Interactions Between Carbon Nanotubes and Liquid Crystals in Binary Nematic Electro-Optic Cells
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Interactions Between Carbon Nanotubes and Liquid Crystals in Binary Nematic ElectroOptic Cells Georgi Y. Georgiev1,2, Erin A. Gombos1, Michael B. McIntyre1, Michael F. Mattera1, Peter A. Gati1, Yaniel Cabrera2 and Peggy Cebe2 1 Department of Natural Sciences, Assumption College, Worcester, MA 01609 2 Department of Physics and Astronomy, Tufts University, Medford, MA 02155 ABSTRACT We studied the effects of multiwalled carbon nanotubes (MWCNTs) on the Freedericksz transition of a liquid crystal (LC) and calibrated the altitudinal angle of CNTs as a function of the electric field. In addition, we directed the azimuthal angle which gave us complete control of the 3D orientation of the CNTs. We constructed anti-parallel electro-optic cells using a nanocomposite at a concentration of 0.01% CNTs with 4-Cyano-4'-pentylbiphenyl (5CB) liquid crystal. This low concentration was necessary to achieve maximum transmission of electromagnetic radiation through the cell and to minimize the Van der Waals attraction between the CNTs responsible for their aggregation. We chose two dimensional microscopic transmission ellipsometry (2D-MTE) to measure the phaseshift of the polarized electromagnetic radiation through the cell and to derive from it the altitudinal angle of the CNTs. Our results show that in the presence of CNTs the Freedericksz transition occurs at 55% of the transitional electric field as compared to the control electro-optic cell without CNTs. The width of the Freedericksz transition narrows by a similar factor. The switching time of the cell decreased in the presence CNTs by 18.5%. INTRODUCTION It has been shown that Carbon Nanotubes self organize as a liquid crystal in their lyotropic phase [1]. Their nematic director couples to that of the nematic liquid crystals when dispersed together [2-7]. Applying external electric or magnetic fields leads to Freedericksz transition in the nanocomposite as in the pure liquid crystal. The Freedericksz transition has been described as a symmetry breaking bifurcation problem [8]. The occurrence of this transition for LC/CNT nanocomposite allows control of the altitudinal angle in pre-oriented electro-optic cells. The azimuthal angle can also be directed using surface effects which in combination with the control on the altitudinal angle is currently one of the few efficient ways of positioning large quantities of nanotubes in a needed 3D orientation. At low concentrations of CNTs, the LC/CNT dispersion is transparent for electromagnetic radiation, which makes it suitable for Two Dimensional Microscopic Transmission Ellipsometry (2D-MTE). In our experiments the total transmission of light and the phaseshift were not affected measurably by the presence of the nanotubes. Because CNTs have mobility in the nanocomposite, they tend to aggregate in clusters due to strong Van der Waals interactions between the nanotubes. Aggregation is thus also minimized at low concentrations of CNTs. Our method of choice for 3D orientation of the CNTs is to preset the azimuthal angle by sur
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