electrorheological properties of polypyrrole and its ... · est owing to their physical and...

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

Introduction1)

Electrorheological (ER) response is defined as the dra-

matic 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 fluids 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 [2-12].

The simplicity of engineering designs based on ER mate-

rials has facilitated the development of specifications for

a broad range of devices, such as dampers, clutches, and

adaptive structures [12]. Although many ER devices

have been brought successfully to the prototype stage,

and despite much industrial activity, the anticipated com-

mercialization of these devices has yet to be realized.

†To whom all correspondence should be addressed.

(e-mail: [email protected])

The main limitation of ER technology development is a

lack of effective ER fluids [11].

During the past decade there has been an increasing

amount of interest in designing effective ER fluids: theo-

retically and synthetically. The general requirements of

an effective ER fluid are the followings [11,13]: 1) there

should be a marked rheological properties change on the

application of an electric field, 2) the off-field viscosity

of the ER fluid should be low, 3) the current flow should

be zero or low to minimize power loss as well as heating

effects, 4) there should be a broad operating temperature

range (hence anhydrous ER fluids have an advantage), 5)

there should be tunability of the particle properties to

control the ER properties as well as the suspension sta-

bility properties, and 6) there should be a strong ER ef-

fect in both dc and ac fields.

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

ER response [14-16]. Useful continuous phases generally

Do-Heyoung Kim and Young Dae Kim880

have as many of the following properties as possible: 1)

high boiling point and low freezing point (in other words,

it should have a wide working temperature range), 2) low

viscosity to keep the viscosity of the ER fluid at a low

level at zero electric field, 3) high electrical resistance

and high dielectric breakdown strength, 4) chemical and

thermal stability to prevent degradation on storage and

service, 5) a high density (particle sedimentation might

not occur until the densities of both the liquid and the

solid match each other), 6) hydrophobicity and low mois-

ture absorbability from the environment, and 7) low tox-

icity and low cost [17,18].

Various mechanisms have been proposed to explain the

ER response. The inter-electrode circulation proposes

that the inter-electrode circulation of particles between

the electrodes, due to the particle charge change by elec-

trochemical reactions at the electrode surface, lead to the

ER response [7,19]. The electro-osmosis suggests that

the ER response arise from the formation of a water

bridge between the particles [20]. The surfactant bridge

model proposes that surfactants enhance the ER response

at low surfactant concentration by the increased surface

polarization and then lead to the nonlinear ER behavior

due to the increased conduction through the surfactant

bridge formed between the particles [21,22]. The electric

double layer proposes that the origin of the ER response

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

the particles due to the field induced polarization of the

particles [25-33]. A conduction model proposes that the

ER effect is determined by the conductivity mismatch

between the particle and liquid phase [34,35]. Among

these mechanisms, the electrostatic polarization model

and conduction model seem to be the suitable ex-

planations for the ER behaviors of semiconducting poly-

mer based ER fluids including PPy based ER fluids.

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 sus-

pensions exhibit a significantly enhanced ER response

with activator present [2,36,37]. Enhancing ER activity

with activators such as water severely limits the allow-

able temperature range of operation, promotes corrosion,

and increases power consumption. Therefore, it is neces-

sary to design ER fluids which show a high ER response

without the limitations imposed by introducing water

based activators.

To overcome the limitations (thermal stability and corro-

sion) of water based systems, dry based systems have been

investigated with anhydrous particles. Inherent semi-

conducting polymers (ICP) are most promising ER materi-

als among various anhydrous materials. Among them, sem-

iconducting polymers including polyaniline [38,39], poly-

pyrrole [38,40] and semiconducting polymer composites

[41-44] have been studied as high-performance anhydrous

ER materials, and they showed superior physical properties,

such as high polarizability and environmental stability.

ICP constitute a class of polymers with particular inter-

est owing to their physical and chemical properties. PPy

is one of the most promising ICP because it has higher

conductivity and environmental stability in the con-

ductive state than many other semiconducting polymers

and hence PPy and its composites are extensively used as

ER materials. To design effective ER fluids by employ-

ing PPy or PPy derivatives, many research groups fo-

cused on the preparation of semiconducting PPy-based

composite materials. Heterogeneous semiconducting pol-

ymer composites, especially for semiconducting polymer

coated organic or inorganic composites and semicon-

ducting polymer-organic or semiconducting polymer-in-

organic nanocomposites, have drawn the attention over

last few years, giving rise to a host of various composites

and nanocomposites with interesting physical properties

and important application potential [10,45,46].

In this paper, ER properties of PPy based ER fluids

(PPy, PPy copolymer, PPy coated particles, and PPy

nanocomposites, etc.) are reviewed. Also, the ER behav-

iors of PPy based ER fluids such as shear, yield, and tran-

sient stress behavior and additive effectives are reviewed.

ER Mechanisms

There are many diverse applications of the ER response.

Although many ER devices have been brought success-

fully to the prototype stage, there are currently no com-

mercially available devices. The main limitation of ER

technology development is a lack of effective fluids

[11,47]. Of primary importance is the development of

suspensions that can perform desired rheological tasks,

for sufficient duration, with minimum power consump-

tion, and with acceptable interactions with environment.

Solutions to these problems require development of new

ER fluids and devices, which in turn require under-

standing the mechanisms controlling ER activity. Various

models or mechanisms were proposed previously to ex-

plain the observed ER phenomena. Although the electro-

static polarization mechanism and conduction model ap-

pear to explain most experimental observations of semi-

conducting polymer based ER fluids, other phenomena

would also influence the ER behaviors of semiconduct-

ing polymer based ER fluids under some conditions.

Inter-Electrode Circulation Model

Inter-electrode circulation model is based on inter-elec-

trode circulation of particles [7,19]. Particles in ER fluids

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

field. Once at the electrode, ions within the particle pores

may migrate out of the particle or the particle surface

may undergo electrochemical reactions, the result in ei-

ther case being that the particle charge can change sign.

