Design and preparation of graphene/poly(ether ether ketone)composites with excellent electrical conductivity

Lilong Yang • Shuling Zhang • Zheng Chen •

Yunliang Guo • Jiashuang Luan • Zhi Geng •

Guibin Wang

Received: 27 October 2013 / Accepted: 30 November 2013 / Published online: 12 December 2013

� Springer Science+Business Media New York 2013

Abstract Graphene/poly(ether ether ketone) (m-TRG/

PEEK) composites with excellent electrical conductivity

were fabricated by hot pressing technique with thermally

reduced graphene nanosheets (m-TRG) which were modi-

fied by poly(ether sulfone). Moreover, the conductive,

thermal, and mechanical properties of PEEK/m-TRG

composites were investigated by the precision impedance

analyzer, thermal gravimetric analyzer, differential scan-

ning calorimetry, and universal tester, respectively. The

electrical conductivity of m-TRG/PEEK composites was

greatly improved by incorporating graphene, resulting in a

sharp transition from electrical insulator to semiconductor

with a low percolation threshold of 0.76 vol.%. A high

electrical conductivity of 0.18 S m-1 was achieved with

3.84 vol.% of m-TRG. The data were compared with those

of composites reduced chemically, and the results showed

that thermal reduction was an effective method to acquire

higher electrical conductive composites. The excellent

electrical property should be attributed to the large specific

surface area of m-TRG, well dispersion of m-TRG in

PEEK matrix, and good compatibility of m-TRG with

PEEK matrix, as proven by scanning electron microscope.

Besides, m-TRG/PEEK composites also exhibited rela-

tively good thermal and mechanical properties.

Introduction

Recently, conductive polymer composites have attracted

tremendous attention due to their various applications such

as electrodes for supercapacitors [1, 2], electromagnetic

interference shielding devices [3], fuel cells [4], photo-

voltaic devices [5], etc. In order to improve the conduc-

tivity of polymer composites and reduce the percolation

threshold (/c), the conducting fillers are usually adopted,

which include natural graphite (NG) flake [6], carbon black

[7], single-walled carbon nanotubes and multi-walled car-

bon nanotubes [8, 9], graphene nanosheets [10], metal

powders, and so on.

Graphene, monolayer of carbon atoms arranging in a

honeycomb network, has recently gained revolutionary

aspirations because of its remarkable electrical [11], ther-

mal [12, 13], and mechanical properties [14, 15]. Prepa-

ration of graphene from colloidal suspensions of graphene

oxide (GO) is still considered as the most effective way to

produce graphene on a large scale. As the precursor of

graphene, GO is reduced into graphene mainly by chemical

or thermal approaches [16–18]. Many reports about

graphene-based composites have confirmed the efficacy of

graphene in obtaining low electrical percolation threshold.

The lowest electrical percolation threshold was 0.1 vol.%

reported by Stankovich et al. [19] for polystyrene (PS)

solution blended with isocyanate-treated GO and followed

by solution-phase reduction with dimethylhydrazine. Ste-

urer et al. [20] found percolation threshold of thermally

reduced graphene (TRG) ranged from 1.3 to 3.8 vol.% for

different polymers even using the similar procedures.

L. Yang � S. Zhang � Z. Chen � Y. Guo � J. Luan � Z. Geng �G. Wang (&)

College of Chemistry, The Key Laboratory of Super Engineering

Plastics, Ministry of Education, Jilin University,

Changchun 130012, China

e-mail: wgb@jlu.edu.cn

J. Luan

College of Quartermaster Technology, Jilin University,

Changchun 130062, China

Z. Geng

College of Environment Science, Northeast Normal University,

Changchun 130024, China

123

J Mater Sci (2014) 49:2372–2382

DOI 10.1007/s10853-013-7940-2

Hu et al. [21] prepared PS/graphene composites by in situ

emulsion polymerization and a reduction of GO using hydra-

zine hydrate, giving a conductivity of 2.9 9 10-2 S m-1 for

composites containing 2.0 wt% graphene.

As the matrix of high-performance composites, poly

(ether ether ketone), PEEK is widely used in the areas of

aerospace, nuclear industries, automobile, machinery and

chemical industries, etc. PEEK is a semi-crystalline engi-

neering thermoplastic with outstanding properties such as

superior thermal stability, good resistance to solvent and

wear, high-glass transition, and mechanical strength [22,

23]. Numerous efforts have been devoted to further improve

the properties of PEEK by incorporation with fillers such as

carbon fibers, glass fibers, and nanoparticles [24, 25].

