Content uploaded by Mustafa Yavuz
Author content
All content in this area was uploaded by Mustafa Yavuz on Mar 31, 2020
Content may be subject to copyright.
This article was originally published in a journal published by
Elsevier, and the attached copy is provided by Elsevier for the
author’s benefit and for the benefit of the author’s institution, for
non-commercial research and educational use including without
limitation use in instruction at your institution, sending it to specific
colleagues that you know, and providing a copy to your institution’s
administrator.
All other uses, reproduction and distribution, including without
limitation commercial reprints, selling or licensing copies or access,
or posting on open internet sites, your personal or institution’s
website or repository, are prohibited. For exceptions, permission
may be sought for such use through Elsevier’s permissions site at:
http://www.elsevier.com/locate/permissionusematerial
Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 124–132
Comparison of electrorheological properties of
some polyaniline derivatives
Bet¨
ul Gercek a, Mustafa Yavuz b, Hasim Yilmazc,∗, Bekir Sari a, Halil Ibrahim Unal a
aDepartment of Chemistry, Faculty of Science, Gazi University, Teknikokullar, 06500 Ankara, Turkey
bDepartment of Chemistry, Faculty of Science, Suleyman Demirel University, 32260 Isparta, Turkey
cDepartment of Chemistry, Faculty of Science, Harran University, Osmanbey, 63190 Sanliurfa, Turkey
Received 28 July 2006; received in revised form 28 October 2006; accepted 14 November 2006
Available online 19 November 2006
Abstract
In this study, electrorheological (ER) properties of various polyaniline derivatives, namely: poly(o-toluidine) (POT), poly(N-methyl aniline)
(PNMAn), poly(N-ethyl aniline) (PNEAn), and poly(2-ethyl aniline) (P2EAn) were investigated. Effects of various parameters such as; particle size,
particle conductivity, suspension’s sedimentation stability, flow times, concentration, electric field strength, shear rate, frequency and temperature
onto ER activity of these polyaniline derivatives/silicone oil (SO) suspensions were investigated. Average particle diameters (d50) of four samples
were determined by dynamic light scattering as 8.51, 9.08, 11.22 and 13.46 m for P2EAn, PNEAn, PNMAn and POT, respectively. It was found
that POT has the highest conductivity with a value of 6.87 ×10−7Sm
−1, and P2EAn has the lowest with 1.01 ×10−7Sm
−1, among the polyaniline
derivatives examined. Fifty four percent of sedimentation ratio was measured after 30 days for all the polyaniline derivatives/SO suspensions studied.
The highest electric field viscosity was observed for POT/SO suspensions as η=1.04 kPa s. The maximum excess shear stress was obtained for the
POT suspensions (25 wt.%) as τ = 917 Pa at E= 2 kV/mm.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Conducting polymers; Polyaniline derivatives; Electrorheological suspensions
1. Introduction
ER fluids refer to a class of smart materials with rheologi-
cal properties that make reversible changes considerably with
changes in external applied electric field strength. Commonly
this kind of fluid is a suspension containing polarizable solid
particles as dispersed medium and non-conducting oil as contin-
ues phase. According to chemical contents, dispersed phase can
be composed of organic or inorganic particles. Application of
external electric field strength can induce polarization of these
suspended particles. As a result, a characteristic chain length
structure along the electric field direction can be formed on a very
short time, and the viscosity and shear stress can be improved
[1]. So ER fluid shows a Bingham behavior under applied elec-
tric field. ER fluids are regarded as smart materials for active
device applications. Because of these excellent characteristics,
such as short response time, low power consumption, smooth-
∗Corresponding author. Fax: +90 312 212 22 79.
E-mail address: hasim@harran.edu.tr (H. Yilmaz).
ness of operation and mechanical simplicity, many researches
have focused on ER fluids [2].
ER fluids have been considered for various applications
in mechanical engineering such as engine mouth, clutches,
shock absorbers, ER valves, robotic arms and several control
systems [3]. Recently, ER fluids have found some newly devel-
oped applications, such as human muscle stimulators, spacecraft
deployment dampers, seismic controlling frame structures, ER
tactile displays, and photonic crystals [3].
Many ER suspensions require additives such as surfactant
or polar promoters, which are called wet ER fluids [4]. These
consist of carboxymethyl cellulose [5], silica [6], zeolite [7] and
titanium dioxide [8], which have been widely utilized as dis-
persed phase in the formulation of the wet ER fluids. Additives
are added to improve colloidal stability of dispersed particles
but also enhance ER activity [9].
To overcome the shortcomings (thermal stability and
corrosion) that wet-base ER systems possess, various dry-
base ER systems have been investigated with anhydrous
particles, including poly(acene qinone) radicals [10], polyani-
line [11], polypyrole [12], copolyaniline [13], poly(methyl
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2006.11.028
B. Gercek et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 124–132 125
methacrylate)–block–polystyrene copolymer [14], polystyrene–
block–polyisoprene copolymer [15], carbonaceous particle, and
intrinsically polarizable semiconducting polymers [16].
