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.

Collective molecular relaxations and electro-optical switching response of a wide room temperature antiferroelectric liquid crystal mixture

Antiferroelectric liquid crystals (AFLCs) are advanced smart materials for displays with unique properties of tristate switching. Thermodynamic, dielectric and switching characteristics of a room temperature bicomponent AFLC mixture at eutectic composition was investigated in the present work. The emergence of various phases in the eutectic composition was identified on the basis of observed dielectric relaxation modes and other parameters. Critical electric field (Ec) to unwind the helix, spontaneous polarization (Ps), torsional viscosity (γt) and switching time (ts) have been determined in the ferroelectric (SmC*) and the antiferroelectric (SmCA*) phases from switching experiments. Spontaneous polarization follows the equation, Ps = Po(1-T/Tc)γ in the temperature range of tilted chiral smectic phases, with values of Tc = 91.8 ± 0.1 °C and γ = 0.410 ± 0.005. Value of γ indicates that the SmA* to SmC* transition is of the first order as observed by the enthalpy measurements from the thermodynamic studies. In the AFLC phase of interest, ts is ∼2–3 ms and Ps∼200 nC/cm2. These parameters suggest that the mixture is appropriate base material for electro-optical applications.

Influence of Ionic Conductivity in Polymer Dispersed Antiferroelectric Liquid Crystals (PDAFLC): A Comparison of Critical Fields

Theoretically critical unwinding field was studied using Landau free energy model of three different geometrical configuration of antiferroelectric liquid crystals. In this study, we considered the influence of ionic conductivity both in pure antiferroelectric liquid crystals and polymer dispersed antiferroelectric liquid crystals. We observed that the modified elastic constant which accounted for the interaction of ionic conductivity and polymer cross-linked chains is very much responsible for the variation of critical field.

Frequency response of dielectric relaxation modes in a room temperature antiferroelectric phase liquid crystalline mixture

Thermodynamic, optical, and electrical characterization of a newly formulated mixture of chiral mesogenic molecules has been carried out. The material exhibits different mesophases like paraelectric smectic (SmA*), ferroelectric chiral smectic (SmC*) with synclinic structure, and wide temperature range antiferroelectric chiral smectic (SmCa*) with anticlinic structure. These studies show that at room temperature, the SmCa* phase is present in a wide temperature range of ∼128 °C (from −35.0 °C to 92.6 °C) followed by the SmC* phase (from 92.6 °C to 110.8 °C) as well as SmA* phase (from 110.8 °C to 113.9 °C). Besides the wide temperature range, the SmCa* phase has also been optimized for high helical pitch length, which is useful for helix unwound cells through surface stabilization for application. Frequency-dependent dielectric parameters have been determined for the material in the frequency range of 1 Hz to 40 MHz. The dielectric spectroscopy shows six different relaxation modes in various phases due to the collective and individual molecular behavior. The SmA* phase exhibits soft mode relaxation associated with variations in the amplitude of the tilt angle. The identified soft mode is significantly temperature-dependent. Using the Curie-Weiss law, the transition temperature for the paraelectric SmA* phase to the ferroelectric SmC* phase (Tc) is found to be 111.4 °C. Two relaxation modes in the SmC* phase have been identified Goldstone mode and a domain mode. These modes have extremely high dielectric strength resulting in high permittivity in comparison with other modes. Three modes of relaxation have been detected in the antiferroelectric SmCa* phase. Different characteristic parameters of these modes are strongly temperature-dependent. This study signifies that the present room-temperature antiferroelectric liquid crystals may be a potential candidate for display applications.

A Review of Developments in Polymer Stabilized Liquid Crystals.

Polymer-stabilized liquid crystals (PSLCs) are multi-functional materials consisting of polymer networks in a continuous phase of liquid crystals (LCs), of which polymer networks provide anchoring energy to align the LCs. A number of improvements are detailed, including polymer-stabilized nematic liquid crystals (PSNLCs), polymer-stabilized cholesteric liquid crystals (PSCLCs), polymer-stabilized blue phase liquid crystals (PSBPLCs), polymer-stabilized smectic liquid crystals (PSSLCs), polymer-stabilized ferroelectric liquid crystals (PSFLCs), and polymer-stabilized antiferroelectric liquid crystals (PSAFLCs) in this review. Polymer stabilization has achieved multiple functionalities for LCs; in smart windows, a sufficiently strong electric field allows the LCs to reorient and enables switching from a scattering (transparent) state to a transparent (scattering) state. For broadband reflectors, the reflection bandwidth of LCs is manually tuned by electric fields, light, magnetic fields, or temperature. PSBPLCs open a new way for next-generation displays, spatial light modulators, sensors, lasers, lenses, and photonics applications. Polymer networks in PSFLCs or PSAFLCs enhance their grayscale memories utilized in flexible displays and energy-saving smart cards. At the end, the remaining challenges and research opportunities of PSLCs are discussed.

