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polyaniline nanocomposites and their electrorheological characteristics under an applied electric field
Current Applied Physics 7 (2007) 352–355 www.elsevier.com/locate/cap www.kps.or.kr
Carbon nanotube/polyaniline nanocomposites and their electrorheological characteristics under an applied electric field C.S. Choi, S.J. Park, H.J. Choi
*
Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Received 10 May 2006; accepted 14 September 2006 Available online 30 October 2006
Abstract A conducting polyaniline (PANI) was synthesized via an oxidative dispersion polymerization technique, using poly(vinyl alcohol) (PVA) as a polymeric stabilizer, in the presence of multi-walled carbon nanotubes (MWNT) purified in acidic solution, and dispersion stability of the MWNT in an aqueous solution of PVA was studied for different PVA concentrations. Their morphology was confirmed by a scanning electron microscope. Its electrorheological (ER) characteristics were also investigated by dispersing the PANI/MWNT composite particles in an insulating silicone oil. Its ER properties were examined using a rotational rheometer under varying applied DC electric field strengths, in which the ER fluid is generally composed of a suspension of conducting particles dispersed in an insulating fluid, which shows a rapid and reversible change in shear viscosity with an applied electric field. Synthesized PANI/MWNT composite particles are observed to enhance interparticular interactions, since the degree of polarization of PANI/MWNT composite particle increases with applied electric field strengths. The shear stresses of the PANI/MWNT nanocomposite based ER fluid increase with the electric field strength for a broad range of shear rates. 2006 Elsevier B.V. All rights reserved. PACS: 73.61.Ph; 47.65.Gx; 83.80.Gv; 83.80.Hj Keywords: Carbon nanotube; Polyaniline; Dispersion; Electrorheological fluid
1. Introduction Carbon nanotubes (CNTs)/polymer nanocomposites have been widely applied for various industrial applications due to their interesting characteristics of good thermal property, size stability, and properties of lightweight, electrical conductivity, mechanical strength and electromagnetic interference (EMI) shielding [1,2]. However, it has been reported that the improvement of the mechanical properties of CNTs/polymer composites is limited owing to the phase separation between the polymer matrix and CNTs. CNTs are regarded to be very difficult to be uniformly dispersed in a polymeric matrix, since they have very large surface areas and strong van der Waals forces, *
Corresponding author. Tel.: +82 32 860 8777; fax: +82 32 865 5178. E-mail address: [email protected] (H.J. Choi).
1567-1739/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2006.09.007
which can lead to the formation of strongly bound aggregates [3]. Therefore, uniform distribution of CNTs throughout the polymer matrix is critical in producing materials with excellent reproducible and optical properties [4–6]. Diverse chemical methods have been proposed for both single-walled carbon nanotubes and multiwalled carbon nanotubes (MWNTs) [7,8]. Concurrently, the PANI is known to be one of the most technologically important materials because of its environmental stability in a conducting form, easiness and low cost of synthesis. The PANI has also been adopted as an electrorheological (ER) material [9,10]. Electrorheological (ER) fluid, typically a suspension of semiconducting or dielectric solid particles in electrically non-conducting liquid media, exhibits rapid and reversible change in shear viscosity under the imposed electric field. This phenomenon originates from the aggregation of the solid
C.S. Choi et al. / Current Applied Physics 7 (2007) 352–355
particles due to attractive forces, induced by the external field, among the dipolar moments. These structures increase both the apparent viscosity of the fluid and the viscoelasticity of the solidified ER fluids [11]. MWNT and PANI nanocomposites were prepared from an oxidative dispersion polymerization of aniline in the presence of MWNT. A surfactant PVA with a hydrophilic OH group was used for a good dispersion of MWNT in water. MWNTs in the solution were dispersed physically under ultrasonication. After preparation of polymer composite, we examined both their dispersion stability and ER properties.
