폴리피롤 현탁액의 전기유변 특성

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

송인철, 김영대

전남대학교 응용화학부

Electrorheological Properties of Polypyrrole Dispersions

In Chul Song and Young Dae Kim

전남대학교 응용화학부

서론

Electrorheological (ER) response is defined as the dramatic change in rheological properties of a suspension of small particles due to the application of a large electric field transverse to the direction of flow. ER suspensions are typically composed of nonconducting or semiconducting particles dispersed in a nonconducting continuous phase. A large ER effect was first reported by Winslow in 1949 [1], and has been reviewed in several publications [2 -4]. The simplicity of engineering designs based on ER materials has facilitated the development of specifications for a broad range of devices, such as dampers, clutches, and adaptive structures [3]. Although many ER devices have been brought successfully to the prototype stage, and despite much industrial activity, the anticipated commercialization of these devices has yet to be realized. The main limitation of ER technology development is a lack of effective fluids [4].

Activators are often used to activate suspensions. Some suspensions display little or no ER activity unless small amount of water or surfactant is added, while other suspensions exhibit a significantly enhanced ER response with activator present [5-8]. Enhancing ER activity with activators such as water severely limits the allowable temperature range of operation, promotes corrosion, and increases power consumption. Therefore, it is necessary to design ER suspensions which show a high ER response without the limitations imposed by introducing activators, especially water.

Recently, anhydrous ER suspensions using polymer particles [9], inorganic-organic composite particles [10], and semiconducting polymer particles [9,11-14] were reported. The ER suspensions of semiconducting polymer (such as, polyaniline [11], copolyaniline [12], copolypyrrole [13], poly(styrene-co-divinylbenzene) [9], and acene quinone radical polymers [14], etc) particles showed good ER responses. The polarizability of these materials is high and no activator is needed. Polypyrrole (PPy) is one of the most promising semiconducting polymer, because it has higher conductivity and environmental stability in the conductive state than many other conducting polymers.

화학공학의 이론과 응용 제8권 제1호 2002년

In this paper, we investigated the ER response and dielectric properties of the PPy dispersions in mineral oil. Various PPy particles were synthesized controlling the amount of oxidant and surfactant to investigate the effect of these variables on the ER response.

[Experimental]

PPy was synthesized by chemical polymerization according to the method reported by Kudoh [15]. 0.187 mole pyrrole monomer (Acros Chemical) and a surfactant were dissolved in 50 ml deionized water in a round-bottomed flask fitted with a magnetic stirrer. An oxidant dissolved in 50 ml deionized water was slowly added to the dissolved solution and polymerization was allowed to proceed at 20 oC for 1 h while stirring. Pyrrole was purified by passing through a column of activated basic alumina and stored at 15 oC prior to use. Sodium p-toluene sulfate (Aldrich) was used as the surfactant and ammonium persulfate (Duksan Chemical) was used as the oxidant. Various amounts of surfactant and oxidant were employed to investigate the effect of these variables on the ER response. The black PPy particles were filtered and washed several times with deionized water to remove by-products of the pyrrole polymerization. After being washed with ethanol several times, the particles were treated with 0.1 M aqueous NH4OH to reduce the particle surface conductivity. The particles were dried in a vacuum oven at 50 oC for 48 h.

ER dispersions were prepared by dispersing the PPy particles in mineral oil (Sunkyung Chemical, [eqn: eta]c = 180 cP, [eqn: rho]c = 0.85 g/cm3) and stored in a desiccator to minimize contact with air. Dispersions were allowed to equilibrate for at least 24 h before experiments. The mineral oil was stored for more than 1 week over molecular sieves and then used for all rheological and dielectric measurements.

Rheological experiments were performed on a Brookfield viscometer DV-II fitted with parallel plates, and modified for the application of large electric fields. Potential differences were supplied by a high-voltage dc power supply. Samples were placed between the parallel plates and sheared for 1 min at a large shear rate (> 30 s-1) and zero field strength to ensure a uniform particle distribution. The desired electric field was then applied for 1 min with no shear prior to measurements. Rheological measurements were performed with decreasing and increasing shear rates, to obtain plots of shear stress as a function of shear rates. Values for the dynamic yield stress were determined by extrapolating the shear stress-shear rate data to zero shear rate.

