Composites Science and Technology 102 (2014) 59–64

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Composites Science and Technology

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Effect of fabrication method on the structure and electromagneticresponse of carbon nanotube/polystyrene composites in low-frequencyand Ka bands

http://dx.doi.org/10.1016/j.compscitech.2014.07.0130266-3538/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +7 383 330 53 52; fax: +7 383 330 94 89.E-mail address: o.sedelnikova@gmail.com (O.V. Sedelnikova).

O.V. Sedelnikova a,⇑, M.A. Kanygin a, E.Yu. Korovin b, L.G. Bulusheva a,b, V.I. Suslyaev b, A.V. Okotrub a,b

a Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Science, 3 Acad. Lavrentiev Ave., Novosibirsk 630090, Russiab Tomsk State University, 36 Lenin Ave., Tomsk 634050, Russia

a r t i c l e i n f o

Article history:Received 5 May 2014Received in revised form 23 June 2014Accepted 11 July 2014Available online 23 July 2014

Keywords:A. Carbon nanotubesA. Polymer–matrix composites (PMCs)B. Electrical propertiesMicrowave absorption

a b s t r a c t

Casting, forge-rolling, and stretching methods have been used for preparation of polystyrene compositeswith 0.25 wt.% loading of multiwall carbon nanotubes (MWCNTs). Electromagnetic properties of thecomposites were studied using impedance spectroscopy and microwave probing in the Ka band. Analysisof impedance spectra revealed capacitive behavior of the forge-rolled composite and charge tunnelingthrough a polystyrene layer separating the nanotubes in two other composite samples. The stretchedcomposite demonstrated anisotropy of microwave response due to predominant orientation of nano-tubes along deformation direction. The near-isotropic shielding efficiency of the forge-rolled compositewas attributed to strong fragmentation of MWCNTs under applied shear forces.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction For filling of polymers with CNTs, several methods have devel-

Polymer composites containing conductive inclusions are veryperspective for electromagnetic applications. Materials, whichhave a predetermined arrangement of the inclusions and providethe electromagnetic response in certain frequency range, are ofparticular interest. In this context, carbon nanotubes (CNTs), con-ductive quasi-one-dimensional nanoparticles, have attracted aspecial attention due to extremely high shielding efficiency ofcomposites in gigahertz [1–6] and terahertz [7–11] regions evenat low nanotube concentration.

Electromagnetic characteristics of CNT-based composites aregoverned by many factors, such as, the polymer nature, geometryof CNTs (length, defectness, etc.), concentration and distributionof nanotubes in matrix. Variation in any of these parameters coulddrastically change property of composite material. For example, ithas been shown that lessening of the defect-free-length in CNTwalls results in reduction of dielectric permittivity, dc conductivityand electromagnetic interference shielding efficiency of compos-ites [2]. Microwave probing of polymethylmethacrylate (PMMA)composite with CNTs detected a strong lowering of sampletransmission with a decrease of CNT diameter [12]. Dispersionand distribution of CNTs in polymer matrix are also importantfor composite properties [13].

oped such as casting, spins coating [14], stretching [1,15], andforge-rolling [9,10,16]. Our previous investigations have shownthat both stretching and forge-rolling results in alignment of nano-tubes in polymer matrix and as a consequence in anisotropic elec-tromagnetic properties of composites [1,2,9,10,16]. However, themeasurement results are difficult to correlate because the CNTshave been produced in different synthetic conditions. Moreover,the properties of anisotropic composites contained the same CNTamount but fabricated by different methods have never been stud-ied comparatively, while it was mentioned that every method hasown advantages and limitations [17].

The present work is devoted to investigation of the electromag-netic response of three types of CNT/polystyrene composites pre-pared using the nanotubes taken from the same batch and in theequal amount. The composites were fabricated by a solution cast-ing and forge-rolling or stretching of soft CNT/polystyrene film. Themeasurements were done in low-frequency (1 kHz–2 MHz) andmicrowave (26.5–40 GHz) regions and analyzed in a light of thenanotube length and orientation.

