Milada Glogarová

The Influence of an External Electric Field on the Structure of Chiral Sm C∗ Liquid Crystal

Abstract The mechanism of a field induced transition from helical Sm C∗ to Sm C structure in a DOBAMBC liquid crystal situated between two glass plates has been studied by optical methods. The important role of a regular system of twist disclination lines (dechiralization lines) in unwinding the helical structure by electric field has been revealed. A simplified model of ±2π twist disclination pairs is proposed which describes the unwinding of the Sm C∗ structure. Under the external electric field the pairs of ±2π twist disclinations move and annihilate each other and thus effectively unwind the helical structure of the sample. Critical fields for switching the helical Sm C∗ structure, as well as the unwound structure, are estimated.

Novel liquid crystals based on [1]benzothieno[3,2- b][1]benzothiophene

Four series of new [1]benzothieno[3,2- b][1]benzothiophene derivatives have been synthesized. In the non-chiral series a SmA phase occurs, while the chiral series exhibits a rather wide antiferroelectric SmCA* phase just below the SmA phase. The SmA-SmCA* phase transition has been studied using DSC and dielectric spectroscopy. In the SmCA* phase the spontaneous quantities have been measured. The tilt angle shows a typical temperature dependence and the values of spontaneous polarization are rather moderate. The length of the helical pitch increases on increasing the length of the non-chiral alkyl chain.

The structure of ferroelectric liquid crystals in planar geometry and its response to electric fields

Abstract The review describes possible structures of the chiral smectic C (Sm C*) liquid crystals (DOBAMBC and a particular Sm C + cholesteric mixture) in samples with planar surface orientation of molecules. In two of them (planar helicoidal PH1 and PH2 configurations) the helicoidal structure is realized in the centre of the plate-like samples. In the other two (planar twisted PT and planar uniform PU configurations) the helix is unwound. Behaviour of these structures in an electric field is discussed. In the field induced transition PH → PT.motion and mutual annihilation of ±2π twist disclinations plays an important role. On the other hand the transition PT → PU or the switching processes between two PU states (PU+ and PU-) with opposite dipole-moments are governed by ferroelectric domain-like mechanisms (nucleation and growth of domains). Hysteresis loop P(E) is observed at frequencies higher than 1 Hz, when the transition PU+ → PU− takes place. The small signal dielectric permittivity of PH or PT con...

A novel type of banana liquid crystals based on 1-substituted naphthalene-2,7-diol cores

The effect of lateral 1-substitution in naphthalene-2,7-diol with CH3, CN and Cl on mesomorphic properties is studied. DSC and X-ray diffraction, texture observation and dielectric measurements have been performed. For the non-substituted core the high-temperature BX phase and the B4 phase occur while in CH3 and Cl substituted compounds the B2 phases are found. In the CN derivatives, besides the crystalline-like phase N, the phase transition between the chiral B2 and racemic B2 phases is observed.

The effect of a lateral substituent on the mesomorphic properties in a series of ferroelectric liquid crystals with a 2-alkoxypropionate unit

A new series of ferroelectric liquid crystal materials containing a lateral methyl group on the aromatic ring of the alkoxybenzoate unit has been synthesized and investigated. These materials exhibit a wide temperature range of ferroelectric SmC* phase on cooling (including supercooling) with a very high spontaneous polarization. All phase transition temperatures are lower when compared with those of similar non-substituted materials. In comparison with the compounds containing the lateral methoxy group, only the SmC*-Cr transition changed significantly. The values of spontaneous polarization are mostly increased compared with both non-substituted and methoxy substituted compounds.

New series of ferroelectric liquid crystals with four ester groups

Three series of ferroelectric liquid crystals have been synthesized, having three ester groups in the mesogenic core and one ester group in the chiral chain. The ester groups introduced into the core decrease the temperature of crystallization, which results in a broad temperature range ferroelectric smectic C phase in homologues with a long non-chiral chain. These phases exhibit a high spontaneous polarization and relatively short pitch of the helical structure.

New ferroelectric and antiferroelectric liquid crystalline materials containing differing numbers of lactate units

A new series of chiral liquid crystalline materials having the same rigid core containing a keto group, and various numbers of (S)-lactate groups in the chiral segment, has been synthesized and studied. All the compounds contain a branched terminal alkyl chain derived from (S)-2-methylbutanol. The influence of the number of lactate units on mesogenic behaviour has been studied. All the compounds show the paraelectric SmA phase and the ferroelectric SmC* phase over rather wide temperature ranges. The antiferroelectric SmCA* phase was observed for the compound possessing two lactate groups.

Novel hockey‐stick mesogens with the nematic, synclinic and anticlinic smectic C phase sequence

New hockey‐stick mesogens have been synthesised and their mesomorphic properties studied. The molecular structure consists of a laterally substituted central naphthalene unit prolonged by two non‐symmetrical arms, both of which contain ester units of identical orientation. The shorter arm possesses only one and the longer arm three benzene rings. Nematic, synclinic smectic C and anticlinic smectic C phases were identified and their physical properties studied. No banana‐type ordering producing dipolar phases has been found.

Thickness dependent low frequency relaxations in ferroelectric liquid crystals with different temperature dependence of the helix pitch

A low frequency relaxation mode 1 has been detected in ferroelectric phases of three materials. The relaxation frequency and the dielectric strength of this mode depend strongly on the sample thickness, but their temperature dependences are qualitatively different in the materials studied, reflecting the behaviour of the helicoidal structure of these materials. This mode has been attributed to a superposition of the Goldstone and thickness modes when the helicoidal structure exists and to the thickness mode only, when the helix is unwound. In addition, with one material, mode 2 has been detected at still lower frequencies, and this also exhibits a strong sample thickness dependence. It was attributed to fluctuations of the director field modified by a non-homogeneous ionic charge distribution across the sample. As both modes 1 and 2 are strongly sample thickness dependent, they do not represent bulk properties of the materials studied, but reflect the structure in real samples, which is determined by surf...

Synthesis and mesomorphic properties of a new ferroelectric liquid crystal with 2-alkoxypropionate chiral groups

Abstract Two series of new chiral substances with 2-alkoxypropionate chiral groups have been synthetized. All compounds exhibit very wide ferroelectric SmC* phases with a high spontaneous polarization.