Nematic-isotropic Transition Research Articles

Hydrogen bonding and the design of twist-bend nematogens

The phase properties of equimolar mixtures consisting of a hydrogen bond donor, a 4-alkoxybenzoic acid (nOBA), and one of two different stilbazole-based hydrogen bond acceptors, either 4-[(E)-2-(4-{[6-(4′-methoxy[1,1′-biphenyl]-4-yl)hexyl]oxy}phenyl)-ethenyl]-pyridine (1OB6OS) or 4-[(E)-4′-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)-[1,1′-biphenyl]-4-carbonitrile (CB6OS) are reported. Neither hydrogen bond acceptor exhibits liquid crystal behaviour whereas the nOBA compounds show smectic and/or nematic behaviour depending on the length of the alkyloxy chain. For the complexes of an nOBA with n = 1–5, both conventional nematic and twist-bend nematic phases were observed, while for n ≥ 6 smectic phases emerged and the twist-bend nematic phase was extinguished. The CB6OS-nOBA mixtures may exhibit the heliconical smectic CTB phase. The local molecular arrangement in the two sets of mixtures are similar and changes on increasing n but this is not reflected in the nematic-isotropic transition temperatures. Birefringence studies of the mixtures are reported. In general the behaviour of the hydrogen-bonded mixtures is similar to that of their covalently bonded counterparts.

Increased Nematic-Isotropic Transition Temperature on Doping a Liquid Crystal with Molecularly Rigid Carboxylic Acids.

A thermotropic nematic liquid crystal (LC) becomes an isotropic liquid at the nematic-isotropic transition temperature (TNI), which depends on the molecular order of the mesophase. By means of a polarized optical microscope and a differential scanning calorimeter, it was found that doping the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) with molecularly rigid carboxylic acids (benzoic, 1-naphthoic, 2-naphthoic, and biphenyl-4-carboxylic acids) increases TNI without modification of the nematic-isotropic transition enthalpy. This increment in TNI is due to the increased order caused by the formation of molecularly rigid and elongated dimers of carboxylic acids in the nematic LC, as confirmed with infrared spectra. Furthermore, TNI increased with the length of the molecularly rigid dimers at the same concentration level. Conversely, doping the LC with molecularly flexible acids caused lowering of TNI. A quantitative correlation was established between the TNI increase of the rigid carboxylic acids and the length of the dimers of these acids; a predictive model for these ΔTNI values as a function of acid molar fraction was developed. It was also demonstrated that the doping of 5CB with rigid carboxylic acids increases the rotational viscosity of the liquid crystal.

Enhancing the thermo-optical response of nematic liquid crystal with a polymer network

As nematic liquid crystals are heated, the change in the nematic order parameter affects the physical properties of the phase such as dielectric permittivity, indices of refraction, and elastic constants. This enables nematic liquid crystals to be used as temperature sensors. Of particular interest is their use as an uncooled thermal imager. In this paper, an optical sensor for this application is considered where the change in birefringence of the liquid crystal with temperature is probed by a visible wavelength. One problem with this type of sensor is the large change in birefringence as the liquid crystal that undergoes a first-order nematic-isotropic phase transition cannot be used. To solve this problem, confinement of the liquid crystal to obtain a second-order phase transition is considered. Confinement of the liquid crystal is accomplished with a polymer network. Through numerical Landau-de Gennes calculations and experimental measurements, it is demonstrated that the average domain size of an appropriate polymer network is ∼250 nm. This confinement results in a doubling of the thermo-optical response of the nematic liquid crystal. Additionally, it is demonstrated that the polymer network suppresses twist fluctuations of the nematic director, which halves the noise of the sensor. The results of this work allow for improved uncooled thermal imagers.

Atomistic Mechanism of Anisotropic Heat Conduction in the Liquid Crystal 4-Heptyl-4'-cyanobiphenyl: All-Atom Molecular Dynamics.

