Competition between micro-segregation and anti-parallel alignment of an amphiphilic rod-like liquid crystal

Recently, we reported that a rod-like molecule, 4-[4-(7-hydroxyheptyloxy)phenyl]-1-(4-hexylphenyl)-2,3-difluorobenzene, exhibited a nematic phase with a layered structure and smectic C phases consisting of three states. We prepared a homologous series of the rod-like molecules in which a 2,3-difluoro-1,4-diphenylbenzene unit and a hydroxyl unit are connected via a flexible methylene spacer. We investigated their physical properties using polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Although they showed nematic, smectic A and smectic C phases, the phase structures were found to depend on the flexible spacer length. Those compounds possessing a shorter spacer length than a heptyl unit exhibited bilayered smectic A and smectic C phases, whereas those possessing a longer spacer length than a heptyl unit showed conventional monolayered smectic A and smectic C phases. A nematic phase with a smectic-like layer ordering was observed for the compound possessing an octyl unit. The structure–property relations of the amphiphilic compounds are discussed.     

The synthesis and characterization of novel non-symmetric dimers with rod-like and disc-like mesogenic units

Theory predicts that a compound whose structure possesses both rod-like and disc-like characteristics should exhibit a biaxial nematic phase. With this in mind we have synthesized and characterized two new series of non-symmetric dimers containing rod-like and disc-like mesogenic units linked by a flexible spacer. The two series differ in that in one the perimeter of the disc-like unit is decorated with alkyl chains, whereas for the other it is not. The liquid crystalline properties of the two series were investigated both as pure systems and in their equimolar binary mixtures with an electron acceptor. The nematic phase formed by the equimolar binary mixtures is monotropic. However, a small entropy change at the nematic-isotropic transition could be determined. The nature and structure of the monotropic nematic phase is discussed.     

Computer simulation of the liquid crystal formation in a semi-flexible polymer system

Molecular dynamic simulations are reported for system of semi-flexible linear rod-like molecules. The molecules are composed of N_c tangent soft spheres, connected by elastic springs. Rigidity is introduced by additional springs between all pairs of spheres along the molecule. The formation of only a nematic LC phase is shown for all systems with N_c = 8 and different flexibility. The effect of flexibility on the order parameter and the volume fraction at the LC phase transition is compared with theoretical predictions by Khokhlov-Semenov and with available simulation data. The dependence of the anisotropy of diffusion on chain flexibility in LC phase was studied. The polymer brushes consisting of flexible and semi-flexible (composed of linear rod-like segments) chains were simulated at different grafting densities. Height of brush, order parameter, distribution of density and chain ends in brush were obtained in both cases and compared with theoretical predictions.     

Computer simulation studies of anisotropic systems. XXII. An equimolar mixture of rods and discs: A biaxial nematic?

In principle, binary mixtures of rod-like and disc-like particles should exhibit a biaxial nematic phase, but in practice phase separation into two uniaxial nematic phases prevents this. Here, we report the results of a computer simulation study of an equimolar mixture of rods and discs in which phase separation is not allowed. The particles are confined to the sites of a simple cubic lattice in which each rod is surrounded by six discs and vice versa. Neighbouring particles interact such that they prefer to align with their respective symmetry axes orthogonal to each other. In contrast, the interaction between next nearest neighbours, which are either rods or discs, is such that their symmetry axes tend to be parallel. Monte Carlo simulations of this model mixture show that an orientationally ordered phase exists at low temperatures. This nematic phase has overall uniaxial symmetry and the particles have a negative second rank orientational order parameter, indicating that they tend to align at right angles to the director. The two interpenetrating sub-lattices containing either rods or discs, however, exhibit a biaxial nematic phase. The results of the simulation are found to be in reasonable agreement with the predictions of a molecular field theory for this model mixture. We have also investigated the behaviour of this mixture when the rods and discs are allowed to exchange between their lattice sites. The mixture is found to separate into two uniaxial nematic phases composed essentially of either rods or discs, as expected.     

The phase behavior, structure, and dynamics of rodlike mesogens with various flexibility using dissipative particle dynamics simulation