Despite some investigations of PEEK composites have been

conducted, reports about graphene/PEEK composite with

excellent electrical conductivity have rarely been published

[26]. The biggest challenge to prepare graphene/PEEK

composites is the difficulty in the dispersion of graphene and

the formation of graphene conductive network in the PEEK

matrix, due to the strong solvent resistance, high melt vis-

cosity, and melt-processing temperature of PEEK matrix.

In this paper, m-TRG/PEEK composites were success-

fully fabricated by hot pressing technique with thermally

reduced graphene nanosheets (m-TRG) which were modi-

fied by poly(ether sulfone), PES. We adopted the method to

mainly consider the following three reasons. Firstly, PES

could be dissolved in the most common solvents, and the

p-bonded interaction occurred between the aromatic struc-

ture of PES and the surface of TRG, which resulted in

avoiding the agglomeration of TRG. Secondly, PEEK will

be compatible with PES, because the molecular structure of

PES is similar to that of PEEK, which insures well disper-

sion of m-TRG in PEEK and strong interfacial adhesion

between m-TRG and PEEK. Finally, PES and TRG have

good thermal stability, and they can meet the melt-pro-

cessing condition of PEEK. The obtained m-TRG/PEEK

composites possessed excellent electrical conductivity and

low percolation threshold. It is well known that before

chemical reduction, GO should be modified with surfactant,

polymer, or silane coupling agent to prevent its aggregation

[27]. As a contrast, 3-triethoxysilylpropyl-amine (KH550)-

functionalized graphene/PEEK composites were prepared,

and their electrical conductive properties were investigated.

Experimental section

Materials

Scheme 1 showed the chemical structures of PEEK and PES,

which were supplied in powder form by Changchun Jilin

University Super Engineering Plastics Research Co. Ltd.

(People’s Republic of China). The melt flow index of PEEK is

24 g 10 min-1 (400 �C, 5 kg load), the density is

1.29 g cm-3, and the inherent viscosity of PES is 0.53 dL g-1.

The graphite was purchased from Bay Carbon, Inc. (USA), and

the density is 2.1 g cm-3. Hydrochloric acid (36–38 wt%),

sulfuric acid (98 wt%), N,N-dimethyl formamide (DMF, 99.5

wt%), hydrazine hydrate (80 wt%), potassium permanganate

(AR), potassium persulfate (AR), phosphorus pentoxide (AR),

and hydrogen peroxide (30 wt%) were bought from Beijing

Chemical Factory (China). KH550 was obtained by Nanjing

Fine Chemical Co. Ltd. (China).

Preparation of graphene

Graphene nanosheets were prepared by complete oxidation of

pristine graphite, then thermal exfoliation, and reduction of

GO. GO was synthesized from graphite powder by a modi-

fied Hummers method [28, 29]. The dried GO powder was

inserted into a tube furnace and slowly heated to 1000 �C, the

temperature was maintained for 10 min to obtain TRG. The

density of the resulted TRG was 1.7 g cm-3.

We have adopted the following procedure to obtain

KH550-functionalized graphene. Firstly, H2O (100 mL) and

GO powder (1.0 g) were added into a three-necked flask

equipped with a magnetic stirring under ultrasonication for

30 min; then, KH550 (6 mL) was added dropwise into the

above solution. The hydrolysis of KH550 and condensation of

GO were carried out simultaneously at 70 �C for 4 h to pro-

duce the functionalized GO solution. Secondly, on completion

of the reaction, 15 mL of hydrazine hydrate (80 %) solution

was added into the flask, which was put into an oil bath

(100 �C) for 10 h. After this step, GO was reduced to graph-

ene. Finally, the solution was filtered and washed by deionized

water. The final products were stored in a vacuum oven at

60 �C until the weight was constant, and the product-marked

CRG–KH550 was acquired by a subsequent low-temperature

thermal reduction at 380 �C for 10 h. The density of the CRG–

KH550 was 1.98 g cm-3. Scheme 2 illustrated the prepara-

tion processes of the KH550-functionalized graphene. The

two-step processes include KH550 hydrolysis in solution and

covalent interaction between GO and KH550, and reduction

of KH550–GO into KH550–graphene.

