Negative birefringence ferroelectric liquid crystals

The synthesis and evaluation of a negative birefringence material which possesses a wide smectic C phase is reported.     

Hydrogen bond-induced ferroelectric liquid crystals

Abstract

Hydrogen-bonded dimers of 4-alkoxy-4′-stilbazole homologues (C _n OSB, proton acceptors) and the mono-(2-methylbutyl)ester of terephthalic acid (MBTA, proton donor) behaved like conventional thermotropic mesogens. For chiral MBTA mixtures, chiral nematic and blue phases were observed with n = 1 and 2, and chiral smectic C phases were observed with n ≥ 5. Achiral phases were observed for mixtures with racemic MBTA as proton donor. Transition temperatures and enthalpies are similar for the chiral and achiral systems.     

Hydrogen bond-induced ferroelectric liquid crystals

Hydrogen-bonded dimers of 4-alkoxy-4'-stilbazole homologues (C_nOSB, proton acceptors) and the mono-(2-methylbutyl)ester of terephthalic acid (MBTA, proton donor) behaved like conventional thermotropic mesogens. For chiral MBTA mixtures, chiral nematic and blue phases were observed with n = 1 and 2, and chiral smectic C phases were observed with n ≥ 5. Achiral phases were observed for mixtures with racemic MBTA as proton donor. Transition temperatures and enthalpies are similar for the chiral and achiral systems.     

Novel frequency dependence of dielectric biaxiality of surface stabilized ferroelectric liquid crystals

The frequency dependence of the dielectric biaxiality of surface stabilized ferroelectric liquid crystals (SSFLCs) was studied. The principal values of the dielectric tensor ε₁, ε₂ and ε₃ were measured by the MOM (molecular orientational model) method. Three dielectric permittivities were measured for each of two samples. These were the permittivity of the homeotropic cell and the permittivity of the planar homogeneous cell with and without the DC bias. Then the dielectric tensor components were calculated based on the molecular orientational models. We present the theory and experimental procedure of the MOM method. Measurements have been performed on Merck FLC compound SCE-8. The following novel dielectric behaviour was observed, as the DC bias voltage was increased the dielectric permittivity of the planar homogeneous cell decreased at the low frequencies (∼ 1 kHz) while increased at the high frequencies (10kHz ∼). The sign of the dielectric biaxiality ∂εε (= ε₂ - ε₁) inverted around 1 kHz, being negative at low frequencies and positive at high frequencies. The roles of the biaxiality on the dielectric behaviour of SSFLC cells are discussed.     

Novel method for order parameter of ferroelectric liquid crystals by image analysis

A novel method for temperature dependence of order parameter calculations at different wavelengths through birefringence measurements for ferroelectric liquid crystals (S)-(-)-2-methylbutyl 4′-(4″-n-alkanoyloxy benzoyloxy) biphenyl-4-carboxylates (where n?=?16 and 18) has been proposed using image analysis and linear regression method of Kuczynski equation. Measurements are made for its liquid crystal phases (SmecticC*, SmecticA) to the isotropic phase of the sample. The birefringence values of the samples at different wavelengths are obtained from the textural analysis of liquid crystal as a function of temperature using MATLAB software.     

Novel ferroelectric liquid crystals based on optically active Preliminary communication propargylic alcohols

Some new calamitic chiral mesogens (2,5-diphenyl-1,3,4-thiadiazoles, 2,5-diphenylpyrimidines and a biphenylyl benzoate) incorporating the 1-alkylpropynyl moiety have been synthesized as enantiomerically enriched materials (ee c. 87%) and/or as racemates. Their liquid crystalline properties were studied by polarizing microscopy, differential scanning calorimetry and electro-optical investigation. Thereby it was found that most of the new compounds exhibit smectic C phases. The spontaneous polarization (Ps) of the optically active materials was measured to be around 10 nCcm-2.     

Photorefractive effect of ferroelectric liquid crystals

This paper reviews our recent work on the photorefractive effect of ferroelectric liquid crystals (FLCs). The photorefractive effect is defined as the optical modulation of the refractive index of a medium as a result of a variety of processes. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon enables the creation of different types of photonic applications. FLCs exhibit fast electric field response, and the orientation of the molecular axis of FLCs changes its direction according to the change in direction of the spontaneous polarization (Ps). When two laser beams interfere in a photoconductive FLC, an orientational grating is formed. The mechanism of the formation of the grating is based on the response of the Ps to the photoinduced internal electric field. The time of formation of the refractive index grating is significantly shorter in FLC materials.     

