Effect of Carbon Nanotubes on Liquid Crystal

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 192 (2017) 935 – 940

TRANSCOM 2017: International scientific conference on sustainable, modern and safe transport

Effect of carbon nanotubes on liquid crystal behavior in electric and magnetic fields studied by SAW Marek Veveričíka*, Peter Burya, Peter Kopčanskýb, Milan Timkob, Zuzana Mitróováb a

Department of Physics, Faculty of Electrical Engineering, University of Žilina,,Univerzitná 1, 010 26 Žilina, Slovakia b Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice, Slovakia

Abstract The effect of multi-walled carbon nanotubes (MWCNT) and functionalized MWCNT (MWCNT/Fe3O4) on structural changes in thermotropic liquid crystal (6CHBT) was studied using the attenuation measurement of surface acoustic wave (SAW) propagating along the liquid crystal. Both kinds of nanoparticles of low volume concentration (1 x 10-4) were added to the liquid crystal during its isotropic phase. Pure 6CHBT liquid crystal was used for the comparison of structural changes and orientational coupling of the liquid crystals molecules with both types of carbon nanotubes. These observations proved, that the doping process significantly affected the behavior of liquid crystal in applied electric and magnetic fields, and indicated potential application. ©2017 2017The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of TRANSCOM 2017: International scientific conference on responsibility the scientific committee of TRANSCOM 2017: International scientific conference on sustainable, Peer-review modern and safe of transport. sustainable,under modern and safe transport Keywords: liquid crystal; carbon nanotubes (MWCNT); structural changes; surface acoustic wave;

1. Introduction The doping of liquids with carbon nanotubes and magnetic nanoparticles have attracted wide interest in many areas of science, technology and medicine [1, 2]. Carbon nanotubes are molecular scaled tubes of graphitic carbon with outstanding properties. The simplest nanotube is composed of a single sheet of a honeycomb network of carbon atoms, called graphene, which is rolled up seamlessly into a tubular form. Single-wall carbon nanotube (SWCNT) was

* Corresponding author. Tel.: +421 41 5132317; fax: +421 41 5131516. E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of TRANSCOM 2017: International scientific conference on sustainable, modern and safe transport