All-atom molecular dynamics simulations were performed on 4-heptyl-4'-cyanobiphenyl (7CB) to study the mechanism of heat conduction in this nematic liquid crystal atomistically. To describe 7CB properly, the AMBER-type force field was optimized for the dihedral parameter of biphenyl and the Lennard-Jones parameters. The molecular dynamics calculation using the optimized force field well reproduced the experimental values of the isotropic-nematic phase transition temperature, density, and anisotropy of the thermal conductivity. Furthermore, the contributions of convection, intramolecular interaction, and intermolecular interaction to the thermal conductivity were determined by performing thermal conductivity decomposition analysis. According to the analysis, the contributions of convection, bond stretching, and bond bending interactions were higher in the direction parallel to the nematic director than that perpendicular to the director, which is the origin of the anisotropy in the nematic phase. This result indicates that the anisotropy is caused by well-aligned covalent bonds and high mobility parallel to the director. This quantitative description of the mechanism of heat conduction of 7CB is foreseen to provide new insights toward designing highly thermally conductive liquid-crystalline materials.

Preparation of liquid crystal nanocapsules by polymerization of oil-in-water emulsion monomer droplets

Liquid crystal nanocapsules (LC-Nanocapsules) were prepared by miniemulsion polymerization of the oil-in-water emulsion monomer droplets dissolving the liquid crystal (LC) compounds. In order to establish the preparation conditions of LC-Nanocapsules exhibiting the liquid crystallinity, the effects of the capsule wall-forming monomers and the crosslinking agent concentration on the capsule structure were investigated in detail. The monodisperse colloidal products covered with the robust polymer shell wall was successfully prepared by the polymerization of the emulsion monomer droplets obtained through the phase inversion temperature emulsification technique using the amphiphilic block copolymer as an emulsifier. The endothermic peak was observed at the nematic-isotropic phase transition temperature (TNI) of the LC in the differential scanning calorimetry diagram of LC-Nanocapsules. The bright- and dark-field images of the dried thin films of LC-Nanocapsules spread on a glass substrate were found to appear repeatedly by the temperature change below and above TNI by polarized optical microscopic analysis. These results revealed that the LC-Nanocapsules with a complete engulfing morphology were successfully formed by the spontaneous coacervation phenomena between the crosslinked polymer and the LC with a progression of the polymerization, as theoretically predicted from the viewpoint of the spreading coefficients.

Inclusion of Mass Density Fluctuations in the Landau-de Gennes Theory of Nematic-Isotropic Phase Transition

Experimental evidences have shown that mass density gradients couple energetically to the liquid crystal director. In this paper, we incorporate this mechanism into the Landau-de Gennes theory of nematic-isotropic transition. In conformity with empirical evidence, the extended theory enables us to explain the enhancement to the amplitude of the singular part of the specific heat above the nematic-isotropic phase transition temperature TNI. It also permits us to obtain a correction term for the Kerr constant B in isotropic phase proportional to 1/(T − T∗)2, where T∗ is the virtual transition temperature. The curve B versus T thus corrected provides a very good fit to experimental data.

Phase Equilibria of Flexible Polymer and Monomer Liquid Crystal Blends

In this study, we explored the phase equilibria in blends of a nematic liquid crystal monomer and a nematic liquid crystal flexible polymer within the whole composition of the phase diagram. The crystal-nematic and nematic-isotropic transition temperatures, as well as the transition enthalpies and entropies, indicated the total miscibility in this mesogenic blend system. We also extracted an empirical axial ratio of 4.52 for the polymer from the nematic-isotropic transitions of the blends. Further, from the biphasic nematic-isotropic transitions of the blends in low concentration regions, we estimated the entropies of pure monomer and polymer from a theoretical lattice model. The calculated and experimental results showed good agreement. This work will be continued with a more detailed study of phase equilibria and comparison with other theoretical models in nematic flexible polymer and non-mesogenic rigid-rod monomer blends will be made.

Electro-optical, dielectric and optical properties of graphene oxide dispersed nematic liquid crystal composites

ABSTRACT Graphene oxide (GO) in various concentrations (0.04 wt.%, 0.1 wt.% and 0.3 wt.%) was dispersed into nematic liquid crystal (LC) 5CB and studied for electro-optical, dielectric and optical responses. The textural and phase transition studies were investigated by optical polarising microscopy in the nematic phase. The effect of GO concentration on the micro-texture, phase transition temperatures, birefringence, threshold voltage, contrast ratio, dielectric permittivity, dielectric loss and conductivity was also investigated. The nematic–isotropic transition temperature slight increases (~3%) in the 0.04 wt.% and 0.1 wt.% GO dispersed 5CB composites as compared to 5CB. Threshold voltage decreases in the GO–5CB composites with the increase of GO concentration. Additionally, the effect of GO concentration on the optical absorbance of UV–visible light and energy bandgap is also considered.

