Cholesteric-nematic Phase Transition Research Articles

Structural transition in a ferroelectric nano-dispersed cholesteric liquid crystal

ABSTRACT We present a theoretical study of the orientational and electrical behaviors of a cholesteric liquid crystal dispersed with ferroelectric nanoparticles. We assume a soft planar coupling between the liquid crystal molecules and the nanoparticles. We consider two spiral structures in the ferroelectric nano-dispersed system under an external electric field. This assumption is due to the fact that the director and the polarization vector would have different pitches of spiral structure. We study the behavior of the average polarization and the pitch of the helical structure as a function of the field strength. The impact of ferroelectric nanoparticles on cholesteric-nematic phase transition is investigated by calculating the critical electric field. The influence of field strength and material parameters on the phase transition is also discussed. The calculations are based on a developed continuum theory and a modified form of free energy for the helical supramolecular structure. The influence of nanoparticle volume fraction on the helix unwinding of both spiral structures is studied. It is found that an electric field with a sufficiently high strength causes an increase in the pitch of the helical structure of polarization. We obtain a critical volume fraction of nanoparticles, after which the pitch of the polarization helical structure differs from the director pitch.

Visible‐Light‐Driven Halogen Bond Donor Based Molecular Switches: From Reversible Unwinding to Handedness Inversion in Self‐Organized Soft Helical Superstructures

AbstractVisible‐light‐driven molecular switches endowing reversible modulation of the functionalities of self‐organized soft materials are currently highly sought after for fundamental scientific studies and technological applications. Reported herein are the design and synthesis of two novel halogen bond donor based chiral molecular switches that exhibit reversible photoisomerization upon exposure to visible light of different wavelengths. These chiral molecular switches induce photoresponsive helical superstructures, that is, cholesteric liquid crystals, when doped into the commercially available room‐temperature achiral liquid crystal host 5CB, which also acts as a halogen‐bond acceptor. The induced helical superstructure containing the molecular switch with terminal iodo atoms exhibits visible‐light‐driven reversible unwinding, that is, a cholesteric–nematic phase transition. Interestingly, the molecular switch with terminal bromo atoms confers reversible handedness inversion to the helical superstructure upon irradiation with visible light of different wavelengths. This visible‐light‐driven, reversible handedness inversion, enabled by a halogen bond donor molecular switch, is unprecedented.

Visible-Light-Driven Halogen Bond Donor Based Molecular Switches: From Reversible Unwinding to Handedness Inversion in Self-Organized Soft Helical Superstructures.

Visible-light-driven molecular switches endowing reversible modulation of the functionalities of self-organized soft materials are currently highly sought after for fundamental scientific studies and technological applications. Reported herein are the design and synthesis of two novel halogen bond donor based chiral molecular switches that exhibit reversible photoisomerization upon exposure to visible light of different wavelengths. These chiral molecular switches induce photoresponsive helical superstructures, that is, cholesteric liquid crystals, when doped into the commercially available room-temperature achiral liquid crystal host 5CB, which also acts as a halogen-bond acceptor. The induced helical superstructure containing the molecular switch with terminal iodo atoms exhibits visible-light-driven reversible unwinding, that is, a cholesteric-nematic phase transition. Interestingly, the molecular switch with terminal bromo atoms confers reversible handedness inversion to the helical superstructure upon irradiation with visible light of different wavelengths. This visible-light-driven, reversible handedness inversion, enabled by a halogen bond donor molecular switch, is unprecedented.

The effect of a modified surface anchoring potential on the cholesteric–nematic transition

As it is known, the Rapini-Papoular potential is suitable for description of anchoring energy between the liquid crystal and bounding surfaces in the case of small deviations of the director from the easy orientation axis. We use modified surface potential that can describe the anchoring energy at large deviations of the director more adequately. The interaction effect of a cholesteric liquid crystal with a bounding surfaces on the nature and features of cholesteric-nematic phase transitions induced by an external magnetic field is studied. We determine the conditions for existence of three different phases in a cholesteric liquid crystal layer with a finite homeotropic anchoring at the boundaries. The dependencies of the threshold fields of orientational transitions between the phases of a liquid crystal on the material parameters of the cell are calculated. Landau expansions of the total free energy of cholesteric near the transition points are constructed. We determine the tricritical values of the anisotropy parameter of the surface anchoring, which characterize the nature of transitions between the phases of a cholesteric liquid crystal.

Phase diagrams of mixtures of a polymer and a cholesteric liquid crystal under an external field.

We present a mean field theory to describe phase behaviors in mixtures of a polymer and a cholesteric liquid crystal under an external magnetic or electric field. Taking into account a chiral coupling between a polymer and a liquid crystal under the external field, we examine twist-untwist phase transitions and phase separations in the mixtures. It is found that a cholesteric-nematic phase transition can be induced by not only the external field but also concentration and temperature. Depending on the strength of the external field, we predict cholesteric-paranematic (Ch+pN), nematic-paranematic (N+pN), cholesteric-nematic (Ch+N) phase separations, etc., on the temperature-concentration plane. We also discuss mixtures of a non-chiral nematic liquid crystal and a chiral dopant.

