Electrorheological Properties of Carbon Nanotube ...

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Journal of Nanoscience and Nanotechnology Vol. 8, 1–5, 2008

Electrorheological Properties of Carbon Nanotube/ Polyelectrolyte Composite Silica Nanoparticles by Layer-by-Layer Self-Assembly Byung-Soo Kim2 † , Bumsu Kim1 † , and Kyung-Do Suh2 ∗ 1

Division of Inorganic Chemistry Exam, Bureau of Chemistry and Biotechnology Exam, Korean Intellectual Property Office (KIPO), Daejeon 302-701, South Korea 2 Division of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, South Korea Multiwall carbon nanotubes (MCNTs)/silica (SiO2 ) composite particles were prepared by layerby-layer (LbL) self-assembly method using polyelectrolytes and functionalized MCNTs (fMCNTs). The fMCNTs prepared by chemical oxidation method were incorporated on the outermost layer of polyelectrolyte-coated SiO2 particles. The amount of fMCNTs was varied by LbL self assembly. In the process the number of fMCNT layers on SiO2 particles could be controlled. The fMCNT-coated SiO2 particles were characterized by zeta-potential analysis, transmission electron microscopy (TEM), and optical microscopy (OM). In addition, the electrorheological (ER) properties of multilayers containing fMCNTs on silica particles were investigated under controlled electric fields. The ER properties of the composite particles were influenced by the amount of fMCNTs in multilayers.

Keywords: Carbon Nanotubes, Silica Particle, Polyelectrolyte, Self-Assembly, Electrorheology.

The carbon nanotubes (CNTs) discovered by lijima have been paid great attention as an advanced material due to their excellent electrical, mechanical, physical, chemical, and thermal properties.1 The CNTs have many potential applications, such as, sensor, hydrogen storage, fuel cell, field emission transistor, and so on.2–4 In recent years, CNT composites with different materials (polymer, organic, inorganic) have been studied.5 6 It is confirmed that CNTs in matrix enhanced the electrical properties, i.e., conductivity, ER effect, electromagnetic interference effect, etc., of the composites.7–12 Recently, CNTs were coated on the surface of polymer substrates and their properties were investigated.13 14 The CNT-coated composites were prepared by LbL selfassembly technique.15–17 It is a method to modify the surface of substrates by adsorbing alternate layers of oppositely charged materials. The technique was employed to fabricate multilayer structures on substrates or colloidal particles18 19 owing to attractive advantages such as, easy adjustment of thickness, size and shape by controlling the layer numbers.20 21 Many potential applications of LbL ∗ †

Author to whom correspondence should be addressed. These authors contributed equally to this work.

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self-assembly have been envisaged in medicines, biosensors, drug release, cosmetics, and various electronics.22 In this study, the negatively charged fMCNTs containing carboxylic groups were prepared by chemical oxidation method. Poly(diallyldimethylammonium chloride) (PDDA)/fMCNT multilayers on SiO2 particles were fabricated by LbL self-assembly using the alternative adsorption of PDDA and fMCNTs. The amount of fMCNTs was varied by controlling the number of PDDA/fMCNT multilayers on polyelectrolyte-coated SiO2 particles. The morphology and the properties of silica particles having polyelectrolyte/fMCNT multilayers were observed by TEM and zeta-potential analysis. The ER behavior of the PDDA/fMCNT self-assembled silica particles in insulation oil was investigated under various electric fields.

2. EXPERIMENTAL DETAILS 2.1. Materials The purified multiwall carbon nanotubes (MCNTs, CVD, 95%) were obtained from Iljin Nanotech Inc. Sulfuric acid (97%) and nitric acid (60–62%) were obtained from Yakuri and Junsei chemical, respectively. The silica (SiO2 ) particles (500 nm ± 50 nm) were purchased from

1533-4880/2008/8/001/005

doi:10.1166/jnn.2008.203

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1. INTRODUCTION

Electrorheological Properties of Carbon Nanotube/Polyelectrolyte Composite Silica Nanoparticles

Sukgyung A.T. Poly(diallyldimethylammonium chloride) (PDDA, 20 wt% in water. Mw = 100,000–200,000) and poly(sodium 4-styrenesulfonate) (PSS, Mw = 70,000) were purchased from Aldrich. The deionized water (resistivity: >18.2 M cm) used in all experiments was purified by Millipore Milli-Q Plus purification systems. 2.2. Chemical Oxidation of MCNTs The MCNTs were functionalized by a sonication treatment in a mixture of sulfuric acid and nitric acid (3:1) for 6 h at room temperature.23 The functionalized MCNTs (fMCNTs) were diluted with deionized water and separated by centrifugation. They were continuously subjected to repeat redispersion/centrifugation in deionized water until there was no sedimentation of the fMCNTs. Finally, the soluble fMCNTs obtained above were diluted to 0.0005 g fMCNTs per 1 g of deionized water.

