Electrorheological Properties of Polypyrrole and its ... · est owing to their physical and...
Transcript of 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
Do-Heyoung Kim and Young Dae Kim882
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 experi- mentally.
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-
Electrorheological Properties of Polypyrrole and its Composite ER Fluids 883
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
Do-Heyoung Kim and Young Dae Kim884
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
Electrorheological Properties of Polypyrrole and its Composite ER Fluids 885
(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
Do-Heyoung Kim and Young Dae Kim886
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].
Electrorheological Properties of Polypyrrole and its Composite ER Fluids 887
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
Do-Heyoung Kim and Young Dae Kim888
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.
Electrorheological Properties of Polypyrrole and its Composite ER Fluids 889
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
Do-Heyoung Kim and Young Dae Kim890
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-
Electrorheological Properties of Polypyrrole and its Composite ER Fluids 891
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-
Do-Heyoung Kim and Young Dae Kim892
terials is bright since the electrical and physical proper-
ties of PPy based material can be easily controlled.
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