We present a systematic dissipative particle dynamics (DPD) study on the phase behavior, structure, and dynamics of rodlike mesogens. In addition to a rigid fused-bead-chain model with RATTLE constraint method, we also construct a semirigid model in which the flexibility is controlled by the bending constant of k(φ). Using this notation, the rigid model has an infinite bending constant of k(φ)=∞. Within the parameter space studied, both two kinds of models exhibit the nematic and smectic-A phases in addition to the isotropic and solid phases. All of the phase transitions are accompanied by the discontinuities in the thermodynamical, structural, and dynamical quantities and the hysteresis around the transition points, and are therefore first order. Note that the obtained solid state exhibits an in-layer tetragonal packing due to the high density. For the rigid model, the simulations show that the liquid crystal phases can be observed for mesogens with at least five beads and the nematic phase is the first one to appear. More importantly, the phase diagram of seven-bead-chain models is obtained as a function of k(φ) and temperature. It is found that decreasing the value of k(φ) reduces the anisotropy of molecular shape and the orientational ordering, and thereby shifts the liquid crystal phases to the lower temperature end of the phase diagram. Due to the different k(φ) dependence of phase transition temperatures, the nematic phase range exhibits a more marked narrowing than the smectic-A phase as k(φ) is reduced, implying that the flexibility has a destabilizing effect on the nematic and smectic-A phases. We also have investigated the anisotropic translational diffusion in liquid crystal phases and its temperature and flexibility dependence. In our study, we find that the phases formed, their statical and dynamic properties, as well as the transition properties are in close accord with those observations in real thermotropic liquid crystals. It is clear that both the rigid and semirigid models we used are valuable models with which to study the behavior of thermotropic liquid crystals using DPD algorithm.     

Interface effects in nematic liquid-crystalline structures of polymer solutions

A solution of long semirigid linear macromolecules was considered. The liquid-crystalline nematic ordering in the solution was analyzed theoretically using an Onsager-type approach. The orientation entropy was calculated in the frameworks of Lifshits' approach, successfully developed for this system originally by Khokhlov and Semenov. For homogeneous liquid-crystalline phase using the third virial approximation for intersegmental steric interaction the orientation distribution function, the free energy density, the isotropic-nematic coexistence and the spinodal conditions were computed numerically for two types of polymer flexibility mechanism: persistent chains and chains of freely joint segments. For the asymptotically exact second virial approximation the applicability region was analyzed. We considered the general equations, which describe the concentration and orientational segment distribution for a semirigid persistent polymer chain at a surface (or interface) of any shape and orientation. These equations were numerically solved for the case when the nematic director axis was perpendicular to a planar interface boundary between the real coexisting nematic and isotropic phases. The coordinate-dependencies of the polymer concentration and of the order-parameter take the smooth two-steps form in the interface region.     

Synthesis and characterization of non‐symmetric chiral dimers

A new class of cholesterol‐based non‐symmetric dimers have been synthesized and characterized. They comprise O‐alkylated cinnamic acid and pro‐mesogenic cholesterol segments interlinked covalently through a methylene spacer varying in its length and parity. All the dimers and some of the key precursors have been studied for their phase behaviour. All the intermediates show mesomorphism. Importantly, the thermal properties of dimers are found to be critically dependent on the parity of the flexible spacer. The dimers with odd‐parity spacer display chiral nematic and/or twist grain boundary phases. In contrast, the dimers with even‐parity spacer are either crystalline or exhibit metastable chiral nematic and/or twist grain boundary phases with the exception of one compound for which two unknown mesophases have been observed. The odd–even effect was found to be indistinct for selective reflection wavelengths of the chiral nematic phase. For some dimers, a variation in the pitch of the chiral nematic phase as a function of temperature was observed. Cyclic voltammetry experiments revealed the electrochemical properties of a representative liquid crystal dimer.     

New lyotropic nematics of double chain surfactants

The phase behaviour of N,N-dialkyl-N,N-dimethyl ammonium bromide double chain surfactants was investigated by polarization microscopy, ²H-N.M.R. spectroscopy, and surface and magnetic field alignment in order to find new lyotropic nematic systems. Decanol was utilized to cause a rod to disc transition in the nematic phase. The sign of the anisotropy of the diamagnetic susceptibility Δx was inverted by substituting the bromide counter-ion with the benzenesulphonate ion. The phase behaviour was modified within a wide range by varying the length of the two alkyl chains. Using these tools we were able to prepare any of the four kinds of uniaxial nematic phases: rod-like and disc-like aggregates with both negative and positive anisotropy of the diamagnetic susceptibility. Several new binary, ternary and quaternary systems of these double chain surfactants and some experiments are described to classify the different nematic phases.     

Visco-elastic properties of reentrant nematic liquid crystalline mixtures

The visco-elastic properties for binary mixtures of 4- n -hexyloxy-4'-cyanobiphenyl (6OCB) and 4- n -octyloxy-4'-cyanobiphenyl (8OCB) are investigated in detail by a light scattering technique. The mixtures exhibit a reentrant nematic (RN) phase between the smectic A (SmA) and crystal phases in the range 22.0-29.5 wt % of 6OCB. The viscosity and the elastic constant increase with cooling in both the nematic and reentrant nematic phases. It is also found that the mixtures exhibit anomalously large values of viscosity and elastic constant near the phase transition, i.e. a pretransitional phenomenon can be observed. The activation energy for viscosity is smaller in the RN phase than in the ordinary nematic phase appearing at higher temperatures, while the normalized elastic constant (defined as the ratio of the elastic constant to the square of the dielectric anisotropy) is larger. Moreover, the visco-elastic behaviour depends on the specific time during which the sample has been kept in the SmA phase. These results confirm that the SmA phase has a strong influence on the formation of molecular aggregates, which plays an important role in the reentrant phenomenon of these systems.     