Mesomorphic properties of non-symmetric three-arm chenodeoxycholic acid-derived liquid crystals

Six three-arm star-shaped liquid crystals (LCs) based on chenodeoxycholic acid (CDCA), termed as G-BH, G-YD, G-FD, G-DJ, G-DZ and G-BX, respectively, have been synthesised. CDCA was used as the chiral core and the nematic side arm, 6-(4-(ethylbenzoyloxy) phenoxy)-6-oxohexanoic acid, was chosen to be introduced into the two hydroxyl of CDCA to synthesise cholesteric LC (CDCA2EA) and different structures were introduced into the carboxyl group of CDCA to prepare the three-arm star-shaped LCs. Chemical structures and LC properties of the six three-arm LCs were characterised by FTIR, ¹H-NMR, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction and polarised optical microscopy. G-BH and G-DJ displayed cholesteric phase with a long helical pitch; G-BX displayed nematic phase; and G-YD, G-FD and G-DZ displayed cholesteric phase on heating and on cooling. These results indicated that not only the chiral core CDCA but also the structures of the side arms played an important role in inducing the cholesteric phase of the CDCA-derived LCs. G-DZ displayed selective reflection, a wider ?λ and red shift on cooling.     

Novel ferroelectric liquid crystals and dopants for induced ferroelectric phases containing 3,4-epoxy-2-octanols as chiral unit

The synthesis of a new class of chiral smectic liquid crystals is described, along with transition temperatures, phase behaviour, spontaneous polarizations and related data. The general structure is shown in A. possessing a chiral unit derived from 3,4-epoxy-2-octanol. Two of these new compounds show seemingly antiferroelectric behaviour. The influence of the mesogenic core structure on mesophase behaviour and ferroelectric properties was investigated. The non-liquid crystalline representatives were examined as dopants for induced ferroelectric phases in an achiral matrix.     

Broad liquid crystalline temperature range of ferroelectric liquid crystals

Novel ferroelectric liquid crystalline compounds, containing the (S)-2-methyl-l -butyl (4-hydroxybiphenyl-4'-carbonyloxy)biphenyl-4-carboxylate mesogenic group and an oligooxyethylene spacer, were synthesized. The mesomorphic properties of these materials were investigated by differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurement. The results indicate that all members of this series exhibit a very broad temperature mesophase range (reaching a maximum around 210°C) including a blue phase (BP), cholesteric (Ch), twist grain boundary A (TGB_A), chiral smectic C (S*_c), and smectic X (S_x) phases. The mesomorphic properties are discussed and a comparison is made with three phenyl rings of ester core analogues.     

Structure-property relationships in dopant-induced ferroelectric liquid crystals

Measurements are reported of the ferroelectric polarization P_s induced in a non-chiral smectic C phase by a variety of chiral dopants having different molecular structural features. Using molecular calculations of contributing dipole moments, ferroelectric order parameters are determined from the experimental results. The relationships between the P_s and various other molecular properties are discussed, and it is shown that restricted rotation of the molecule due to its shape and internal energy barriers to rotation can result in relatively high values of P_s. In contrast dipolar groups flexibly attached to a chiral centre may have their contribution to P_s greatly reduced through internal rotation.     

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.     

Orientational ordering of ferroelectric thiobenzoate liquid crystals

The orientational ordering of a series of ferroelectric thiobenzoate liquid crystals was studied by natural abundance ¹³C NMR spectroscopy. The technique used was a combination of variable angle spinning (VAS) and separated local field spectroscopy (SLF). With rapid sample spinning about an axis forming an angle of c. 45° with respect to the magnetic field, the smectic director aligns parallel to the spinning axis, leading to narrow peaks in the ¹³C NMR spectrum. The two-dimensional NMR spectroscopic method SLF allows the observation of decoupled ¹³C signals in the ω2 dimension and first-order Csbnd;H splitting patterns in the ω1 dimension, from which the dipolar C-H coupling constants for individual bonds can be obtained. The order parameters for different molecular segments of eight different compounds, all containing two phenyl rings linked by a thioester group, were obtained this way. A considerable influence of length, branching and chirality of the aliphatic chain on the order parameters was observed.     

