composites were prepared with a twin-screw extruder (Werner & Pfleiderer, ZSK-25) at 210 °C and at the screw speed of 350 rpm in the presence or absence of PP-g-MAH compatibilizer. The composite melt was quenched in water and then pelletized by a cutting machine. Before melt compounding, the PP-g-MAH compatibilizer and pristine masterbatch chips were dried in an oven for 4 h at 80 °C to avoid void formation and degradation induced by mois-
Table 1 Materials used in this study
2. Experimental procedures 2.1. Materials PP/MWCNT composites were prepared by diluting highly concentrated MWCNT masterbatch chips. An isotactic PP homopolymer manufactured for fiber spinning (MOPLENE HP552R, MFR 25.0 g/10 min, Polymirae, Korea) was used as the base polymer resin. Polypropylene masterbatch chips (Fibril Nanotubes MB3020-01, HYPERION Catalysis, USA) containing 20 wt% multi-walled carbon nanotubes were used for spinning of PP/MWCNT composite fibers. According to the supplier [10], the MWCNTs conTM
Materials
Technical Information
Supplier
Polymer matrix
Isotactic polypropylene MOPLENE HP552R MFR = 25.0 g/10 min. PP based maleic anhydride (PP-g-MAH) POLYBONDÒ-3150 MAH content: 0.5 wt% Masterbatch chip (MB 3020-01) MWCNT concentration of 20 wt% High load MFR = 39.2 g/10 min
Polymirae (Korea)
Compatibilizer
Multi-walled CNTs
Crompton (USA)
HYPERION Catalysis (USA)
Table 2 Compositions of the prepared composites based on pure PP matrix, PP-g-MAH compatibilizer, and MWCNTs (wt%) Composites
MOPLENENHP 552R
Pure PP PP + PB blend Composites without compatibilizer
100 97 99.5 99 98 97 95 96.5 96 95 94 92 96 92 90
Composites with compatibilizer
PP-g-MAH
MWCNTs
Abbreviation
0.5 1.0 2.0 3.0 5.0 0.5 1.0 2.0 3.0 5.0 3.0 3.0 3.0
Pure PP PP/PB03 PP/MWCNT0.5 PP/MWCNT1.0 PP/MWCNT2.0 PP/MWCNT3.0 PP/MWCNT5.0 PP/03/MWCNT0.5 PP/03/MWCNT1.0 PP/03/MWCNT2.0 PP/03/MWCNT3.0 PP/03/MWCNT5.0 PP/01/MWCNT3.0 PP/05/MWCNT3.0 PP/07/MWCNT3.0
3.0
3.0 3.0 3.0 3.0 3.0 1.0 5.0 7.0
MACROMOLECULAR NANOTECHNOLOGY
and X-ray diffraction methods. The homogeneous dispersion of CNTs in PP matrix is achieved by strong hydrogen bonding between hydroxyl groups of the CNTs and maleic anhydride groups of PP-g-MAH depending on the chemical similarity of PP matrix and the grafted PP [6]. Homogenizing mixers or extruders are employed frequently as processing equipments for mechanical mixing of CNT particles with polymer melts [7]. One of the most common processing methods is the melt compounding as a means to embed CNT particles directly into thermoplastic polymer melts. Another method is the masterbatch process which is one of the simplest and most economical methods in processing of CNT composites. It can be often used where it is necessary to improve processing characteristics and physical properties of composites or to disperse concentrated fillers uniformly in polymer matrix. Pötschke et al. [8,9] prepared PC/MWCNT composites by diluting highly concentrated masterbatch chips and investigated morphological, rheological, and electrical properties as well as orientation of MWCNTs by using dielectric spectroscopy and Raman spectroscopy. It is necessary to understand effects of MWCNTs and compatibilizers on physical properties of polymer/MWCNT composites. However, there have been few reports on the role of compatibilizer for improving wrapping and dispersion of MWCNTs in PP/MWCNT composites prepared by diluting highly concentrated MWCNT masterbatch chips.
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ture. All samples were compression molded into a disk of 25 mm diameter and 1.5 mm thickness by using a hot press (WABASH, 25-1212-2TMB) at the same temperature as the barrel temperature of the extruder. The compression molded samples were used for analysis of the internal structure and measurement of the rheological properties. Compositions of the prepared composite samples based on PP matrix, PP-g-MAH compatibilizer, and MWCNTs are listed in Table 2.
