Accepted Manuscript Processing of Graphene nanoribbon based hybrid composite for electromagnetic shielding Anupama Joshi, Anil Bajaj, Rajvinder Singh, Anoop Anand, P.S. Alegaonkar, Suwarna Datar PII: DOI: Reference:
S1359-8368(14)00416-8 http://dx.doi.org/10.1016/j.compositesb.2014.09.014 JCOMB 3186
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Composites: Part B
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3 July 2014 1 September 2014 21 September 2014
Please cite this article as: Joshi, A., Bajaj, A., Singh, R., Anand, A., Alegaonkar, P.S., Datar, S., Processing of Graphene nanoribbon based hybrid composite for electromagnetic shielding, Composites: Part B (2014), doi: http:// dx.doi.org/10.1016/j.compositesb.2014.09.014
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Processing of Graphene nanoribbon based hybrid composite for electromagnetic shielding Anupama Joshi1, Anil Bajaj1, Rajvinder Singh1, Anoop Anand2, P. S. Alegaonkar1, and Suwarna Datar1* 1
Department of Applied Physics, Defence Institute of Advanced Technology, Deemed University, Girinagar, Pune 411 025, India 2 Research & Development Establishment, DRDO, Pioneer Lines Dighi, Pune-411 015 *
Corresponding author: [email protected]
The advent of graphene heralded by the recent studies on carbon based conducting polymer composites has been a motivation for the use of graphene as an electromagnetic interference (EMI) shielding material. One of the variants of graphene, graphene nanoribbon (GNR) shows remarkably different properties from 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 frequency range of 8.2 - 12.4 (GHz). We have performed detailed studies of the EMI shielding effect and the performance of the composite and found that the composite shows ~ -40dB shielding which is sufficient to shield more than 95% of the EM waves in X Band. We checked the shielding effectiveness of the composite film by varying the GNR percentage and the thickness of the film. The strength properties of the synthesized composited were also studied with a aim to have a material having both high strength and EMI shielding properties. Keywords: A. Nanocomposite A. Graphene nanoribbon A. Polyaniline A. Epoxy C. Electromagnetic interference
1. Introduction Electromagnetic Interference (EMI) has become a serious problem whose impact can be seen from day to day life like the interference of mobile signals with laptop, television or speakers causing flickering of picture or disturbance in sound to the space exploration, military applications and so on [1-4]. The impact of EMI is not limited to the malfunctioning of electronic gadgets but it also affects human health, for example, continuous exposure of electromagnetic radiation increases the risk of cancer, asthma, heart problems, migraine and even leads to miscarriage . To provide solutions to these problems there is a very active quest for making materials which are light weight, resistant to corrosion, flexible, as well as have effective and practical shielding applications. Carbon nanofibers , carbon nanotubes [7-12], graphene [13, 14], carbon foam [15, 16] etc. have proved to be useful nanofillers in polymer composites. Increase in the concentration of such fillers increases the cost of the material and degrades the strength of the composite . An EMI shielding material may not turn out to be a high strength material. Therefore, for an application such as aerospace industry which requires a high strength material as well as EMI shielding capabilities, a very special polymer filler combination is needed to provide both the capabilities reasonably well. Epoxy resins are thermosetting polymers and hence offer a wide range of applications owing to their excellent mechanical properties like high stiffness, specific strength, dimensional stability, low cost, good adhesion to many substrates, chemical resistance and so on . Many groups have worked on the use of epoxy based carbon composites to enhance its properties. Nanocomposite with carbon filler in polymer, faces issues related to interfacial interaction between the filler and the polymer for high strength applications. Cooper et.al have investigated the detachment of MWCNT from