Dye guest-host effects in ferroelectric liquid crystals

Here we present the optic and electro-optic characteristics of ferroelectric dye guest-host devices incorporating different dye and S*_C host materials. We demonstrate fast response time (< 100 μs) single polarizer and zero-polarizer device configurations for high order parameter dichroic dyes and single polarizer fluorescent dye devices. The dynamic data include measurements of response times as functions of voltage, temperature and dye concentration, from which we conclude that the inclusion of certain dyes does not adversely affect the ferroelectric phase. The contrast ratios of these novel guest-host devices are also presented and are shown, as expected, to be functions of tilt angle (and therefore temperature). However, we demonstrate that the high tilt angle of 45° is not necessary for high contrast devices, and in fact for certain devices configurations θ = 22½° is optimum. These results are discussed.     

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.     

Electro-optic pulse response of ferroelectric liquid crystals

The electro-optic response of ferroelectric smectic C* liquid crystals has been studied. Anomalous switching behaviour of such materials which possess a negative dielectric anisotropy has been reported. These materials show a minimum in response time at a sufficiently high field. We present results showing the dependency of this minimum upon spontaneous polarisation and the effect of AC bias. Calculations based upon the equation of motion of the director around the cone are presented which describe this effect and its dependence on the relative magnitudes of the spontaneous polarization and dielectric anisotropy of the material. Good agreement with the experimental results is found.     

New chiral dopants for ferroelectric liquid crystals

New chiral dopants for application in ferroelectric liquid crystal mixtures are presented. They derive from chirally modified biphenylcyclohexylcarbonitriles, lactates and optically active 2-cyano-2-methylalkanoates. Synthetic routes to the new compounds are shown and the physical properties of the new compounds in a non-chiral smectic C host are reported.     

Non-linear dielectric spectra of ferroelectric liquid crystals

Abstract

The non-linear dielectric relaxation spectroscopy has been recently developed and applied to soft materials such as polymers. We have applied this new method to the S*_C phases of some ferroelectric liquid crystals. Under a weak AC electric field, the original and third order harmonic frequency components of electric displacement are proportional to the first and third powers of the applied electric field, respectively. The linear spectrum obtained from the original frequency component shows the relaxation of Debye type and the third order non-linear spectrum shows the relaxation with an extended form of Debye type to the non-linear case. The third order non-linear dielectric increment is found to be negative, which implies that the dielectric non-linearity of the liquid crystal in the S*_C phase is due to the saturation of molecular dipole moments induced by the applied electric field. The temperature dependence of the linear and third order non-linear spectra in the S*_C phase are also studied. Both spectra do not change their forms much through the whole temperature range of the S*_C phase. In the vicinity of the S_A–S*_C transition temperature, the critical behaviour is more remarkable in the third order spectrum than in the linear one.     

Thermal behavior of selected ferroelectric liquid crystals

Thermal properties of three ferroelectric liquid crystals, namely: 3-octyloxy-6-[2-fluor-4-(fluoroctyloxy)phenyl]-pyridine (FFP), 3-(3-fluor-nonyl)-6[4-heptyloxyphenyl]-pyridine (FNHPh-P), 4-[(S,S)-2,3-epoxyhexyloxy]phenyl 4-(decyloxy)benzoate (EHPhDB) were studied using heat flux differential scanning calorimetry method. All the transitions expected in this compounds, except the SmC^*-SmA^* and SmC^*-S₃ transitions, were detected in the DSC curves. The temperatures of the phase transitions and the enthalpy changes associated with them were determined. The transition from the liquid crystalline to the crystalline state showed significant hysteresis for all three compounds studied. The following transitions: SmA^*-Is, SmG^*-SmC^* for FNHPh-P, N^*-Is for EHPhDB, N^*-Is and SmC^*-N^* for FFP also showed a small hysteresis basing on which first-order character of all the above transitions was implied. All three substances studied in this work are characterized by a complex polymorphism in the solid state.The author is grateful to Dr. Stanislaw Wróbel for his stimulating interest, valuable discussions and supplying the samples. This work has been done in the framework of the KBN grant 2 P302 139 07.