Facile Synthesis of Polypyrrole/Titanate Core-Shell Nanorods and Their Electrorheological Characteristics

doi:10.4236/msa.2011.22015

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Materials Sciences and Applicatio ns, 2011, 2, 111-115

doi:10.4236/msa.2011.22015 Published Online February 2011 (http://www.SciRP.org/journal/msa)

111

Facile Synthesis of Polypyrrole/Titanate Core-Shell

Nanorods and Their Electrorheological

Characteristics

Jiexu Zhang1, Ying He2, Lijun Ji1

1College of Chemical Engineering, East China University of Science and Technology, Shanghai, China; 2Key Laboratory for Ul-

trafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and

Technology, Shanghai, China.

Email: [email protected]

Received November 9th, 2010; revised January 14th, 2011; accepted January 24th, 2011.

ABSTRACT

In this work, polypyrrole (PPy)/titanate (TN) composite nanorods were successfully synthesized using cetyl trimethyl-

ammonium bromide (CTAB) as a structure-directing agent by in situ chemical oxidative polymerization. The structural

characterization indicated that the new composite rods were core (TN)-shell (PPy) nanostructure with the average di-

ameter in the range of 250-300 nm. Further, this semiconducting composite can be used as a dispersed phase in sili-

cone oil for a new electrorheological (ER) fluid, and its ER behavior was investigated under steady and oscillatory

shear. It was found that the PPy/TN fluid showed typical ER characteristics under an external electric field.

Keywords: Polypyrrole, Titanate Nanorods, Conducting Composite, Electrorheological Fluid, Viscoelastic Properties

1. Introduction

In recent years, considerable efforts have been made to-

wards the design and synthesis of one-dimensional (1D)

nanostructured materials (e.g. nanotube, nanowire and

nanorod) due to their potential applications in nanoscale

electronic and photonic devices, as well as in biotech-

nology. More recently, interesting ER properties induced

by 1D materials -based fluids have also attracted wide

attention [1-3].

As one of the important smart materials, ER fluids

have been intensively investigated over the past decades.

ER fluids are commonly known as suspensions consist-

ing of polarizable particles dispersed in an insulating

liquid medium [4]. Their rheological properties can rap-

idly and reversibly change upon application of an electric

field. This rapid and reversible electric-controlled me-

chanical response makes ER fluids to be used in the

automotive industry for clutch, brake and damping sys-

tems [5]. To facilitate the practical application of ER

technology, current research has focused on the devel-

opment of optimal anhydrous ER materials with high-

performance. Among these materials, 1D nanomaterials

have had the advantage of the ER materials with a good

ER efficiency due to their large aspect ratio (large in-

duced dipole moments) and good suspended stability. In

particular, 1D conducting nanocomposites, containing

inorganic nanotubes/nanowires coated with conducting

polymer, provide an exciting system exhibiting improved

chemical and physical properties over those of core or

shell materials [6]. Since conducting polymers (e.g.

polyaniline and polypyrrole) have been developed for ER

application in the past years because of their high po-

larizability, good thermal stability and controllable con-

ductivities and dielectric properties. Therefore, it is ex-

pected that 1D conducting nanocomposites with core-

shell structure could be used as advanced ER fluids.

In this paper, we present a facile way to prepare a new

conducting nanocomposite in which titanate (TN) rods

are encapsulated in the shell of polypyrrole (PPy). TiO2-

derived TN nanorods with wide band-gaps have also

recently received increasing interest due to their impor-

tant photocatalytic, photovoltaic, and semiconducting

properties [7]. Hydrothermal treatment of TiO2 precur-

sors in a highly alkaline medium is an effective way to

the synthesis of 1D TN nanostructures. The PPy/TN

composite nanorods were obtained by chemical oxidative

polymerization directed by CTAB in the presence of TN.

Facile Synthesis of Polypyrrole/Titanate Core-Shell Nanorods and Their Electrorheological Characteristics

112

The structure and morphology of the nanorods were

characterized. Furthermore, the potential application of

PPy/TN nanorods as an ER fluid under both steady and

dynamic shear is also investigated, which might be help-

ful in the design and fabrication of ER materials with

high-performance.