the epoxy matrix. They observed that the shear strength depended on the size of the interface. Good mechanical properties require homogeneous dispersion of carbon nanofiller in epoxy matrix and strong interfacial interaction between the two . Sufficient stress transfer from polymer matrix to carbon nanostructure is required for this. This can be achieved by chemical modification of the carbon nanostructures. Several groups have reported enhancement in mechanical properties using epoxy/carbon nanostructure composite. Liao and his co-workers have studied the thermo mechanical property of epoxy based nanocomposite of SWCNT . Allaoui’s group has investigated the influence of MWCNTs in rubber epoxy . Gallego and co-worker used cationic photo polymerization technique to enhance the mechanical properties of functionalized graphene/ epoxy composite . Zaman et.al also showed the effect of change in temperature while sonication on the mechanical properties of Epoxy/graphene platelets nanocomposite . Rafiee et.al have investigated the tensile strength, Young’s modulus, ductility, and toughness of an epoxy polymer reinforced with thermally treated GNRs. They have compared their results with multiwalled carbon nanotube (MWNT) epoxy composites to establish the effect of unzipping of the MWNTs on the mechanical properties of the composite . Organic polymers having extended π-conjugated network when doped with suitable material having either electrons or holes as charged carriers show enhancement in their conducting properties . Among various conducting polymers, Polyaniline (PANI) has been considered as one of the most promising candidates as shielding material due to its ease of synthesis, good environmental stability, low specific mass, relatively high conductivity and economically feasibility [26, 27]. The properties of PANI can further be tuned by controlling the polymerization reaction and the degree of doping. Among
the various fillers, carbon based fillers, like carbon nanotubes (CNT) [7-12], carbon fibers [6, 28], carbon black [29, 30] and graphene [13, 14] have been extensively reported to have enhanced both EMI and strength capabilities. Graphene sheets are 2D structures of sp2 hybridized carbon atoms and have prompted intensive study for potential engineering applications. Particularly, the electrical, mechanical, electronic and various other exciting properties of graphene and graphene based composites offer a new arena for the development of advanced engineering materials . Much interest has been shown towards the use of graphene based composites in aerospace applications such as electrostatic dissipation, electromagnetic interference (EMI) shielding and conducting coating [13, 14]. One of the derivatives of graphene is graphene nanoribbon (GNR) which shows amazingly different electronic and mechanical properties compared to graphene due to the contribution of the edge states and is a promising candidate for a plethora of applications [32, 33]. In one of our publications we have shown very good EMI properties of GNR in PVA matrix . In the present work, we have tried to enhance the EMI shielding properties of PANI by adding a small percentage of GNR in PANI during the formation of composite with Epoxy matrix. We have studied the shielding performance of the GNR/PANI composite in the epoxy matrix particularly in the frequency range of 8.2-12.4 GHz i.e. X-band since this band is useful for many military and commercial applications like TV transmission, Doppler weather radars, defence tracking etc. The main objective of the present work is to come up with a composite which possesses EMI shielding properties along with reasonably good strength. Since EMI shielding is the attenuation of the incident electromagnetic waves produced by the shielding material. It is a measure of the losses due to reflection, absorption and
multiple internal reflections suffered by the incident electromagnetic wave at the interface. For shielding due to reflection the material should have mobile charge carriers (electrons or holes) to interact with the incident EM wave. Shielding due to absorption is the secondary mechanism and depends on the thickness of the shield. The electrical or magnetic dipoles in the shielding material interact with the incident EM wave and help in enhancing the shielding due to absorption. Apart from shielding due to reflection and absorption, multiple reflection also plays a part in shielding. It represents the internal reflection within the shielding material and requires large surface area or interface area in the shield. However losses due to multiple reflections can be neglected if the thickness of the shielding material is greater than the skin depth . The shielding effectiveness is also measured in terms of logarithmic ratio of incident and transmitted electromagnetic powers (electric or magnetic) and can be expressed as
SET (dB) = 10 ଵ 20 ଵ ு
= 20 ଵ ு