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2.3. Characterization Melt flow rates of PP/MWCNT composites and pristine masterbatch chips with 20 wt% MWCNTs were carried out according to ASTM D1238 using Melt Indexer (Göttfert, MI-2) at standard load of 2.13 kg and high load of 23.0 kg, respectively. Steady and dynamic oscillatory shear properties were investigated by using a rotational rheometer (TA Instruments, AR-2000). Steady shear measurement was performed at the shear rate of 0.001–10 s 1 and dynamic oscillatory shear measurement was carried out at the frequency of 0.1–200 rad/s with the constant strain of 10%, which is within the linear viscoelastic range of the materials. The electrical conductivity was measured by the fourprobe method (Agilent Technologies, Agilent 4339B highresistance meter) at ambient condition to characterize electrical percolation threshold of PP/MWCNT composites. Surface of the melt compounded PP/MWCNT composite chips was observed using an FE-SEM (JEOL, JSM-6330F). FE-SEM specimens were coated with platinum using a sputter coating instrument (Cressington Scientific Instruments, Sputter Coater 108) in vacuum for 5 min prior to the observation and the FE-SEM was operated at 10.0 kV. 3. Results and discussion 3.1. Melt flow rate The melt flow rate (MFR) was measured for the pure PP matrix and PP/MWCNT composites with the loading of 2.16 kg and at 230 °C. However, the MFR of the pristine masterbatch chip could be measured only at high loading condition of 21.6 kg because there was no running flow across capillary at the loading of 2.16 kg. The measured MFR was 39.2 g/10 min at the high CNT loading. Fig. 1a shows the effects of MWCNT loading on the MFR of PP/ MWCNT composites with or without the compatibilizer. The MFR of PP/MWCNT composite melts continuously decreased from 24.0 to 7.5 as the loading of MWCNTs increases from 0 to 5 wt%. Since MFR of the PP/MWCNT composite melt is sensitive to CNT loading, melt processing of highly concentrated MWCNT/PP composites would be difficult due to extremely high viscosity. This is because porous and agglomerated MWCNT particles prevented composite melts from flowing [11,12]. According to recent report [13], it was shown that the MFR was decreased with increasing filler loading in the case of polymer composites containing porous fillers such as carbon fibers or carbon blacks as shown in the case of MWCNT filled composites. The effect of PP-g-MAH loading on the MFR of PP/ MWCNT03 composites containing MWCNTs of 3 wt% was
shown in Fig. 1b. The melt flow rates are monotonically increased up to the compatibilizer loading of 5 wt% and decreased thereafter because PP-g-MAH compatibilizer has slightly higher MFR than pure PP matrix and acts as a lubricating agent between the PP matrix and MWCNT particles. According to Lee and Youn [14] and Gnatowski and Koszkul [15], it was shown that the MFR of fully exfoliated and compatibilized polypropylene and polyamide composites was decreased with increasing compatibilizer loading because of increased interfacial interaction and molecular bonding between the polymer matrices and fillers. However, the MFR of poorly compatibilized composites was remained as constant irrespective of the amount of compatibilizer loading. In this study, the MFR of compatibilized PP/MWCNT03 composites was monotonically increased with increasing PP-g-MAH loading due to higher MFR of compatibilizer as shown in Fig 1b, indicating poor interfacial interaction between PP-g-MAH and MWCNT particles. 3.2. Steady shear flow behavior In Fig. 2a, the effect of MWCNT loading on the steady shear viscosity of PP/03/MWCNT composites containing the same PP-g-MAH weight fraction of 3 wt% was examined by changing the MWCNT weight fraction from 0.5 to 5 wt%. While the pure PP matrix, PP/PB03 blend, and PP/ PB/MWCNT composites containing MWCNTs of 0.5 wt% (PP/03/MWCNT0.5) and 1 wt% (PP/03/MWCNT1.0) exhibited Newtonian behavior at low shear rates (0.001– 1.0 s 1) and slight shear thinning behavior at above 1.0 s 1, PP/PB/MWCNT composites with MWCNT loading of higher than 2 wt% exhibited non-Newtonian behavior at all shear rate ranges e.g., initial shear-thickening and then yield stress behavior at low shear rate ranges. As the MWCNT loading was increased, it was shown that steady shear viscosity was increased because of the increased MWCNT–MWCNT interaction. PP/PB/MWCNT composites with MWCNT loading of 3 wt% showed shear thinning behavior at lower shear rates. Effects of the PP-g-MAH compatibilizer for PP/PB/ MWCNT03 composites were investigated by changing the compatibilizer loading from 1 to 7 wt% and are shown in Fig. 2b. As the PP-g-MAH loading increases, steady shear viscosity of the PP/PB/MWCNT03 composite melt is slightly increased at low shear rate ranges. At higher shear rate ranges, steady shear viscosity showed almost the same values irrespective of the amount of PP-g-MAH loading. From the above results, it is concluded that the compatibilizer does not play an important role in improvement of the interaction between PP matrix and MWCNT particles because MWCNT particles in pristine masterbatch chips are not chemically functionalized or modified [16]. 3.3. Dynamic oscillatory shear flow behavior It is well known that highly concentrated and agglomerated composites show the yield stress and non-terminal solid-like behavior at lower frequency ranges and these phenomena arise from the increase of chain entanglements and interactions between particles and polymer matrix as
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a
30
Pure PP matrix PP/PB03 PP/03/MWCNT composites PP/MWCNT composites
MFR (g/10min)
25
20
15
10
5 1
2
3
4
5
6
MWCNT loading (wt%)
b
15 MWCNT loading = 3.0 wt%
MFR (g/10min)
14
13
12
11
10 0
2
4
6
8
Compatibilizer loading (wt%) Fig. 1. Effects of (a) MWCNT loading on the MFR of PP/MWCNT composite melts and (b) PP-g-MAH compatibilizer loading on the MFR of PP/03/MWCNT composite melts.
well as between particles [17]. Fig. 3 shows the effect of MWCNT on linear viscoelastic properties of PP/03/MWCNT composites containing PP-g-MAH of 3 wt%. Although terminal slope of pure PP matrix shows a typical unity at low frequency ranges, terminal slopes of PP/PB/MWCNT composites decrease with increasing MWCNT loading and the plateau regimes are obtained at MWCNT loadings of above 2 wt%. Complex viscosity of the PP/PB/MWCNT composite melt shows a similar tendency to the steady shear viscosity as previously shown in Fig. 2. As the MWCNT loading increases, complex viscosity of the PP/PB/MWCNT composite is increased in proportion to the MWCNT load-
ing at low frequency ranges and frequency thinning is initiated at lower frequency ranges. The effect of PP-g-MAH compatibilizer loading on PP/ PB/MWCNT03 composites containing MWCNTs of 3 wt% was characterized by changing the loading from 1 to 7 wt%. As shown in Fig. 4, even though much more than 3 wt% PP-g-MAH was added, storage modulus and complex viscosity of PP/PB/MWCNT composites show similar values to those of PP/MWCNT03 specimens without the compatibilizer irrespective of the loading amount of PP-g-MAH at entire shear rates. In highly concentrated and agglomerated MWCNT/PP composites, it is found that PP-g-MAH
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0
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a
1e+6 Pure PP PP+PB03 PP/03/MWCNT0.5 PP/03/MWCNT1.0 PP/03/MWCNT2.0 PP/03/MWCNT3.0 PP/03/MWCNT5.0
Shear viscosity (Pa . s)
1e+5
1e+4
1e+2 0.001
0.01
0.1
1
10
Shear rate (s-1)
b
Shear viscosity (Pa . s)
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1e+3
1e+5
Pure PP PP+PB03 PP/01/MWCNT03 PP/03/MWCNT03 PP/05/MWCNT03 PP/07/MWCNT03
1e+4
1e+3
1e+2 0.001
0.01
0.1
1
10
Shear rate (s-1) Fig. 2. Effects of (a) MWCNT loading and (b) PP-g-MAH compatibilizer loading on the steady shear viscosities of PP/03/MWCNT and PP/PB/MWCNT03 composite melts, respectively.
