Free surface-induced bilayer smectic A phase in polar liquid crystals

Abstract

Here the influence of the free surface on both a thick (semi-infinite) layer and a thin freely suspended film of a polar liquid crystal is investigated. It is shown that within the temperature range of the monolayer smectic A phase (S_A1 ) the interaction between polar molecules and the free surface of the liquid crystal gives rise to a bilayer smectic A, a structure with long range antiferroelectric order (S_A2 ) in the surface region of the semi-infinite layer. The dependence of the bilayer smectic order parameter on the strength of the interaction between the constituent molecules and the free surface as well as temperature and the distance to the free surface are determined. Sufficiently far from the S_A1 -S_A2 transition the latter dependence has an exponential character and the depth of the S_A2 phase penetration into bulk liquid crystal is equal to the longitudinal correlation length for the bilayer smectic A structure fluctuations in the S_A1 phase. However, near the S_A1 -S_A2 transition the bilayer smectic order parameter decays non-exponentially and more rapidly with increasing distance to the free surface. In addition, it is found that the bilayer S_A2 phase can form several smectic layers at the free surface of a semi-infinite polar liquid crystal layer with the S_A1 phase. Finally, it is shown that in a freely suspended film the free surface-induced S_A2 phase can completely occupy the volume of the sample. Hence in a freely suspended polar liquid crystal film the S_A1 -S_A2 transition occurs with decreasing film thickness.     

Dielectric studies under high pressure on strongly polar liquid crystals exhibiting monolayer smectic A phase

Dielectric studies have been performed at elevated pressures on two compounds of a homologous series possessing a strongly polar terminal group and three phenyl rings in their molecular structure. This is the first high pressure dielectric study on such systems that exhibit a monolayer smectic A phase. Also notable is the fact that these compounds show a dual frequency response, a promising feature for fast liquid crystal display devices. The low frequency relaxation recorded in the nematic and smectic A phases is attributed to the reorientation of the molecules about the short axis. The temperature and pressure dependence of the relaxation frequency of this mode as well as the involved activation parameters are discussed. At a given relative temperature the relaxation frequency decreases as the pressure is increased. However, the parameter that characterizes the temperature dependences of the relaxation frequency, viz., the activation enthalpy, has a very weak dependence on pressure. In contrast, the activation volume shows a linear decrease with temperature.     

Plenary Lecture. Phase transitions and critical phenomena in polar smectic A liquid crystals

Since the discovery of a phase transition between two polymorphic forms of the smectic A phase, there has been a considerable amount of experimental as well as theoretical work on strongly polar materials exhibiting different smectic A phases. These studies have led to the observation of a variety of new phase transitions and critical phenomena which are rarely encountered in any other area of condensed matter physics. We review briefly some of these important results.     

Plenary Lecture. Phase transitions and critical phenomena in polar smectic A liquid crystals

Since the discovery of a phase transition between two polymorphic forms of the smectic A phase, there has been a considerable amount of experimental as well as theoretical work on strongly polar materials exhibiting different smectic A phases. These studies have led to the observation of a variety of new phase transitions and critical phenomena which are rarely encountered in any other area of condensed matter physics. We review briefly some of these important results.     

A backflow effect in smectic C liquid crystals

This paper discusses the influence upon a smectic C liquid crystal cell of backflow induced by the relaxation of alignment following the removal of a strong electric or magnetic field. Our study, based upon a recently proposed continuum theory, concentrates upon the homeotropic configuration in which the smectic layers are parallel to the boundary plates, but some consideration is also given to the bookshelf geometry. Although the governing equations prove to be rather complex, some progress is possible analytically by repeating an approximation made in the corresponding problem for a nematic.     

Point-like impurity-dislocation interactions in smectic A liquid crystals

The displacement field created in the neighbourhood of a point-like impurity, its self-energy and point-like impurity-dislocation interaction are calculated for a smectic A liquid crystal in the approximation of small deformations. The binding energy of a point defect to an edge dislocation is also estimated. The use of the Peach-Kochler formula as a basis for the calculation of the dislocation interaction with other defects is discussed.     

Point-like impurity-dislocation interactions in smectic A liquid crystals

Abstract

The displacement field created in the neighbourhood of a point-like impurity, its self-energy and point-like impurity-dislocation interaction are calculated for a smectic A liquid crystal in the approximation of small deformations. The binding energy of a point defect to an edge dislocation is also estimated. The use of the Peach-Kochler formula as a basis for the calculation of the dislocation interaction with other defects is discussed.     