According to electromagnetic theory, the EM wave incident on the shielding material splits into four parts: reflected wave, internally reflected wave, absorbed wave and transmitted wave. Therefore, total shielding effectiveness (SET) is measured in dB and can be described as SET = SER + SEA + SEM
Where SER, SEA and SEM are the contribution due to reflection, absorption and multiple internal reflection. When SER is ≥ 10dB SEM is neglected and Shielding effectiveness is given by SET ≈ SER + SEA From the two port network system we can obtain the scattering parameters (Sparameters) which are related with the reflectance and transmittance i.e., ் ଶ |ଶଵ |ଶ |ଵଶ |ଶ ூ ோ ଶ |ଵଵ |ଶ |ଶଶ |ଶ ூ Therefore effective absorbance (Aeff) is given by
SER and SEA are also expressed in terms of reflectance, transmittance and effective absorbance as R 10 1 A 10 1 eff =
2. Experimental 2.1 Materials Potassium permanganate (KMnO4), sulphuric acid (H2 SO4), aniline, (1S)-(+)Camphor-10-Sulfonic acid (CSA), ammonium peroxydisulfate, Epoxy LY 1564, Hardener XB 3486 were of analytical grade. Aqueous solution was prepared using
doubledistilled water. Thin Multiwalled Carbon Nanotubes (t-MWCNT) were prepared using water assisted chemical vapor deposition (CVD) technique. 2.2 Synthesis of PANI functionalized GNR The method proposed by D.V. Kosynkinet. al  was followed for unzipping of CNT using KMnO4 and H2 SO4. The non-covalent functionalization of synthesized GNR was performed by a coating of CSA doped PANI by in-situ polymerization of aniline using ammonium peroxydisulfate as polymerising agent under ambient conditions . For this, two different weight percentages (2.5 and 5) of GNR were added to aniline monomer to prepare two different samples. The above synthesized PANI coated GNR samples were filtered and washed with ethanol and vacuum dried for 24hrs at 80°C. 2.3 Synthesis of PANI functionalized GNR/ Epoxy solution blend Figure. 1 shows the schematic for the synthesis of composite. 0.1wt % of PANI/GNR loaded Epoxy composite was prepared by solution blend technique by separately dispersing PANI/GNR in chloroform and then probe sonicating the mixture for 30 min with Mechanical Probe Sonicator (13mmVibra Cell Processor VCX 750) operating at 40% of the max power 750W.Thereafter the solution was added to the Epoxy LY 1564 and stirred for 10 minutes and the solution was heated at approx 70oC to evaporate the chloroform. Subsequently, XB 3486 hardener was added to the above solution in the known ratio and stirred for 5 min followed by degassing for 10 min. The solution was then poured in the molds for preparing the desired dog bone shape of 1.7mm and 3.4mm thickness. The mouldswere made as per ASTM D 638 Type-I standard. The sample took 24 hrsto cure after which post curing of the sample was done for 8 hrs at 80oC.
2.4 Characterizations The morphological details of the PANI/ GNR samples were characterized using scanning electron microscope (SEM). Raman spectroscopy was performed using unpolarized Raman spectroscopic technique. The spectra was recorded at wavelength λ = 633 nm using Horiba HR800 Raman spectrometer.The molecular structure of the synthesized sample was obtained by Fourier transform infrared (FTIR) spectroscopy using Brucker Tensor 37 spectroscope. The thermal decomposition behaviour of the PANI/GNR composite as well as epoxy blended PANI/ GNR composite was studied using thermogravimetric analysis (TGA) under a nitrogen atmosphere from room temperature to 650 °C operated at a heating rate of 20°C min-1. The tensile test and the Young’s modulus of elasticity were measured using the Servo Hydraulic Universal Testing Machine (BISS India) with total cell 10kN capacity.
The EMI shielding
effectiveness of the composite films was measured using Rhode & Schwarz ZVA-40 10MHz-40MHz vector network analyser. The calibration was performed using OSL (Open-Short-Load) technique. Electromagnetic waves were injected directly into the film using 354B X- band wave guide of standard dimension of the window 0.9" X 0.4". The frequency was scanned from 8.2 to 12.4 GHz. The EMI shielding effectiveness was measured using Rohde & Schwarz Vector 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 polymer matrix depends on the distribution and dispersion of the filler inside the matrix. To get good EMI shielding, it is necessary for the composite to have closed packed network with