TRG nanosheets modified with PES (m-TRG)

Firstly, 0.1 g TRG nanosheets were added into a small

beaker with 10 mL DMF under ultrasonication for 30 min.

Scheme 1 Chemical structures of PEEK (left) and PES (right)

J Mater Sci (2014) 49:2372–2382 2373

123

Subsequently, 0.5 g PES was added to the beaker, and the

ultrasonication was continued for another 1 h. Lastly, the

mixture was poured into 40 mL deionized water, and the

precipitate came into being, which was collected by fil-

tration. The product was washed with deionized water five

times to remove DMF completely.

Fabrication of graphene/PEEK composites

Graphene/PEEK composites were fabricated according to

the following process. First, 2.0 g PEEK powder was dis-

persed with 20 mL ethanol in a beaker. Second, different

quantity of graphene was dispersed with 10 mL ethanol in

another beaker under ultrasonication for 30 min. Third, the

two above-mentioned dispersions were mixed with mag-

netic stirring, then filtered, and dried in a vacuum oven at

60 �C for 24 h, and the desired samples with different

contents of PEEK composites were obtained. The com-

posite film was fabricated by hot pressing technique under

370 �C with the pressure of 15 MPa. The weight and

corresponding volume fraction of graphene in PEEK

matrix were summarized in Table 1.

Characterization

The alternating current (AC) conductivity of graphene/

PEEK composites was measured with an Agilent 4294A

precision impedance analyzer. For AC conductivity mea-

surement, parts of the samples were cut into small round

specimens with diameter of 10 mm, and two opposite

surfaces of the samples were coated with silver conductive

glue to reduce contact resistance between sample and

electrodes. Four specimens of each sample were tested, and

the most accurate value was reported as AC conductivity.

The density of the samples was measured by an Alfa

Mirage SD-200L electronic densimeter.

The morphology of graphene was characterized using

transmission electron microscopy (TEM JEOL JEM-1200

EX) at an accelerating voltage of 200 kV. Graphene

nanosheets were dispersed in DMF by ultrasonication, and

some pieces were collected on carbon-coated copper grids

for observation.

The Fourier-transform infrared (FTIR) spectrum was

measured via the KBr pellet method using a Nicolet Impact

410 FTIR spectrometer.

X-ray diffraction (XRD) patterns were obtained directly

on the powder sample of graphite, GO, and graphene using

an Empyrean II X-ray diffractometer (PANalytical B.V.

Netherlands). Samples were scanned at 3� min-1 using Cu

Ka radiation (0.154 nm) with a filament voltage of 45 kV

and current of 40 mA.

The dispersion of the m-TRG in the PEEK matrix and

the microstructure of the composites were observed by

scanning electron microscope (SEM HITACHI-SU8020).

m-TRG/PEEK composites were freeze-fractured in liquid

nitrogen, and the fractured section was coated with a thin

layer of gold before observation. The surface microstruc-

ture of m-TRG/PEEK composites and element analysis

were observed from SSX-550 Shimadzu SEM equipped

with energy dispersive X-ray.

Tensile test was carried out on a Shimadzu AG-1 uni-

versal testing machine without strain gauge type exten-

someter at room temperature. The rate for the tensile test

was 10 mm min-1. The tensile strength, elongation at

break, and Young’s modulus were obtained directly from

the stress–strain curves. The average values of five speci-

mens were reported. The samples were molded by an

injection-molding machine into a dumbbell-shaped bar of

30.0(length) 9 4.0 (width) 9 2 (thickness) mm3.

Scheme 2 Illustration of the reaction between GO and KH550, and

the reduction of KH550–GO into KH550–graphene

Table 1 The weight and

corresponding volume fraction

of graphene in PEEK matrix

m-TRG/PEEK (wt%) 0 0.5 0.8 1 1.5 2 3 4 5

m-TRG/PEEK (vol.%) 0 0.38 0.61 0.76 1.14 1.53 2.29 3.06 3.84

CRG–KH550/PEEK (wt%) 0 0.3 0.5 1 2 3 5 6.5 8

CRG–KH550/PEEK (vol.%) 0 0.2 0.33 0.65 1.32 1.98 3.34 4.38 5.43

2374 J Mater Sci (2014) 49:2372–2382

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Thermal gravimetric analysis (TGA) measurement was

determined under nitrogen at a heating rate of 10 �C

min-1. A sample was contained within an open platinum

pan on a Perkin-Elmer Pyris 1 TGA. Differential scanning

calorimetry (DSC) measurements were performed on a

Mettler-Toledo DSC 821e instrument at a heating rate of

20 �C min-1 under nitrogen.