The polyelectrolyte multilayers (PDDA/PSS/PDDA) on the SiO2 nanoparticles were produced by LbL selfassembly. For the purpose the concentration of the polyelectrolyte (PDDA and PSS) solution in water was maintained at 10 mg/mL. The positively charged PDDA was adsorbed on SiO2 particles (0.1 g) for 30 min. Excess PDDA was removed with deionized water by centrifuging (15,000 rpm, 15 min) and repeating the process three times. The negatively charged PSS was adsorbed on PDDA-coated SiO2 particles for 30 min. Excess PSS was removed by three repeated centrifugation (15,000 rpm, 15 min) with deionized water. Finally, PDDA was adsorbed on PDDA/PSS self-assembled SiO2 , and excess PDDA was removed by centrifugation.

2.5. Characterizations The morphology of fMCNT/polyelectrolyte self-assembled particles was observed by TEM (Cal Zeiss LIBRA 120). The zeta potential of fMCNT/polyelectrolyte selfassembled particles was measured by a zeta-potential analyzer (otsuka, ELS 8000) with different outermost layer of particles. All samples were sonificated for 3 min before measurements to improve dispersion. 2.6. Measurements of Shear Stress and Viscosity ER suspensions containing 6 wt% fMCNT/polyelectrolyte coated particles in silicon oil (KF-96, 50cS, Shinetsu) were prepared by mechanical stirring and sonication. The rheological properties of the suspensions were measured at room temperature by an ARES concentric cylindrical rheometer (ARES4, Rheometric Scientific, Inc.) equipped with a high-voltage power generator (EL5P8L, Glassman High Voltage Inc.). ER fluids were placed in the gap between the rotating outer cup and the stationary inner bob. An electric field was applied for 5 min to obtain an equilibrium structure before applying the shear. Flow curves were obtained with the rheometer operating in the controlled shear rate (CSR) mode.

3. RESULTS AND DISCUSSION Zeta-potential measurements were carried out to confirm the assembly of fMCNT/polyelectrolyte multilayers on SiO2 particles. Figure 1 shows the zeta-potential of each layer on silica particles. Before attaching fMCNTs on the surface of silica particles, PDDA/PSS/PDDA multilayers were self-assembled on SiO2 particles. Because two types of polyelectrolytes (PDDA, PSS) were coated 80

2.4. Synthesis of the fMCNT/Polyelectrolyte Self-Assembled Silica Particles Positively charged PDDA/PSS/PDDA-coated SiO2 particles were dispersed in deionized water. To this the fMCNT dispersion (0.1 wt%) was added and sonicated for 1 hour. Excess fMCNTs were removed by centrifugation after redispersing particles for 3 min by sonication. The fMCNT/polyelectrolyte self-assembled particles were washed with excess amount of deionized water by centrifuging (15,000 rpm, 15 min) for five times. The PDDA (10 mg/mL) was self-assembled on the outermost layer of fMCNT for 30 min. Excess PDDA was removed with deionized water by centrifuging (15,000 rpm, 15 min) three times. By using the same procedure repeatedly, PDDA and fMCNTs were alternately adsorbed on the surfaces of SiO2 particles. In the process the number of adsorbed fMCNT layers on SiO2 particles was controlled. 2

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2.3. Multilayers on SiO2 Nanoparticles by LbL Self Assembly

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20 0 –20 –40 –60 –80

0

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Layer number Fig. 1. The zeta-potential values of fMCNT/polyelectrolyte selfassembled SiO2 particles as a function of layer numbers. 0 layer (): uncoated SiO2 particles; 1, 3, 5, 7 layers (•): PDDA; 2 layer (): PSS; 4, 6, 8 layers (): fMCNT.

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on SiO2 particles, zeta-potential values were altered from +60 mV to −57 mV until the third layer. The alternating value of zeta-potential was also observed after adsorbing fMCNTs on PDDA/PSS/PDDA self-assembled SiO2 particles. When negatively charged fMCNTs were coated on the third layer (positively charged PDDA layer), the zeta-potential value was −50.04 mV. It was confirmed that the fMCNT layer was well organized on the outermost PDDA layer by LbL process. Because the fMCNT layer had the negative surface charge, the positively charged PDDA layer was self-assembled on the layer of fMCNTs by LbL process. The PDDA/fMCNT multilayers on silica particles were obtained by using alternative adsorptions of oppositely charged PDDA and fMCNTs. No matter what the number of (PDDA/fMCNT) multilayer was altered, the zeta-potential value of samples was nearly similar, going