doi:10.1016/j.proeng.2017.06.161

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discovered in 1993 [3], however, carbon nanotubes as multi-tubes nesting (MWCNT) in a concentric fashion were discovered already in 1991 [4]. The diameter of SWCNT ranges from 0.4 nm to 2.0 nm and the length from 20 nm to about 1000 nm. The multitubes are bigger objects with diameters in the range of 1.4 nm–100 nm and length from 1 mm to several milimeters. The exact structure of a nanotube depends on the different angles and curvatures in which a graphene sheet can be rolled on. The structure is determined by a vector, called a chiral vector, which discriminates the nanotubes into ‘‘zigzag’’, ‘‘armchair’’, or ‘‘chiral’’ forms. The electrical properties of nanotubes strongly depend on their structure: the armchair nanotubes are metallic, while zigzag and chiral ones can be either metallic or semiconducting by nature. In general, SWCNTs exhibit a mixture of metallic and semiconducting properties, depending sensitively on their geometrical features, while MWCNTs are regarded as metallic conductors. In principle, carbon nanotubes are insoluble in solvents due to the strong Van der Waals interactions that tightly hold them together, forming bundles. Nanotubes can undergo chemical functionalization to enhance their solubility in various solvents and to produce novel hybrid materials potentially suitable for applications. The main approaches for the functionalization of the nanotubes can be grouped into two principal categories: (a) the covalent attachment of chemical groups through reactions on the conjugated skeleton of nanotubes, and (b) the noncovalent supramolecular adsorption or wrapping of various functional molecules onto the tubes. Nematic liquid crystalline phases occur as additional, thermodynamically stable states of matter between the liquid state and the crystal state in some materials. They can be characterized by a long-range orientational order of the molecules and, as a consequence, by an anisotropy in their physical properties. Liquid crystals (LCs) can be oriented under electric or magnetic fields due to the anisotropy of dielectric permittivity or diamagnetic susceptibility. However, because of the small value of the anisotropy of the diamagnetic susceptibility, the magnetic fields necessary to align liquid crystals should reach rather large values (B > 1 T). In an effort to enhance the magnetic susceptibility of liquid crystals, the idea of doping them with fine magnetic particles was introduced [5]. The authors predicted that a rigid anchoring m ⁄⁄ n, where the unit vector n (director) denotes the preferential direction of the nematic molecules and the unit vector m denotes orientation of the magnetic moment of the magnetic particles, would result in ferromagnetic behavior of the nematic matrix. The experiments confirmed the existence of considerable orientation and concentration effects in liquid crystals doped with magnetic particles as well as the fact that the essential feature of these systems is a strong orientational coupling between the magnetic particles and the LC matrix [6-8]. The applied magnetic field changes the n the orientation of magnetic particles and due to the coupling between magnetic particles and LC molecules the director follows it. Carbon-based nanostructured materials and their relationship with liquid crystals are another topic in current research. It is worth mentioning the recently described connection between graphene oxide and liquid crystals [9] as well as the highly active topic of LC structures doped with carbon nanotubes and the possibility of reorienting them with external fields. LC–carbon nanotube suspensions increasingly rely on improving electro-optic properties of LCs and developing nano sized electrochemical systems [10-12]. Acoustic methods are useful tool for the characterization of LCs, particularly their elastic and viscous parameters near phase transitions. Concerning the surface acoustic waves (SAW) they were used to determine the viscosity distribution in LC layer depending on applied electric field, as the SAW-driven LC light shutter or SAW sensor [13, 14]. Recently the SAW technique has been shown to be a useful tool for investigation of structural changes in ferronematics [15, 16]. In this contribution, we present the utilization of SAW to study structural changes in nematic LCs doped with MWCNT and composites MWCNT/Fe3O4 induced by electric and weak magnetic fields. 2. Experimental Chemical vapor deposited multi-walled carbon nanotubes (MWCNT) were purchased from Sigma Aldrich Co. (length from 0.5 μm to 2 μm, outer diameter from 20 nm to 30 nm, wall thickness from 1 nm to 2 nm). Other reagents (FeCl3.6H2O, FeSO4.7H2O, NH4OH, HNO3, and H2SO4) were of analytical grade. MWCNT/Fe3O4 composites were prepared in two steps; a) functionalization of MWCNT with carboxyl groups and b) subsequently labeling with magnetic nanoparticles [17]. The whole process was performed under nitrogen atmosphere. The reaction mixture was then several times centrifuged; the product was washed with deionized water to neutralize pH and dried at 50 ºC for 24 hours. The scheme of the functionalization is shown in Fig.1. The total iron concentration of the MWCNT/Fe3O4 was determined spectrophotometrically after HCl/H2O2 induced oxidation of FeII to FeIII and an addition of 1% ammonium thiocyanate followed by absorption measurement of the


thiocyanate complex at λ = 495 nm. The amount of magnetic nanoparticles in the prepared samples was 25% w/w. The morphology and size distribution of the prepared MWCNTs and magnetically labeled MWCNTs were measured by transmission electron microscopy (TEM Tesla BS 500). Fourier-transform infrared (FTIR) spectra of MWCNT, carboxyl functionalized MWCNT (MWCNT-COOH), and magnetite labeled nanotubes (MWCNT/Fe3O4) were collected using KBr pellets with an FTLA2000-100 instrument (from ABB) acquiring 32 scans per specimen at a nominal resolution of 4 cm-1. A spectrum of a pure KBr pellet was used as a background to generate the sample spectra. Magnetization measurements were recorded by a SQUID magnetometer (Quantum Design MPMS 5XL) at room temperature. Samples were prepared by wrapping a few tenths mg of sample in about 15 mg of cling film. The known diamagnetic contribution of the film was subtracted from the magnetization curve of each sample.

Fig. 1. The scheme of MWCNT/Fe3O4 preparation.

The surface acoustic waves of fundamental frequencies 10 and 20 MHz and their harmonics could be generated by an interdigital transducer (IDT) prepared on the LiNbO3 delay line using the Pulse Modulator and Receiver - MATEC 7700 and another IDT was used for receiving the surface wave. However, the harmonic frequency 30 MHz appeared to be the most sensitive SAW frequency for structural changes study. The acoustic attenuation was measured using Matec Attenuation Recorder 2470 A. The samples of LCs were placed on the top of the LiNbO3. Fig. 2 shows the experimental arrangement of the LC layer of thickness D ≈ 100 μm was located on the center of the acoustic delay line and sandwiched between the delay line and the glass plate, both coated with gold electrodes. Both edges of the LiNbO3 delay line were skewed (45 °) in order for reflected acoustic waves not to be detected by IDTs.

