Cholesteric-nematic Phase 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.

Tuning Chiral Nematic Pitch of Bioresourced Photonic Films via Coupling Organic Acid Hydrolysis

AbstractControlling the iridescent photonic film generated by self‐assembly of colorless cellulose nanocrystal (CNC) from the nanoscale to macroscale is challenging. This study combines experimental and computational approaches to systematically investigate the correlation between electrostatic interactions and chiral nematic structures. The chiral nematic order of the CNC colloidal is preserved in solid films after the evaporation of water. The cross‐sections of the iridescent film show a clear left‐handed helical arrangement of nanocrystals. The helical structure with the aid of acrylic acid exhibits longer organized patterns. This work reveals that compared to CNC prepared by pure sulfuric acid (zeta potential −37.1 mV), the CNC prepared using coupled mineral sulfuric and organic acrylic acid has a higher zeta potential (−67.2 mV), which induces the increase of helical pitch of the cholesteric nematic phase from 312 to 409 nm and a red shift of the iridescent film. Consequently, the tuning of reflected light wavelength lies in the variation of chiral nematic pitch inside the layered structure, which gives rise to different iridescent colors. The bioresourced photonic film is appealing to both academia and industry where optical and photonic components are essential.

Phase Separations in Mixtures of a Flexible Polymer and a Cholesteric Liquid Crystal in the Presence of an External Field

We present a mean field theory to describe phase behaviors in mixtures of a flexible polymer and a cholesteric liquid crystal under an external magnetic or electric field. Taking into account a chiral coupling between liquid crystals under the external field, we examine twist-untwist phase transitions and phase separations in the mixtures. Depending on the strength of the external field, we predict cholesteric-paranematic (Ch + N), nematic-paranematic (N + pN), cholesteric-nematic (Ch + N) phase separations on the temperature-concentration plane. We also calculate spinodal regions, which are important in phase separation kinetics inside binodal lines.

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.

Dielectric relaxation modes in cholesteryl oleyl carbonate observed by non-linear dielectric spectroscopy

The non-linear dielectric spectra of chiral crystal cholesteryl oleyl carbonate (COC) were recorded in the frequency range of 1–100 MHz in the presence of strong static electric field V/m. The measurements were performed in the cholesteric nematic phase N*. The results revealed two regions in the temperature dependence of the chemical relaxation time behaviour, corresponding to two-phase coexistence region (blue phase + N*) and homogenous cholesteric phase N*, respectively. In the non-linear dielectric relaxation spectra, three absorption bands, related to the different molecular rotations of individual molecules around axes and director, have been observed. The negative components of the frequency dependence of the non-linear dielectric phenomena concern the Langevin saturation of the dipoles orientation in strong electric field.

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.

Phase Effects on the Polymerization of a Styrloxy Cholesteric Liquid Crystalline Monomer

A styryloxy liquid crystalline monomer was synthesized and photopolymerized via free radical and cationic initiators with continuous and pulsed light sources. A phase description of monomer/polymer mixtures was used to help assist in the interpretation of the polymerization data. The monomer (a cholesteryl ester) exhibits isotropic, cholesteric nematic, and crystalline phases. For conversions of up to about 20% polymer, the polymerization media appear to exhibit the same cholesteric phase as the monomer, allowing polymerization rates to be measured in a purely cholesteric nematic phase. Free-radical polymerization exotherms show rate accelerations at conversions corresponding to compositions where changes are observed in the medium opacity. No accelerations are observed for cationic polymerization: Differences are attributed to the types of termination processes in free-radical and cationic polymerization. While polymerization rate changes differ for the free-radical and cationic polymerizations, real-time laser light scattering indicates that polymerization induced phase changes occur at the same degree of conversion in both cases. Rates for free-radical polymerization were also measured as a function of temperature. Arrhenius plots indicate that the polymerization activation energy at low conversions is the same for polymerization in the cholesteric nematic and isotropic phases.

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...

Induction of nematic cholesteric (N∗) phases by charge transfer complexation of disc-shaped multiynes [1

Abstract Based on charge transfer interactions with the (chiral) electron acceptor (-)-2-(2,4,5,7-tetranitro-9-fluorenylidenaminooxy)propionic acid ((-)-TAPA), our easily accessible disc-shaped electron rich multiynes (for example, 1 and 2) give rise to two types of cholesteric nematic phases. Thus, the binary mixture of the nematic discotic (ND) pentakis-(4-methylphenylethynyl)phenyl hexadecyl ether (2) with (-)-TAPA exhibits its twisted variant (N∗D). Likewise, but now in a ternary mixture, the homologue 1—peripherically unsubstituted and non-mesogenic—with the two electron acceptors (-)- TAPA and 2,4,7-trinitrofluorenone—both also non-liquid crystalline—shows cholesteric nematic properties, most probably however of a columnar type (N∗C). First results concerning these charge transfer induced cholesteric-nematic properties, including phase diagrams and the helical twist of the two systems presented here, are discussed on the basis of data obtained by polarizing microscopy and differential scanning calor...

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.