2. Experimental

2.1. Carbon nanotube synthesis

Arrays of aligned multiwall CNTs (MWCNTs) were grown on sil-icon substrates using aerosol-assisted catalytic chemical vapor

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deposition (CCVD) method. Synthesis was carried out in a quartztube inserting into a horizontal tubular reactor with a length of800 mm and diameter of 36 mm. Construction of set-up and detailsof synthesis are described elsewhere [18]. In brief, a 2 wt.% solutionof ferrocene in toluene was injected into reactor heated up to800 �C. The pyrolysis was performed in an argon flow during a halfhour. The structure of the product was characterized by scanningelectron microscopy (SEM) with a Hitachi S-3400N microscope.SEM analysis revealed a vertical alignment of MWCNTs relativeto the surface of silicon substrate (Fig. 1). The height of arrayswas �130 lm. Concentration of iron nanoparticles in the nanotubechannels determined by elemental analysis was �5 wt.%.

2.2. Composite fabrication

Three types of polystyrene composites with 0.25 wt.% MWCNTloading were prepared. MWCNT array was gently detached fromsilicon substrate and portion of sample taken in the requiredamount was stirred with a toluene solution of polystyrene for com-plete polymer dissolving. Then, the composite suspension was son-icated for �2 min using a high-power sonic tip (200 W) with thepurpose to improve nanotube dispersion. Obtained slush was castonto metallic plate and dried at ambient conditions. The plate pro-duced after complete solvent evaporation is believed to containrandomly distributed nanotubes and in further discussion it willbe referred as isotropic composite. The thickness of this compositeplate was �220 lm. To provide predominant orientation of nano-tubes in polymer matrix we used a procedure of forge-rolling orstretching. The details of forge-rolling procedure for polymer fillingwith different carbon nanostructures are described elsewhere[16,19,20]. A composite plate dried to a viscous state was repeat-edly forge-rolled along a certain direction under the following con-ditions. A linear speed of the rolls was �10–15 cm/s, a clearancebetween rolls was�500 lm during the several first cycles and thenit was decreased to �200 lm. As the stretching procedure wasused, a soft composite plate was uniaxially stretched at a heating(�70 �C), which was provided by a hot air gun. A microscrew setupled to stretching of the plate in half. Finally, the composites weredried under a light load at room temperature. The thickness ofthe rolled and stretched plates was �180 and �390 lm, corre-spondingly. All the prepared plates had visually homogeneous graycolor.

Aspect ratio is an important parameter when interaction ofCNTs with electromagnetic field is studied. We used the nanotubes,which grew simultaneously and hence initially had the near samelength. However, during the treatment they can be shortened. First,effect of sonication was checked. Polystyrene matrix of isotropic

Fig. 1. SEM image of MWCNT array used for composite fabrication.

composite plate was dissolved in toluene and nanotubes wereextracted. Fig. 2a shows typical SEM image of the sediment. A pres-ence of polymer in the sample indicates good adhesion betweenpolystyrene and CNT surface. The sonication causes shortening ofMWCNTs down to �50–70 lm, although nanotubes with a lengthof �20 lm were also observed. Since during the stretching onlyelastic field from the polymer matrix is acted on nanotubes [21],their length should be determined by the sonication treatment.In contrast, a shear deformation applied to the composite duringforge-rolling procedure disentangles nanotubes from their agglom-erates [20] and distributes the filler evenly [19]. SEM analysisshows that strong forces of the rolls may break the nanotubesdown to �3–10 lm (Fig. 2b). One can assume that such fragmen-tation of nanotubes should change electromagnetic response ofthe MWCNT/polystyrene composite noticeably in comparison withthe casted and stretched samples.

2.3. Measurement details

Measurements of real (ReZ) and imaginary (ImZ) parts of theimpedance of MWCNT/polystyrene composites were done regard-ing a two-contact method with an impedancemeter Z-2000 (madeby ‘‘Elins’’, Chernogolovka, Russia) in a frequency range from 1 kHzto 2 MHz. The examined region of composite plate located in cen-tral part of the measuring cell was �0.38 cm2.

