Macromol. Rapid Commun. 19,271-273 (1998) 27 1

Effect of ionic pendent groups on a polyaniline-based electrorheological fluid

Min S. Cho', Hyoung J. Choi*', Kiwing To2

I Department of Polymer Science and Engineering, Inha University, Inchon, 402-75 1, Korea

* Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

(Received: November 11, 1997; revised manuscript of March 9, 1998)

(e-mail: hjchoi@munhak.inha.ac.kr)

SUMMARY Particles of semiconducting polyaniline and a copolyaniline bearing ionic substituents were synthesized. Electrorheological (ER) fluids using these particles were compared with each other with respect to their rheological properties and dielectric spectra. In the steady shear rheological experiment conducted at 3 kV/mm (DC) at 25 "C, the copolymer system showed higher stress than the polyaniline system in the whole shear rate region. This result was interpreted in terms of the conductivities of the particles and their dielectric spectra. Especially, the different behavior in the high shear rate region can be related to the electrical relaxa- tion phenomena observed in the dielectric spectra.

Introduction Most electrorheological (ER) fluids are suspensions of dielectric particles in a non-conducting liquid. In the pre- sence of an electric field, the suspended particles are polarized due to the dielectric mismatch between the par- ticles and the suspending fluid. The interaction among the polarized particles causes them to arrange themselves in the form of strings along the electric field. The fibrilla- tion of particles due to the electric field produces a large and reversible increase in the apparent viscosity'). Clearly the ER performance of an ER fluid depends crucially on the electrical properties of the suspended particles. Recently, Goodwin et a1.2) have studied the electrorheol- ogy of ER fluids containing copolypyrrole latices synthe- sized from pyrrole and N-methylpyrrole. In their study, N-methylpyrrole was used to control the conductivity of the polymer particles.

In this paper, we compare the electrical and ER proper- ties of an ER fluid using semiconducting polyaniline par- ticles to a similar ER fluid using copolyaniline particles. Polyaniline has been generally recognized as an easily polymerizable and thermally stable organic conducting polymer. Furthermore, by varying the degree of doping, its conductivity can be controlled continuously. Polyani- line particles have to be semiconductive if they are used in an ER fluid. Compared to other ER fluids, ER fluids using polyaniline have relatively low density, better ther- mal stability and a controllable conductivity. Hence, it is considered as one of the most promising materials of ER fluids, especially for anhydrous system^^.^).

Recently an ER fluid made of copolyaniline particles has been investigated5). The copolyaniline bearing sodium sulfonate groups attached to the backbone has better processibility than polyaniline because the copo-

Macromol. Rapid Commun. 19, No. 6, June 1998

0 1998, Hiithig & Wepf Verlag, Zug

lyaniline is more soluble than the homopolymer. Note that the sulfonate ion acts as a self-dopant6) and the con- ductivity of the copolyaniline is mainly due to the sulfo- nate ions. For this semiconducting copolyaniline, one can expect an enhanced induced polarization due to the ionic groups at the polymer backbone. Therefore, when it is used as the dispersed part in an ER fluid, one should expect to observe better ER performance.

In this paper, we synthesized both semiconducting polyaniline and its copolymer having ionic substituents, and prepared ER fluids by dispersing these particles into silicone oil. Both the ER characteristics and the dielectric properties of these ER systems were then investigated.