Stimulated light scattering in smectic A liquid crystals

The non-linear interaction of arbitrary polarized light with smectic layer deformations in smectic A liquids (S_A) is considered. It is shown that the combined effect of anisotropy, fluidity and a characteristic kind of deformation cause a number of specific non-linear optical phenomena. Two-wave mixing in S_A transforms into a partly degenerate four-wave mixing (FWM) when the polarization and the direction of propagation of the coupled electromagnetic (EM) waves are arbitrary. The interference of the EM waves gives rise to a dynamic grating of layer deformations without a change of mass density of S_A. In the resonant case a propagating mode of a second sound (SS) is excited. The non-linear phenomena are analysed by solving the self-consistent system of the Maxwell equations for the non-linear anisotropic inhomogeneous medium and the hydrodynamic equations of S_A in the external EM field. The explicit expressions of the EM and SS waves amplitudes are obtained. It is shown that the coupled fundamental EM waves undergo the parametric amplification and the phase cross-modulation, and their amplitudes as well as the SS wave amplitude are spatially localized. The energy transfer between the coupled EM waves is non-reciprocal. The scattering of the fundamental EM waves by the dynamic grating results in the appearance of additional harmonics with combination frequencies and wavevectors. The light induced dynamic grating also generates a longitudinal electric field due to the flexoelectric effect.     

Energetics of screw dislocations in smectic A liquid crystals

We derive the self-energy of a single screw dislocation in smectic A liquid crystals allowing for bend curvature in the bulk. For the core region two models are investigated: a nematic one including bend and twist curvature and an isotropic one including surface curvature energy. The former is energetically favourable. For both models the interaction force between two parallel screw dislocations is zero within the linear theory. Taking into account non-linearities perturbatively, an interaction potential is obtained, which is proportional to the logarithm of the distance of the screw dislocations.     

Preliminary communication A novel class of heterocyclic liquid crystals with broad smectic C phase

A successful approach in molecular design through fusing aromatic/heterocyclic rings for broadening the smectic C (SmC) phase and the synthesis of the first heterocyclic liquid crystalline (LC) molecules containing 3,7-disubstituted quinoline with one of the widest SmC phases are reported. Their naphthalene analogues are compared. The nitrogen atom in quinoline system introduces attractive forces to aid the formation of the tilted SmC phase and the flexible chain length also influences the tilted SmC phase effectively.     

Electro-optic effects in smectic C liquid crystals

Electro-optic effects are observed in the smectic C states of 4-n-heptyloxybenzoic acid, 4-n-octyloxybenzoic acid. 4,4′-bis-n-heptyloxyazoxybenzene, and 4-n-decyloxy cinnamic acid under the application of an external ac electric field. The stripe domain pattern is observed for all the compounds studied above the first threshold. In the smectic C state there exists a second threshold at which the direction of the domain lines changes.     

Surface tensions of smectic liquid crystals

We determine surface tensions σ of smectic liquid crystals from the curvature pressure of smectic films. A new technique is introduced for the comparison of surface tensions of different smectic materials. The method is based on the relation of curvatures of smectic films drawn on communicating vessels. The measurement of the temperature dependence of σ reveals anomalies in the vicinity of phase transitions to low temperature smectic modifications. This anomalous slope dσ/dT can be related to the surface excess entropy of the material in the corresponding temperature range. The surface tension values determined for a number of mesogens fit well into the classification proposed by Mach et al.     

Photomechanically induced ferroelectricity in smectic liquid crystals

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm^-2) UV light. In the case of a sample initially in the S*_c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P_s) and changes in electro-optic switching characteristics. If the sample is illuminated in the S_A phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the S_A phase, a reversible, isothermal transition to a biphasic S*_c/isotropic state occurs. In this case, the P_s is seen to rise from zero in the S_A phase to a finite value(∼2 nC cm^-2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*_c phase and UV illuminated, a more complicated variation of P_s occurs with the sample again undergoing a transition to a biphasic S*_c/isotropic state. Possible mechanisms for the transition are discussed.     