Microwave response of MWCNT/polystyrene composites wasstudied using a vector network analyzer Agilent N5247A. The mea-surement cell was a standard waveguide (7.112 � 3.566 mm2)

Fig. 2. SEM images of MWCNT sediment extracted from isotropic (a) and forge-rolled composite by polystyrene dissolving. Numbers indicate the nanotube length.

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supporting H10 oscillation. The composite samples placed in abreak of waveguide line were irradiated by electromagnetic wavewith the transverse electric field oriented either parallel or perpen-dicular to the deformation direction (rolling or stretching). Thereflected (S11) and transmitted (S21) power was registered within26.5–40 GHz frequency range (Ka band).

3. Results and discussion

3.1. Impedance spectroscopy data

Impedance diagrams of the MWCNT/polystyrene composites arepresented in Fig. 3a and b. Real ReZ and imaginary ImZ componentsof impedance of isotropic composite are considerably smaller ascompared to those for anisotropic composite plates. The forge-rolled sample shows monotonic decrease of both ReZ and ImZ parts

Fig. 3. Frequency dependence of real ReZ (a) and imaginary ImZ (b) parts ofimpedance (Z) for MWCNT/polystyrene composites produced by stretching (greensquares) and forge-rolling (blue triangles). (c) Complex impedance of MWCNT/polystyrene composites. The insets show impedance of isotropic composite (redcircles). Numbers in (c) indicates logarithm of the frequency. (For interpretation ofthe references to color in this figure legend, the reader is referred to the web versionof this article.)

with frequency increase. The impedance plotted in the Nyquistcoordinates (ImZ vs ReZ) is close to a straight vertical line (Fig. 3c)that indicates substantial capacitive nature of the composite. Thefrequency dependence of impedance spectrum of the stretchedMWCNT/polystyrene plate is quite different. The spectrum showsno dispersion in the range from 10 kHz to 2 MHz, however, around�4.1 kHz (the correlation frequency fc) the ReZ component has aninflection, while the ImZ component passes through a maximum(Fig. 3(a and b)). This behavior is similar to that observed for the iso-tropic MWCNT/polystyrene composite. The correlation frequency inthat case was�0.4 MHz. The complex impedance spectra of the iso-tropic and stretched samples show well-defined semicircles withdiameters of �13.2 kX and �0.7 MX (Fig. 3c), respectively, charac-teristics of bulk resistance of a material.

The impedance data can be analyzed using an equivalent circuitconsisted of resistor and capacitor [22]. Depending on geometricaland electronic structure of material, network may be organizedthrough a series or parallel connection. In particular, the imped-ance of CNT percolation film can be described by two parallel RCcircuits in series corresponded to conductivity inside of a nanotubebundle and tunneling between the bundles [23]. In our case, wherethe composites have a rather low concentration of nanotubes,which are mainly isolated (see Fig. 2), the charge transfer from ananotube to another one takes place via a dielectric layer.

Frequency dependence of dielectric constant e and resistivity R ofthe MWCNT/polystyrene composites are presented in Fig. 4a and b.Reconstruction of dielectric characteristics was done as describedby Gavrilov et al. [20]. The measurement cell was considered astwo parallel Voigt elements, which correspond to own loss andcapacitance of experimental cell and composite material. For thesame content of MWCNTs, the dielectric constant of stretched com-posite is two or three times larger than that of the forge-rolled sam-ple, which properties are mostly governed by permittivity of theunfilled polystyrene plate (epolystyrene = 1.9). At the same time, thestretched composite was found to be more conducting than theforge-rolled one. Further increase of the dielectric constant anddecrease of the resistivity were found for the isotropic composite.

