Communication

894

Functionalized Graphene–PVDF FoamComposites for EMI Shieldinga

For my mother, Lakshmidevi

Varrla Eswaraiah, Venkataraman Sankaranarayanan,Sundara Ramaprabhu*

Novel foam composites comprising functionalized graphene (f-G) and polyvinylidene fluoride(PVDF) were prepared and electrical conductivity and electromagnetic interference (EMI)shielding efficiency of the composites with different mass fractions of f-G have been inves-tigated. The electrical conductivity increases with the increase in concentration of f-G ininsulating PVDF matrix. A dramatic change in the conductivity is observed from 10�16 S �m�1

for insulating PVDF to 10�4 S �m�1 for 0.5wt.% f-Greinforced PVDF composite, which can be attributed tohigh-aspect-ratio and highly conducting nature of f-Gnanofiller, which forms a conductive network in thepolymer. An EMI shielding effectiveness of �20dB isobtained in X-band (8–12GHz) region and 18dB in broad-band (1–8GHz) region for 5wt.% of f-G in foam compo-site. The application of conductive graphene foamcomposites as lightweight EMI shielding materials forX-band and broadband shielding has been demonstratedand the mechanism of EMI shielding in f-G/PVDF foamcomposites has been discussed.

Introduction

Plastic foams or cellular plastics also referred as expanded

or sponge plastics are having many attractive properties

such as low density, flexibility, thermal insulation, impact

damping, and good structure stability.[1] It is hard to go a

V. Eswaraiah, Prof. S. RamaprabhuDepartment of Physics, Alternative Energy and NanotechnologyLaboratory (AENL), Nanofunctional Materials Technology Centre(NFMTC), Indian Institute of Technology, Madras 600036, IndiaFax: (þ91) 44 22570509; E-mail: ramp@iitm.ac.inV. Eswaraiah, Prof. V. SankaranarayananDepartment of Physics, Low Temperature Physics Laboratory,Indian Institute of Technology, Madras 600036, India

a Supporting Information for this article is available from the WileyOnline Library or from the author.

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day without coming across some sort of polymer foam.

Polymer foams are found virtually everywhere in the

modernworldandareused inawidevarietyof applications

such as in packaging of food, cushioning of furniture,

protective equipment,medical devices, transportation, and

in thermal insulation.[2–7] There are various reports on

preparation routes and enormous applications of polymer

foams in different areas.[8–11] However, limited studies

havebeendone inthearenapertainingpolymer foamswith

conductingnanofillerswhichfind tremendousapplications

in various fields such as electrostatic discharge, electro-

magnetic interference shielding,[12] strain sensor, flexible,

and light weight actuators. In recent years electronics field

has diversified in telecommunication systems, cellular

phones, high speed communication systems, military

devices, wireless devices, etc.[13] Due to the increase in

use of high operating frequency and band width in

library.com DOI: 10.1002/mame.201100035

Functionalized Graphene–PVDF Foam Composites for EMI Shielding

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electronic systems, especially in X-band and broad band

frequencies, there are concerns and more chances of

deterioration of the radio wave environment known as

electromagnetic interference (EMI). This EMI has adverse

effects on electronic equipments such as false operation

due to unwanted electromagnetic waves and leakage of

information in wireless telecommunications.[14–17] Con-

ventionally, metals and metallic composites are used as

EMI shielding materials as they have high shielding

efficiency owing to their good electrical conductivity. Even

thoughmetals are good for EMI shielding, they suffer from

poorchemical resistance, oxidation, corrosion,highdensity,

and difficulty in processing.[18] Hence attention has been

paid to the development of effective EMI shielding

materials using polymer nanocomposites. Polymer compo-

sites containing carbon-based nanofillers have been

studied extensively for EMI shielding due to the unique

combination of electrical conductivity of the nanofiller and

the flexibility of polymer.[19,20]

Graphene sheets,[21] one atom thick two-dimensional

layers of sp2 bonded carbon atoms are predicted to

have unusual properties. One possible route to harness

these properties for applications would be to incorporate

graphene sheets in a composite material.[22] Graphene-

based composites havemany advantages for EMI shielding

due to its low cost, high aspect ratio, and light weight.

