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ProcessingofGraphenenanoribbonbasedhybridcompositeforelectromagneticshielding

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Composites: Part B 69 (2015) 472–477

Contents lists available at ScienceDirect

Composites: Part B

journal homepage: www.elsevier .com/locate /composi tesb

Processing of graphene nanoribbon based hybrid compositefor electromagnetic shielding

http://dx.doi.org/10.1016/j.compositesb.2014.09.0141359-8368/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: suwarna.datar@gmail.com (S. Datar).

Anupama Joshi a, Anil Bajaj a, Rajvinder Singh a, Anoop Anand b, P.S. Alegaonkar a, Suwarna Datar a,⇑a Department of Applied Physics, Defence Institute of Advanced Technology, Deemed University, Girinagar, Pune 411 025, Indiab Research & Development Establishment, DRDO, Pioneer Lines Dighi, Pune 411 015, India

a r t i c l e i n f o

Article history:Received 3 July 2014Received in revised form 1 September 2014Accepted 21 September 2014Available online 28 September 2014

Keywords:A. Nano-structuresA. Polymer-matrix composites (PMCs)B. Mechanical propertiesEMI shielding

a b s t r a c t

The advent of graphene heralded by the recent studies on carbon based conducting polymer compositeshas been a motivation for the use of graphene as an electromagnetic interference (EMI) shielding mate-rial. One of the variants of graphene, graphene nanoribbon (GNR) shows remarkably different propertiesfrom graphene. The EMI shielding effectiveness of the composite material mainly depends on fillers’intrinsic conductivity, dielectric constant and aspect ratio. We have synthesized graphene nanoribbon(GNR) – Polyaniline (PANI) – epoxy composite film for effective shielding material in the X-band fre-quency range of 8.2–12.4 (GHz). We have performed detailed studies of the EMI shielding effect andthe performance of the composite and found that the composite shows ��40 dB shielding which is suf-ficient to shield more than 95% of the EM waves in X Band. We checked the shielding effectiveness of thecomposite film by varying the GNR percentage and the thickness of the film. The strength properties ofthe synthesized composited were also studied with a aim to have a material having both high strengthand EMI shielding properties.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Electromagnetic interference (EMI) has become a serious prob-lem whose impact can be seen from day to day life like the inter-ference of mobile signals with laptop, television or speakerscausing flickering of picture or disturbance in sound to the spaceexploration, military applications and so on [1–4]. The impact ofEMI is not limited to the malfunctioning of electronic gadgets butit also affects human health, for example, continuous exposure ofelectromagnetic radiation increases the risk of cancer, asthma,heart problems, migraine and even leads to miscarriage [5]. To pro-vide solutions to these problems there is a very active quest formaking materials which are light weight, resistant to corrosion,flexible, as well as have effective and practical shielding applica-tions. Carbon nanofibers [6], carbon nanotubes [7–12], graphene[13,14], carbon foam [15,16], etc. have proved to be useful nanof-illers in polymer composites. Increase in the concentration of suchfillers increases the cost of the material and degrades the strengthof the composite [17]. An EMI shielding material may not turn outto be a high strength material. Therefore, for an application such asaerospace industry which requires a high strength material as well

as EMI shielding capabilities, a very special polymer filler combina-tion is needed to provide both the capabilities reasonably well.

Epoxy resins are thermosetting polymers and hence offer awide range of applications owing to their excellent mechanicalproperties like high stiffness, specific strength, dimensional stabil-ity, low cost, good adhesion to many substrates, chemical resis-tance and so on [18]. Many groups have worked on the use ofepoxy based carbon composites to enhance its properties.Nanocomposite with carbon filler in polymer, faces issues relatedto interfacial interaction between the filler and the polymer forhigh strength applications. Cooper et al. have investigated thedetachment of MWCNT from the epoxy matrix. They observed thatthe shear strength depended on the size of the interface. Goodmechanical properties require homogeneous dispersion of carbonnanofiller in epoxy matrix and strong interfacial interactionbetween the two [19]. Sufficient stress transfer from polymermatrix to carbon nanostructure is required for this. This can beachieved by chemical modification of the carbon nanostructures.Several groups have reported enhancement in mechanical proper-ties using epoxy/carbon nanostructure composite. Liao and hisco-workers have studied the thermo mechanical property of epoxybased nanocomposite of SWCNT [20]. Allaoui’s group has investi-gated the influence of MWCNTs in rubber epoxy [21]. Gallegoand co-worker used cationic photo polymerization technique to

Fig. 1. Schematic of the synthesis of composite.

