Thresholdless switching induced by polar anchoring in antiferroelectric liquid crystals

We present a novel thresholdless switching mode in an antiferroelectric liquid crystal cell which is stabilized by the presence of polar anchoring at the cell surfaces and the antiferroelectric nature of the material. We also suggest other possible configurations which are induced by strong polar anchoring and possess quite different director structures and optical characteristics.     

Influence of anchoring at a nematic cell surface on threshold spatially periodic reorientation of a director

We have analysed the influence of surface director anchoring in a planar flexoelectric nematic cell on the threshold spatially periodic reorientation of the director in an external dc electric field. By minimizing the free energy of the nematic cell we obtained the equations for a director and numerically solved them in the one elastic constant approximation. The dependences of the threshold electric field and the spatial period of director structure on the azimuthal and polar anchoring energy, as well as the flexoelectric parameters, are determined. It is shown that the domain of the flexoelectric parameter values, at which the spatially periodic reorientation of a director takes place, increases with decreasing azimuthal anchoring energy and increasing polar anchoring energy.     

Twist deformation in anticlinic antiferroelectric structure in smectic B2 imposed by the surface anchoring

A simplified model is used to estimate the energies of observed structures in the smectic B₂ phase composed of bent-shaped molecules. An approximate twist solution connecting anticlinic antiferroelectric structure in the sample bulk with synclinic ferroelectric order induced at the sample surfaces by anchoring is proposed and the elastic and anchoring energies of this structure are determined. It is shown that uniform, twisted and partly twisted (mixed) anticlinic antiferroelectric structures can coexist with nearly the same energies.     

New method for measuring polar anchoring energy of nematic liquid crystals

A new method for measuring a polar anchoring energy of nematic liquid crystals (LCs) is proposed. A variation of LC tilt angle on the surface with an applied electrical field was determined by a reflective method. The twisted LC cell configuration was selected to compensate a contribution of the induced birefringence in the reflective spectra. The electrical field controlled reflectance was used to analyse the potential form of the polar anchoring energy and to define the anchoring strength. The proposed method is applicable for 2–5 μm thick LC cells.     

Relationship between surface anchoring strength and surface ordering on weakly rubbed polyimide surfaces

We have investigated the relationship between the polar anchoring strength and surface ordering in a nematic liquid crystal on two kinds of weakly rubbed polyimide (PI) surfaces. The polar anchoring strength of 5CB on weakly rubbed PI surfaces, both with and without side chains, increases with rubbing strength and with decreasing temperature. The surface order parameter of 5CB on these surfaces increases with rubbing strength, suggesting that the polar anchoring strength on rubbed PI surfaces is related to the surface order parameter.     

Relationship between surface anchoring strength and surface ordering on weakly rubbed polyimide surfaces

We have investigated the relationship between the polar anchoring strength and surface ordering in a nematic liquid crystal on two kinds of weakly rubbed polyimide (PI) surfaces. The polar anchoring strength of 5CB on weakly rubbed PI surfaces, both with and without side chains, increases with rubbing strength and with decreasing temperature. The surface order parameter of 5CB on these surfaces increases with rubbing strength, suggesting that the polar anchoring strength on rubbed PI surfaces is related to the surface order parameter.     

An extension of the McMillan theory to a bilayer smectic A phase with long range antiferroelectric order

Here an extension of the McMillan theory to describe both a monolayer smectic A phase with short range antiferroelectric order and a bilayer smectic A phase with long range antiferroelectric order in polar liquid crystals is proposed. In a molecular field approximation the single particle pseudopotential and self consistent equations which determine the usual orientational and translational order parameters and a new order parameter taking into account the antiferroelectric character of a bilayer smectic A structure are derived from a modified pair potential. The regions of existence for the isotropic, nematic, as well as both monolayer and bilayer smectic A phases together with the transitional characteristics are determined by numerical solution of the self consistent equations. In addition the temperature dependence of all the order parameters is determined. These results are compared with those from experiment.     

