Short range order in the isotropic phase of antiferroelectric liquid crystals

Optical activity measurements in the isotropic phase of two antiferroelectric liquid crystal systems in which the chirality can be varied reveal unusual behaviour of the short range order. In one system the phase sequence as the chirality is increased is smectic A, smectic C^* _A, and smectic Q. In the other system the phase sequence is smectic C^*, smectic C^* _A, and smectic Q as the chirality is increased. The short range order of the isotropic phase behaves similarly for these systems, showing mean field behaviour at low chirality and far above the phase transition, but deviating from this behaviour significantly as the chirality is increased and the phase transition is approached. These optical activity results indicate how different is the short range order in the isotropic phase for these antiferroelectric liquid crystal systems and demonstrates the crucial role played by chirality. Past theoretical work that includes smecticlike fluctuations in the calculation of short range order in the isotropic phase is capable of qualitatively explaining these results.     

Experimental characterization of hexatic smectic phases through electro-optic studies and dielectric relaxation spectroscopy

The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4′-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmC_A* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmC_A* and hexatic smectic phases were determined. The dielectric strength at the SmC_A* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmC_A* and hexatic smectic I* phases.     

Systematic study of the chiral smectic phases of a fluorinated compound

ABSTRACT

Physical properties of the partially fluorinated compound 3F5FPhF, with hockey stick-like molecules, were studied by complementary methods. Apart from the already reported paraelectric SmA*, ferroelectric SmC* and antiferroelectric SmC*_A phases, the presence of the smectic C*_α subphase in the phase sequence was proved by differential scanning calorimetry, polarising optical microscopy, electro-optic and dielectric spectroscopy methods. The temperature dependence of the smectic layer thickness and correlation length of the lateral short-range order was determined by X-ray diffraction. Based on dielectric measurements three relaxation processes were revealed in the antiferroelectric SmC*_A phase (two collective: P_L, P_H and one molecular: s-process), two collective ones (Goldstone and soft modes) were found both in the ferroelectric SmC* phase and SmC*_α subphase while one relaxation process (soft mode) in the paraelectric SmA* phase. The results were compared with that obtained for other structurally similar compounds, and it was shown that even addition of one methylene group to the side chain influences much on the physical properties.     

Antiferroelectric alignment and mechanical director rotation in a hydrogen-bonded chiral SmC* A elastomer

A new type of chiral smectic elastomer based on poly[4-(6-acryloyloxyhexyloxy)benzoic acid] is discussed. The layer structure and the molecular tilt stabilized by hydrogen bonding between side groups are identified by X-ray measurements. Well aligned and optically clear monodomain samples with smectic layers in the film plane are obtained by uniaxial stretching and then frozen-in by additional gamma-radiation crosslinking. In this monodomain state, two opposite orientations of director tilt are distributed through the sample thickness and alternate between neighbouring layers in a zigzag fashion. This structure of the stress-aligned chiral smectic C elastomer is similar to that of antiferroelectric liquid crystals of the smectic C^* _A type. Further mechanical stretching in the layer plane induces a gradual c-director reorientation along the new stress axis, when a threshold deformation ~ 20% is exceeded. The (reversible) transition proceeds as a director azimuthal rotation around the smectic C cone, with the layers essentially undistorted and the tilt angle of the side mesogenic groups preserved.     

Amplitude and phase fluctuation modes in the smectic Cα* phase of an antiferroelectric compound

Dielectric measurements have been carried out on the chiral smectic C_α (SmC_α*) phase of a MHPOBC analogous compound. Two relaxation modes have been observed in this phase for planar orientation of the molecules. One process has been observed at frequency lower than that of the soft mode of the chiral smectic A (SmA*) phase. This relaxation process is connected with the helicity of the SmC_α* phase. In the high‐frequency region, another relaxation process has been observed in the SmC_α* phase for which bias field dependence is similar to that of the soft mode at the SmA*–SmC* phase transition. The experimental observations are in agreement with a recently proposed dielectric theory for the SmC_α* phase and theoretical dielectric results obtained by numerical simulations. Thus, we report here experimental verification of two theoretically predicted dielectric modes in the SmC_α* phase.     

