Ferroelectric Smectic Liquid Crystals Research Articles

Ferroelectric Smectic Liquid Crystals

Since the discovery of the first ferroelectric liquid crystal (FLC) in the chiral smectic C (SmC*) phase, ferroelectricity in liquid crystals has attracted much attention due to not only the fundamental interest but also the applications. This review focuses on the evolution of the design concept for ferroelectric smectic liquid crystals. It progresses from considering macroscopic phase symmetry to designing intermolecular interactions. For the purpose of understanding the molecular organization in smectic phases, we propose a dynamic model of constituent molecules in the smectic A (SmA) and SmC* phases based on 13C NMR studies. Then, we follow the structure–property relationship in ferroelectric SmC* liquid crystals for FLC displays. We reconsider de Vries-like materials that can provide defect-free alignment. We pay attention to the electro-optical switching in the chiral de Vries smectic A phase. Finally, we show several liquid crystals exhibiting polar smectic A phases and discuss how the polar order occurs in the highest symmetric smectic A phase.

Ferroelectric Smectic Liquid Crystals as Electrocaloric Materials

The 1980s saw the development of ferroelectric chiral smectic C (SmC*) liquid crystals (FLCs) with a clear focus on their application in fast electro-optic devices. However, as the only known fluid ferroelectric materials, they also have potential in other applications, one of which is in heat-exchange devices based on the electrocaloric effect. In particular, ferroelectric liquid crystals can be both the electrocaloric material and the heat exchanging fluid in an electrocaloric device, significantly simplifying some of the design constraints associated with solid dielectrics. In this paper, we consider the electrocaloric potential of three SmC* ferroelectric liquid crystal systems, two of which are pure materials that exhibit ferroelectric, antiferroelectric, and intermediate phases and one that was developed as a room-temperature SmC* material for electro-optic applications. We report the field-induced temperature changes of these selected materials, measured indirectly using the Maxwell method. The maximum induced temperature change determined, 0.37 K, is currently record-breaking for an FLC and is sufficiently large to make these materials interesting candidates for the development for electrocaloric applications. Using the electrocaloric temperature change normalised as a function of electric field strength, as a function of merit, the performances of FLCs are compared with ferroelectric ceramics and polymers.

Solitons in surface stabilized ferroelectric liquid crystals and the determination of the twist elastic constant

Excitation of solitary waves and their propagation in surface-stabilized ferroelectric liquid crystal cells under alternating external electric field is investigated both theoretically and experimentally. The effect of solitary waves on electro-optic response spectra is analyzed for different amplitudes of applied fields, temperatures, and sample thicknesses. It is shown that solitons can only be excited within narrow ranges of frequencies of the sufficiently strong electric fields. The minimal frequency, at which soliton waves appear in ferroelectric smectic liquid crystals, is found to be related to the material constants of these systems. It is proved that measuring this threshold frequency gives the possibility to determine one of the material parameters, if the others are known. In this way, the intra-smectic-layer elastic constant is found for systems with the chevron geometry.

Ferroelectric Response and Induced Biaxiality in the Nematic Phase of Bent‐Core Mesogens

AbstractThe still undiscovered fluid ferroelectric nematic phase is expected to exhibit a much faster and easier response to an external electric field compared to conventional ferroelectric smectic liquid crystals; therefore, the discovery of such a phase could open new avenues in electro‐optic device technology. Here, experimental evidence of a ferroelectric response to a switching electric field in a low molar mass nematic liquid crystal is reported and connected with field‐induced biaxiality. The fluid is made of bent‐core polar molecules and is nematic over a range of 120 °C. Combining repolarization current measurements, electro‐optical characterizations, X‐ray diffraction and computer simulations, ferroelectric switching is demonstrated and it is concluded that the response is due to field‐induced reorganization of polar cybotactic groups within the nematic phase. This work represents significant progress toward the realization of ferroelectric fluids that can be aligned at command with a simple electric field.

