Enhanced Electrorheology of Conducting Polyaniline Confined in

Coaxial cable-like polyaniline@titania nanofibers: facile synthesis and low power electrorheological fluid application. Journal of Materials Chemistry...
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Langmuir 2004, 20, 202-207

Enhanced Electrorheology of Conducting Polyaniline Confined in MCM-41 Channels Min S. Cho, Hyoung J. Choi,* and Wha-Seung Ahn† Department of Polymer Science and Engineering, Inha University, Incheon, 402-751, Korea Received June 14, 2003. In Final Form: November 3, 2003

A composite material of a silica-based mesoporous molecular sieve, MCM-41, with conducting polyaniline (PANI) inside the uniformly aligned one-dimensional channels (PANI/MCM-41) was prepared and its nanocomposite formation was confirmed through an electrical conductivity measurement. This nanocomposite particle was adopted for a dispersed phase in electrorheological (ER) fluids, and the ER property was measured using a Couette-type rotational rheometer equipped with a high voltage generator. Suspension of PANI/MCM-41 showed ER properties more enhanced than those of MCM-41 or PANI alone as a result of the anisotropic polarization of the PANI/MCM-41 nanocomposite.

Introduction Electrorheological (ER) fluids have been the subjects of much research as one of the smart materials showing interesting properties of rheology under an external field (e.g., an electric field). They show a reversible transition from a fluidlike state to a solidlike state under an applied electric field,1-4 thus causing a yield stress following a shear stress increase of the fluid above the yield point. Polarized particles behave as electric dipoles, which attract each other to form chains, and the formation of particle chains in the direction of the applied electric field produces the observed rheological changes. These rheological changes in ER fluids are very fast and reversible within the order of milliseconds so that ER fluids have been considered for various applications in electromechanical devices including shock absorbers and clutches,5 which can be controlled in an active way. A typical ER fluid is the heterogeneous dispersion of polarizable dielectric particles in a liquid of a low dielectric constant. Since the early works of research in ER fluids, extrinsic polarizable particles such as silica, alumina, mesoporous molecular sieves, and starch have been used, which contain various kinds of additives generating polarizability,6-8 for example, water, surfactant, and alcohols. The operation of these hydrous ER fluids are limited by the narrow operation temperature, device corrosion, water evaporation, and dispersion instability, which may be caused either by the presence of additives or by the much higher density than that of the medium oil. These drawbacks have, however, been improved by * Corresponding author. Phone: 82-32-860-7486. Fax: 82-32865-5178. E-mail: [email protected]. † Department of Chemical Engineering, Inha University, Incheon, 402-751, Korea. (1) Hao, T. Adv. Mater. 2001, 13, 1847. (2) Sim, I. S.; Kim, J. W.; Choi, H. J.; Kim, C. A.; Jhon, M. S. Chem. Mater. 2001, 13, 1243. (3) Parthasarathy, M.; Klingenberg, D. J. Mater. Sci. Eng. 1996, R17, 57. (4) Loudet, J. C.; Poulin, P. Phys. Rev. Lett. 2001, 87, 165503. (5) Hong, S. R.; Choi, S. B.; Han, M. S. Int. J. Mech. Sci. 2002, 44, 2027. (6) Kim, Y. D.; Klingenberg, D. J. J. Colloid Interface Sci. 1996, 183, 568. (7) Choi, H. J.; Cho, M. S.; Kang, K. K.; Ahn, W. S. Microporous Mesoporous Mater. 2000, 39, 19. (8) Yethiraj, A.; van Blaaderen, A. Int. J. Mod. Phys. B 2002, 16, 2328.