2. Experimental

2.1. Synthesis of PPy/TN Composite Nanorods

TN rods were prepared according to the report [8], with a

minor modification. In a typical synthesis, 1.5 of TiO2

powder (Degussa P25, Germany) was mixed with 70 ml

of 10 M NaOH solution, which was followed by hydro-

thermal treatment of the mixture at 200˚C in a 100 ml

Teflon-lined autoclave for 24 h. The white precipitate

was separated by filtration and washed with distilled wa-

ter until pH was around 7. The samples were then dried

in an oven at 60˚C for 12 h.

To obtain PPy/TN composite nanorods, 0.5g of TN

nanorods were dispersed in 50 ml of 0.1 M CTAB solu-

tion under ultrasonication at room temperature for 1 h.

After that, the mixture was cooled to 0-5˚C with vigorous

stirring. Then, a precooled solution of pyrrole monomer

was introduced into the above-mentioned solution. After

0.5 h, 50 ml of ammonium persulfate (APS) precooled

solution was added dropwise to the mixture (molar ratio

of Py/APS = 1), and the polymerization reaction was

allowed to proceed at 0-5˚C for 24 h. The resulting pre-

cipitate was filtered and washed with distilled water and

acetone. Finally, the product was dried in a vacuum oven

at 60˚C for 24 h.

2.2. Characterization of Materials

FT-IR spectra of the samples were obtained on Nicolet

Magna-550 spectrometer in the range of 4000-400 cm−1,

the morphology of the nanocomposite was studied using

JEOL JSM-6360LV scanning electron microscope (SEM).

To prepare the ER fluid, PPy/TN particles were firstly

dedoped by immersion in 3 vol.% ammonia and then

dried in a vacuum oven at 60˚C for 12 h. The dried parti-

cles were dispersed in silicone oil (Fluid 200, Dow

Corning, UK; viscosity

c = 108 mPa⋅s, density dc =

0.965 g/cm3) to form the fluid concentration of 10 wt.%.

Measurements of rheological properties of the prepared

fluids were carried out under controlled shear rate (CSR)

mode using a coaxial cylinder viscometer (Bohlin

GEMINI, Malvern Instruments, UK). The suspensions

were placed in the Couette cell with the rotating inner

cylinder of 14 mm in diameter and the outer cylinder

separated by a 0.7 mm gap. They were connected to a

DC power supply producing the following field strength:

E = 0.5-3 kV/mm. All the experiments were carried out

at the temperature of 25˚C in the shear rate range 0.1-700

s−1. Static yield stress was obtained from measurements

performed in CSS mode. Further, dynamic viscoelastic

tests were performed through dynamic strain sweeps and

frequency sweeps. The strain sweep was carried out with

applied strains of 10−4 to 1.0 at a frequency of 10 Hz un-

der an electric field in order to determine the linear vis-

coelastic region. The rheological parameters were then

obtained from the frequency sweep tests (0.1 to 50 Hz) at

fixed strain amplitude in the linear viscoelastic region.

3. Results and Discussion

3.1. Structure of Materials

The chemical structure of PPy/TN composite nanorods

was investigated using FT-IR spectroscopy. Figure 1

shows the IR spectra of the PPy and PPy/TN nanorods. It

is evident that the composite shows similar FT-IR char-

acteristic bands to those of polypyrrole in the 900-1600

cm−1 range, i.e., 1550, 1460, 1300 and 1040 cm−1 bands,

which are assigned to the pyrrole ring stretching, C–N

stretching mode, the in-plane vibrations of C–H, respec-

tively [9]. In addition, a broad band at 3430 cm−1 is at-

tributed to the N–H stretching mode, and the bands at

1640 cm−1 correspond to the bending mode of H–O–H.

On the other hand, other bands at 914 and 465 cm−1 sug-

gest the occurrence of the TiO6 octahedra [10]. The

above results indicate that the as-synthesized composite

contain both PPy and TN components.

The morphologies of TN and PPy/TN nanorods char-

acterized by SEM are shown in Figure 2. The as-syn-

thesized TN nanorods with diameters of about 200 nm

and lengths of up to several micrometers are observed

(Figure 2(a)). In contrast to them, PPy/TN composite

exhibits larger diameter than that of TN, i.e. 250-300 nm.

PPy particles are almost deposited on the surface of TN

4000 3500 3000 2500 2000 1500 1000500

Transmitt ance ( %)

Wavenumber (cm-1)

(a)

(b)

3430

1640

1550 1460

1300

1040

465

914

Figure 1. FT-IR spectra of (a) PPy and (b) PPy/TN compos-

ite nanorods.