First order transitions to a lyotropic biaxial nematic

The phase diagram of the sodium dodecylsulphate/decanol/water system is studied by²H NMR spectroscopy in the range between the calamitic nematic (N⁺_C) and discotic nematic (N^-_D) phases. In this narrow range a nematic biaxial phase (N_BX) is observed. The phase transitions between the nematic phases are all of first order. The shape of the surfactant aggregates in the nematic phases varies with composition and temperature.     

Preliminary Communication : Induction of smectic A phases by electron donor-acceptor interaction between calamitic mesogens and 2,4,7-trinitrofluorenone

The mesomorphic properties of conventional rod-like liquid crystals (diphenyl1,3,4-thiadiazoles, diphenylpyrimidines, diphenyltriazines, diphenyltetrazines and p-terphenyl derivatives), of macrocyclic liquid crystals and of dimesogens can be influenced by addition of the electron acceptor 2,4,7-trinitrofluorenone (TNF). Thereby nematic and smectic C phases are suppressed and smectic A phases can be stabilized or induced. Long and branched terminal chains result in a strong stabilization of the SA phase, whereas no smectic phase is induced to accompany the nematic phases of mesogens with short terminal chains.     

Visco-elastic properties of reentrant nematic liquid crystalline mixtures

The visco-elastic properties for binary mixtures of 4-n -hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) are investigated in detail by a light scattering technique. The mixtures exhibit a reentrant nematic (RN) phase between the smectic A (SmA) and crystal phases in the range 22.0–29.5 wt % of 6OCB. The viscosity and the elastic constant increase with cooling in both the nematic and reentrant nematic phases. It is also found that the mixtures exhibit anomalously large values of viscosity and elastic constant near the phase transition, i.e. a pretransitional phenomenon can be observed. The activation energy for viscosity is smaller in the RN phase than in the ordinary nematic phase appearing at higher temperatures, while the normalized elastic constant (defined as the ratio of the elastic constant to the square of the dielectric anisotropy) is larger. Moreover, the visco-elastic behaviour depends on the specific time during which the sample has been kept in the SmA phase. These results confirm that the SmA phase has a strong influence on the formation of molecular aggregates, which plays an important role in the reentrant phenomenon of these systems.     

Supermolecular chiral mesogenic tripedes

A novel series of chiral liquid crystalline tripedes Glucoside and Mannoside derivatives G(n) and M(n) (n=1-3) have been synthesised. The inner cores consist of methyl α-D-Glucoside G or methyl α-D-Mannoside M, regioselectively functionalised at the less hindered position C6, with tert-butyldimethylsilyl (TBDMS), hydroxyl or carboxylic acid moieties. The cores, which can acquire several flexible conformations, are attached to rod-like smectogenic-preferring cyanobiphenyl units, by means of a flexible hexanoyl spacer. These Glyco-Supermolecules exhibit chiral nematic (N*) and smectic A (SmA) phases. The combined effects of core chirality and functional groups on thermal and mesomorphic characteristics are discussed.     

Synthesis of UV-curable liquid crystalline diacrylates for the application of polarized electroluminescence

Two series of UV-curable rod-like diacrylates containing bis-tolane (M1-M5) and bis-stilbene (M6-M10) mesogens were synthesized. All of the diacrylates were prepared through convergent syntheses involving palladium-catalysed coupling reactions. The thermal transitions and mesomorphic properties of the diacrylates were characterized by differential scanning calorimetry and optical microscopy. All the bis-tolane-based monomers reveal nematic phases, except M5 which shows no mesomorphic behaviour. For the bis-stilbene based monomers, most show no mesomorphic behavior and only M7 and M9 exhibit a SmA phase.     

Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals

The effects of dipolar interactions and molecular flexibility on the structure and phase behavior of bent-core molecular fluids are studied using Monte Carlo computer simulations. Some calculations of flexoelectric coefficients are also reported. The rigid cores of the model molecules consist of either five or seven soft spheres arranged in a "V" shape with external bend angle gamma. With purely repulsive sphere-sphere interactions and gamma = 0 degrees (linear molecules) the seven-sphere model exhibits isotropic, uniaxial nematic, and untilted and tilted smectic phases. With gamma > or = 20 degrees the untilted smectic phases disappear, while the system with gamma > or = 40 degrees shows a direct tilted smectic-isotropic fluid transition. The addition of electrostatic interactions between transverse dipole moments on the apical spheres is generally seen to reduce the degree of molecular inclination in tilted phases, and destabilizes the nematic and untilted smectic phases of linear molecules. The effects of adding three-segment flexible tails to the ends of five-sphere bent-core molecules are examined using configurational-bias Monte Carlo simulations. Only isotropic and smectic phases are observed. On the one hand, molecular flexibility gives rise to pronounced fluctuations in the smectic-layer structure, bringing the simulated system in better correspondence with real materials; on the other hand, the smectic phase shows almost no tilt. Lastly, the flexoelectric coefficients of various nematic phases--with and without attractive sphere-sphere interactions--are presented. The results are encouraging, but a large computational effort is required to evaluate the appropriate fluctuation relations reliably.     

Phase diagrams of binary mixtures of biaxial nematogens

A mean field theory is used to describe nematic phases of binary mixtures of biaxial molecules. Using a general pseudopotential consistent with the D_2h symmetry of the constituent particles, the theory is used to calculate the elements of the order tensors necessary to describe the orientational order in binary mixtures in both uniaxial and biaxial nematic phases. For a single component, the model only requires one parameter, r₂, a ratio of anisotropic interaction strengths, to predict the temperature dependence of the four order parameters. The temperature dependence of the orientational distribution functions is illustrated for both rod-like and plate-like molecules. For binary mixtures, three anisotropic interaction strengths, r₁, r₂, and r₃, are needed to calculate the order parameters of both components as a function of concentration and temperature. The free energy is evaluated to predict the phase stability of the mixture. By systematically varying the anisotropic interaction strengths, temperature-concentration phase diagrams for a variety of molecular shapes are presented. The theoretical predictions suggest that binary mixtures of molecules with highly asymmetric shapes will display stable biaxial nematic phases.     

Synthesis and liquid crystalline properties of novel laterally connected trimesogens and tetramesogens

Three types of laterally connected triplet mesogens and one quadruplet mesogen incorporating rigid p-terphenyl units have been synthesized. Their liquid crystalline behaviour was investigated by polarizing microscopy, differential scanning calorimetry and X-ray scattering. The lateral fixation of three rod-like 4,4'-didecyloxy-p-terphenyl units mostly gives liquid crystalline materials with considerably increased mesophase stabilities with respect to the parent 4,4'-didecyloxy-2'-methyl-p-terphenyl. The mesophase stability strongly depends on the type of connection. The highest clearing temperatures were observed for triplets which are connected in line with each other (type I) and triplets which are laterally connected in a peripheral manner. Only the oligomesogens of type III are not liquid crystalline. All compounds incorporating exclusively decyloxy chains exhibit smectic phases (S_A and S_c). For the ethoxy derivatives the nematic phase was found.     

First order transitions to a lyotropic biaxial nematic

Abstract

The phase diagram of the sodium dodecylsulphate/decanol/water system is studied by²H NMR spectroscopy in the range between the calamitic nematic (N⁺ _C) and discotic nematic (N^? _D) phases. In this narrow range a nematic biaxial phase (N_BX) is observed. The phase transitions between the nematic phases are all of first order. The shape of the surfactant aggregates in the nematic phases varies with composition and temperature.     

Paramagnetic liquid-crystalline complexes based on novel enaminoketone ligands

Calamitic ligands having aminopropenone-3 ring substituted with alkyl-trans-cyclohexyl-phenyl and alkyl groups are low melting nematogens. Copper(II) complexed to the ligands gives thermally stable rod-like liquid crystals which exhibit an enantiotropic paramagnetic nematic phase. It is shown that modifications of the terminal groups as well as synthesis of asymmetric complexes enables us to extend the nematic range and depress the melting points.     

Towards Nematic Phases in Ionic Liquid Crystals – A Simulation Study

Ionic liquid crystals (ILCs) are soft matter materials with broad liquid crystalline phases and intrinsic electric conductivity. They typically consist of a rod-shaped mesogenic ion and a smaller spherical counter-ion. Their mesomorphic properties can be easily tuned by exchanging the counter ion. ILCs show a strong tendency to form smectic A phases due to the segregation of ionic and the non-ionic molecular segments. Nematic phases are therefore extremely rare in ILCs and the question of why nematic phases are so exceptional in existing ILCs, and how nematic ILCs might be obtained in the future is of vital interest for both the fundamental understanding and the potential applications of ILCs. Here, we present the result of a simulation study, which highlights the crucial role of the location of the ionic charge on the rod-like mesogenic ions in the phase behaviour of ILCs. We find that shifting the charge from the ends towards the centre of the mesogenic ion destabilizes the liquid crystalline state and induces a change from smectic A to nematic phases.