adopting intrinsic polarizable particles to ER fluids, which have their own polarizable species such as electrons and ions. As ER materials for anhydrous systems, various semiconducting polymers such as polyaniline (PANI)9-11 and its derivatives,12 poly(acene quinine) radicals,13 polyphenylenediamine,14 and inorganic materials including polymer/clay nanocomposites with polyaniline, polypyrrole, and styrene-acrylonitrile copolymers have been used for particulates in ER fluids.15-21 In addition, polymeric particles possessing polar groups such as amino, hydroxyl, and amino-cyan have been adopted. These polar groups may affect the ER behavior by playing the role of electronic donor so that the chemical structures of the organic materials become an important factor in the ER performance.22 Calcined mesoporous silica, MCM-41, suspended7 in silicone oil and the PANI/mesoporous SBA-15 nanocomposite suspended23 in silicone oil were also recently reported to show interesting ER properties. Essentially, electric properties such as the conductivity and dielectric constant of ER materials have governing influences on their ER performance, and the ER performance is maximized under an optimum set of conditions.24 (9) Choi, H. J.; Kim, T. W.; Cho, M. S.; Kim, S. G.; Jhon, M. S. Eur. Polym. J. 1997, 33, 699. (10) Lengalova, A.; Pavlinek, V.; Saha, P.; Stejskal, J.; Kitano, T.; Quadrat, O. Physica A 2003, 321, 411. (11) Bocinska, M.; Wycislik, H.; Osuchowski, M.; Plocharski, J. Int. J. Mod. Phys. B 2002, 16, 2461. (12) Choi, H. J.; Kim, J. W.; To, K. Polymer 1999, 40, 2163. (13) Sohn, J. I.; Cho, M. S.; Choi, H. J.; Jhon, M. S. Macromol. Chem. Phys. 2002, 203, 1135. (14) Trlica, J.; Saha, P.; Quadrat, O.; Stejskal, J. Physica A 2000, 283, 337. (15) Zhao, X. P.; Yin, J. B. Chem. Mater. 2002, 14, 2258. (16) Kim, J. W.; Kim, S. G.; Choi, H. J.; Jhon, M. S. Macromol. Rapid Commun. 1999, 20, 450. (17) Lu, J.; Zhao, X. J. Mater. Res. 2002, 17, 1514. (18) See, H.; Kawai, A.; Ikazaki, F. Colloid Polym. Sci. 2002, 280, 24. (19) Jun, J. B.; Lee, C. H.; Kim, J. W.; Suh, K. D. Colloid Polym. Sci. 2002, 280, 744. (20) Kim, J. W.; Liu, F.; Choi, H. J. J. Ind. Eng. Chem. 2002, 8, 399. (21) Kim, J. W.; Jang, L. W.; Choi, H. J.; Jhon, M. S. J. Appl. Polym. Sci. 2003, 89, 821. (22) Sung, J. H.; Choi, H. J.; Jhon, M. S. Mater. Chem. Phys. 2002, 77, 778. (23) Cho, M. S.; Choi, H. J.; Kim, K. Y.; Ahn, W. S. Macromol. Rapid Commun. 2002, 23, 713. (24) Atten, P.; Foulc, J. N.; Gonon, P. Int. J. Mod. Phys. B 2002, 16, 2662.

10.1021/la035051z CCC: $27.50 © 2004 American Chemical Society Published on Web 12/05/2003