The difference between dry-base and wet-base systems is
the carrier species for particle polarization. The particle chain
structure formed is by the migration of ions in the adsorbed
water in wet-base ER fluids, whereas the electrons move inside
the molecules of the particles in the dry-base ER fluids.
Polyaniline (PAn) has several advantages over other polymer
particles such as low density, ease of the conductivity control,
thermal and environmental stability [17]. Therefore, it has been
applied to various electronic devices, such as chemical sen-
sors, electrochromic devices, smart windows, and light emitting
diodes [13,18,19].
Kim et al. have investigated the ER properties of dodecylben-
zenesulfonic acid doped PAn/SO suspensions [20]. Langelova et
al. reported the effect of concentration on ER activity of PAn/SO
suspensions [11]. Lee et al. studied the effect of polymerization
temperature on PAn suspensions [21]. Choi et al. investigated
the effect of ionic and nonionic substituents on the ER char-
acteristics of N-substituted copolyaniline [22]. Lee et al. also
studied the effect of surfactants on ER properties of Pan/SO
suspensions [23]. Kim et al. investigated the ER characteris-
tics of o-methoxy, o-methyl, o-ethoxy and o-ethyl derivatives of
PAn/SO suspensions [24].
In the present study, ER properties of chemically synthesized
four kinds of polyaniline derivatives, namely: POT, PNMAn,
PNEAn, and P2EAn were investigated. Sedimentation sta-
bilities of these polyaniline derivatives/SO suspensions were
determined; effects of particle size, conductivity, flow times,
dispersed particle concentration, electric field strength, shear
rate, frequency, temperature and promoter onto ER activity were
discussed.
Coding of the polymers:
POT: poly(ortho-toluidine)
PNMAn: poly(N-methylaniline)
PNEAn: poly(N-ethylaniline)
P2EAn: poly(2-ethylaniline)
2. Experimental
2.1. Materials
Silicone oil (ρ= 0.963 g/cm3,η= 1000 mPa s, ε= 2.61) was
used after drying at 130 ◦C for 3h in a vacuum oven, to remove
any moisture present. The polymerizations of four alkyl-anilines
(o-toluidine, N-methyl aniline, N-ethyl aniline and 2-ethyl
aniline) were carried out in our laboratory recently, and their syn-
thesis and characterizations are published elsewhere [25–27].
2.2. Characterization
The characterization of POT, PNEAn, PNMAn and P2EAn
samples were carried out by various techniques such as FTIR
spectroscopy, thermal analysis, wide angle X-ray diffraction
(WAXRD) (LabX, Midland, Canada) and magnetic suscepti-
bility measurements and their details are given in the literature
[25–27] and some of the characterization results are discussed
in Section 3of this study.
For ER purposes, the following further characterization
studies were carried out: dielectric constant and conductivity
measurements were carried out with HP 4192 A 6F Impedance
Analyzer (UK). The current–potential measurements were per-
formed on polymers as discs (20 mm long, 5 mm wide and 1 mm
thick) with a Keithley 220 programmable current source and a
Keithley 199 digital multimeter (Ohio, USA) at ambient temper-
ature. The capacitance, C, of ER particles was measured with an
HP 4192 A LF Impedance Analyzer at frequency of 1.0 MHz at
constant temperature (20.0 ±0.1 ◦C).
Particle sizes of all the polyaniline derivative samples were
determined using a Malvern Mastersizer E, version 1.2b parti-
cle size analyzer (UK). During the particle size measurements,
some samples were dispersed in ethanol and stirred at a constant
temperature of 20 ◦C. The data collected were evaluated accord-
ing to Fraunhofer diffraction theory by the Malvern Software
computer.
Scanning electron micrograph (SEM) was used with JEOL
JSM-6360 LV scanning electron microscope (Japan).
2.3. Preparation of suspensions
Suspensions of PAn derivative particles were prepared in
SO at a series of concentration (c= 5–25% m/m), by dispers-
ing definite amount of dispersed phase in calculated amount of
continuous phase according to formula:
(m/m, %) =mdispersed phase
mdispersed phase +moil ×100.(1)
2.4. Sedimentation stability measurements
The gravitational stability of PAn derivatives/SO suspensions
against sedimentation was determined at constant temperature
(25 ◦C). Glass tubes containing the suspensions were immersed
into a constant temperature water bath and formation of first
precipitates was taken to be the indication of sedimentation
instability.
2.5. Electrorheological measurements
Suspensions were mechanically stirred before each mea-
surement against sedimentation. Flow rate measurements were
126 B. Gercek et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 124–132
Table 1
Conductivity and dielectric constant values of polymers
POT PNEA PNMA P2EA
Conductivity (S m−1)×1076.8 5.9 2.4 1.2
Dielectric constant 318 107 86 74
carried out between two brass electrodes, which were connected
to a high voltage dc power supply (0–12.5 kV, with increments).
