pubs.acs.org/Langmuir © 2010 American Chemical Society
Core-Shell Structured Semiconducting PMMA/Polyaniline Snowman-like Anisotropic Microparticles and Their Electrorheology Ying Dan Liu, Fei Fei Fang, and Hyoung Jin Choi* Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea Received March 24, 2010. Revised Manuscript Received June 19, 2010 Core-shell structured semiconducting snowman-like particles were synthesized, and their electrorheological (ER) characteristics under an applied electric field were examined. Monodispersed snowman-like poly(methyl methacrylate) (PMMA) particles were fabricated previously using a seed emulsion polymerization procedure. These anisotropic particle-based ER fluids, which were tested using a rotational rheometer, exhibited unusual ER properties in the flow curves at various electric field strengths when analyzed using the Cho-Choi-Jhon model. The dielectric spectra, as supporting data for the ER effect, were measured using a LCR meter. The relaxation time of the ER fluid was relatively shorter than typical ER fluids.
Introduction Colloid particles with shape and chemical composition anisotropy have been investigated extensively for their special properties and different applications from isotropic particles. Among them, nonspherical particles, which may be dumbbell-like, snowman-like, raspberry-like, acorn-like, ocellatus-like, and hemispherical, have been introduced1-3 to induce unique properties in optics,4,5 dielectrophoresis,6 and suspension rheology.7,8 However, colloid particles are typically prepared in a spherical shape due to the minimization of interfacial tension energy.1 Therefore, further investigations, including a range of methods and mechanisms for nonspherical particles, have been examined. Both seeded emulsion9 and dispersion polymerization (or dynamic swelling)10 have been suggested for the synthesis of monodispersed snowmanlike particles. Kim et al.11 also introduced a new route for surface functional snowman-like particles that were useful for molecular reorganization and self-assembly processes. Analyses in terms of thermodynamics and kinetics showed that an elastic effect with increasing temperature was the key factor for phase separation of swollen polymer seeds. On the other hand, cross-linking in the seed particles also plays an important role in controlling the morphology of particles. *Corresponding author. E-mail:
[email protected]. (1) Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E.; Duguet, E. J. Mater. Chem. 2005, 15, 3745. (2) Perro, A.; Reculusa, S.; Bourgeat-Lami, E.; Duguet, E.; Ravaine, S. Colloids Surf., A 2006, 284-285, 78. (3) Tanaka, T.; Komatsu, Y.; Fujibayashi, T.; Minami, H.; Okubo, M. Langmuir 2010, 26, 3848. (4) Lu, Y.; Yin, Y.; Xia, Y. Adv. Mater. 2001, 13, 415. (5) Hosein, I. D.; Ghebrebrhan, M.; Joannopoulos, J. D.; Liddell, C. M. Langmuir 2010, 26, 2151. (6) Winter, W. T.; Welland, M. E. J. Phys. D: Appl. Phys. 2009, 42, 045501. (7) Wolf, B.; Frith, W. J.; Singleton, S.; Tassieri, M.; Norton, I. T. Rheol. Acta 2001, 40, 238. (8) Raiskinm€aki, P.; Shakib-Manesh, A.; Koponen, A.; J€asberg, A.; Kataja, M.; Timonen, J. Comput. Phys. Commun. 2000, 129, 185. (9) Mock, E. B.; Bruyn, H. De; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Langmuir 2006, 22, 4037. (10) Okubo, M.; Fujibayashi, T.; Yamada, M.; Minami, H. Colloid Polym. Sci. 2005, 283, 1041. (11) Kim, J. W.; Larsen, R. J.; Weitz, D. A. J. Am. Chem. Soc. 2006, 128, 14374. (12) (a) Kanu, R. C.; Shaw, M. T. J. Rheol. 1998, 42, 657. (b) Bao, W.; Wu, F.; Cao, G.; Zhou, W.; Gao, Y.; Huang, P.; Yang, J.; Ren, N.; Tang, Y. J. Phys.: Conf. Ser. 2009, 149, 012001. (c) Parmar, K. P. S.; Meheust, Y.; Schjelderupsen, B.; Fossum, J. O. Langmuir 2008, 24, 1814. (d) Yin, J.; Zhao, X.; Xia, X.; Xiang, L.; Qiao, Y. Polymer 2008, 49, 4413.
