Phases Of Antiferroelectric Liquid Crystal Research Articles

New examples of ferroelectric nematic materials showing evidence for the antiferroelectric smectic-Z phase

We present a new ferroelectric nematic material, 4-((4′-((trans)-5-ethyloxan-2-yl)-2′,3,5,6′-tetrafluoro-[1,1′-biphenyl]-4-yl)difluoromethoxy)-2,6-difluorobenzonitrile (AUUQU-2-N) and its higher homologues, the molecular structures of which include fluorinated building blocks, an oxane ring, and a terminal cyano group, all contributing to a large molecular dipole moment of about 12.5 D. We observed that AUUQU-2-N has three distinct liquid crystal phases, two of which were found to be polar phases with a spontaneous electric polarization Ps of up to 6 µC cm–2. The highest temperature phase is a common enantiotropic nematic (N) exhibiting only field-induced polarization. The lowest-temperature, monotropic phase proved to be a new example of the ferroelectric nematic phase (NF), evidenced by a single-peak polarization reversal current response, a giant imaginary dielectric permittivity on the order of 103, and the absence of any smectic layer X-ray diffraction peaks. The ordinary nematic phase N and the ferroelectric nematic phase NF are separated by an antiferroelectric liquid crystal phase which has low permittivity and a polarization reversal current exhibiting a characteristic double-peak response. In the polarizing light microscope, this antiferroelectric phase shows characteristic zig-zag defects, evidence of a layered structure. These observations suggest that this is another example of the recently discovered smectic ZA (SmZA) phase, having smectic layers with the molecular director parallel to the layer planes. The diffraction peaks from the smectic layering have not been observed to date but detailed 2D X-ray studies indicate the presence of additional short-range structures including smectic C-type correlations in all three phases—N, SmZA and NF—which may shed new light on the understanding of polar and antipolar order in these phases.

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

ABSTRACTThe 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.

PA02 Gradual Transition from Ferroelectric to Antiferroelectric Liquid Crystal Phase for Binary Mixture System

PA02 Gradual Transition from Ferroelectric to Antiferroelectric Liquid Crystal Phase for Binary Mixture System

Enhanced photoluminescence of antiferroelectric liquid crystals doped with sodium titanate nanoparticles

We have observed the enhancement in photoluminescence (PL) intensity of the antiferroelectric liquid crystals (AFLC) doped with Sodium titanate nanoparticles (NPs). The enhancement in PL intensity of AFLC strongly depends on the doping concentration of NPs. The Sodium titanate NPs were prepared such that the emission from NPs together with AFLC molecules gives enhanced PL intensity. On the other hand, smectic CA* phase of AFLC provides strong light scattering and hence incident photons get scattered to excite more NPs and give enhanced PL counts. We have also observed the shift in emission energy band of AFLC doped with NPs.

Non-linear dielectric effect in the isotropic phase of antiferroelectric liquid crystals

Abstract We have theoretically studied the non-linear dielectric effect in the isotropic phase of antiferroelectric liquid crystals on the basis of phenomenological theory. We find an analytical expression for the non-linear dielectric effect in the isotropic phase of the I – SmC A * transition. The temperature dependence of the non-linear dielectric effect is presented in the isotropic phase of the I – SmC A * transition.

Frequency dispersion of dielectric permittivity–voltage hysteresis during field driven ferroelectric-antiferroelectric-ferroelectric transition in antiferroelectric liquid crystal (SmCA*)

Dielectric permittivity at different polarizing states during field driven ferroelectric-ferroelectric (F-F) transition and the dielectric relaxation spectra in the antiferroelectric liquid crystal (AFLC) phase have been studied to elucidate the interaction processes involved during the transition. The unwinding process of antiferroelectric helix has been found to be governed by the polar coupling between the applied field and the antiferroelectric polarization vector followed by nonpolar interaction with the dielectric anisotropy of the tilted molecules. Frequency dependence of the permittivity-voltage hysteresis behavior observed during the F-F transitions is concluded to be due to different relaxation processes involved in the AFLC phase.

Electro-optical switching of a thin antiferroelectric liquid crystal film in the planar geometry with evaporated top electrode

An antiferroelectric liquid crystal phase formed by bent-shape molecules was investigated in a geometry typical of solid-state technology. A thin Langmuir film of the compound was transferred onto a conductive glass and supplied with a top electrode evaporated in vacuum. Upon heating the sandwich keeps its integrity even in a high-temperature B2 phase which, despite strong confinement, manifests antiferroelectric and electrooptic properties similar to those in bulk samples. This approach fills the gap between investigations of solid and liquid crystal ferroelectrics and open new ways for studying dielectric, ferro- and antiferroelectric properties of thin and ultrathin films of liquid crystals.