The gap between the electrodes was 0.5 cm, the width of the
electrodes was 1.0 cm, and the height of the liquid on the elec-
trodes was 5.0 cm. During the measurements the electrodes
were immersed into a vessel containing suspensions, with a
specific concentration, and after a few seconds the vessel was
removed and flow time for complete drainage measured, using
a digital stop-watch under E= 0 kV/mm and E= 0 kV/mm con-
ditions. This procedure was repeated under various electric field
strengths.
Rheological properties of the suspensions were determined
with a Termo-Haake RS600 parallel plate Electro-rheometer
(Germany). The gap between the parallel plates was 1.0 mm
and the diameters of the upper and lower plates were 35 mm.
All the experiments were carried out at a controlled rate
(CR) mode [except for the shear modulus (G) versus fre-
quency (f) graph, which is carried out at controlled stress
(CS) mode] and at various temperatures (25–125 ◦C, with
25 ◦C increments.). The voltage used in these experiments
was also supplied by a 0–12.5 kV (with 0.5 kV increments)
dc electric field generator (Fug Electronics, HCL 14, Ger-
many), which enabled resistivity to be created during the
experiments.
3. Results and discussion
3.1. Characterization
Conductivity and dielectric constant are important param-
eters on ER effect [28].Table 1 shows the conductivity and
dielectric constant values of polyaniline derivatives. As reflected
from the table, POT has the highest conductivity and dielectric
constant among the polyaniline derivatives examined. This is in
accordance with the ER properties of POT/SO suspensions, i.e.,
it has the highest flow time, ER efficiency, excess shear stress
and electric field viscosity, which are discussed in detail in the
following sections. Block et al. proposed in their study that, sus-
pended particles with conductivity around 10−7Sm
−1should
usually be accompanied by the largest ER effect [29]. The rea-
son is that the strength of interfacial polarization can reach to a
maximum.
Average particle diameters (d50) of the P2EAn, PNEAn,
PNMAn and POT determined to be 8.51, 9.08, 11.22 and
13.46 m, respectively.
SEM micrographs of four polyaniline derivatives are shown
in Fig. 1(a–d). As seen from the micrographs, morphological
structures of polyaniline derivatives are similar, in closed-packed
structures, which resulted in the closer conductivity values
[30,31].
3.2. Sedimentation stability
When the density of particles is not the same as that of
medium, the particles with micron size settle down according
to Stoke’s law [32]. In order to solve the traditional problem of
particle sedimentation, several works have developed different
solutions [33]. Density mismatch between dispersed phase and
continuous phase plays an important role in sedimentation ratio
of the ER fluids [34].Fig. 2 shows the change of sedimentation
ratio with time for POT, P2EAn, PNMAn, and PNEAn/SO ER
active suspensions.
The sedimentation stability ratios of POT, P2EAn, PNMAn,
and PNEAn were closer to each other; this is due to the closer
particle sizes of the dispersed particles in the SO. Fifty four
percent of polyaniline derivative/SO suspensions were deter-
mined to remain unsettled after 30 days. These sedimentation
stability results are satisfactory and meet the industrial require-
ments. Similar results were reported by Unal et al. [35] Zhao and
Duan [36] and Dong et al. [37] for (poly(lithium-2-acrylamido-
2-methyl propane sulfonic acid) organic/inorganic colloidal
hybrid, fluid catalytic cracking slurry and BaTiO3in silicone oil,
respectively. On the other hand, total sedimentation stability was
reported by Lu and Zhao for the polyaniline–montmorillonite
clay/SO suspensions [38], but it seems unreliable for a clay
suspension.
3.3. Electrorheology
3.3.1. Flow rate
To observe the effect of dc electric field on the ER activity,
flow rate measurements are carried out on the polyaniline deriva-
tive/SO suspensions. For this purpose, POT, P2EAn, PNMAn,
and PNEAn were prepared at a series of particle concentrations
(5–25 wt.%) in SO and flow times measured under E= 0 kV/mm
and E= 0 kV/mm conditions. Results obtained just from
25 wt.% suspensions in SO are depicted in Fig. 3. As seen from
the graph, flow times of suspensions increased with increasing
external electric field strength. We observed Ethreshold = 500 V
threshold energy for all the polyaniline derivatives examined.
Flow times given for the suspensions are the maximum flow
times, which could be measured under the applied external
electric field. When the electric field was further increased,
a highly stronger bridge formation was occurred for all the
suspensions and no flow was observed. Similar trends were
observed for the other concentrations of the suspensions exam-
ined. Similar behaviors were observed in our previous studies
for poly(Li-2-hydroxy ethyl methacrylate)/SO, calcium car-
bonate/SO, polystyrene–block–poly(methyl methacrylate)/SO
suspensions [34,39].