Langmuir 2010, 26(15), 12849–12854
Anisotropic dielectric particles, such as ellipsoidal or fibrous particles, have been studied as electrorheological (ER) materials with enhanced dielectric properties and an increased ER effect.12 Therefore, this study focused on synthesizing the anisotropic (snowman-like) particles and applying them to an ER fluid to determine their ER properties in an external electric field. An ER fluid is such a smart material that can be changed from a liquid-like state to solid-like state by applying an electric field. Generally, it is composed of semiconducting (or polarizable) particles dispersed in an insulated liquid. These particles can be polarized and tend to form chain or column-like structures (fibrillation) by connecting with each other in the direction of the external electric field. These structural changes in an ER fluid increase the shear viscosity,13 which is rapid, reversible, and controllable. This property gives ER fluids many potential application in mechanical and intelligent devices, involving automotive dampers and clutches,14 break and actuator devices in orthotic devices,15 haptic devices for telesurgery,16 and recent uses in microfluidics.17 Core-shell structured colloidal particles with an ultrathin conducting overlayer are alternatives for conducting polymers, which combine the morphological effect of core particles with the electrical properties of conducting layers. A thin conducting layer reduces the cost of conducting polymers and enhances the transparency of the particles, which is favorable for the applications of the photographic industry.18 In addition, core-shell (13) (a) Shen, R.; Wang, X.; Lu, Y.; Wang, D.; Sun, G.; Cao, Z.; Lu, K. Adv. Mater. 2009, 21, 4631. (b) Yin, J.; Zhao, X.; Xiang, L.; Xia, X.; Zhang, Z. Soft Matter 2009, 5, 4687. (c) Yin, J.; Zhao, X. Nanotechnology 2006, 17, 192. (d) Zhang, W. L.; Park, B. J.; Choi, H. J. Chem. Commun. DOI: 10.1039/C0CC00557F. (14) (a) Khanicheh, A.; Mintzopoulos, D.; Weinberg, B.; Tzika, A. A.; Mavroidis, C. IEEE-ASME Trans. Mechatron. 2008, 13, 286. (b) Coulter, J. P.; Weiss, K. D.; Carlson, J. D. J. Intell. Mater. Syst. Struct. 1993, 4, 248. (c) Chung, S. K.; Shin, H. B. IEEE Trans. Veh. Technol. 2004, 53, 206. (d) Sims, N. D.; Peel, D. J.; Stanway, R.; Johnson, A. R.; Bullough, W. A. J. Sound Vibr. 2000, 229, 207. (15) (a) Choi, S. B.; Lee, D. Y. Proc. Inst. Mech. Eng., Part C 2005, 219, 627. (b) Tan, K. P.; Bullough, W. A.; Stanway, R.; Sims, N.; Johnson, A. R.; Tozer, R. C. J. Intell. Mater. Syst. Struct. 2002, 13, 533. (16) (a) Nikitczuk, J.; Weinberg, B.; Mavroidis, C. Smart Mater. Struct. 2007, 16, 418. (b) Mavroidis, C.; Pfeiffer, C.; Lennon, J.; Paljic, A.; Celestino, J.; Bar-Cohen, Y. Robotics 2000, 2000, 174. (17) (a) Niu, X.; Liu, L.; Wen, W.; Sheng, P. J. Intell. Mater. Syst. Struct. 2007, 18, 1187. (b) Zhang, M.; Gong, X.; Wen, W. Electrophoresis 2009, 30, 3116. (18) Huijs, F. M.; Vercauteren, F. F.; Ruiter, B. de; Kalicharan, D.; Hadziioannou, G. Synth. Met. 1999, 102, 1151.