On the temperature dependence of the tilt and spontaneous polarisation in high tilt antiferroelectric liquid crystalsElectronic supplementary information (ESI) available: derivation of the expression for the free energy expansion in the biaxial SmC* phase. See http://www.rsc.org/suppdata/jm/b3/b314747a/

The temperature-dependent physical properties of four related liquid crystalline materials are reported. The materials differ chemically only in the degree of fluorination of one of the core aromatic rings and all materials exhibit both chiral smectic-C (ferroelectric) and antiferroelectric liquid crystal phases. The steric tilt angles, calculated from the layer spacing, are relatively low, saturating at a maximum of 25°, while the optical tilt angles are much higher, achieving values of 42.5°. Most obviously, the tilt angle and spontaneous polarisation exhibit very different temperature dependencies, the former appearing to be discontinuous at the orthogonal to tilted transition, while the latter follows a simple power law. It is noted that this behaviour has been observed in other similar materials, but that it is not in agreement with the simple coupling between spontaneous polarisation and tilt that is often assumed in ferroelectric liquid crystals. A theoretical explanation is given for the different temperature dependence of these two physical quantities, suggesting that the biaxial order parameter, not directly determined by the tilt angle, is important in these systems.

On the phase sequence of antiferroelectric liquid crystals and its relation to orientational and translational order

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this material and, in particular, about the presence or absence of SmC* in its phase sequence. By means of dielectric spectroscopy and polarizing microscopy, we show that whereas the pure compound lacks the SmC* phase, this phase rapidly replaces the SmC*β subphase through the reduced purity resulting from temperature-induced chemical degradation which is hard to avoid under standard experimental conditions. X-ray investigations furthermore show that this change in phase sequence is coupled to a decrease in translational order. This explains the large variations in the reported phase sequence and electro-optic behaviour of MHPOBC, in particular concerning the SmC*β phase which has been said to exhibit ferro-, ferri- as well as antiferroelectric properties. It is likely that the sensitivity of the AFLC phase sequence to sample purity is a general property of AFLC materials. We discuss the importance of optical and chemical purity as well as tilt and spontaneous polarization for the observed phase sequence and propose that one of the key features determining the existence of the different tilted structures is the antagonism between orientational (nematic) and translational (smectic) order. The decreased smectic order (increased layer interdigitation) imposed by chemical impurities promotes the synclinic SmC* phase at the cost of the AFLC phases SmC*α, SmC*β, SmC*γ and SmC*a. We also propose that the SmA* phase in FLC and AFLC materials may actually have a somewhat different character and, depending on its microstructure, some of the tilted phases can be expected to appear or not to appear in the phase sequence. AFLC materials exhibiting a direct SmA* -SmC*a transition are found to be typical ‘de Vries smectics’, with very high orientational disorder in the SmA* phase. Finally, we discuss the fact that SmC*β and SmC*γ have two superposed helical superstructures and explain the observation that the handedness of the large scale helix may very well change sign, while the handedness on the unit cell level is preserved.

Short range order in the isotropic phase of antiferroelectric liquid crystals

Optical activity measurements in the isotropic phase of two antiferroelectric liquid crystal systems in which the chirality can be varied reveal unusual behaviour of the short range order. In one system the phase sequence as the chirality is increased is smectic A, smectic C* A, and smectic Q. In the other system the phase sequence is smectic C*, smectic C* A, and smectic Q as the chirality is increased. The short range order of the isotropic phase behaves similarly for these systems, showing mean field behaviour at low chirality and far above the phase transition, but deviating from this behaviour significantly as the chirality is increased and the phase transition is approached. These optical activity results indicate how different is the short range order in the isotropic phase for these antiferroelectric liquid crystal systems and demonstrates the crucial role played by chirality. Past theoretical work that includes smecticlike fluctuations in the calculation of short range order in the isotropic phase is capable of qualitatively explaining these results.

Polymer-Dispersed Ferroelectric and Antiferroelectric Liquid Crystals

PDLC structures containing ferroelectric and antiferroelectric liquid crystal phase of the same material have been obtained. The initial studies of electrooptical properties of those materials have been done.

Field-induced phase transitions in an antiferroelectric liquid crystal using the pyroelectric effect

The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical polar phases. Measurements of pyroelectric response of these phases as a function of temperature and bias field have elucidated the ability of this method for investigating the nature of antiferroelectric phases and phase transitions under the bias field. The pyroelectric signal as a function of the bias field at fixed temperatures and as a function of temperature for fixed bias fields was measured for different phases of the investigated AFLC material. A theoretical model describing the pyroelectric response in different phases of AFLC is given, and the experimental results are interpreted. The threshold fields for field induced phase transitions are determined. The type of field induced phase transition from the AF phase in particular is found to be dependent on the temperature within its range. The properties of an unusual ferrielectric phase existing between ferrielectric chiral smectic-C (SmC*) and antiferroelectric AF phases are studied in a great detail. The results confirm that this phase is one of the incommensurate phases, predicted by the axial next-nearest neighbor Ising model and Landau model for this temperature region.

