Aqueous ferric chloride doped poly(p-phenylene) for electrorheological material

Abstract

An electrorheological (ER) fluid is a suspension of polarizable solid particles dispersed in a non-conducting liquid, exhibiting drastic and reversible change in rheological properties when an external electric field is applied [1, 2] via ordering of the microstructure into particulate columns. This rapid and reversible response has potential application in many electrically-controlled mechanical devices, which transform electrical energy to mechanical energy [3]. The basic mechanism of the ER phenomenon is believed to be that the applied external field induces electric polarization within each particle relative to the suspending medium, and the resulting electrostatic interaction between the particles lead to the formation of aggregates aligned in the direction of the field [4]. The presence of these particle aggregates results in an increase of viscosity. In general, polarization may arise from various charge transport mechanisms, such as orientation of atomic/molecular dipoles or interfacial polarization, with the latter generally considered as the main contributor to ER behavior under dc or ac fields. This basic model of particle polarization leading to aggregation and increased flow resistance appears to have gained general acceptance [2]. Recently, considerable emphasis has been placed on the development of optimal materials for ER, especially anhydrous materials, which have a broad operating temperature range. Among various polarizable particles for anhydrous ER materials, semiconducting polymers including polyaniline [5, 6], N-substituted copolyaniline [7, 8], sulfonated poly(styrene-co-divinylbenzene) [9], copolystyrene particles with a polyaniline coating [10], poly(aniline-co-o-ethoxyaniline) [11] and poly(acene quinone) radicals [12] have been investigated. Recently, polymer-clay nanocomposites [13, 14] have been adopted as materials for dry-base ER fluids. Poly(p-phenylene) (PPP), which has been extensively studied as a conducting and electroluminescent material, also exhibits ER properties [15]. PPP is a structurally simple conjugated polymer consisting of phenylene rings, and is an insoluble and infusible material. Plocharski et al. [16] pointed out that its ER application seems to be particularly interesting, since it possesses advantages over typical conductive polymers (e.g., moderate conductivity, powder form and insolubility). For PPP treated with anhydrous ferric chloride (FeCl3) dissolved in dry nitromethane, Plocharski et al. [16] observed that the magnitude of the ER effect increases with dielectric constant (when saturation occurs, the dielectric constants exceed 100). Despite the fact that the electrical conductivity of PPP particles doped with FeCl3 in aqueous solution is much less than those doped with FeCl3 in nitromethane solution, doping of FeCl3 in aqueous solution has not been wellinvestigated. Therefore, in this study, we examine the aqueous FeCl3 doped PPP system in different dopant solutions. Semiconductive PPP particles were synthesized from benzene in the presence of anhydrous AlCl3 and Cu2Cl2 following a procedure similar to that of Kovacic and Oziomek [17]. The reaction mixture was maintained at 45 ◦C for 2 h. After polymerization, the mixture of PPP particles and aqueous acid was washed with water and ethyl alcohol to remove Cu2Cl2 and aqueous acid completely. To increase the conductivity of PPP in the semiconductive region for ER application, it was doped with two different aqueous FeCl3 solutions (2.5 and 5.0 wt%) at 25 ◦C for 48 h. After doping, the PPP particles were filtered and dried. It is clear that the concentration of dopant should, in general, influence ER behavior. Indeed, we observed an increase in ER effect with doping level. However, higher doping only leads to higher electric currents during an ER measurement and not to an increase in yield stress [16]. To determine the chemical structure of the synthesized PPP, FT-IR spectroscopy (Bruker, IFS 48) was used (Fig. 1). The 1,4-disubstituted benzene peaks appear at 800 and 1,033 cm−1, and the ring stretching structure peaks appear at 1,478 and 1,403 cm−1. The average particle size of the polymer was measured to be approximately 20 μm using a particle size analyzer (Malvern, MS 20). Furthermore, the conductivity of the pellets for each sample was measured by using a picoammeter (Keithley, Model 487) with a customdesigned cell (2 probe). ER fluids with 10 wt% PPP particles dispersed in silicone oil (kinematic viscosity of 30 cS) were prepared by suspending the mixture using a pearl mill (Shinil Co., Korea). Rheological properties of the ER fluids were measured by a rotational rheometer (Physica, MC-120, Germany) with a Couette-type cylinder equipped with a high