3.3.2. Effect of concentration
Suspension concentration exerts a principle effect on the ER
activity. The change in ER efficiency ((ηE−η0)η0), with sus-
pension concentrations of dispersed particles at constant shear
rate ( ˙
γ=0.2s
−1) and temperature (T=25◦C) is depicted in
Fig. 4. ER efficiency of polyaniline derivative/SO suspensions
was observed to increase with increasing suspended particle con-
B. Gercek et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 124–132 127
Fig. 1. SEM micrograph of polymers. (a) PNEAn, (b) POT, (c) PNMAn, and (d) PNEAn.
Fig. 2. Sedimentation stabilities of suspensions. ()POT,() P2EAn, ()
PNMAn, (×) PNEAn, c= 5 wt.%, and T=20◦C.
Fig. 3. The changes in flow times with electric field strength. ()POT,()
P2EAn, () PNMAn, (×) PNEAn, c= 25 wt.%, and T=25◦C.
128 B. Gercek et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 124–132
Fig. 4. Dependence of ER efficiency on concentration. ()POT,() P2EAn,
() PNMAn, (×) PNEAn, E= 0 and 2 kV/mm, T=25◦C, and ˙
γ=0.2s
−1.
centration. This tendency may be attributed to the effects of
increased polarization forces between particles with increasing
suspension concentration. In dilute suspensions (large distance
between the suspended particles), the magnitude of this polar-
ization force in the direction of the applied electric field (E)is
[40]:
F=6ε2r6E2
ρ4(2)
where ε2is the dielectric constant of the particle, ρthe distance
between the particles, and ris the radius of the particle. As
shown by this equation, an increased suspension concentration
will decrease the distance between the particles, which will result
in an increased polarization force.
Fig. 6. (a) The changes in shear stress with shear rate. ()POT,() P2EAn, () PNMAn, (×) PNEAn, E= 0 kV/mm, c= 15 wt.%, and T=25◦C. (b) The changes
in shear stress with shear rate. ()POT,() P2EAn, () PNMAn, (×) PNEAn, E= 2 kV/mm, c= 15 wt.%, and T=25◦C.
Fig. 5. The changes in excess shear stress with concentration. ()POT,()
P2EAn, () PNMAn, (×) PNEAn, E= 2 kV/mm, ˙
γ=0.2s
−1, and T=25◦C.
Maximum ER efficiency of polyaniline derivatives/SO
suspensions were observed to be: POT (45.5)> P2EAn
(39.6) > PNMAn (30.5) > PNEAn (18.0), respectively.
Fig. 5 shows the change of excess shear stress (τ =
τE= 0−τE=0) with suspension concentrations. As seen from
graph, τ increases with increasing suspension concentration,
and the highest increment was observed for POT/SO suspension
system as τ = 917 Pa at E= 2 kV/mm for c= 25 wt.%. This may
be attributed to the highest electrical conductivity of POT par-
ticles, which leads to higher polarisability, when subjected to
external electric field strength.
3.3.3. Effect of shear rate
Change of shear stress of suspensions (POT, P2EAn,
PNMAn, and PNEAn/SO) with shear rate at constant conditions
B. Gercek et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 124–132 129
Fig. 7. The changes in ER efficiency with log(shear rate). ()POT,() P2EAn,
() PNMAn, (×) PNEAn, E= 0 and 2 kV/mm, T=25◦C, and c= 5 wt.%.
(c= 15 wt.%, T=20◦C) is shown in Fig. 6(a and b). As seen from
Fig. 6(a), shear stresses of all the suspensions examined increase
with increasing shear rate and show a Newtonian flow behav-
ior in the absence electric field (E= 0kV/mm). But, as reflected
from Fig. 6(b), Bingham plastic behavior was observed under the
influence of external electric field strength (E= 2 kV/mm). This
is caused by the role of induced polarization forces, which is a
typical rheological characteristic of ER fluids under the influence
of external electric field strength [41]. This means that flows have
occurred only after exceeding a minimum yield stress (τy). The
yield stresses determined from Fig. 6(b) are as follows: τyPOT
(201 Pa) > τyPNEAn (171 Pa) > τyPNMAn (134 Pa) > τyP2EAn
(73 Pa).
In Fig. 7, the ER efficiency is plotted against log(shear
rate) for (POT, P2EAn, PNMAn, and PNEAn/SO) samples.
As reflected from the graph, viscosity of all the suspen-
sions decreases with increasing shear rate until ˙
γ=10 s−1,
and then become shear independent. This is a typical shear
thinning non-Newtonian visco-elastic behavior of polyaniline
derivatives/SO suspensions. POT, P2EAn, PNMAn and PNEAn
particles are affected by the hydrodynamic interactions and
the viscous forces (F), which have the following magnitude
[38]:
F=6πηsr2˙
γ(3)
where ηsis the viscosity of the suspension and ˙
γis the average
shear rate.