The particle will then move rapidly toward the oppositely

charged electrode. As this process is repeated con-

tinually, the back and forth motion of the particles be-

tween the electrodes generates a secondary flow and

hence an additional mode of energy dissipation, resulting

in an increased suspension viscosity. The rapid circu-

lation of particles between the electrodes has been ob-

served for dilute suspensions in large dc electric fields,

but this motion disappears as the concentration is in-

creased [48] or when an alternating electric field of suffi-

cient frequency is applied. As ER activity is still ob-

served under these conditions, the inter-electrode circu-

lation of particles cannot produce the ER response.

Electric Double Layer Model

Another mechanism proposed as the origin of the ER

response is the overlap of electric double layers [23,24].

The model was primarily proposed to explain why water

played a key role in the ER response and why the ER ef-

fect could take place on a millisecond time scale. The fi-

brillation process, proposed by Winslow [1], was thought

to be rather slow compared with the ER response time

[23,24] and thus the fibrillation model seems to be in-

adequate to describe ER phenomena. As mentioned

above, particles in ER fluids tend to bear a net charge. As

a result, each particle is surrounded by a diffuse counter

ion cloud balancing the particle charge (an electric dou-

ble layer). Under the applied field, this counter ion cloud

distorts and overlaps with the counter ion clouds of

neighboring particles. This enhances the electrostatic re-

pulsion between particles which must be overcome in or-

der for particles to flow past one another, giving rise to

the ER response. A more refined model was developed

by Uejima [49] and by Deinega [7]. One criticism of this

mechanism is that, in typical ER fluids, the thickness of

the double layer is often greater than the distance separat-

ing the electrodes [11]. No quantitative theory based on

this mechanism has been developed, but as electric dou-

ble layer distortion is a polarization phenomenon, this

mechanism is simply a special case of the electrostatic

polarization model discussed below.

Electro-Osmosis Model

One proposed mechanism is based on electro-osmosis

[20,50,51]. Most ER fluids are composed of porous par-

ticles suspended in a nonaqueous fluid. Within the par-

ticle pores are water-solvated ions. Upon application of

the electric field, the ions respond by moving toward the

oppositely charged electrode, carrying water with them.

The water migrates to the particle surface, forming a wa-

ter bridge with any particle with which it contacts. This

electro-osmosis model suggests that the water bridges

must be broken (overcome interfacial tension) to make

the suspension flow, giving rise to the ER response. This

theory has received support because most ER fluids do

contain some mobile ions and it has been shown that

many suspensions require added water in order to ob-

serve an ER response [23,52,53]. See and coworkers [56]

expanded the suggestion to a multiple water bridge for-

mation model using a condenser concept and proposed

that the water-enhanced ER behavior arises from the sum

of surface tensions of multiple water bridges. However,

recent experiments have been reported on ER fluids that

display a significant response while being essentially an-

hydrous [36,54,55]. These results have caused serious

doubt to be expressed about the electro-osmosis model.

Surfactant Bridge Model

Surfactants are added to ER fluids for a variety of rea-

sons and can be used to tailor suspension properties [3,4,

7,36,57-59]. They are often used to promote colloidal

stability and to control rheological properties. Surfactants

are also used to activate suspensions [57,58]. Surfactant

influences the ER response in two different ways. At

small surfactant concentrations, it enhances the ER re-

sponse by enhancing the particle polarizability; at large

concentrations, the response degrades (nonlinear ER re-

sponse). The ER enhancement at small surfactant con-

centrations arises from the enhanced interfacial polariza-

tion. The nonlinear ER response arises from the for-

mation of surfactant-rich phase between particles in-

duced by the applied electric field [57]. The surfactant

bridge model proposes that surfactants added as an addi-

tive enhance the ER response at low surfactant concen-

tration by the increased surface polarization and then

lead to the nonlinear ER behavior due to the surfactant

bridge formed between the particles [21,22]. The model

successfully predicted the ER behavior of surfactant-acti-

vated ER fluids. The model was expanded to predict the

electric field frequency effect on the surfactant activated

ER behavior [60].

Electrostatic Polarization Model

Winslow [1] proposed the fibrillation model based on

the observation that the fibrillated chains were formed

between the electrodes in the ER fluid under an applied

electric field. The fibrillated chains were formed as par-

ticles polarized under the applied electric field and

aligned as an induced dipole along the direction of the

electric field. The interaction force between the polarized

particles increased dramatically with the increasing elec-

tric field strength, resulting in the obvious ER effect.

This model is a kind of electrostatic polarization model


because the particle polarization is important.

The best supported explanation of the ER response is

given by the electrostatic polarization mechanism. In this

model, an ER fluid is assumed to consist of a dispersed

and continuous phase, each composed of a different di-

electric material. Particles polarize under the applied

electric field, to a different extent than the continuous

phase, making the particles appear to a first approx-

imation as dipoles aligned with the applied electric field.

The interaction between dipoles is such that they prefer

to align head-to-tail, forming particle chains that span the

electrode gap. This theory claims that the ER response

arises from the necessity of breaking the particulate

chains (overcoming the electrostatic interactions between

particles) to make the suspension flow. The electrostatic

force on a dielectric particle was found to be dependent

on the dielectric constant mismatch between the particle

and continuous medium [25-33] and scales as

F = 12πεoεc a2β

2E

2 (1)

where εo is the permittivity of free space, εc is the dielec-

tric constant of the continuous phase, a is the particle ra-

dius, E is the applied electric field strength, and β = (εp-

εc)/(εp + 2εc) is the relative polarizability of the particle

where εp is the particle dielectric constant. Hence, the

yield stress should vary with the square of the electric

field strength as commonly observed experimentally

[3,4,7,11,36,61]. This explanation was suggested by

Winslow in his original report [1], and is the theory on

which several modeling studies are based [25,48, 62-65].

It is supported by the observations that particles indeed

tend to form chains that span the electrode gap and that

yield stresses are often found to vary with E2 [1,48,62,

66,67]. The major criticism has been that it cannot pre-

dict the rapid response time observed experimentally.

Indeed, dipolar forces between micron sized particles can

give rise to relatively slow phenomena [36]. However, it

has been shown that the characteristic response time pre-

dicted by this theory is in agreement with experimental

observations [48,62]. The electrostatic polarization mod-

el, which was based on the idealized ER fluid system,

was modified extensively to compensate its many limi-

tations due to the idealization. If the applied electric field

is dc or low frequency ac, the conductivity mismatch be-

tween the particle and liquid phase was considered to be

a main factor rather than the dielectric constant mismatch

[28,31,33]. A thorough overview of the calculated results

on the basis of various slightly different polarization

models was given by Parthasarathy and Klingenberg [5].

