Mater. Res. Soc. Symp. Proc. Vol. 12 © 2011 Materials Research SocietyDOI: 10.1557/opl.2011.

π-π Stacking Molecular Interactions Between Carbon Nanotubes and 5CB Liquid Crystal

Georgi Y. Georgiev1,2, Erin A. Gombos1, Michael B. McIntyre1 and Peggy Cebe2

1Department of Natural Sciences, Assumption College, Worcester, MA 01609, U.S.A.

2Department of Physics and Astronomy, Tufts University, Medford, MA 02155, U.S.A.

ABSTRACT

Multiwall Carbon Nanotubes (MWCNT) align by coupling to the liquid crystals’ (LC) nematic director in LC/MWCNT dispersions. This coupling is so strong that the LC molecules act as molecular motors to reorient the MWCNTs when an electric field is applied across oriented electro optic cells. On the other hand, MWCNTs also improve the LC order and modify the crystal phase of LCs. We investigate the physical reasons for those strong effects by studying the molecular interactions between a host LC and MWCNTs. It has been predicted theoretically that the aromatic rings could stack with their π orbitals in 4-Cyano-4'-pentylbiphenyl (5CB) and MWCNT nanocomposites. Experimentally 5CB modifies the MWCNTs Raman breathing modes in the same nanocomposites. In turn, we look for evidence of this interaction between MWCNTs and LCs at the 5CB molecules. Using FTIR spectroscopy we found that the modes corresponding to 5CB aromatic rings vibrations are affected in the presence of MWCNTs which confirms that π-π stacking of 5CB’s biphenyl rigid core to the carbon rings on the MWCNTs’ surface may indeed be major mechanism for MWCNT/LC nematic coupling. It shows also that the Raman breathing mode effects on MWCNTs can be due to this π- π stacking interaction with 5CB. Further investigations of the MWCNTs interactions with 5CB can lead to developing of a complete model of this phenomenon and help applications for electro optic cells, nanoswitches, new crystal forms for optics, communication technology and others.

INTRODUCTION

It has been reported in Raman studies that the breathing mode of MWCNTs is changed in the

presence of 5CB LC [1]. These authors also report polarized microscopy measurements confirming induced nematic order around the CNTs at temperatures above the nematic to isotropic transition temperature where the rest of the sample is already isotropic. Our work confirms that this interaction in turn leads to changes in the 5CB molecular vibrational modes of the atoms participating in phenol rings due to the proposed π- π stacking [2]. Specifically, we measure the C-C=C stretch of the carbon atoms in the aromatic rings and C-H stretch and bend vibrations of the hydrogen atoms connected to the rings. Our data confirm experimentally the theory of π- π stacking as simulated in [2] reposnsible for the changes found in the aromatic rings of the 5CB and the MWCNTs. It is important to answer the question: How LCs couple so strongly to MWCNTs that LCs can be used as molecular motors for MWCNT upon application of electric field [3, 4]? We find that the peaks associated with vibrations of the aromatic rings of the 5CB molecule change proportionally to the concentration of MWCNTs, through FT-IR spectroscopy. This interaction may be one of the reasons for the modified crystal structure of 5CB at low temperarure in presence of MWCNTs [5].

It is important to study the MWCNTs because of their extraordinary electronic and

mechanical properties and their use in many promising application [6]. In recent years there is

29293

great interest in LC/MWCNTs nanocomposites [7]. New research for improvements in the scientific understanding for the utilization of carbon nanotubes for nanotechnology devices is an extremely active area of science [8, 9].

MATERIALS AND METHODS

5CB and MWCNTs

4-Cyano-4'-pentylbiphenyl (5CB) LC was obtained from Tokyo Kasei Kogyo Co. LTD (Catalog #C1550). MWCNTs were purchased from MER Corporation, produced by catalytic chemical vapor deposition (CVD) with a diameter of 140 ± 30 nm and length of 7 ± 2 µm, with purity of >90% (Catalog #MRCSD).

Purifying MWCNTs by Oxidation

One gram of the MWNTs was suspended in a mixture of concentrated sulfuric acid and nitric acid (3:1 vol. ratio). This solution was sonicated in a Misonix water bath sonicator for 24h at 50˚C [10]. The resultant suspension was diluted with 400mL of deionized water and filtered through a 400 nm pore membrane (PTFE) until the water passing through the filter had a pH between 6 and 7. The dispersions were subsequently filtered or diluted to the desired concentration. The resulting MWNTs had a pH of between 3 and 3.5 and were stable. Scanning Electron Microscopy (SEM) analysis showed that the MWCNTs were shortened between 30% and 40% of their original length, as expected.

Preparation of 5CB/MWCNT Nanocomposites

The 5CB/MWCNT samples with different concentrations based upon weight were prepared using sonication of the nanocomposite at room temperature.