Although the viscous forces are proportional to the shear
rate, at higher shear rates, the suspensions viscosities come
less dependent of on E. This suggests that at higher shear
rates, the viscous forces are dominant to the polarization
forces, and the suspensions structures do not vary appreciably
with E.
Similar behaviors were reported for the studies of polyani-
line/SO [17], poly(naphtalane quinine)/SO [42] and saponite/SO
[28] suspensions.
Fig. 8. The change of viscosity with electric field strength. ()POT,() P2EAn,
() PNMAn, (×) PNEAn, c= 15 wt.%, ˙
γ=0.2s
−1, and T=25◦C.
3.3.4. Effect of electric field strength
Fig. 8 shows the change in the electric field viscosity
with electric field strength at constant conditions: ˙
γ=0.2s
−1,
c= 15 wt.% and T=25◦C. As seen from the graph, electric field
viscosity (ηE) increases with increasing electric field strength
and reaches to ηE= 1.04 kPa s for POT. Under applied electric
field strength, the magnitude of the polarization forces between
particles increases, and in turn, the particles rapidly aggregate
into the chain length (formed by the polarized particles) perpen-
dicular to the electrodes, hence resulting in the improvement of
the viscosity. A similar trend was observed by D¨
urrschmith and
Hoffmann [43] in ER studies of saponite suspensions prepared
in SO, by Unal et al. [35] in ER studies of poly(Li-2-hydroxy
ethyl methacrylate) suspensions prepared in SO and Lengalova
et al. [44] in polyaniline suspensions.
Fig. 9. The changes in polarization forces with electric field strength. ()POT,
() P2EAn, () PNMAn, (×) PNEAn, c= 15 wt.%, ˙
γ=0.2s
−1, and T=25◦C.
130 B. Gercek et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 124–132
Fig. 10. Dependence of shear stress on electric field strength. ()POT,()
P2EAn, () PNMAn, (×) PNEAn, c= 15 wt.%, ˙
γ=0.2s
−1, and T=25◦C.
Fig. 9 shows the changes in polarization forces (redistribu-
tion of the charges of molecules) with externally applied electric
field strength at constant conditions (c= 15 wt.%, ˙
γ=0.2s
−1,
T=25◦C). It has known that, in two component ER systems
(containing dispersed and continues phases), the interfacial
polarization is reported to be responsible for ER effect [28].
As seen from Fig. 9, electric field induced polarization forces
increase with increasing electric field strength for all the polyani-
line derivatives/SO suspensions examined in this work. This
trend is due to the increased magnitude of polarization forces act-
ing between the charged particles [30]. When electric field was
applied to the polyaniline derivatives/SO suspensions, polariza-
tion forces caused the aggregation of polyaniline particles and
a chain formation between the upper and lower plates occurred.
The relation of the magnitude of viscous forces (F) with viscos-
ity of suspensions (ηs) is given in Eq. (3). Similar behavior was
reported for the polyaniline suspensions [45].
Fig. 10 shows the change in shear stress as a function
of external electric field strength, keeping shear rate (˙
γ=
0.2s
−1), suspension concentration (c= 15 wt.%) and tempera-
ture (T=25◦C) constant. It can be seen from the graph that, shear
stress linearly related to square of the electric field strength (E2).
These trends originate from the stronger interaction between par-
ticles induced by the electric field strength. This is suitable with
theoretical predictions proposed by Klingenberg and Zukoski
[46].
τ∝En(4)
The change of electric field induced shear stress (τE)
of the polyaniline derivatives/SO suspensions were as
follows: τEPOT (246 Pa) > τEPNEAn (195 Pa) > τEPNMAn
(180 Pa) > τEP2EAn (65 Pa). As reflected from the graph, POT
suspension shown the highest electric field induced shear stress
among the polyaniline derivatives/SO suspensions examined. In
the literature, Zhang et al. also reported that, POT/SO suspension
shown higher shear stress than polyaniline/SO and brominated
polyaniline/SO suspensions [47].
Fig. 11. The changes in elastic modulus with frequency. ()POT,() P2EAn,
() PNMAn, (×) PNEAn, c= 15 wt.%, E= 2.0 kV/mm, and T=25◦C.
Maziopa et al. [48]. and Kim et al. [24]. reported similar
trends for electric field induced shear stress of polyanilines pre-
pared with various dopants, and various polyaniline derivatives.
3.3.5. Effect of frequency
The external stress frequency is an essential factor for char-
acterizing the dynamic visco-elastic properties of ER fluids
[42].Fig. 11 demonstrates the change of electric field induced
complex shear modulus (G) with frequency at c= 15 wt.%,
E= 2.0 kV/mm and T=25◦C. The setting shear stress for this
experiment was τ= 10 Pa, which can ensure the measure-
ments are conducted in the small strain region. As reflected
from Fig. 10,Gincreased with external frequency. This
behavior was reported in the literature [49] as the typi-
cal characteristic of a visco-elastic material composed of
polyaniline–Na+–montmorillonite. Similar behavior was also
observed in studies of zeolite/SO suspensions [50].
