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Superior conductive polystyrene – chemically converted graphenenanocomposite†

Viet Hung Pham,a Tran Viet Cuong,a Thanh Truong Dang,a Seung Hyun Hur,a Byung-Seon Kong,b

Eui Jung Kim,a Eun Woo Shina and Jin Suk Chung*a

Received 17th March 2011, Accepted 17th May 2011

DOI: 10.1039/c1jm11146a

The polystyrene–chemically converted graphene composite (PS-CCG) prepared by solution blending

followed by compression molding, exhibited a percolation threshold as low as 0.19 vol.% and an

electrical conductivity as high as 72.18 S m�1 at only �2.45 vol.%. The superior electrical conductivity

of PS-CCG is the result of the combination of high electrical conductivity of CCG and the good

dispersion of the nanofiller in PS matrix. The thermal properties of polystyrene were greatly improved

upon addition of a small amount of CCG. The onset decomposition temperature of the PS-CCG

increased by approximately 60 �C at 0.19 vol% of CCG loading. The mechanical properties of the PS-

CCG were also affected by CCG loading. The storage modulus in the glassy region was enhanced by

about 28% at 1.94 vol.% of CCG loading.

1. Introduction

Graphene, a two-dimensional monolayer of sp2-hybridized

carbon atoms, has attracted tremendous attention in both

scientific and industrial areas due to its excellent mechanical,

electrical, thermal and optical properties, as well as its high

specific surface area.1–3 One of the most promising applications

of graphene is in polymer nanocomposites.4–8 Graphene has been

made by four different methods including micromechanical

cleavage, chemical vapor deposition, liquid-phase exfoliation of

graphite and reduction of graphene oxide by chemical reducing

agent and thermal treatment.2 Among these, the reduction of

graphene oxide is considered as large-scale method which is

suitable for production of graphene to use of nanofiller for

polymer nanocomposites.2,5–7 However, the key challenge in

preparation and processing of bulk-quantity graphene sheets by

chemical reduction of graphene oxide is aggregation.9 The low

dispersibility of chemically converted graphene (CCG) in organic

solvents10–12 is a major obstacle in the preparation of polymer-

CCG composites by solution blending since most of engineering

polymers are soluble in organic solvents.6,7,12 Functionalization

can improve the dispersibility of CCG,13–15 but it is also detri-

mental to the electrical property of CCG, leading to decreasing

aSchool of Chemical Engineering and Bioengineering, University of Ulsan,Namgu, Daehakro 93, Ulsan, 680-749, Republic of Korea. E-mail:jschung@mail.ulsan.ac.kr; Fax: +82 522 591 689; Tel: +82 522 592 249bKCC Central Research Institute, Mabookdong 83, Giheunggu, Yonginsi,Gyunggido, 446-716, Republic of Korea

† Electronic supplementary information (ESI) available: Materials,graphene oxide synthesis, morphology of fresh fractured surface ofPS-CCG and Tg of PS-CCG determined by DSC. See DOI:10.1039/c1jm11146a

11312 | J. Mater. Chem., 2011, 21, 11312–11316

electrical conductivity of polymer nanocomposite. For example,

the polystyrene nanocomposite with reduced phenyl isocyanate-

functionalized graphene oxide exhibited electrical conductivity

of only �1 S m�1 at 2.5 vol.%.4

We have recently reported that it is possible to prepare high-

quality CCG suspension in various organic solvents with the

concentration up to 0.7 mg mL�1 by hydrazine reduction of

aqueous suspension of graphene oxide at low temperature, fol-

lowed by redispersing the CCG filter cake in organic solvents.16

Herein, taking advantage of this possibility, we prepared the

polystyrene-CCG composite (PS-CCG) by solution blending.

The obtained PS-CCG did not only exhibit superior electrically

conductivity but also have high thermal stability and significant

enhancement of mechanical properties with small amount of

CCG loading.

2. Experimental

2.1 Preparation of colloidal suspension of CCG in N-methyl-

pyrrolidone

The preparation of colloidal suspension of CCG in N-methyl-

pyrrolidone (NMP) is described elsewhere.16 Briefly, as-synthe-

sized graphene oxide was diluted to a concentration of

4 mg mL�1 with the aid of sonication in an ultrasonic bath

(Jeiotech UC-10, 200 W) for 10 min in order to create

a homogenous colloidal suspension. Then, the hydrazine reduc-

tion was carried out by adding 4 mL of hydrazine hydrate to

a 100 mL aqueous suspension of graphene oxide with stirring for

24 h at ambient temperature (�30 �C). The resulting CCG was

filtered and washed copiously with NMP, and the resultant CCG

filter cake was re-dispersed in NMP by sonication for 1 h.

