Solution of a molecular statistical model for chiral nematic liquid crystals

A molecular statistical model for the chiral nematic phase of liquid crystals is investigated. The model is treated in the molecular field approximation. The resulting set of coupled integral equations for the order parameters and the pitch determining equation are solved numerically. The model hamiltonian consists of a nematic and a twist producing term. If only the nematic term is present, the model is known to have a first order phase transition. The model containing only the twist producing term is shown to exhibit a second order phase transition. The order parameters and wave length are presented for three ratios of the coupling constants in the case that both interactions are present.     

Diffusion measurements in chiral liquid crystals

Abstract

We present results of diffusion investigations in twisted nematics (nematic-cholesteric mixtures). The dependence of the diffusion coefficient along the helix on the concentration of the cholesteric compound, the sample temperature and the diffusion time are measured by a new nuclear magnetic resonance method. The diffusion coefficient was found to decrease with diffusion time in the studied range of about 0·5–50 ms. The results are consistent with data from ¹³C-N.M.R. experiments, and with field gradient measurements by other authors.     

Effect of introducing thioether linkages on the molecular organization of chiral twin liquid crystals

Novel symmetric and non-symmetric chiral twin compounds possessing one or two thioether groups in a central spacer were prepared, and the effect of substituting oxygen for sulphur on the liquid crystalline properties investigated. Chiral twin compounds possessing an alkylsulphanyl spacer showed an antiferroelectric phase exclusively. However, replacing the alkyloxy chain of the analogous monomer by the alkylsulphanyl chain has no significant effect on the phase transition behaviour, i.e. both of the monomers showed the ferro- and ferri-electric phases as well as the antiferroelectric phase. Thus, different effects of introducing the thioether linkage were for the first time observed between twin and monomeric systems. The introduction of oxygen or sulphur atoms into the central alkyl spacer of the chiral twin was also investigated, and these modifications were found to stabilize the SmA phase. Furthermore, the twin compound possessing a thiaalkyl spacer showed two different molecular assemblies in the smectic A phase.     

Effect of introducing thioether linkages on the molecular organization of chiral twin liquid crystals

Novel symmetric and non-symmetric chiral twin compounds possessing one or two thioether groups in a central spacer were prepared, and the effect of substituting oxygen for sulphur on the liquid crystalline properties investigated. Chiral twin compounds possessing an alkylsulphanyl spacer showed an antiferroelectric phase exclusively. However, replacing the alkyloxy chain of the analogous monomer by the alkylsulphanyl chain has no significant effect on the phase transition behaviour, i.e. both of the monomers showed the ferro- and ferri-electric phases as well as the antiferroelectric phase. Thus, different effects of introducing the thioether linkage were for the first time observed between twin and monomeric systems. The introduction of oxygen or sulphur atoms into the central alkyl spacer of the chiral twin was also investigated, and these modifications were found to stabilize the SmA phase. Furthermore, the twin compound possessing a thiaalkyl spacer showed two different molecular assemblies in the smectic A phase.     

Solute-solvent interactions and chiral induction in liquid crystals

The induction of a cholesteric phase by doping an achiral nematic liquid crystal with an enantiopure solute is a phenomenon that, as in all general supramolecular phenomena of chiral amplification, depends in a subtle way on intermolecular interactions. The micrometric helical deformation of the phase director in the cholesteric phase is generated by the interplay of anisotropy and chirality of probe-medium interactions. In the case of a flexible chiral dopant, the solvent can influence the twisting power in two ways, difficult to disentangle: it is responsible for the solute orientational order, an essential ingredient for the emergence of phase chirality; but also it can affect the dopant conformational distribution and then the chirality of the structures present in the solution. In this work we have investigated methyl phenyl sulfoxide, a flexible, chiral molecule that, when dissolved in different nematics, can produce cholesteric phases of opposite handedness. This peculiar, intriguing sensitivity to the environment makes MPS a suitable probe for a thorough investigation of the effects of solute-solvent interactions on chiral induction in liquid crystals. NMR experiments in various nematic solvents have been performed in addition to twisting power measurements. From the analysis of partially averaged 1H-1H and 13C-1H dipolar couplings, the effects of solvent on solute conformation and orientational order are disentangled, and this information is combined with the modeling of the chirality of intermolecular interactions, within a molecular field theory. The integration of different techniques allows an unprecedented insight into the role of solvent in mediating the chirality transfer from molecule to phase.     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light‐emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Light-induced structural changes in chiral liquid crystals

