Antiferroelectric Smectic Liquid Crystal Research Articles

The orthoconic antiferroelectric smectic liquid crystals and their engineering by doping with homo- and heterostructural compounds

The attempt of the engineering of modern liquid crystalline orhoconic antiferroelectric smectic material OAFLC with remarkable optical properties is reported. The high tilted anticlinic antiferroelectric parent mixtures were formulated and studied. Subsequently those parent mixtures were doped with homo- and heterostructural dopants with selected properties including a molecular shape, molecular dipole moment μ value and orientation and molecular conformation. The effects of the doping are described. The doping affecting a single phase parameter is reported. Examples of functional dopants are presented and their influence on the physical and structural properties of doped mixture is shown.

Asymmetric switching in a ferrielectric liquid crystal device

The fast multistate switching associated with ferroelectric and antiferroelectric smectic liquid crystals makes them important materials for electro-optic devices, and asymmetric electro-optic responses are of particular interest for some analog switching applications. We report intrinsic asymmetric switching in a ferrielectric liquid crystal device observed using time-resolved small angle x-ray scattering and electro-optic measurements. The experiments reveal a marked variation in the response time depending on field polarity not present in the ferroelectric or antiferroelectric phase. We suggest that this is a consequence of the ferrielectric structure itself and interactions with the device surfaces.

Anticlinic Antiferroelectric Smectic Liquid Crystal Formed by an Asymmetric Banana‐Shaped Molecule

Anticlinic Antiferroelectric Smectic Liquid Crystal Formed by an Asymmetric Banana‐Shaped Molecule

Electric field‐induced birefringence, optical rotatory power and conoscopic measurements of a chiral antiferroelectric smectic liquid crystal

Using a photoelastic modulator‐based novel set‐up, the electric field‐induced in‐plane birefringence and the optical rotatory power (ORP) were measured of an antiferroelectric liquid crystalline compound (12OF1M7) in its various phases using 30 µm homeotropic cells. Some specific signatures of the in‐plane birefringence and of the ORP for the various phases are being established. A relatively small threshold field is needed for the unwinding process of the antiferroelectric phase with a unit cell of four layers [SmCA*(1/2)] compared with that for two layers [SmCA*(0)]. On application of the electric field on the high temperature side of the SmCA*(1/2) phase (80.1–81.5°C), a field‐induced phase transition is shown to occur directly to the SmC* phase, whereas on the lower temperature side (79.4–80.1°C) the transition takes place to SmC* via the SmCA*(1/3) phase. The in‐plane birefringence exhibits a critical power law dependence for the SmC*–SmA transition. The ORP changes sign within the temperature range of the phase with a unit cell of three layers, reflecting a change in the handedness during this phase. Using tilted conoscopy, the results for the biaxiality and the apparent tilt angle for a smectic liquid crystal with a tilt angle greater than 18° in the ferroelectric phase are reported. The biaxiality implies the difference in the refractive indices between the two minor axes of the refractive index ellipsoid. The optical transmittance at visible and IR wavelengths for free‐standing films reveal characteristic reflection bands for these phases. The modulated structures of the reflected bands appear just above the SmCA* phase and below SmCA*(1/3); these are possibly due to an easy deformation of the phase by the surfaces.

Molecular Alignment in SmLC Cells Having a Surface Layer with INS Phase Sequence

In smectic liquid crystals (SmLCs) having no nematic (N) phase in the phase sequence, such as antiferroelectric SmLCs and SmLCs with V-shaped electrooptical switching, it is hard to obtain the LC media which have a uniform molecular alignment structure. This is because the transition of isotropic (Iso) phase directly to Sm phase during the cooling gives rise to a non-uniform Sm layer structure due to the both occurance of molecular alignment and Sm layer formation at the same time. We propose an idea to fabricate defect-free SmLC cells in which the uniform Sm layer structure is realized by the use of an intervening surfacial LC layer with Iso-N-Sm (INS) phase sequence.

Polymer-Stabilized Antiferroelectric Liquid Crystals

In smectic liquid crystals (SmLCs) with no nematic phase in the phase sequence, such as antiferroelectric SmLCs and SmLCs with V-shaped electrooptical switching, it is hard to obtain LC cells with a uniform molecular alignment structure. The direct transition from the isotropic phase to the Sm phase during cooling gives rise to a nonuniform Sm layer structure due to the dispersion of the layer normal. We propose a method of fabricating defect-free SmLC cells with a uniform Sm layer structure using polymer stabilization or the phase separation of the SmLC and the polymer.

