Orthoconic liquid crystals — A case study

Since the early investigations on liquid crystals it was realized how the confining surfaces often determine the textures and even properties of the material. This influence is particularly complex and important for chiral materials. When we come to chiral smectics the surfaces may have dramatic effects. These are illustrated on the ferroelectric liquid crystals; they then again increase in importance for the antiferroelectric liquid crystals where the most recent example is given by the orthoconic liquid crystals.     

A long-pitch orthoconic antiferroelectric mixture modified by isomeric and racemic homostructural dopants

Orthoconic Antiferroelectric Liquid Crystals (OAFLC) are recognised as a promising medium for display and photonic applications due to their unique electrooptical properties. The application of OAFLC is still hampered by a number of parasitic effects deteriorating the electrooptical performance of contemporary available materials. An attempt elaborating of a working OAFLC mixture with the helical pitch longer than the typical cell gap is reported. Using this mixture, near-perfect optical uniformity and excellent dark state at the zero electric field applied were obtained. Basic OAFLC mixture was doped with two homostructural analogues to study the influence of the molecular polarity and chirality on the electrooptical performance and chosen physical parameters. The physical and structural properties of admixtures and their electrooptical performance are presented and discussed.     

The influence of antiferroelectric compounds on helical pitch of orthoconic W-1000 mixture

The influence of six antiferroelectric compounds on the helical pitch of mixture W-1000, which was reported as long pitch orthoconic antiferroelectric liquid crystalline mixture, was checked by spectrophotometry and polarimetry methods. The electro-optical properties for the mixture with the longest pitch were measured. An improvement in electro-optical response due to the long pitch is reported. The novelty in electro-optical properties is the good symmetry response.     

Helix parameters in bi- and multicomponent mixtures composed of orthoconic antiferroelectric liquid crystals with three ring molecular core

Eight types of bicomponent systems composed of antiferroelectric compounds with different polarity of achiral chain and different temperature dependence of helical pitch as well as three multicomponent mixtures composed of antiferroelectric compounds with opposite helical twist sense were studied. The phase miscibility was tested by polarising optical microscopy. The results of the helical pitch and helical twist sense measurements obtained by a spectrophotometric method of selectively reflected light and by polarimetric method, respectively, are presented. It was found that it is possible to control helical pitch length and temperature of helix twist inversion in antiferroelectric mixtures by controlling the ratio of compounds with left- and right-handed helix in such mixtures, although all of them have the same chiral terminal chain.     

Dielectric relaxation spectroscopy and electro-optical studies of a new,partially fluorinated orthoconic antiferroelectric liquid crystal material exhibiting V-shaped switching

In this paper we report the results from detailed electro-optical and dielectric studies in various antiferroelectric and ferroelectric phases of an orthoconic antiferroelectric liquid crystal (OAFLC) material . The material possesses high tilt and high spontaneous polarisation. Such an OAFLC, because of its high tilt, provides an excellent dark state. The material exhibits V-shaped switching in the SmC* phase. Dielectric studies reveal the existence of another phase during heating in the range between 78.6 and 92°C which did not appear in the DSC curve and in polarising microscopy. This phase has been identified as the SmC_γ* phase and is extremely sensitive with respect to the cell conditions, aligning material, purity, etc. Three dielectric modes have been assigned in the above-mentioned temperature range and their origins are discussed.     

Chiral HPLC and physical characterisation of orthoconic antiferroelectric liquid crystals

New orthoconic antiferroelectric liquid crystalline materials were synthesised and characterised in their racemic forms and as (S) enantiomers. The materials possess oligo-methylene spacers of different lengths in semi-fluorinated achiral chains and lateral substitution by fluorine at two different positions of the molecular core. For comparison purposes, analogical materials without fluorine lateral substitutions were also prepared. Polysaccharide chiral stationary phases based on two different chiral selectors were used for the separation of the enantiomers of the individual racemic mixtures by high-performance liquid chromatography. A baseline separation of (S) and (R) enantiomers was obtained for four of the six studied liquid crystalline materials. Two of the materials were partially separated under the optimised separation conditions. The elution order of the individual enantiomers in the racemic mixtures was successfully assigned, as pure (S) enantiomers of all the studied materials were available. Both the position of the fluorine atom within the molecular core and the size of the achiral moiety had significant effects on the separation of the individual enantiomers of the studied compounds. Moreover, it was also found that the structure of the chiral stationary phase selector significantly influenced the enantiomeric resolution.     