A. Joshi et al. / Composites: Part B 69 (2015) 472–477 473

enhance the mechanical properties of functionalized graphene/epoxy composite [22]. Zaman et.al also showed the effect of changein temperature while sonication on the mechanical properties ofepoxy/graphene platelets nanocomposite [23]. Rafiee et al. haveinvestigated the Tensile strength, Young’s modulus, ductility, andtoughness of an epoxy polymer reinforced with thermally treatedGNRs. They have compared their results with multiwalled carbonnanotube (MWNT) epoxy composites to establish the effect ofunzipping of the MWNTs on the mechanical properties of the com-posite [24].

Organic polymers having extended p-conjugated network whendoped with suitable material having either electrons or holes ascharged carriers show enhancement in their conducting properties[25]. Among various conducting polymers, Polyaniline (PANI) hasbeen considered as one of the most promising candidates as shield-ing material due to its ease of synthesis, good environmental sta-bility, low specific mass, relatively high conductivity andeconomically feasibility [26,27]. The properties of PANI can furtherbe tuned by controlling the polymerization reaction and the degreeof doping. Among the various fillers, carbon based fillers, like car-bon nanotubes (CNT) [7–12], carbon fibers [6,28], carbon black[29,30] and graphene [13,14] have been extensively reported tohave enhanced both EMI and strength capabilities. Graphenesheets are 2D structures of sp2 hybridized carbon atoms and haveprompted intensive study for potential engineering applications.Particularly, the electrical, mechanical, electronic and various otherexciting properties of graphene and graphene based compositesoffer a new arena for the development of advanced engineeringmaterials [31]. Much interest has been shown towards the use ofgraphene based composites in aerospace applications such as elec-trostatic dissipation, electromagnetic interference (EMI) shieldingand conducting coating [13,14]. One of the derivatives of grapheneis graphene nanoribbon (GNR) which shows amazingly differentelectronic and mechanical properties compared to graphene dueto the contribution of the edge states and is a promising candidatefor a plethora of applications [32,33]. In one of our publications wehave shown very good EMI properties of GNR in PVA matrix [34]. Inthe present work, we have tried to enhance the EMI shielding prop-erties of PANI by adding a small percentage of GNR in PANI duringthe formation of composite with epoxy matrix. We have studiedthe shielding performance of the GNR/PANI composite in the epoxymatrix particularly in the frequency range of 8.2–12.4 GHz i.e.X-band since this band is useful for many military and commercialapplications like TV transmission, Doppler weather radars, defencetracking, etc. The main objective of the present work is to come up

with a composite which possesses EMI shielding properties alongwith reasonably good strength.

Since EMI shielding is the attenuation of the incident electro-magnetic waves produced by the shielding material. It is a measureof the losses due to reflection, absorption and multiple internalreflections suffered by the incident electromagnetic wave at theinterface. For shielding due to reflection the material should havemobile charge carriers (electrons or holes) to interact with the inci-dent EM wave. Shielding due to absorption is the secondary mech-anism and depends on the thickness of the shield. The electrical ormagnetic dipoles in the shielding material interact with the inci-dent EM wave and help in enhancing the shielding due to absorp-tion. Apart from shielding due to reflection and absorption,multiple reflection also plays a part in shielding. It represents theinternal reflection within the shielding material and requires largesurface area or interface area in the shield. However losses due tomultiple reflections can be neglected if the thickness of the shield-ing material is greater than the skin depth [11].