Currently, even though nanocomposites employing

carbon-based reinforced materials are dominated by

carbon nanotubes (CNTs), the inherent bundling, intrinsic

impurities from catalysts, and high cost are hampering

their application. Graphene reinforced polymer composites

exhibit outstanding structural, electrical, and mechanical

properties compared with that of CNTs and carbon

nanofibers (CNFs) reinforced polymer composites.[23] The

low price and availability of the pristine graphite in large

quantities coupled with relatively simple solution process

makes graphene a potential choice as conductive filler

in the preparation of conductive foam composites. The

superior aspect ratio of graphene helps in reinforcing the

polymer at lower loadings. EMI shielding properties of

puregrapheneandCNTfilmshavebeen tested in our earlier

report,[24] and the study aims at mechanism of EMI

shielding. Most of the practical EMI shielding applications

demand delicate and light weight materials and hence,

graphene-based polymer composites are promising mate-

rials. Liang et al.[25] studied EMI shielding effectiveness (SE)

of graphene–epoxy composite over a frequency range of

8.2–12.4GHz, and an EMI SE of 21 dB was reported for

15wt.-% loadingof graphene. This loading is quitehigher as

compared with CNTs reinforced polymer composites for

EMI shielding. Herein we demonstrate the first graphene-

based polymer foam composite that can be used for broad

band EMI shielding applications with lowest loadings of

graphene.

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Macromol. Mater. Eng. 2

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In the present study, a polymer foam nanocomposite

with the combination of functionalized graphene (f-G) and

polyvinylidene fluoride (PVDF) has been used for EMI

shielding. The surface of the graphene was modified with

hydroxyl and carbonyl groups in order to avoid the

aggregation of graphene nanofillers in the polymer and

to disperse graphene in the solvent without prolonged

sonication since it was reported that functionalized

graphene sheets in polymer matrices improves the

solubility of the f-G in polymers.[26] Selection of PVDF

was based on its unique properties, i.e., highest chemical

resistance and high temperature sustainability and also

considering its applications inwide variety of fields such as

piezoelectric, pyroelectric, etc. The present study therefore

aims to develop a novel, low cost, light weight, and flexible

EMI shielding material with functionalized graphene

reinforced PVDF foam composite.

Experimental Section

Materials

A few layered graphene was prepared in our laboratory which is

having a thickness of 2–5nm and lateral dimensions of 20–

40mm.[27] The polymer matrix used in this work was poly(viny-

lidene fluoride) powder and foaming agent (2,20-azobisisobutyr-

onitrile, AIBN) obtained from Alfa Aesar, Ward Hill, MA. Dimethyl

fomamide (DMF) was obtained from RFCL limited, New Delhi.

Sulfuric acid (98% H2SO4) and nitric acid (69% HNO3) in analytical

grade were purchased from Ranbaxy, India. Millipore water was

used for washing purposes.

Synthesis of f-G–PVDF Foam Composites

The method of preparation of the composites was according to

literature with slight modification.[12] The experimental procedure

can be divided into two segments. In the first segment, pure

graphene was functionalized with a combination of H2SO4 and

HNO3 in3:1 ratio at70 8C for 6h followedbywashingwithMillipore

water several times. Then the sample was dried in vacuum at 70 8Cfor 6h and this sample is labeled as f-G. In the second segment, f-G

was dispersed in 80ml of DMF solution by ultrasonicating for 1h to

obtain a good dispersion and it is labeled as sample 1. Desirable

amount of PVDF along with 5wt.-% foaming agent, AIBN, was

dissolved in80mlofDMFandit is labeledassample2.Samples1and

2 were mixed together and ultrasonicated to achieve black colored

suspension. In order to avoid sedimentation of the particles, the

mixed solution was transferred into a shear mixer and stirred at

2000 rpm for 2h. Upon completion of stirring, the composite

solutionwaspoured intopetri dishandkept in theovenat70 8Covernight. Afterwards the dried thin filmwas thermally cured in hot air

oventoformthethinfilmwithoutanysolvent. Finallythecomposite

film was folded and cut into pieces and kept in stainless steel mold

and hot pressed to form thicker structures. In the hot pressing

process, the foaming agent was decomposed and gave off nitrogen

gas within the f-G/PVDF to eventually generate foam composite

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Scheme 1. Schematic representation of the functionalization of graphene and itsreinforcement in the PVDF matrix.

896

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V. Eswaraiah, V. Sankaranarayanan, S. Ramaprabhu

(Scheme 1). By following the procedurementioned above, a series of

foam composites were prepared starting from 0 to 11wt.-% of f-G.

Further prepared samples were cut into required dimensions

(rectangular and annular) for the EMI shielding measurements in

the particular frequency bands and for conductivity studies.

Characterization

Field emission scanning electron microscope (FESEM, QUANTA 3D)

was used to observe the morphology of the foam composite.