Highly tilted antiferroelectric (R) enantiomers useful for the formulation of eutectic mixtures

ABSTRACT

Novel chiral three-ring (R) enantiomers were synthesised using optically active (R)-(?)-2-octanol. Properties, such as the sequence of phases, the transition temperatures and enthalpies, were tested by a polarising optical microscope and differential scanning calorimeter. An antiferroelectric smectic phase (SmC_A*) with a direct transition from the antiferroelectric to the isotropic phase (SmC_A*-Iso) was observed for three esters with an achiral C₃F₇CH₂O(CH₂)₃O– terminal chain. Bi- and multicomponent mixtures with a broad temperature range of the antiferroelectric phase and good electro-optical properties were formulated. Helical pitch of pure esters and mixtures was measured by spectrophotometry method.     

Effect of alkyl chain length in the terminal ester group on mesomorphic properties of new chiral lactic acid derivatives

Two series of new liquid crystalline lactic acid derivatives with a terminal ester group have been synthesised. The effect of this ester unit and the length of its alkyl chain on the mesomorphic and dielectric properties of the compounds exhibiting a broad temperature range of chiral smectic phases have been studied. We found that the mesomorphic behaviour and phase transition temperatures are strongly affected by the molecular architecture. Depending on the alkyl chain length in the terminal ester unit, the studied materials exhibited paraelectric smectic A*, ferroelectric tilted smectic C* and antiferroelectric smectic C_A* phases over a broad temperature range. The physical properties of the compounds have been studied by optical polarising microscopy, differential scanning calorimetry, electro-optic measurements, small-angle X-ray scattering and dielectric spectroscopy. Furthermore, the homologues with short terminal alkyl chains showed a very small layer shrinkage at the transition from the orthogonal SmA* to the tilted SmC* phase, which is a characteristic feature of ‘de Vries-type’ behaviour.     

Effect of terminal perfluorocarbon chain containing mesogens on phase behaviors of chiral comb-like liquid crystalline polymers

A novel perfluorinated liquid crystal 4′-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctanoyloxy)biphenyl-4-yl undec-10-enoate (PFOBU) was synthesized, which exhibited smectic C phase. Several liquid crystalline polymers (PI–PVI) were synthesized by use of poly(methylhydrogeno)siloxane, PFOBU, and cholesteryl 3-(4-allyloxy-phenyl)-acryloate. The chemical structures and liquid crystalline (LC) properties of the monomers and polymers, and some ferroelectric properties of the chiral smectic C (S_C^*) phase were characterized by use of various experimental techniques. The effect of perfluorocarbon chains on phase behaviors of the fluorinated LC polysiloxanes was studied as well. PI and PII showed single chiral nematic (N^*) mesophase when they were heated and cooled, but PIII, PIV, PV, and PVI containing more perfluorocarbon chain units exhibited S_C^* phase besides N^* mesophase. Introduction of perfluorocarbon chain containing mesogens to the chiral cholesteryl-containing polymer systems resulted in a S_C^* mesophases, indicating that the fluorophobic effect could lead to microphase segregation and modifications of smectic mesophases from the chiral nematic phase.     

Shear flow and magnetic field effects on smectic C,C*, CM and C*M liquid crystals

Abstract

The continuum equations of Leslie et al. [1] for smectic C, and the extension of this theory for chiral smectic C* [2], are applied to problems involving simple planar layer configurations which accommodate uniform layer thickness constraints. The chiral smectic C*_M and non-chiral smectic C_M [3] are considered as either biaxial smectic A phases or antiferroelectric smectic C phases and are therefore included as interesting degenerate cases of the smectic C* and C phases, respectively. The effects of static and time dependent magnetic fields on these materials are compared with related deformations occurring in nematics [4] and cholesterics [5,6]. Their reaction to applied shears is also investigated yielding examples of flow alignment, induced secondary flows and unwinding of the chiral helix and testing the validity of enforcing a constant layer thickness.     

Freestanding ferroelectric elastomer films

A ferroelectric crosslinkable liquid crystalline copolysiloxane exhibiting broad chiral smectic C^* and smectic A phases was synthesized and aligned by spin-coating from solution. The homeotropic orientation of the mesogens was checked by polarizing microscopy and X-ray scattering measurements. Crosslinking in the smectic A phase using UV-irradiation stabilizes the spin-coated films mechanically and fixes the monodomain orientation of the smectic A phase. Freestanding Ferroelectric Elastomer Films (FFEF) can be obtained by removing the substrate.     