Surface effects in thin films of antiferroelectric smectic liquid crystals in terms of the short-pitch long-pitch competition model.

A theoretical model based on the competition between short-pitch and long-pitch types of helical order is developed for thin films of antiferroelectric smectic liquid crystals. In the case of the "bookshelf" structure of the film and non-polar surfaces, subsurface perturbations of the ordering are found out analytically. Corresponding contributions to the free energies of the different phases are analysed. The possibility of sufficient influence of the boundaries on phase sequences is predicted even in the case of weak surface anchoring. A consistent explanation of the controversial experimental information is given; further ways of experimental justification of the model assumptions are discussed.

Flexoelectricity and piezoelectricity: the reason for the rich variety of phases in antiferroelectric smectic liquid crystals.

The free energy of antiferroelectric smectic liquid crystals which takes into account polar order explicitly is presented. Steric, van der Waals, piezoelectric, and flexoelectric interactions to the nearest layers, and dipolar electrostatic interactions to the nearest and to the next-nearest layers, induce indirect tilt interactions with chiral and achiral properties, which extend to the third- and to the fourth-nearest layers. Although the strength of microscopic interactions changes monotonically with decreasing temperature, the effective interlayer interactions change nonmonotonically and give rise to a nonmonotonic change of the modulation period through various phases. Increased chirality changes the phase sequence.

Transient13C–1H Nuclear Overhauser Effect in Liquid Crystal

Significant transient 13 C– 1 H nuclear Overhauser effect (NOE) is observed in liquid crystal, and is explained by a local 13 C– 1 H dipole-dipole interaction and relaxation. Solomon's classical two-pulse experiment gives a significant maximum in signal intensity of 2.5 times the equilibrium magnetization, and also gives a way of estimating zero-quantum transition probability for a dipole-coupled 13 C– 1 H pair. The conventional inversion recovery sequence is recommended for the experimental determination of 13 C spin-lattice relaxation rates. Magnetization recovery after the Hartmann-Hahn signal enhancement deviates strongly from exponential behavior. Experimental data are shown for an antiferroelectric chiral smectic liquid crystal MHPOBC.

Freezing of Intramolecular Motion around the Chiral Center of an Antiferroelectric Liquid Crystal as Evidenced by Damped Transient Oscillations in 13C- 1H Cross Polarization

Transient oscillations in 13 C- 1 H cross-polarization dynamics in NMR is studied for an antiferroelectric chiral smectic liquid crystal MHPOBC. Decay time of oscillation, which is determined by proton spin-diffusion, probes intramolecular motions. Different behaviors are found for the chain conformational motions around the chiral center in the paraelectric smectic A(S A ) and the antiferroelectric chiral smectic CA(S CA * ) phases: Intramolecular motion of the chiral chain in the S A phase is frozen in the S CA * phase. This enables a large transverse dipole moment of a molecule to come out.

Short-pitch and long-pitch modes as a key for the understanding of phase sequences and types of ordering in antiferroelectric smectic liquid crystals

A novel theoretical approach is suggested for the description of phase transitions and structures observed in antiferroelectric smectic liquid crystals. Allowance is made for two possible types of ordering: short-pitch and long-pitch helical modes. The thermodynamic behavior of the material is treated as a result of the competition between these two modes. The formulated theory reproduces the main features of the experimentally observed phase sequences and structures, thus explaining their physical nature.

Metamagnetism and V-Shaped Response in Anti-Ferroelectric Smectic Liquid Crystals

In order to find a clue to elucidation of the thresholdless response or V-shaped response observed in antiferroelectric smectic materials, an antiferroelectric phase of the Ising model with layered structure, which is known as metamagnetism, is studied statically and dynamically under an external field. It is shown that the phase exhibiting a continuous change driven by the field is not necessarily a disordered phase, but such phenomenon occurs in the phase with long range order.

Field-induced phase transitions in antiferroelectric liquid crystals.