Electrical conductivity of the foam composites was measured with

four probe technique in order to avoid the contact resistance. A

constant current was applied using Keithley 2400 source meter to

the outer probes of the four contacts and corresponding voltagewas

measuredwith Keithley 2182 nanovoltmeter between inner probes

at roomtemperature. EMI shieldingmeasurementswereperformed

with E8362B vector network analyzer with waveguide (X 281A

adapter, Agilent Technologies) in X-band (8–12GHz) and coaxial

sample holder for the EMI shielding measurements in 1–8GHz

range. Sample holders for the measurement of EMI SE in respective

regions are shown in Supporting Information Figure S4 and S5.

Figure 1. a) Field-emission scanning electron micrographs (FES-EMs) of graphene, b) low-magnification micrograph showingthe formation of micropores and cell-size distribution, c) high-magnification micrograph showing the ultra-thin graphenenanofillers within the pores of the PVDF foam composite, andd) transmission electron micrograph of graphene.

Results and Discussion

Functionalization of graphene is an essential step in the

preparation of graphene reinforced polymer composites as

it can improve stress transfer between polymer and

graphene. The confirmation of functionalization has been

achieved by analyzing the FTIR spectra (Figure S1). FESEM

images of f-Gand5wt.-%graphene–PVDF foamcomposites

are shown in Figure 1a–c. Figure 1a shows the nature of

graphene which is wrinkled in nature. Figure 1b and c

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micrographs provide visual evidence of

the foam structure and the dispersion of

f-G nanofiller in PVDF matrix. Transmis-

sion electronmicrographof the graphene

shown in Figure 1d suggests that gra-

phene nanofillers are 3–4nm in width. It

is evident from Figure 1b that foam

structure has been formed throughout

the composite film. The estimated cell

sizes are in the range of 0.5–2mm. The

foaming agent (AIBN) used might have

helped in the formation of such foam like

structure in the polymer matrix. This

decomposes readily at higher tempera-

ture to give off large volume of nitrogen

gas. The main constituents of conven-

tional polymer foams are a solid polymer

phase and a gaseous phase derived from

the blowing agent. In the present case,

the foam composite consists of three

phases, two solid phases (base polymer

and f-G) and one gaseous phase (blowing

agent). FESEM image at higher magnification (Figure 1c)

clearly shows f-G nanofillers embedded in PVDF matrix

with sufficient interconnections. The huge surface area

(�400m2 � g�1) and hence high aspect ratio (�1000) of the

present f-G nanofiller provide large number of interconnec-

tions. This helps in achieving percolation in electrical

conduction at lower loadings leading the composite

electrically conductive.

im www.MaterialsViews.com

Figure 2. Log DC conductivity versus mass fraction of f-G compo-sites measured at room temperature. Inset: log–log plot ofconductivity versus (p – pc)/pc for the same composites.

Functionalized Graphene–PVDF Foam Composites for EMI Shielding

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The electrical conductivity of the foam composites as a

function ofmass fraction of f-G is displayed in Figure 2. It is

observed fromthefigure that below0.5wt.-%of f-G in PVDF

matrix, the conductivity changes dramatically displaying

an increase of 13 orders of magnitude, indicating the

formation of conductive percolating network. The con-

ductivity rises from 10�16 S �m�1 for pure PVDF to

�10�3 S �m�1 for f-G/PVDF foam composite with the

addition of small amount (0.5wt.-%) of f-G. The plot in the

insetofFigure2showsthat theelectrical conductivityof the

foam composites obeys power law[28]

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s / ðn�ncÞb (1)

where s is the electrical conductivity of the composite, n is

the volume fraction of f-G, nc is the critical volume fraction

of f-G, and b is the critical exponent. We assume that

mass fraction and volume fraction of polymer and f-G

Figure 3. EMI SE for f-G–PVDF composites in the a) broadband range, 1–8 GHz, b) X-bandrange, 8–12 GHz.

are almost the same. As shown in the

inset of Figure 2, log s vs. log(p�pc/pc)

plot, the conductivity of f-G/PVDF com-

posite agrees well with the percolation

behavior predicted by Equation (1). The

straight line in the Figure 2 with

percolation threshold (pc)¼ 0.5wt.-%

and t¼ 2.66 gives best fit to the data

with a correlation factor 0.98 and it

matches well with the reported percola-

tion threshold values.[29] The percolation

threshold is the critical factor above

which a continuous connected network

is formed for the transport of electrons

throughout the matrix. Apart from low

percolation threshold, we also noticed

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the conductivity of 2wt.-% f-G/PVDF foam composite

reaches 10.16 S �m�1which is 17 orders ofmagnitudemore

than electrical conductivity of PVDF.