Conduction Model

The electrostatic polarization model sometimes fails to

explain the ER behavior of some ER fluids, especially if

the particle and liquid phase are conductive. If the con-

tinuous phase dissociates under very high electric field

strengths, the fluid between the particles becomes con-

ductive allowing current flow as the gap between the par-

ticles decreases since the applied electric field is lo-

calized on the fluid between the particles. As a result, the

electric response of the fluid between the particles be-

comes to show electric breakdown or particle discharge

under the high electric field strength. This phenomenon

would lead to a decreased ER behavior which deviates

from the electrostatic polarization model. Atten [34] and

Foulc [35] proposed a conduction model, where the ER

effect was determined by the conductivity mismatch be-

tween the particle and liquid phase, which is dominant

factor for the dc and low frequency ac excitation, rather

than the dielectric mismatch. Tang [68] and Wu [69] ex-

tensively developed this model further. The conduction

model could explain some ER phenomena, especially for

the semiconducting polymer based ER fluids, that the

electrostatic polarization model fails to explain. However,

some experimental results provide evidence against this

model [61]. Furthermore, Khusid and Acrivos [71] noted

that the conduction model could be suitable for the static

situation only where the suspension microstructure had

been fully formed and could not give an explanation of

the dynamic phenomena.

Dielectric Loss Model

The dielectric loss model was proposed by Hao and

coworkers [6,72-76] to explain their experimental results.

This model is an extended electrostatic polarization

model. Two dynamic processes were emphasized in this

model. The first step was the particle polarization proc-

ess, in which the particle dielectric constant was do-

minant. The second step was particle turning, i.e. the po-

larized particle could have the capability to align along

the direction of the electric field. This step was de-

termined by the particle dielectric loss. The second step

was the most important one, which distinguished the ER

particle from non-ER particle. Both the ER particle and

non-ER particle could be polarized under an electric

field, however, the ER particle could re-orientate along

the electric field direction, building the fibrillated bridges

between two electrodes. The non-ER particle does not

have such ability.

PPy as an ER Material

ER fluids are suspensions of polarizable nonconducting

or semiconducting particles in a nonconducting con-

tinuous phase of low relative polarizability [6-10,40,43,

44,53,77]. Activators are often used to activate suspen-

sions. Enhancing ER activity with activators such as wa-


ter severely limits the allowable temperature range of op-

eration, promotes corrosion, and increases power con-

sumption. To overcome the limitations of water based

systems, dry based systems have been investigated with

anhydrous particles. Anhydrous ER fluids using polymer

particles [78], inorganic-organic composite particles [85],

and semiconducting polymer particles [38,78-84] were

reported. Recently, semiconducting polymer coated in-

organic or organic composites particles (polyaniline-

coated inorganic particles [83,86]) and semiconducting

polymerinorganic nanocomposites [80,84] were used for

ER fluids and they showed promising ER responses.

Semiconducting polymers are usually intractable materi-

als particularly in the doped semiconducting state.

Dispersions of the semiconducting polymers in aque-

ous/non aqueous media have attracted considerable re-

search interest. Among the semiconducting polymers, by

far the largest amount of research thrust appears to have

been directed to PPy based composite systems [54,55,

87-97]. The prospect of developing an effective ER fluid

using PPy based materials is bright since the electrical

and physical properties of PPy based material can be

easily controlled.

Synthesis

During the past decade, various researchers have de-

scribed the preparation of sterically stabilized colloidal

dispersions of air stable intrinsically semiconducting pol-

ymers such as PPy by several methods [10]. The syn-

thesis of sterically stabilized polymer particles via dis-

persion polymerization was achieved in aqueous medium.

Polyvinylalcohol-stabilized particles of PPy have consid-

erable potential as ER materials [43] and novel marker

particles in immunodiagnostic strip assays [98]. Synthesis

of colloidal PPy-particles was achieved using a tai-

lor-made reactive copolymer stabilizer based on 2-

(dimethylamino) ethylmethacrylate [99].

Incorporation of inorganic particles inside the core of

organic polymers became a popular and interesting meth-

od for preparation of polymer based nanocomposites dur-

ing the last decade. PPy coated SiO2 [100] and PPy coat-

ed SnO2 [101] nanocomposites were prepared by in-situ

polymerization of water soluble pyrrole in stirred sol-

utions containing FeCl3 and the respective nano-oxide.

Methylcellulose stabilized PPy coated SiO2 [102] and

SnO2 [103] nanocomposites were prepared by in-situ pol-

ymerization of water soluble pyrrole in stirred solutions

containing FeCl3⋅6H2O, the respective nano-oxide, and

methylcellulose and used as ER materials [102,103].

PPy-MnO2 [104] and PPy-Al2O3 [105] nanocomposites

were prepared via polymerization of pyrrole by using ox-

idants such as FeCl3 in aqueous medium in which re-

spective nano-metal oxide was suspended.

Numerous attempts appeared to have been made for the

modification of one (semiconducting or nonconducting)

polymer with another semiconducting polymer. The ba-

sic idea was to prepare a composite that would possess

the combined properties of either polymer components.

Binary polymer composites of PNVC-PPy [106] were

prepared by simultaneous polymerization of a mixture of

water insoluble monomers in a solvent like THF and an

aqueous metal oxide suspension in the presence of an

oxidant. The mixed polymer composites of PPy-(PNVC-

Al2O3) [105], PPy-(PAN-SiO2) [107], and PPy-(PMMA-

SiO2) [108] were subsequently obtained by adding non-

aqueous solution of a preformed polymer with sonication

or by polymerization of monomer in the medium.

Clays are the most abundant minerals and available as

inexpensive materials that have high physical and me-

chanical strengths as well as high chemical resistance

[10]. An inverted emulsion pathway was developed to

prepare PPy-clay nanocomposites by using dodecylben-

zenesulfonic acid (DBSA) and used as ER materials

[80,91]. A highly soluble PPy was produced [109] by an

in-situ polymerization method using Na+DEHS

- in water

and an aqueous APS solution. PPy-(PF-MMT) and PPy-

(PTP-MMT) composites were recently obtained by add-

ing pyrrole monomers to preformed PTP-MMT and

PF-MMT composites in HCl solution and in aqueous sol-

ution respectively and used as ER materials [88].