Published on Web 07/01/2010
DOI: 10.1021/la101165k
12849
Article
Liu et al.
particles are a type of interesting design for ER materials, expecting better ER performance.19 The conducting polymers used for this purpose include polypyrrole, polyaniline (PANI), poly(3,4-ethylenedioxythiophene), etc.20,21 To the best of our knowledge, latex core particles with various materials, such as polystyrene, poly(methyl methacrylate) (PMMA), and poly(butyl methacrylate), are always spherical22,23 in a general study of core-shell particles. This paper reports a simple way of preparing conducting PANI-coated anisotropic snowman-like particles using a twostep process. In the first step, cross-linked PMMA particles were synthesized in two steps using a dispersion polymerization method instead of the most frequently reported seed polymerization method.24,25 Seed emulsion polymerization was used to prepare snowman-like particles. In the second step, these particles were applied as cores for the coating by PANI to prepare conducting snowman-like core-shell particles. The products obtained are electroresponsive and can be applied as dispersed particles in an ER fluid. The fibrillation phenomenon and other ER performances (flow curves and dielectric spectra) of this new particle-based ER fluid are also reported.
Experimental Section Materials. Methyl methacrylate (MMA) was used after purification. The initiator R,R0 -azobisisobutyronitrile (AIBN) was recrystallized in ethanol prior to use. The other materials, including allyl methacrylate (AMA, 98%), poly(vinylpyridine) (PVP 55 000), poly(vinyl alcohol) (PVA 1700), ethylene glycol dimethacrylate (EGDMA, 98%), aniline, ammonium persulfate (APS), sodium dodecyl sulfate (SDS), HCl (35%), and NaOH, were used as received. Ethanol, methanol, and diwater were used in all experiments. Synthesis of Cross-Linked PMMA Seeds. PVP (28 g), as a stabilizer, was dispersed in methanol in a double-layered glass reactor. A monomer solution (104.8 g of MMA, 0.7 g of AMA, 1.12 g of AIBN) was added when the temperature of the system was heated to 60 C through a circulator. The reaction was kept at 60 C for 12 h. The seeds were washed with methanol and diwater several times and dried in an oven at 60 C for 24 h. Synthesis of Snowman-like Particles. The PMMA seeds (11.09 g) obtained were dispersed in a PVA aqueous solution. The monomer solution, including 99.81 g of MMA, 12.32 g of EGDMA, and 1.12 g of initiator, was added to the seed emulsion with rapid mechanical stirring. After swelling for 12 h at room temperature, the system was heated to 75 C and held at that temperature for 24 h. Snowman-like particles were obtained by centrifuging and washed with methanol several times. Conducting Coating by PANI to Snowman-like Particles (SPMPANI). The synthesized snowman-like particles were dispersed in the SDS containing aqueous solution. Mild stirring was maintained to allow adsorption of the SDS to the particle surface. Aniline (1:5 in mass to snowman-like particle) was then added, and the pH was adjusted to 0.7 with 1 M HCl. After transferring the mixture to a 500 mL reactor, polymerization was started by dropping an APS solution (1.25:1 in mole to aniline) in HCl at 0 C. The reaction was held at that temperature for 24 h, and the final products were dedoped with a 1 M NaOH solution to (19) (a) Gonon, P.; Foulc, J. N.; Atten, P.; Boissy, C. J. Appl. Phys. 1999, 86, 7160. (b) Cho, M. S.; Choi, H. J.; Kim, K. Y.; Ahn, W. S. Macromol. Rapid Commun. 2002, 23, 713. (20) Khan, M. A.; Armes, S. P. Adv. Mater. 2000, 12, 671. (21) Lee, J. M.; Lee, D. G.; Lee, S. J.; Kim, J. H.; Cheong, I. W. Macromolecules 2009, 42, 4511. (22) Schartl, W. Adv. Mater. 2000, 12, 1899. (23) Choi, H. J.; Cho, M. S.; Park, S. Y.; Cho, C. H.; Jhon, M. S. Des. Monomers Polym. 2004, 7, 111. (24) Park, J. G.; Forster, J. D.; Dufresne, E. R. Langmuir 2009, 25, 8903. (25) Kim, J. W.; Suh, K. D. J. Ind. Eng. Chem. 2008, 14, 119.