compatibilizer does not have any significant influence on homogeneous dispersion of MWCNTs and improvement of interfacial interaction between PP matrix and MWCNT particles. The reason is that there is little molecular interaction between maleic anhydride functional groups and pristine MWCNTs because MWCNT particles used in this study were not chemically functionalized and modified at all. Therefore it is understood that rheological properties
are more affected by MWCNT loadings than compatibilizer loadings in highly concentrated MWCNT/PP composite system. 3.4. Electrical conductivity To determine the optimum structure for electrically conductive composites, the internal structure and rheological
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a
1e+6
Storage modulus (Pa)
1e+5
1e+4
1e+3 Pure PP PP/PB03 PP/03/MWCNT0.5 PP/03/MWCNT1.0 PP/03/MWCNT2.0 PP/03/MWCNT3.0 PP/03/MWCNT5.0
1e+2
1e+1
0.1
1
10
100
Angular frequency (rad/s)
Complex viscosity (Pa . s)
b
1e+5 Pure PP PP/PB03 PP/03/MWCNT0.5 PP/03/MWCNT1.0 PP/03/MWCNT2.0 PP/03/MWCNT3.0 PP/03/MWCNT5.0
1e+4
1e+3
1e+2
1e+1 0.1
1
10
100
Angular frequency (rad/s) Fig. 3. Effects of MWCNT loading on (a) storage modulus and (b) complex viscosity of PP/03/MWCNT composite melts containing compatibilizer of 3.0 wt%.
behavior of electronic composites should be understood before processing of the composites. Fig. 5 shows electrical conductivity measured by the four-probe method. Electrical conductivity of PP/MWCNT composites without the compatibilizer was abruptly increased when the MWCNT loading was varied from 0.5 to 2 wt% as shown in Fig. 5a. However, electrical percolation concentration of PP/03/MWCNT composites containing PP-g-MAH of 3 wt% was shifted toward higher MWCNT loading and electrical conductivity had slightly lower values compared with PP/MWCNT composites without the compatibilizer.
As explained previously for rheological properties, this trend is determined by whether continuous network structure of MWCNTs is successfully formed in PP/MWCNT composites or not. It is frequently assumed that the network structure is slightly hindered because wrapping of the MWCNT is increased sufficiently by the PP-g-MAH compatibilizer, especially at low concentration of MWCNTs. Therefore it is understood that the number of electrical contacts between the MWCNTs is reduced and the tunneling electron effect is diminished because MWCNTs are easily wetted and coated with an insula-
MACROMOLECULAR NANOTECHNOLOGY
1e+0
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a
1e+6 PP/PB03 PP/MWCNT3.0 PP/01/MWCNT3.0 PP/03/MWCNT3.0 PP/05/MWCNT3.0 PP/07/MWCNT3.0
Storage modulus (Pa)
1e+5
1e+4
1e+3
1e+2
1e+1
0.1
1
10
100
Angular frequency (rad/s)
b
Complex viscosity (Pa . s)
MACROMOLECULAR NANOTECHNOLOGY
1e+0
1e+5 PP/PB03 PP/MWCNT3.0 PP/01/MWCNT3.0 PP/03/MWCNT3.0 PP/05/MWCNT3.0 PP/07/MWCNT3.0
1e+4
1e+3
1e+2
1e+1 0.1
1
10
100
Angular frequency (rad/s) Fig. 4. Effects of PP-g-MAH compatibilizer loading on (a) storage modulus and (b) complex viscosity of PP/MWCNT03 composite melts containing MWCNTs of 3.0 wt%.
tion layer of the compatibilizer in PP/03/MWCNT systems [18]. The functionalization of CNTs involves chemical modification of their surface and electrical conductivity is improved by the several modification methods such as acid or amine treatment [19] and polymer wrapping [20]. Especially, polymer wrapping utilizes p–p stocking interaction with surfactant or compatibilizer and has advantages because disruption of the sp2 structure does not occur and the CNT properties are preserved. To compare the wrap-
ping effect of PP-g-MAH compatibilizer, electrical conductivity of PP/MWCNT03 composites was investigated by changing the PP-g-MAH loading from 1 to 7 wt%. As shown in Fig. 5b, when the compatibilizer was added into the PP/ MWCNT03 composites, electrical conductivity was decreased compared with that of the PP/MWCNT03 composite without PP-g-MAH and remained almost constant irrespective of the PP-g-MAH loading. From above experiment results, it is understood that the compatibilizer did not alter the electrical conductivity
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a
100
Electrical conductivity (S/cm)
10-1 10-2 10-3 10-4 10-5
without compatibilizer with compatibilizer
10-6 10-7 10-8 10-9 10-10
0
1
2
3
4
5
6
MWCNT loading (wt%)
b
102 101
Pure PP PP/PB/MWCNT composites
Electrical conductivity (S/cm)
100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 0
2
4
6
8
Compatibilizer loading (wt%) Fig. 5. Effects of (a) MWCNT loading on electrical conductivity of PP/MWCNT composites with or without compatibilizer and (b) compatibilizer loading on electrical conductivity of PP/MWCNT03 composites containing MWCNTs of 3.0 wt%.