Simulations of energy and switching in AFLCDs with different boundary conditions

Using standard expressions for the various terms in the Gibbs free energy, the switching in antiferroelectric liquid crystal (AFLC) displays is simulated and the time evolution of various energy terms and of the liquid crystal director distributions are calculated. It is shown that when returning from a strong positive voltage to zero, one can reach two types of antiferroelectric state: the normal alternating state with the two bulk polarizations perpendicular to the electrodes and opposite to each other, and the alternative splayed symmetric state with two bulk polarizations parallel to the electrodes and again opposite to each other. The former case gives rise to tri-state switching characteristics, the latter to V-shaped switching. In general strong polar interaction with the alignment layer favours V-shaped switching while weak or no polar interaction give rise to tri-state switching characteristics. Since the V-shaped characteristic has so far only been demonstrated experimentally in ferroelectric liquid crystals (or antiferroelectric liquid crystals being in the ferroelectric state), the difference in AFLCs is discussed and the conditions for continuous switching are modelled. The simulations show that the switching characteristics of the antiferroelectric display can be controlled by the surface parameters.     

Cyclic ureas as novel building blocks for bent-core liquid crystals

Cyclic ureas represent a new class of bent-core liquid crystals which, depending on the ring size and other structural parameters, can form a series of polar (ferroelectric and antiferroelectric), as well as non-polar, tilted and non-tilted smectic and undulated smectic phases.     

On the way to polar achiral liquid crystals

A brief review is presented of theoretical predictions and experimental observations concerning polar (pyroelectric, ferroelectric and antiferroelectric) achiral mesomorphic phases.     

The first liquid crystalline dimers consisting of two banana-shaped mesogenic units: a new way for switching between ferroelectricity and antiferroelectricity with bent-core molecules

Depending on the number of dimethylsiloxane units in the spacer connecting two banana-shaped molecules either ferroelectric or antiferroelectric switchable polar smectic C phases have been obtained.     

Achiral bent-core azo compounds: observation of photoinduced effects in an antiferroelectric tilted smectic mesophase

We report the synthesis and physical properties of four novel bent-core compounds, containing an azo linkage and alkyl terminal chains. It is found that, except for the lowest homologue, all the materials are liquid crystalline, exhibiting columnar or tilted polar smectic phases. In one of the compounds a reversible photoinduced variation of the spontaneous polarization of the antiferroelectric state was observed.     

Achiral bent-core azo compounds: observation of photoinduced effects in an antiferroelectric tilted smectic mesophase

We report the synthesis and physical properties of four novel bent-core compounds, containing an azo linkage and alkyl terminal chains. It is found that, except for the lowest homologue, all the materials are liquid crystalline, exhibiting columnar or tilted polar smectic phases. In one of the compounds a reversible photoinduced variation of the spontaneous polarization of the antiferroelectric state was observed.     

The carbosilane unit as a stable building block for liquid crystal design: a new class of ferroelectric switching banana-shaped mesogens

New banana shaped liquid crystals with a carbosilane unit at one end were synthesised and depending on the number of Si-atoms either antiferroelectric (AF) or ferroelectric (FE) switching polar smectic C phases have been obtained.     

Influence of weak anchoring on flow instabilities in nematic liquid crystals

We analyse the homogeneous instabilities in a nematic liquid crystal subjected to plane steady Couette or Poiseuille flow in the case when the director is prealigned perpendicular to the flow plane taking into account weak anchoring at the confining surfaces. The critical shear rate decreases for decreasing anchoring strength and goes to zero in the limit of torque-free boundary conditions. For Poiseuille flow two types of instability arise depending on the values of the azimuthal (W_a) and polar (W_p) surface anchoring strengths. The critical line in (W_a,W_p) space which separates the two instabilities regimes is obtained.     

A new class of bent‐shaped mesogens exhibiting unusual mesophase behaviour

Examples of a new class of bent‐core compounds are presented, the central fragment of which consists of a benzoyl derivative of a secondary cyclic amine. The mesophase behaviour has been studied by differential scanning calorimetry, polarizing optical microscopy, X‐ray diffraction and electro‐optical measurements. It was found that the five‐ring compounds form smectic CP or B₁ phases which are typical for bent‐core mesogens. The six‐ring compounds exhibit, in some cases, only conventional non‐polar smectic phases; in other cases non‐polar smectic phases as well as banana phases are seen. Of particular interest is the occurrence of a switchable uniaxial semectic A‐like phase with an antiferroelectric structure.     