Novel light-sensitive chiral dopants are studied as a light-sensitive component in chiral liquid crystals which may be used in tunable optical devices. Light-induced cis-trans- isomerization of chiral dopants results in changes of helical twisting power which translates into variations of helical pitch. Due to the light absorption in the liquid crystal cell the pitch variation is non-uniform across the cell, which leads, at first, to a deformation of cholesteric layers, and then to the formation of cholesteric bubbles. The sequence of structural changes has a distinct visual pattern and occurs at the surface close to the UV light source. Small deformations of cholesteric layers and bubbles are unstable and disappear after removing UV irradiation. The increasing size of the cholesteric bubbles results in better stability; large bubbles do not disappear after removing UV light. A theoretical model is suggested to describe the undulations of cholesteric layers.     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light-emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Structure of liquid crystal droplets with chiral propeller texture

We experimentally studied a nematic liquid crystal whose molecules form twisted head-to-head H-bonded dimers. We observed that when the material transformed from the isotropic to nematic phase, it formed droplets with chiral propeller textures. We carried out a computer simulation to investigate the liquid crystal director configuration inside the droplets and to study the effects of elastic constants and chirality on the droplet texture. Results of our study show it is likely that the material in the droplets had nonzero chirality due to spontaneous chiral phase separation.     

New chiral dopants for ferroelectric liquid crystals

New chiral dopants for application in ferroelectric liquid crystal mixtures are presented. They derive from chirally modified biphenylcyclohexylcarbonitriles, lactates and optically active 2-cyano-2-methylalkanoates. Synthetic routes to the new compounds are shown and the physical properties of the new compounds in a non-chiral smectic C host are reported.     

Cholesteric pitch of rigid and semi-flexible chiral liquid crystals

We calculate using mean field theory, the cholesteric pitch in systems composed of chiral molecules, which we model as 'corkscrews'. We consider both the rigid and semi-flexible limits. Our result for the cholesteric pitch depends on the intrinsic molecular pitch length as well as the concentration of molecules, but does not depend on the flexibility of the molecule.     

Twist-grain-boundary phase diagrams in chiral liquid crystals

The experimental phase diagrams of several chiral systems are compared with a theoretical diagram based on the chiral Chen-Lubensky model, which predicts at least two kinds of twist-grain-boundary phase, TGBA and TGBC. Also shown for comparison are typical nonchiral phase diagrams that exhibit a nematic-smectic A-smectic C multicritical point. Several aspects of experiment and theory agree, but there appear to be common experimental features that differ from those predicted by current theory.     

Synthesis and mesomorphic properties of chiral fluorinated liquid crystals

Some fluorinated chiral liquid crystals were synthesized and the compounds characterized by IR, ¹H NMR, ¹⁹F NMR and mass spectroscopy and elemental analysis. Their phase transition behaviour was investigated by differential scanning calorimetry and polarizing optical microscopy. Nearly all of the compounds synthesized are liquid crystals with an enantiotropic cholesteric phase. Some of them exhibit a blue phase. Lateral tetrafluoro substitution decreases the clearing point and molecular polarity affects the formation of liquid crystalline phases.     

Synthesis and mesomorphic properties of chiral fluorinated liquid crystals

Some fluorinated chiral liquid crystals were synthesized and the compounds characterized by IR, ¹H NMR, ¹⁹F NMR and mass spectroscopy and elemental analysis. Their phase transition behaviour was investigated by differential scanning calorimetry and polarizing optical microscopy. Nearly all of the compounds synthesized are liquid crystals with an enantiotropic cholesteric phase. Some of them exhibit a blue phase. Lateral tetrafluoro substitution decreases the clearing point and molecular polarity affects the formation of liquid crystalline phases.     