Experimental part The polyaniline and copolyaniline were synthesized by using an aqueous solution of HCl, ammonium peroxysulfate ((N&)&08, oxidant or initiator) and monomers. Polyaniline was synthesized through oxidative polymerization using the modified method suggested by Leclerc et al.') A prechilled solution of 0.36 mol ammonium peroxysulfate in 200 ml of 1 M HC1 was added dropwise through a dropping funnel to a 3-neck, 2-liter, round-bottom flask containing 0.6 mol aniline in 400ml of HCl, with continuous stirring for 1 h. When synthesizing the copolyaniline, 5 mol-% of sodium 4-ani- linobenzenesulfonate was added to the reaction system as a comonomer. Note that sodium 4-anilinobenzenesulfonate has better reactivity than aniline, so there may be more than 5 mol-%*) of comonomer unit in the main chain of the copolya- niline. Polymerization temperature was kept at 0 * 0.1 "C, and initiator solution was added dropwise. Vigorous stirring was applied to reduce the temperature gradient during reac- tion. Polyaniline was obtained as dark-green (emeraldine) precipitates from the reaction system. C1- ion is a dopant in these polymers, and the doping degree can be controlled by

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212 M. S. Cho, H. J. Choi, K. To

the pH of the aqueous solution containing the polymer parti- cles. To make semi-conductive particles suitable for ER fluids, pH was fixed at 9.0 for both the polyaniline and the copolyaniline. The synthesized polymer was then washed 3 times using distilled water to remove the initiator (oxidant), the unreacted monomer, and the oligomer. The products were finally put into a vacuum oven for approximately two days for drying.

The particle size of the polymers was adjusted by a bead mill and a 38 pm sieve. The densities of particles were mea- sured by a pycnometer to be 1.30 g/cm3 and 1.41 g/cm3 for the polyaniline and the copolyaniline, respectively. We then obtained the conductivities of the polyaniline particles (=lo-'' Skm) and copolyaniline particles S/cm) by 2- probe measurement on a pressed disk of the respective poly- mer. Presumably, the ionic substituents in the copolyaniline contribute to the higher conductivity, whereas the doping level in the copolymer is too low to affect the conduction of electrons through the backbone.

ER fluids were prepared by dispersing these particles into silicone oil whose viscosity was 30 CS at 25 "C. The silicone oil was dried by molecular sieves before use, and the particle concentration was fixed at 20 vol-% in this study. ER charac- terizations were carried out using a rotational rheometer (Physica; MC 120) with a high voltage generator. Shear stress was measured as a function of shear rate up to 1000 s-'. The measuring unit was of concentric cylindrical type, with a 0.59 mm gap between the bob and the cup.

The dielectric spectra of the ER fluids were measured with an impedance analyzer (HP 4284A) with a liquid test fixture in the frequency range from 10 Hz to lo6 Hz.

Results and discussion Successful copolymerization was confirmed by FT-IR analysis. Fig. 1 presents the FT-IR spectrum determined using KBr pellets. The peaks at 1586 cm-' and 1490 cm-I originate from the aromatic C - C stretching vibrations, whereas those at 1309 cm-' and 1 144 cm-' are due to aro- matic amine stretching. The peak at 824 cm-' comes from the out-of-plane H deformation of aromatic rings in poly- aniline homopolymer unit sequences. In addition, charac- teristic peaks of the copolymer appear at 1029 cm-' and 1004 cm-' due to the vibration of the sulfonate ion.

Fig. 2 shows the shear stress behaviors of the polyani- line and the copolyaniline ER fluids under an applied electric field of 3 kV/mm at 25°C. In the low shear rate (<I s-I) region, both samples show the typical nonlinear behavior due to the fibrillar structures of particles in an electric field, with the copolyaniline sample having slightly better performance than the homopolymer sam- ple. On the other hand, in the high shear rate (>1 s-') region, stress decreases with increasing shear rate for the homopolymer sample, while that for the copolymer sam- ple remains unchanged. Thus, the copolyaniline system is a much better ER fluid than the homopolymer system, especially in the high shear rate regime.