12850 DOI: 10.1021/la101165k
Figure 1. OM images of swelling process of cross-linked PMMA particles by monomer solution (MMA, EGDMA, and AIBN) in PVA aqueous solution: (a) dispersed PMMA particles; (b) after addition of monomer solution for 30 min; (c) swelling for 5 h; (d) swelling for 12 h. decrease their conductivity for the electrorheological (ER) test and dried for 3 days after washing. Preparation of ER Fluid. The snowman-like particles were dispersed in silicone oil to make good dispersions by mechanical shaking and ultrasonication. The fluid was then stored in a vacuum oven at room temperature for 10 min to eliminate air bubbles from the fluid. ER fluids with two different concentrations, 13 and 18 wt %, were prepared. Characterization. The swelling process of the PMMA particles to form snowman-like particles was observed by optical microscopy (OM) at different periods. Scanning electron microscopy (SEM, S-4300, Hitachi, Japan) with a voltage of 15 kV at a working distance of 15 mm was also performed. The thermal properties were examined using a thermal gravimetric analyzer (TA Instruments, Q50). The ER properties of the snowman-like particle-based ER fluid was observed using a rotational rheometer (MCR 300, Physica) equipped with a high-voltage power supply (HCP 14-12500, fug) and a CC17 geometry (gap size: 0.71 mm). A direct current (dc) electric field was applied before testing for 30 s to ensure that the chainlike structure had achieved equilibrium. The shear stresses of the fluid were measured as the shear rate was increased from 0.01 to 1000 (1/s) under various electric field strengths (0, 0.1, 0.5, 1.0, 1.5, and 2.0 kV/mm). A dielectric study was carried out using a LCR meter (Agilent HP 4284A) over frequency range of 20-106 Hz.
Results and Discussion Snowman-like particles were fabricated by seed emulsion polymerization. However, the final shapes of the particles were formed by a swelling process at room temperature, as confirmed by the OM images at different swelling times in Figure 1. The cross-linked PMMA particles in Figure 1a were used as seeds for the snowman-like particles. Before adding the monomer solution (monomer, initiator, and cross-linker), the seeds were quite small. When the monomer solution was added, small droplets were formed in the emulsion system by stirring at very high speeds, as shown in Figure 1b. On the other hand, as the monomers diffused into the PMMA seed particles, the seeds increased dramatically in size, almost 2 times compared to the original seeds, but they still maintained their spherical shape. After swelling for 5 h (Figure 1c), snowman-like particles appeared, possessing smaller heads and larger bodies. However, which of the two parts concerns the original seeds could not be determined. After 12 h, all the seeds were transformed to snowman-like Langmuir 2010, 26(15), 12849–12854
Liu et al.
Figure 2. SEM images of cross-linked PMMA microseeds (a), snowman-like particles (b), PANI coated snowman-like particles (c), and the high-magnification image (d).