Fourier Transformed Infrared (FTIR) spectroscopy

A Thermo Scientific Nicolet 6700 FT-IR spectrometer was used to obtain the infrared

absorption data. The nanocomposites were deposited on a thin layer on CaF windows. A blank CaF window signal was used for the background. All of the subtractions and graphs were done on the Thermo Scientific Nicolet 6700 FT-IR spectrometer software.

RESULTS AND DISCUSSION

Our results confirm the observed changes in the Raman breathing mode of CNTS attributed

to π-π stacking with the phenol rings of 5CB [1], by measuring the signals from the aromatic core of 5CB itself. Figures 1 to 3, show strong differences between the signals of atoms participating or connected to the aromatic rings of 5CB samples with and without MWCNTs. The difference features amplify with increase of MWCNT contents, which further confirms that they are due to the presence of MWCNTs in 5CB. This interaction matches well with observation of the MWCNTs breathing mode in the presence of 5CB and with theoretical simulations about π- π stacking [1, 2]. These results evidence a strong coupling which is the

reason for improved order parameter and other effects, like a shift in the Fréedericksz transition voltage in 5CB /MWCNTs nanocomposites [4].

Wavenumber (cm-1)

Ab

sorb

an

ce

Figure 1. FT-IR Absorbance vs. wavenumber for two concentrations of MWCNTs in 5CB. The range 870-675 cm-1 is typical for C-H vibration for phenyl ring substitution corresponding to out of plane bending of the hydrogen atoms [11]. The topmost trace (purple) is the absorbance of 1% MWCNTs, while the lower one (blue) is of 0.1% MWCNTs nanocomposite. From the lowest two curves the upper one (orange) is the spectrum of 1% MWCNTs with the one of the pure 5CB subtracted. The lowest spectrum (red) is the spectrum of 0.1% MWCNTs with the one of the pure 5CB subtracted.

On Figure 1 are shown results from FTIR absorbance studies on the 5CB/MWCNTs nanocomposites at low concentrations of 0.1% and 1% MWCNTs and their difference spectra with 5CB with 0% MWCNTs. We see a clear increasing trend of the difference peaks with increase of the MWCNTs concentration. This result indicates that in the C-H vibrations of the 5CB phenyl rings are affected proportionally to the amount of MWCNTs in the sample. This trend can be explained with the π- π stacking of the 5CB phenyl rings to those on the surface of the MWCNTs. The trend is consistent with the trends of increasing smectic order with increase of MWCNTs content in our results for polymers [12]. At higher than 1% concentrations when most of the 5CB molecules are already in contact with MWCNTs, we expect this trend to saturate.

Wavenumber (cm-1)

Ab

sorb

ance

Figure 2. FT-IR Absorbance vs. wavenumber for two concentrations of MWCNTs in 5CB. The 1606cm-1 peak corresponds to a C-C=C symmetric stretch and the 1494cm-1 corresponds to a C-C=C asymmetric stretch [11]. The appearance of both peaks and the similar effect of the MWCNTs addition to both of them confirm their assignment. The topmost purple trace is the absorbance of 1% MWCNTs, while the lower one blue is of 0.1% MWCNTs nanocomposite. From the lowest two curves the upper one (red) is the spectrum of 0.1% MWCNTs with the one of the pure 5CB subtracted. The lowest spectrum (orange) is the spectrum of 1% MWCNTs with the one of the pure 5CB subtracted.

We investigate also the signature of π-π stacking in other regions of the FT-IR absorption spectra of 5CB and 5CB/MWCNTs nanocomposites. On Figure 2 we present the FT-IR data for the C-C=C stretch in the aromatic rings of 5CB in the absence and presence of MWCNTs. They show a second confirmation that the vibrations of the 5CB phenol rings are affected by the MWCNTs. Again, we see a trend of increase in the amplitude of the difference peaks with increase of the MWCNTs concentration. This result indicates that in the C-C=C vibrations of the 5CB phenyl rings are affected proportionally to the amount of MWCNTs in the sample similar to the C-H vibrations on Figure 1. This trend confirms the π-π stacking of the 5CB phenyl rings to those on the surface of the MWCNTs.

Figure 3. FT-IR Absorbance vs. wavenumber for two concentrations of MWCNTs in 5CB. The range from 3000 to 3100 cm-1 is typical for C-H stretch in aromatics [11]. The topmost trace (purple) is the absorbance of 1% MWCNTs, while the lower one (blue) is of 0.1% MWCNTs nanocomposite. From the lowest two curves the upper one (red) is the spectrum of 0.1% MWCNTs with the one of the pure 5CB subtracted. The lowest spectrum (orange) is the spectrum of 1% MWCNTs with the one of the pure 5CB subtracted.