Results

Characterization of graphene

The TGA curves of GO and graphene were shown in

Fig. 1. Obviously, the difference in structure and chemical

composition directly influenced the thermal stability of GO

and graphene. The weight loss of 47.5, 11.5, and 2.2 % for

GO, CRG–KH550, and TRG at 600 �C, respectively,

illustrated the different contents of the residual oxygen-

containing functional groups on GO and graphene surface.

The weight loss occurred after 600 �C related to the ther-

mal decomposition of graphene. It could also be seen that

GO showed a higher weight loss than graphene, which

demonstrated that most of the oxygen-containing func-

tional groups had been eliminated in the process of

reduction of GO [30]. Thereby, graphene with higher

thermal stability could be fully suitable for the conditions

of PEEK melt processing.

FTIR spectra revealed the presence of the covalent

bonds between graphene and KH550. As shown in Fig. 2,

the FTIR spectrum of CRG exhibited three characteristic

peaks at 3443, 1660, and 1089 cm-1, indicating the pre-

sence of the hydroxyl, benzene carboxyl, and epoxy

groups, respectively. The existence of these groups might

be due to the incomplete reduction of GO. From the CRG–

KH550 FTIR spectrum, we could see the peaks at 2926 and

2894 cm-1 belonging to C–H aliphatic stretching vibra-

tions and the peak at 1080 cm-1 for Si–O–C stretching

vibrations, which indicated that the KH550 was attached to

the graphene successfully. The disappearance of the –OH

stretching vibrations (3443 cm-1) in the FTIR spectrum of

the CRG–KH550 showed that the –O(C2H5) group of

KH550 has hydrolyzed and coupled completely to

graphene.

The TEM micrographs of chemical reduction (a: CRG–

KH550), thermal reduction (b: TRG) graphene and

graphene nanosheets modified with PES (c: m-TRG) were

shown in Fig. 3. The results indicated that both of CRG–

KH550 and TRG existed in the form of transparent ultra-

thin film with a few ripples within the plane, and PES

between the graphene sheets prevented the stack of

graphene sheets and improved the dispersion of graphene.

It was known that perfect two-dimensional graphene

crystal was thermodynamically unstable; therefore, corru-

gations and ripples in the two dimension of the graphene

were formed for thermodynamic stability [31, 32]. Com-

pared with the images of the CRG–KH550 (Fig. 3a) and

TRG (Fig. 3b), we could easily find that CRG–KH550 had

a smaller size and uniformly dispersed in the solution

because of the coupling effect between the graphene layers.

Compared to KH550-functionalized graphene, the p-bon-

ded interaction between TRG and PES did not disrupt the

pristine structure of graphene sheets. Consequently, TRG

still retained high conductivity and good thermal stability.

Nevertheless, the transparency and rippled feature of the

graphene suggested that the graphene prepared was few-

layer or even monolayer.

Fig. 1 TGA curves of GO and graphene Fig. 2 FTIR absorption spectra of CRG and CRG–KH550

J Mater Sci (2014) 49:2372–2382 2375

123

To further verify the nanostructure change of the

graphene by different reduction methods, SEM observa-

tions were performed, and Fig. 4 showed the SEM images

of NG and graphene prepared under chemical reduction

and thermal reduction conditions. Figure 4a showed the

compact layers of graphene sheets in graphite, while

Fig. 4b, c showed the highly wrinkled morphologies of

graphene with loose and porous nanostructure, which were

due to the opening of plain graphitic networks during

oxidation and exfoliation. It could be obviously observed

that compared with TRG, CRG–KH550 was sufficiently

exfoliated to thin graphene sheets with obvious larger

interlayer spacing, which was in relation to the reduction

method.

XRD was used to characterize the changes in the

nanostructure of GO reduced under different conditions.