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 Cross-sectional SEM images of the freshly-fractured surfaces of

PS (a) and PS-CCG pellets with various CCG loading (vol.%): (b) 0.48,

(c) 0.96, (d) 1.45, (e) 1.94 and (f) high magnification of (c) imaging in the

charge contrast mode.

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2.2 Preparation of PS – CCG composite by solution blending

Typically, 5.0 g of PS (Mw �192 000 g mol�1) was added to a pre-

determined volume of the colloidal suspension of CCG in NMP

(0.7 mg mL�1) with stirring for 3 h. The PS-CCG suspension was

then sonicated for 15 min. Coagulation was subsequently ach-

ieved by the PS-CCG suspension dropwise to a large volume of

methanol with vigorous stirring. The resultant PS-CCG was

filtered, washed with methanol, and then dried in vacuum at

80 �C for 12 h. The PS-CCG powder was ground to a fine powder

using a mortar and pestle. The PS-CCG powder was molded in

a hydraulic hot press (Collin Press 200G) at 30 bar and 210 �C for

3 min to obtain a PS-CCG pellet.

The mass fraction of CCG loading was converted to a volume

fraction using the following equation:17

v ¼ wrp

wrp þ ð1� wÞrgwhere v and w are the volume fraction and mass fraction of CCG,

respectively. rp and rg are the density of PS and CCG, whose

value can be set to 1.05 g cm�3 and 2.2 g cm�3,4,17 respectively.

2.3 Characterizations

The morphology of the freshly fractured surface of PS-CCG

pellets was observed by FE-SEM (Quanta 200, FEI). The elec-

trical resistances of the PS-CCG pellets (sample size �2 � 2 �0.4 mm3) were measured using the four-point probe method

(CMT-10 MP, Advanced Instrument Technology). The thick-

nesses of the PS-CCG pellets were measured using a caliper

(Absolute Digimatic, Mitutoyo). The thermal properties of the

PS-CCGweremeasured by TGA (Q50, TA Instrument) andDSC

(Q20, TA Instrument) under a nitrogen atmosphere at a heating

rate of 10 �Cmin�1. The dynamicmechanical properties of the PS-

CCG composite were measured using a dynamic mechanical

analyzer (DMA-Q800, TA Instrument) in the single cantilever

deformation mode at a frequency of 1 Hz. The temperature was

swept from 0 to 180 �C at a heating rate of 3 �C min�1.

3. Results and discussion

The dispersion of nanofiller in polymer matrix closely relates to

its effectiveness for improving its mechanical, thermal, electrical

and other properties.4,18 The solution blending has been widely

used to prepare various kinds of polymer-CCG composites

because it is a simple and effective route to disperse single-layer

CCG platelets into the polymer matrix.19–21 To verify the

homogeneous dispersion of CCG sheets in the PS matrix, the

fresh-fractured surface of the PS-CCG pellets was characterized

by SEM imaging in charge contrast mode, which allows the

visualization of the overall conductive nanofiller within the

polymer matrix.18 The surface of PS sample was coated with

platinum as conductive layer since PS is insulator, while no

additional sample treatment was applied to the PS-CCG

samples. As shown in Fig. 1(b–e), the clearance of the SEM

images of the PS-CCGs increased with the CCG loading due to

the increase of the electrical conductivity which decreases the

charge accumulated on the surface of the sample during the

imaging operation. The SEM images of PS-CCG clearly show

that crumpled and wrinkled CCG sheets were homogenously

This journal is ª The Royal Society of Chemistry 2011

dispersed in the PS matrix. The density of CCG sheets propor-

tionally increase with its loading. The good dispersion of CCG

sheets in the PS matrix indicates high compatibility between the

CCG sheets and PS. In addition, it is clear that the CCG sheets

connected with each other and created a conducting network.