TheEMI SE is definedas the logarithmic ratio of incoming

(Pi) to outgoingpower (Po) of radiation. Ingeneral, efficiency

of any shielding material is expressed in decibels (dB).

Higher thedecibel level of EMI SE, less energy is transmitted

through shielding material. The EMI SE of f-G/PVDF foam

composites in X-band region and broad band are displayed

in Figure 3a and b. Details of the experimental set up for

measuring EMI SE are provided in the Supporting Informa-

tion. It is observed that the SE of each composite is almost

constant in the entire frequency range. And also EMI SE

increases with increase in f-G loading in PVDF foam

composite. The SE of 1 and 5wt.-% f-G in PVDF foam

composite is found to be �7 and �18dB respectively

over a frequency range of 8–12GHz. Similarly an EMI

shielding efficiency of 20 and 28dB has been obtained for

5 and 7wt.-% f-G, respectively, in PVDF foam composite in

broadband frequency range (1–8GHz). The higher EMI

shielding efficiency (28 dB) of the foam composites in

broadband range (1–8GHz) for 7wt.-% f-G/PVDF in

comparison to EMI shielding efficiency (20 dB) of 7wt.-%

f-G/PVDF composite inX-bandmaybedue to the skin effect

of the foamcompositeathigher frequencies. The increase in

EMI SE can be attributed to the increase in conductivity of

the foam composite since graphene nanofillers formed a

conducting network in PVDF matrix. As the loading of f-G

increases in the polymer, the number of conducting f-G

interconnections increases resulting in more interaction

between the nanofillers and incoming radiation. This could

improve the shielding effectively. Yang et al.[12] reported an

EMI SE of 19 dB at 15wt.-% loading of CNFs in CNF–

polystyrene (PS) foam composites for EMI shielding

applications. The CNF–PS composites were more reflective

to EM radiation thanabsorptive. The primary EMI shielding

mechanism in the present f-G/PVDF composites is reflec-

tion and which is confirmed by the observation of

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V. Eswaraiah, V. Sankaranarayanan, S. Ramaprabhu

reflectivity of 5wt.-% f-G/PVDF foam composite. In the

present study, the reflectivity (R), transmissivity (T),

and absorptivity (A) are 0.78, 0.01, and 0.21, respectively

for the PVDF foam composite containing 5wt.-% f-G

and the result is consistent with the EMI shielding

mechanism in pure graphene film.[24] As the mass

fraction of f-G increases from 1 to 5wt.-%, the reflectivity

increases from10 to�80% of the incident electromagnetic

radiation. This reflection can be due to high electrical

conductivity, may be due to change in the permittivity

and dielectric losses of the f-G/PVDF foam composites

(Figure S2 in Supporting Information). On the basis

of this result, we infer that such f-G/PVDF foam

composites are more reflective and less absorptive

to electromagnetic radiation in both X-band and broad-

band frequencies, that is, the primary EMI shielding

mechanism of such foam composites is reflection

rather than absorption in the X-band frequency region.

The present higher EMI shielding efficiency at lower

loadings of f-G over CNT-based polymer foams can be

attributed to the high aspect ratio and high electrical

conductivity of the f-G nanofiller. The EMI SE required

for most of the commercial applications is 20 dB.

Hence the current 5wt.-% f-G reinforced PVDF foam

composites can be used as light weight EMI shielding

materials. The EMI SE at higher volume fractions of f-G

are falling in the range 21–23 dB, which is very close to

the EMI SE of 5wt.-% f-G/PVDF composite, hence the

results are not shown in the study.

Conclusion

Novel foam composites comprising f-G and PVDF

were prepared by simple technique and the typical

percolation behavior from insulating to conducting

nature was observed with the addition of low wt.-% of

f-G. EMI SE of �20 dB is obtained with 5wt.-% functio-

nalized graphene reinforced foam composite in X-band

region. The present study demonstrates the use of

functionalized graphene–PVDF foam composites for

light weight EMI shielding with lower loadings of the

nanofiller.

Acknowledgements: The authors gratefully acknowledge finan-cial support from Department of Science and Technology (DST),India and also thanks to the IIT Madras for supporting thisresearch. Special thanks to the Prof. Harishankar Ramachandranfor helping in EMI shielding measurements.

Received: January 26, 2011; Revised: April 4, 2011; Publishedonline: June 3, 2011; DOI: 10.1002/mame.201100035

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� 2011 WILEY-VCH Verlag Gmb

Keywords: composites; conductivity; electromagnetic interfer-ence; foams; functionalized graphene

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