Nature of smectic A*-C* phase transitions in a series of ferroelectric liquid crystals with little smectic layer shrinkage

The smectic layer spacing of two homologous series of ferroelectric liquid crystal compounds was characterized by small-angle x-ray diffraction and different degrees of smectic layer shrinkage on cooling from the SmA* into the SmC* phase were observed. The smectic A*-smectic C* phase transition was further studied by measuring the thermal and electric field effects on the optical tilt angle and the electric polarization. With decreasing length of the alkyl terminal chain the phase transition changes from tricritical exhibiting high layer shrinkage to a pure second-order transition with almost no layer shrinkage. This is explained by the increased one-dimensional translational order of the smectic layers, which seems to promote the "de Vries"-type [Mol. Cryst. Liq. Cryst. 41, 27 (1977)] smectic A*-C* phase transition with no or little layer shrinkage.     

Temperature gradient-cooled smectic A phase orientation and its effects on Cl orientation in ferroelectric liquid crystals

The temperature gradient cooling of ferroelectric liquid crystals (FLCs) confined to a closed long narrow rectilinear space causes the molecules constituting the smectic layers to acquire significant movement. The ordinary layer structure of the SmA phase consequently undergoes deformation which in turn determines the mode of the SmA SmC* phase transition. SmA stripe-shaped texture was found to result from molecular movement only when the direction of the temperature gradient is the same as the rubbing direction. For FLCs whose SmA temperature range exceeds 20 C, the SmA phase undergoes virtually defect-free C1 orientation without change to C2 orientation, when the direction of the temperature gradient is opposite that of rubbing. Defect-free C2 orientation is possible irrespective of the SmA temperature range in temperature gradient cooling. C1 and C2 orientations may combine with no zigzag defects through the use of such FLCs.     

Polyimide aligned ferroelectric smectic C liquid crystals

Abstract

27 alkyl-aromatic polyimides have been synthesized and tested as alignment layers for surface stabilized ferroelectric liquid crystals with a number of room temperature ferroelectric mixtures. The cone angles are used as a quantitative measure of the bistability obtained in these cells. The effect of the structure of the polyimides on the cone angles obtained and the stability of the alignment is discussed.     

Density studies on various smectic liquid crystals

Density measurements as a function of temperature for four homologues of the 5-n-alkyl-2-(4-n-alkyloxy-phenyl)-pyrimidines (PYP nOm) which exhibit nematic, smectic A and smectic C phases are reported. Furthermore 1-butyl-c-4-(4'-octyl-biphenyl-4-yl)-r-cyclo-hexan- carbonitrile (NCB84) is studied; this has additionally a smectic G phase. From these data the thermal expansion coefficients are calculated. Comparing PYP 907 and PYP 709, differing in their exchanged alkyl chains, we observe in the smectic A and the smectic C phase a distinctly lower density for PYP 709 whereas their densities nearly agree in the isotropic phase. The pyrimidines PYP 709 and PYP 808 exhibit a continuous volume change on crossing the smectic A-smectic C transition which differs dramatically from PYP 909 which shows a small volume jump. Furthermore a binary mixture of PYP 708 and PYP 706 is analysed which shows only a nematic and a smectic C phase. The associated phase transition is probably first order revealing nearly no pretransitional behaviour. The smectic A-smectic C transition of NCB84 seems to be second order exhibiting a continuous change of volume across the transition whereas the smectic C-smectic G transition shows a volume discontinuity and is first order. In order to induce ferroelectric smectic C* phases all smectic C materials were doped with a chiral pyrimidine dopant. Astonishingly the thermal expansion coefficient across the smectic A-smectic C* transition is influenced by the dopant in a very different way.     

Twin-like domains in chiral smectic C liquid crystals

In the chiral smectic C phase of liquid crystals with the phase transition N*-SmC*, texture development depending on the sample thickness is reported. In very thin samples, domains of rectangular-like shape are observed. As two possible tilts of smectic layers are possible for one anchoring direction, smectic layers inside a domain, called twin-like domains, are tilted with respect to layers in outer regions, similarly to crystalline planes in solid crystalline twins. An elastic model of such a twin domain is proposed and its energy determined.     

Polyimide aligned ferroelectric smectic C liquid crystals

27 alkyl-aromatic polyimides have been synthesized and tested as alignment layers for surface stabilized ferroelectric liquid crystals with a number of room temperature ferroelectric mixtures. The cone angles are used as a quantitative measure of the bistability obtained in these cells. The effect of the structure of the polyimides on the cone angles obtained and the stability of the alignment is discussed.