Fig. 2 shows the electrical conductivity of PS-CCG as a func-

tion of CCG volume fraction at room temperature which

demonstrates the percolation behavior. The percolation

threshold of PS-CCG was around 0.19 vol.%, which is only

a little higher than the lowest percolation of polystyrene-reduced

phenyl isocyanate-treated graphene oxide composite.4

The percolation threshold depends largely on the loading of

the conductive nanoparticle, the extent of nanoparticle disper-

sion, the aspect ratio and orientation of the nanoparticle.22,23 A

low percolation value of PS-CCGmight be due to the high aspect

ratio of the CCG sheet and good dispersion of CCG in the PS

matrix. The electrical conductivity of PS-CCG then sharply

increased with the CCG volume fraction, especially at low

loading region. The electrical conductivity of PS-CCG was more

than 1 S m�1 at only 0.48 vol.% and reached 15 S m�1 at only

0.96 vol.%. By increasing the CCG fraction to 2.45 vol.%,

a conductivity of 72.18 S m�1 was achieved. To the best of our

knowledge, this is the best conductivity of polymer-graphene

composites with low graphene loading (Table 1). The inset in

Fig. 2 reveals that the electrical conductivity of PS-CCG obeyed

the power law:

J. Mater. Chem., 2011, 21, 11312–11316 | 11313

Fig. 2 Electrical conductivity of PS-CCG as a function of CCG volume

fraction. The inset is a log-log plot of the electrical conductivity versus

(f�fc).

Fig. 3 TGA thermograms of PS-CCGs as a function of CCG volume

fraction. The inset is onset and midpoint decomposition temperature of

PS-CCG composites.

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sc ¼ sf[(f�fc)/(1�fc)]t

where sf is the conductivity of the filler, f is the filler volume

fraction, fc is the percolation threshold volume fraction and t is

the universal critical exponent.4,19 The linear fit to the log-log plot

of conductivity versus (f � fc) resulted in t ¼ 2.83 � 0.23 and

sf ¼ 106.96 � 0.54 (S m�1). The t value is similar with that of poly-

styrene-reduced phenyl isocyanate-treated graphene oxide

composite (t¼ 2.74� 0.20),4 indicating a similar geometry of the

conducting nanoparticle and electron transport behavior.22,23 In

other words, the dispersion of CCG sheets in the PS matrix was

as complete as that of reduced phenyl isocyanate-treated gra-

phene oxide. However, the sf is 2 orders magnitude higher than

that of the polystyrene-reduced phenyl isocyanate-treated gra-

phene oxide composite (sf ¼ 104.92 � 0.52 (S m�1)),4 suggesting

a high electrical conductivity of the CCG sheet.

Graphene-based nanofillers have been found to significantly

increase the thermal stability of the host polymer due to the

restriction of the polymer chain mobility near the graphene

surface. During combustion, graphene sheets act as inflammable

anisotropic nanoparticles forming a jammed network of char

layers that retards transport of the decomposition products.6,19

Improvement in the thermal stability of the PS-CCG was char-

acterized by thermogravimetric analysis under non-oxidative

Table 1 Electrical conductivity of polymer-graphene composites

Polymer-graphenecomposite Graphene Processin

PS-graphene Reduced phenylisocyanate-treated graphene oxide

Solution

PS/Gr CCG Latex tecPET/graphene Functionalized graphene sheet Melt comPolycarbonate-graphene Functionalized graphene sheet Latex tecPolycarbonate-graphene Functionalized graphene sheet SolutionPS-CCG CCG Solution

11314 | J. Mater. Chem., 2011, 21, 11312–11316

condition. As shown in the inset of Fig. 3, the onset decompo-

sition temperature (Tonset) of PS-CCG increased remarkably in

comparison with pristine PS. The Tonset of PS-CCG increased

�60 �C at only 0.19 vol.% of CCG loading. However, Tonset

slightly decreased with increasing CCG volume fraction due to

the thermal decomposition of residual labile oxygen functional

groups of CCG.16 The midpoint decomposition temperature

(Tmidpoint) of PS-CCG increased �30 �C in comparison with

pristine PS, irrespective of the CCG volume fraction. The great

improvement in Tonset at very low CCG loading and the fact that

no changes in Tmidpoint were observed, irrespective of the CCG

loading, indicate that the CCG sheets were well-dispersed in the

PS matrix and well-interacted with the PS chains. Consequently,

a small amount of CCG is sufficient to create the inflammable

jammed network that retards transport of the decomposition

products.

