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Influence of AC Electric Field on Macroscopic Network

of Carbon Nanotubes in PolystyreneXizhi Yang

a, Yuefeng Zhu

b, Lijun Ji

a, Chan Zhang

a& Ji Liang

a

aKey Laboratory for Advanced Materials Processing Technology , Ministry of Education , P.

ChinabDepartment of Mechanical Engineering , Tsinghua University , Beijing, P. R. China

Published online: 23 Dec 2010.

To cite this article:Xizhi Yang , Yuefeng Zhu , Lijun Ji , Chan Zhang & Ji Liang (2007) Influence of AC Electric Field onMacroscopic Network of Carbon Nanotubes in Polystyrene, Journal of Dispersion Science and Technology, 28:8, 1164-1168,

DOI: 10.1080/01932690701526740

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Influence of AC Electric Field on Macroscopic Network of CarbonNanotubes in Polystyrene

Xizhi Yang, Yuefeng Zhu, Lijun Ji, Chan Zhang, and Ji LiangKey Laboratory for Advanced Materials Processing Technology, Ministry of Education, P. R. China and Department of

Mechanical Engineering, Tsinghua University, Beijing, P. R. China

CVD-grown multiwall carbon nanotubes are dispersed in styrene monomer. During thepolymerization of styrene, an AC electric field is applied to induce the CNTs to align alongthe electric field line to form a macroscopic nanotube network in polystyrene matrix. The die-lectrophoresis force and the electric field redistribution at the CNTs apexes are responsible foralignment of the CNTs as well as bonding between the CNTs. Parameters such as field strengthand nanotube weight fraction are varied. The results indicate that the macroscopic CNTsalignment along electric field direction can be observed only if the AC voltage reaches or ishigher than certain values, and the higher the electric field frequency is, the more uniformlythe CNTs align along electric field direction. In addition, nanotube concentration also affects

the alignment of CNTs. According to the results of this study, the CNTs will align into a devel-oped network in polystyrene matrix under a proper combination of three parameters of theelectric field voltage, frequency, and the CNTs concentration.

Keywords Carbon nanotubes, styrene, electric field, composites

1 INTRODUCTION

Carbon nanotubes (CNTs) have remarkable properties that

have aroused much interest for a wide variety of potential

applications in composites. Their unusual properties include

high moduli of elasticity and strength, high aspect ratios, excel-

lent thermal and electrical conductivities, and magnetic proper-

ties.[110] One of the key requirements for composites is thewell dispersion of CNTs,[1115] and the controllable alignment

of CNTs also is an important requirement, especially for the

integration of nanotube-based devices.

Qin et al.[16] used grafting to and grafting from methods to

functionalize single-walled carbon nanotubes (SWNTs) with

polystyrene (PS). According to their tests, the final functiona-

lized SWNTs dissolved well in organic solvents, and the

original SWNTs bundles were broken into very small ropes

or even individual tubes as revealed by AFM. Kimura

et al.[17] showed that multiwalled carbon nanotubes

(MWNTs) were aligned in a polyester matrix through

polymerizing a MWNTs-monomer dispersion inside a

magnet. The authors[18,19] reported that dispersion status of

carbon nanotubes in liquid media could be evidently

improved by effect of an electric field.

Recently, electric field was applied for orientation and array

of CNTs. Du et al.[20] prepared CNTs films by electrophoretic

deposition (EPD) with an external direct current (DC) electricfield and investigated primarily the electric properties of the

films. Yamamoto et al.[21,22] and other researchers[2325]

aligned CNTs between electrodes using DC and alternating

current (AC) electric fields due to the depolarization of

CNTs induced by electric fields. Chen et al.[26] demonstrated

the controllable inter connection of SWNTs under AC

electric field. It was reported that the interconnected carbon

nanotubes were found to be parallel with the electric flux.

Martin et al.[27] exerted both AC and DC electric fields

during nanocomposite curing and aligned conductive

nanotube networks in epoxy matrix were observed.

The results mentioned above demonstrate an obvious effect

of electric fields on dispersion and distribution morphology of

CNTs in different matrix. In this paper, we exert an external

AC electric field on the suspension of CNTs during the

polymerization of styrene to prepare composites with a macro-

scopic CNTs network in PS matrix, and demonstrate the

relationship between the formation of the CNTs network and

the AC electric field conditions. Moreover, the effect of the

CNTs concentration is preliminarily studied.

The project was sponsored by the Foundation of National NaturalScience, P. R. China (Grant No. 10332020) and the Innovation Fundfor Outstanding Scholar of Henan Province, P. R. China (GrantNo. 0521001000).

Received 1 October 2006; Accepted 15 October 2006.