particles with a uniform size and morphology, as shown in Figure 1d. Parts a and b of Figure 2 show SEM images of both PMMA seeds and snowman-like particles, respectively. The PMMA seeds are spherical but with plicate surfaces, just like dried prunes. The seeds were nearly monodisperse and ∼5 μm in diameter. The snowman-like particles were prepared by swelling the PMMA seeds with 9 times the massed MMA monomer, as described in the Experimental Section. Therefore, these particles are much larger than the seeds. Every snowman-like particle has a porous and rough head as well as relatively smooth body that is much larger than the hemispherical shaped head. The appearance of these particles, such as the difference between the head and body of the “snowman” as well as the small bulges in the body, might be caused by the different cross-linkers used during the preparation of PMMA seeds and snowman-like particles. In addition, it could be proof of our presumption regarding the dynamics of swelling. The shapes (snowman) of the particles were formed in the swelling process within the monomer emulsion system, where the seeds were over swollen by the monomers. While relaxation of the crosslinked polymer chains causes the seeds to shrink and expels the monomers from the seeds, nonspherical (or anisotropic) particles were formed. Indeed, the shapes of the particles vary subtly according to internal and external factors, such as the monomer/ polymer ratios and wetting properties, mass of cross-linkers, and temperature.9,10 After coating with a conducting PANI layer, the core-shell SPMPANI particles were prepared, as shown in Figure 2c,d. The morphology of the particles was significantly different from the uncoated particles due to the nonuniform shell of PANI. It appears that PANI grows more randomly after coating in the near surfactant layer, which gives the particle surface a rough and irregular appearance. This may be related to the solubility of aniline in water, even though it is very low. In the high-magnification image, we can confirm the coating of particles by PANI more clearly. Scheme 1 illustrates the entire process from the PMMA seeds to the SPMPANI particles. Snowman-like particles were synthesized by swelling PMMA seeds without a template or shape controlled agent. With the help of SDS, the surfaces of the snowmanlike particles were negatively charged, whereas the anilines were positive in an HCl solution. Therefore, anilines are adsorbed on the particle surface through electrostatic interactions and then polymerized by dropping an ammonium persulfate (APS) solution. Langmuir 2010, 26(15), 12849–12854
Article
Figure 3 shows the TGA data of both snowman-like particles and their PANI-coated product (SPMPANI). The SPMPANI particle appears to show better thermostability up to 280 C, whereas the snowman-like particles begin to degrade at about 240 C. This may be due to thermal protection of the more thermostable PANI shell. In addition, two steps of weight loss in the TGA curve of the SPMPANI were observed. The first step in the temperature range of 280-407 C was due to the degradation of cross-linked PMMA, whereas the degradation of PANI occurred in the second range, 407-600 C. Therefore, the mass fraction of the PANI shell was confirmed to be 14% from the residual weight in the different ranges of the curve. This is lower than the input amount of aniline monomer (∼17%). The reason may be that aniline itself polymerizes separately in the water phase. On the other hand, the SPMPANI particle-based ER fluid in silicone oil exhibited a fibrillation phenomenon under an applied electric field (Figure 4), where the ER fluid was injected between the gap of the two electrodes (∼350 μm) and observed by optical microscopy. The randomly dispersed particles (a) transform to fibrillated chain structures (b) parallel to the electric field immediately after the application of an electric voltage of 300 V on both sides of the aluminum electrodes. Interestingly, the chains were connected particles aligning in their long axis in the form of a head-to-head or head-to-tail rather than random particle clusters.26,27 This orientation will give the largest induced dipole moment in the polarization process under an applied electric field.28 To observe the responses of the ER fluid to various electric field strengths, the flow curves were tested using a rotational rheometer, and the results are shown in Figure 5. ER fluids generally behave as a Newtonian fluid in the absence of an electric field with shear stress increasing linearly with increasing shear rate without yielding. While under an electric field, yield stresses are needed to initiate shear flow, and the shear stresses become stable or increase slightly with increasing shear rates. These phenomena are typical characteristics of Bingham fluid behavior,29 which is a general model for an ER suspension. The constitutive equation for the Bingham fluid model is expressed as follows: _ τ ¼ τy þ ηγ, γ_ ¼ 0,
τ g τy τ < τy
ð1Þ