And, the XRD patterns of NG, GO, and graphene were

given in Fig. 5. As shown in Fig. 5a, the strong and sharp

diffraction peak of pristine graphite at 26.56� completely

disappeared after oxidization and a new peak at 11.36�appeared instead, indicating a complete oxidization of

graphite, which was a prerequisite to obtain exfoliated

graphene. After reduction, there was no significant dif-

fraction peak, which demonstrated a complete exfoliation

of GO with the disruption or collapse of long-range

periodic-layered structure caused by the decomposition of

various oxygen-containing functional groups. On the other

hand, the TRG and CRG–KH550 exhibited almost the

same diffraction curve, indicating that the different

reduction methods had no obvious influence on the long-

range structure of graphene. However, when the diffraction

pattern was further analyzed, as shown in Fig. 5b the low-

diffraction peaks in the vicinity of 2h = 25� might be

ascribed to the accumulation of reduced sheets due to weak

interactions. Irrespective of the broadening effects caused

by the defects, the diffraction peaks of graphene with low

intensity were analyzed with Bragg equation (2dsinh = k),

as well as NG and GO for comparison [33]. The diffraction

angle (2h), interlayer distance (d), and full width at half

maximum (FWHM) were summarized in Table 2.

Energy dispersive spectrometry (EDS) is an effective

method to provide information regarding the distribution

and quantity of each element in the sample. Thus, the line

and element-mapping photographs of (a, c) CRG–KH550

and (b, d) m-TRG were shown in Fig. 6. The sample was

coated with a thin layer of gold before observation. The

main components of graphene are carbon and oxygen, the

presence of oxygen might be the result of incomplete

reduction of GO. This result could be used to explain why

the peak of hydroxyl (3443 cm-1) appeared in the FTIR

Fig. 3 TEM micrographs of CRG–KH550 (a), TRG (b), and m-TRG nanosheets (c). The black dots on the surface of graphene sheet in c are

PES

Fig. 4 SEM images of natural graphite (a), CRG–KH550 (b), and TRG (c)

2376 J Mater Sci (2014) 49:2372–2382

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spectrum of graphene. Compared with CRG–KH550, it

could be seen obviously that m-TRG had a higher ratio of

carbon to oxygen atom, indicating a higher reduction

degree and higher amount of restored sp2-carbon networks,

which consequently resulted in a higher electrical con-

ductivity. The well-dispersed blue dots, which represented

the position of the silicon of KH550 and sulfur of PES

element in graphene, and the more number of the blue dots

illustrated the higher element content in sample.

Discussion

Electrical properties of graphene/PEEK composites

In the research of inorganic–organic conducting-polymer-

based composites, the critical volume fraction at the per-

colation threshold is a key parameter [34]. Figure 7 pro-

vided the relationship between the electrical conductivity

of the composites and the graphene content; both graphene/

PEEK composites exhibited a transition from insulator to

semiconductor, the conductivity of graphene/PEEK com-

posites sharply improved at lower graphene contents, fol-

lowed by slowly raising at higher concentrations. The inset

of Fig. 7 which indicated their electrical conductivity (r)

obeyed the power law [35]:

r / r0 /� /cð Þt for / [ /c; ð1Þ

where r0 is the bulk electrical conductivity of the filler, /is the filler volume fraction, and t is the critical exponent

describing the rapid variation of r near percolation

threshold (/c). The percolation threshold is the critical

content, above which a continuous connected network

formed for the transport of electrons throughout the matrix.

As shown in the inset of Fig. 7, the double-logarithmic plot

of the power law, the conductivity of graphene/PEEK

composites agreed with the percolation behavior predicted

by Eq. 1. Especially, for m-TRG/PEEK composites, when

/c = 0.76 vol.% and t = 5.79, the straight line fitted well

with the experimental data. The relatively high t value was

owing to the high aspect ratios of the m-TRG and its dis-

persion state. For the CRG–KH550/PEEK system, the

percolation threshold was as low as 0.2 vol.% and

t = 3.24; the reason was that the CRG–KH550 was well

dispersed. The result was also confirmed by Fig. 3b that the

m-TRG with the large specific surface area was inclined to

aggregate, which partly destroyed the electron transport

networks in the composites, and limited the enhancement

of conductivity.

In comparison to CRG–KH550, when the m-TRG

nanosheets were incorporated into PEEK matrix, the

electrical conductivity of composites quickly increased to

1.78 9 10-1 S m-1 at the content of merely 3.84 vol.%.