The shielding effectiveness is also measured in terms of loga-rithmic ratio of incident and transmitted electromagnetic powers(electric or magnetic) and can be expressed as

SETðdBÞ ¼ 10log10PT

PI

� �¼ 20log10

ET

EI

� �¼ 20log10

HT

HI

� �

According to electromagnetic theory, the EM wave incident onthe shielding material splits into four parts: reflected wave, inter-nally reflected wave, absorbed wave and transmitted wave. There-fore, total shielding effectiveness (SET) is measured in dB and canbe described as

SET ¼ SER þ SEA þ SEM

where SER, SEA and SEM are the contribution due to reflection,absorption and multiple internal reflection. When SER is P10 dBSEM is neglected and Shielding effectiveness is given by

SET � SER þ SEA

From the two port network system we can obtain the scatteringparameters (S-parameters) which are related with the reflectanceand transmittance i.e.,

T ¼ ET

EI

� �2

¼ jS21j2 ¼ jS12j2

R ¼ ER

EI

� �2

¼ jS11j2 ¼ jS22j2

Therefore effective absorbance (Aeff) is given by

Aeff ¼ð1� R� TÞð1� RÞ

SER and SEA are also expressed in terms of reflectance, transmit-tance and effective absorbance as

SER ¼ �10 logð1� RÞ

SEA ¼ �10 logð1� AeffÞ ¼ � logT

1� R

2. Experimental

2.1. Materials

Potassium permanganate (KMnO4), sulfuric acid (H2SO4), ani-line, (1S)-(+)-Camphor-10-Sulfonic acid (CSA), ammonium peroxy-disulfate, Epoxy LY 1564, Hardener XB 3486 were of analyticalgrade. Aqueous solution was prepared using doubledistilled water.

Fig. 2. SEM image of (a) pure Epoxy and (b) 0.1 wt% GNR/PANI composite in the epoxy matrix.

Fig. 3. (a) Raman spectra of GNR/PANI epoxy and only GNR. Inset show the Raman spectra of epoxy (b) FTIR spectra of GNR/PANI epoxy composite.

Table 1Assignment of functional group to various peaks of Fig. 3(b).

Peak number Wavenumber (cm�1) Group

1 3358 COOAH/OH2 �3000, 2860 c CAH epoxy stretching3 2182 AN@C@NA4 1735 C@O stretch5 1608, 1506 C@C in ring6 1454 CAH bending7 1371 NAO symmetric stretch8 1300 CAN stretch aromatic amine9 1089 c CAO epoxy

10 1020 CAN med11 820 CH and @CH2 out of plane bending12 540 Alkyl halide

Fig. 4. TGA trace PANI/GNR, Plain epoxy and epoxy PANI/GNR composite.

474 A. Joshi et al. / Composites: Part B 69 (2015) 472–477

Thin Multiwalled Carbon Nanotubes (t-MWCNT) were preparedusing water assisted chemical vapor deposition (CVD) technique.

2.2. Synthesis of PANI functionalized GNR

The method proposed by Kosynkin et al. [35] was followed forunzipping of CNT using KMnO4 and H2SO4. The non-covalent func-tionalization of synthesized GNR was performed by a coating ofCSA doped PANI by in-situ polymerization of aniline usingammonium peroxydisulfate as polymerizing agent under ambientconditions [36]. For this, two different weight percentages (2.5and 5) of GNR were added to aniline monomer to prepare twodifferent samples. The above synthesized PANI coated GNR sam-

ples were filtered and washed with ethanol and vacuum driedfor 24 h at 80 �C.

2.3. Synthesis of PANI functionalized GNR/epoxy solution blend

Fig. 1 shows the schematic for the synthesis of composite.0.1 wt% of PANI/GNR loaded epoxy composite was prepared bysolution blend technique by separately dispersing PANI/GNR inchloroform and then probe sonicating the mixture for 30 min withMechanical Probe Sonicator (13 mm Vibra Cell Processor VCX 750)operating at 40% of the max power 750 W. Thereafter the solution

Table 2Comparison of the Tensile strength and Young’s modulus ofdifferent samples.