where γ_ is the shear rate, τ is the shear stress, η is the shear viscosity, and τy is the yield stress, which is related to the electric field strength. However, for many different ER fluids, this simple model cannot be fitted to the experimental data of shear stress well over the entire shear rate region.30-33 ER fluids with a weak ER effect often show a decrease in shear stress after the yield value until a critical shear rate is reached34 and then exhibit an increase in shear stress with increasing shear rate. In other words, the flow (26) Jin, H. J.; Choi, H. J.; Yoon, S. H.; Myung, S. J.; Shim, S. E. Chem. Mater. 2005, 17, 4034. (27) Choi, H. J.; Lim., J. Y.; Zhang, K. Diamond Relat. Mater. 2008, 17, 1498. (28) Blair, M. J.; Patey, G. N. J. Chem. Phys. 1999, 111, 3278. (29) Cheng, Q.; Pavlinek, V.; He, Y.; Li, C.; Saha, P. Colloid Polym. Sci. 2009, 287, 435. (30) (a) Fang, F. F.; Cho, M. S.; Choi, H. J.; Yoon, S. S.; Ahn, W. S. J. Ind. Eng. Chem. 2008, 14, 18. (b) Sung, J. H.; Cho, M. S.; Choi, H. J.; Jhon, M. S. J. Ind. Eng. Chem. 2004, 10, 1217. (31) Kim, S. G.; Lim, J. Y.; Sung, J. H.; Choi, H. J.; Seo, Y. Polymer 2007, 48, 6622. (32) Wang, B. X.; Zhou, M.; Roaynek, Z.; Fossum, J. O. J. Mater. Chem. 2009, 19, 1816. (33) Yin, J. B.; Zhao, X. P. Colloids Surf., A 2008, 329, 153. (34) Kim, J. W.; Noh, M. H.; Choi, H. J.; Lee, D. C.; Jhon, M. S. Polymer 2000, 41, 1229.
DOI: 10.1021/la101165k
12851
Article
Liu et al.
Figure 3. TGA curves of snowman-like PMMA particles and PANI-coated snowman-like particles (SPMPANI) operated under air atmosphere at ramp rate of 10 C/min from 30 to 700 C.
Figure 4. Optical microscopic images of SPMPANI-based ER fluid. Very dilute ER fluid was dropped between two parallel electrodes (350 μm) and observed by OM. The pictures were captured when the electric field (300 V) is off (a) and on (b).
curves of this type of ER fluid are concave, either slightly or seriously, which cannot be fitted using a simple Bingham fluid model. This is the same with this SPMPANI particle-based ER fluid, as shown in Figure 5a. In other words, decreases in shear stress in the low shear rate regime are also observed at all electric field strengths. The previously reported model, known as the Cho-Choi-Jhon (CCJ) model,35 was suggested for this situation and found to better explain the special flow curves. The constitutive rheological equation of state for this type of ER fluid under an applied electric field can be written as ! τy 1 þ η¥ 1 þ γ_ ð2Þ τ ¼ _ R 1 þ ðt2 γÞ _ β ðt3 γÞ where t2 and t3 are the time constants and η is the shear viscosity at an infinite shear rate and is also considered to be the shear viscosity in the absence of an electric field. R and β are in charge of the decrease and increase in shear stress, respectively, in the low and high shear rate regions. β is in the range of 0 and 1 because dτ/dγ_ g 0 above the critical shear rate. Compared to the dotted lines generated by fitting the Bingham model to the flow curves, the solid lines from the CCJ model fit the special flow curves quite well in both the low and high shear rate regions. The fitting parameters are listed in Table 1. The fibrillation of ER particles in shear flow is a breaking and re-formation process caused by the cooperation of electrostatic and hydrodynamic interactions, which are reduced by an external electric and flow field. In the low shear rate region, where the electrostatic interaction is dominant, the aligned particles begin to break with shear deformation and the broken structures tend to re-form chains again.36 However, the rate of destruction may be faster than the rate of re-formation. Therefore, the shear stress generated decreases with increasing shear rate. Hydrodynamic interactions dominate in the high shear rate region, where fibril particle structures are fully destroyed without re-formation, and the suspension behaves like a pseudo-Newtonian fluid. Herein, for this semiconducting PANI-coated snowman-like particles, the large insulated core may play an important role as well in the flow behavior of their ER fluid under applied electric field. In addition, contrary to the fact that particles with larger aspect ratio are generally known to have a better ER effect through preferential alignment, we observe a weak ER effect. As demonstrated in Scheme 2, due to anisotropic snowman-like shape not only less number of particles will be placed between two electrodes compared to spherical microparticle but also the polarized interaction between particles also will be reduced due to unsymmetric polarization density. In Figure 5b, similar flow curves were obtained when the condensed ER fluid (18 wt %) was tested, except for enhanced yield shear stress at every electric field strength. Therefore, the decrease in shear rate may be independent of the particle content (if it is sufficient for an ER response) but related to the response time of the dispersed SPMPANI particles according to the increasing in shear rate. However, the bulky insulating
Scheme 1. Schematic Representation of the Synthesis of PANI-Coated Snowman-like Particles
12852 DOI: 10.1021/la101165k
Langmuir 2010, 26(15), 12849–12854
Liu et al.