3.3.6. Effect of temperature
TGA results of polyalkylaniline derivatives [26] were shown
that, initial degradation temperatures of all the samples were
≥148 ◦C, which are desirable values for high temperature ER
fluid applications.
Fig. 12 shows the changes in the shear stress of POT,
P2EAn, PNMAn and PNEAn/SO suspensions under var-
ious temperatures at constant conditions (E= 2 kV/mm,
˙
γ=0.2s
−1,c= 15 wt.%). As reflected from the figure, the
shear stresses of all the suspensions examined in this work
decrease with increasing temperature. The shear stress losses
(τ =τT=0◦C−τT= 125 ◦C) calculated from the graph were
as follows: PNEAn (τ = 55.28 Pa) < POT (τ = 61.11 Pa)
< P2EAn (τ = 77.21 Pa) < PNMAn (τ = 101.08 Pa). Com-
monly, the temperature has two effects on the ER fluids: one
is the effect on polarization force and another one is Brownian
motion. The increase of temperature results, both in decreased
activation energy of polarization of suspended particles, and
B. Gercek et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 124–132 131
Fig. 12. The changes in shear stress with temperature. ()POT,() P2EAn,
() NMAn, (×) PNEAn, E= 2 kV/mm, ˙
γ=0.2s
−1, and c= 15 wt.%.
also on the polarisability of particles, which results with a
decrease in the electric field induced shear stress. On the other
hand, Brownian motion does not contribute to chain formation
of suspended particles.
Choi [51] and Lu [38] reported shear stress increases with
increasing the temperature, Unal [35] and Liu and Shaw [52]
reported shear stress decrease with increasing the temperature.
The effect of promoter on the ER activity of polyaniline
derivatives/SO suspensions were also investigated and it was
observed that, addition of any promoter caused an electrical
brake-down and had no positive ER increment. That is why,
the polyaniline derivatives examined in this study was classified
as dry-base ER fluids, which is an important property from an
industrial point of view.
4. Conclusions
This study was conducted to investigate the ER behav-
ior of four ER fluids, namely: poly(o-toluidine), poly(N-
methylaniline), poly(N-ethylaniline), and poly(2-ethylaniline)
suspensions in SO, and the following conclusions were drawn.
Sedimentation stability of suspensions was determined as
30 days. ER activity of all the suspensions was observed
to increase with increasing electric field strength, concen-
tration and decreasing shear rate. All the suspensions were
shown Newtonian flow behavior in the absence electric field,
and shown Bingham plastic flow behavior in the presence of
externally applied electric field. The viscosities of polyani-
line derivatives/SO suspensions were decreased sharply with
increasing shear rate, hence showing a typical shear thinning
non-Newtonian visco-elastic behavior. Complex shear modulus
of the samples was observed to increase with external frequency
and showing a typical characteristic of a visco-elastic material.
The highest ER effect was observed for POT suspensions. The
ER strength of all the polyaniline derivatives/SO suspensions
was observed to sensitive to high temperature, and they were
classified as dry-base ER fluids.
Acknowledgements
This work was supported by the State Planning Organiza-
tion of Turkey (Grant No. 2001K 120580) and Gazi University
Research Fund (Grant No. FEF 05/2006-45). Special thanks to
Prof. Dr. Metin G¨
ur¨
u for SEM analysis.
References
[1] T.C. Hasley, W. Torr, Structure of electrorheological fluids, Phys. Rev. Lett.
65 (1990) 2820–2823.
[2] Y. Chen, A.F. Sprecher, H. Conrad, Electrostatic particle–particle inter-
actions in electrorheological fluids, J. Appl. Phys. 70 (1991) 6796–
6803.
[3] S.B. Choi, Y.T. Choi, E.G. Chang, S.J. Han, C.S. Kim, Control charac-
teristics of a continuously variable ER damper, Mechatronics 8 (1998)
143–161.
[4] H. Tao, Adv. Mater., Electrorheol. Fluids 13 (2001) 1847–1857.
[5] H.I. Unal, H. Yilmaz, Electrorheological properties of poly(lithium-2-
acrylamido-2-methyl propane sulfonic acid) suspensions, J. Appl. Polym.
Sci. 86 (2002) 1106–1112.
[6] M. Edali, M.N. Esmail, G.H. Vatistas, Rheological properties of high con-
centrations of carboxymethyl cellulose solutions, J. Appl. Polym. Sci. 79
(2001) 1787–1801.
[7] M. Parthasatyh, D.J. Klingenberg, Electrorheology: mechanism and mod-
els, Mater. Sci. Eng. 17 (1996) 57–103.
[8] V. Pavlinek, O. Quadrat, B. Porsch, P. Saha, Electrorheological behaviour
of suspensions of various surface-modified porous silica particles, Colloids
Surf. A: Physicochem. Eng. Aspects 155 (1999) 241–247.
