Order parameters and densities in the smectic C, smectic A and nematic phases of a liquid crystal mixture

Measurements of the orientational and translational order parameters for the nematic, smectic A and smectic C phases of a commercial liquid crystal material are reported. The order parameters have been obtained by analysis of the angular distribution of the intensity of X-rays scattered by a sample aligned by a magnetic field. Results are presented as a function of temperature, and it is found that the apparent orientational order parameter in the smectic C phase decreases with decrease in temperature. This is explained using a model of random tilt. The experimental order parameters are compared with those calculated from a mean field model.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

On the relationship between electro-optic and dielectric parameters of a smectic C* ferroelectric liquid crystal

A relationship between the electro-optic switching time and dielectric parameters of a S*_c ferroelectric liquid crystal (FLC) is obtained. This relationship is derived in terms of spontaneous polarization P_s, relaxation time τ_G and dielectric strength Δε_G of the Goldstone mode. It shows clearly that the switching phenomenon in FLCs is governed by the dielectric behaviour of the Goldstone mode. Based on the Landau model, the switching time has also been related to the material parameters of the FLC.     

A novel liquid crystal showing antiferroelectric smectic C* and twist grain boundary phases

A novel antiferroelectric liquid crystal, 4-[5-(4-octyloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(4-methoxyphenyl)butanoate (TFMB) showing twist grain boundary phases was found and investigated. In optically active TFMB, a diffuse liquid-liquid transition was observed above the clearing point. TFMB exhibited a stable antiferroelectric smectic C* phase. The relationship between antiferroelectricity and the molecular structure is discussed.     

Induced smectic C* phases

Induced ferroelectric S*_C phases are formed by non-chiral S_C host phases doped with chiral dipolar guest molecules. In those mixtures the spontaneous polarization P_s and the tilt angle Θ has been investigated as a function of the mole fraction x_G of the chiral dopant. In most cases the reduced polarization P₀ = P_S/ sin Θ has been found to depend linearly on x_G. The polarization power which is defined by δ_P=(∂P₀/∂x_G)ΔT is discussed in terms of the molecular structure of the chiral dopants. There are systems in which P₀(x_G) deviates positively from linearity. This behaviour can be understood by considering a local field correction to P₀. By assuming a local field of Lorentz type a theoretical relation for P₀(x_G) has been derived which explains the experimental results. The effect of a local field is considerable if the transverse dipole moment and the polarizability of the chiral dopant are large.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4'-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral-racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic-smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.     

Computer simulation of interactions between liquid crystal molecules and polymer surfaces I. Alignment of nematic and smectic A phases

Results are presented from computer simulations of liquid crystal molecules in contact with polymeric surfaces. These form part of a study of the complex alignment interactions which operate in liquid crystal displays. The liquid crystal molecules considered are 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-n-octyl-4'-cyanobiphenyl (8CB); the polymeric surfaces simulated were crystalline polyethylene, polypropylene, poly(vinyl alcohol) and Nylon 6. Additional simulations were performed using graphite as a substrate. Polyethylene, poly(vinyl alcohol) and Nylon 6 were all found to induce orientation of the 5CB and 8CB molecules parallel to the polymer chain axes, as would be expected from experimental studies. On the other hand, polypropylene induces many different orientations with no clear preference for either. No evidence was found for the alignment of 8CB molecules on graphite substrates, in disagreement both with experimental findings and the results from previous simulations. The nature of the alignment interactions and possible reasons for the observed discrepancies are discussed.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

Abstract

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4′-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral–racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic–smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.     

Computer simulation of interactions between liquid crystal molecules and polymer surfaces I. Alignment of nematic and smectic A phases

Results are presented from computer simulations of liquid crystal molecules in contact with polymeric surfaces. These form part of a study of the complex alignment interactions which operate in liquid crystal displays. The liquid crystal molecules considered are 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-n-octyl-4'-cyanobiphenyl (8CB); the polymeric surfaces simulated were crystalline polyethylene, polypropylene, poly(vinyl alcohol) and Nylon 6. Additional simulations were performed using graphite as a substrate. Polyethylene, poly(vinyl alcohol) and Nylon 6 were all found to induce orientation of the 5CB and 8CB molecules parallel to the polymer chain axes, as would be expected from experimental studies. On the other hand, polypropylene induces many different orientations with no clear preference for either. No evidence was found for the alignment of 8CB molecules on graphite substrates, in disagreement both with experimental findings and the results from previous simulations. The nature of the alignment interactions and possible reasons for the observed discrepancies are discussed.     

Orientation control over bent-core smectic liquid crystal phases

Bent-core smectic liquid crystal (LC) phases (B phases) have been widely studied since their unusual polar and chiral properties were discovered in the 1990s. Relatively few studies have examined the mechanisms by which the orientation of the B phase may be controlled to produce the type of macroscopic domain that is essential for organic semiconductor, optical device and patterning applications. This review is intended to cover recent progress towards controlling the B phases upon layering, including anisotropic treatment methods and topographical confinement methods. Finally, this review closes with a discussion of B smectic phases that have been fabricated for use in certain applications.     

Phase transitions between smectic A phases as well as smectic C* phases and undulated structures in terminal non-polar compounds

Two types of smectic A and smectic C phases, respectively, have been proved by X-ray and DSC methods for compounds that are terminal non-polar, but consist of tuning-fork shaped molecules. The structural models of the phases are discussed on the basis of steric interaction and of a dense packing. The X-ray patterns of oriented samples point to an undulation of the smectic layers in the case of the smectic A and smectic B phase.     