During recent years a great deal of research interest was

paid to studies on carbon based nanocomposites of vari-

ous polymers which exhibit interesting bulk properties.

After the discovery of CNTs the macromolecular analogs

of fullerene research on CNT containing polymer nano-

composites intensified in the global context. PPy-CNT

composite nanowires [110] were prepared by a template

directed electrochemical synthetic route involving plat-

ing of PPy into the pores of the host membrane.

PPy based ER Fluids

PPy and PPy Copolymer

PPy and its derivative are used as ER materials, as it

can be easily prepared with a controllable conductivity

using conventional chemical and electrochemical methods.

The possibilities of using PPy as an ER material were re-

ported [13,17,40,87]. The ER fluids of PPy particles in

silicon oil showed a significant ER effect only when the

particle conductivity was within a certain range. The ER

effect increased with particle volume fraction and electric

field strength, but decreased with the increasing shear

rate [87].

Figure 1 shows the scanning electron microscope (SEM)

image of PPy particles [40]. The PPy particles were

synthesized by chemical polymerization according to the

method reported by Kudoh [111]. The particle shape is


Figure 1. SEM micrographs of the PPy particles. The PPy par-

ticles were synthesized by chemical polymerization using so-

dium p-toluene sulfate as the surfactant and ammonium persul-

fate was used as the oxidant [40].

irregular but almost spherical, and the particles present

in aggregates. The PPy particle size is around 300 nm.

The average diameter of the PPy aggregates in mineral

oil was 53 µm, indicating that the PPy particles in the

ER dispersion were present as large aggregates. The µm

sized PPy agglomerate morphology was also reported for

the PPy prepared by the polymerization of pyrrole in

strongly acidic conditions using HF, HCl, HBr, and

HNO3 [38].

The ER behavior of the ER fluids of PPy particles syn-

thesized by cationic addition polymerization showed that

continuous phases affected strongly their ER behavior. It

was reported that trioctyltrimellitate showed promising

ER effect among the continuous phases of silicone oil,

mineral oil, trioctyltrimellitate, dioctylphthalate, and ma-

rlotherm-s [14].

The ER fluids prepared by suspending PPy particles

synthesized by pyrrole polymerization in strongly acidic

conditions using HCl and HBr in 1-chloronaphthalene-1-

bromonaphthalene showed an electric field dependent

ER behavior. While the ER fluids prepared from the PPy

prepared using HF and HNO3 did not exhibit an ER re-

sponse [38].

Goodwin and coworkers [112] studied the ER behaviors

of ER fluids prepared by using a series of copolymer par-

ticles synthesized from pyrrole and N-methylpyrrole. The

ER fluids were prepared by suspending the particles sta-

bilized by a graft copolymer with poly(12-hydroxystearic

acid) as the stabilizing moieties in dodecane. The de-

pendence of the static yield stress on electric field

strength was E2 at the higher volume fractions, while the

static yield stress was proportional to Em where m is

slightly higher than 2 at low volume fractions.

The dependence of the dynamic yield stress on oxidant

amount (ammonium persulfate) is presented in Figure 2

for 1 wt% PPy dispersions in mineral oil (ηc = 180 cP,

Figure 2. Dynamic yield stress as a function of the oxidant

amount for 1 wt% PPy dispersions of various oxidant amounts

in mineral oil. The PPy particles were synthesized by chemical

polymerization using sodium p-toluene sulfate as the surfactant

and ammonium persulfate was used as the oxidant [40].

ρc = 850 kg/m3) [40]. The dynamic yield stress was de-

termined by extrapolating the shear stress-shear rate data

to zero shear rate. The yield stress increases with the oxi-

dant amount, passes through a maximum, and then de-

creases with the oxidant amount. The same ER behaviors

of the increasing and then decreasing yield stress show-

ing a maximum with the increasing oxidant amount were

reported for PPy coated polyethylene ER fluids [43] and

PPy coated polyethylmethacrylate ER fluids [113].

Also, the yield stress increases with the electric field

strength and is proportional to E1.8

. At the oxidant

amount of 0.09 mol, the decrease in the ER response is

very significant. Furthermore, at oxidant amounts larger

than 0.09 mol, the ER measurements could not be per-

formed because particle strands formed between the elec-

trode gap acted as a short circuit in the applied electric

field. The conductivities of the PPy particles increased

with the increasing oxidant amount. Therefore, the yield

stress increase at low oxidant amounts arises from the en-

hanced particle polarization due to the increased PPy

conductivity. The decrease in the ER response at large

oxidant amounts seems to arise from the increased con-

duction between the PPy particles [40].

PPy Coated Particles

PPy was coated on polymer particles to enhance the

particle polarization by increasing the particle surface

conductivity, which would lead to an enhanced ER

response. It was reported that the increased particle sur-

face conductivity enhanced the particle polarization and

hence increased the ER response [21,22,57,59,60]. In-

vestigations showed that the coating should be a materi-

al of high dielectric constant, high electrical breakdown


(a)

(b)

Figure 3. SEM micrographs of (a) the PEMA particles (× 200)

and (b) PPy-coated PEMA particles (× 200) [113].

strength, and reasonable level of conductivity, which is

used to increase the density of electrostatic energy [86,

114-121], suggesting the use of semiconducting poly-

mers for the coating materials.

PPy-coated polymer particles were synthesized by the

pyrrole polymerization on polymer particles by control-

ling the amount of pyrrole or oxidant [43,113]. Figure 3

shows the SEM images of the polyethylmethacrylate

(PEMA) (Figure 3(a)) and PPy-coated PEMA particles

(Figure 3(b)). Compared to the PEMA particles, the PPy-

coated PEMA particles show PPy coverage on the par-

ticle surfaces. The average diameters of PEMA particles

and PPy-coated PEMA particles were 40 and 43 µm, re-

spectively [113].