Actually, at 1.53 vol.% of m-TRG content, the conductivity

of m-TRG/PEEK composites was 2.00 9 10-4 S m-1,

which was higher than 10-6 S m-1 of the antistatic crite-

rion. However, for the CRG–KH550/PEEK system, the

conductivity was recorded as 1.0 9 10-2 S m-1, and the

amount of graphene was up to 5.43 vol.%. This could be

attributed to the higher conductivity of the m-TRG nano-

sheets that originated from their extensive conjugated

Fig. 5 XRD patterns of natural graphite, GO, and GO reduced at different methods

Table 2 Principal characteristics of NG, GO, and graphene obtained

from XRD

NG GO TRG CRG–KH550

2h (�) 26.56 11.36 25.79 23.84

d (nm) 0.34 0.78 0.35 0.37

FWHM (�) 0.27 1.73 5.06 10.24

J Mater Sci (2014) 49:2372–2382 2377

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Fig. 6 EDS patterns of CRG–KH550 (a), m-TRG (b), Si-mapping (c), and S-mapping (d). Blue dots indicated silicon and sulfur (Color figure online)

Fig. 7 Dependence of the conductivity of the graphene/PEEK composites on the graphene volume fraction (/G), measured at room temperature

and 103 Hz. The inset indicated the best fits of the conductivity to Eq. 1

2378 J Mater Sci (2014) 49:2372–2382

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sp2-carbon networks, which was absent in the coupling

agent-functionalized graphene nanosheets. The CRG–

KH550 nanosheets contained large fraction of distorted

sp3-hybridized carbon units which limited the p–p inter-

actions [36]. As evidenced by Fig. 6b, the high carbon to

oxygen atomic ratio of m-TRG illustrated the higher

graphitization.

The dependency of the AC conductivity of graphene/

PEEK composites on the frequency was shown in Fig. 8.

Apparently, the AC conductivity of graphene/PEEK com-

posites increased almost linearly with increasing the fre-

quency when the content of graphene was lower, i.e., less

than 1 wt% for m-TRG/PEEK composites, and the 0.3 wt%

for CRG–KH550/PEEK system, which demonstrated their

insulating behavior [37]. However, for semiconductors, the

conductivity of the composites exhibited a plateau at first

and then increased a little at high-frequency regime, for

instance, the m-TRG/PEEK composites at 1.5 wt%, CRG–

KH550/PEEK system at 0.5 wt%. When the content of

graphene continued to increase, the conductivity became

independent on frequency which ranged from 100 Hz to

1 MHz. This could be attributed to the graphene nano-

sheets which effectively provided the tunneling for the hop

of electrons.

Morphology of m-TRG/PEEK composites

Figure 9 showed the surface micrographs of pristine and

fracture samples of m-TRG/PEEK composites with the

5 wt% m-TRG content. As shown in Fig. 9a, b, the graphene

sheets were uniformly dispersed and embedded into the

PEEK matrix with some bundles and scarcely any aggrega-

tion on the composite surface. Apparently, wrinkled layers of

m-TRG/PEEK composites were observed by their cross-

section SEM micrographs (Fig. 9c, d). The SEM images of

the fracture surface also provided additional evidence of

good dispersion of graphene. In addition, the boundary

between the m-TRG and the PEEK matrix was obscure,

because the TRG was wrapped or covered by polymer

matrix, implying strong interfacial adhesion between the

PEEK matrix and TRG sheets. As discussed above, the well

dispersion of graphene played an important role in achieving

excellent conductive property of the composites. On the

contrary, any heterogeneity or aggregation could result in

structural defects which would make detrimental effects on

the mechanical properties of the composites.

Mechanical properties of m-TRG/PEEK composites

In general, mechanical performance of composite basically

depends on the features of the reinforcement filler, such as

shape, size, strength and modulus, the influence of filler on

matrix crystallization ability, the dispersion of filler in matrix,

the interaction of filler and matrix, and the efficiency of

interfacial stress transfer. It could be seen in Fig. 10 that the

composites of the m-TRG/PEEK had a tensile strength of

96.6–106.2 MPa, Young’s modulus 2.21–3.07 GPa, and

elongation at break 4.7–98.3 %. Especially, the elongation at

break of m-TRG/PEEK composites drastically decreased with

increasing the weight fraction of m-TRG, which was normal

for composites. The m-TRG restricted the plastic deformation

of the matrix, which led to the drop of elongation at break.