Electro-optical properties of a new series of fluorinated antiferroelectric orthoconic liquid crystalline esters

The electro-optical properties of orthoconic antiferroelectric liquid crystals of some analogues of (S)?MHPOBC ((S)?4?(1?methylheptyloxy)carbonylphenyl 4'-octyloxy-4-biphenylcarboxylate) have been investigated and discussed in terms of the structure property correlations of liquid crystalline molecules. Polarisation field reversal technique has been used for their characterisation. In this study the occurrence of such a high value of spontaneous polarisation (P_s) for the investigated compound is especially noteworthy as they comprise a bulky chiral unit, zigzag ester linkage and polar fluorinated terminal. Lengthening of oligomethylene spacer in the terminal chain and placement of fluorine atom in the molecular core structure leads to reduction in the value of P_s. The P_s data suggest that the nature of the chiral smectic C (SmC*) to smectic A (SmA) phase transition is of second order and the critical exponent β is in the range 0.19–0.34. The ‘V-shaped’ mode or thresholdless switching in such high-polarisation ferroelectric liquid crystals has also been discussed.     

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.     

The dielectric relaxation processes in an antiferroelectric liquid crystal under cylindrical confinement

In the cylindrical pore geometry of inorganic Anopore membranes the collective relaxation processes observed in a bulk antiferroelectric liquid crystal change considerably under confinement. The frequency degeneration of the soft and Goldstone modes present at the smectic A* (SmA*)-chiral smectic C (SmC*) phase transition in the bulk phase is removed under geometrical restrictions. The relaxation rate of the soft mode is strongly modified due to the deformation of the smectic layers in the curved geometry of the pores and is superimposed by the molecular relaxation process in the SmA* and SmC* phases. The soft mode in confinement splits into two relaxation processes, which are present through all other mesophases (SmC* and SmC_a*). One of them is nearly temperature independent and slightly decreases in frequency in the SmC_a* phase. This Goldstone-like process can be assigned to the highly deformed helical structure fluctuations. The second one exhibits the characteristic features for the molecular and soft mode relaxation processes depending on the temperature range. The biquadratic and the piezoelectric coupling between the tilt angle and spontaneous polarization are revealed in their temperature dependence.     

A new mesogenic mixture with antiferroelectric phase only at a broad temperature range

New compounds (S)-4?-(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-[3-(2,2,3,3,4,4,5,5,5-nonafluoropentyloxy)prop-1-oxy]benzoates were synthesised and characterised. Their mesomorphic properties were tested by means of polarising optical microscopy and differential scanning calorimetry. The multicomponent antiferroelectric high tilt mixture with long pitch has been also formulated and characterised. The helical pitch of prepared mixture was estimated with selective reflection method.     

The temperature and concentration dependence of helical pitch in the mixtures of antiferroelectric compounds with the opposite helical twist sense

The temperature dependence of helical pitch for members of an homologous series of orthoconic antiferroelectric compounds (3FmHPhF) and their bicomponent mixtures (m = 3 and 5 as well as m = 3 and 7) has been measured by the method of selective light reflection. The compounds have the same chiral centre but they have different temperature dependence of the helical pitch as well as the helical twisting sense in SmC*_A phase: right for member m = 3, left for m = 7, and both right and left for m = 5. For this last homologue, the change of twisting sense is connected with the appearance of the unwound structure. Concentration dependence on helical pitch was investigated in the mixtures. It was demonstrated that it is possible to obtain mixtures with desirable high pitch and also with totally unwound structure.     

Three‐dimensional structure in holographic polymer‐dispersed liquid crystals

Polymer‐dispersed liquid crystals, heterogeneous structures consisting of a liquid crystal and a polymer, are promising materials for generating holograms with switchable diffraction efficiency. The experiments presented here reveal that the generation of a face‐centered cubic structure can be achieved by exposing a liquid crystal/reactive monomer mixture to the interference pattern of four coherent laser beams under appropriate geometrical conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Metastable states in antiferroelectric liquid crystalline mixtures

We report on the observation of relaxation phenomena with extremely long relaxation times, amounting to several hours. These effects take place in some liquid crystal mixtures exhibiting ferroelectric and antiferroelectric dipole order. The observed phenomena are connected with the transformation from the supercooled ferroelectric state to another, metastable state. This transition may be described using a Debye type relaxation formula. At low temperatures, a second slow transition takes place: from the metastable intermediate state to the antiferroelectric phase. This transition is characterized by unidimensional growth of the antiferroelectric domains with a constant velocity. Close to the lower temperature limit of existence of the ferroelectric phase, a direct transition from the ferroelectric to the antiferroelectric phase takes place. This transition is described by an Avrami model, hence it is governed by the creation and growth of nuclei of the antiferroelectric phase.     

