J. Ind. Eng. Chem., Vol. 13, No. 6, (2007) 879-894 REVIEW Electrorheological Properties of Polypyrrole and its Composite ER Fluids † Do-Heyoung Kim and Young Dae Kim Faculty of Applied Chemical Engineering, Chonnam National University, Kwangju 500-757, Korea Received October 31, 2007 Abstract: Electrorheological (ER) fluids are suspensions of polarizable nonconducting or semiconducting par- ticles in a nonconducting continuous phase of low relative polarizability. In the absence of an electric field, they have the properties of suspensions of neutral solid particles. Upon the application of an electric field, an organized structure of particles is formed and the ER fluids exhibit a remarkable change in rheological prop- erties, including a drastic increase in apparent viscosity as well as yield stress. Various mechanisms have been proposed to explain the ER behavior to understand the ER behaviors and design effective ER fluids. Polypyrrole (PPy) is one of the most promising semiconducting polymers because it has higher conductivity and environmental stability than many other semiconducting polymers. PPy and its composites have been ex- tensively used as ER materials and their ER fluids showed promising ER responses. ER properties of PPy based ER fluids (PPy, PPy copolymer, PPy coated particles, and PPy nanocomposites, etc.) and the ER be- haviors of PPy based ER fluids such as shear, yield, and transient stress behavior and additive effect are reviewed. Keywords: electrorheological fluid, electrorheology, semiconducting polymer, polypyrrole, yield stress, semi- conducting polymer composite Introduction The main limitation of ER technology development is a 1) lack of effective ER fluids [11]. Electrorheological (ER) response is defined as the dra- During the past decade there has been an increasing matic change in rheological properties of a suspension of amount of interest in designing effective ER fluids: theo- small particles due to the application of a large electric retically and synthetically. The general requirements of field transverse to the direction of flow. ER fluids are an effective ER fluid are the followings [11,13]: 1) there typically composed of nonconducting or semiconducting should be a marked rheological properties change on the particles dispersed in a nonconducting continuous phase. application of an electric field, 2) the off-field viscosity A large ER effect was first reported by Winslow in 1949 of the ER fluid should be low, 3) the current flow should [1], and has been reviewed in several publications [2-12]. be zero or low to minimize power loss as well as heating The simplicity of engineering designs based on ER mate- effects, 4) there should be a broad operating temperature rials has facilitated the development of specifications for range (hence anhydrous ER fluids have an advantage), 5) a broad range of devices, such as dampers, clutches, and there should be tunability of the particle properties to adaptive structures [12]. Although many ER devices control the ER properties as well as the suspension sta- have been brought successfully to the prototype stage, bility properties, and 6) there should be a strong ER ef- and despite much industrial activity, the anticipated com- fect in both dc and ac fields. mercialization of these devices has yet to be realized. The continuous phase of an ER fluid is usually a non- conducting liquid phase such as insulating oils. In some cases the continuous phase properties strongly affect the † To whom all correspondence should be addressed. ER response [14-16]. Useful continuous phases generally (e-mail: [email protected]) 880 Do-Heyoung Kim and Young Dae Kim have as many of the following properties as possible: 1) pyrrole [38,40] and semiconducting polymer composites high boiling point and low freezing point (in other words, [41-44] have been studied as high-performance anhydrous it should have a wide working temperature range), 2) low ER materials, and they showed superior physical properties, viscosity to keep the viscosity of the ER fluid at a low such as high polarizability and environmental stability. level at zero electric field, 3) high electrical resistance ICP constitute a class of polymers with particular inter- and high dielectric breakdown strength, 4) chemical and est owing to their physical and chemical properties. PPy thermal stability to prevent degradation on storage and is one of the most promising ICP because it has higher service, 5) a high density (particle sedimentation might conductivity and environmental stability in the con- not occur until the densities of both the liquid and the ductive state than many other semiconducting polymers solid match each other), 6) hydrophobicity and low mois- and hence PPy and its composites are