Influence of SiO2 nanoparticles on electro-optical and thermophysical properties of polyacrylate/liquid crystal composites

ABSTRACTIn this paper, SiO2 nanoparticle-doped polymer dispersed liquid crystal (PDLC) lenses are made from a mixture of prepolymer, 5CB liquid crystal and SiO2 nanoparticles by the polymerisation induced phase separation (PIPS) process. The effect of SiO2 nanoparticles on the electro-optical properties of PDLC films are studied. It is established that SiO2 nanoparticles affect the microstructure of PDLC films significantly because of the formed agglomerates of SiO2 nanoparticles. Results show an improvement in the electro-optical properties and a decrease in the response time for doped systems with small amount of SiO2 nanoparticles. We also observe a shift of nematic–isotropic transition temperature as a function of SiO2 nanoparticle contents. A good agreement between the electro-optical study and thermophysical properties is reached.

Dispersion of multiferroic BiFeO3 nanoparticles in nematic liquid crystals

Dispersion of ferroelectric- and ferromagnetic nanoparticles in liquid crystals (LCs) is investigated in the last decades. Recently, doping multiferroic-BiFeO3 nanoparticles in the LCs has become of interest. These nanoparticles have coupled ferroelectric- and ferromagnetic properties at ambient temperature. In the present experiments, a nematic LC was doped with a low concentration of BiFeO3 nanoparticles, synthesized using a modified Pechini method. The particles were characterized and their dispersions in heptane with an added surfactant were used for doping of the LC. The measured dielectric constants in the doped LCs varied as compared to the pure LC. It was shown that the presence of a magnetic field in the cooling process (cooling down from isotropic to anisotropic phase) of the LC cells leads to a significant increase of dielectric anisotropy. Also, the transition temperature decreased in the nematic-isotropic transition temperature for the doped sample.

Effect of superhydrophobic nanoplatelets on the phase behaviour of liquid crystals

We investigate the effect of superhydrophobic Perfluorinated Silica Nanoplatelets (PFSNP) on the Isotropic-Nematic-Smectic-A-Smectic-C phase transitions of the liquid crystalline compound 4-n-pentyloxyphenyl-4′-n-octyloxybenzoate by means of polarised optical microscopy, differential scanning calorimetry, x-ray scattering, and birefringence measurements. We measure the phase transition temperature shift, coexistence temperature span variation, and the orientational order parameter of the nanocomposites, as functions of the mass fraction of nanoplatelets. These variations are non-monotonic function of PFSNP mass fraction and they are definitely related among them with linear laws. We show that the non-monotony may be related with PFSNP dispersion quality, partial-aggregation phenomena of PFSNP, and/or to nanoplatelet organization. The isotropic-nematic transition becomes softer in presence of the nanoplatelets. Nematic and smectic-C phase temperature span is practically independent from the PFSNP volume fraction. In contrary, the smectic-A phase is further stabilized for its temperature window becomes much wider in the presence of the PFSNP. A phenomenological model accounts for the experimental observations.

Structures and phase transition in a two-dimensional system of Gay-Berne molecules

We have used Percus-Yevick liquid integral equation theory and classical density functional theory of freezing to study the structure and isotropic-nematic phase transitions in a two-dimensional system of soft ellipses interacting via full Gay-Berne interaction potential. The accuracy of the pair-correlation functions given by integral equation theory has been tested by those obtained by NVT Monte-Carlo simulation. The theory is found to overestimate the structure as we move to either higher density or to the case of a longer aspect ratio. Isotropic-nematic transition parameters have been calculated for three different systems of ellipses characterized by their major to minor axis ratio κ=3.0,3.5 and 4.0 at temperature T*=2.0. The same has been calculated at T*=1.0 for the system with κ=3.0. Nematic phase is found to stabilize for all the 2D systems of GB ellipses. Isotropic-nematic transition is found to be weakly first order.