A theoretical study is made of the process by which an antiferroelectric smectic liquid crystal undergoes a field-induced transition to ferroelectric alignment. We find that for cells of moderate thickness the initial departure from antiferroelectric alignment occurs as a continuous Fréedericksz transition. The following transition from partial alignment to complete ferroelectric ordering may occur as either a first-order or continuous transition, depending on the relative strength of some of the model parameters. The case where the transition is continuous provides a possible mechanism for some recently observed thresholdless transitions in these systems.

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.

Ground State of the Classical XY Model with Twofold Symmetry Breaking Field Pertaining to the Devil's Staircase

The Ising model and the classical XY model with frustrating second nearest neighbour interaction in an axial direction have qualitatively different properties to each other both at ground state and in the finite temperature. The XY model exposed to the symmetry breaking field with twofold symmetry is here introduced, which is reduced to the Ising model, that is, the ANNNI model in the limit of the strong field strength. The ground state of this model is examined, in which the freedom of the XY spin is shown to be suppressed at rather small strength of the symmetry breaking field and accordingly the Ising properties dominate in large area on an interaction parameter versus field diagram, while the freedom of XY spin survives only at an area with weak field. This result seems to explain why the successive phase transitions observed in ferro- and antiferro-electric smectic liquid crystals can occur.

Surface-Anchortng Solitary Waves in Antiferroelectric Liquid Crystals

By considering an expression for the energy of a cell containing an antiferroelectric smectic liquid crystal with only nearest-neighbor interactions between layers we conclude that while the transition to and from ferroelectric alignment can propagate as a stable solitary wave in the bulk of the material, under certain conditions a separate surface wave can also exist.

Deflection of light by phase boundaries in antiferroelectric liquid crystals

Light deflection phenomena in an antiferroelectric chiral smectic liquid crystal MHPOBC were investigated. At electric field induced phase transitions between antiferroelectric (AF) state to ferroelectric (F) one, characteristic deflected lights were observed; dependence of the angle of deflection on temperatures and on angles of incidence were quite different from those of diffraction lights due to a periodical change of refractive index caused by helical structure of chiral liquid crystals. On the basis of numerical analysis for domain walls of ferroelastic crystals, it is concluded that the light deflection is due to refraction and reflection phenomena at the phase boundaries between AF and F states coexisting at the field induced phase transition.

Reflection and refraction of light at the boundaries between ferroelectric and antiferroelectric phases in chiral smectic liquid crystals

Abstract Light deflection phenomena in antiferroelectric chiral smectic liquid crystals were investigated. Characteristic deflected lights were observed at electric field induced phase transitions between antiferroelectric (AF) state to ferroelectric (F) one. On the basis of a numerical analysis of Huygens construction at the phase boundaries between coexisting AF and F states, it is concluded that the light deflection is not due to diffraction but due to refraction and reflection at the phase boundaries. The characteristic dependence of the deflection angle on the angle of incidence was proposed as a possible evidence of coexistence of phases or domains in materials.

Devil's Staircase, Critical Thickness, and Propagating Fingers in Antiferroelectric Liquid Crystals

From an expression for the energy of a cell containing an antiferroelectric smectic liquid crystal with only nearest-neighbor interactions between layers we conclude the following: (1) a critical cell thickness exists at which the commensurability of the system changes from unity to higher values; (2) for negative dielectric anisotropy there is another critical thickness below which a transition to ferroelectric alignment cannot be homogeneously nucleated at any field; and (3) the transition to and from ferroelectric alignment propagates as a stable solitary wave under certain conditions.

Molecular Orientational Structures in Ferroelectric, Ferrielectric and Antiferroelectric Smectic Liquid Crystal Phases as Studied by Conoscope Observation

Molecular orientational structures in MHPOBC were studied by means of conoscope observation. Contrary to a ferroelectric response of the conoscope to an electric field in the smectic C* phase, the conoscopic figure in the antiferroelectric smectic CA* phase does not shift its center and is biaxial with its optic plane perpendicular to the field direction. This conoscope change is only due to a dielectric contribution, indicating the existence of the inherent threshold in the electric field induced transition to the ferroelectric phase. In between the ferroelectric and antiferroelectric phases, a kind of ferrielectric phase was clearly distinguished in the field response of the conoscopic figure; the shift of the conoscope center perpendicular to the applied field and the biaxial optic plane parallel to the field suggest a novel molecular orientational structure.