Address correspondence to Yuefeng Zhu, Department of Mechan-ical Engineering, Tsinghua University, Beijing 100084, P. R. China.E-mail: yfzhu@tsinghua.edu.cn

Journal of Dispersion Science and Technology, 28:11641168, 2007

Copyright# Taylor & Francis Group, LLC

ISSN: 0193-2691 print/1532-2351 online

DOI: 10.1080/01932690701526740

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2 EXPERIMENTAL

The CNTs used in this article were prepared by chemical

vapor depositing (CVD) method. The prepared CNTs were

treated in HF for 24 hours to remove catalyst particles and

other impurities, then, the conglomerations of the entangled

CNTs were ball milled slightly. Thereafter, the so-called

original CNTs were obtained. Morphology of the so-called

original CNTs was carried out with LEO 1530 field emissionscanning electron microscope (FE-SEM) operating with an

accelerating voltage of 10 kV. The morphology is shown in

Figure 1. The styrene monomer used was produced by Guang-

dong Shantou Xilong Chemical Factory (Shantou, Guangdong

Province, China).

The suspension was prepared by dispersing the original

CNTs in styrene monomer for various CNTs concentrations,

and 2,2-azoisobytyronitrile (AIBN) also was added into the sus-

pension as a reaction initiator. A rough-dispersed CNT suspen-

sion was achieved by sonicating the solution for 8 10

minutes. Then, the rough-dispersed CNT suspension was

dropped into a Bunsen beaker in which electrodes were set for

introducing electric field as shown in Figure 2. The wall ofthe Bunsen beaker was accessorized with a piece of aluminum

foil as one electrode, and a copper rod with a diameter of

1 mm was set in the center of the beaker as another electrode.

The beaker was put into water bath with a temperature of

758C during the polymerization of the styrene monomer. At

the same time, an AC electric field was exerted on the suspen-

sion. The applied AC voltage was varied from 100 V to 300 V

(virtual value) while the AC frequency was tuned from 45 Hz

to 500 Hz. The concentration of CNTs in the styrene was

changed from 0.05 wt% (called No. 1) to 0.1 wt% (called No.

2). After two hours polymerization process of the styrene

monomer, layers of CNTs-styrene composites were prepared.

An AC electric field was supplied by an AC power supply(Instek APS 9301, Taiwan Guwei Electric Com Taiwan,

China). Morphologies of the samples were recorded by

means of a scanner (Microtek Artixscan 1010plus). A visc-

ometer (NDJ-1, Shanghai Precision & Scientific Instrument

Co. Ltd., Shanghai, China) was used to test the viscosity of

the suspensions.

3 RESULTS AND DISCUSSIONS

3.1 Influence of AC Voltage on the CNTs NetworkThe morphologies of the sample No. 2 prepared under

different electric field conditions are shown in Figure 3. In

the samples prepared under the AC frequency of 500 Hz and

different voltages, there are few CNTs aligning along the direc-

tion of the electric field with the AC voltage of 100 V and most

of the CNTs are entangled and aggregated near the aluminum

foil (see Figure 3a). Powering up the voltage to 250 V, most of

the CNTs are obviously aligned along the electric field direc-

tion and the alignment is actinoid starting from the copper

rod (see Figure 3b). Although some CNTs are entangled and

aggregated, they are dispersed more uniformly than that

shown in Figure 3a. As the AC voltage reaches 300 V, the

alignment of CNTs becomes fine and dense especially in thecentral region near the copper rod, although there exist some

heterogeneous aggregations of CNTs in the fringe region

near the aluminum foil (see Figure 3c). When the voltage

rises to 300 V, the alignment of CNTs under frequency of

500 Hz is not as good as that under 450 Hz. So the samples

are prepared under voltage of 300 V and frequency of

450 Hz in this experiment.

Martin et al.[27] proposed that dielectrophoresis effect could

induce nanotube movement towards the electrode for those

nanotubes in close proximity to the electrodes under effect of

an AC field. In case of CNTs bundles, dielectrophoresis also

can make them to array along the electric field direction.

When an electric field is applied across two electrodes, depo-

sition of CNTs on the surface of one electrode occurs, which

lead to a remarkable change of the electric field distribution,

and the nanotube apex demonstrates extremely high strength

and gradient of electric field.[26] This results in the directional

movement of CNTs in liquid and axially or breadthwise con-

nection to the surface-fixed nanotube apexes along the

electric field gradient. As a result, CNTs bundles will bridge

FIG. 2. Schematic drawing of the experimental devices.

FIG. 1. Morphology of so-called original CNTs (FE-SEM).

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the two electrodes. In addition, the AC voltage must be high

enough to overcome thermal energy of Brownian motion and

resistance against rotation of the CNTs bundles in the

viscous suspension environment. Due to the limitation of the

AC power supply, the highest voltage is 300 V in this exper-

iment. According to the experimental results, the higher theAC voltage is, the more CNTs align along the electric field

line. However, the strength and gradient of electric field at

the CNTs apexes will be enhanced as the voltage increases,

and transverse movement of CNTs to the surface-fixed

nanotube will augment. This will result in that the CNTs

bundles align along the electric field line and meanwhile

connect each other breadthwise. This will form rather a dendri-

tic network than electric-field-lined network of the CNTs.