Fig.2. Schematic arrangement of LC cell on LiNbO3 delay line for SAW investigation.

3. Results and discussion Structural changes in LC under both electric and magnetic fields were monitored using measurements of the attenuation of SAW propagating along the interface between the LiNbO3 delay line and LC cell. As it was shown in our previous works [8, 15], there is an interaction between the magnetic moment of the magnetic particles and the liquid crystal molecules which favors a parallel initial orientation of magnetic moments and the director. The initial intrinsic arrangement of LC was supposed to have a planar alignment when the director n was parallel to the electrodes and electric or magnetic fields were then applied perpendicular to them. The applied fields turned the director to its direction so that LC molecules changed orientation to perpendicular to the surface of electrodes and the SAW

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attenuation subsequently changed. However, from the viscosity distribution measurement it follows that molecules change the orientation gradually and starting from the center [18]. The applied magnetic field turned the magnetic moment of the magnetic nanoparticles to its direction and consequently due to the orientational coupling between LC molecules and magnetic nanoparticles the director n changed orientation, too. It can be supposed that the same mechanism of reorientation occurs also in the case of carbon nanotubes.

Fig.3. Effect of applied magnetic field on SAW attenuation for 6CHBT doped with MWCNT and MWCNT/Fe 3O4 (Φ =10-4).

Fig. 3 shows the effect of applied magnetic field on SAW attenuation changes 'D in 6CHBT doped with MWCNT, MWCNT/Fe3O4 volume concentration Φ=10-4 as well as pure 6CHBT. This increase of magnetic field was applied continuously during 10 minutes from 0 to 400 mT. It can be seen, that the magnitude of this effect depends on the magnitude of applied magnetic field. The structural changes registered by acoustic attenuation practically starts at magnetic field 100 mT and for fields higher than 300 mT the saturation is gradually coming up. However, it is interesting that in the case of MWCNT changes of acoustic attenuation are bigger than in the case of MWCNT/Fe3O4 that is in the contrast with results obtained on SWCNT and SWCNT/Fe3O4 using capacitance measurements [8]. It seems that Fe3O4 magnetic nanoparticles do not increase magnetic moments of MWCNTs. Similar developments of acoustic attenuation depending on the applied magnetic field were detected also for different concentrations of MWCNTs. The applied magnetic field, due to the low diamagnetic anisotropy of LC molecules, did not induce any response in pure 6CHBT. The progress of the switching processes and the comparison of effects of applied voltage (10V) and magnetic field (250 mT) on the SAW attenuation for 6CHBT doped with MWCNT (Φ =10-4) is illustrated in Fig. 4. These developments indicate that LC molecules can be reoriented, except the electric field, also by magnetic field perpendicular to electrode surfaces. The larger time constants connected with structure changes in the case of applied magnetic field indicate also the presence of different process and the role of magnetic moments of MWCNTs. The process of reorientation of LC molecules is in this case influenced by their coupling with MWCNTs. While the relaxation time of the process occurring after the electric field activation was τe1 = 1.6 s, the relaxation time of process initiated by the magnetic field was noticeably longer, τm1 = 20.2 s. After removing of the magnetic fields, the registered process had the relaxation time τm2 = 9.5 s, the process after removing of electric field had the relaxation time τe2 = 12.2 s, indicating very similar process as in the case of magnetic field. However, the second process with longer relaxation time could be distinguished, too. These processes are slower than in the case of spherical particles [15] but comparable with processes when rod-like magnetic particles were used for doping of 6CHBT liquid crystal [16].


Fig.4. Effects of applied voltage (10V) and magnetic field (250 mT) on SAW attenuation for 6CHBT doped with MWCNT (Φ =10-4).

The rate of processes occurring after the application of both electric and magnetic fields depends on the magnitudes of applied fields. The relaxation time τe1, for example, changed its value for voltages in the region 3-10 V from 4.1 s to 1.6 s. This fact coincides with electro-optical investigations [19], however, the different rates are probably due to the different LCs. The experimental observations confirm the role of MWCNTs in LCs and their interaction with liquid crystal molecules and coincide with previous results [11, 12, 19]. Results obtained on MWCNT/Fe3O4 were very similar with those presented in Fig. 4, but with the difference that observed changes were weaker.

Fig.5. Effect of successive application of electric and/or magnetic fields on SAW attenuation for doped 6CHBT (Φ =10-4).