The results presented show that dielectric response of MWCNT/polystyrene composite is very sensitive to fabrication method. Themost important parameters responsible for this property are thedispersion state and the size of carbon filler. Casting gives no pre-dominant orientation of MWCNTs in polymer matrix and some ofthe nanotubes can be perpendicular to the plate surface. As theresult, dielectric constant and conductivity in the cross-section ofthe composite are considerably enhanced relative to those foranisotropic composite plates. Despite the different orientation ofMWCNTs, the isotropic and stretched composites have similar fre-quency dependence of impedance, although the shift of the maxi-mum of the ImZ component from 0.4 MHz to 4.1 kHz indicates thelarger effective size of conducting inclusions in the latter compos-ite [24]. The spectrum of the forge-rolled sample is distinct due todiffering in the length distribution of MWCNTs. Indeed, accordingto the SEM investigation of sediments extracted from polystyreneplates, the used fabrication methods provide effective disentan-gling of nanotubes from their agglomerates. However, strongforces of rolls break the nanotubes from the initial length of�130 lm to �3–10 lm (Fig. 3b), while average length of MWCNTsconstituting the isotropic and stretched composites is �70 lm. Thehighly capacitive spectrum of the forge-rolled composite indicatesthat distance between these short nanotubes is larger than that isnecessary for tunneling of electrons [25], and as the result thecharge carriers are blocked by rather thick polystyrene layer pro-viding an insulating state for the composite (i.e. forge-rolled sam-ple is below the percolation threshold). Response of the isotropicand stretched composites with longer nanotubes can be attributedto activation of conduction mechanism (i.e. the samples are above

Fig. 4. Frequency dependence of dielectric constant (a) and resistivity (b) forMWCNT/polystyrene produced by casting (red circles), stretching (green squares),and forge-rolling (blue triangles) procedures. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

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the percolation threshold). Therefore we suggest that in thesecases average distance between MWCNTs becomes small enoughto provide efficient electron tunneling across the plate. Previousinvestigations showed that percolation effect can be observed bothfor the smaller [26] and larger [27] content of CNTs, than that wehave taken here.

Fig. 5. Transmitted/input (S21) and reflected/input (S11) signals of isotropic (a),forge-rolled (b), and stretched (c) MWCNT/polystyrene composites in frequencyrange of 26.5–40 GHz. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

3.2. Microwave probing

The electromagnetic response of MWCNT/polystyrene compos-ites as a ratio of transmitted/input (S21) and reflected/input (S11)signals is shown in Fig. 5. Both reflection and transmission spectrain the Ka band are strongly dependent on the fabrication method ofcomposite. Reflectance (R), transmittance (T), and absorbance (A)of the composites were reconstructed from the measured S-param-eters for a thickness of a plate of 100 lm (Fig. 6). The isotropicsample shields a one third of microwave radiation mainly due toreflection mechanism (R � 24%, A � 10%). Ordering of nanotubesin polystyrene matrix during the deformation causes decrease inreflectance (no more than 5%) and increase in absorbance (no lessthan 15%). The shielding efficiency of the anisotropic compositesstrongly depends on the fabrication procedure: the transmittanceof the forge-rolled sample deteriorates as compared to the castedone, while electromagnetic response of the plate produced bystretching demonstrates noticeable enhancement in the shieldingefficiency. We relate the poor shielding ability of the forge-rolledcomposite with strong fragmentation of MWCNTs.

Orientation of nanotubes in polymer matrix results in aniso-tropic behavior for both reflection and transmission modes. Theforge-rolled composite has a negligible difference in microwavetransmission for parallel (T|| � 79%) and perpendicular (T\ � 82%)orientations of the plate relative to polarization of the electric field.A minor orientational dependence of S-parameters (Fig. 5b) indi-cates poor ordering of nanotubes in polystyrene matrix after the

rolling. However, previously we found a considerable increase ofthe permittivity of rolled MWCNT-based composites measuredalong the deformation direction relative to the value measuredacross [2,16]. Thus, we suggest that the forge-rolling procedureresults in arrangement of nanotubes in the plane direction of sam-ple rather than along the deformation direction. Probably, the usedrolling regime was too fast to align MWCNTs in polystyrene matrix.The stretched composite showed noticeable dependence of the Tand R parameters on the composite plate orientation (T|| � 40%,A|| � 55%, and T\ � 60%, A\ � 37%). The value T\ is close to thetransmittance of the isotropic composite that indicates insuffi-ciency of the applied deformation for full straighten of the nano-tubes in polystyrene matrix. This technological disadvantagecould be eliminated by choosing more flexible polymer matrix.