2. Experimental part The MWNT (Iljin Nanotec Co., Korea) with length of 10–50 lm and diameter of 10–20 nm, synthesized by a thermal chemical vapor deposition method, was purified prior to use. Purity of the pristine MWNT used in our study was 97%. It was treated in 3 M HNO3, followed by a reflux process in 5 M HCl at 120 C for 6 h with stirring [12]. After this acidic treatment, MWNT was washed via deionized water. Purity of the as-treated MWNT was observed to be increased to be 99%. Then the acid-treated MWNT (0.1 wt% for aniline), aniline monomer, and dopant HCl were added in PVA solution. Since MWNTs are difficult to be dispersed uniformly, we used a surfactant of PVA with hydrophilic and hydrophobic groups to disperse hydrophobic MWNT into water. PVA was initially dissolved in deionized water. Ultrasonication and stirring via ultrasonic generator were applied to break up MWNT aggregates for 5 h at 25 C. The deionized water containing ammonium peroxydisulfate was added later, with continuous stirring and ultrasonication for 24 h at 20 C. After polymerization, the MWNT/PANI composites were washed by deionized water and separated three times by centrifugation at 7000 rpm for 0.5 h. The conductivity of the MWNT/PANI composite (1 wt% PVA) measured by two-probe method was 2.8 · 106 S/cm which was an appropriate conductivity value for our ER application. Furthermore, in order to examine the dispersion stability of MWNT, we put the MWNT in water with an aniline monomer and HCl for different PVA concentration. The concentrations of PVA in deionized water were 0.5, 1, 5 and 10 wt%. Initially, the PVA was dissolved in water at
353
90 C and MWNTs were added in the solution under ultrasonication for 5 h for dispersion. An ER fluid was prepared by suspending MWNT/PANI particles (10 wt%) in an insulating silicone oil, and its rheological properties were examined for several electric field strength. The flow curves of MWNT/PANI nanocomposites based ER fluid were measured by a rotational rheometer (Physica, MCR 300, Germany) equipped with a high DC voltage generator. 3. Results and discussion For the dispersion stability test, MWNTs were dispersed by a surfactant of PVA in the solution because a hydrophobic part of the PVA contacted with MWNTs and aniline monomer and a hydrophilic part contacted with water. The 0.5 and 1 wt% PVA dispersed MWNTs finely in each solution. On the other hand, the sedimentation of 5 and 10 wt% PVA occurred due to interconnection of the MWNTs. The sedimentation of MWNTs is believed to happen as the increased viscosity of a higher PVA concentration interrupted its dispersion. Two months later, degree of dispersion and sedimentation of MWNTs in PVA solution were observed as shown in Fig. 1. Sedimentation of the MWNTs in 0.5 wt% PVA solution occurred at bottom of the vial due to the insufficient amount of PVA for suspending MWNTs in water. However, 1 wt% of PVA was found to stabilize the MWNT dispersion. In addition, the effect of PVA as a surfactant was also confirmed using toluene/water solution with separated two regions. The 1 wt% PVA solution dispersed MWNTs was dropped in the vial containing toluene/water solution. The drops of PVA solution with MWNT passed through toluene region and then were dispersed well spontaneously with black color in water region. Even after the vial was shaken strongly, the solution was clearly separated into toluene region, and water region with dispersed MWNTs by the PVA. Fig. 2 represents the size distributions of dispersed MWNTs in each solution which were measured by light scattering and presented using conversion distribution. The size distributions of 0.5 and 1 wt% PVA solution are narrower than those of 5 and 10 wt% PVA solutions. Although semitransparent PVA solution (5 wt%) has a similar size distribution as that of 0.5 and 1 wt%, there exists another CNT clusters of larger size (above 1000 nm) in 5 wt% solution (bimodal distribution). Size distribution
Fig. 1. Visualization of MWNT in the solution (PVA concentrations of 0.5, 1, 5 and 10 wt%) after two months.
354
C.S. Choi et al. / Current Applied Physics 7 (2007) 352–355
of PVA (10 wt%) solution as a condition for MWNT/ PANI composite precipitated mostly at the bottom. Thereby, 1 wt% of PVA was found to stabilize the MWNT dispersion relatively well. The rate balance between PANI polymerization and adsorption of stabilizer PVA might determine the shape and size of PANI nanocomposites during the synthesis. Fig. 3 represents the TEM image of MWNT/PANI composites. PANI seems to be pierced with MWNT like a pearl necklace structure. As for the ER characteristics of the MWNT/PANI particles (1 wt% PVA) in insulating silicone oil, the flow curves under four different DC electric field strengths are given in Fig. 4. DC electric field was maintained for 3 min at 25 C
Shear Stress [pa]
Fig. 2. Size distribution of the dispersed MWNTs by light scattering method for four PVA concentrations.