Effect of doping ferroelectric BaTiO3 nanoparticles on dielectric, electro-optical, and ionic properties of antiferroelectric bent-core liquid crystal

Nano-doping in polar bent-core liquid crystals is the topic of current interest for designing multifunctional smart materials. Here, we study the effect of doping of two different-sized (5–15 nm and 50–70 nm) ferroelectric BaTiO3 nanoparticles on the dielectric, electro-optical and ionic properties of antiferroelectric smectic A bent-core liquid crystal. Doping with both the sizes of nanoparticles results in lowering of the transition temperature of the liquid crystal mesophases. Both, the pure as well as doped samples show a soft mode in their antiferroelectric smectic A phase. In comparison to pure liquid crystal, the dielectric strength of the soft mode in doped samples is decreased, while their relaxation frequencies show an increasing behavior. Additionally, a decrement has been observed in the spontaneous polarization of the doped samples. A drop in polarization as well as soft mode dielectric strength values in doped samples, simultaneously point to an antiparallel dipole–dipole correlation between the ferroelectric nanoparticles and antiferroelectric liquid crystal molecules. Higher values of molecular mode relaxation frequencies obtained in nano-doped samples indicate a decrement in viscosity and dilution of pure liquid crystal. An important electro-optical parameter, the response time shows a remarkable decrement for the higher size (50–70 nm) nanoparticles doped sample. An enhancement in direct current conductivity and ion diffusion coefficient is observed for both the nano-doped samples. The smaller size (5–15 nm) ferroelectric nanoparticles reduce the mobile ion concentration and act as ion-capturing agents for the liquid crystal. A systematic investigation of the nano-doped bent-core liquid crystal can catalyze the use of the combined media for faster switching electro-optic applications.

High tilted antiferroelectric liquid crystals: Polymer-based approach for phase stabilisation and device development

On doping orthoconic antiferroelectric liquid crystal (OAFLC) mixtures with a small concentration of reactive mesogens, new liquid crystalline material was engineered with improved opto-electronic performance. In-situ photo polymerisation of the mixtures resulted in the creation of a polymer network that helped removal of the metastability bought on by F-states (Synclinic Ferroelectric State) in tristate switching devices. These metastable states causes the switching timings of the device to be asymmetrical, with the fall time being significantly longer than the rise time due to the difficulty in relaxing from the Ferroelectric-synclinic to the Antiferroelectric-anticlinic state. The opto-electronic parameters of the device cells were measured before and after the mixtures were stabilised by Ultraviolet light (UV) polymerisation. Polymer stabilisation of the orthoconic state yielded symmetric optical rise and fall times, accompanied by a reduction in the electrical response times. Spontaneous polarisation in the OAFLC mixtures were found to decrease after UV polymerisation; however, this is an important trade-off vis-à-vis symmetric optical response times, considering the overall performance of the material from an application standpoint.

Electro-Optic Effect of Laser Photobleaching on Viscoelastic Properties of Chiral Liquid Crystals

Viscoelastic properties are one of the most fundamental properties of chiral liquid crystals. In general, their determination is not a straightforward task. The main problem is the multitude of physical parameters needed to determine the value of the elasticity and viscosity constants. It is also necessary to consider the character of a respective phase. This problem is particularly important in the case of chiral phases such as ferroelectric and antiferroelectric phases or in the blue phases. There are several experimental methods to measure viscosity and elasticity constants in chiral phases. These methods use various phenomena to detect deformation, e.g., light transmission, polarization current, light modulation, dielectric constant and helix deformation or helix unwinding. Commonly, an external electric field is used to induce deformation, the homogeneity of which inside the cell is essential. This study is focused on the analysis of the effect of laser photobleaching on the electro-optic properties of the antiferroelectric liquid crystal and on the homogeneity of the electric field. The results obtained by confocal microscopy as a function of the cell depth are presented. The influence of the stabilization procedure of the isolated region performed by controlled laser photobleaching on the electro-optic properties has been studied. The observation was conducted using a polarizing microscope, and numerical analysis of two-dimensional colored textures was performed. The obtained results suggest that laser photobleaching can produce an anchoring effect, which has a positive effect on the electro-optic properties of antiferroelectric liquid crystal.