of PP/PB/MWCNT03 composites containing MWCNTs of 3 wt% which is above the electrical percolation threshold (1–2 wt%). The MWCNTs form a percolated network structure near and above the percolation threshold, which generates a solid-like plateau behavior of the PP/MWCNT composites. Although the percolated network structure is formed, it is found that the tunneling electron effect is not diminished in spite of sufficient wrapping of the MWCNTs by the PP-g-MAH compatibilizer as shown in dynamic oscillatory shear results. Therefore generation of additional chemical bonds is needed through a proper
functionalization of the MWCNT surface to enhance homogeneous dispersion of MWCNTs and to improve the electrical conductivity, especially for low concentrated MWCNT composite system with the compatibilizer. 3.5. Morphological observation Fig. 6 shows the FE-SEM images of well dispersed and highly entangled regions of pristine MWCNT masterbatch chips, respectively. Although some MWCNTs were randomly dispersed, some MWCNTs remained as severely
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Fig. 6. FE-SEM images of (a) well dispersed and (b) highly entangled regions of pristine masterbatch chips containing MWCNTs of 20 wt%.
aggregated and entangled bundles in PP matrix. It is inferred that novel dispersion methods are needed to overcome aggregation problems, e.g., chemical treatment or functionalization of CNT bundles [17], application of sufficient mechanical force during melt compounding [16], significant increase in wetting of CNTs [6], and so on. Fig. 7 shows the FE-SEM pictures of PP/MWCNT01 composites and PP/03/MWCNT01 composites containing PP-g-MAH of 3 wt%. In Fig. 7a, FE-SEM image of PP/MWCNT01 specimen showed significantly agglomerated and entangled MWCNT bundles like the pristine masterbatch chips even though the MWCNT composites were melt compounded twice with the twin-screw extruder. Two distinct phases were formed in the flowing direction of composite melts
because MWCNTs were not mixed with PP matrix in the case of uncompatibilized PP/MWCNT composites. However, in the case of PP/03/MWCNT01 composites, FE-SEM images showed improved dispersion of MWCNTs compared with uncompatibilized PP/MWCNT01 composites as shown in Fig. 7b.
4. Conclusions Two types of melt compounded composites were prepared by diluting highly concentrated masterbatch chips in the absence and presence of the compatibilizer containing maleic anhydride group. In highly concentrated MWCNT
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Fig. 7. FE-SEM images of (a) PP/MWCNT01 composites without compatibilizer and (b) PP/03/MWCNT01 composites with PP-g-MAH compatibilizer of 3.0 wt%.
composites, the steady and dynamic viscosities of PP/PB/MWCNT composites showed similar results to PP/MWCNT composites. Although electrical conductivity of the PP/MWCNT composite was abruptly increased at MWCNT loadings between 0.5 and 2 wt%, electrical percolation threshold of the PP/03/MWCNT composite was shifted toward higher MWCNT loading (between 1 and 2 wt%) and its conductivity showed slightly lower values than that of the PP/MWCNT composite. FE-SEM observation of the MWCNT composite was carried out to understand the rheological results in steady and dynamic oscillatory shear flows and electrical properties of PP/PB/ MWCNT composites.
Acknowledgements This study was supported by the Korea Science and Engineering Foundation through the Applied Rheology Center (ARC). The authors are grateful for the support. References [1] Thostenson ET, Ren Z, Chou TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61(13):1899–912. [2] Song YS, Youn JR. Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites. Carbon 2005;43(7): 1378–85.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Rheological and electrical properties of polypropylene/MWCNT composites prepared with MWCNT masterbatch chips Seung Hwan Lee, Myung Wook Kim, Sung Ho Kim, Jae Ryoun Youn * Research Institute of Advanced Materials (RIAM), Department of Materials Science and Engineering, Seoul National University, 56-1, Shillim-dong, Gwanak-gu, Seoul 151-744, Republic of Korea
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a r t i c l e
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Article history: Received 17 October 2007 Received in revised form 15 February 2008 Accepted 24 March 2008 Available online 29 March 2008
Keywords: Carbon nanotubes Masterbatch Compatibilizer Rheology Electrical percolation
a b s t r a c t Melt compounded PP/MWCNT (polypropylene/multi-walled carbon nanotube) composites were prepared by diluting highly concentrated masterbatch chips. Maleic anhydride grafted polypropylene (PP-g-MAH) was used as a compatibilizer to promote dispersion and interaction of MWCNTs. Rheological properties were investigated with respect to the MWCNT and compatibilizer loadings, and related to morphological and electrical properties. As the MWCNT loading was increased, shear viscosity and yield stress were increased at low shear rate region because of increased interaction between MWCNT particles. When the MWCNT loading was low, MWCNT dispersion was improved by the PP-gMAH compatibilizer because MWCNTs were wetted sufficiently due to the presence of the compatibilizer. However, rheological and electrical properties of highly concentrated MWCNT composites with the compatibilizer were not improved compared with PP/ MWCNT composites without the compatibilizer because the compatibilizer did not provide sufficient wrapping of MWCNT particles. Electrical and morphological properties of PP/ MWCNT composites were correlated with the rheological properties in steady and dynamic oscillatory shear flows. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Polymer/carbon nanotube (CNT) composites have attracted considerable interest due to their good electrical conductivity and superior reinforcing capability. However, it is difficult to dissolve and disperse CNTs in solvent or polymer melt because of their stable structure and strong resistance to wetting. A suitable functionalization method of CNTs has become an interest subject for developing new composites because it can improve solubility and processability [1]. Many studies have been reported on effective methods for improving the dispersion and interfacial interaction between the CNTs and polymer matrix. Song et al. [2] investigated the influence of solvent treatment
