Electric-Field-Induced Phase Change in Cholesteric Liquid Crystals

Abstract

assumption that the "excited" beam consisted entirely of excited molecules. While it was argued that, on the basis of the almost resonant charge transfer, the excited N 2 A S Z should be highly preferred, the possibility that some groundstate N 2 molecules are produced cannot be eliminated. It should be noted, however, that corrections for this effect would make the excitedstate ionization-cross-section and emission coefficient even larger. These arguments contain the implicit assumption that the N 2 + ions were in the ground state. In order to verify that excited N 2 + ions did not dominate the results, the ionizing electron energy was varied from 18 to 24 eV in the charge-transfer cross-section measurement (35-eV ion beam) and from 19.5 to 24 eV in Several workers 1 " 3 have reported the influence of electric fields upon the scattering of light by cholesteric liquid crystals. In this Letter, we wish to report a phase transition which was observed in cholesteric liquid crystals subjected to high dc electric fields. To our knowledge, a change of phase in these materials due to electric fields has not been previously identified.Results were obtained as follows. A cholesteric liquid crystal was put between, and in contact with, tin-oxide-coated glass plates separated by 0.2 mm. The liquid-crystal materials were mixtures of cholesteryl chloride, nonanoate, and oleyl carbonate with typical proportions of 30, 56, and 14% by weight, respectively. 4 The normal appearance of the layer was that of a milky white, opalescent sheet from which light was diffusely scattered. When plane-polarized light was normally incident on the layer, it was impossible to extinguish the field of view with a linear analyzer. When an average electric field of «10 5 V/cm was applied, the liquid crystal assumed a clear, colorless appearance. Normally incident, plane-polarized light now emerged plane-polarized and thus could be extinguished. However, the material was not optically isotropic. Plane-the ionization cross-section measurement (35-eV neutral excited beam). No change outside ±5 and ±10%, respectively, could be detected up to 22-eV electron energy. Above 22 eV, however, both cross sections increased markedly. This is interpreted as being due to the production of long-lived N 2 + excited ions, which transfer charge with NO to produce highly excited metastable N 2 molecules. 3 N. G. Utterback and G. H. Miller, Phys. Rev. 124, 1477 (1961), and 129, 219 (1963. 4 N. G. Utterback and T. Griffith, Jr., Rev. Sci. Instr. 37, 866 (1966). 5 This result has immediate practical importance in calculation of bulk ionization rates through integration of cross sections over velocity distributions. It has been shown (C. F. Hansen, private communication) that if this result holds generally, bulk ionization rates can be dominated by collision between excited particles even though relatively few of these are present. It would then be essential to know the cross sections for collisions between all the excited-state possibilities in order to calculate reliab...

Synthesis and properties of mono-substituted liquid crystalline polyacetylene derivatives—doping, magnetic orientation, and photo-isomerization

“…Color upon insertion of a 1/4 wavelength plate, black second and fourth quadrants, and blue first and third quadrants; yellow second and fourth quadrants upon insertion of a gypsum first-order red plate demonstrate that the polymer is an optically positive uniaxial material (see supplementary data) [6]. The symmetry of the Maltese cross indicates that the mesogenic side-chains have no tilt-angle with respect to the glass substrate.…”

Applications of Smectic and Cholesteric Liquid Crystals

“…Polarizers are not required if a long pitch cholesteric is used with' homeotropic surface alignment and a pleochroic dye is added [10]. Examination of (3) shows that the layer is spontaneously homeotropic with no applied field if 2k22 Po KJ3 < a (4) and that it transforms canonically to a cholesteric above a threshold field given by (3) provided Ell < E1. The inclusion of a pleochroic dye is not a trivial exercise in this di'splay since many nematics with Ell < 1>1 seem to significantly reduce the optical order parameter of the dye.…”

“…- With certain boundary conditions the off (E=O) state is a scattering focal conic texture, and the transition between this and the optically clear pseudonematic on (E>Ec) state forms the basis of a display device [4]. A significant improvement in the optical performance of this display is achieved by changing this scattering effect into one showing absorption of light by the addition of a pleochroic dye [5].…”

The Effects of Magnetic Fields on the Structure of Cholesteric Liquid Crystals of Polypeptides

“…For this field, the pitch becomes infinite, as has been experimentally ascertained on amyloxy-azoxybenzene 22 and a mixture of the derivatives of cholesterol. 23 When the polymer concentration is increased, k 22/(xp-Xt) decreases and Fe increases, probably due to increasing stability of the cholesteric structure. This represses the possible brownian movement of the polymer molecules and would increase an apparent magnetic anisotropy and make the pitch smaller as well.…”