Article
Figure 5. Flow curves of a 13 wt % SPMPANI-based ER fluid (open) under various electric field strengths (a) and comparison to a denser ER fluid (18 wt %, closed) (b). Solid and dotted lines in (a) are generated by fitting CCJ and Bingham models, respectively, to the flow curves. Inset of (b) is the yield stress (τy) of the two ER fluids versus electric field strength. Table 1. Optimal Parameters in the Equations of Bingham and CCJ Model Obtained by Fitting the Models to the Flow Curves of ER Fluid (13 wt %) at Various Electric Field Strengths electric field strength [kV/mm] model
parameters
0.1
0.5
1.0
1.5
2.0
Bingham
τy η τy η R β t2 t3
8.67 0.067 8.67 0.078 0.57 0.85 1.21 0.016
11.98 0.065 12.23 0.081 0.44 0.87 1.89 0.018
17.56 0.065 19.14 0.10 0.43 0.91 2.50 0.019
22.56 0.060 25.30 0.080 0.41 0.68 1.10 0.011
30.45 0.060 35.20 0.080 0.40 0.70 1.30 0.012
CCJ
snowman-like core retards the polarization of the entire particle. As a result, the polarization of the particles lags behind the changes in shear rate. The yield stresses of the two ER fluids with different mass ratios were plotted as a function of the electric field strength. As shown in the inset graph in Figure 5b, the yield values of each fluid increase with increasing electric field strength. The Cole-Cole equation,37,38 which is a dielectric relaxation model for analyzing the dielectric properties of materials, has been introduced to describe the relationship between the ER effects and dielectric properties.39,40 The model is described in terms of complex dielectric constant as follows: ε ¼ ε0 - iε00 ¼ ε¥ þ
Δε ð1 þ iωλÞ1 - R
ð3Þ
(35) Cho, M. S.; Choi, H. J.; Jhon, M. S. Polymer 2005, 46, 11484. (36) Sung, J. H.; Park, D. P.; Park, B. J.; Choi, H. J.; Jhon, M. S. Biomacromolecules 2005, 6, 2182. (37) Cole, K. S.; Cole, R. H. J. Chem. Phys. 1941, 9, 341. (38) Choi, H. J.; Hong, C. H.; Jhon, M. S. Int. J. Mod. Phys. B 2007, 21, 4974. (39) Cho, M. S.; Cho, Y. H.; Choi, H. J.; Jhon, M. S. Langmuir 2003, 19, 5875. (40) Zhao, X. P.; Yin, J. B. J. Ind. Eng. Chem. 2006, 12, 184.