A molecular theoretical derivation of the Landau free energy of chiral smectic C liquid crystals

A molecular theoretical derivation of the Landau free energy of chiral smectic C liquid crystals is presented on the basis of a molecular theoretical approach. The excess free energy concerned with the ferroelectric helicoidal structure is derived within a mean field approximation. Several material constants such as the Curie temperature, the elastic constant, the helical pitch, the piezo- and flexoelectric constants and the dielectric susceptibility are expressed in terms of some effective intermolecular interaction parameters between molecules. A molecular theoretical consideration of the chiral smectic C-smectic A transitions is given in comparison with a few previous works. By comparing with some available experimental data for DOBAMBC, effective interaction parameters are numerically estimated.     

Macroscopic properties of mixed chiral smectic liquid crystals

Binary mixtures of chiral and non-chiral smectogens, DOBAMBC and DBHP respectively, exhibit a chiral smectic C phase in two separated concentration regions (about 0-20 and 60-100 mol per cent of DOBAMBC) and a chiral smectic I phase in the region of 60-100 mol per cent of DOBAMBC. For the S*_cphase, the temperature and the concentration dependences of the spontaneous polarization, the tilt angle and the helical pitch were determined. The electric field switching properties of the S*_iphase have been studied. For higher concentrations, the switching is similar to that of the S*_iphase of pure DOBAMBC. When the concentration of DOBAMBC is lowered, bistable switching is observed.     

Dynamics of chiral liquid crystals under applied shear

We simulate the alignment dynamics of cholesteric (chiral) rod-like liquid crystals by using a Landau-de Gennes (LdG) expression for microstructure evolution in response to flow. This study is motivated by recent advances in novel cholesteric nanorod dispersions. Prior work on the modelling of cholesterics has suffered from the restriction of helicity to only a single direction, often with a pre-imposed pitch, due to numerical difficulties. This has severely limited cholesteric modelling in regard to both accuracy and experimental relevance. Our simulations avoid this limitation. Relevant forces on rods include solvent-rod drag, nematic alignment, microstructure elasticity and chiral twist. Phase diagrams are developed to demonstrate the response of these systems to variations in chiral and flow forces. Our results indicate that for low shear rates, chiral and elastic forces prevent the rods from moving in response to flow. At high shear rates, the rods tumble and form unique transient structures (combinations of tumbling and cholesteric phases) as flow forces and chiral forces compete. Even if slight alignment is induced at the boundaries, the phase diagram substantially changes, chiefly by constraining the possible chiral phases. This work has immediate relevance to applications which exploit the optical properties of films solidified from cholesteric dispersions.     

Photoinduced hyper-reflective cholesteric liquid crystals enabled via surface initiated photopolymerization

A structured polymer was synthesized by surface initiated photopolymerization in the presence of a cholesteric liquid crystal (CLC). The templated helical polymer (traversing 2/3 the cell thickness) was backfilled with an opposite handedness, photoresponsive CLC mixture yielding a photo-induced, large contrast, hyper-reflective (R > 99%) CLC film.     

Photochromic chiral liquid crystalline systems containing spiro-oxazine with a chiral substituent II. Photoinduced behaviour

The photoinduced behaviour of low molecular mass liquid crystalline (LC) systems doped with the chiral photochromic spiro-oxazine, 9'-{5-(-)-menthoxyacetoxy-1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3'-[3H]naphth-[2,1 -b][1,4]oxazine]} 4-[11-(tetrahydro-2-pyranyloxy)undecyloxy]biphenyl-4'-carboxylate (THP11SOM) was observed by means of transmittance measurements at 21°C. The experimental results revealed that the ring-opened merocyanine form of THP11SOM resulting from the photochromism first disturbed the helical planar orientation of the LC molecules and then induced a new helical pitch. The magnitudes of both the disturbance in orientation of the LC molecules and the shift in the helical pitch were significantly influenced by the concentration of THP11SOM. The photoinduced behaviour was reversible due to the photochemical and thermal back reaction of the THP11SOM. In addition, the effect of UV light on the nematic-isotropic transition temperatures of the mixtures with or without a hindered amine light stabilizer was studied.