Theory of self-assembly-driven nematic liquid crystals revised

ABSTRACTConcentrated solutions of short blunt-ended DNA duplexes at room temperature can form liquid crystal phases due to stacking interactions between duplex terminals which induce the aggregation of the duplexes into semi-flexible linear chains. Mesophases observed in these systems include nematic, columnar and cholesteric ones. This experimental system is just one of many examples, where liquid crystals ordering emerges as a result of molecular self-assembly into linear chains. In the attempt to go beyond a simple Onsager theoretical approach to understand the thermodynamic behavior of this system, we introduced some years ago a general theoretical description, which models the isotropic-nematic transition by properly taking into account molecular self-assembly, and we carefully verified the theoretical predictions against numerical simulations of patchy hard cylinders. Here, we provide a revised version of the theory in the attempt of understanding which assumptions are worth to be improved. In particular, we focus on the Parsons-Lee approximation and the modeling of orientational entropy. We compare the results from the revised version of the theory against original ones, showing that the present version of the theory is able to capture more accurately the phase boundaries of the isotropic-nematic transition.

Temperature dependence of voltage-controlled polarization plane rotation in a magnetic and electro-optic heterostructure

We propose a theoretical study of the influence of temperature on the state of polarization of a near-infrared light beam reflected from or transmitted through a voltage-controlled nematic liquid crystal (LC) cell deposited on a magnetic film. In both cases, the change in polarization with respect to that of the incoming beam originates in the linear Kerr or Faraday effects taking place in the magnetic layer, but the overall value of the polarization plane rotation depends on the reflectivity and transmittivity of the whole heterostructure. Temperature manifests itself in all layers of the structure through both thermal expansion and the thermo-optic effect when it varies between room temperature and the temperature of nematic-isotropic phase transition in the LC. The thermo-optic effect is strongest in the LC cell, where it is shown to induce noticeable variations of the polarization plane rotation of both reflected and transmitted light beams in that temperature range. Such temperature-dependent changes of the magneto-optical response can be compensated for by adjusting the dc voltage applied to the LC. On the other hand, we show that they may be exploited for sensitive temperature monitoring.

Thermoresponsive liquid crystalline polymer membranes that undergo phase transition at body temperature

Biological membranes regulate their properties and functions by drastic changes in their organized structure and mobility in response to changes in environmental factors such as pH, temperature, and biomolecule concentration. Liquid crystalline polymers (LCPs) are widely used as functional materials in a variety of fields because, similarly to biological membranes, they undergo abrupt changes in their organized structures and mobilities at their phase transition temperatures. In order to design smart membranes that regulate drug permeability as a function of temperature, we focused on well-organized and dynamic LCPs that undergo liquid crystalline-isotropic phase transition around body temperature. In this study, side-chain LCPs composed of a flexible siloxane main chain and pendant mesogens were synthesized and LCP membranes were prepared by casting the LCPs/xylene on porous support membranes. The nematic-isotropic phase transition temperature (TNI) of the resulting LCPs strongly depended on the mesogen content. Although the permeation of drugs through the LCP membranes was hindered at temperatures below TNI, their drug permeability was drastically enhanced over TNI. Thermoresponsive regulation of drug permeation through the LCP membranes is attributed to the drastic changes in their well-organized and dynamic structures that are induced by the nematic-isotropic phase transition.

Multiparticle collision dynamics for tensorial nematodynamics.

Liquid crystals establish a nearly unique combination of thermodynamic, hydrodynamic, and topological behavior. This poses a challenge to their theoretical understanding and modeling. The arena where these effects come together is the mesoscopic (micron) scale. It is then important to develop models aimed at capturing this variety of dynamics. We have generalized the particle-based multiparticle collision dynamics (MPCD) method to model the dynamics of nematic liquid crystals. Following the Qian-Sheng theory [Phys. Rev. E 58, 7475 (1998)1063-651X10.1103/PhysRevE.58.7475] of nematics, the spatial and temporal variations of the nematic director field and order parameter are described by a tensor order parameter. The key idea is to assign tensorial degrees of freedom to each MPCD particle, whose mesoscopic average is the tensor order parameter. This nematic MPCD method includes backflow effect, velocity-orientation coupling, and thermal fluctuations. We validate the applicability of this method by testing (i) the nematic-isotropic phase transition, (ii) the flow alignment of the director in shear and Poiseuille flows, and (iii) the annihilation dynamics of a pair of line defects. We find excellent agreement with existing literature. We also investigate the flow field around a force dipole in a nematic liquid crystal, which represents the leading-order flow field around a force-free microswimmer. The anisotropy of the medium not only affects the magnitude of velocity field around the force dipole, but can also induce hydrodynamic torques depending on the orientation of dipole axis relative to director field. A force dipole experiences a hydrodynamic torque when the dipole axis is tilted with respect to the far-field director. The direction of hydrodynamic torque is such that the pusher- (or puller-) type force dipole tends to orient along (or perpendicular to) the director field. Our nematic MPCD method can have far-reaching implications not only in modeling of nematic flows, but also to study the motion of colloids and microswimmers immersed in an anisotropic medium.