The dependence of the magnitude of the spontaneous polarization on the cell thickness in ferroelectric liquid crystals

The magnitude of the spontaneous polarization (P) was measured for S-3,7-dimethyloctyl 4-n-hexadecyloxybenzoyloxybenzoate, a material with an isotropic to smectic C phase transition (T_c), using a triangular wave method. The temperature dependence of the measured values was well represented by the expression P = P₀(T_c-T)^α· P₀ was found to depend on the cell thickness for thicknesses below 18 μm, but saturated to a constant value above this limit. The value of the exponent α, on the other hand, was found to be insensitive to the cell thickness. The decreased value of the polarization in the thin cells is thought to be a result of polarization screening due to ionic impurities.     

A bent-shape liquid crystal compound with antiferroelectric triclinic-monoclinic phase transition

Electro-optical and polarization current measurements on 1,3-phenylene-bis[4-(3-fluoro-4-decyloxyphenyliminomethyl) benzoate] (3F-10-O-PIMB) revealed a second order phase transition between two antiferroelectric ‘smectic banana’ phases. The observations show that the switching between the ferroelectric states in the higher temperature (HT) phase requires higher thresholds than in the lower temperature (LT) phase. It is hypothesized that the HT phase has a lower (triclinic, C₁) symmetry, than that of the LT phase (monoclinic, C₂). It is also shown that electric fields can induce transitions between different ‘smectic banana’ phases.     

Influence of the bias field on dielectric properties of the SmCA* in the vicinity of the SmC*–SmCA* phase transition

In a large class of smectic mixtures prepared at our University, the phase transition between chiral ferroelectric smectic C (SmC*) and chiral antiferroelectric smectic C (SmC_A*) phases can be observed on cooling. Under bias field the temperature of the phase transition SmC*?SmC_A* decreases (ca. 100°C in the investigated mixture). The transition is called: unwound SmC*?twisted SmC_A* phase transition. The Goldstone mode in SmC* phase is reduced by a direct current field while two modes (P_H and P_L) in the SmC_A* phase are amplified. The amplitude of the fast X mode observed in the SmC_A* phase is reduced. The aim of this paper is to show how parameters of the modes in SmC_A* phase (calculated from Cole–Cole model) change with bias voltage—when twisted structure in SmC_A* phase is gradually unwound. The character of the modes observed in SmC_A* is discussed. A new effect is shown: a high value of dielectric loss is detected in the unwound SmC* phase, which is very close to SmC_A*.     

Viscoelastic behaviour of ferro- and antiferroelectric phases

We have investigated the viscoelastic behaviour exhibited by chiral smectic C* phases in homeotropic orientation. For an appropriate scattering geometry and using the light scattering technique, the orientational diffusivities associated with the Goldsone mode in ferro- and antiferroelectric phases of C₁₁thiobenzoate and C₈tolane compounds have been measured. (SmC_A*) is about 100 times weaker than (SmC*). This divergence may be largely attributed to the mode of the molecular arrangement in ferro- and antiferroelectric phases, in spite of the same helical structure observed in the two phases.     

Synthesis of new smectic C* liquid-crystalline block copolymers

A series of smectic C^* liquid-crystalline (LC) block copolymers were successfully synthesized via the living anionic polymerization of polystyrene with optically active methacrylate monomers containing (S)-2-methylbutyl 4-(4-hydroxyphenylcarbonyloxy)-biphenyl-4′-carboxylate mesogens. These materials are the first reported smectic C^* block copolymers. Anionic polymerization in tetrahydrofuran (THF) at −70°C leads to LC block molecular weights of approximately twenty repeating units. The number-average molecular weight of the polystyrene block was varied from 7000 to 20000 to adjust the composition in the block copolymers. Differential scanning calorimetry and optical microscopy indicate that the smectic C^* phase is present in the systems over broad temperature ranges.     

On the origin of high optical director tilt in a partially fluorinated orthoconic antiferroelectric liquid crystal mixture

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A* (SmA*) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C* (SmC*) and smectic C_a* (SmC_a*) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA* phase but the tilting directions are there randomly distributed, giving the phase its uniaxial characteristics. At the transition to the SmC* phase, the distribution is biased such that the molecular tilt already present in SmA* now gives a contribution to the macroscopic tilt angle. In addition, there is a certain increase of the average tilt angle, leading to a slightly smaller layer thickness in the tilted phases. Analysis of the wide angle scattering data show that the molecular tilt in SmC_a* is about 20° larger than in SmA*. The large optical tilt (45°) in the SmC_a* phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations.     