Silicon-containing polyphilic bent-core molecules: the importance of nanosegregation for the development of chirality and polar order in liquid crystalline phases formed by achiral molecules

Polyphilic molecules composed of a bent aromatic core, oligo(siloxane) units, and alkyl segments were synthesized, and the self-organization of these molecules was investigated. Most materials organize into polar smectic liquid crystalline phases. The switching process of these mesophases changes from antiferroelectric for the nonsilylated compounds via superparaelectric to surface-stabilized ferroelectric with increasing segregation of the silylated segments. It is proposed that the siloxane sublayers stabilize a polar synclinic ferroelectric (SmC(s)P(F)) structure, and the escape from a macroscopic polar order as well as steric effects leads to a deformation of the layers with formation of disordered microdomains, giving rise to optical isotropy. Another striking feature is the spontaneous formation of chiral domains with opposite handedness. For two compounds, a temperature-dependent inversion of the optical rotation of these domains was found, and this is associated with an increase of the tilt angle of the molecules from < 45 degrees to > 50 degrees. This observation confirms the recently proposed concept of layer optical chirality (Hough, L. E.; Clark, N. A. Phys. Rev. Lett. 2005, 95, 107802), which is a new source of optical activity in supramolecular systems. With increasing length of the alkyl chains, segregation is lost and a transition from smectic to a columnar phase is found. In the columnar phase, the switching process is antiferroelectric and takes place by rotation of the molecules around the long axes, which reverses the layer chirality; that is, the racemic ground-state structure is switched into a homogeneous chiral structure upon application of an electric field.     

Nematic structures in cylindrical cavities

We present the results of Monte Carlo simulations of a nematic liquid crystal confined to a cylinder with homeotropic surface anchoring. The nematic is modelled using the LebwohlLasher model. For low values of the cylinder radius or anchoring strength, a stable planar polar configuration with two line defects is readily formed, consistent with the predictions of elastic theory. At larger values of the radius or anchoring strength we observe a metastable escaped radial configuration. However, this structure eventually collapses to a planar configuration, contrary to the predictions of elastic theory.     

Liquid crystal alignment capabilities on rubbed organic solvent soluble polyimide surfaces with trifluoromethyl moieties

High pretilt angles, polar anchoring energy (out of plane-tilt), and surface ordering in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) were investigated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. It was found that the pretilt angle of 5CB is about 15° in the wide rubbing region of rubbed soluble PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings. It is suggested that the microscopic surface structure of the polymer contributes to the LC pretilt angle generation at the surface. Also, the polar anchoring energy of 5CB is dependent on the molecular structure of these unidirectionally rubbed soluble PI surfaces. The polar anchoring strength of 5CB on rubbed soluble PI surfaces is as weak with trifluoromethyl moieties attached to the lateral benzene rings weak as when the trifluoromethyl moieties are attached to the polymer backbone. Finally, the polar anchoring energy of 5CB strongly depends on the surface ordering of rubbed soluble PI surfaces.     

Oscillating Poiseuille flow in photo‐aligned liquid crystal cells

We present an experimental study of thin liquid crystal (LC) layers under the action of a harmonically varied pressure gradient. Optical measurements were performed to register the linear oscillations of a nematic director related to homeotropic and homeoplanar (hybrid) initial states. In the latter case one of the inner surfaces of the rectangular channels was treated by ultraviolet light to provide a relatively weak planar anchoring. The optical response of hybrid and homeotropic LC cells under an oscillating pressure gradient was investigated in relation to on the amplitude and frequency of the pressure gradient. A hydrodynamic model is developed taking into account the LC polar anchoring strength and the surface viscosity responsible for a fast LC surface dynamics. Our estimates show that the thickness of the boundary layer corresponding to the surface viscosity does not exceed 10^?6 m, and further experiments are needed with thinner LC cells and higher frequency oscillations to achieve a more precise value. An oscillating Poiseuille flow in the hybrid cell was found to be useful for characterizing elastic and viscous properties of a weakly anchoring LC surface layer in a fast surface dynamic process.