(a)

positive and negative alternately. These values mean that each layer of polyelectrolyte and fMCNTs was coated well on SiO2 particles. The morphology of fMCNTs self-assembled SiO2 particles was observed by TEM. Figure 2 shows TEM images of the (PDDA/fMCNT)1 layer and the (PDDA/fMCNT)3 layer on PDDA/PSS/PDDA self-assembled SiO2 particles, respectively. The bundles of fMCNTs anchored on polyelectrolyte coated SiO2 particles were observed. The diameter of bundles was about 20–50 nm. The length distribution of fMCNTs fell into several hundreds of nanometers. The (PDDA/fMCNT)3 layer (Fig. 2(b)) had more adsorbed bundles of the fMCNTs on the surface than the (PDDA/fMCNT)1 layer (Fig. 2(a)). Figure 3 shows the ER behavior of 2 wt% PDDA/ fMCNT multilayer self-assembled SiO2 particles in the silicon oil between two parallel electrodes. The gap between the two electrodes was 450 m, and the applied electric field was 500 V/mm. The images of ER fluid were obtained under the optical microscope. By applying the voltage between two electrodes, PDDA/fMCNT multilayer coated SiO2 particles were aligned through the medium and formed columns across medium. It was demonstrated that SiO2 particles having multilayer (PDDA/fMCNT)3 (a) 100 µm

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(b)

(b) 100 µm

Fig. 2. TEM images of SiO2 particles coated with (PDDA/fMCNT)1 multilayer (a) and (PDDA/fMCNT)3 multilayer (b), scale bar: 100 nm.


Fig. 3. OM images of 2 wt% fMCNT/polyelectrolyte self-assembled SiO2 particles dispersed in silicone oil under applied electric fields (500 V/mm). The distance between two electrodes was 450 m. a: (PDDA/fMCNT)1 multilayer; b: (PDDA/fMCNT)3 multilayer.

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formed more and thicker columns than SiO2 particles containing multilayer (PDDA/fMCNT)1 under applied electric fields. The ER properties of SiO2 particles coated with fMCNT/polyelectrolyte multilayer in insulating silicon oil were measured by applying different voltages as shown in Figure 4. The shear stress of (PDDA/fMCNT)1 self-assembled SiO2 particles increased with increasing shear rates (Figs. 4(a, b)). At low shear rates, the shear stress of (PDDA/fMCNT)1 multilayer suspension decreased under 1 kV/mm as shown in Figure 4(b). These phenomena were reported and explained.24 25 The SiO2 particles coated with (PDDA/fMCNT)3 multilayer showed the typical behavior of ER fluid (Figs. 4(c, d)). The viscosity and the shear stress of ER fluids increased gradually at the same shear rate by increasing the strength of the electric fields. In addition, the shear stress of (PDDA/fMCNT)3 self-assembled SiO2 particles

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became almost uniform constant regardless of the applied shear rates, when voltage increased. This non-Newtonian behavior is the rheological behavior of Bingham fluid as the typical behavior of ER fluids.26 The shear stress behavior of Bingham fluid is express as Eq. (1), •

= y +

(1)

where is the shear stress, is the shear viscosity and

is the shear rate, y is the electric field-induced dynamic yield stress. The dynamic yield stresses can be obtained by extrapolating the shear rate to zero (here, the plateau stress at a low shear rate, = 01 s−1 ) under a controlled shear rate test. These various ER properties with different number of multilayers on particles were well consistent with the ER behavior observed by optical microscopy in Figure 3.

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Viscosity (Pa–s)

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0 kV/mm 0.5 kV/mm 1.0 kV/mm 100

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(s–1)

10–1 10–1

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Shear rate

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Fig. 4. The viscosity (a, c) and the shear stress (b, d) versus shear rate for 6 wt% ER suspension of SiO2 particles coated with fMCNT/polyelectrolytes multilayer at various electric fields. a, b: (PDDA/fMCNT)1 layer; c, d: (PDDA/fMCNT)3 layer; : 0 kV, •: 0.5 kV, : 1 kV.

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Because (PDDA/fMCNT)3 self-assembled SiO2 particles had more amount of fMCNTs in the shell than (PDDA/fMCNT)1 self-assembled SiO2 particles, the ER property of (PDDA/fMCNT)3 multilayer was better than that of (PDDA/fMCNT)1 multilayer. In addition, the amount of attached fMCNTs on the surface of particles increased with increasing the number of PDDA/fMCNT multilayers as shown in Figure 2. It was suggested that the amount of fMCNTs on the surface of SiO2 particles was a critical factor to control the ER properties of multilayer self-assembled SiO2 particles. Furthermore, the ER properties of particles may be controlled by changing the amount of fMCNTs with varying the number of (PDDA/fMCNT) self-assembled layers. After all, the amount of fMCNTs was efficiently controlled by changing the number of PDDA/fMCNT layers in our system.