Sample Tensile strength(MPa)

Young’s modulus(GPa)

Epoxy 63.72 3.089C1 59.63 2.609C2 56.20 2.363C3 44.18 2.136

Fig. 6. Comparison shielding effectiveness of the composite.

A. Joshi et al. / Composites: Part B 69 (2015) 472–477 475

was added to the Epoxy LY 1564 and stirred for 10 min and thesolution was heated at approx 70 �C to evaporate the chloroform.Subsequently, XB 3486 hardener was added to the above solutionin the known ratio and stirred for 5 min followed by degassingfor 10 min. The solution was then poured in the molds for prepar-ing the desired dog bone shape of 1.7 mm and 3.4 mm thickness.The molds were made as per ASTM D 638 Type-I standard. Thesample took 24 h to cure after which post curing of the samplewas done for 8 h at 80 �C.

2.4. Characterizations

The morphological details of the PANI/GNR samples were char-acterized using scanning electron microscope (SEM). Raman spec-troscopy was performed using unpolarized Raman spectroscopictechnique. The spectra was recorded at wavelength k = 633 nmusing Horiba HR800 Raman spectrometer. The molecular structureof the synthesized sample was obtained by Fourier transform infra-red (FTIR) spectroscopy using Brucker Tensor 37 spectroscope. Thethermal decomposition behavior of the PANI/GNR composite aswell as epoxy blended PANI/GNR composite was studied usingthermogravimetric analysis (TGA) under a nitrogen atmospherefrom room temperature to 650 �C operated at a heating rate of20 �C min�1. The tensile test and the Young’s modulus of elasticitywere measured using the Servo Hydraulic Universal TestingMachine (BISS India) with total cell 10 kN capacity. The EMI shield-ing effectiveness of the composite films was measured using Rhode& Schwarz ZVA-40 10–40 MHz vector network analyser. The cali-bration was performed using OSL (Open-Short-Load) technique.Electromagnetic waves were injected directly into the film using354B X-band wave guide of standard dimension of the window0.900 � 0.400. The frequency was scanned from 8.2 to 12.4 GHz. TheEMI shielding effectiveness was measured using Rohde & SchwarzVector Network analyser in the range of 8.2–12.4 GHz.

3. Results and discussion

3.1. Morphological study

The formation of conductive network in an insulating polymermatrix depends on the distribution and dispersion of the fillerinside the matrix. To get good EMI shielding, it is necessary forthe composite to have closed packed network with continuouschain of conducting filler. Fig. 2(a) and (b) show the SEM imagesof plain epoxy and the composite respectively. From the SEMimage, Fig. 2(b), one cannot distinguish between the filler andthe matrix epoxy. This could be due to the fact that the fillerpercentage is very low in the matrix. But one can observe a big

Fig. 5. Stress–Strain curves of the PANI/GNR composite in the Epoxy matrix.

difference Fig. 2(a) and (b). There is very little contrast in case ofonly epoxy film (Fig. 2(a)) whereas the image becomes clear oncethe filler is added (Fig. 2(b)). The small percentage of GNR–PANIin epoxy is completely changing the microstructure of the epoxyas observed from these images. A good interaction between epoxyand GNR/PANI is the key in getting good dispersion which canbe achieved by ultrasonic dispersion and controlled solventevaporation.