Impact of carbon quantum dots on self-assembly and dielectric relaxation modes of a room temperature tri-component fluorinated antiferroelectric liquid crystal mixture.

We investigated the impact of carbon quantum dots (CQDs) of sizes ∼2-5 nm in a room temperature tri-component fluorinated antiferroelectric liquid crystal (AFLC) mixture. The synthesised CQDs have been characterised by high-resolution transmission electron microscopy and X-ray diffraction spectroscopy. An increase in the transition temperature and enthalpy to the isotropic liquid phase indicates stabilization of the AFLC mixture in the presence of CQDs. The dielectric studies have been carried out in the frequency range of 1 Hz-40 MHz under the planar anchoring conditions of the molecules. An appreciable increase in the permittivity, dielectric strength, and conductivity has been observed owing to the existence of sp2/sp3 hybridization in the CQDs which form a strong coupling and develop a dipolar ordering in the systems. Various relaxation frequencies were increased with the incarceration of CQDs in the AFLC mixture. Overall, the different studies suggest that the doped CQDs are very well settled in between the host molecules without disturbing the molecular ordering of the hosts. Such results are encouraging and reveal the potential applicability of the CQD doped systems to produce highly efficient tuneable optical devices and other multifaceted applications.

The comparison of self-assembling behaviour of phenyl biphenylcarboxylate and biphenyl benzoate compounds with the different length and shape of chiral terminal chain

Our recent advances on polymer-stabilised orthoconic antiferroelectric liquid crystals (PSOAFLCs) eliminated the asymmetry of switching times in electro-optical effect based on surface stabilized OAFLCs and paved the way for their effective use in many photonic and optoelectronic devices requiring very fast switching. To fabricate new modifications of PSOAFLCs, besides bi- and tetrafunctional monomers, multicomponent AFLCs mixtures with a very low meting point, high stability of the antiferroelectric phase at lower temperatures and higher helical pitch length are desirable. Herein, a very convenient synthetic method of new AFLC compounds with a phenyl biphenylcarboxylate structure of the rigid core with different length and shape of terminal chains is reported. The resulting mesomorphic properties and helical parameters are compared with those of analogues with a biphenyl benzoate structure of the rigid core. The designed AFLC compounds clearly possess definite advantages. Specifically, they exhibit lower meting points, higher clearing points, broader temperature range of the antiferroelectric phase (SmCA*) and much longer helical pitch length, especially for analogues with the shortest achiral terminal chain, but unexpectedly lower tendency to give the direct transition between the antiferroelectric and isotropic phase. The introduction of the cycloalkyl ring at the end of the chiral terminal chain increases the melting points, flattens the temperature dependence of the helical pitch and changes the sense of the helical twist, but also enhances the ability of the direct SmCA*-Iso phase transition, regardless the type of the rigid core. The obtained results highlight a possibility of utilizing compounds with the phenyl biphenylcarboxylate structure of the rigid core, designed under this work, together with previously prepared compounds with the biphenyl benzoate rigid core to formulate multicomponent OALFC mixtures with a very low melting point and a very high helical pitch length.

A wide range room temperature antiferroelectric liquid crystal mixture with moderate spontaneous polarization and sub-millisecond switching time

A room-temperature antiferroelectric liquid crystal (AFLC) mixture has been prepared by doping two chiral fluorinated compounds into a four-component achiral phenyl pyrimidine-based host matrix. The suitability of the mixture for practical applications has been tested by optical polarizing microscopy, synchrotron X-ray diffraction, frequency-dependent dielectric, and electro-optic studies. The mixture possesses an antiferroelectric phase over a wide temperature range (nearly 82 °C) followed by a paraelectric phase for 13.7 °C. It is found to display very fast switching between 243 and 45 μs at the driving voltage of 3.2 V/μm in the SmC*A phase. At 20 °C, the mixture exhibits a moderately high spontaneous polarization (137 nC/cm2), low viscosity (1.01 Pa s) and fast switching (240μs). The formulated mixture is thus expected to be suitable for use in fast-response AFLC-based display devices.