Three different successive applications of electric and/or magnetic fields are illustrated in Fig.5. The curve shows the application of magnetic (250 mT) and electric (10 V) fields one after another. It is evident that the effects of both electric and magnetic fields can be combined and completed together up to reaching of the saturate state. So that in the case of lower electric field the magnetic field can contribute to the reorientation of LC molecules and oppositely in the case of lower magnetic field the electric field can contribute to the reorientation.

4. Conclusions In this contribution we have presented the utilization of SAW to study the structure changes in 6CHBT doped with carbon nanotubes (MWCNTs) and functionalized MWCNTs, induced by electric and weak magnetic fields. Obtained

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results confirmed previously supposed orientational coupling between MWCNTs and liquid crystal matrix. Doping of a nematic liquid crystal with MWCNT or MWCNT/Fe3O4 causes an effective orientational coupling between both electric and magnetic moments of carbon nanotubes and the director of the nematic liquid crystal. The obtained results also showed that in the 6CHBT liquid crystal doped with carbon nanotubes the orientation of the magnetic moments of carbon nanotubes and molecules of the liquid crystal are parallel. However, the director of the liquid crystal at some distance from the surface of the carbon nanotubes depends on the strength of the applied external fields. In conclusion, our results showed that the combination of electric and magnetic fields can be used to the control of the orientation of LC molecules in doped samples unlike pure liquid crystals. The doping of LCs can impact also their magneto-optical properties. These results could be interesting for some practical applications particularly as nanosensors or magnetooptical devices for various industrial branches including transport industry as a part of electronic devices. This could lead to new kind of sensors for autonomous driving what is main goal for today’s road safety policy. Acknowledgements This work was supported by VEGA projects 2/0016/17 and 1/0510/17. Authors also would like to thank to Mr. František Černobila for technical assistance. References [1] K. Bradley, J. P. Gabriel and G. Gruner, Nano Lett., 3 (2003) 1353–1355. [2] Z. Kuang, S. N. Kim, W. J. Crookes-Goodson, B. L. Farmer and R. R. Naik, NANO 4 (2010) 452. [3] S. Iijima and T. Ichihashi, Nature 363 (1993) 603–605. [4] S. Iijima, Nature 354 (1991) 56–58. [5] F. Brochard, P. G. de Gennes , J. Physique 31 ( 1970) 691. [6] J. Liebert, A., Martinet, J. Phys. Lett. 40 (1979) 363. [7] S. V. Burylov, Y. L., Raikher, Mol. Cryst. Cryst. 258 (1995) 123. [8] N. Tomašovičová, M. Timko, Z. Mitróová, M. Koneracká, M. Rajňak, N. Éber, T. Tóth-Kaona, X. Chaud, J. Jadzyn, P. Kopčanský, Phys. Rev. E87 (2013) 014501. [9] T. Z. Shen, S. H. Hong, J. K. Song, Nat. Mater. 13 (2014) 394. [10] M. Rahman, W. J. Lee, Phys. D: Appl. Phys. 42 (2009) 063001. [11] R. Basu, G. S. Ianncchione, Appl. Phys. Lett. 95 (2009) 173113. [12] R. Basu, G. S. Ianncchione, Appl. Phys. Lett. 93 (2008) 183105. [13] Y.J. Liu, X. Ding, Sz-Chin Steven Lin, J. Shi, I-Kao Chiang, T.J. Huang, Advanced materials 23, 1656 (2011) [14] S.J. Patrash, E.T. Zellers, Analytica Chemica Acta 288, 167 (1994) [15] P. Bury, Š. Hardoň, J. Kúdelčík, M. Veveričík, P. Kopčanský, M. Timko, V. Závišová, J. of Magnetism and Magnetic Materials 423 , 57 (2017) [16] P. Bury, M. Veveričík, J. Kúdelčík, P. Kopčanský, M. Timko, V. Závišová, Acta Phys. Polonica, in press. [17] Z. Mitróová, N. Tomašovičová, M. Timko, M. Koneracká, J. Kováč, J. Jadzyn, I. Vávra, N. Éber, T. Tóth-Katona, E. Beaugnon, X. Chaude and P. Kopčanský, Magnetohydrodynamics 45, (2009) 353 [18] H. Moritake, R. Ozaki, K. Chiba. H. Yamamoto, J. Ogawa, K. Yoshino, IEEE International conference on dielectric liquids (2011) [19] F. M. Ion, Ch. Berezovski, R. Berezovski, G. Heimann, M. Moisescu, Romanian Rep. Phys. 64 (2012) 1011-1018..