3.2. Raman and FTIR analysis

To confirm the interaction between epoxy and GNR/PANI,Raman spectroscopy and FT-IR were carried out. Since the twotechniques complement each other they can be very helpful in cre-ating a reasonable picture about the interaction between theseentities. Both these techniques are powerful, fast, non destructiveand capable of providing detailed information about the molecularstructure of the sample. Fig. 3(a) and (b) shows the Raman spectraand FT-IR of the composite. Couple of Raman active bands can beobserved. One important band is the G band which is due to thein phase vibration of the graphite lattice i.e. E2g mode at1577 cm�1 which is broad in case of GNR and shifts to1607 cm�1 in case of the composite in the epoxy matrix. Thebroadening of G band is due to the oxidation of GNR during synthe-sis and the reduction in the size of in-plane sp2 domains [37]. Thedotted dark green line shows the broadening in case of compositewhere pink dotted line is for GNR. Half width in case of compositeis 107.5 cm�1 where as in case of GNR is 78 cm�1. The shift in the Gband after the formation of the composite could be due to com-pressive stress on the GNR caused by the polymer matrix [38].The other important band is the D band which is due to the pres-ence of defects and edge effects and is considered as A1g mode [39].The half width of the D peak in case of composite is 78.075 cm�1

where as for GNR it is 45.465 cm�1 shown by blue dotted line forthe composite and green dotted line for GNR. The Id/Ig ratio of onlyGNR is 1.5 whereas that of GNR/PANI in the epoxy matrix is 0.45.This significant reduction in Id/Ig ratio signifies the formation ofbonds reducing the overall defects in the GNR. This is alsoconsistent with the observation of shift in G band after formation

476 A. Joshi et al. / Composites: Part B 69 (2015) 472–477

of composite which could be the stress caused in GNR by bondformation.

To further verify the conjugation of GNR/PANI composite in theepoxy matrix we performed the FTIR analysis of the end product.From the spectra shown in Fig. 3(b) we observe the typical signa-tures of GNR, PANI as well as Epoxy. These signatures are recog-nized as COOAH/AOH stretching of oxidized GNR in the rangefrom 3600 to 3085 cm�1and C@O stretching at �1743 cm�1. Thisconfirms the presence of edge carboxylic acid in GNR [35]. Thepresence of NH2 bending or scissoring band �1605 cm�1, the modeat 1237 cm�1 may be assigned partly to CAN stretching and partlyto the ring stretching vibrations and the band at 1463 cm�1 whichis characterized as typical ring stretching in PANI. The presence ofepoxy in the IR spectroscopy was established by the presence ofstrong band at �3000 cm�1 (mC–H epoxy) and 1089 (cC–O epoxy).The 1,4 substitution of the aromatic ring is seen at 830 cm�1 forthe epoxy resin. Further the IR spectra articulates the occurrenceof absorption bands and NAH stretching and bending vibrations.Table 1 show the details of the peak assignment. Thus from theabove discussion we can confirm the presence of GNR/PANI inthe Epoxy matrix.

3.3. Thermogravimetric analysis (TGA)

The effect of epoxy in the 0.1 wt% of PANI/GNR composite’sthermal stability was studied using TGA in a nitrogen atmosphere.Fig. 4 shows the thermogram (TG) of PANI/GNR, plain epoxy andepoxy PANI/GNR. From the TG plots it is clear that the weight lossoccurs in several systematic steps each corresponding to loss ofparticular species. In case of PANI/GNR composite, the first steploss (25–100 �C) may be attributed to the loss of adsorbed watermolecules. The second loss step (106–185 �C) involves the loss ofdopants. The third loss step (190–310 �C) involves the loss of lowmolecular weight fragment and onset of polymeric degradation.The final loss step (310–700 �C) corresponds to the completebreakdown of polymeric backbone as well as heavier fragments.Further the result reveals that thermal degradation stability ofthe nanocomposite increases when it is loaded in the epoxy matrix.These results indicate that the epoxy plays an important role in theformation of stable and strong physical barrier for thermal transferin the composite. From the TGA results we may conclude that thisnanocomposite of epoxy PANI/GNR may be used as a good EMIshielding material especially in aerospace and radiation technologyindustries where high thermal stability is the perquisite.