The dependence of the dynamic yield stress on electric

field strength is presented in Figure 4 for 10 wt% PPy-

coated polyethylene (PE) suspensions of various PPy-

coated particles. The PPy-coated PE particles were syn-

thesized by using various amounts of pyrrole and 1.5 g of

FeCl3⋅6H2O [43]. Symbols represent experimental data

and lines indicate the linear regression of the data.

Compared to the yield stresses of the uncoated PE sus-

Figure 4. Yield stress as a function of electric field strength for

10 wt% various PPy-coated PE suspensions in mineral oil

(FeCl3⋅6H2O = 1.5 g, symbol: pyrrole amount, n is the slope

of the regression line) [43].

pension, those of the PPy-coated PE suspensions are

greatly enhanced by coating PPy on the PE particles.

The yield stresses increase with the increasing pyrrole

amount. The increase in the ER response with pyrrole

amount is due to the enhanced particle polarization with

the increasing particle surface conductivity. Lascelles

and coworkers [122] also reported that the conductivity

of PPy-coated polystyrene particles and the PPy coating

thickness increased with the amount of pyrrole during the

pyrrole polymerization. The linear regression lines in

Figure 4 show that the yield stress is fitted with E2 when

the amount of pyrrole is less than 0.075 g, consistent

with the electrostatic polarization. At larger pyrrole

amounts, the yield stress is proportional to En where n <

2. The value of n decreased with the increasing pyrrole

amount. This behavior arises from the increased con-

duction between the PPy-coated PE particles due to the

increased particle surface conductivity at large pyrrole

amounts. However, this phenomenon is different from

the nonlinear conduction [34,35,68,69] in that the in-

creased conduction arises from the high particle surface

conductivity, not from the field dissociation of the con-

tinuous phase [34,35,68,69]. Even at the pyrrole amount

of 0.3 g, strands of coated particles short-cut the circuit

in the electric fields and the ER experiments could not be

performed. Similar ER behavior and yield stress depend-

ence on the electric field strength were reported for PPy-

coated PEMA ER fluids [113].

The yield stress of PPy-coated polymer based ER fluids

depended on oxidant (FeCl3⋅6H2O) amount during

synthesis. The yield stress initially increased with the ox-

idant amount, passed through a maximum, and then de-

creased with the oxidant amount [43,113]. The increase


Figure 5. Yield stress as a function of electric field squared for

10 wt% PPy-coated PE and double coated PPy suspensions in

mineral oil [43].

in the ER response with oxidant amounts was explained

by the enhanced particle polarization with the increased

particle surface conductivity. The decrease in ER re-

sponse at large oxidant amounts was explained by the in-

creased conduction between the PPy-coated polymer

particles. It was noted that the yield stress was propor-

tional to E2 at lower oxidant amounts, but proportional to

En (n < 2) at higher oxidant amounts. Also, the value of n

decreased with the increasing oxidant amount. As the

conduction between the particles increased, the effective

electric field between the particles decreased, leading to

the decreased ER response. As a result, the ER response

and the value of n decreased as the conduction between

the particles increased.

Double Coated PPy Particles

The decreased ER response with the increased con-

duction between the particles was observed, even though

the increased surface conductivity still enhanced the par-

ticle polarization [40,43,57,102,103,113]. The decreased

ER response would be prevented if the increased con-

duction between the PPy-coated PE particles could be

restricted. Double coated PPy particles were used as an

ER material to restrict the increased conduction between

the particles and thereby to enhance the ER response

[43]. Double coated PPy particles were prepared by coat-

ing poly(vinyl alcohol) PPA on the PPy-coated PE par-

ticles (3.0 g FeCl3⋅6H2O), which showed the decreased

ER response due to the increased conduction. The de-

pendence of the yield stress on electric field squared is

presented in Figure 5 for 10 wt% PPy-coated PE suspen-

sions and its double-coated PPy suspension. For compar-

ison, the ER response of the PPy-coated PE suspension

of the FeCl3⋅6H2O amount of 1.5 g, which showed the

most enhanced ER response, was included. The ER re-

sponse of the double coated PPy suspension is greatly

enhanced compared to that of the PPy-coated PE suspen-

sion. The ER response is even higher than that of the

PPy-coated PE suspension of the FeCl3⋅6H2O amount

of 1.5 g.

PPy-inorganic Composites

Neither inorganic nor polymeric materials have perfect

material properties as ER materials. Inorganic-polymer

composites might show the advantages of both the in-

organic and the polymeric materials, showing an en-

hanced ER effect and dispersing stability. Theoretical in-

vestigations showed that the outer coating should be a

material of high dielectric constant, high electrical break-

down strength, and reasonable level of conductivity,

which is used to increase the density of electrostatic en-

ergy [86,114-121].

The yield stress and current density of the PPy-Na+-

montmorillonite nanocomposite ER fluids were found to

increase with electrical field strength, where nanocom-

posites of PPy with inorganic Na+-montmorillonite (MMT)

clay were synthesized using DBSA as both a dopant and

an emulsifier [80,88]. The ER behavior of the PPy/clay

nanocomposite ER fluid was investigated, where an in-

verted emulsion pathway was employed to synthesize

PPy into a layer of inorganic clay within a nano level us-

ing DBSA as both an emulsifier and a dopant. The static

yield stress of the PPy/clay nanocomposite ER fluid was

proportional to E1.5

[89,123]. PPy encapsulated in the

channels of mesoporous silica (MCM-41) was synthe-

sized. ER and dielectric properties of PPy-MCM-41

based ER fluids showed that the PPy-MCM-41 ER fluid

exhibited better ER behavior than that without PPy [93,

124]. PPy-SBA-15 nanocomposites in which PPy was

confined in ordered mesoporous silica SBA-15 channels

were synthesized by an in-situ polymerization technique.

PPy-SBA-15 nanocomposite ER fluids in silicone oil dis-

played notable ER characteristics under external electric

fields [125].

Sterically stabilized PPy-inorganic nanocomposite par-

ticles were prepared to improve the ER response by en-

hancing the particle properties by forming inorganic-

semiconducting polymer hybrid nanocomposite between

inorganic and PPy and sterically stabilizing the particles.