It is of interest to compare the obtained results with microwaveelectromagnetic response of other polymeric composites with car-bon nanostructures. Due to light-weight, flexibility, chemical inert-ness and high-performance, such materials are becoming more

Fig. 6. Reflectance (a), transmittance (b), and absorbance (c) of MWCNT/polysty-rene composites produced by different fabrication methods. Symbols ‘‘||’’ and ‘‘\’’indicate orientation of MWCNTs in polystyrene matrix relative to the polarizationof microwave electric field.

Fig. 7. Models showing CNT distribution in polystyrene matrix of isotropic (a),forge-rolled (b) and stretched (c) composites. Dashed lines in (a) and (c) showpotential conductive path, circles in (a) and (c) indicate electron hopping regions.Circles in (b) indicate capacitors.

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preferred in comparison with the conventional metallic coating.Reasonably good efficiency was found for the 700-lm-thick epoxycomposites filled with 1.5 wt.% of CNTs (T � 60%) [4,5]. Such shield-ing is comparable with the values T and T\ for our isotropic andstretched composites, which, however, were thinner. PMMA andepoxy composites with a 10 wt.% of onion-like carbon showed�60% microwave transmittance for the 1-mm-thick plate [28] thatalso is close to our values. It was found that shielding efficiency ofMWCNT/PMMA and MWCNT/phosphate composites increases witha fraction of nanotubes, and at �2 wt.% loading the composites arealmost opaque for radiation in the Ka band [2,6]. Addition of1.5 wt.% of single-walled CNTs in epoxy resin improved the micro-wave transmittance by �35% [5] that is the same as the value T||

for our stretched sample. The shielding efficiency of nanometer filmof amorphous pyrolytic carbon was found to be very similar to a con-ventional mm-thick polymer filled with nanosized carbon struc-tures [29,30]. To our best knowledge, there are only a fewinvestigations on the polarization anisotropy of electromagneticresponse of CNT-based materials. Thus, stretching of a MWCNT/

PMMA composite had caused considerable increase of the dielectricpermittivity in the Ka band [1]. Textile composites from CNT yarnexhibited considerable dependence of both shielding efficiencyand reflectance on the yarn package: they were almost transparentfor perpendicular orientation relative to polarization of the electricfield, while for parallel orientation the sample transmittance wascompletely suppressed (S21 � �17 and �13 dB) mainly due toreflection mechanism [31]. Considerable angular dependence ofthe transmitted and reflected THz radiation has been demonstratedfor the rolled MWCNT/polystyrene composite [9,10]. Hence, weshould conclude that electromagnetic properties of the preparedcomposites are comparable with those for shielding materials filledwith different nanocarbon forms, and this despite the fact that theinvestigated plates were thinner and less loaded with carbon filler.Moreover, we expect high THz anisotropy for CNT-based compositesprepared by mechanical stretching because this fabrication tech-nique preserves long length of the nanotubes.

4. Conclusion

We have performed a comparative study of electromagneticproperties of low-loaded MWCNT/polystyrene composites fabri-cated by casting, forge-rolling and stretching techniques. We foundthat fabrication method influences the size and distribution pat-tern of nanotubes in polystyrene matrix (Fig. 7): the casting yieldsisotropic arrangement of nanotubes in polymer matrix, the forge-rolling results in fragmentation of nanotubes and alignment of fil-ler in the plane direction of sample, the stretching results inarrangement of nanotubes along the deformation direction. Forthe same content of MWCNTs, the dielectric properties of compos-ite with short nanotubes is governed by the properties of polymer

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matrix, while the transition from the insulating to conductive statewas detected for the both composites with longer nanotubes.MWCNT arrangement in polystyrene matrix enhances absorbanceof composite plate, while transmittance depends on the length andalignment of nanotubes. Noticeable anisotropic transmissiondetected for the stretched composite indicates high potential ofthe mechanical stretching technique for the production of giga-hertz polarizers.

Acknowledgments

This work was supported by the Russian Foundation for BasicResearch (Grant 14-03-90028 in a part of low-frequency measure-ments and Grant 13-02-90711 in a part of the microwavemeasurements).

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