* Corresponding author. Tel.: +82 2 880 8326; fax: +82 2 885 1748. E-mail address: [email protected] (J.R. Youn). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.03.017
and CNT loadings on dispersion of CNTs through rheological measurements and reported that the CNT suspension in epoxy resin with good dispersion exhibited lower storage modulus and complex viscosity than that with poor dispersion because interaction between CNTs and epoxy matrix is decreased. Compatibilization is one of the most effective noncovalent functionalization methods to improve CNT dispersion and interfacial interaction between CNTs and polymer matrix. The interaction between functional groups of the compatibilizer and carboxyl or amine groups of CNTs stabilized the morphology and improved the interfacial adhesion between CNTs and the polymer matrix [3,4]. Bellayer et al. [5] prepared well-dispersed PS/CNT nanocomposites via melt extrusion using various compatibilizers containing trialkylimidazolium salt and examined effects of the compatibilizers on internal structure of PS/CNT composites by using FE-SEM, Fourier-transform infrared spectrometer,
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tained in the masterbatch chips were produced by the chemical vapor deposition (CVD) method and were typically consisted of 8–15 graphitic layers wrapped around a hollow core of several nanometer scales. The masterbatch chip was melt compounded using a MDK/E 46-11D Buss-Kneader (L/D = 11/1) and was supplied in pellet form. PP-g-MAH (PolybondÒ3150, CROMPTON, USA) was used as a compatibilizer to promote the interaction between PP matrix and MWCNTs and to improve the wetting of MWCNT particles. Its MFR was 50.0 g/10 min and maleic anhydride content was 0.5 wt%. Materials used in this study are listed in Table 1. 2.2. Preparation of PP/MWCNT composites Two types of melt compounded PP/MWCNT composites were prepared with a twin-screw extruder (Werner & Pfleiderer, ZSK-25) at 210 °C and at the screw speed of 350 rpm in the presence or absence of PP-g-MAH compatibilizer. The composite melt was quenched in water and then pelletized by a cutting machine. Before melt compounding, the PP-g-MAH compatibilizer and pristine masterbatch chips were dried in an oven for 4 h at 80 °C to avoid void formation and degradation induced by mois-
Table 1 Materials used in this study
2. Experimental procedures 2.1. Materials PP/MWCNT composites were prepared by diluting highly concentrated MWCNT masterbatch chips. An isotactic PP homopolymer manufactured for fiber spinning (MOPLENE HP552R, MFR 25.0 g/10 min, Polymirae, Korea) was used as the base polymer resin. Polypropylene masterbatch chips (Fibril Nanotubes MB3020-01, HYPERION Catalysis, USA) containing 20 wt% multi-walled carbon nanotubes were used for spinning of PP/MWCNT composite fibers. According to the supplier [10], the MWCNTs conTM
Materials
Technical Information
Supplier
Polymer matrix
Isotactic polypropylene MOPLENE HP552R MFR = 25.0 g/10 min. PP based maleic anhydride (PP-g-MAH) POLYBONDÒ-3150 MAH content: 0.5 wt% Masterbatch chip (MB 3020-01) MWCNT concentration of 20 wt% High load MFR = 39.2 g/10 min
Polymirae (Korea)
Compatibilizer
Multi-walled CNTs
Crompton (USA)
HYPERION Catalysis (USA)
Table 2 Compositions of the prepared composites based on pure PP matrix, PP-g-MAH compatibilizer, and MWCNTs (wt%) Composites
MOPLENENHP 552R
Pure PP PP + PB blend Composites without compatibilizer
100 97 99.5 99 98 97 95 96.5 96 95 94 92 96 92 90
Composites with compatibilizer
PP-g-MAH
MWCNTs
Abbreviation
0.5 1.0 2.0 3.0 5.0 0.5 1.0 2.0 3.0 5.0 3.0 3.0 3.0
Pure PP PP/PB03 PP/MWCNT0.5 PP/MWCNT1.0 PP/MWCNT2.0 PP/MWCNT3.0 PP/MWCNT5.0 PP/03/MWCNT0.5 PP/03/MWCNT1.0 PP/03/MWCNT2.0 PP/03/MWCNT3.0 PP/03/MWCNT5.0 PP/01/MWCNT3.0 PP/05/MWCNT3.0 PP/07/MWCNT3.0
3.0
3.0 3.0 3.0 3.0 3.0 1.0 5.0 7.0
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and X-ray diffraction methods. The homogeneous dispersion of CNTs in PP matrix is achieved by strong hydrogen bonding between hydroxyl groups of the CNTs and maleic anhydride groups of PP-g-MAH depending on the chemical similarity of PP matrix and the grafted PP [6]. Homogenizing mixers or extruders are employed frequently as processing equipments for mechanical mixing of CNT particles with polymer melts [7]. One of the most common processing methods is the melt compounding as a means to embed CNT particles directly into thermoplastic polymer melts. Another method is the masterbatch process which is one of the simplest and most economical methods in processing of CNT composites. It can be often used where it is necessary to improve processing characteristics and physical properties of composites or to disperse concentrated fillers uniformly in polymer matrix. Pötschke et al. [8,9] prepared PC/MWCNT composites by diluting highly concentrated masterbatch chips and investigated morphological, rheological, and electrical properties as well as orientation of MWCNTs by using dielectric spectroscopy and Raman spectroscopy. It is necessary to understand effects of MWCNTs and compatibilizers on physical properties of polymer/MWCNT composites. However, there have been few reports on the role of compatibilizer for improving wrapping and dispersion of MWCNTs in PP/MWCNT composites prepared by diluting highly concentrated MWCNT masterbatch chips.