Electrorheology of Conducting Polyaniline

In this current study, as a continuation of our brief report,25 we synthesized an organic-inorganic nanocomposite in which conducting PANI is confined within the channels of mesoporous MCM-41, and its potential use as an ER fluid system based on both steady shear and dynamic oscillation experimentation was evaluated. Originally, nanocomposites adopting nanoscale alignments of the encapsulated molecules inside the host channels26 have been investigated using various mesoporous hosts with guest materials such as carbon, polymer, or metal. Among them, composites containing either conducting polymers or metals, in particular, have been actively investigated as “molecular wires” in electronic devices27,28 or as a candidate for use in optoelectronic devices.29 The composite of PANI in MCM-41 (PANI/MCM-41) is a new system for ER materials in which a conducting PANI exists only within insulating MCM-41 channels. In other words, overall particles are dielectric, and conducting components are placed inside of the uniform channels of the MCM-41 particles. Experimental Section Preparation of the PANI/MCM-41 Nanocomposite. The MCM-41 host material was synthesized in the procedure as follows.30 Colloidal silica (40 wt %, SiO2) in water (Ludox AS-40, Dupont) was added to a 40 wt % tetraethylammonium hydroxide solution with vigorous stirring. This solution mixture was combined with a 25 wt % cetyltrimethylammonium chloride solution while applying agitation at room temperature for 1 h. The mixture was then placed in an autoclave and kept at 373 K for 24 h in a conventional oven. The reaction mixture was cooled to room temperature, and acetic acid was added dropwise under vigorous stirring until the pH becomes about 10.2. This mixture was then heated again to 373 K for 24 h. The solid product obtained was filtered off, and surfactant was extracted from MCM-41 in a EtOH-HCl solution at room temperature (1 g solid/ 20 mL EtOH + 2 mL HCl). After complete drying, it was finally calcined in air at 823 K. To prepare the PANI/MCM-41 nanocomposite, the dried MCM41 host was contacted with aniline gas at 313 K for 24 h.27 The MCM-41 containing 15% (w/w) of aniline was then immersed in 0.2 M HCl aqueous solution to 2% (w/w) concentration, and the oxidant initiator, ammonium peroxysulfate (APS), was added to the reaction system with continuous stirring at room temperature. The PANI was synthesized through oxidation polymerization,9 and the mole ratio of APS to aniline was 1.25. The pale yellow of the reaction suspension turns to a green color within 0.5 h after the addition of APS, and the reaction was continued to 24 h. Note that the PANI synthesized in this study was in a doped oxidized state called emeraldine salt.9 The PANI/MCM-41 obtained was washed with an aqueous HCl solution and methanol and dried at room temperature under a reduced pressure of 10 mmHg. Furthermore, the homo-PANI particle for the comparison in this study was also synthesized using the conventional oxidation polymerization of aniline to produce a fine emeraldine hydrochloride form.9 A solution of aniline monomer in 1 M HCl was chilled, and polymerization was initiated at 0 °C by a pre-chilled solution of APS in 1 M HCl. The reaction was maintained for an additional 2 h to complete the reaction to produce the emeraldine hydrochloride form. A portion of the PANI particles were dedoped by reducing the pH of the aqueous medium, which contained the particles, to pH 9.0 using an aqueous NaOH solution, which (25) Choi, H. J.; Cho, M. S.; Ahn, W. S. Synth. Met. 2003, 135-136, 711. (26) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. (27) Wu, C. G.; Bein, T. Science 1994, 264, 1757. (28) MacLachlan, M. J.; Aroca, P.; Coombs, N.; Manners, I.; Ozin, G. A. Adv. Mater. 1998, 10, 144. (29) Nguyen, T.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652. (30) Whang, M. S.; Kwon, Y. K.; Kim, G. J. J. Ind. Eng. Chem. 2002, 8, 262.

Langmuir, Vol. 20, No. 1, 2004 203 allowed them to be used as dispersed particles for ER fluid directly because of their high conductivity (approximately 1 S/cm). ER Characterization. The dried PANI/MCM-41 particle was dispersed in silicone oil to become 5% (v/v) loading. The ER property of the PANI/MCM-41 suspension in silicone oil was measured by a Couette-type rotational rheometer (Physica, MC120) equipped with a direct current (dc) high-voltage generator. The measuring geometry was a concentric cylinder, and all measurements were conducted at 25 °C. The ER fluid was placed in the gap between the stationary outer cup and the rotating measuring bob. The dielectric spectra of ER fluids were also measured by an impedance analyzer (Hewlett-Packard, HP4284A) within the frequency range from 20 Hz to 1 MHz using a measuring fixture (HP16452A) for liquids to investigate their interfacial polarization. The 1 V of electrical potential was applied to the ER fluid during measurement. It was small enough that no particle structure formation within the ER fluid was induced, and we could obtain the true behavior of the interfacial polarization between the particles and the medium.