2
10
3kv/mm 2kv/mm 1kv/mm 1
10 -1 10
0
10
1
10
2
10
Shear Rate [1/s] Fig. 4. Fit of model equations to flow curves for MWNT (0.1 wt%)/PANI suspension in silicone oil. Dashed line is for Bingham model Eq. (1) and bold solid line for our suggested model Eq. (2).
Fig. 3. TEM Image of the MWNT/PANI nanocomposite.
for an equilibrium columnar structure prior to the rheological measurement. The ER effect can be characterized through its yield stress with a controlled shear rate mode which in general measures a dynamic yield stress. In the absence of an electric field, the ER fluid showed a shearthinning behavior. In a low shear rate, the shear stress remained almost constant (plateau region) as the shear rate increased up to a certain value. The plateau region shows Bingham fluid behavior with a non-vanishing yield stress under an applied electric field. The Bingham fluid equation was described below [13].
C.S. Choi et al. / Current Applied Physics 7 (2007) 352–355
s ¼ s0 þ g0 c_ s P s0 c_ ¼ 0 s < s0
ð1Þ
Here s is the shear stress and sy is the yield stress which is a function of an applied electric field strength. c_ is the shear rate and g is the shear viscosity. The MWNT/PANI particles show an enhancement of interparticle interactions when an electric field is applied. As we increase the electric field strength, the yield stress of ER fluid gets higher. After the plateau region, shear stress increases with shear rate because the hydrodynamic force dominates the electrostatic force. MWNT/PANI particle chains begin to break up due to the hydrodynamic shear and the particles do not have a sufficient time to realign along the electric field direction. Fig. 4 shows the shear stress as a function of a shear rate for the ER fluid at four different electric field strengths. In order to describe complicated flow curve of such ER systems, recently we proposed the following constitutive equation of state for the ER fluids under an applied electric field [14]: ! 1 1 s ¼ s0 þ g1 1 þ c_ ð2Þ a 1 þ ðt1 c_ Þ ðt2 c_ Þb Here a is related to the decline in the shear stress at a low shear rate region, and the exponent b ranges from 0 < b 6 1, because ds=d_c P 0 above the critical shear rate at which the shear stress becomes a minimum. t1 and t2 are time constants, in which t1 is considered to the inverse of the shear rate at which the shear stress shows a minimum at a low shear rate region and t2 is related to the inverse of the shear rate at which a pseudo-Newtonian behavior starts. 1 is the viscosity at an infinite shear rate. The first term of the right-hand side in Eq. (2) described the decline of shear stress behavior at a low shear rate region while the second term explains the shear stress at a high shear rate region. In a low c_ region, the electrostatic interactions among particles induced by external electric fields are dominant compared to the hydrodynamic interactions induced by the external flow field. The aligned particular structures begin to break with shear deformation, and the broken structures tend to reform the chains by the applied electric field, depending on the magnitude of the applied shear and particle–particle interaction in the fibrils. The decrease in shear stress is observed when the destruction rate of the fibrils becomes faster than the reformation rate as the c_ increases.