Pristine graphene is the strongest material with an in-plane

elastic modulus approximately 1.1 TPa.19,26,27 CCG has lower in-

plane stiffness compared to pristine graphene, ranging from 200

to 650 GPa depending on the extent of oxidation. However, these

values are still much higher than those of most polymeric

materials. CCG incorporates into the polymer matrix and acts as

the primary load-bearing component of the polymer nano-

composite, leading to the enhancement of mechanical proper-

ties.19 The mechanical properties of the PS-CCG measured by

gPercolationthreshold

Highest reportedconductivity Ref.

blending 0.12 vol.% �1 S m�1 at 2.5 vol.% 4

hnology 0.6 wt (%) �15 S m�1 at 2 wt (%) 24pounding 0.47 vol.% 2.1 S m�1 at 3.0 vol.% 25hnology 0.14 vol.% 51.2 S m�1 at 2.2 vol.% 23blending 0.38 vol.% 22.6 S m�1 at 2.2 vol.% 23blending 0.19 vol.% 72.2 S m�1 at 2.5 vol.% Present work

This journal is ª The Royal Society of Chemistry 2011

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DMA are shown in Fig. 4. The storage modulus of the PS-CCG

gradually increased with the CCG loading in both glassy and

rubbery regions. In the glassy region, a relative increase in the

storage modulus was observed with a maximum value of

approximately 28% corresponding to 1.94 vol.% of CCG loading

at 90 �C (inset of Fig. 4b). It is notable that, in rubbery regions,

the storage modulus significantly increased with CCG loading.

Fig. 4 (a) Storage modulus, (b) Relative storage modulus and (c) Tan

d curves of PS-CCG composites.

This journal is ª The Royal Society of Chemistry 2011

For 1.94 vol.% of CCG loading, the storage modulus increased

by about 25 times in rubbery plateau region and by about 40

times in rubbery flow region.

An increase in storage modulus of the PS-CCG in the glassy

region could be explained by the uniform dispersion of CCG

sheets in the PS matrix (Fig. 1) and the moderate interaction

between the CCG and the PS matrix (Fig. S1, see ESI†).

Although visual observation of the morphology of the fractured

surface of the PS-CCG pellets showed a strong interaction

between the CCG sheets and PS matrix, the main interaction

force between the CCG sheet and PS matrix is the van der Waals

force. This is because most of the functional groups of CCG had

been removed during the hydrazine reduction9,10 and PS has no

functional group. In addition, the conformation of the CCG

sheets dispersed in the PS matrix was crumpled and wrinkled

(Fig. 1), which is not an ideal conformation for stretch transfer

because these conformations have the tendency to unfold rather

than stretch in-plane under an applied tensile stress.19 In

contrast, a remarkable enhancement in the storage modulus of

PS-CCG in the rubbery regions may arise from a large difference

in the modulus between the CCG and PS at temperature above

the glass transition temperature (Tg).19

Fig. 4c shows the Tan d curves for PS-CCG as a function of

CCG loading. It is evident that the Tan d significantly decreases

with an increase in CCG loading. The addition of CCG resulted

in a sharp decrease in the area under the damping peak indicating

that the segmental mobility of the PS chains during glass tran-

sition was significantly limited and obstructed by the presence of

the CCG sheets.28,29 However, Tg determined from the Tan

d peak decreased slightly with an increase in CCG content, from

115.2 to 114.7, 114.5, 114.0 and 112.0 �Cwith 0.24, 0.48, 0.96 and

1.94 vol.% of CCG loading, respectively. The Tg determined by

DSC also showed a gradual decrease from 107.1 to 101.6 �C at

2.45 vol.% of CCG loading (Fig. S2, see ESI†). The decrease of

the Tg could be explained by the poor wetting of the CCG surface

with the PS matrix.23

Conclusions

The PS-CCG prepared by solution blending exhibited a perco-

lation threshold as low as 0.19 vol.% and an electrical conduc-

tivity as high as 72.18 S m�1 at only 2.45 vol.%, the highest

conductivity of a polymer-graphene nanocomposite with a low

graphene volume fraction ever reported. The superior electrical

conductivity of the PS-CCG is a result of the combination of

CCG’s high electrical conductivity and the good dispersion of the

CCG in the PS matrix. A great improvement in the thermal

properties of polystyrene was achieved with a small amount of

CCG loading. In addition, the mechanical properties of the PS

were also improved by incorporation with CCG. The superior

conductive PS-CCG composite has great potential for applica-

tion as an electromagnetic interference shielding material.

Notes and references

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