Phase-field model for front propagation in a temperature gradient: selection and competition between the correlation and the thermal lengths.

A phase-field model is presented to study the propagation and the selection of a front in directional growth. The phase transition can be first or second order and is described by a nonconserved order parameter. In general, the thermal length l(u) (inversely proportional to the temperature gradient) is much larger than the correlation length l(phi), which gives the width of the front, and there is no direct competition between them (epsilon =l(phi)/l(u)<<1). In this paper, we consider a situation where these two lengths can be of the same order of magnitude (epsilon =l(phi)/l(u) close to 1). This happens in liquid crystals at the nematic-cholesteric phase transition. The problem of the front selection is solved theoretically by first performing an asymptotic analysis of the governing equations in the limit epsilon-->0, and then by solving the equations numerically. The main result is that the front is selected in a single way (no continuum of solutions) as long as epsilon not equal 0, whatever the velocity and the order of the phase transition. Finally, we show that the order parameter profile and the front temperature can change significantly when epsilon approaches 1.

Effect of Nematic Liquid Crystals on Hysteresis Width Using Nematic-Cholesteric Phase Transition Mode

The hysteresis width of nematic liquid crystals (NLCs) on a polyimide (PI) surfaces without side chains in nematic-cholesteric phase transition (NCPT) mode was investigated. The generated pretilt angle of ZLI-1132 was almost the same as that of 4-n-pentyl-4′-cyanobiphenyl (5CB) on a PI surface with both sides rubbed. The phase transition time of ZLI-1132 using NCPT mode was similar to that of 5CB on PI surfaces without rubbing-treatment. However, the hysteresis width increased with relation concentration of ZLI-1132 in a ZLI-1132-5CB mixture. The hysteresis width of NCPT mode was not dependent on pretilt angle. Consequently, the large hysteresis width of NCPT mode is attributed to the high elastic constants of the NLCs.

Topological Model for Dipole–Quadrupole Polarizability and Its Application to Analysis of Discriminating Intermolecular Interactions

A simplified model for calculating the dipole and dipole–quadrupole polarizabilities Ai,jk is suggested. The model is based on the mean energy method and on the topological approximation to the bond order matrix. Pseudoinvariants of the Ai,jk tensor are calculated to evaluate the degree of molecular dissymmetry, which is responsible for the discriminating intermolecular forces. The method is illustrated by calculations for a number of chiral molecules and comparison of the calculated indices with the experimentally evaluated twisting ability of molecules during the nematic–cholesteric phase transition.

Phase Transitions in a Cholesteric Close to a Disordering Surface

The thermodynamic behaviour of a semi-infinite cholesteric sample close to a disordering surface is analyzed within the frame of the Landau-Ginzburg-de Gennes theory. In particular, the effect of the boundary on the field-induced cholesteric-nematic phase transition is considered. The appearance of a nematic layer on the top of the cholesteric bulk phase is predicted and an experiment which could possibly demonstrate this phenomenon is proposed.

The field-induced cholesteric-nematic phase transition and its dependence on layer thickness, boundary conditions, and temperature

Chiral-nematic liquid-crystal layers which have a short pitch, homeotropic boundary conditions, and bistable properties are investigated with respect to the temperature dependence of their field-induced cholesteric-nematic phase transition. Hereto the threshold field needed for the transition is studied as a function of the layer thickness. This is done for various temperatures. The threshold field hardly depends on the layer thickness. Moreover, it sharply rises below −5 °C. Both the value of the threshold field and its temperature dependence can be fully described in terms of liquid-crystal bulk material parameters, the conditions at the homeotropic boundary layer used do not play a role. From the threshold field data the twist elastic constant is determined as a function of temperature. This temperature dependence is compared to the temperature dependence as determined from the square of the long range order parameter calculated from temperature dependent refractive index data. A good correlation is found between both data sets of the twist elastic constant.

Driving voltage of reflective cholesteric liquid crystal displays

The reflective cholesteric texture mode between planar and homeotropic textures, which can achieve a high contrast ratio, needs to be coupled with an active matrix drive scheme such as a method using thin film transistors. In this article, the threshold voltage of the cholesteric–nematic phase transition, an important characteristic related to the requirements of active matrix drive schemes, is discussed. We investigated the role of the cell gap in determining the reflectance, and studied the effects of varying the nematic liquid crystal and chiral component of the cholesteric mixture. The cell gap needs to be optimized for lower threshold voltage and higher reflectance. The chiral component and the nematic liquid crystal both have a strong effect on the threshold voltage. On the basis of these results, novel chiral materials are proposed. This study demonstrates that some of these materials successfully contribute to the reduction of the threshold voltage.

Liquid Crystal Modulator of Laser Radiation for High-Power Lasers of Infra-Red Range of Spectrum

The possibility of high-power IR-radiation modulation by cholesteric-nematic phase transition in LC is discussed.