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ture. All samples were compression molded into a disk of 25 mm diameter and 1.5 mm thickness by using a hot press (WABASH, 25-1212-2TMB) at the same temperature as the barrel temperature of the extruder. The compression molded samples were used for analysis of the internal structure and measurement of the rheological properties. Compositions of the prepared composite samples based on PP matrix, PP-g-MAH compatibilizer, and MWCNTs are listed in Table 2.
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2.3. Characterization Melt flow rates of PP/MWCNT composites and pristine masterbatch chips with 20 wt% MWCNTs were carried out according to ASTM D1238 using Melt Indexer (Göttfert, MI-2) at standard load of 2.13 kg and high load of 23.0 kg, respectively. Steady and dynamic oscillatory shear properties were investigated by using a rotational rheometer (TA Instruments, AR-2000). Steady shear measurement was performed at the shear rate of 0.001–10 s 1 and dynamic oscillatory shear measurement was carried out at the frequency of 0.1–200 rad/s with the constant strain of 10%, which is within the linear viscoelastic range of the materials. The electrical conductivity was measured by the fourprobe method (Agilent Technologies, Agilent 4339B highresistance meter) at ambient condition to characterize electrical percolation threshold of PP/MWCNT composites. Surface of the melt compounded PP/MWCNT composite chips was observed using an FE-SEM (JEOL, JSM-6330F). FE-SEM specimens were coated with platinum using a sputter coating instrument (Cressington Scientific Instruments, Sputter Coater 108) in vacuum for 5 min prior to the observation and the FE-SEM was operated at 10.0 kV. 3. Results and discussion 3.1. Melt flow rate The melt flow rate (MFR) was measured for the pure PP matrix and PP/MWCNT composites with the loading of 2.16 kg and at 230 °C. However, the MFR of the pristine masterbatch chip could be measured only at high loading condition of 21.6 kg because there was no running flow across capillary at the loading of 2.16 kg. The measured MFR was 39.2 g/10 min at the high CNT loading. Fig. 1a shows the effects of MWCNT loading on the MFR of PP/ MWCNT composites with or without the compatibilizer. The MFR of PP/MWCNT composite melts continuously decreased from 24.0 to 7.5 as the loading of MWCNTs increases from 0 to 5 wt%. Since MFR of the PP/MWCNT composite melt is sensitive to CNT loading, melt processing of highly concentrated MWCNT/PP composites would be difficult due to extremely high viscosity. This is because porous and agglomerated MWCNT particles prevented composite melts from flowing [11,12]. According to recent report [13], it was shown that the MFR was decreased with increasing filler loading in the case of polymer composites containing porous fillers such as carbon fibers or carbon blacks as shown in the case of MWCNT filled composites. The effect of PP-g-MAH loading on the MFR of PP/ MWCNT03 composites containing MWCNTs of 3 wt% was
shown in Fig. 1b. The melt flow rates are monotonically increased up to the compatibilizer loading of 5 wt% and decreased thereafter because PP-g-MAH compatibilizer has slightly higher MFR than pure PP matrix and acts as a lubricating agent between the PP matrix and MWCNT particles. According to Lee and Youn [14] and Gnatowski and Koszkul [15], it was shown that the MFR of fully exfoliated and compatibilized polypropylene and polyamide composites was decreased with increasing compatibilizer loading because of increased interfacial interaction and molecular bonding between the polymer matrices and fillers. However, the MFR of poorly compatibilized composites was remained as constant irrespective of the amount of compatibilizer loading. In this study, the MFR of compatibilized PP/MWCNT03 composites was monotonically increased with increasing PP-g-MAH loading due to higher MFR of compatibilizer as shown in Fig 1b, indicating poor interfacial interaction between PP-g-MAH and MWCNT particles. 3.2. Steady shear flow behavior In Fig. 2a, the effect of MWCNT loading on the steady shear viscosity of PP/03/MWCNT composites containing the same PP-g-MAH weight fraction of 3 wt% was examined by changing the MWCNT weight fraction from 0.5 to 5 wt%. While the pure PP matrix, PP/PB03 blend, and PP/ PB/MWCNT composites containing MWCNTs of 0.5 wt% (PP/03/MWCNT0.5) and 1 wt% (PP/03/MWCNT1.0) exhibited Newtonian behavior at low shear rates (0.001– 1.0 s 1) and slight shear thinning behavior at above 1.0 s 1, PP/PB/MWCNT composites with MWCNT loading of higher than 2 wt% exhibited non-Newtonian behavior at all shear rate ranges e.g., initial shear-thickening and then yield stress behavior at low shear rate ranges. As the MWCNT loading was increased, it was shown that steady shear viscosity was increased because of the increased MWCNT–MWCNT interaction. PP/PB/MWCNT composites with MWCNT loading of 3 wt% showed shear thinning behavior at lower shear rates. Effects of the PP-g-MAH compatibilizer for PP/PB/ MWCNT03 composites were investigated by changing the compatibilizer loading from 1 to 7 wt% and are shown in Fig. 2b. As the PP-g-MAH loading increases, steady shear viscosity of the PP/PB/MWCNT03 composite melt is slightly increased at low shear rate ranges. At higher shear rate ranges, steady shear viscosity showed almost the same values irrespective of the amount of PP-g-MAH loading. From the above results, it is concluded that the compatibilizer does not play an important role in improvement of the interaction between PP matrix and MWCNT particles because MWCNT particles in pristine masterbatch chips are not chemically functionalized or modified [16]. 3.3. Dynamic oscillatory shear flow behavior It is well known that highly concentrated and agglomerated composites show the yield stress and non-terminal solid-like behavior at lower frequency ranges and these phenomena arise from the increase of chain entanglements and interactions between particles and polymer matrix as
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a
30
Pure PP matrix PP/PB03 PP/03/MWCNT composites PP/MWCNT composites
MFR (g/10min)
25
20
15
10
5 1
2
3
4
5
6
MWCNT loading (wt%)
b
15 MWCNT loading = 3.0 wt%
MFR (g/10min)
14
13
12
11
10 0
2
4
6
8
Compatibilizer loading (wt%) Fig. 1. Effects of (a) MWCNT loading on the MFR of PP/MWCNT composite melts and (b) PP-g-MAH compatibilizer loading on the MFR of PP/03/MWCNT composite melts.