Dispersion capacitance and loss were measured using a Fluke impedance analyzer (Fluke 6306A RLC meter), which probes frequencies in the range of 50 Hz to 106 Hz and operates with potential differences in the range of 0.05  2.0 V (rms). A three-terminal, guarded dielectric cell was employed. Suspension dielectric constants and dissipation factors were determined for decreasing and increasing field frequencies.

화학공학의 이론과 응용 제8권 제1호 2002년

[Results and Discussion]

PPy polymerization was confirmed by FT-IR analysis. The FT-IR spectrum of the PPy particles treated with KBr showed the characteristic peaks of PPy at 1573, 1306, 1190, 1043, 931, and 783 cm-1 [16]. The SEM image of the PPy particles showed that the particle shape is irregular but almost spherical, and the particles present in aggregates. The PPy particle size is around 300 nm. The average size of the PPy aggregates dispersed in mineral oil was measured by a particle size analyzer (Malvern PSA). The average diameter of the PPy aggregates in mineral oil was 53 [eqn: mu]m, indicating that the PPy particles in the ER dispersion were present as large aggregates.

Without an electric field, the PPy dispersions behaved like a Newtonian fluid. By applying an electric field to the dispersions, the shear stress for the PPy dispersions dramatically increased and the suspensions showed a yield stress, showing shear thinning behavior. The dependence of the dynamic yield stress on oxidant amount (ammonium persulfate) is presented in Fig. 1 for 1 wt% PPy dispersions in mineral oil. The PPy particles were synthesized using various amounts of oxidant and the surfactant amount of 0.016 mole. The yield stress increases with the oxidant amount, passes through a maximum, and then decreases with the oxidant amount. Also, the yield stress increases with the electric field strength (E) and is proportional to E1.8. At the oxidant amount of 0.09 mole, the decrease in the ER response is very significant. Furthermore, at oxidant amounts larger than 0.09 mole, the ER measurements could not be performed because particle strands formed between the electrode gap acted as a short circuit in the applied electric field. The yield stress behavior seems to arise from the increased PPy conductivity with the increase in the oxidant amount.

The dynamic yield stress is related to polarization forces. The polarization force per pair interaction under a dc electric field is given by [17]

(1)

where [eqn: epsilon]o is the permittivity of free space, [eqn: epsilon]c is the dielectric constant of the continuous phase, a is the particle radius, E is the applied electric field strength, and  is the relative polarizability of the particle where [eqn: sigma]c is the conductivity of the continuous phase and [eqn: sigma]p is the particle conductivity. Therefore, the yield stress will increase by enhancing the relative polarizability  and hence particle polarization which is related to [eqn: sigma]p.

Figure 1.Dynamic yield stress as a function of oxidant amount for 1 wt% PPy dispersions of various oxidant amount in mineral oil.

The conductivities of the PPy particles were 2.93x10-8, 3.1x10-8, 3.21x10-8, and 3.55x10-8 S/cm for 0.03, 0.05, 0.07, and 0.09 mole of the oxidant amount, respectively, increasing with the oxidant amount. Therefore, the yield stress increase at low oxidant amounts arises from the enhanced particle polarization due to the increased PPy conductivity.

결론

The yield stress increases with the oxidant and surfactant amount, passes through a maximum, and then decreases with the oxidant and surfactant. However, the decrease in the ER response at large surfactant amounts is not so significant compared to that at large oxidant amounts. The increase in the ER response with the oxidant and surfactant amount arises from the enhanced particle polarization and the particle polarization is originated from the increased PPy conductivity. The decrease in the ER response at large oxidant amounts seems to arise from the increased conduction between the PPy particles, while the slight decrease in the ER response at large surfactant amounts arises from the decreased PPy conductivity at large surfactant amounts.

화학공학의 이론과 응용 제8권 제1호 2002년