Results and Discussion Regular hexagonal pore openings of the mesoporous channels and particle morphology of the synthesized pristine MCM-41 host are shown in Figure 1a,b via transmission electron microscope (TEM, CM200, Philips) and scanning electron microscope (SEM, FE-SEM S-4200, Hitachi) images, respectively. An SEM image of PANI/ MCM-41 nanocomposite particles is also given in Figure 1c. Even though some aggregates of MCM-41 particles were visible, the size of the primary particles was less than 10 µm and they took the shape of a twisted rectangular bar. Virtually no difference in particle surface morphology between the pristine MCM-41 host and the PANI/MCM-41 composite material was observed, which can be served as an evidence of PANI confinement within MCM-41 channels rather than surface adsorption and subsequent growth of the PANI. The polymer product confined within the channels was also confirmed by a nitrogen sorption measurement. The N2-adsorption isotherm plots of both the empty pristine MCM-41 and the PANI/MCM-41 composites are represented in Figure 2.25 The residual pore volume of PANI/ MCM-41 is reduced to 0.63 cm3/g from 0.96 cm3/g for the empty pristine MCM-41 as a consequence of pore filling by PANI. The amount of PANI in the PANI/MCM-41 composite was estimated by thermogravimetric analysis (TGA; TGA7, Perkin-Elmer) to be about 10% (w/w). During the TGA measurement, the temperature was raised from 30 to 900 °C with a heating rate of 20 °C/min in air. The decomposition of the main chain of PANI occurred above 700 °C.9 The dc electrical conductivity of PANI/MCM-41 was measured by two probe methods using a pressed disk of the PANI/MCM-41 sample and found to be 10-9 S/cm. This electrical conductivity of PANI/MCM-41 is considered to be much lower than that of pristine doped PANI itself placed within the channel of MCM-41. Note that the electrical conductivity of doped PANI with HCl is generally known to be about 1 S/cm.31 This fact not only provides more supporting evidence that the most PANI synthesized is located only inside the MCM-41 channels without any PANI adsorption outside the MCM-41 surface but also indicates that this method of preparing the PANI/MCM41 composite is an alternative way of controlling the conductivity of the ER fluid without doping and dedoping steps, which are very time-consuming. Microencapsulated PANI particles with melamine-formaldehyde have been (31) Kim, B. H.; Jung, J. H.; Hong, S. H.; Joo, J.; Epstein, A. J.; Mizoguchi, K.; Kim, J. W.; Choi, H. J. Macromolecules 2002, 35, 1419.

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Figure 1. TEM image of a cross section of MCM-41 (a) and SEM image of the MCM-41 host (b) and PANI/MCM-41 (c).

Figure 2. N2-adsorption isotherm curves before (pristine MCM41; 0) and after (PANI/MCM-41; O) aniline polymerization. Solid symbols represent the adsorption processes and open symbols the desorption processes.

also adopted to improve the particle preparation method for PANI.32 A schematic diagram of a PANI/MCM-41 particle pictured from the characterization work mentioned so far is shown in Figure 3. MCM-41 was synthesized from rodlike micelles of cationic surfactant as a template and composed of the uniform channels of 3.1-nm pore diameter aligned through the longitudinal axis of the primary particles. PANI is believed to be partially filling the channels of the MCM-41 particles and forming long conductive wires within the insulating channels made of silicate on the basis of both TGA and the N2-adsorption isotherm. PANI/MCM-41 nanocomposite material can be electrically anisotropic as a result of PANI forming predominantly inside the channels, as illustrated in Figure 3. Therefore, the induced dipole moment of PANI/MCM41 particles in the longitudinal direction is larger than that in the lateral direction. It is also noteworthy in that (32) Lee, Y. H.; Kim, C. A.; Jang, W. H.; Choi, H. J.; Jhon, M. S. Polymer 2001, 42, 8277.

Figure 3. Proposed schematic diagram of a PANI/MCM-41 composite particle, in which conducting PANI is filled in the uniform one-dimensional channels of the MCM-41.

the PANI/montmorillonite nanocomposite,31 the intercalated conducting PANI into the intergallery of the clay, has also been reported to show an anisotropic quasi-onedimensional variable hopping model on dc electrical conductivity because of a confined geometrical effect similar to that of our PANI/MCM-41 nanocomposite. Flow curves of shear stress and shear viscosity for PANI-, PANI/MCM-41-, and MCM-41-based ER fluids, which were measured using a Couette-type rotational rheometer, are given in Figure 4, as a function of shear rate under both an applied electric field of 3 kV/mm and without an applied electric field. The shear-thinning behavior of these ER fluids is clearly observed for all three suspensions under an electric field, compared to the cases without an electric field as shown in Figure 4b. The PANI used in this ER fluid experiment for comparison was dedoped, in which the pH of the suspension containing PANI during synthesis was controlled to 9.0 with aqueous NaOH solution. After the dedoping process, the PANI has a conductivity on the order of 10-9 S/cm, which is close to the conductivity of PANI/MCM-41 prepared. Even though particles of the PANI/MCM-41 and MCM-41 were found

Electrorheology of Conducting Polyaniline

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Figure 4. Flow curves of shear stress (a) and shear viscosity (b) versus shear rate for each ER fluid at 25 °C under 0 and 3 kV/mm. Solid symbols represent the data at 0 kV/mm (9, PANI/MCM-41; 2, PANI; b, MCM-41), and open symbols represent the data at 3 kV/mm.