[9] N. Sakai, Y. Komoda, Tata N. Rao, D.A. Tryk, A. Fujishima, Effect of
adsorbed water on the photoelectrorheology of TiO2particle suspensions,
J. Electroanal. Chem. 445 (1998) 1–6.
[10] H.J. Choi, M.S. Cho, M.S. Jhon, Electrorheological properties of
poly(acene quinone) radical suspensions, Polym. Adv. Technol. 8 (1997)
697–700.
[11] A. Langelova, V. Pavlinek, P. Saha, O. Quadrat, T. Kitano, J. Stejskal,
Influence of particle concentration on the electrorheological efficiency of
polyaniline suspensions, Eur. Polym. J. 39 (2003) 641–645.
[12] D. Sahin, B. Sari, H.I. Unal, An investigation of some parameters on elec-
trorheological properties of polypyrrole suspensions, Turk. J. Chem. 26
(2002) 113–124.
[13] H.J. Choi, M.S. Cho, K. To, Electrorheological and dielectric characteris-
tics of semiconductive polyaniline–silicone oil suspensions, Physica A 254
(1998) 272–279.
[14] H. Yilmaz, M. Degirmenci, H.I. Unal, Electrorheological properties of
PMMA–b–PSt copolymer suspensions, J. Colloid Interface Sci. 293 (2006)
489–495.
[15] M. Yavuz, H.I. Unal, Y. Yildirir, Electrorheological properties of suspen-
sions prepared from porystyrene–block–polyisoprene copolymer, Turk. J.
Chem. 25 (2001) 19–32.
[16] H.J. Choi, J.W. Kim, S.H. Yoon, R. Fujiura, M. Komatsu, M.S. Jhon,
Synthesis and electrorheological characterization of carbonaceous particle
suspensions, J. Mater. Sci. Lett. 18 (1999) 1445–1447.
[17] W.H. Jang, J.W. Kim, H.J. Choi, M.S. Jhon, Synthesis and electrorheology
of camphorsulfonic acid doped polyaniline suspensions, Colloid Polym.
Sci. 279 (2001) 823–827.
[18] I.S. Lee, J.Y. Lee, J.H. Sung, H.J. Choi, Synthesis and electrorheological
characteristics of polyaniline–titanium dioxide hybrid suspension, Synth.
Met. 152 (2005) 173–176.
[19] M.G. Han, S.S. Im, Dielectric spectroscopy of conductive polyaniline salt
films, J. Appl. Polym. Sci. 82 (2001) 2760–2769.
[20] S.G. Kim, J.W. Kim, M.S. Cho, H.J. Choi, M.S. Jhon, Viscoelastic
characterization of semiconducting dodecylbenzenesulfonic acid doped
132 B. Gercek et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 124–132
polyaniline electrorheological suspensions, J. Appl. Polym. Sci. 79 (2001)
108–114.
[21] H.J. Lee, M.S. Cho, H.J. Choi, M.S. Jhon, Effect of polymerization temper-
ature on polyaniline based electrorheological suspensions, Colloid Polym.
Sci. 277 (1999) 73–76.
[22] H.J. Choi, M.S. Cho, J.W.Kim, R.M. Webber,M.S. Jhon, Effect of ionic and
nonionic substituents on the electrorheological characteristics of polyani-
line derivatives, Int. J. Mod. Phys. B 15 (6–7) (2001) 988–995.
[23] H.J. Lee, B.D. Chin, S.M. Yang, O.O. Park, Surfactant effect on the stabil-
ity and electrorheological properties of polyaniline particle suspension, J.
Colloid Interface Sci. 206 (1998) 424–438.
[24] J.W. Kim, W.H. Jang, H.J. Choi, J. Joo, Synthesis and electrorheological
characteristics of polyaniline derivatives with different substituents, Synth.
Met. 119 (2001) 173–174.
[25] B. Sari, A. Gok, D. Sahin, Synthesis and properties of conducting
polypyrrole, polyalkylanilines, and composites of polypyrrole and poly(2-
ethylaniline), J. Appl. Polym. Sci. 101 (2006) 241–249.
[26] B. Sari, M. Talu, A. G¨
ok, Synthesis and characterization of
Nylon6/polyalkylaniline conducting composites, J. Appl. Polym. Sci. 87
(2003) 1693–1701.
[27] F. Catoldo, P. Maltese, Synthesis of alkyl and N-alkyl-substituted polyani-
lines: a study on their spectral properties and thermal stability, Eur. Polym.
J. 38 (9) (2002) 1791.
[28] T. Hao, Electrorheological suspensions, Adv. Colloid Interface Sci. 97
(2002) 1–35.
[29] H. Block, J.P. Kelly, A. Qin, T. Watson, Materials and mechanisms in
electrorheology, Langmiur 6 (1990) 6–14.