3.2 Influence of AC Frequency

The alignment morphologies of the CNTs in the compositesare shown in Figure 4. It can be seen that even though the AC

voltage is 300 V, alignment of the CNTs along the electric field

direction cannot be obtained under low frequencies of the AC

electric field (see Figures 4a and 4b). Only when the frequency

is up to 250 Hz, there will be CNTs alignment along electric

field direction (see Figures 4c4e).

FIG. 3. Morphologies of the CNTs network under different AC voltages and frequencies.

FIG. 4. Morphologies of the CNTs under an AC voltage of 300 V with various frequencies.

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It is observed that CNTs alignment started from live wire.

Aluminum foil is made to be live wire. The distinction of the

CNTs network structure under different AC frequency is

clear. As the frequency increases (see Figure 4), the CNTs

network structure is more regular and more CNTs align

along the electric field line.

As shown in Figure 4a, there is no alignment of CNTs with

the AC frequency of 45 Hz. Some CNTs are entangled and

aggregated in the central region. As the AC frequency

reaches 100 Hz, a few of CNTs align along the electric field

direction, especially in the fringe region near the aluminum

foil (see Figure 4b). Under the AC frequency of 250 Hz, the

CNTs near the fringe region are obviously aligned along the

electric field direction, and in the central region, some CNTs

are free of alignment (see Figure 4c). With the AC frequency

increasing further, the alignment of CNTs is denser (see

Figures 4d and 4e). In Figure 4d, the threads composed of

CNTs are curved, and this phenomenon is attributed to the

occasional change of the electric field distribution. Once a

piece of the thread becomes curved after the alignment of

CNTs completes, the electric field distribution around it willbe changed and the other threads nearby will be influenced in

turn.

The influence of the frequency on the CNTs alignment is

due to the perfection of the CNTs orientation at right angle

to the electrode increased with increase of applied AC fre-

quency. Kumar et al.[25] reported that an improved CNT orien-

tation could be achieved with AC electric field of high

frequency, this was due to alternating force exerted rapidly

on field-induced dipoles of the nanotubes and the nanotubes

oriented nearly at right angle to the metal electrodes. Accord-

ing to the experiment results of this paper, it is believed that a

congruent voltage and frequency can induce CNTs aligned har-

moniously along the electric field direction. As discussedabove, low voltages cannot provide enough power for CNTs

to conquer thermal energy of Brownian motion and resistance

against rotation of the CNTs bundles in the viscous suspension

environment, and low frequencies cannot lead right angle of

the CNTs orientation to the electrode. In our tests, an AC

electric field of 300 V and 450 Hz is proper for alignment of

the CNTs in PS matrix.

3.3 Influence of CNTs Concentration

Effects of CNTs concentration on their network structure

are shown in Figure 5. It is seen that, as the CNTs concentration

is 0.05 wt% (No. 1), under the same conditions, the alignment

of CNTs is more uniform than that of sample No. 2. With the

AC voltage of 250 V and frequency of 500 Hz, there are less

CNTs entangled in sample No. 1 than that in sample No. 2

(see Figures 5a and 5b). Under the AC voltage of 300 V and

frequency of 450 Hz, the network structure of CNTs in

sample No. 1 is more homogeneous than that in sample No.

2 (see Figures 5c and 5d).

FIG. 5. Morphologies of CNTs network formed under different AC elec-tric field conditions.

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Viscosities of the suspension with different CNTs concen-

tration are measured. The viscosities of samples No. 1 and No.

2 are 8.67 mPas and 11.34 mPas, respectively. The lower the

viscosity of the suspension is, the lower the resistance against

the rotating CNTs is, and the more freely the CNTs move in

the suspension. As a result, alignment of CNTs is more likely

along the electric field direction. According to the viscosity

test, the viscosity of sample No. 1 is lower than that of sample

No. 2. So the CNTs alignment in sample No. 1 is more

uniform than that in sample No. 2 under the same conditions.

4 CONCLUSIONS

CNTs are dispersed in styrene monomer and then an AC

electric field is applied to induce the CNTs to align along the

electric field line to form a macroscopic network in PS

matrix after polymerization. Dielectrophoresis force and the

electric field redistribution at the CNTs apexes are considered

to be responsible for alignment of the CNTs as well as bonding

between pieces of the CNTs. Under different electric field con-

ditions, various morphologies of the CNTs network structures

can be obtained. The results indicate that only when the ACvoltage reaches certain intensity, which is high enough to

overcome thermal energy of Brownian motion and resistance

against rotation of the CNTs bundles in the viscous suspension

environment, a macroscopic CNTs alignment along the electric

field direction can be observed. And high electric field fre-

quency is good for CNTs alignment, for the perfection of the

CNT orientation at right angle to the electrode increased with

increase of the applied electric field frequency. In addition,

CNTs concentration also affects the alignment of CNTs

because the viscosity of the suspension is different with

various CNTs concentrations and there is less resistance

against rotation of the CNTs bundles in the low viscous suspen-

sion environment. The CNTs will align into a developednetwork in PS matrix under a proper combination of three par-

ameters of the electric field voltage, frequency, and the CNTs

concentration.

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