On the thickness dependence of ferroelectric polarization in smectic C* liquid crystal cells

The thickness dependence of ferroelectric polarization in a SmC* liquid crystal exhibiting a large value of polarization has been investigated. It is observed that the saturation value of the polarization shows no significant thickness dependence. This observation is in accordance with a recent theoretical framework developed by Galerne. It is shown that the wall thickness, where the polarization undergoes a reversal, and hence the self-screening effect, is insignificant in the material studied.     

On the thickness dependence of ferroelectric polarization in smectic C* liquid crystal cells

The thickness dependence of ferroelectric polarization in a SmC* liquid crystal exhibiting a large value of polarization has been investigated. It is observed that the saturation value of the polarization shows no significant thickness dependence. This observation is in accordance with a recent theoretical framework developed by Galerne. It is shown that the wall thickness, where the polarization undergoes a reversal, and hence the self-screening effect, is insignificant in the material studied.     

Twisted smectic A phases in side chain liquid crystal polymers

The syntheses of two side chain liquid crystal polymers, a polyacrylate and a polymethacrylate, are reported. In each of the polymers the liquid-crystalline side group carries an asymmetric carbon atom, thereby making some of the liquid crystal phases formed by the polymers optically active and chiral. For the chiral polyacrylate smectic A and chiral ferroelectric smectic C phases are observed, however for the chiral polymethacrylate a cholesteric phase is detected above the smectic A phase. It is found that the smectic A to cholesteric phase transition is mediated by the formation of an intermediary twisted smectic A phase. This intermediary phase is a liquid-crystalline analogue of the Abrikosov flux phase found in Type II superconductors.     

Aliphatic polyimides as alignment layers for surface stabilized ferroelectric smectic C liquid crystal cells

Eight aliphatic polyimides have been synthesized and tested as alignment layers for surface stabilized ferroelectic liquid crystals with a number of room temperature ferroelectric mixtures. The cone angles are used as a quantitative measure of the bistability obtained in these cells. The effect of the structure of the polyimides on the cone angles obtained is discussed. It is shown that the structure of the polymer alignment layer strongly influences the cone angle found between the relaxed states in a surface stabilized ferroelectric liquid crystal cell. Highly crystalline polymers give a larger cone angle than less crystalline homologues.     

Twisted smectic A phases in side chain liquid crystal polymers

Abstract

The syntheses of two side chain liquid crystal polymers, a polyacrylate and a polymethacrylate, are reported. In each of the polymers the liquid-crystalline side group carries an asymmetric carbon atom, thereby making some of the liquid crystal phases formed by the polymers optically active and chiral. For the chiral polyacrylate smectic A and chiral ferroelectric smectic C phases are observed, however for the chiral polymethacrylate a cholesteric phase is detected above the smectic A phase. It is found that the smectic A to cholesteric phase transition is mediated by the formation of an intermediary twisted smectic A phase. This intermediary phase is a liquid-crystalline analogue of the Abrikosov flux phase found in Type II superconductors.     

Side-on fixed polysiloxanes and 'diluted' copolysiloxanes with nematic and smectic C phases

The influence of the molecular parameters of side-on fixed liquid crystal polymers (LCP), namely the spacer length, the aliphatic tail length and the proportion of mesogenic groups ('dilution' effect), on the mesomorphic, thermodynamic and structural properties of the nematic and smectic C phases has been studied. This shows in particular the stabilization of the SmC phase on increasing the terminal chain lengths. Moreover, this paper provides evidence for an original behaviour in the SmC organization of side-on LCP with a nonstructural monotonous evolution of the thermodynamic and properties with the 'dilution'. Finally, a comparison with side-end LCP can be established : the question of the participation of the polymer backbone in the smectic organization in side-on LCP is raised.     

Symmetric bent-shaped liquid crystal dimers showing transitions between optically uniaxial and biaxial smectic phases

We report the synthesis of a new homologous series of symmetric bent-shaped liquid crystal dimers bearing oxybiphenylcarboxylate mesogens linked by a pentamethylene spacer. The dimers were characterised by optical, thermal, X-ray diffraction and computational techniques. Shorter dimers showed an I-N-SmA-SmC phase sequence with strong first-order phase transitions. For the longer dimers, the unconventional uniaxial SmA phase present in shorter members collapsed. A 9:1 binary mixture of a bent dimer with a non-mesogenic dimer exhibited upon the SmA-SmC phase transition a maximum layer contraction of 1%.     

Dielectric relaxation in coexisting nematic and smectic B phases

Mixtures of a polar solute 4-n-pentyl-4'-cyanobiphenyl in a non-polar nematic solvent exhibit two separated low frequency dielectric relaxations for concentrations of the solute between 2 mol% and 20 mol% over a limited temperature range. This behaviour is attributed to coexisting nematic and smectic B phases, in which the polar solute probe has different relaxation frequencies. The observed dielectric spectra can be accurately fitted to two Debye-like relaxations, and the strengths of the absorptions are proportional to the amounts of the coexisting phases. A microscopic determination of the phase diagram confirms the assignment of the coexisting phases, and it is concluded that there is a preference of the dipolar probe molecule for the smectic B phase, which is induced as a result of solute-solvent interactions.