PPy-SiO2-methylcellulose and PPy-SnO2-methylcellu-

lose nanocomposite particles were synthesized by con-

trolling the ratio of pyrrole, SiO2 or SnO2, and methyl-

cellulose amounts [102,103]. The ER response of the

PPy-SnO2-methylcellulose nanocomposite suspensions

increased with the increasing SnO2/pyrrole ratio and also

depended on the amount of methylcellulose amount,

showing a maximum ER behavior [103].


Figure 6. Yield stress as a function of volume fraction for PPy-

silica-methylcellulose nanocomposite suspensions under vari-

ous electric field strengths (silica/pyrrole weight ratio during

the polymerization = 7.0) [102].

The dynamic yield stress as a function of particle vol-

ume fraction is presented in Figure 6 for PPy-SiO2-

methylcellulose nanocomposite suspensions under vari-

ous electric field strengths [102]. The PPy-SiO2- methyl-

cellulose nanocomposite particles were polymerized with

the SiO2/pyrrole weight ratio of 7.0 and 0.15 g methyl-

cellulose. A power-law dependence on the volume frac-

tion τo = Kϕm fits adequately the dependence of the yield

stress on the particle volume fraction. ϕ is the particle

volume fraction and K is the electric field strength de-

pendent constant. The values of m are 1.23, 1.24, 1.38,

and 1.54 for E = 500, 1000, 1500, and 2000 V/mm, re-

spectively, increasing with the electric field strength. The

value of m is larger than 1 and increases with the electric

field strength. Block and coworkers [79] also showed

that the value of m was larger than 1 and increased with

the electric field strength. According to the electrostatic

polarization model and the conduction model, the yield

stress is proportional to the volume fraction [5]. Varia-

tion of the value of m is probably related to the structure

change with the electric field strength. As the particle

volume fraction increases, structure formed between the

electrodes is more complex than an ideal chain structure

(particles would form cluster).

ER Behaviors of PPy System

Shear Stress

The ER behaviors under various electric field strengths

are presented in Figure 7 for a 10 wt% PPy-coated PE

suspension in mineral oil [43]. Without an electric field,

the suspension behaves like a Newtonian fluid with the

Figure 7. Shear stress as a function of shear rate for 10 wt%

PPy-coated PE suspension in mineral oil (pyrrole = 0.1 g and

FeCl3⋅6H2O = 1.5 g) [43].

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 in-

crease 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 response-an apparent yielding phenomenon at

low shear rates and shear thinning behavior approaching

a constant viscosity at large shear rates. 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. This anomalous behavior was

observed for almost all PPy based ER fluids; PPy/clay

nanocomposite ER fluid [89], PPy-MCM-41 ER fluid

[93], PPy-SBA-15 nanocomposite ER fluid [125], and

PPy/Na+

montmorillonite nanocomposite ER fluid

(MMT) [80]. See and coworkers [90] reported that a de-

creased in the shear stress with an increasing shear rate

only occurred under dc current electric fields

Marshall and coworkers [126] showed that the depend-

ence of suspension viscosity on electric field strength and

shear rate could be combined into a single curve in terms

of the Mason number, Mn = ηc/2εoεc (βE)

2 and the

rheological data were correlated with the Bingham con-

stitutive equation

∞

∞

(2)

Here, η∞ is the high shear rate viscosity of the suspen


Figure 8. Relative suspensions viscosity as a function of Mason

number at several electric field strengths [43].

sion under no electric field, and Mn* is a material prop-

erty of the suspension depending on dielectric properties

and volume fraction. Mn is a measure of the relative im-

portance of hydrodynamic and polarization forces.

As shown in Figure 8, the data in Figure 7 reduce to a

single linear curve with the slope of 1.0 well approxi-

mated by the equation (2). Mn* is found to be 0.48 by

performing a least-squares fit of the data. At low shear

rates (Mn ≪ 1), polarization forces are dominant over

hydrodynamic forces. The stress is determined by polar-

ization forces and the shear stress is independent of shear

rate, showing a plateau (refer to Figure 7). At large shear

rates (Mn ≫ 1), hydrodynamic forces are dominant.

Therefore, the stress arises from purely hydrodynamic

forces and the suspension viscosity is independent of the

electric field strength, leading to a Newtonian behav-

ior-the shear stress is proportional to shear rates and sus-

pension viscosities at various electric fields approach to

η∞.

When polarization forces and hydrodynamic forces are

comparable (e.g., at the intermediate shear rates [Mn*/Mn

≈1]), they might influence indirectly each other, leading

to a synergistic or negative synergistic interaction. The

(a) (b) (c)

Figure 9. Configurations of 1 wt% PPy-coated PEMA particles in mineral oil at (a) E = 0 V/mm, (b) E = 500 V/mm, and (c) E = 1000

V/mm [113].

indirect influence seems to arise from the dynamics of

structural rearrangements. At low Mn, the stress arises

from breaking particle strands between the electrode gap.

With increasing shear rate, hydrodynamic forces begin to

influence the structure of particle strands, forming par-

ticle strand aggregates due to the rearrangements of par-

ticle strands. The formation of particle strand aggregates

at the intermediate shear rates may cause the negative

synergistic interaction, leading to the shear stress de-

crease with shear rate. The change in the shear stress at

the intermediate shear rates was referred as forming

small strand-like aggregates [127] or swirling motion

[81,128]. Since anomalous behavior occurs when Mn ≈

Mn*, we can estimate the critical shear rate,

= 2εoεc

(βE)2Mn

*/ηc, at which the shear stress shows the anom-

alous behavior. The values of c were estimated in the

range of 9∼90 s-1

and increased with the electric field

strength.

It was also observed 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. The electric

field-induced alteration of the suspension structure was

presented in Figure 8 for 1 wt% PPy-coated PEMA par-

ticles in mineral oil [113]. Without the applied electric

field, the suspension shows random particle config-

uration (Figure 9(a)). When an electric field is applied,

particles are polarized due to the imposed electric field

and form particle strands spanning between the electro-

des due to the dipole interactions between the particles

(Figures 9(b) and (c)). Compared to the particle strands

under E = 500 V/mm (Figure 9(b)), the particle strands

under E = 1000 V/mm are more uniform and completely

spanning between the electrodes, indicating that the ER

response will increase with the increase in the electric

field strength.