The inversion phenomenon of the helical twist sense in antiferroelectric liquid crystal phase from electronic and vibrational circular dichroism

The investigation of the helical pitch and the helical twist sense for several liquid crystal compounds in antiferroelectric phase have been performed. Electronic circular dichroic (ECD) and vibrational circular dichroic (VCD) spectroscopies have proved the existence of unwound helical structure in antiferroelctric phase. Obtained results may confirm the assumption connected via the inversion phenomena in liquid crystalline chiral phase with the change of the concentration of different conformers promoting opposite handedness. Two examples of such conformers, obtained by conformational analysis, have been proposed.     

From colloids in liquid crystals to colloidal liquid crystals

ABSTRACT

Liquid crystals in combination with nanoparticles are a fascinating topic of research, because of the wealth of aspects and questions to study. These range from simple effects of nanoparticles on phase transitions and phase diagrams, to the tuning of physical properties, adding of novel functionalities, all the way to the formation of spontaneous order by nanoparticles themselves and the possibilities that templating has for future materials design and applications. This article intends to provide a flavour of the multiplicity, variety and diversity that these thermotropic and lyotropic systems have to offer in the area of materials development, which we believe will become increasingly important, especially for switchable non-display applications and nanotechnology. It is not intended to provide a conclusive overview, which would be a presumptuous attempt considering the limited space available, but rather to place our own work into a wider context and to point out some more recent developments and trends in liquid crystal – nanoparticle dispersions.     

The new high tilt mixtures with antiferroelectric phase at a broad temperature range and a long helical pitch

Five new bicomponent eutectic mixtures based on compounds different in the bridge between a rigid core and a chiral centre (-COO- or -CH₂O-) or in the optical purity (‘S’ or racemic ‘R,S’) were prepared and characterised. Their mesomorphic properties were tested by means of polarising optical microscopy and differential scanning calorimetry. The helical pitch of prepared mixtures was estimated with selective reflection method. The optical tilt angle Θ of mixtures was also measured and discussed.     

Switchable anti-peeping film for liquid crystal displays from polymer dispersed liquid crystals

Liquid crystals displays (LCDs) currently dominate the display market, wherein a wide viewing angle is considered as one of the most important characteristics. However, for LCDs with wide viewing angles, some private information inevitably becomes more visible; thus, an LCD with a switchable viewing angle has attracted greater interest. Here, we report a novel switchable viewing angle film that can make the viewing angle of an LCD electrically switchable between ±30° and ±60°, i.e. between an anti-peeping mode and a share mode, by 5.0 V is turned on and off, respectively. The response time necessary to change between the modes is in milliseconds. It is believed that it has potential applications in LCDs with high quality.     

Intercalated smectic a phases in banana-shaped liquid crystals with carbonate end groups.

Novel, symmetrical, bent-core mesogens, namely substituted 1,3-phenylenebis{4'-[(ethoxycarbonyl)oxy]-1,1'-biphenyl}-4-carboxylates, are prepared with carbonate groups at both ends of the molecules. The mesophase properties are investigated for different electron-donating and -withdrawing substituents connected to the central ring at different positions. These compounds exhibit nematic and/or smectic A intercalated (SmA(int)) mesophases. Electro-optical studies show a very significant electric-field-induced biaxiality that is fast (<1 ms) and large (0.03) associated with a high birefringence (0.33 at 500 nm, 10 V mum(-1) field) for the unsubstituted analogue. For one substance this effect is observable from above 100 degrees C down to room temperature. Such electric-field-induced biaxiality may find practical applications in fast electro-optic modulation devices.     

Characterization of a new chiral antiferroelectric liquid crystal with a lateral bromo substituent

In this paper we present the characterization of a new chiral mesogen with a lateral bromo substituent. The identification of the various smectic phases is achieved by texture observation, miscibility studies and DSC. The optic and electro-optic properties are also investigated. The real part of the complex dielectric permittivity as a function of the temperature was also measured in the planar geometry for thin samples. The dielectric behaviour will be qualitatively discussed on the simple assumption of different anchorings. These results point out the importance of anchoring conditions in relation to the macroscopic properties observed in very thin cells of antiferroelectric liquid crystals.