Thermal stabilization of the smectic-Cα* phase by doping with photo-active reactive mesogen

• The incorporation of monomers into liquid crystals changes their physical properties. • Dielectric properties and switching behaviour of liquid crystal molecules depend on the monomer concentration. • A new method for thermal stabilization of the smectic-Ca* phase is presented. • The admixture of monomer causes a relatively small decrease in spontaneous polarization of the liquid crystal system. The effect of a photo-active reactive mesogen 4-(3-acryloyloxypropyloxy)-benzoesure2-methyl-1,4-phenylester on an antiferroelectric liquid crystal was investigated in the context of possible broadening of the temperature range in which the smectic-C α * phase exists in newly synthesized hybrid materials promising for optical applications. In the mixtures of various reactive mesogen concentrations from 1 to 9 wt. %, dielectric spectroscopy and reversal current method were used to determine the temperature range and switching time of the smectic-C α * phase. As a result of reactive mesogen doping, the thermal stabilization of the smectic-C α * phase over 21.5 °C, around a room temperature, i.e. between 26.5 and 48 °C, was confirmed in the modified liquid crystal system.

Gamma-radiation exposure-induced modifications in the thermal, dielectric, and electro-optical properties of two-room temperature antiferroelectric liquid crystal mixtures

ABSTRACT The influence of different doses of gamma radiation on the phase transition temperatures, permittivity, conductivity, tilt angle, anchoring energy coefficients, spontaneous polarisation, rotational viscosity, and switching time of two-room temperature antiferroelectric liquid crystal (AFLC) mixtures has been investigated. At low radiation exposure, phase transition temperatures and optical tilt angles of the systems decrease slightly, but the effect is significant at high radiation doses. The permittivities as well as the ac and dc conductivities of the mixtures were significantly altered by γ-irradiation. Anchoring energy coefficients, spontaneous polarisation, rotational viscosity and switching time of the mixtures are shown to have a similar type of behaviour in both mixtures at low radiation doses, but their behaviour becomes random at higher levels of γ-irradiation.

Nanocomposites Based on Antiferroelectric Liquid Crystal (S)-MHPOBC Doping with Au Nanoparticles.

Modification of the physical properties of the (S)-MHPOBC antiferroelectric liquid crystal (AFLC) by doping with low concentrations of gold nanoparticles is presented for the first time. We used several complementary experimental methods to determine the effect of Au nanoparticles on AFLC in the metal–organic composites. It was found that the dopant inhibits the matrix crystallization process and modifies the phase transitions temperatures and switching time, as well as increases the helical pitch and spontaneous polarization, while the tilt angle slightly changes. We also showed that both the LC matrix and Au nanoparticles show strong fluorescence in the green light range, and the contact angle depends on the temperature and dopant concentration.

Twist elastic constant in the SmC* phase of an antiferroelectric liquid crystal with and without photo-polymer networks.

Chiral liquid crystalline phases with pitches that are comparable to visible wavelengths, all Bragg-reflect visible light. We measured Bragg reflections from the chiral SmC* phase of the antiferroelectric liquid crystal (AFLC), AS612, and explored how applied electric fields and polymer networks modify these reflections. From the critical field that unwinds the helix and destroys Bragg reflections, the twist elastic constant of the SmC* phase is determined. We estimate from this result, the elastic constant of polymer network, which when introduced into the liquid crystal matrix, fixes the helical pitch (and hence, Bragg peaks). These parameters are crucial for designing both FLC and AFLC displays since networks enhance the mechanical properties of the liquid crystal host while not affecting their optical properties.

Phase transition properties of ferroelectric and antiferroelectric liquid crystals

ABSTRACT Thermodynamic models based on the Landau free-energy density are popular and well suited to study the phase transition properties of ferroelectric and antiferroelectric liquid crystals. Ferroelectric and antiferroelectric liquid crystals have molecular chirality which play an important role in the technology of liquid crystals for the application of devices performance. The structural properties of ferroelectric phase transitions which give rise to the macroscopic tensors coupled to the macroscopic fields, the parameters describing the antiferroelectric phase transitions are still under investigation due to the lack of precise symmetrical properties characterising their existence. The study of ferroelectric and antiferroelectric liquid crystals has been substantial experimental strides. In theoretical approach, there has been relatively inappreciable work on this fascinating class of material. The present review paper focused on a comprehensive overview of the current scenario of the phase transition properties of ferroelectric and antiferroelectric liquid crystals for the fundamental and technological applications. The Landau Ginzburg theory of the phase transitions between ferroelectric and antiferroelectric liquid mesophases is investigated. The important experimental and theoretical developments of ferroelectric and antiferroelectric mesogens are also summarised.