3000 2500 2000 1500 1000 500

WAVENUMBER [cm']

Fig. 1. FT-IR spectrum of the copolyaniline particles

-* Copolymer -0- Homopolymer

l o 2 lo-' 10" 10' lo2 lo3 SHEAR RATE[s-']

Fig. 2. Comparison of the flow behaviors of the polyaniline and the copolyaniline ER systems with 20% particles (v/v) in silicone oil at 3 kV/mm

To understand the difference in the rheological proper- ties shown in Fig. 2, we studied the polarization process of these ER fluids. Several inve~tigators~.'~) suggested the presence of interfacial polarization in the suspension of conducting particles in a non-conducting oil. It was Block et a1.I) who showed that the interfacial polarization can be affected by a flow field so that the ER phenomenon is the result of the interaction between the flow field and the polarization induced by an external electric field. In order to study the mechanism of electrostatic interaction and the polarization properties of our samples, we measured the dielectric spectra of the ER fluids in the frequency range from 10 Hz to 106Hz using an impedance analyzer.

Fig. 3(a) and 3(b) show the variations of the permittiv- ity (8') and loss factor (E"), respectively, for our samples. In these figures, we observed dielectric relaxation in the copolyaniline ER fluid, as evidenced from the shape of E'

in Fig. 3(a) and the broad peak of E" in Fig. 3(b). The dielectric relaxation time z of the polarization for the

Effect of ionic pendent groups on a polyaniline-based ...

o Copolyaniline 0 Polyaniline 0

0 0 9 8 0

273

W

8-

6-

4 -

2-

Copolyaniline 0 Polyaniline

0 10' lo2 l o 3 lo4 lo5 lo6

FREQUENCY [Hz]

10' 1 o2 1 0' 1 0' 1 o5 1 o6 FREQUENCY [Hz]

Fig. 3. Electric permittivity E' (a) and loss factor E" (b) versus frequency for ER fluids using the polyaniline and the copolyani- line particles

copolyaniline suspension can be obtained from the rela- tion z = 1/(271fmax) ?P 0.3 ms, wheref,,, ?P 300 Hz is the frequency when E" is maximum. In contrast, similar to the result obtained by Webber"), no dielectric relaxation was observed in the homopolymer ER fluid. We believe that the difference in the dielectric spectra of our samples is due to the presence of the sulfonate ions and sodium ions in the copolymer. These counter ions, which are localized at different positions along the backbone of the copoly- aniline, may contribute a permanent dipole moment to the particles of this copolymer.

Presumably, the homopolymer, which does not have counter ions along the backbone, possesses negligible permanent dipole moment. Therefore one observes, by comparing Fig. 3(a) and Fig. 3(b), that 8' and d' for the ER fluid using the homopolymer are significantly smaller than those using the copolymer at low frequency. How-

ever, at a frequency much higher than the inverse relaxa- tion time of the copolymer ER system, the dipole moment of the ER fluid can no longer follow the external electric field. Hence we observe E' (and E") for the two samples to approach the same value at the high frequency limit.

The above discussions on the dielectric data imply that the ER performance of the copolyaniline system should be better than that of the polyaniline system when the characteristic time scale is longer than the dielectric relaxation time of the copolymer system. This may explain the different rheological behaviors, as shown in Fig. 2, especially in the shear rate region from 1 s-l to lo3 S S ' . The drop in the shear stress at 10 s-' shear rate for the homopolymer system suggests a relaxation time shorter than 0.1 s when the induced dipole moment of the rotat- ing particles or clusters in this shear field cannot follow the external electric field. Hence shear stress decreases as shear rate increases. On the other hand, because the relaxation time (=lo" s) of the copolymer is shorter than the inverse shear rate of the shear field, the polarization of copolymer ER system can stay in phase with the exter- nal electric field so that the shear stress remains unchanged. In conclusion, the better ER performance of the copolyaniline system, having ionic substituents at the backbone, can be explained from the electrical relaxation analysis obtained from the dielectric spectrum.

Acknowledgement: We are grateful to Dr. C. K. Chan for his help in our dielectric experiment, and we thank one of the refer- ees for bringing ref.2' to our attention. This work was supported by the Research Fund for Advanced Materials (1997) through the Korean Ministry of Education.

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