Figure 3 displays the C-H stretch signals for hydrogen atoms connected to the aromatic rings. The C-H bonds connected to the aromatic ring may be affected to a different degree by the π-π stacking of the phenol rings themselves, and provide important information about the interaction. It provides a third confirmation that the 5CB phenol rings participate in the interaction between 5CB and MWCNTs. This Figure shows a similar trend of increased effect on the amplitude with increased MWCNTs concentration.

CONCLUSIONS

We studied the effect of MWCNTs at low concentrations of 0.1% and 1% on the FT-IR spectral properties of 5CB nematic liquid crystal and compared them to those of the pure 5CB. We observed differences in the frequencies assigned to the vibrations of atoms participating in the aromatic rings of 5CB upon adding a minute amount of MWCNTs of 1%. The increase of MWCNTs content led to increase in those difference signals, confirming that the observed effects are due to interactions between MWCNTs and 5CB. Comparison of those data with ones from other investigators [1] showing that the Raman breathing mode of the MWCNTs

themselves is affected in the presence of 5CB confirms this finding. Also these results are in line with a theoretical prediction for a π- π stacking between aromatic rings of the MWCNTs and 5CB molecules [2]. We conclude that one of the reasons for the alignment of the nematic directors of 5CB and MWCNTs is this π- π stacking interaction, which also may lead to other effects on the properties of liquid crystals, such as improved order parameter and changed transition electric field in the Fréedericksz transition [4, 12].

In another paper [5] we study we study the effect of MWCNTs on the phase diagram of 5CB and specifically on the N-C transition which also can be explained with the π-π stacking interaction as described in this paper. Our results here and in [5] provide support to a theory that there are even more aspects to the interactions between 5CB and MWCNTs and that the MWCNTs may change the LC order of 5CB from nematic to smectic aligning 5CB in layers on its surface [13]. Further Raman and FT-IR spectroscopic studies can provide information about interactions of other parts of the 5CB molecules with the MWCNTs and lead to a complete model of the 5CB/MWCNT coupling in their nanocomposites.

ACKNOWLEDGMENTS

For support of this research, the authors thank: Assumption College for a Faculty Development Grant; The Natural Science Department at Assumption College for the summer student stipends; The Petroleum Research Fund, 44149-AC7; The National Science Foundation, Polymers Program of the Division of Materials Research, DMR-0602473, and MRI Program under DMR-0520655 for thermal analysis instrumentation.

REFERENCES

1. G. Scalia, J. P. F. Lagerwall, M. Haluska, U. Dettlaff-Weglikowska, F. Giesselmann and S. Roth, Phys. Status Solidi B, 243(13), 3238–3241 (2006).

2. W. Gwizdała, K. Górny, Z. Gburski, Journal of Molecular Structure 887 148–151 (2008). 3. I. Dierking, G. Scalia and P. Morales, J. Appl. Phys. 92, 8 (2004). 4. G. Georgiev, E. A. Gombos, M. McIntyre, M. Mattera, P. Gati, Y. Cabrera and P. Cebe, in

Nanoscale Pattern Formation, edited by E. Chason, R. Cuerno, J. Gray, K.-H. Heinig, (Mater. Res. Soc. Symp. Proc. 1228E, Warrendale, PA, 2010), KK11-81.

5. G. Georgiev, M. McIntyre, E. A. Gombos, and P. Cebe, submitted to Liquid Crystal

Materials - Beyond Displays (Mater. Res. Soc. Symp. Proc. Fall 2010 meeting in Boston, MA), ID: 900325. L 9.6.

6. A. Eletskii, Physics - Uspekhi 50(3) 225-261 (2007). 7. J. Lagerwall, and G. Scalia, Journal of Material Chemistry 18, 2890-2898 (2008). 8. M. Rahman, W. Lee, J. Phys. Appl. Phys. 42, 063001 (2009). 9. R. Basu, K. Sigdel and G. Iannacchione, arXiv:0905.2779 (2009). 10. G. Georgiev, Y. Cabrera, L. Wielgus, Z. Iftikhar, M. Mattera, P. Gati, A. Potter and P.

Cebe, in Artificially Induced Grain Alignment in Thin Films, edited by V. Matias, R. Hammond, S.-H. Moon, R. Hühne (Mater. Res. Soc. Symp. Proc. 1150, Warrendale, PA, 2009), RR04-16, p. 185-190.

11. L. J. Bellamy, The Infrared Spectra of Complex Molecules, 3rd ed., (Springer, 1975). 12. G. Georgiev, R. Judith, M. McIntyre, E. A. Gombos, and P. Cebe, submitted to Symposium

DD: Artificially Induced Crystalline Alignment in Thin Films and Nanostructures (Mater. Res. Soc. Symp. Proc. Fall 2010 meeting in Boston, MA), ID: 913554. DD 7.9.

13. A. Dawid, W. Gwizdała, Journal of Non-Crystalline Solids 355 1302–1306 (2009).