However, the tensile strength of m-TRG/PEEK composites

was almost not changed. For semi-crystalline polymer, the

crystallization ability could also affect the mechanical prop-

erties, a higher crystalline fraction meaning a higher strength

[38]. Additionally, the Young’s modulus of m-TRG/PEEK

composites increased compared with that of pure PEEK,

owing to high aspect ratio and stiffness of graphene. The data

of the mechanical properties indicated that these composites

still exhibited relatively good mechanical properties, which

were guarantees for their practical application.

Fig. 8 Dependence of alternating current conductivity of graphene/PEEK composites on frequency

J Mater Sci (2014) 49:2372–2382 2379

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Thermal properties of m-TRG/PEEK composites

The thermal properties of m-TRG/PEEK composites with

various m-TRG contents were given in Table 3. It could be

seen that there was only a little change in the glass tran-

sition temperature (Tg), just from 148 to 156 �C, which

could be ascribed to the strong interaction between the

filler and the matrix that hindered the segmental motion of

polymer chains.

In order to examine the thermal stability of m-TRG/

PEEK composites, several TGA experiments were per-

formed under N2 atmosphere. As it could be seen in

Table 3, 5 % weight loss decomposition temperature (DT5)

and 10 % weight loss decomposition temperature (DT10) of

the composites were much higher than those of pure PEEK,

which might be attributed to the restriction of the thermal

mobility of PEEK chains near the graphene surface.

Simultaneously, the layered graphene with the outstanding

Fig. 9 SEM images with low (left column) and high (right column) magnification obtained from the surface (a, b) and fracture surface (c, d) of

composite sample with m-TRG content 5 wt%. The selected regions in b and d are graphene sheets

Fig. 10 The mechanical properties of m-TRG/PEEK composites

2380 J Mater Sci (2014) 49:2372–2382

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gas barrier properties confined the transport of the

decomposition products [39].

Conclusion

In summary, the composites with excellent electrical con-

ductivity were fabricated by hot pressing technique based

on PEEK matrix and graphene. The electrical conductivity

of the composites with the m-TRG content 3.84 vol.% was

0.18 S m-1 , and the percolation threshold of the m-TRG/

PEEK composites was 0.76 vol.%. The performance of the

TRG and CRG–KH550 greatly affected the electrical

conductivity of composites. Difference of surface chemical

condition resulted in the different interfacial adhesion and

dispersion of graphene. Meanwhile, m-TRG/PEEK com-

posites exhibited relatively good thermal and mechanical

properties. The remarkable properties of the m-TRG/PEEK

composites should be attributed to the homogeneous dis-

persion of m-TRG and strong interfacial interaction

between m-TRG and PEEK matrix. The composites we

prepared may be particularly suitable for some harsh

environments in the electrical power industry.

Acknowledgements This work was supported by PhD Program

Foundation of Ministry of Education of China (No. 20120061110017)

and Jilin Provincial Science and Technology Development Project of

China (20126025).

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Table 3 Thermal properties of m-TRG/PEEK composites

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PEEK 148.55 ± 0.38, n = 3 575.3 ± 0.2, n = 3 581.5 ± 0.5, n = 3

0.5 % m-TRG/PEEK 148.61 ± 0.34, n = 3 581.8 ± 0.3, n = 3 589.3 ± 0.3, n = 3

1 % m-TRG/PEEK 156.61 ± 0.42, n = 3 586.4 ± 0.2, n = 3 596.0 ± 0.2, n = 3

3 % m-TRG/PEEK 154.46 ± 0.43, n = 3 599.5 ± 0.2, n = 3 609.3 ± 0.3, n = 3

5 % m-TRG/PEEK 149.46 ± 0.30, n = 3 598.3 ± 0.3, n = 3 608.0 ± 0.1, n = 3

a After quenching in liquid nitrogen from the heating trace of DSC measurements conducted at a heating rate of 20 �C min-1

b 5 % weight loss temperature measured by TGA at a heating rate of 10 �C min-1 in N2

c 10 % weight loss temperature measured by TGA at a heating rate of 10 �C min-1 in N2

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