Contribution of dechiralization lines to electro-optical response and dielectric permittivity in the SmC* phase

In the ferroelectric SmC* phase of antiferroelectric liquid crystal in planar geometry, a new contribution to the electro-optical and dielectric responses was found. This mode which we propose to be called a “surface layer mode'', appears with the apparition of 2π-disclinations (dechiralization lines) in the texture. Some evidences of the existence of this mode are given. A simple model, taking into account a transformation of a pair of π-disclination on the surface to one 2π-disclination in the bulk, is discussed.

Observation of an SmCα* phase in an antiferroelectric liquid crystal using pyroelectrics and dielectrics

The measurements of the pyroelectric and dielectric properties as functions of frequency, bias voltage and temperature in the region of the phase transition from the SmA to phases of antiferroelectric liquid crystal (AFLC) are presented. On the basis of the results of pyroelectric and dielectric response, the nature of the SmC α * phase in the AFLC material (R)-12OBP1M7 is being established.

Field induced phase transition in smectic Cβ*phase of antiferroelectric liquid crystals

Electric field dependence of complex dielectric constant and temperature dependence of switching current were measured in antiferroelectric liquid crystals for various optical purities. A two-step change in the dielectric constant of bias field and current peaks representing SmC α* phase were observed in high optical purity MHPOBC. A molecular orientational model to explain results of the two-step change is proposed. The first and the second decrease in the dielectric constant correspond to the reorientation to the ferroelectric and antiferroelectric orderings, respectively, and the two-step change in dielectric constant for electric field is related to the generation of antiferroelectric ordering in SmC β* phase.

On the seemingly antiferroelectric behaviour of cfrtain ferroelectric liquid crystals

Abstract Double current peak, double hysteresis loop and even in some sense three electro-optic states, the main characteristics of the antiferroelectric liquid crystal phase, are found also in the ferroelectric smectic C* phase, observed so far on several liquid crystal materials possessing a C*—N* phase sequence. In the observed cases, the confining surfaces bring about a pronounced monostable surface anchoring of the liquid crystal molecules forcing the layers to be broken in two alternating directions, corresponding to the equivalent surface directions of positive and negative molecular tilt relative to the layer normal. The resulting striped texture is very reminiscent of certain domain patterns in solid ferroelectrics. The seemingly anomalous electric switching behaviour of this C* texture mimics an antiferroelectric and may thus easily lead to an incorrect designation of the phase if not supplemented by other identification methods. Hysteresis shape and switching current peaks are thus by themselve...

Erratum: Orientation of alkyl chains and hindered rotation of carbonyl groups in the smectic-C* phase of antiferroelectric liquid crystals studied by polarized Fourier transform infrared spectroscopy

Erratum: Orientation of alkyl chains and hindered rotation of carbonyl groups in the smectic-C* phase of antiferroelectric liquid crystals studied by polarized Fourier transform infrared spectroscopy

The molecular aspect of the double absorption peak in the dielectric spectrum of the antiferroelectric liquid crystal phase

Abstract The dielectric spectrum of the antiferroelectric smectic C∗ phase exhibits a low and a high frequency absorption peak (Pl, Ph) which have been studied as a function of temperature and bias electric field. Measurements from 10 Hz to 10 MHz were carried out with smectic layers parallel and quasi-perpendicular to the cell plates for the multicomponent mixture CS-4000 (Chisso). In addition to the orthogonal smectic A∗ phase, this material has four tilted phases, three narrow phases with a dielectric behaviour permitting us to classify two of them as C∗α (82·80° to 81·91°C) and C∗γ (80·10° to 79.17°C), and one broad antiferroelectric phase (79.17° to −10°C). On applying an increasing bias field, Δ∊ for both processes first increases by about a factor of two, then exhibits a maximum at a threshold field E c corresponding to the antiferroelectric-ferroelectric transition at which it decreases by almost one order of magnitude. In fact, at E c the PH peak vanishes and the PL peak shows up at a frequency s...

Free-standing films of ferroelectric and antiferroelectric liquid crystals

Abstract We give a short review of the experimental behavior of ferroelectric and antiferro-electric liquid crystal phases in free-standing films with emphasis on the smectic-A - ferroelectric smectic-C and ferroelectric smectic-C - antiferroelectric smectic-CA phase transitions.