Orientation of a chiral smectic C elastomer by mechanical fields

It is well known that, with respect to the director, nematic elastomers can be macroscopically aligned by uniaxial mechanical fields. Extending this method to a chiral smectic C elastomer, depending on the experimental set-up either smectic layer orientation or director orientation parallel to the stress axis occurs. In order to align the director and the smectic layers a biaxial mechanical field (e.g. shear field) consistent with the phase symmetry has to be used to achieve a macroscopically uniform orientation of the untwisted smectic C^* structure.     

Fast-switching ferrolectric liquid crystalline polymers containing one or two centres of chirality in the side chain

In a liquid crystalline side chain polyacrylate containing one center of chirality in the terminal alkyl chain of the mesogenic part, switching times of 200–400 μs were measured in the SmC^* phase. Below this phase an unidentified phase exists, which shows electroclinic-like switching. The phase transition between those two phases can be shifted by applying an electric field. At higher molecular weights three subphases emerge in the SmC^* region. Variation of the spacer length revealed, for the first time, ferroelectric switching even at a spacer length of only two CH₂ groups. By shifting the centre of chirality into the spacer of the side group a polymer resulted, which shows electroclinic switching in the SmC^* phase, changing to ferroelectric switching when the voltage is increased. Incorporation of an oxirane ring as chiral building block into the spacer yielded a polymer that shows a sign inversion of the spontaneous polarization in the SmC^* phase. A polymer containing a dioxolane carbonic ester as chiral unit exhibits three switching states, with the third state existing at a low or zero electric field. This phenomenon is known for antiferroelectric liquid crystals. By doping a racemic LC polymer with a chiral monomeric LC we induced a spontaneous polarization. Colored FLC polymers were obtained by two different approaches. In an FLC–dye copolymer, increasing switching speed in three different chiral smectic phases was observed when increasing the dye concentration.     

X-ray diffraction and dielectric spectroscopy studies on a partially fluorinated ferroelectric liquid crystal from the family of terphenyl esters

The fluorinated compound, (S)-4′′-(6-perfluoropentanoyoxyhexyl-1-oxy)-2′,3′-difluoro-4-(1-methylheptyloxycarbonyl)-[1,1′:4′,1′′]-terphenyl, which exhibits antiferroelectric SmC_A*, ferroelectric SmC* and paraelectric SmA* phases, has been investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and frequency-dependent dielectric spectroscopy methods. X-ray studies have revealed that the layer thickness remains almost constant in the SmA* phase but within the SmC* and SmC_A* phases it decreases with decreasing temperature, a step jump being observed only at the SmA*–SmC* transition. The tilt angle in the SmC_A* phase decreases from 22.2° to 19.5°, and in the SmC* phase it decreases from 18.8° to 5.5°. Spontaneous polarisation is found to be quite high and varies between 74.1 and 118.7 nC cm^?2. The variation in ε′ and ε′′ with temperature shows a discontinuous change at the transition temperatures. Goldstone mode relaxation is only observed in the ferroelectric and antiferroelectric phases and is found to be of the Cole–Cole type. The soft mode is observed on application of a bias field near the SmC*–SmA* transition. Neither the soft mode nor the anti-phase azimuthal angle fluctuation mode is observed in SmC_A*. Rotational viscosity decreases quite rapidly with temperature but in a different manner in the ferroelectric and antiferroelectric phases. Activation energy for this process is found to be 48.14 kJ mol^?1 in the SmC* phase.     

Anomalous behaviour in the SmA*–SmCA* pre‐transitional regime of a chiral swallow‐tailed antiferroelectric liquid crystal

The temperature‐ and electric field‐dependent dielectric relaxation and polarisation of a new chiral swallow tailed antiferroelectric liquid crystal, i.e. 1‐ethylpropyl (S)‐2‐{6‐[4‐(4′‐decyloxyphenyl)benzoyloxy]‐2‐naphthyl}propionate (abbreviated as EP10PBNP), were investigated. The electric field‐induced dielectric loss spectra of EP10PBNP revealed electroclinic and anomalous dielectric behaviour in the chiral smectic A (SmA*)–chiral antiferroelectric smectic C (SmC_A*) pre‐transitional regime. From an analysis of thermal hysteresis of the dielectric constant, electric field‐induced polarisation and dielectric loss spectra, the appearance of a ferrielectric‐like mesophase is observed followed by an unstable SmC_A* phase in the SmA*–SmC_A* pre‐transitional regime.