355
In conclusion, MWNT/PANI particles were prepared by an oxidative dispersion polymerization using PVA as a polymeric stabilizer [15]. Since MWNT is very difficult to be dispersed in water, a surfactant PVA (1 wt%) was used, based on the dispersion stability test of the MWNT. The PVA placed on an interface between MWNT and water enabled higher affinity and better dispersion. The synthesized MWNT/PMMA particles exhibited typical ER responses, i.e., shear stress increase with an increase in applied DC electric field strength. In addition, our proposed model was capable of fitting the flow curves more accurately than Bingham fluid model, and successfully explained shear stress by describing the decline of shear stress at low shear rate of ER fluid. Acknowledgement This work was supported by research grants from the Korea Science and Engineering Foundation through the Applied Rheology Center (ARC) at Korea University, Seoul, Korea. References [1] W. Feng, X.D. Bai, Y.Q. Lian, J. Liang, X.G. Wang, K. Yoshino, Carbon 41 (2003) 1551. [2] W. Liang, Nature 411 (2001) 665. [3] H.J. Choi, S.J. Park, S.T. Kim, M.S. Jhon, Diam. Relat. Mater. 14 (2005) 766. [4] J. Sandler, M.S.P. Shaffer, T. Prasse, W. Bautjofer, K. Schulte, A.H. Windle, Polymer 40 (1999) 5967. [5] S.D. Oh, S.H. Choi, B.Y. Lee, A. Gopalan, K.P. Lee, S.H. Kim, J. Ind. Eng. Chem. 12 (2006) 156. [6] R. Bandyopadhyaya, E. Nativ-Roth, O. Regev, R. YerushalmiRozen, Nano Lett. 2 (2002) 25. [7] H.J. Jin, H.J. Choi, S.H. Yoon, S.J. Myung, S.E. Shim, Chem. Mater. 17 (2005) 4034. [8] S.J. Park, M.S. Cho, S.T. Lim, H.J. Choi, M.S. Jhon, Macromol. Rapid Commun. 24 (2003) 1070. [9] H.J. Choi, M.S. Cho, J.W. Kim, C.A. Kim, M.S. Jhon, Appl. Phys. Lett. 78 (2001) 3806. [10] M.S. Cho, Y.H. Cho, H.J. Choi, M.S. Jhon, Langmuir 19 (2003) 5875. [11] X.P. Zhao, J.B. Yin, J. Ind. Eng. Chem. 12 (2006) 184. [12] M.S. Cho, H.J. Choi, K.Y. Kim, W.S. Ahn, Macromol. Rapid Commun. 23 (2002) 713. [13] M.S. Cho, H.J. Choi, M.S. Jhon, Polymer 46 (2005) 11484. [14] H. See, A. Kawai, F. Ikazaki, Colloid Polym. Sci. 280 (2002) 24. [15] S.J. Park, S.Y. Park, M.S. Cho, H.J. Choi, M.S. Jhon, Synth. Met. 152 (2005) 337.
Carbon nanotube/polyaniline nanocomposites and their electrorheological characteristics under an applied electric field C.S. Choi, S.J. Park, H.J. Choi
*
Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Received 10 May 2006; accepted 14 September 2006 Available online 30 October 2006
Abstract A conducting polyaniline (PANI) was synthesized via an oxidative dispersion polymerization technique, using poly(vinyl alcohol) (PVA) as a polymeric stabilizer, in the presence of multi-walled carbon nanotubes (MWNT) purified in acidic solution, and dispersion stability of the MWNT in an aqueous solution of PVA was studied for different PVA concentrations. Their morphology was confirmed by a scanning electron microscope. Its electrorheological (ER) characteristics were also investigated by dispersing the PANI/MWNT composite particles in an insulating silicone oil. Its ER properties were examined using a rotational rheometer under varying applied DC electric field strengths, in which the ER fluid is generally composed of a suspension of conducting particles dispersed in an insulating fluid, which shows a rapid and reversible change in shear viscosity with an applied electric field. Synthesized PANI/MWNT composite particles are observed to enhance interparticular interactions, since the degree of polarization of PANI/MWNT composite particle increases with applied electric field strengths. The shear stresses of the PANI/MWNT nanocomposite based ER fluid increase with the electric field strength for a broad range of shear rates. 2006 Elsevier B.V. All rights reserved. PACS: 73.61.Ph; 47.65.Gx; 83.80.Gv; 83.80.Hj Keywords: Carbon nanotube; Polyaniline; Dispersion; Electrorheological fluid
1. Introduction Carbon nanotubes (CNTs)/polymer nanocomposites have been widely applied for various industrial applications due to their interesting characteristics of good thermal property, size stability, and properties of lightweight, electrical conductivity, mechanical strength and electromagnetic interference (EMI) shielding [1,2]. However, it has been reported that the improvement of the mechanical properties of CNTs/polymer composites is limited owing to the phase separation between the polymer matrix and CNTs. CNTs are regarded to be very difficult to be uniformly dispersed in a polymeric matrix, since they have very large surface areas and strong van der Waals forces, *
Corresponding author. Tel.: +82 32 860 8777; fax: +82 32 865 5178. E-mail address: [email protected] (H.J. Choi).