Figure 5. (a) Dielectric spectrum (permittivity, ′, versus frequency) and (b) Cole-Cole plot for each ER fluid.

to have irregular shapes from the SEM images (Figures 1b,c), because the shape of the PANI particle has also been reported to be irregular, the shape will not affect their flow curves. Note that the flow behavior was not largely different with ER fluids containing spherical particles.33 According to our measurements, both PANIand PANI/MCM-41-based ER fluids show a higher shear stress than MCM-41-based ER fluid under the whole shear-rate range investigated. Polarizing species under the electric fields in this work must be conducting PANI for the PANI/MCM-41-based ER fluid and adsorbed moisture for the MCM-41-based ER fluid,7 both of which amounted to about 10% (w/w), as confirmed by TGA. It was shown that the developed shear stress for PANI/MCM41 by an applied electric field is consistently larger in the whole shear-rate regime than that of MCM-41. These experimental findings clearly demonstrate the superior ER performance of the PANI/MCM-41 system to MCM(33) Cho, M. S.; Cho, Y. H.; Choi, H. J.; Jhon, M. S. Langmuir 2003, 19, 5875.

41-based ER fluid. Apparently, PANI must be more efficient than water as polarizing species in these ER fluids. In addition, MCM-41 is considered to be a better host of PANI for ER fluids comparing with SBA-15,23 because PANI/MCM-41-based ER fluid showed a much larger shear stress increase under an applied electric field than PANI/SBA-15-based ER fluid. Furthermore, Figure 4a under an applied electric field exhibits Bingham fluid behaviors, compared to the flow curves without an applied electric field. Furthermore, note that, at 0 kV/mm, all three ER fluids show Newtonian behavior; that is, the shear stress is linearly proportional to the shear rate.34 When the performance of the PANI/MCM-41 system is compared with ER fluid based on PANI particles, their shear stresses in a low shear-rate region are almost same. However, while the shear stress decreased abruptly with a shear rate for the PANI system, the PANI/MCM-41based ER fluid still maintained its shear stress level, up to a high shear rate above 100 s-1. The structures formed by the applied electric field sustained the shear deformation up to 50 s-1, and these were totally destroyed by the faster deformation larger than 50 s-1. The stress decrease observed by the PANI-based ER fluid is a well-known phenomenon among various ER materials because of the sudden rupture of particular structures within the fluid,18 and it is a characteristic of an ER fluid having insufficient attraction between particles.2 In a low shear-rate 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 particleparticle interaction in the fibrils. The decrease in shear stress is observed when the increase in the reformed structures with the shear rate is not as complete as those before applying shear flow.33,34 In other words, as the shear rate increases, the destruction rate of the fibrils becomes faster than the reformation rate,21 and the shear rate, which gives a minimum shear stress, has been designated as the critical shear rate.35 In addition, the PANI/MCM-41-based ER system exhibits an outstanding enhancement of ER property when (34) Choi, H. J.; Cho, M. S.; Kim, J. W.; Kim, C. A.; Jhon, M. S. Appl. Phys. Lett. 2001, 78, 3806. (35) Kim, J. W.; Noh, M. H.; Choi, H. J.; Lee, D. C.; Jhon, M. S. Polymer 2000, 41, 1229.