Phase transitions in ferroelectric liquid crystals

Molecular chirality plays an important role in the science of liquid crystals, leading to cholesteric liquid crystal, blue phases, ferroelectric and antiferroelectric smectic phases and twist grain boundary phases. In all of these mesogens, chirality is an intrinsic property built into the chemical structure of mesogenic molecules. The study of ferroelectric liquid crystals has seen substantial experimental strides. In theoretical aspects, there has been relatively little basic work on this fascinating class of material. This review will try to present a comprehensive overview of the current status of the phase transitions in ferroelectric smectic liquid crystals. The article begins with a brief introduction about the symmetry and structure of ferroelectric mesophases. An attempt is made to identify a range of problems and related questions associated with the study of phase transitions. In the remaining parts of the article the important experimental and theoretical developments are summarized. Finally, some of the future directions have been identified.

V-shaped electro-optic response observed in a chiral ferroelectric smectic liquid crystal

We report on the observation of V-shaped switching in a ferroelectric liquid crystal cell over a wide range of temperatures. Results of the optical transmittance in the visible region give us the helical pitch for various temperatures of the ferroelectric liquid crystalline compound used. We show that the helical pitch, in addition to the spontaneous polarization (PS) and thickness of the alignment layer of the cell, is an important factor in giving V-shaped switching. A longer or helical compensated mixture gives a better V-shaped switching.

Electric-field-induced unwinding of ferroelectric helix in thin smectic C* layers with soft and rigid anchoring of molecules

The unwinding of a helical structure in thin films of a ferroelectric smectic liquid crystal (LC) by an external electric field has been theoretically studied using a discrete model in which every LC layer is characterized by a two-dimensional vector ξ i (describing the orientation of molecules) and by the polarization P i . It is established that the unwinding of the LC helix in thin films significantly differs from the well-known behavior of thick samples. In particular, discrete intermediate states (differing by an integer or half-integer number of turns) are formed in thin films for both weak and strong anchoring of molecules to a substrate surface. The physical factor responsible for this behavior is the presence of near-surface regions with thicknesses below the helix pitch and the corresponding uncompensated polarization.

Electromigration of microspheres in ferroelectric smectic liquid crystals

When an electric field is applied to microspheres which are dispersed in a ferroelectric smectic liquid crystal, particle translation along the smectic layer plane, i.e., in a direction nearly perpendicular to that of the director, can be observed. Under certain electric field conditions the translation is shown to be linear in time. We have determined the stability regime of linear particle displacement in the parameter space of amplitude and frequency for various applied wave forms. This regime enlarges for increasing electric field amplitude and frequency, with a threshold behavior observed for small parameters. The upper stability boundary is related to the reciprocal ferroelectric switching time. The microspheres translational velocity is independent of the applied electric field amplitude, but increases linearly with applied frequency. The microsphere velocity also increases with increasing temperature, which is indicative of the respective decrease in liquid crystal viscosity. Possible mechanisms of electric-field-induced particle motion are discussed.

Dynamics of the molecular orientation field coupled to ions in two-dimensional ferroelectric liquid crystals

Molecular orientation fluctuations in ferroelectric smectic liquid crystals produce space charges, due to the divergence of the spontaneous polarization. These space charges interact with mobile ions, so that one must consider the coupled dynamics of the orientation and ionic degrees of freedom. Previous theory and light scattering experiments on thin free-standing films of ferroelectric liquid crystals have not included this coupling, possibly invalidating their quantitative conclusions. We consider the most important case of very slow ionic dynamics, compared to rapid orientational fluctuations, and focus on the use of a short electric field pulse to quench orientational fluctuations. We find that the resulting change in scattered light intensity must include a term due to the quasistatic ionic configuration, which has previously been ignored. In addition to developing the general theory, we present a simple model to demonstrate the role of this added term.