well as between particles [17]. Fig. 3 shows the effect of MWCNT on linear viscoelastic properties of PP/03/MWCNT composites containing PP-g-MAH of 3 wt%. Although terminal slope of pure PP matrix shows a typical unity at low frequency ranges, terminal slopes of PP/PB/MWCNT composites decrease with increasing MWCNT loading and the plateau regimes are obtained at MWCNT loadings of above 2 wt%. Complex viscosity of the PP/PB/MWCNT composite melt shows a similar tendency to the steady shear viscosity as previously shown in Fig. 2. As the MWCNT loading increases, complex viscosity of the PP/PB/MWCNT composite is increased in proportion to the MWCNT load-
ing at low frequency ranges and frequency thinning is initiated at lower frequency ranges. The effect of PP-g-MAH compatibilizer loading on PP/ PB/MWCNT03 composites containing MWCNTs of 3 wt% was characterized by changing the loading from 1 to 7 wt%. As shown in Fig. 4, even though much more than 3 wt% PP-g-MAH was added, storage modulus and complex viscosity of PP/PB/MWCNT composites show similar values to those of PP/MWCNT03 specimens without the compatibilizer irrespective of the loading amount of PP-g-MAH at entire shear rates. In highly concentrated and agglomerated MWCNT/PP composites, it is found that PP-g-MAH
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0
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a
1e+6 Pure PP PP+PB03 PP/03/MWCNT0.5 PP/03/MWCNT1.0 PP/03/MWCNT2.0 PP/03/MWCNT3.0 PP/03/MWCNT5.0
Shear viscosity (Pa . s)
1e+5
1e+4
1e+2 0.001
0.01
0.1
1
10
Shear rate (s-1)
b
Shear viscosity (Pa . s)
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1e+3
1e+5
Pure PP PP+PB03 PP/01/MWCNT03 PP/03/MWCNT03 PP/05/MWCNT03 PP/07/MWCNT03
1e+4
1e+3
1e+2 0.001
0.01
0.1
1
10
Shear rate (s-1) Fig. 2. Effects of (a) MWCNT loading and (b) PP-g-MAH compatibilizer loading on the steady shear viscosities of PP/03/MWCNT and PP/PB/MWCNT03 composite melts, respectively.
compatibilizer does not have any significant influence on homogeneous dispersion of MWCNTs and improvement of interfacial interaction between PP matrix and MWCNT particles. The reason is that there is little molecular interaction between maleic anhydride functional groups and pristine MWCNTs because MWCNT particles used in this study were not chemically functionalized and modified at all. Therefore it is understood that rheological properties
are more affected by MWCNT loadings than compatibilizer loadings in highly concentrated MWCNT/PP composite system. 3.4. Electrical conductivity To determine the optimum structure for electrically conductive composites, the internal structure and rheological
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a
1e+6
Storage modulus (Pa)
1e+5
1e+4
1e+3 Pure PP PP/PB03 PP/03/MWCNT0.5 PP/03/MWCNT1.0 PP/03/MWCNT2.0 PP/03/MWCNT3.0 PP/03/MWCNT5.0
1e+2
1e+1
0.1
1
10
100
Angular frequency (rad/s)
Complex viscosity (Pa . s)
b
1e+5 Pure PP PP/PB03 PP/03/MWCNT0.5 PP/03/MWCNT1.0 PP/03/MWCNT2.0 PP/03/MWCNT3.0 PP/03/MWCNT5.0
1e+4
1e+3
1e+2
1e+1 0.1
1
10
100
Angular frequency (rad/s) Fig. 3. Effects of MWCNT loading on (a) storage modulus and (b) complex viscosity of PP/03/MWCNT composite melts containing compatibilizer of 3.0 wt%.
behavior of electronic composites should be understood before processing of the composites. Fig. 5 shows electrical conductivity measured by the four-probe method. Electrical conductivity of PP/MWCNT composites without the compatibilizer was abruptly increased when the MWCNT loading was varied from 0.5 to 2 wt% as shown in Fig. 5a. However, electrical percolation concentration of PP/03/MWCNT composites containing PP-g-MAH of 3 wt% was shifted toward higher MWCNT loading and electrical conductivity had slightly lower values compared with PP/MWCNT composites without the compatibilizer.