Goodwin and coworkers [112] studied the ER behaviors

of ER fluids prepared by using a series of copolymer

particles synthesized from pyrrole and N-methylpyrrole.

They described the ER behavior of PPy and N-methyl

PPy copolymer ER fluids by the Bingham model and the

high shear behavior by the Dougherty-Krieger equation.


Figure 10. Fitting of model equations to flow curves of poly-

pyrrole/clay based ER fluids for two different electric fields.

The dashed line and the solid line are from Bingham model and

our proposed model at two different electric field strengths of

2.5 kV/mm and 1.5 kV/mm, respectively [123].

By combining the Bingham equation and the Dougherty-

Krieger equation, the following constitutive equation was

obtained

τ = τy + ηc

(3)

where τy is the yield stress, ηc is the viscosity of the con-

tinuous phase, [η] is the intrinsic viscosity, ϕ is the par-

ticle volume fraction, ϕm is and the maximum packing

fraction, and is the shear rate. They showed that over the experimental range of shear rates, the combined mod-

el provided an adequate description of the experimental

data. The dependence of the static yield stress on field

was E2 at the higher volume fractions. However, this

model has a disadvantage for applying to the semi-

conducting polymer based ER fluid system since it can-

not predict the anomalous behavior where the shear

stress decreases with shear rate.

There have been many quantitative analyses used to de-

scribe both the yield stress and shear stress behaviors

[129-134,140], but only a few reports describe the con-

stitutive equation [132,134]. It is prevailing that the ob-

tained shear stress often exhibited complicated behavior

[133]. In addition, many of the reported constitutive

equations are too complicated for use.

Fang and coworkers [123] studied the ER behavior of

polypyrrole/clay nanocomposite-based ER fluids. They

analyzed the flow curves of the ER fluids using the mod-

el constitutive rheological equation of state suggested by

Cho and coworkers [134] for analyzing the ER fluids un-

der an applied electric field more comprehensively as

follows

τ =

∞

(4)

where t1, t2, α, and β are constants. Figure 10 shows

that the data obtained in both the high and low shear rate

ranges picked in reference [89] were fitted very accu-

rately by the proposed empirical constitutive equation

model, suggesting that this model can successfully pre-

dict the typical anomalous behavior of semiconducting

polymer based ER fluids where the shear stress decreases

with shear rate.

Yield Stress

Assuming pairwise additivity and only nearest neighbor

interactions, the dynamic yield stress can be represented

for the electrostatic polarization model as [5]

τ = 18ϕεoεcβ2E

2 fm

(5)

where ϕ is the particle volume fraction, l is the electrode

separation, fm is the maximum in the dimensionless re-

storing force, and θm is the angle at the maximum. fm and

θm are functions of only εp/εc. This result shows that the

dynamic yield stress increases quadratically with the

electric field strength. The estimated dynamic yield

stresses of the PPy-coated PE ER fluids were not com-

parable to the experimental data and the discrepancy be-

tween the experimental and estimated value increased

with the increasing pyrrole amount [43]. The under-

estimation of the dynamic yield stress could arise from

the neglect of the multiple interactions between the par-

ticles in the equation (5) and the nonlinear conduction

between the semiconducting particles. Therefore, the ap-

plication of the yield stress of the electrostatic polar-

ization model is somewhat limited for semiconducting

polymer based ER fluids.

The conduction model of Felici and coworkers [135]

was extended by Davis and Ginder [136] to determine

the static yield stress of the single chain model by con-

sidering the influence of electric field-dependent fluid

conductivity. If the applied electric field strength was

large enough that the electric field strength in the inter-

particle gap was limited by non-linear conduction, the

static yield stress was given by

τs =

εε

(6)

where Em is the maximum electric field strength in the in-

terparticle gap, equivalent to the fluid’s breakdown

strength. Yield stresses appeared to approach E3/2

at large

electric field strength. Felici and coworkers [135] sug-

gested the value of Em as 30∼40 kV/mm. Almost all of


Figure 11. Transient shear stress behavior at the shear rate of

0.1 s-1

for 10 wt% PPy-coated PE suspensions in mineral

(FeCl3⋅6H2O = 1.5 g, ■ : E = 1500 V/mm, Θ: E = 2000

V/mm; the inset figure is for FeCl3⋅6H2O = 0.75 g) [44].

PPy-based ER fluids showed the nonlinear behavior (τy

∝ En, 1 < n < 2) [40,43,102,103,113,125], supporting

that the conduction could predict the yield stress behav-

iors of the PPy-based ER fluids.

Choi and coworkers [92] developed a hybrid yield

stress equation by extending the conduction model. To

represent the yield stress data for a broad electric field

strength range, the simple hybrid yield stress equation

was given by

τy = αE2

(7)

where α depends on the dielectric constant of the fluid

and particle volume fraction, and Ec is proportional to the

particle conductivity. τy in Eq. (7) has two limiting be-

haviors: E2 for E ≪ Ec and E

1.5 for E ≫ Ec. The ER ex-

perimental result of PPy-MCM-41 ER fluids were in

good agreement with the model [80,93]. More supporting

results have been reported in other studies [39,94].

Transient Behavior

Transient stress behavior in ER fluids was reported for

ER fluids of organic [95,96,137] and inorganic particles

[132]. Hysteresis was considered to be related to the loss

rate of chain structure caused by the applied shear and to

the rate of structure build-up by an applied electric field

[96]. Aizawa and coworkers [137] associated the hyste-

resis in ER fluids to different extents of cluster breaking

and lamellae formations. Following hysteresis measure-

ments, they observed particle agglomeration. Field-de-

pendent hysteresis was also observed for polymethylani-

line-based ER fluids [79].

The transient shear stress behavior is presented in

Figure 11 for 10 wt% PPy-coated PE suspensions at the

shear rate = 0.1 s-1

[44]. The desired electric field was

applied for 1 min prior to shearing the suspensions to al-

low the formation of particle strands between the elec-

trode gap. The shear was imposed at t = 0 sec. Typical

stress growth curves for ER fluids are shown in the inset

figure in Figure 10, which is the transient ER response of

the PPy-coated PE suspension of the oxidant (FeCl3⋅

6H2O) amount of 0.75 g. By applying the shear, the

stress instantaneously increases and reaches a steady-

state value. The steady-state shear stress increases with

the electric field strength. However, the ER responses of

the PPy-coated PE suspension of the oxidant amount of

1.5 g show notable behaviors where the stress instanta-

neously reaches a maximum and then slowly decreases to

a steady-state value. The magnitudes of the overshoot

and the steady-state shear stress increased with the elec-

tric field strength.