Evidence for de Vries structure in a smectic-Aliquid crystal observed by polarized Raman scattering

The second- and fourth-order apparent orientational order parameters of the core part of the molecule P2 (app) and P4 (app) , have been measured by polarized vibrational Raman spectroscopy for a homogeneously aligned ferroelectric smectic liquid crystal with three dimethyl siloxane groups in the achiral terminal chain, which shows de Vries-type phenomena, i.e., very large electroclinic effect in the smectic- A (Sm-A) phase and a negligible layer contraction at the phase transition between the Sm-A and Sm- C(*) phases. The apparent orientational order parameters of the rigid core part of the molecule are extremely small both with and without the external electric field in Sm-A . These results provide evidence for the existence of the de Vries Sm-A phase, where the local molecular director is tilted at a large angle.

Viscoelastic director rotation of a low molecular mass liquid crystal

We show evidence for a viscoelastic rotation of the spontaneous polarization in a low molecular mass chiral ferroelectric smectic liquid crystal (SmC*). A weak elastic modulusw of ~102 Pa is sufficient to cause a unique effect: the polarization turns perpendicular to sufficiently strong a.c. electric fields. Viscoelasticity is unusual for low molecular mass materials. We suggest that it is connected to the anisotropy of the positional order. This is an inherent property of liquid crystals, but has never been taken into account in macroscopic descriptions.

Potential curves of two-molecule interactions in some smectic and ferroelectric liquid crystals

The atom-atomic potential method is used to study the potential curve of pairwise intermolecular interactions in smectic crystals C [C5H11O-(OH)C6H3-CH=N-C6H4-C5H11 (A), C10H21O-C6H4-CH=CH-C6H4-OC10H21 (B)], ferroelectric smectic liquid crystals C* [C7H15O-C6H4-C6H4-COOCH2C*H(CH3)C2H5 (C), C8H17O-C6H4-C6H4-C2H4C*H(CH3)C2H5 (D)], and binary systems A+C and A+D. Both inter- and intramolecular parameters as well as their mutual effects are taken into consideration. The intermolecular interactions are shown to significantly affect the conformations of isolated molecules. For the CC, DD, and AD interactions, the antiparallel ⇅ packing of molecules is found to be most favorable. In the case of the AA and AC interactions, the ⇈ and ⇅ packings are equally favorable in terms of energy (energy difference Upair does not exceed 0.8 kcal/mole). Despite the structural similarity between the chiral molecules C and D, the presence of the highly polar groups -O- , OH, CH=N, and COO leads to pronounced changes in the position and orientation of molecules in their packing in the AC and AD pairs. The pair interaction energy Upair near the minimum point was studied as a function of the shift of molecules relative to each other along the long axis X and of the angle 9 between the long molecular axes. The curve of U(pair) vs the shift along X has a few deep local extrema. The possibility of formation of energetically favorable molecular associates (dimers) in the compounds is demonstrated. The associates are stabilized by dipole-dipole intermolecular interactions.

Supermolecular stereochemistry in ferroelectric liquid crystals

Until recently, it was an empirical fact that chiral liquid crystal phases were produced only from enantiomerically enriched liquid crystal materials. Ferroelectric smectic liquid crystals have always been composed of enantiomerically enriched molecules as well. Here we describe the first example of the formation of chiral supermolecular liquid crystalline structures from achiral bent-core molecules (bow phases or banana phases). In one case, a metastable antiferroelectric bow-phase structure is macroscopically chiral, with bulk samples composed of chiral macroscopic domains of either handedness. This system represents the first known liquid conglomerate. In addition, based upon a directed design approach exploiting control of interlayer clinicity, a ferroelectric liquid crystal conglomerate has been created from a racemic mixture. In this case bulk ferroelectricity is obtained by spontaneous polar symmetry breaking. Copyright © 2000 John Wiley & Sons, Ltd.

The director structure and electroclinic fast optical switching in a grating-coupled ferroelectric smectic liquid crystal cell

Optical excitation of guided modes in a liquid crystal layer using grating-coupling gives sharp features in the angle-dependent reflectivity data. These features are strengthened by using a metallized grating to enhance coupling to the guided modes in the liquid crystal. In the present study the liquid crystal has a smectic A phase which exhibits fast electroclinic switching. Combining the sharp features in the reflectivity together with the electroclinic effect leads to fast, high contrast, optical switching which may open up potential for novel device structures.