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Figure 6. (a) Strain amplitude versus dynamic moduli under various electric field strengths at 10 rad/s of angular frequency (solid symbols, G′; open symbols, G′′) and (b) angular frequency versus dynamic moduli under 1 (9, 0) and 2 (b, O) kV/mm using a strain of 3 × 10-5.

we consider the amount of PANI within ER fluids. In the case of PANI/MCM-41-based ER fluid, PANI is only about 0.5% (v/v), which is much lower than the minimum concentration of PANI for showing ER performance. Although only a small amount of PANI is contained, it shows ER properties because the PANI is uniformly distributed in the MCM-41 particles by constructing nanosized bundles. Furthermore, it makes the PANI/ MCM-41-based ER fluid superior to the PANI-based ER fluid. Focusing on the flow behavior above 100 s-1 of shear rate, PANI encapsulated within the channels of MCM-41 is in a highly doped state so that it has low electrical resistance and electrons within the PANI chains can move somewhat freely under an external electric field. Thus, the polarization within PANI/MCM-41 particles under an applied electric field would be fast. In addition, the electrical anisotropy of PANI/MCM-41 must be superior to PANI because of aligned PANI structures through the nanoscale channel of the MCM-41 host. This combined effect is expected to produce a higher attraction force between particles under the electric field and consequently generates the higher shear stress of PANI/MCM-41 observed than that for the PANI system in the high shearrate region, and these will be confirmed in terms of dielectric spectrum analysis in the following. Parts a and b of Figures 5 show the permittivity (′) as a function of the electrical frequency and Cole-Cole plots for each ER fluid, respectively. It is a typical result from the interfacial polarization of suspensions such as ER fluids consisting of a polarizable-dispersed phase and insulating medium. Both the relaxation of the interfacial polarization with the decrease of ′ from 0 to ∞ and the hemicyclic shape of the Cole-Cole plot (Figure 3b) were observed in these ER suspensions. The lines in Figure 5 are fitted results by the following Cole-Cole formulaa:36,37

* ) ′ + i′′ ) ∞ +

0 -  ∞ 1 + (iωλ)1-R

(0 e R < 1) (1)

Because eq 1 represents the two-phase systems such as ER fluids, the fitting results can be considered to be quite valid for the dielectric spectra of these fluids. In addition, (36) Lee, J. H.; Cho, M. S.; Choi, H. J.; Jhon, M. S. Colloid Polym. Sci. 1999, 277, 73. (37) Cho, Y. H.; Cho, M. S.; Choi, H. J.; Jhon, M. S. Colloid Polym. Sci. 2002, 280, 1062.

even though the data points given in Figure 5a are obtained for a limited range because of the frequency limit of our impedance analyzer from 20 Hz to 1 MHz, the fitted lines can be extended to cover the entire range of each of the graphs on the basis of eq 1. Among the parameters in the Cole-Cole formula, the relaxation time of interfacial polarization (λ) and an achievable polarizability (∆ ) 0 - ∞) are generally known to be related with the yield stress and stress enhancement under an applied electric field.33 As the shorter λ within adequate range and the bigger ∆ are applied, the higher stress enhancement by an applied electric field is achieved,36,37 and these are dependent on each other. λ can be roughly estimated from the relaxation of ′ (decrease in ′ from low to high frequency) in Figure 5a, in which a reciprocal frequency at the maximum decreasing slope of ′ is linearly related by λ. On the other hand, ∆ can be directly obtained from Cole-Cole plots (Figure 5b), in which ∆ is the width of the Cole-Cole arc. In addition, λ reflects the rate of interfacial polarization when an external electric field is applied, so that it is mainly related to the stress increase during deformation under a shear field, and ∆ shows the degree of polarization, which is related to the electrostatic interaction between particles. From the fitting results, the λ values were 0.1, 0.02, and 0.0005 s for MCM-41-, PANI/MCM-41-, and PANIbased ER fluids, and the ∆ values were 1.31, 1.63, and 2.98 for PANI-, MCM-41-, and PANI/MCM-41-based ER fluid, respectively. The difference in flow behavior and ER performance of ER fluids can be interpreted by these parameters, obtained from the dielectric spectrum of ER fluids.37,38 The highest ∆ of PANI/MCM-41-based ER fluid means that it has the strongest particulate attraction among ER fluids tested in this study so that it shows the best ER performance as the results in Figure 4. However, on the basis of λ, the polarization rate of PANI-based ER fluid might be faster than that of PANI/MCM-41 under an applied electric field. Therefore, considering both λ and ∆, it can be concluded that the ∆ value is more effective and dominant for ER performance because it is directly related to the strength of particulate fibril structures.38 Furthermore, it can also be noted that the λ of PANI/ MCM-41 (0.02 s) is fast enough to have good ER performance37 in a high shear-rate region. On the other hand, the R values in eq 1 were determined to be 0.45, 0.29, and 0.40 for MCM-41, PANI/MCM-41, and PANI-based ER (38) Hao, T.; Kawai, A.; Ikazaki, F. Langmuir 1998, 14, 1256.