1567-1739/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2006.09.007
which can lead to the formation of strongly bound aggregates [3]. Therefore, uniform distribution of CNTs throughout the polymer matrix is critical in producing materials with excellent reproducible and optical properties [4–6]. Diverse chemical methods have been proposed for both single-walled carbon nanotubes and multiwalled carbon nanotubes (MWNTs) [7,8]. Concurrently, the PANI is known to be one of the most technologically important materials because of its environmental stability in a conducting form, easiness and low cost of synthesis. The PANI has also been adopted as an electrorheological (ER) material [9,10]. Electrorheological (ER) fluid, typically a suspension of semiconducting or dielectric solid particles in electrically non-conducting liquid media, exhibits rapid and reversible change in shear viscosity under the imposed electric field. This phenomenon originates from the aggregation of the solid
C.S. Choi et al. / Current Applied Physics 7 (2007) 352–355
particles due to attractive forces, induced by the external field, among the dipolar moments. These structures increase both the apparent viscosity of the fluid and the viscoelasticity of the solidified ER fluids [11]. MWNT and PANI nanocomposites were prepared from an oxidative dispersion polymerization of aniline in the presence of MWNT. A surfactant PVA with a hydrophilic OH group was used for a good dispersion of MWNT in water. MWNTs in the solution were dispersed physically under ultrasonication. After preparation of polymer composite, we examined both their dispersion stability and ER properties.
2. Experimental part The MWNT (Iljin Nanotec Co., Korea) with length of 10–50 lm and diameter of 10–20 nm, synthesized by a thermal chemical vapor deposition method, was purified prior to use. Purity of the pristine MWNT used in our study was 97%. It was treated in 3 M HNO3, followed by a reflux process in 5 M HCl at 120 C for 6 h with stirring [12]. After this acidic treatment, MWNT was washed via deionized water. Purity of the as-treated MWNT was observed to be increased to be 99%. Then the acid-treated MWNT (0.1 wt% for aniline), aniline monomer, and dopant HCl were added in PVA solution. Since MWNTs are difficult to be dispersed uniformly, we used a surfactant of PVA with hydrophilic and hydrophobic groups to disperse hydrophobic MWNT into water. PVA was initially dissolved in deionized water. Ultrasonication and stirring via ultrasonic generator were applied to break up MWNT aggregates for 5 h at 25 C. The deionized water containing ammonium peroxydisulfate was added later, with continuous stirring and ultrasonication for 24 h at 20 C. After polymerization, the MWNT/PANI composites were washed by deionized water and separated three times by centrifugation at 7000 rpm for 0.5 h. The conductivity of the MWNT/PANI composite (1 wt% PVA) measured by two-probe method was 2.8 · 106 S/cm which was an appropriate conductivity value for our ER application. Furthermore, in order to examine the dispersion stability of MWNT, we put the MWNT in water with an aniline monomer and HCl for different PVA concentration. The concentrations of PVA in deionized water were 0.5, 1, 5 and 10 wt%. Initially, the PVA was dissolved in water at
353
