폴리피롤이 코팅된 폴리에칠렌 입자로 제조된 전기유변유체의 특성

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

박동협,김영대

전남대학교 응용화학부

The Electrorheological Response of PPy-coated PE Suspensions

Dong Hyup Park and Young Dae Kim

Faculty of Applied Chemistry, Chonnam National University

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 [2-5]. 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 suspensions [5].

Various mechanisms have been proposed to explain the ER response. The electro-osmosis suggests that the ER response arise from the formation of a water bridge between the particles [6]. The inter-electrode circulation proposes that the inter-electrode circulation of particles between the electrodes, due to the particle charge change by electrochemical reactions at the electrode surface, lead to the ER response [2]. The electric double layer proposes that the origin of the ER response is the overlap of electric double layers [7]. The electrostatic polarization model explains that the ER response arises from the electrostatic interactions between the particles due to the field induced polarization of the particles [8,9]. Among these mechanisms, the electrostatic polarization model seems to be the most suitable explanation for the ER phenomena.

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 [6,10-13]. 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.

화학공학의 이론과 응용 제8권 제1호 2002년

Recently, anhydrous ER suspensions using polymer particles [14], inorganic-organic composite particles [15], and semiconducting polymer particles [14, 16-19] were reported. The ER suspensions of semiconducting polymer (such as, polyaniline [16], copolyaniline [17], copolypyrrole [18], poly(styrene-co-divinylbenzene) [14], and acene quinone radical polymers [19], etc) particles showed good ER responses.

In this paper, we investigated the ER response and dielectric properties of the suspensions of polypyrrole(PPy)-coated polyethylene (PE) particles. PPy was coated on PE particles to enhance the particle polarization by increasing the particle surface conductivity, which would lead to the enhanced ER response. It was reported that the increased particle surface conductivity enhanced the particle polarization and hence increased the ER response [12, 13]. PPy-coated PE particles were synthesized by the pyrrole polymerization on PE particles. Various PPy-coated PE particles were synthesized by controlling the pyrrole or FeCl3□6H20 amount during the pyrrole polymerization, and the ER responses and dielectric properties of their suspensions were investigated. Also, poly(vinyl alcohol) was coated on the PPy-coated PE particles to restrict the increased conduction between the particles.

[Experimental]

2.5 g PE particles (Aldrich) were dispersed in 200 ml deionized water containing 1.0 g methylcellulose (Showa chemical) in a round-bottomed flask fitted with a magnetic stirrer. FeCl3□6H20 (Kanio chemical) dissolved in 50 ml deionized water was slowly added to the PE particle dispersed solution. Pyrrole (Acros Chemical) was added and polymerization was allowed to proceed at 25 oC for 24 h while stirring. Pyrrole was purified by passing through a column of activated basic alumina and stored at 15 oC prior to use. The black PPy-coated PE particles were filtered and washed several times by deionized water to remove by-products of the pyrrole polymerization. The particles were dried in a vacuum oven at 50 oC for 24 h. Particle sizes of PE and PPy-coated PE particles were measured by a particle size analyzer (Malvern PSA). The average diameters of PE particles and PPy-coated PE particles were 45 [eqn: mu]m and 50 [eqn: mu]m, respectively. The SEM images of the PE and PPy-coated PE particle surface indicated that the PE particle surface is effectively covered with PPy.

ER suspensions were prepared by dispersing PPy-coated PE 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. Suspensions 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.

화학공학의 이론과 응용 제8권 제1호 2002년

Rheological experiments were performed at 25 oC on a Rheometric ARES rheometer 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 (> 40 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 by shearing the suspension at a constant shear rate under the applied electric field, and recording the shear stress transmitted by the suspension. Experiments 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.

Suspension capacitance and loss were measured using a Fluke impedance analyzer (Fluke 6306A RLC meter), which probes frequencies in the range of 50 Hz to 100 kHz, and operates with potential differences in the range of 0.01  1.0 V (rms). A three-terminal, guarded dielectric cell was employed. Suspension dielectric constants and dissipation factors were determined for decreasing and increasing field frequencies. The conductivities of the PPy-coated PE particles were measured by the two-probe method using compressed disks with a picoammeter (Keithley 485).

[Results and Discussion]

The ER behaviors under various electric field strengths are presented in Fig. 1 for a 10 wt% PPy-coated PE suspension in mineral oil. The PPy-coated PE particles were synthesized by using 0.1 g of pyrrole and 1.5 g of FeCl3□6H20. Without an electric field, the suspension behaves like a Newtonian fluid with the slope of log (shear stress) to log (shear rate) of 1.0. By applying an electric field to the suspension, the shear stresses for the ER suspension dramatically increase and even the suspension shows a yield stress, showing shear thinning behavior. The shear stresses and yield stress increase with the increase in the electric field strength. The steady-shear rheological response can be described as that of Bingham fluid, showing the prevalent features of the ER responsean apparent yielding phenomenon at low shear rates and shear thinning behavior approaching a constant viscosity at large shear rates. It was also observed by using a CCD camera that the ER response was related to the electric field-induced alteration of the suspension structure, where strands of particles formed spanning the electrode gap under the applied electric field. At intermediate shear rates (5-50 s-1), however, anomalous behaviors where the shear stress decreases with shear rate are observed. The anomalous behavior might arise from a negative synergistic interaction between hydrodynamic and polarization forces.

Fig. 1. Shear stress as a function of shear rate for 10 wt% PPy-coated PE suspensions in mineral oil.

결론

PPy-coated PE particles were prepared by the pyrrole polymerization on PE particles. PPy coating on the PE particles was observed by SEM. It was found that the ER response (yield stress and shear stress) increased with the coating of PPy on the PE particles. The ER response initially increases with the pyrrole or FeCl3□6H20 amount, passes through a maximum, and then decreases with the pyrrole or FeCl3□6H20 amount. The increase in the ER response with pyrrole or FeCl3□6H20 amounts is due to the enhanced particle polarization with the increased particle surface conductivity. The decrease in ER response at large pyrrole or FeCl3□6H20 amounts arises from the increased conduction between the PPy-coated PE particles. PVA was coated on the PPy-coated PE particles to restrict the increased conduction.

화학공학의 이론과 응용 제8권 제1호 2002년