4. CONCLUSION

References and Notes 1. S. Iijima, Nature 354, 56 (1991). 2. R. H. Baughman, A. A. Zakhidov, and W. A. Heer, Science 297, 787 (2002). 3. V. Derycke, R. Martel, J. Appenzeller, and P. Avouris, Nano Lett. 1, 453 (2001). 4. Y. Qiao, C. M. Li, S. J. Bao, and Q. L. Bao, J. Power Sources 170, 79 (2007). 5. L. Dai and A. W. H. Mau, Adv. Mater. 13, 899 (2001). 6. J. Sun, L. Gao, and W. Li, Chem. Mater. 14, 5169 (2002). 7. O. Breuer and U. Sundararaj, Polym. Compos. 25, 630 (2004). 8. D. A. Makeiff and T. Huber, Synth. Met. 156, 497 (2006). 9. S. J. Park, S. T. Lim, M. S. Cho, H. M. Kim, J. Joo, and H. J. Choi, Curr. Appl. Phys. 5, 302 (2005). 10. Y. D. Kim and D. H. Park, Synth. Met. 142, 147 (2004). 11. S. J. Park, M. S. Cho, S. T. Lim, H. J. Choi, and M. S. Jhon, Macromol. Rapid Commun. 26, 1563 (2005). 12. H. J. Jin, H. J. Choi, S. H. Yoon, S. J. Myung, and S. E. Shim, Chem. Mater. 17, 4034 (2005). 13. M. A. Correa-Duarte, A. Kosiorek, W. Kandulski, M. Giersig, and V. Salgueirino-Maceira, Small 2, 220 (2006). 14. S. Qin, D. Qin, W. T. Ford, J. E. Herrera, and D. E. Resasco, Macromolecules 37, 9963 (2004). 15. X. Wang, H. X. Huang, A. R. Liu, B. Liu, T. Wakayama, C. Nakamura, J. Miyake, and D. J. Qian, Carbon 44, 2115 (2006). 16. Y. Lan, E. Wang, Y. Song, Z. Kang, M. Jiang, L. Gao, S. Lian, D. Wu, L. Xu, and Z. Li, J. Colloid Interface Sci. 284, 216 (2005). 17. K. J. Loh, J. Kim, J. P. Lynch, N. W. S. Kam, and N. A. Kotov, Smart Mater. Struct. 16, 429 (2007). 18. H. Trotter, A. A. Zaman, and R. Partch, J. Colloid Interface Sci. 286, 233 (2005). 19. C. Jiang, S. Markutsya, and V. V. Tsukruk, Adv. Mater. 16, 157 (2004). 20. F. Caruso, R. A. Caruso, and H. Mohwald, Science 282, 1111 (1998). 21. G. Decher, Science 277, 1232 (1997). 22. W. Yang, D. Trau, R. Renneberg, N. T. Yu, and F. Caruso, J. Colloid Interface Sci. 234, 356 (2001). 23. Z. Shi, Y. Lian, X. Zhou, Z. Gu, Y. Zhang, S. Iijima, Q. Gong, H. Li, and S. L. Zhang, Chem. Commun. 6, 461 (2000). 24. M. S. Cho, H. J. Choi, and M. S. Jhon, Langmuir 19, 5875 (2003). 25. M. S. Cho, H. J. Choi, and M. S. Jhon, Polymer 46, 11484 (2005). 26. S. M. Chen and C. G. Wei, Smart Mater. Struct. 15, 371 (2006).

Received: 4 February 2007. Accepted: 24 November 2007.


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The fMCNT/polyelectrolyte self-assembled SiO2 particles were prepared by LbL self-assembly method with fMCNTs and polyelectrolytes. Each layer of oppositely charged polyelectrolyte and fMCNTs was alternately adsorbed on silica particles because of electrostatic interactions. The amount of fMCNTs was efficiently controlled by varying the number of layers on SiO2 particles with LbL process. It was confirmed that fMCNTs were well attached on SiO2 particles by LbL self-assembly. In addition, the amount of fMCNTs on SiO2 particles controlled by LbL self-assembly was observed by TEM. The ER properties of the fMCNT/polyelectrolyte selfassembled SiO2 particles in insulating oil were measured under applied electric fields. SiO2 particles coated with (PDDA/fMCNT)3 multilayer showed the typical behavior of ER fluid. Because (PDDA/fMCNT)3 self-assembled SiO2 particles had more amount of fMCNTs in the shell than (PDDA/fMCNT)1 self-assembled SiO2 particles, the ER property of (PDDA/fMCNT)3 multilayer was better than that of (PDDA/fMCNT)1 multilayer. It was believed that the amount of fMCNTs in the shell of SiO2 particles was a critical factor to control the ER properties of multilayer self-assembled SiO2 particles. In addition, it was

suggested that the ER properties of particles could be controlled by changing the mount of fMCNTs with varying the number of PDDA/fMCNT self-assembled layers.