continuous chain of conducting filler. Figure 2 (a) and (b) show the SEM images of plain epoxy and the composite respectively. From the SEM image, Figure 2 (b), one cannot distinguish between the filler and the matrix Epoxy. This could be due to the fact that the filler percentage is very low in the matrix. But one can observe a big difference Figure 2 (a) and (b). There is very little contrast in case of only epoxy film (Figure 2 (a)) whereas the image becomes clear once the filler is added (Figure 2 (b)). The small percentage of GNR-PANI in epoxy is completely changing the microstructure of the epoxy as observed from these images. A good interaction between epoxy and GNR/PANI is the key in getting good dispersion which can be achieved by ultrasonic dispersion and controlled solvent evaporation. 3.2 Raman and FTIR Analysis To confirm the interaction between epoxy and GNR/PANI, Raman spectroscopy and FT-IRwere carried out. Since the two techniques complement each other they can be very helpful in creating a reasonable picture about the interaction between these entities. Both these techniques are powerful, fast, non destructive and capable of providing detailed information about the molecular structure of the sample. Figure 3 (a) and (b) shows the Raman spectra and FT-IR of the composite. Couple of Raman active bands can be observed. One important band is the G band which is due to the in phase vibration of the graphite lattice i.e. E2g mode at 1577cm-1 which is broad in case of GNR and shifts to 1607cm-1 in case of the composite in the epoxy matrix. The broadening of G band is due to the oxidation of GNR during synthesis and the reduction in the size of in-plane sp2 domains .The dotted dark green line shows the broadening in case of composite where pink dotted line is for GNR. Half width in case of composite is 107.5cm-1 where as in case of GNR is 78cm-1. The shift in the G band
after the formation of the composite could be due to compressive stress on the GNR caused by the polymer matrix .The other important band is the D band which is due to the presence of defects and edge effects and is considered as A1gmode .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 for the composite and green dotted line for GNR. The Id/Ig ratio of only GNR 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 of bonds reducing the overall defects in the GNR. This is also consistent with the observation of shift in G band after formation of composite which could be the stress caused in GNR by bond formation. To further verify the conjugation of GNR/PANI composite in the epoxy matrix we performed the FTIR analysis of the end product. From the spectra shown in figure 3 (b) we observe the typical signatures of GNR, PANI as well as Epoxy. These signatures are recognized as COO-H/-OH stretching of oxidized GNR in the range from 3600-3085 cm-1and C=O stretching at ~ 1743 cm-1.This confirms the presence of edge carboxylic acid in GNR . The presence of NH2 bending or scissoring band ~ 1605cm-1, the mode at 1237cm-1 may be assigned partly to C-N stretching and partly to the ring stretching vibrations and the band at 1463cm-1which is characterized as typical ring stretching in PANI. The presence of epoxy in the IR spectroscopy was established by the presence of strong band at ~3000cm-1 (νC-H epoxy) and 1089(γC-O epoxy). The 1,4 substitution of the aromatic ring is seen at 830 cm-1 for the epoxy resin. Further the IR spectra articulates the occurrence of absorption bands and N-H stretching and bending vibrations.Table I show the details of the peak assignment. Thus from the above discussion we can confirm the presence of GNR/PANI in the Epoxy matrix.
3.3 Thermogravimetric analysis (TGA) The effect of epoxy in the 0.1 wt% of PANI / GNR composite’s thermal stability was studied using TGA in a nitrogen atmosphere. Figure 4 shows the thermogram (TG) of PANI/GNR, Plain epoxy and Epoxy PANI/GNR. From the TG plots it is clear that the weight loss occurs in several systematic steps each corresponding to loss of particular species. In case of PANI /GNR composite, the first step loss (25-100 ºC) may be attributed to the loss of adsorbed water molecules. The second loss step (106-185ºC) involves the loss of dopants. The third loss step (190-310ºC) involves the loss of low molecular weight fragment and onset of polymeric degradation. The final loss step (310ºC -700ºC) corresponds to the complete breakdown of polymeric backbone as well as heavier fragments. Further the result reveals that thermal degradation stability of the nanocomposite increases when it is loaded in the epoxy matrix. These results indicate that the epoxy plays an important role in the formation of stable and strong physical barrier for thermal transfer in the composite. From the TGA results we may conclude that this nanocomposite of Epoxy PANI/ GNR may be used as a good EMI shielding material especially in aerospace and radiation technology industries where high thermal stability is the perquisite.