Switching in a Biaxial Smectic A - like Phase

ABSTRACT Comments and the work presented in this paper aim at initiating discussion within the liquid crystalline community about the nature of different biaxial phases and their switching mechanisms. The mechanisms have the potential for applications in switching devices for displays and photonics. Claire Meyer et. al. interestingly report recently a fast Freedericksz-like transition of the secondary director in the biaxial Smectic Ab phase of a mixture of two compounds. Fast switching in liquid crystals occurs through a coupling of the biaxiality with the electric field. To realise this potential, one of the major goals of the liquid crystal community had been to discover biaxial nematic liquid crystals. In a biaxial liquid crystalline phase, the refractive indices along the three orthogonal axes are all different [n11≠ n22 ≠ n33] as opposed to these being different only along the two orthogonal directions n11 ≠ n22 = n33 in uniaxial nematics. In this notation 1, 2 and 3 are the three orthogonal axes that coincide with the major and the two minor directors. The biaxial nematic phase has been elusive for over half a Century. It has therefore proven to be almost impossible to meet the goal of achieving fast switching using biaxial nematics. Ferroelectric and antiferroelectric liquid crystals were discovered in 1975 and 1989 by R. Meyer in the USA and Atsuo Fukuda in Japan, respectively, for fast switching. These have been used in devices for niche but limited high-resolution applications as the devices using these liquid crystals suffered not only from achieving proper alignment at surfaces but also showed poor recovery of alignment following repeated cycles of switching.

Modification in different physical parameters of orthoconic antiferroelectric liquid crystal mixture via the dispersion of hexanethiol capped silver nanoparticles

Herein we investigate an orthoconic anti ferroelectric liquid crystal (OAFLC) material, W1000 and its mixtures with the dispersion of hexanethiol capped silver nanoparticles (Ag NPs) in three different concentrations of 0.1 wt%, 0.25 wt% and 0.50 wt%. The pristine AFLC and its mixtures are characterized by the dielectric, electro-optical and spectroscopic techniques. Various parameters viz. dielectric permittivity, electrical conductivity, spontaneous polarization, rotational viscosity, response time, anchoring energy coefficients, and photoluminescence of the pristine AFLC are altered due to the addition of Ag NPs. The polarizing optical micrographs show modification in the molecular distribution of the host AFLC after the addition of Ag NPs. The spontaneous polarization of the Ag NPs dispersed system was reduced due to the anti-parallel correlation amid dopant and AFLC dipoles as a consequence the dielectric permittivity of the Ag NPs-AFLC mixtures has also been decreased. The electrical conductivity of the pristine AFLC has increased after the dispersion of Ag NPs. Faster switching response of Ag NPs dispersed system comparing to that of the pristine AFLC is manifested by the decrement in the rotational viscosity. The anchoring energy coefficients were found to be reduced for the Ag NPs dispersed system. The photoluminescence (PL) intensity of the pristine AFLC material has been enhanced after the dispersion of Ag NPs which strongly depends upon the dopant concentration. Faster switching response, enhanced electrical conductivity and PL Intensity of the Ag NPs-AFLC mixtures are the remarkable findings of this study.

Computational, dielectric and electro-optical analysis of an orthoconic antiferroelectric mesogen having superstructure in its reduced symmetry phase

The study presents a comprehensive analysis of a novel octan-2-yl 4'-(4-{4-[3-(butanoyloxy)propoxy]benzoyloxy}benzoyloxy)-[1,1'-biphenyl]-4-carboxylate (abb. as OAFLC-108), an orthoconic antiferroelectric liquid crystal. Its paramorphic sequence is estimated from defect studies. Its optimised structure, interatomic charge distribution and dynamics, global parameters and nonlinear optical properties are calculated by density functional theory. Vibrational modes are determined experimentally and theoretically. Low-frequency dielectric spectra are studied in view of molecular fluctuations, activation energy and relaxation time. The moderate value (29.5 nC/cm2) of spontaneous polarisation and a response time of 2 ms support the candidature of OAFLC-108 as a competitive optoelectronic material.

Investigation of the tilt angle and spontaneous polarisation of antiferroelectric liquid crystals with a chiral centre based on (S)-(+)-3-octanol

The electro-optical properties (tilt angle of the director and spontaneous polarisation) of eight smectic liquid crystalline compounds with a chiral centre based on (S)-(+)-3-octanol and different substitution of the benzene ring and different length of the oligomethylene spacer (−(CH2)nO−) were studied. The compounds possess the self-assembling behaviour with an antiferroelectric ordering in smectic Ca* phase at a broad temperature range, some of materials have also a ferroelectric smectic C* phase and/or an orthogonal smectic A* phase. Due to high chemical stability and very high values of the optical tilt angle (40–44°) at saturation, the tested compounds can be used as chiral components for orthoconic antiferroelectric liquid crystalline mixtures.