extensively used as ture absorbability from the environment, and 7) low tox- ER materials. To design effective ER fluids by employ- icity and low cost [17,18]. ing PPy or PPy derivatives, many research groups fo- Various mechanisms have been proposed to explain the cused on the preparation of semiconducting PPy-based ER response. The inter-electrode circulation proposes composite materials. Heterogeneous semiconducting pol- that the inter-electrode circulation of particles between ymer composites, especially for semiconducting polymer the electrodes, due to the particle charge change by elec- coated organic or inorganic composites and semicon- trochemical reactions at the electrode surface, lead to the ducting polymer-organic or semiconducting polymer-in- ER response [7,19]. The electro-osmosis suggests that organic nanocomposites, have drawn the attention over the ER response arise from the formation of a water last few years, giving rise to a host of various composites bridge between the particles [20]. The surfactant bridge and nanocomposites with interesting physical properties model proposes that surfactants enhance the ER response and important application potential [10,45,46]. at low surfactant concentration by the increased surface In this paper, ER properties of PPy based ER fluids polarization and then lead to the nonlinear ER behavior (PPy, PPy copolymer, PPy coated particles, and PPy due to the increased conduction through the surfactant nanocomposites, etc.) are reviewed. Also, the ER behav- bridge formed between the particles [21,22]. The electric iors of PPy based ER fluids such as shear, yield, and tran- double layer proposes that the origin of the ER response sient stress behavior and additive effectives are reviewed. is the overlap of electric double layers [23,24]. The elec- trostatic polarization model explains that the ER re- sponse arises from the electrostatic interactions between ER Mechanisms the particles due to the field induced polarization of the particles [25-33]. A conduction model proposes that the There are many diverse applications of the ER response. ER effect is determined by the conductivity mismatch Although many ER devices have been brought success- between the particle and liquid phase [34,35]. Among fully to the prototype stage, there are currently no com- these mechanisms, the electrostatic polarization model mercially available devices. The main limitation of ER and conduction model seem to be the suitable ex- technology development is a lack of effective fluids planations for the ER behaviors of semiconducting poly- [11,47]. Of primary importance is the development of mer based ER fluids including PPy based ER fluids. suspensions that can perform desired rheological tasks, Activators are often used to activate suspensions. Some for sufficient duration, with minimum power consump- suspensions display little or no ER activity unless small tion, and with acceptable interactions with environment. amount of water or surfactant is added, while other sus- Solutions to these problems require development of new pensions exhibit a significantly enhanced ER response ER fluids and devices, which in turn require under- with activator present [2,36,37]. Enhancing ER activity standing the mechanisms controlling ER activity. Various with activators such as water severely limits the allow- models or mechanisms were proposed previously to ex- able temperature range of operation, promotes corrosion, plain the observed ER phenomena. Although the electro- and increases power consumption. Therefore, it is neces- static polarization mechanism and conduction model ap- sary to design ER fluids which show a high ER response pear to explain most experimental observations of semi- without the limitations imposed by introducing water conducting polymer based ER fluids, other phenomena based activators. would also influence the ER behaviors of semiconduct- To overcome the limitations (thermal stability and corro- ing polymer based ER fluids under some conditions. sion) of water based systems, dry based systems have been investigated with anhydrous particles. Inherent semi- Inter-Electrode Circulation Model conducting polymers (ICP) are most promising ER materi- Inter-electrode circulation model is based on inter-elec- als among various anhydrous materials. Among them, sem- trode circulation of particles [7,19]. Particles in ER fluids iconducting polymers including polyaniline [38,39], poly- often bear a net charge, and therefore move rapidly to- Electrorheological Properties of Polypyrrole and its Composite ER Fluids 881 ward the oppositely charged electrode in a strong electric The water migrates to the particle surface, forming a wa- field. Once at the electrode, ions within the particle pores