Facile Synthesis of Polypyrrole/Titanate Core-Shell Nanorods and Their Electrorheological Characteristics 113

(a)

(b)

Figure 2. SEM images of (a) TN nanorods and (b) PPy/TN

composite nanorods.

nanorods to form a core-shell structure shown in Figure

2(b). Also, the aspect ratios of PPy/TN nanorods are es-

timated to be 10-15.

3.2. ER Behavior of PPy/TN Fluid

This new core-shell nanocomposite can be used for ER

application. Figure 3 provides the flow curve for PPy/

TN based ER fluid measured in CSR mode under four

different electric field strengths. Without an electric field,

the suspension shows a slight departure from Newtonian

fluid behavior due to the relatively high particle concen-

tration. When an electric field is applied to the suspen-

sion, the shear stress increases quickly with electric field,

and yield stresses appear like in a Bingham fluid, which

is the typical rheological characteristic of ER fluid under

an electric field [11]. The figure also shows that, in the

low-shear-rate region the shear stress slightly decreases

with increasing shear rate up to a critical shear rate, and

100101102103

10-1

100

101

102

103

Shear stress (Pa)

Shear rate (s-1)

0.0 kV/mm

1.0 kV/mm

2.0 kV/mm

3.0 kV/mm

Figure 3. Shear stress versus shear rate for PPy/TN-based

fluid under different electric field strengths.

then increases with shear rate. The critical shear rate is a

transition point at which the fluid starts to exhibit

pseudo-Newtonian behavior. This phenomenon is related

to the structural change of ER fluid under shear. The

dispersed particles are polarized and formed chain-like

structures as a result of electrostatic forces generated

between the dipoles induced by the external electric field.

In the low-shear-rate region, with increasing shear rate

the slight decrease of shear stress is a consequence of the

destruction rate of chain-like structures exceeding their

reformation rate. In the high-shear-rate region, the shear

stress increases with shear rate showing pseudo-Newto-

nian, implying that the hydrodynamic forces rather than

the electrostatic forces begins to dominate the flow be-

havior. This is similar to other reports [12,13]. Further,

one can note that the critical shear rate shifts to high

value with increasing electric fields, which is related to

the magnitude of attractive interactions between the dis-

persed particles.

The change of the microstructure of PPy/TN compos-

ite fluid can also be detected under dynamic shear. Fig-

ure 4(a) shows the storage modulus (G′) and loss mo-

dulus (G″) as a function of strain for 10 wt.% PPy/TN

fluid. It can be found that both moduli increase with in-

creasing electric field strength and G′ is larger than G″ in

the linear viscoelastic range. This is mainly caused by the

elasticity of PPy/TN based fluid. The chain-like struc-

tures formed by the polarized PPy/TN particles become

more elastic and stiffer at higher electric field, which

makes the fluid behave like solid, resulting in a larger G′.

When the strain is increased, for example, exceeds the

critical strain, the chain-like structures will collapse and

the fluid shows liquid behavior, i.e. G″ > G′. Figure 4(b)

presents G′ and G″ of PPy/TN fluid as a function of fre-

quency at a small strain of 2 ×10–4 in the linear viscoelas-

Facile Synthesis of Polypyrrole/Titanate Core-Shell Nanorods and Their Electrorheological Characteristics

114

(a) (b)

Figure 4. G′ and G″ as a function of strain amplitude (a) and (b) frequency for PPy/TN fluid under different electric field

strengths: open symbols for G′; shaded symbols for G″.

tic range. At the same electric field strength, both moduli

are almost independent of frequency in the low fre-

quency range due to rigid chain-like structure, while with

increasing frequency they increase with frequency show-

ing a transition to a nonlinear region because of the onset

of chain rupture [14,15].

4. Conclusions

A new polypyrrole/titanate composite nanorod was syn-

thesized via chemical oxidative polymerization of pyr-

role directed by cetyl trimethylammonium bromide in the

presence of titanate nanorods. FTIR and SEM charac-

terization confirmed that polypyrrole was deposited on

the surface of titanate particles and formed a core-shell

structure. Further, it was observed that the suspension of

polypyrrole/titanate exhibited typical ER behavior under

an external electric field. Shear stress and dynamic

moduli increase with electric field due to elasticity of the

ER fluid. And in the linear viscoelastic range, storage

modulus is larger than loss modulus under an electric

field.

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