Cholesteric-nematic Phase Research Articles

A Boundary Induced Cholesteric-Nematic Phase Transition

Abstract Long pitch cholesteric liquid crystals were prepared in thin samples with homeotropic boundary conditions. When the sample thickness was less than the cholesteric pitch, the cholesteric twist was destroyed, and a homeotropically aligned nematic resulted.

New optical storage mode in liquid crystals

A novel electrically reversible storage effect in doped nematic liquid crystals is described. The ``off state'' is homeotropic nematic and the ``on state'' a single layer of densely packed spherulites. Both states are stable in time. The effect was observed in a narrow range of pitch and cell thickness combinations, and is interpreted as a nematic-cholesteric phase transition.

The pitch-independent critical field in cholesteric liquid crystalline poly-γ-ethyl-L-glutamate

The critical magnetic field for the cholesteric-nematic phase transition of the lyotropic liquid crystal poly- gamma -ethyl-L-glutamate in ethyl acetate has been determined by the use of light diffraction patterns. In spite of the variation of the zero-field helix pitch with concentration, the critical field is nearly constant at about 29 kOe.

Optical properties of the Magnetotropic Nematic Phase of Poly-γ-Benzyl-ζ-Glutamate

Abstract Above a critical concentration in certain solvents poly-γ-benzyl-L-glutamate (PBLG) forms a lyotropic liquid crystal of the cholesteric structure. In the presence of sufficiently strong electric or magnetic fields this liquid crystal undergoes a mesomorphic phase transition to nematic order. Reported are the results of our long term observations of bulk samples of liquid crystalline PBLG subjected to strong magnetic fields. We have observed visually magnetohydrodynamic processes that occur on several very different time scales: (a) the slow cholesteric-nematic phase transition, (b) a more rapid reorientation in the acquired nematic condition, and (c) a critical orientation of nematic PBLG with unusual anisotropic optical effects and high, but directional, thermal sensitivity. The phenomena are accounted for in terms of molecular order and dynamics of the associated polypeptide helices.

Cholesteric twist in a model system

A highly idealized model is studied to gain some understanding of the structure of cholesteric liquid crystals. Asymmetric molecules, characterized by two perpendicular axes, the ``long'' and the ``short'' molecular axes, are considered; the centers of mass of the molecules lie on a rectangular lattice. An effective interaction that takes account of left-right asymmetry is assumed between nearest neighbors; also, an external field, correcting for a deficiency of the simple interaction, is chosen so that in the absence of a cholesteric twist the system is purely ``nematic.'' The results of the model (treated in mean field approximation) are (i) there is no cholesteric-nematic phase transition, an experimentally well-established fact; (ii) S∼ γ θ where S is a measure of asymmetry in the orientations of the ``short'' molecular axes, θ is the rotation (in radians) of the cholesteric helix from one layer of molecules to the next, and γ =O(10−2); (iii) the pitch of the helix is insensitive to temperature changes. An argument showing the reason for result (iii) in the present model is given.

Liquid Crystal Matrix Display

It is shown that the electric-field-induced cholesteric-nematic phase transition of mixed liquid crystal possesses two important properties; (i) a sharp threshold in the scattering-vs.-voltage characteristic and (ii) an electrically controllable relaxation time. The mixed liquid crystal is composed of p-n-butoxybenzylidene-p'-cyanoaniline, p-n-octoxybenzylidene-p'-cyanoaniline, p-methoxybenzylidene-p'-n-butylaniline, and cholesteryl oleyl carbonate. A matrix panel with 3,500 display elements is constructed by using the mixed liquid crystal. When the matrix panel is scanned by a line-at-a-time addressing, the contrast of displayed characters is about 15: 1.

AC Electric-Field-Induced Cholesteric-Nematic Phase Transition in Mixed Liquid Crystal Films

The electric-field-induced cholesteric-nematic phase transition is investigated in mixed liquid crystals consisting of p-n-butoxybenzylidene-p'-cyanoaniline, p-methylbenzylidene-p'-n-butyl-aniline, and cholesteryl oleyl carbonate. The relaxation transient of mixed liquid crystal films from a transparent nematic state back to a cloudy cholesteric state is studied by measuring the transmission of plane-polarized monochromatic light through the films. Optical rotation occurs in the early stage of the relaxation, followed by the formation of small birefringent regions randomly oriented with respect to each other. It is found that the optical rotation is due to the inverse process of cholesteric-to-nematic transition induced by the electric field parallel to a helical axis. An ac bias field above a certain value has an effect of prolonging the relaxation time. An interpretation of the results is presented.

Cholesteric-Nematic Phase Transitions in Mixtures of Cholesteryl Chloride and Nematic Liquid Crystals

Cholesteric-Nematic Phase Transitions in Mixtures of Cholesteryl Chloride and Nematic Liquid Crystals

Cholesteric-Nematic Phase Transitions in Compensated Liquid Crystals

Cholesteric-Nematic Phase Transitions in Compensated Liquid Crystals

Electric-Field-Induced Color Changes and Pitch DILATION in Cholesteric Liquid Crystals

A new electric-field-induced color range, in which colors change from blue to red with increasing field, has been observed in cholesteric liquid crystals. Observations of this new color range quantitatively confirm the theories of Meyer and de Gennes and enable identification of the specific nature of the electric-field-induced cholesteric-nematic phase transitions previously observed by other workers.

Alternating-Current-Field Induced Cholesteric–Nematic Phase Transitions

In order to determine whether interaction between the permanent or the induced molecular dipole moments of a cholesteric liquid crystal and an applied electric field is responsible for inducing a transition to a nematic phase, ac-field measurements were performed. In a mixture of cholesteryl chloride and cholesteryl myristate, phase transitions can be induced by an ac field of a frequency up to a threshold value fpt; fpt is approximately equal to the relaxation frequency for rotation of cholesteryl chloride molecules around a short molecular axis which has recently been determined by dielectric measurements. A relation between the rms field and fpt is derived and experimentally verified. The absence of ferroelectric ordering in the induced and aligned nematic phase is demonstrated.

Electric-Field-Induced Cholesteric–Nematic Phase Change in Liquid Crystals

Electric-Field-Induced Cholesteric–Nematic Phase Change in Liquid Crystals

Relationship Between Electric Field Strength and Helix Pitch in Induced Cholesteric-Nematic Phase Transitions

A cholesteric-nematic phase transition can be induced by relatively weak electric fields, and the threshold field varies inversely with the pitch of the helix in corroboration of recent theoretical suggestions. The phase change is accompanied by a change in the activation energy for charge-carrier production and can easily be observed by both conductivity measurements and direct optical examination.