Nanoparticle's size, surfactant and concentration effects on stability and isotropic-nematic transition in ferronematic liquid crystal

Isotropic-nematic phase transition in composites based on 4-cyano-4-hexylbiphenyl (6CB) liquid crystal and spherical magnetic iron oxide nanoparticles with various nanoparticle sizes and various concentrations were investigated. The iron oxide nanoparticles were covered by oleic acid surfactant that prevents their aggregation. Dependences of the phase transition temperature shift on the nanoparticle size and the nanoparticle volume fraction were acquired by optical measurements, differential scanning calorimetry measurements and capacitance measurements. TEM and SEM examination showed a nonuniform thickness of oleic acid layers for different nanoparticle sizes. At the same nanoparticle volume fraction, therefore, the volume fraction of oleic acid varies with nanoparticle size. Although oleic acid surfactant prevents the magnetic aggregation, it modifies isotropic-nematic phase transition temperature significantly. The theoretical considerations of oleic acid provided in the present paper give the interpretation of obtained experimental results.

Uniaxial and biaxial nematic phases of banana-shaped molecules and the effects of an external field

ABSTRACTA mean field theory is introduced to describe uniaxial and biaxial nematic phases of banana-shaped liquid crystalline molecules. From a geometric point of view, we derive the tensor order parameter of the banana-shaped molecules as a function of a bend angle and four orientational order parameters. Using the nematic free energy of the Maier-Saupe type, we calculate the phase diagram on the temperaturebend angle () plane and find three uniaxial nematic () and three biaxial nematic () phases, where is the primary axis of the orthogonal molecular axis. We also derive the approximate formulas of a nematic-isotropic phase transition () and a biaxial-uniaxial phase transition () temperature as a function of the bend angle. We also discuss the effects of the external field on the temperature and find the negative dependence of the temperature on the external field.

Fluid-fluid phase behaviour in the explicit solvent ionic model: Hard spherocylinder solvent molecules

We study a fluid-fluid phase transition of the explicit solvent model represented as a mixture of the restricted primitive model (RPM) of ionic fluid and neutral hard spherocylinders (HSC). To this end, we combine two theoretical approaches, i.e., the scale particle theory (SPT) and the associative mean spherical approximation (AMSA). Whereas the SPT is sufficient to provide a rather good description of a reference system taking into account hard-core interactions, the AMSA is known to be efficient in treating the Coulomb interactions between the ions. Alternatively, we also use the mean spherical approximation (MSA) for comparison. We restrict our attention to the HSC with aspect ratios L2/σ2 = 5 and 10, when the diameter of RPM particles is set to σ1 = σ2. We find the conditions at which the fluid-fluid phase transition is accompanied by the isotropic-nematic phase transition occurring in the solvent component. Furthermore, the effect of asphericity of solvent molecules on the fluid-fluid phase transition is studied by considering an “equivalent” mixture in which the HSC particles are replaced by hard spheres (HS) of the same volume. We discuss an importance of accounting for the association effects which appear between oppositely charged ions.

Fumed alumina-in-nematic liquid crystal suspensions under shear and electric field

We study rheology, microstructure, and response to an applied electric field (E) in suspensions of fumed alumina (Al2O3) nanoparticles in a nematic liquid crystal (NLC) made of N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Fumed Al2O3/MBBA suspensions exhibit flowability with nanoparticle volume fraction (ϕ) = 0.001 and 0.007, and become solid-like gels at a nanoparticle ϕ = 0.014 and beyond. The dynamic rheology of gel-like suspensions follows the soft glass rheology (SGR) model. The effective noise temperature remains close to 1 for these Al2O3/MBBA suspensions, which serves as an indication of the presence of glassy dynamics. Further, the optical microscopy and the differential scanning calorimetry (DSC) reveal that the incorporation of fumed Al2O3 nanoparticles causes a significant depression in the nematic-isotropic phase transition temperature (TNI). The gel-like suspensions are less sensitive to pre-shear and show a large structural recovery after shear when compared to Al2O3/silicone oil suspensions. At and above a critical nanoparticle ϕ = 0.005, the suspensions exhibit a reversible nematic-isotropic phase transition under the application of E.