[30] A. Gok, B. Sari, M. Talu, Chemical preparation of conducting
polyfuran/poly(2-chloroaniline) composites and their properties: a com-
parison of their components, polyfuran and poly(2-chloroaniline), J. Appl.
Polym. Sci. 88 (2003) 2924–2931.
[31] Y. Haba, E. Segal, M. Narkis, G.I. Titelman, A. Siegmann,
Polyaniline–DBSA/polymer blends prepared via aqueous dispersions,
Synth. Met. 110 (2000) 189–193.
[32] S. Uemura, K. Minagawa, J. Takimoto, K. Koyama, Opposite electro-
rheological effects between urethane-based polymers having different
terminal groups, J. Chem. Soc. Faraday Trans. 91 (6) (1995) 1051–1052.
[33] M. Qi, M.T. Shaw, Sedimentation-resistant electrorheological fluids based
on PVAL-coated microballoons, J. Appl. Polym. Sci. 65 (1997) 539–547.
[34] Q. Wu, B.Y. Zhao, L.S. Chen, K. Hu, Preparation and electrorheological
property of alkaline earth metal compounds modified amorphous TiO2
electrorheological fluid, Scripta Mater. 50 (2004) 635–639.
[35] H.I. Unal, O. Agirbas, H. Yilmaz, Electrorheological properties of poly(Li-
2-hydroxyethyl methacrylate) suspensions, Colloids Surf. A: Physicochem.
Eng. Aspects 274 (2006) 77–84.
[36] X.P. Zhao, X. Duan, A new organic/inorganic hybrid with high electrorhe-
ological activity, Mater. Lett. 54 (2002) 348–351.
[37] P. Dong, C. Wang, S. Zhao, Preparation of high performance electrorheo-
logical fluids with coke-like particles from FCC slurry conversion, Fuel 84
(2005) 685–689.
[38] J. Lu, X. Zhao, Electrorheological behaviors of polyaniline–
montmorillonite clay nanocomposite, Int. Mod. Phys. B 16 (17–18) (2002)
2521–2527.
[39] H. Yilmaz, H.I. Unal, M. Yavuz, An investigation of electrorheological
properties of calcium carbonate suspensions in silicone oil, Colloid J. 67
(2) (2005) 236–241.
[40] S. Wu, J. Shen, Electrorheological properties of chitin suspensions, J. Appl.
Polym. Sci. 60 (1996) 2159–2164.
[41] Y.T. Lim, J.H. Park, O.O. Park, Improved electrorheological effect in
polyaniline nanocomposite suspensions, J. Colloid Interface Sci. 245
(2002) 198–203.
[42] J.H. Sung, C.H. Hong, B.J. Park, H. Choi, M.S. Jhon, Comment on
preparation and electrorheological property of alkaline earth metal com-
pounds modified amorphous TiO2electrorheological fluid, Scripta Mater.
53 (2005) 1101.
[43] T. D¨
urrschmidt, H. Hoffmann, Electrorheological effects in suspensions of
hydrophobically modified saponite, Colloids Surf. A: Physicochem. Eng.
Aspects 156 (1998) 257–269.
[44] A. Lengalova, V. Pavlinek, P. Saha, J. Stejskal, T. Kitano, O. Quadrat, The
effect of dielectric properties on the electrorheology of suspensions of silica
particles coated with polyaniline, Physica A 321 (2003) 411.
[45] C.J. Gow, C.F. Zukoski, The electrorheological properties of polyaniline
suspensions, J. Colloid Interface Sci. 136 (1990) 175–188.
[46] D.J. Klingenberg, F.C. Zukoski, Studies on the steady-shear behavior of
electrorheological suspensions, Langmiur 6 (1990) 15–24.
[47] L. Zhang, K. Su, K.X. Li, Electrorheological effects of polyaniline-type
electrorheological fluids, J. Appl. Polym. Sci. 87 (2003) 733–740.
[48] A.K. Maziopa, M. Ciszewski, J. Plocharrski, Electrorheological fluids
based on polymer electrolytes, Electrochim. Acta 50 (2005) 3838–3842.
[49] J.W. Kim, S.G. Kim, H.J. Choi, M.S. Suh, M.J. Shin, M.S. Jhon,
Synthesis and electrorheological characterization of polyaniline and
Na+–montmorillonite clay nanocomposite, Int. J. Mod. Phys. B 15 (6–7)
(2001) 657–664.
[50] M.S. Cho, H.J. Choi, I.J. Chin, W.S. Ahn, Electrorheological characteri-
zation of zeolite suspensions, Microporous Mesoporous Mater. 32 (1999)
233–239.
[51] U.S. Choi, Electrorheological properties of chitosan suspension, Colloids
Surf. A: Physcochem. Eng. Aspects 157 (1999) 193–202.
[52] B. Liu, M.T. Shaw, Electrorheology of filled silicone elastomers, J. Rheol.
45 (2001) 641–657.