Langmuir 2010, 26(15), 12849–12854
Figure 6. (a) Dielectric spectra (permittivity, ε0 : red circle; dielectric loss, ε00 : blue square) and (b) Cole-Cole curve of the ER fluid (13 wt %) at room temperature. The solid lines are plotted by the fitting data of Cole-Cole model equation to the dielectric spectra of the ER fluid. Scheme 2. Fibril Structures of Both Spherical- and Snowman-like ER Particles in Applied Eletric Field
where Δε = ε0 - ε is the achievable polarizability of an ER fluid, ε0 is the dielectric constant when the frequency (ω) approaches 0, ε is the dielectric constant at the high frequency limit, λ = 1/2πfmax is the dielectric relaxation time, in which fmax is defined by a local maximum of the dielectric loss of an ER fluid, (1 - R) characterizes the broadness of the relaxation time distribution, and R is a value in the range 0-1. The dielectric properties of the snowman-like particle-based ER fluid (13 wt %) were measured using a LCR meter in the frequency range of 20-106 Hz and plotted in Figure 6. The experimental data were fitted using eq 3, and the parameters were obtained to help understand the ER properties of the fluid discussed above. In the fitting result, ε0 and ε were 4.76 and 3.20, respectively. Therefore, Δε is a relatively low value (1.56) that leads to a weak ER effect (low yield stresses). The values of R (0.29) and λ (3.5 10-6 s-1), particularly λ, were much lower than DOI: 10.1021/la101165k
12853
Article
those reported in a previous study41-43 and indicate a more rapid response to an electric field in this nonspherical particle-based ER fluid. As a result, it is suggested that the snowman-like particles can be polarized in the direction of the long axis in a short time, which may contribute to the ER effect.44-46 However, the large bulk mass of the insulating PMMA cores reduce the polarizability of the particles and obstruct the interactions between them, resulting in low yield stresses. The Cole-Cole curve plotted as ε00 vs ε0 in Figure 6b is a circular arc that also indicates the value of Δε by the difference between the starting and termination points of the arc. Note that in the case of inorganic based core/shell structured nanocomposite Wang and Zhao47 also correlated its ER performance with dielectric characteristics reporting that a larger dielectric constant enhancement may (41) Fang, F. F.; Sung, J. H.; Choi, H. J. J. Macromol. Sci., Part B: Phys. 2006, 45, 923. (42) Fang, F. F.; Kim, J. H.; Choi, H. J.; Seo, Y. J. Appl. Polym. Sci. 2007, 105, 1853. (43) Sung, J. H.; Lee, I.; Choi, H. J. Int. J. Mod. Phys. B 2005, 19, 1128. (44) Sun, Y.; Thomas, M.; Masounave, J. Smart Mater. Struct. 2009, 18, 024004. (45) Stenicka, M.; Pavlı´ nek, V.; Saha, P.; Blinova, N. V.; Stejskal, J.; Quadrat, O. Colloid Polym. Sci. 2009, 287, 403. (46) Sanchis, A.; Sancho, M.; Martinez, G.; Sebastian, J. L.; Munoz, S. Colloids Surf., A 2004, 249, 119. (47) Wang, B.; Zhao, X. Langmuir 2005, 21, 6553.
12854 DOI: 10.1021/la101165k
Liu et al.
increase the interfacial polarizability of particles and induce a high ER effect.
Conclusions Snowman-like particles were fabricated using a seed emulsion polymerization method. After coating the seed particles with a semiconducting PANI layer, the core-shell snowman-like particles were applied as the dispersed phase of the ER fluid. This new ER fluid exhibited interesting flow behavior in the presence of an electric field, such that after the appearance of the yield stress, the shear stress decreased in the low shear rate region and then increased when the shear rate was higher than the critical value. We refer this ER phenomenon to the large insulating PMMA cores which reduce the achievable polarizability of the particles even though the ER fluid shows a very short relaxation time in dielectric analysis. In addition, the CCJ model, a constitutive equation proposed for this special kind of ER fluids, can fit the flow curves much better than Bingham model, indicating the importance of our CCJ model in analyzing ER behaviors. Acknowledgment. This study was supported by the Industrial Strategic Technology Development Program funded by the Ministry of Knowledge Economy, Korea (2009).
Langmuir 2010, 26(15), 12849–12854