Electrorheology of Conducting Polyaniline

fluids, respectively, and it means that all of them have multiple relaxation times rather than a single relaxation time (R ) 0). Another advantage of the PANI/MCM-41 system worth mentioning as a promising ER fluid is its low current density; 0.1 µA/cm2 for PANI/MCM-41-based ER fluid at 3 kV/mm is 1/6 of that (0.6 µA/cm2) of PANI-based ER fluid. It is due to the insulating silica wall of the MCM-41 host and is an additional advantage of saving the electrical power during ER operation. Dynamic oscillation tests adopting the shear flow mode were also conducted, and they were used to study the viscoelastic properties of the solidified PANI/MCM-41based ER fluid under various electric fields. The results of the strain amplitude sweep, which is a measurement of stress as a function of sinusoidal strains at 10 rad/s of deformational frequency, are shown in Figure 6a. Prior to the dynamic oscillation measurement, the strain amplitude sweep test was performed to find a linear viscoelastic region. The storage modulus (G′), that is, the in-phase stress component with the strain, is observed to be larger than the loss modulus (G′′), that is, the out-ofphase stress component, and these values were independent of the strain within the regions of strain applied, which are so-called linear viscoelastic regions. In the linear viscoelastic region of ER fluids, structures developed by the electric fields are unchanged and, therefore, the elasticity is dominant compared with the viscosity (i.e., G′ > G′′). With increasing electric field strength, G′ increased and the linear viscoelastic regions become wider so that the particular microstructures formed in the ER fluid become more elastic and tougher to sustain the larger strain. As we increased the applied strain, G′′ becomes bigger than G′ and these moduli sharply decreased. As the strain is increased, the deformation starts to distort the structure, the structure breaks down beyond a certain degree of deformation, and finally, the elasticity of the ER fluid disappears abruptly. Figure 6b shows respective plots of G′ and G′′ as a function of the frequency with small strain of 3 × 10-5 in the linear viscoelastic region. It was shown that the G′ values are

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either constant or increased slightly as the deformation frequency was increased up to 100 rad/s. This is typical behavior of cross-linked rubbers39 and associated colloidal dispersions,40 which do not relax at a given range of frequency of the strain. Because the relaxation time for a deformation was too long, it is expected that the internal chain structures of ER fluids are not destroyed by deformation under the given conditions. The increase of G′ with the applied electric field indicates that the ER fluid becomes more elastic with the electric fields under its linear viscoelastic conditions. Conclusions A nanocomposite (PANI/MCM-41) consisting of mesoporous silica MCM-41 and conductive PANI confined within the channels of MCM-41 was synthesized, and it was adopted as an ER material. The insertion of PANI into the MCM-41 channels was confirmed by the N2adsorption isotherm, conductivity measurement, and SEM results. Compared with ER fluids based on MCM-41 and PANI, PANI/MCM-41-based ER fluid showed an enhanced ER performance, and it seems to be due to the anisotropic characteristics of the electrical properties of PANI/MCM41 particles. These ER characteristics were also analyzed via the dielectric spectra of ER fluids. Viscoelastic properties of PANI/MCM-41 ER fluid under an applied electric fields were also investigated. The elasticity, stemming from the microstructures formed in the ER fluids, was increased with the strength of the external electric fields, and the toughness of the structures was also enhanced. Acknowledgment. This study was supported by research grants from the Korea Science and Engineering Foundation (Project No. R01-203-000-10382-0). LA035051Z (39) Dealy, J. M.; Wissbrun, K. F. Melt Rheology and Its Role in Plastics Processing: Theory and Applications; Van Nostrand Reinhold: New York, 1990; p 18. (40) Gisler, T.; Ball, R. C.; Weitz, D. A. Phys. Rev. Lett. 1999, 82, 1064.