90 C and MWNTs were added in the solution under ultrasonication for 5 h for dispersion. An ER fluid was prepared by suspending MWNT/PANI particles (10 wt%) in an insulating silicone oil, and its rheological properties were examined for several electric field strength. The flow curves of MWNT/PANI nanocomposites based ER fluid were measured by a rotational rheometer (Physica, MCR 300, Germany) equipped with a high DC voltage generator. 3. Results and discussion For the dispersion stability test, MWNTs were dispersed by a surfactant of PVA in the solution because a hydrophobic part of the PVA contacted with MWNTs and aniline monomer and a hydrophilic part contacted with water. The 0.5 and 1 wt% PVA dispersed MWNTs finely in each solution. On the other hand, the sedimentation of 5 and 10 wt% PVA occurred due to interconnection of the MWNTs. The sedimentation of MWNTs is believed to happen as the increased viscosity of a higher PVA concentration interrupted its dispersion. Two months later, degree of dispersion and sedimentation of MWNTs in PVA solution were observed as shown in Fig. 1. Sedimentation of the MWNTs in 0.5 wt% PVA solution occurred at bottom of the vial due to the insufficient amount of PVA for suspending MWNTs in water. However, 1 wt% of PVA was found to stabilize the MWNT dispersion. In addition, the effect of PVA as a surfactant was also confirmed using toluene/water solution with separated two regions. The 1 wt% PVA solution dispersed MWNTs was dropped in the vial containing toluene/water solution. The drops of PVA solution with MWNT passed through toluene region and then were dispersed well spontaneously with black color in water region. Even after the vial was shaken strongly, the solution was clearly separated into toluene region, and water region with dispersed MWNTs by the PVA. Fig. 2 represents the size distributions of dispersed MWNTs in each solution which were measured by light scattering and presented using conversion distribution. The size distributions of 0.5 and 1 wt% PVA solution are narrower than those of 5 and 10 wt% PVA solutions. Although semitransparent PVA solution (5 wt%) has a similar size distribution as that of 0.5 and 1 wt%, there exists another CNT clusters of larger size (above 1000 nm) in 5 wt% solution (bimodal distribution). Size distribution
Fig. 1. Visualization of MWNT in the solution (PVA concentrations of 0.5, 1, 5 and 10 wt%) after two months.
354
C.S. Choi et al. / Current Applied Physics 7 (2007) 352–355
of PVA (10 wt%) solution as a condition for MWNT/ PANI composite precipitated mostly at the bottom. Thereby, 1 wt% of PVA was found to stabilize the MWNT dispersion relatively well. The rate balance between PANI polymerization and adsorption of stabilizer PVA might determine the shape and size of PANI nanocomposites during the synthesis. Fig. 3 represents the TEM image of MWNT/PANI composites. PANI seems to be pierced with MWNT like a pearl necklace structure. As for the ER characteristics of the MWNT/PANI particles (1 wt% PVA) in insulating silicone oil, the flow curves under four different DC electric field strengths are given in Fig. 4. DC electric field was maintained for 3 min at 25 C
Shear Stress [pa]
Fig. 2. Size distribution of the dispersed MWNTs by light scattering method for four PVA concentrations.
2
10
3kv/mm 2kv/mm 1kv/mm 1
10 -1 10
0
10
1
10
2
10
Shear Rate [1/s] Fig. 4. Fit of model equations to flow curves for MWNT (0.1 wt%)/PANI suspension in silicone oil. Dashed line is for Bingham model Eq. (1) and bold solid line for our suggested model Eq. (2).
Fig. 3. TEM Image of the MWNT/PANI nanocomposite.
for an equilibrium columnar structure prior to the rheological measurement. The ER effect can be characterized through its yield stress with a controlled shear rate mode which in general measures a dynamic yield stress. In the absence of an electric field, the ER fluid showed a shearthinning behavior. In a low shear rate, the shear stress remained almost constant (plateau region) as the shear rate increased up to a certain value. The plateau region shows Bingham fluid behavior with a non-vanishing yield stress under an applied electric field. The Bingham fluid equation was described below [13].