As explained previously for rheological properties, this trend is determined by whether continuous network structure of MWCNTs is successfully formed in PP/MWCNT composites or not. It is frequently assumed that the network structure is slightly hindered because wrapping of the MWCNT is increased sufficiently by the PP-g-MAH compatibilizer, especially at low concentration of MWCNTs. Therefore it is understood that the number of electrical contacts between the MWCNTs is reduced and the tunneling electron effect is diminished because MWCNTs are easily wetted and coated with an insula-
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1e+0
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a
1e+6 PP/PB03 PP/MWCNT3.0 PP/01/MWCNT3.0 PP/03/MWCNT3.0 PP/05/MWCNT3.0 PP/07/MWCNT3.0
Storage modulus (Pa)
1e+5
1e+4
1e+3
1e+2
1e+1
0.1
1
10
100
Angular frequency (rad/s)
b
Complex viscosity (Pa . s)
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1e+0
1e+5 PP/PB03 PP/MWCNT3.0 PP/01/MWCNT3.0 PP/03/MWCNT3.0 PP/05/MWCNT3.0 PP/07/MWCNT3.0
1e+4
1e+3
1e+2
1e+1 0.1
1
10
100
Angular frequency (rad/s) Fig. 4. Effects of PP-g-MAH compatibilizer loading on (a) storage modulus and (b) complex viscosity of PP/MWCNT03 composite melts containing MWCNTs of 3.0 wt%.
tion layer of the compatibilizer in PP/03/MWCNT systems [18]. The functionalization of CNTs involves chemical modification of their surface and electrical conductivity is improved by the several modification methods such as acid or amine treatment [19] and polymer wrapping [20]. Especially, polymer wrapping utilizes p–p stocking interaction with surfactant or compatibilizer and has advantages because disruption of the sp2 structure does not occur and the CNT properties are preserved. To compare the wrap-
ping effect of PP-g-MAH compatibilizer, electrical conductivity of PP/MWCNT03 composites was investigated by changing the PP-g-MAH loading from 1 to 7 wt%. As shown in Fig. 5b, when the compatibilizer was added into the PP/ MWCNT03 composites, electrical conductivity was decreased compared with that of the PP/MWCNT03 composite without PP-g-MAH and remained almost constant irrespective of the PP-g-MAH loading. From above experiment results, it is understood that the compatibilizer did not alter the electrical conductivity
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a
100
Electrical conductivity (S/cm)
10-1 10-2 10-3 10-4 10-5
without compatibilizer with compatibilizer
10-6 10-7 10-8 10-9 10-10
0
1
2
3
4
5
6
MWCNT loading (wt%)
b
102 101
Pure PP PP/PB/MWCNT composites
Electrical conductivity (S/cm)
100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 0
2
4
6
8
Compatibilizer loading (wt%) Fig. 5. Effects of (a) MWCNT loading on electrical conductivity of PP/MWCNT composites with or without compatibilizer and (b) compatibilizer loading on electrical conductivity of PP/MWCNT03 composites containing MWCNTs of 3.0 wt%.
of PP/PB/MWCNT03 composites containing MWCNTs of 3 wt% which is above the electrical percolation threshold (1–2 wt%). The MWCNTs form a percolated network structure near and above the percolation threshold, which generates a solid-like plateau behavior of the PP/MWCNT composites. Although the percolated network structure is formed, it is found that the tunneling electron effect is not diminished in spite of sufficient wrapping of the MWCNTs by the PP-g-MAH compatibilizer as shown in dynamic oscillatory shear results. Therefore generation of additional chemical bonds is needed through a proper
functionalization of the MWCNT surface to enhance homogeneous dispersion of MWCNTs and to improve the electrical conductivity, especially for low concentrated MWCNT composite system with the compatibilizer. 3.5. Morphological observation Fig. 6 shows the FE-SEM images of well dispersed and highly entangled regions of pristine MWCNT masterbatch chips, respectively. Although some MWCNTs were randomly dispersed, some MWCNTs remained as severely
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Fig. 6. FE-SEM images of (a) well dispersed and (b) highly entangled regions of pristine masterbatch chips containing MWCNTs of 20 wt%.
aggregated and entangled bundles in PP matrix. It is inferred that novel dispersion methods are needed to overcome aggregation problems, e.g., chemical treatment or functionalization of CNT bundles [17], application of sufficient mechanical force during melt compounding [16], significant increase in wetting of CNTs [6], and so on. Fig. 7 shows the FE-SEM pictures of PP/MWCNT01 composites and PP/03/MWCNT01 composites containing PP-g-MAH of 3 wt%. In Fig. 7a, FE-SEM image of PP/MWCNT01 specimen showed significantly agglomerated and entangled MWCNT bundles like the pristine masterbatch chips even though the MWCNT composites were melt compounded twice with the twin-screw extruder. Two distinct phases were formed in the flowing direction of composite melts
because MWCNTs were not mixed with PP matrix in the case of uncompatibilized PP/MWCNT composites. However, in the case of PP/03/MWCNT01 composites, FE-SEM images showed improved dispersion of MWCNTs compared with uncompatibilized PP/MWCNT01 composites as shown in Fig. 7b.
4. Conclusions Two types of melt compounded composites were prepared by diluting highly concentrated masterbatch chips in the absence and presence of the compatibilizer containing maleic anhydride group. In highly concentrated MWCNT
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Fig. 7. FE-SEM images of (a) PP/MWCNT01 composites without compatibilizer and (b) PP/03/MWCNT01 composites with PP-g-MAH compatibilizer of 3.0 wt%.
composites, the steady and dynamic viscosities of PP/PB/MWCNT composites showed similar results to PP/MWCNT composites. Although electrical conductivity of the PP/MWCNT composite was abruptly increased at MWCNT loadings between 0.5 and 2 wt%, electrical percolation threshold of the PP/03/MWCNT composite was shifted toward higher MWCNT loading (between 1 and 2 wt%) and its conductivity showed slightly lower values than that of the PP/MWCNT composite. FE-SEM observation of the MWCNT composite was carried out to understand the rheological results in steady and dynamic oscillatory shear flows and electrical properties of PP/PB/ MWCNT composites.
Acknowledgements This study was supported by the Korea Science and Engineering Foundation through the Applied Rheology Center (ARC). The authors are grateful for the support. References [1] Thostenson ET, Ren Z, Chou TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61(13):1899–912. [2] Song YS, Youn JR. Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites. Carbon 2005;43(7): 1378–85.
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