The pictures of the lower parallel plate of the rheometer

after ER experiments are presented in Figure 12 for the

suspensions of various PPy-coated PE particles [44]. The

ER experiments were performed by shearing the suspen-

sions at = 0.1 s-1

under the electric field of 2000 V/mm.

The pictures were taken when a steady state in the shear

stress was reached. The suspension of the oxidant

amount of 0.75 g shows uniform particle distribution on

the plate after the ER experiment. However, the suspen-

sions of larger oxidant amounts show nonuniform par-

ticle distributions and the nonuniformity increases with

the oxidant amount. The suspension of the oxidant

amount of 1.5 g shows a doughnut type particle dis-

tribution and that of the oxidant amount of 3.0 g shows

islands of particle cluster. However, all of the suspen-

sions showed uniform particle distributions if only the

electric field was applied without shearing, indicating

that the nonuniform particle distributions arose from

shear-induced particle strand aggregations.

The transient overshoot may be explained as the rapid

formation of single width particle strands, which would

lead to the maximum shear stress, followed by slower

shear-induced particle aggregation of particle strands.

The particle structure just before shearing would be sin-

gle width particle strands as (a) in Figure 11. By shear-

ing, the stress is transferred to break the particle strands

((b) in Figure 11), showing the instantaneous stress in-

crease up to a maximum. When the shear-induced par-

ticle aggregation is negligible, the maximum stress

would be maintained (inset figure in Figure 11) and the

particle structure would be like (b) in Figure 11, showing

no overshoot. If the shear-induced particle aggregation is

significant, the particle structure would slowly change

from (b) to (c) in Figure 11 and hence the stress would

slowly decreases from the maximum to a steady-state val-


Figure 12. Images of the lower parallel plate after ER experiment at the shear rate of 0.1 s-1

and under the electric field of 2000 V/mm

for the PPy-coated PE suspensions of (a) FeCl3⋅6H2O = 0.75 g, (b) FeCl3⋅6H2O = 1.5 g, (c) FeCl3⋅6H2O = 3.0 g [44].

ue, showing a transient overshoot. However, the notable

increase in the transient overshoot and the dramatic de-

crease in the steady-state shear stress with the increasing

particle conductivity indicate that the shear-induced par-

ticle aggregation cannot be the only explanation for this

phenomenon. It was reported that the particle aggrega-

tion attributed to the increased conduction between the

particles [86] and the increased conduction between the

particles led to the decrease in the steady-state ER re-

sponse [21,35,57]. Therefore, the significant decrease in

the steady-state ER response seems to arise from the

combined effect of the shear-induced particle strand ag-

gregation and the resulting increased conduction between

the particles, but mainly from the increased conduction

[44].

Additive Effect

Additives are polar materials that can adsorb on the sur-

face of the dispersed particles. Many ER additives are

discussed in the literature [2,36,37]. The amount of addi-

tive is very important. Less than 0.01 wt% would not

give any enhancement and greater than 5 wt% would

give a large electric current [37,57]. Surfactants have two

roles in an ER fluid: improving the particles’ sedimenta-

tion properties and enhancing the ER effect [57,138,139].

Models on ER additives has been proposed that takes in-

to account the surface tension and dielectric [138] and

conduction effects on ER fluid performance [21]. Based

on these models, an additive should have a higher dielec-

tric constant, lower conductivity, and larger surface ten-

sion than that of the carrier fluid.

The dependence of the yield stress on surfactant amount

was reported for 1 wt% PPy ER fluid [40]. The yield

stress initially increased with the surfactant amount,

passed through a maximum, and then slowly decreased

with the surfactant amount. Many ER fluids showed the

same yield stress dependence on the surfactant amount

[21,22,57,60]. It was noted that the PPy conductivity be-

havior with the surfactant amount was consistent with the

yield stress behavior, showing a maximum [40]. Kudoh

[111] reported that the conductivity of PPy initially in-

creased with the surfactant amount, showed a maximum

near the maximum doping concentration, and then de-

creased to a constant value with the surfactant amount.

The same yield stress dependence on the surfactant

amount (i.e., the initial increase, passing through a max-

imum, and then slow decrease with the increasing surfac-

tant amount) were also observed for various PPy based

ER fluids: PPy-SnO2 nanocomposite ER fluids [103] and

PPy-silica nanocomposite ER fluids [102].

Conclusion

Semiconducting polymers constitute a class of polymers

with particular interest owing to their physical and chem-

ical properties. PPy is one of the most promising semi-

conducting polymers, because it has higher conductivity

and environmental stability than many other semi-

conducting polymers. Therefore, PPy in various mod-

ifications is suitable as an active solid phase in ER fluids.

To control ER properties by adjusting dielectric proper-

ties of the particles by the introduction of PPy, many re-

searches focused on the preparation of semiconducting

PPy-based composite materials. Heterogeneous-semicon-

ducting polymer nanocomposites have drawn the atten-

tion over last few years, giving rise to a host of PPy com-

posites with interesting physical properties and important

application potential as ER materials. ER properties of

PPy based ER fluids (PPy, PPy copolymer, PPy coated

particles, and PPy naocomposites, etc.) were reviewed.

The ER behaviors of PPy based ER fluids such as shear,

yield, and transient stress behavior and additive effects

were also reviewed. PPy based ER fluids typically

showed anomalous behaviors where the shear stress de-

creases with shear rate were observed. The anomalous

shear stress behavior arises from a negative synergistic

interaction between hydrodynamic and polarization

forces. The yield stress of PPy based ER fluids usually

showed nonlinear ER behavior because of their high con-

ductivity and the transient ER behavior arose from both

the shear-induced particle strand aggregation and the in-

creased conduction between the particles. The prospect

of developing an effective ER fluid using PPy based ma-


terials is bright since the electrical and physical proper-

ties of PPy based material can be easily controlled.

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