Surprising Dynamics of Some Cholesteric Liquid Crystal Patterns

Under homeotropic anchoring conditions, the first order nematic-cholesteric phase transition gives rise to localized spatial patterns called cholesteric fingers. In this Letter we describe a new equilibrium structure which, to our knowledge, has never been reported either numerically or analytically, and we investigate its dynamics. This chiral pattern, which has broken the parity symmetry, is found to move when submitted to an ac electric voltage or to a white noise. Our result may explain, at least qualitatively, the recent observations of spontaneous spiral wave formations in cholesteric liquid crystal under the application of an ac electric voltage.

Hysteresis behaviour of the nematic-cholesteric phase transition for liquid crystals on polyimide films without use of the rubbing technique

The hysteresis behaviour of the nematic-cholesteric phase transition in liquid crystals is closely connected with the alignment of the liquid crystal molecules on the surfaces of treated solid subs...

Surface Effect on the Threshold Electric Fields of Cholesteric-Nematic Phase Transition and Its Reverse Process

Abstract Electric-field-induced cholesteric-nematic (Ch-N) phase transition and its reverse process (N-Ch) are discussed by taking account of the surface interaction effect. Threshold electric fields of a cholesteric to nematic and a nematic to cholesteric phase transition are investigated in conjunction with a liquid crystal layer thickness and surface anchoring strength. By performing a fitting of the theoretical curves to the experimental results of the threshold electric field strengths, we concluded that in a homcotropically aligned thin cell whose liquid crystal layer thickness is below 10 μm, the surface anchoring energy, A 0, contributes significantly to reduce the threshold electric fields both for the Ch-N phase transition and that of N-Ch.

ネマティック‐コレステリック相転移液晶を用いた１００万画素フルカラー投影装置

Full-color projection display with one million pixels has been developed and the optical system has been simplified by using conventional OHPs. This display uses a nematic-cholesteric phase-transition (NCPT) liquid crystal panel with a micro-color filter placed outside the panel This simple configuration provides a color OHP display with high performance and low cost One picture element of this color-projection display consists of 9 pixels in the NCPT black and white panel. Thus gray scale is also obtained by dithering using the 9 pixels, resulting in a full-color projection display The contrast ratio of 5 : 1 and the line-addressing time of 2.2 msec were achieved

Photoconductive Polymers in Liquid Crystal Spatial Light Modulators with High Resolution

Abstract Various types of organic polymer photoconductors have been successfully used in optically addressed liquid crystal (LC) light valves based on the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering, and polymer dispersed LC. Record regime optimization in projection or holographic manner is necessary to obtain the best device characteristics. The highest parameters obtained in the pulse holographic regime were diffraction efficiency of 36%, limiting resolutions of 1500 mm−1, and spectral sensitivity of 10−8 J/cm2 %. Photoconductive polymers themselves may be used for LC alignment simplifying the light valve construction. High aperture devices were realized side by side with the small ones. Polymer dispersed LC's connected with the photoconductive polymers allow for the realization of entirely solid state valves. High resolution, mechanical and elecrical stability, and the possibility for sensitization make the use of the polymer photoco...

Electric-field-induced phase transitions in frustrated cholesteric liquid crystals of negative dielectric anisotropy.

The phase diagram of a large-pitch cholesteric liquid crystal sandwiched between two parallel glass plates with homeotropic anchoring is determined both experimentally and theoretically. Contrary to previous experiments [P. Ribi\`ere and P. Oswald, J. Phys. (Paris) 51, 1703 (1990)], the materials chosen have a negative dielectric anisotropy so that experiments are performed at smaller thickness than the critical value ${\mathit{d}}_{\mathit{c}}$ that is necessary to unwind the helical structure completely at vanishing electric field. The two control parameters are the frustration ratio d/p of the thickness over the quiescent pitch of the cholesteric liquid crystal and the applied voltage V. The main difference with previous experiments is that the cholesteric-nematic phase transition can be second order or first order depending on the sample thickness. There is thus a Landau tricritical point in the phase diagram. Furthermore, we have found a triple point where the nematic phase coexists with both a translationally invariant configuration and a periodic structure. A theoretical model, including two order parameters, allows us to calculate the coordinates of these two particular points as a function of the Frank elastic constants as well as the general aspect of the phase diagram. The agreement between theory and experiment is satisfactory.

Field hysteresis of the cholesteric-nematic phase transition

Abstract Field hysteresis of the cholesteric-nematic phase transition in mixtures with a small (up to 2%) amount of optically active dopant was investigated. The relative value of the hysteresis loop was used to evaluate the transition hysteresis properties. It is shown that this value does not depend on the ratio of the cell thickness to the value of the pitch of the undeformed helix and is linearly dependent on the elastic constant ratio (K22/K33).

Phase Transitions and Solitons in Liquid Crystals

Abstract Phase transitions occurring in liquid crystals are reviewed from a viewpoint of solitons. First, cholesteric-nematic phase transition is explained as a condensation process of solitons. This mechanism of phase transition is common to the so-called commensurate-incommensurate phase transition. In the next place, not only liquid crystals such as chiral smectic C and smectic A phases but also ferroelectrics and some other systems belonging to this category and its generalized versions arc introduced, and discussed especially in relation to an order of phase transition, where a symmetry of periodic phases on an order parameter space gives the basis of the consideration.