3.4. Mechanical properties

Stress–strain behavior from the tensile test is shown in Fig. 5.The specimens of plain epoxy failed immediately after the tensile

Fig. 7. (a) SE of GNR/PANI composite of 2.5 wt% and (b) 5 wt% in

stress reached its maximum value, while there is elongation atthe point of break in case of samples of composites. Table 2 dis-plays the Tensile strength and Young’s modulus comparison ofthe various samples i.e. epoxy, C1 – Epoxy PANI 0.1 wt%, C2 –Epoxy PANI GNR (2.5%) 0.1 wt%, C3 – Epoxy PANI GNR (5%)0.1 wt%. It is observed that the Tensile Strength and Young’s mod-ulus reduces with the increase of the GNR/PANI combination in thecomposite. This is due to the particle size of GNR which leads tostress points in the sample. Also high loading of conducting poly-mer based nanocomposites lead to phase segregation and extremeof physical properties of host matrices leading to poor mechanicalproperties. It can be seen that the Tensile Strength has reduced by6.42%, 11.80% & 30.66% respectively in sample C1, C2 & C3 and theYoung’s modulus has reduced by 15.54%, 23.50% & 30.85% respec-tively in sample C1, C2 & C3 in comparison to the plain epoxy. Eventhough the strength properties are compromised in this compositebut they may still serve well for certain applications if the materialhas good EMI shielding properties.

3.5. EMI shielding effectiveness measurements

Fig. 6 shows the EMI shielding results of the GNR/PANI compos-ite in the epoxy matrix. It is clear from the figure that the plainepoxy is transparent to the electromagnetic waves and does notexhibit any shielding. As the percentage of the GNR/PANI compos-ite in the epoxy matrix increases from 2.5 wt% to 5 wt% the shield-ing effectiveness increases from an average value of �34 dB to�44 dB for 3.4 mm thickness. We observed a dip in absorption at10.5 GHz in the GNR/PANI composite in the epoxy matrix. It isinteresting to see that this was completely absent in PANI in epoxymatrix. It has been reported in the literature that the dielectricbehavior of PANI in different polymer matrix drastically changes.Dielectric relaxation measurements were performed to supportthese results. Additional relaxation processes were observed bythem which were related to interfacial polarization relaxationeffects. The nature of polymer matrix was found to influence theserelaxations by frequency shift, change in relaxation strength andactivation energy. At high frequency, conductivity relaxation wasobserved to be connected to the conductivity in the PANI cluster.When composite of PANI is made with GNR these effects maytranslate into absorption dip at particular frequency [40,41].

Further we have done the comparative study of the variation inthe shielding effectiveness due different thickness of the compositefilms. Fig. 7(a) and (b) shows the variation of shielding effective-ness with thickness. We observed that for 2.5 wt% sample the aver-age shielding effectiveness increases from �34 dB to �50 dB andfor 5 wt% sample shielding increases from �44 dB to �68 dB.

The interaction of the electromagnetic wave causes volumetricelectronic polarization and thus forces the electron to vibrate with

epoxy matrix of different thickness i.e. 1.7 mm and 3.4 mm.

A. Joshi et al. / Composites: Part B 69 (2015) 472–477 477

the wave frequency, which results in the absorption of EM waves[42]. Due to the presence of GNR/PANI in the epoxy matrix thereis increase in electric dipole which results in absorption. Fromthe results obtained we can conclude that absorption is the pri-mary mechanism for EMI shielding for the synthesized composite.

4. Conclusion

From the various studies done on the synthesized composite weconclude that due to the absorption dominant shielding featurethis composite may be used in such areas where EMI shieldingfrom outside radiation is needed but at the same time they shouldbe protected from the EM radiation which they themselves gener-ate. Further the mechanical results are not much comparable withthe data so far reported but the resulting composite will find itsapplication in the aerospace industry where both shielding as wellas strength is the prime requirement. The dispersion of GNR in thematrix has to be worked out to ensure improved mechanical prop-erties along with the EMI shielding.

Acknowledgements

Authors are thankful to Defence Research and DevelopmentOrganization (DRDO), Ministry of Defence, Government of India,for their financial assistance. In our work authors are also thankfulto Dr. Prahalada, Vice Chancellor Defence Institute of AdvanceTechnology (DU) Pune. SSD and PSA would like to acknowledgeDIAT-DRDO program on Nanomaterials by ER-IPR, DRDO for thefinancial assistance.

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