C.S. Choi et al. / Current Applied Physics 7 (2007) 352–355
s ¼ s0 þ g0 c_ s P s0 c_ ¼ 0 s < s0
ð1Þ
Here s is the shear stress and sy is the yield stress which is a function of an applied electric field strength. c_ is the shear rate and g is the shear viscosity. The MWNT/PANI particles show an enhancement of interparticle interactions when an electric field is applied. As we increase the electric field strength, the yield stress of ER fluid gets higher. After the plateau region, shear stress increases with shear rate because the hydrodynamic force dominates the electrostatic force. MWNT/PANI particle chains begin to break up due to the hydrodynamic shear and the particles do not have a sufficient time to realign along the electric field direction. Fig. 4 shows the shear stress as a function of a shear rate for the ER fluid at four different electric field strengths. In order to describe complicated flow curve of such ER systems, recently we proposed the following constitutive equation of state for the ER fluids under an applied electric field [14]: ! 1 1 s ¼ s0 þ g1 1 þ c_ ð2Þ a 1 þ ðt1 c_ Þ ðt2 c_ Þb Here a is related to the decline in the shear stress at a low shear rate region, and the exponent b ranges from 0 < b 6 1, because ds=d_c P 0 above the critical shear rate at which the shear stress becomes a minimum. t1 and t2 are time constants, in which t1 is considered to the inverse of the shear rate at which the shear stress shows a minimum at a low shear rate region and t2 is related to the inverse of the shear rate at which a pseudo-Newtonian behavior starts. 1 is the viscosity at an infinite shear rate. The first term of the right-hand side in Eq. (2) described the decline of shear stress behavior at a low shear rate region while the second term explains the shear stress at a high shear rate region. In a low c_ region, the electrostatic interactions among particles induced by external electric fields are dominant compared to the hydrodynamic interactions induced by the external flow field. The aligned particular structures begin to break with shear deformation, and the broken structures tend to reform the chains by the applied electric field, depending on the magnitude of the applied shear and particle–particle interaction in the fibrils. The decrease in shear stress is observed when the destruction rate of the fibrils becomes faster than the reformation rate as the c_ increases.
355
In conclusion, MWNT/PANI particles were prepared by an oxidative dispersion polymerization using PVA as a polymeric stabilizer [15]. Since MWNT is very difficult to be dispersed in water, a surfactant PVA (1 wt%) was used, based on the dispersion stability test of the MWNT. The PVA placed on an interface between MWNT and water enabled higher affinity and better dispersion. The synthesized MWNT/PMMA particles exhibited typical ER responses, i.e., shear stress increase with an increase in applied DC electric field strength. In addition, our proposed model was capable of fitting the flow curves more accurately than Bingham fluid model, and successfully explained shear stress by describing the decline of shear stress at low shear rate of ER fluid. Acknowledgement This work was supported by research grants from the Korea Science and Engineering Foundation through the Applied Rheology Center (ARC) at Korea University, Seoul, Korea. References [1] W. Feng, X.D. Bai, Y.Q. Lian, J. Liang, X.G. Wang, K. Yoshino, Carbon 41 (2003) 1551. [2] W. Liang, Nature 411 (2001) 665. [3] H.J. Choi, S.J. Park, S.T. Kim, M.S. Jhon, Diam. Relat. Mater. 14 (2005) 766. [4] J. Sandler, M.S.P. Shaffer, T. Prasse, W. Bautjofer, K. Schulte, A.H. Windle, Polymer 40 (1999) 5967. [5] S.D. Oh, S.H. Choi, B.Y. Lee, A. Gopalan, K.P. Lee, S.H. Kim, J. Ind. Eng. Chem. 12 (2006) 156. [6] R. Bandyopadhyaya, E. Nativ-Roth, O. Regev, R. YerushalmiRozen, Nano Lett. 2 (2002) 25. [7] H.J. Jin, H.J. Choi, S.H. Yoon, S.J. Myung, S.E. Shim, Chem. Mater. 17 (2005) 4034. [8] S.J. Park, M.S. Cho, S.T. Lim, H.J. Choi, M.S. Jhon, Macromol. Rapid Commun. 24 (2003) 1070. [9] H.J. Choi, M.S. Cho, J.W. Kim, C.A. Kim, M.S. Jhon, Appl. Phys. Lett. 78 (2001) 3806. [10] M.S. Cho, Y.H. Cho, H.J. Choi, M.S. Jhon, Langmuir 19 (2003) 5875. [11] X.P. Zhao, J.B. Yin, J. Ind. Eng. Chem. 12 (2006) 184. [12] M.S. Cho, H.J. Choi, K.Y. Kim, W.S. Ahn, Macromol. Rapid Commun. 23 (2002) 713. [13] M.S. Cho, H.J. Choi, M.S. Jhon, Polymer 46 (2005) 11484. [14] H. See, A. Kawai, F. Ikazaki, Colloid Polym. Sci. 280 (2002) 24. [15] S.J. Park, S.Y. Park, M.S. Cho, H.J. Choi, M.S. Jhon, Synth. Met. 152 (2005) 337.