Properties of Liquid Crystals in Photonic Crystal, Synthetic Opal

Liquid crystals such as nematic liquid crystals and ferroelectric smectic liquid crystals are well infiltrated in photonic crystal, synthetic opal. Dielectric and optical properties of liquid crystals have been found to exhibit remarkable change upon infiltration in the opal. Diffraction and transmission of light through opals have been found to change upon infiltration of liquid crystal in opals and also upon field application. Relaxation of dielectric response in liquid crystals infiltrated in opal shifted to higher frequency and its temperature dependence has also changed. Electro-optic effect of the opal infiltrated with liquid crystals have been also discussed. These results have been discussed in terms of interaction of liquid crystal molecule in nano-space and the inner surface of the nano-scale void in the photonic crystal, synthetic opal.

Electric field induced birefringence in a ferroelectric liquid crystal

We have measured the d.c. electric field dependence of the birefringence and conoscopic images for the smectic C* phase of a partially racemized ferroelectric chiral smectic liquid crystal CE-8. The experiments were performed using 50mu m thick homeotropic cells with lateral electrodes which created a d.c. electric field parallel to the smectic layers. The observed field induced birefringence shows a characteristic step-like behaviour which is due to the stepby-step unwinding of the helical structure in a sample with finite dimensions along the helical axis. In conoscopic observations we observe that these steps are associated with moving disclination lines that traverse the sample in the direction of the smectic layers. The observed electric field dependence of the birefringence is discussed in terms of the soliton-like unwinding of helical smectic structures and compared with the predictions of the Landau theory. A qualitatively good agreement is obtained.

Layer Structures of Ferroelectric Smectic Liquid Crystals Formed by Bent-Core Molecules

Achiral bent-shaped molecules may form chiral and polar smectic phases, due to a spontaneous tilt of the planes of the molecules. The lowest energy-state is racemic, and in the textures of the ground state usually fine fringes appear parallel to the smectic layers. Recently it was found that the formation of the fringes could be suppressed resulting in a uniform texture in the ground state. In this paper we show evidences for field induced tilted and horizontally zigzagging layer structures. We discuss properties of various layer structures and compare them to our experimental observations. Finally we give a qualitative explanation for the observed field-induced-uniform-racemic ground state.

Crossover from Frustrated XY Model to ANNNI Model at Finite Temperature

To clarify the mechanism of successive phase transitions occurring in ferroelectric and antiferroelectric smectic liquid crystals, a crossover behaviour of the XY model with competing second nearest neighbour interaction exposed to a twofold field to the Ising model (axial next nearest neighbour Ising model) is studied at finite temperature. The phase transition line from an ordered phase with Ising symmetry to the one preserving characteristics of the XY model is shown to lie at the low-temperature side of a critical curve between the former and a disordered phase, which leads to the result that the freedom of the XY spin is suppressed more strongly at finite temperature than at the absolute zero temperature. A phase with a structure of period 6 is analysed in detail as a typical case of modulated phases with the freedom of XY spin.

Distinct Ferroelectric Smectic Liquid Crystals Consisting of Achiral Molecules with Banana Shape

We found three smectic phases, SmX1, SmX2 and SmX3, in achiral compounds with banana shape. The SmX1 phase has a biaxial single molecular layer, along which polarization exists. The polarization precesses along the layer normal, and is aligned by applying an electric field. The spontaneous polarization is about 60 nC/cm2 and shows a slight temperature dependence. The dielectric constant shows a larger value in SmX1 than those of the other phases, suggesting the Goldstone mode. The relaxation frequency is below 100 Hz.

Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules

The synthesis of a banana-shaped molecule is reported and it is found that the smectic phase which it forms is biaxial with the molecules packed in the best direction into a layer. Because of this characteristic packing, spontaneous polarization appears parallel to the layer and switches on reversal of an applied electric field. This is the first obvious example of ferroelectricity in an achiral smectic phase and is ascribed to the C2v symmetry of the molecular packing.