3.4 Mechanical properties Stress- strain behaviour from the tensile test is shown in figure 5. The specimens of plain epoxy failed immediately after the tensile stress reached its maximum value, while there is elongation at the point of break in case of samples of composites. Table II displays the Tensile strength and Young’s Modulus comparison of the 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 Modulus reduces with the increase of the GNR/PANI combination in the composite. This is due to the particle size of GNR which leads to stress points in the sample. Also high loading of conducting polymer based nanocomposites lead to phase segregation and extreme of physical properties of host matrices leading to poor mechanical properties. It can be seen that the Tensile Strength has reduced by 6.42%, 11.80% & 30.66% respectively in sample C1, C2 & C3 and the Young’s Modulus has reduced by 15.54%, 23.50% & 30.85% respectively in sample C1, C2 & C3 in comparison to the plain Epoxy. Even though the strength properties are compromised in this composite but they may still serve well for certain applications if the material has good EMI shielding properties. 3.5 EMI shielding effectiveness measurements Figure 6 shows the EMI shielding results of the GNR/ PANI composite in the epoxy matrix. It is clear from the figure that the plain epoxy is transparent to the electromagnetic waves and does not exhibit any shielding. As the percentage of the GNR / PANI composite in the epoxy matrix increases from 2.5 wt% to 5 wt% the shielding effectiveness increases from an average value of -34dB to -44dB for 3.4mm thickness.We observed a dip in absorption at 10.5 GHz in the GNR/ PANI composite in the epoxy matrix. It is interesting to see that this was completely absent in PANI in epoxy matrix. It has been reported in the literature that the dielectric behaviour of PANI in different polymer matrix drastically changes. Dielectric relaxation measurements were performed to support these results. Additional relaxation processes were observed by them which were related to interfacial polarization relaxation effects. The nature of polymer matrix was found to influence these relaxations by frequency shift, change in
relaxation strength and activation energy. At high frequency, conductivity relaxation was observed to be connected to the conductivity in the PANI cluster. When composite of PANI is made with GNR these effects may translate into absorption dip at particular frequency [40, 41]. Further we have done the comparative study of the variation in the shielding effectiveness due different thickness of the composite films. Figure 7(a), (b) shows the variation of shielding effectiveness with thickness. We observed that for 2.5wt % sample the average shielding effectiveness increases from -34dB to -50dB and for 5wt% sample shielding increases from -44dB to -68dB. The interaction of the electromagnetic wave causes volumetric electronic polarization and thus forces the electron to vibrate with the wave frequency, which results in the absorption of EM waves . Due to the presence of GNR /PANI in the epoxy matrix there is increase in electric dipole which results in absorption. From the results obtained we can conclude that absorption is the primary mechanism for EMI shielding for the synthesized composite. 4. Conclusion From the various studies done on the synthesized composite we conclude that due to the absorption dominant shielding feature this composite may be used in such areas where EMI shielding from outside radiation is needed but at the same time they should be protected from the EM radiation which they themselves generate. Further the mechanical results are not much comparable with the data so far reported but the resulting composite will find its application in the aerospace industry where both shielding as well as strength is the prime requirement. The dispersion of GNR in the
matrix has to be worked out to ensure improved mechanical properties along with the EMI shielding. Acknowledgements Authors are thankful to Defence Research and Development Organization (DRDO), Ministry of Defence, Government of India, for their financial assistance. In our work authors are also thankful to Dr.Prahalada, Vice Chancellor Defence Institute of Advance Technology (DU)Pune. SSD and PSA would like to acknowledge DIATDRDO program on Nanomaterials by ER-IPR, DRDO for the financial assistance.
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Figure Captions Figure 1. Schematic of the synthesis of composite Figure 2. SEM image of (a) pure Epoxy (b) 0.1wt% GNR/PANI composite in the epoxy matrix. Figure 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. Figure 4. TGA trace PANI/GNR, Plain epoxy and epoxy PANI/GNR composite. Figure 5. Stress – Strain curves of the PANI/GNR composite in the Epoxy matrix. Figure 6. Comparison shielding effectiveness of the composite. Figure 7. (a) SE of GNR/PANI composite of 2.5wt % and (b) 5wt% in epoxy matrix of different thickness i.e 1.7mm and 3.4mm.
Table caption Table 1:Assignment of functional group to various peaks of figure 3(b). Table 2: Comparison of the Tensile strength and Young’s Modulus of different samples
Table 1. Assignment of functional group to various peaks of figure 3(b)
γ C-H epoxy stretching
C=C in ring
N-O symmetric stretch
C-N stretch aromatic amine γ C-O epoxy
CH and =CH2 out of plane bending Alky halide
Table 2:Comparison of the Tensile strength and Young’s Modulus of different samples
Sample Epoxy C1 C2 C3
Tensile Strength (MPa) 63.72 59.63 56.20 44.18
Young's Modulus (GPa) 3.089 2.609 2.363 2.136