Natalia Podoliak

The effect of lactate unit number in compounds with azo group in the molecular core

A liquid crystalline compound with the azo group in the molecular core and three lactic acid units in the chiral chain (n = 3) was synthesised and its mesomorphic properties established. Spontaneous polarisation and tilt angle were determined in the ferroelectric and antiferroelectric smectic C phases. Dielectric spectroscopy in the frequency range of 1 Hz–10 MHz was performed within the temperature range of smectic phases showing the characteristic modes in each phase. Small-angle X-ray diffraction was performed and temperature dependences of layer spacing value calculated. Comparison with previously studied compounds with one (n = 1) and two lactate units (n = 2) is presented to establish the effect of lactate unit number on the liquid crystalline properties.

Antiferroelectric phase in liquid crystalline compounds containing an azo group in their molecular core

Novel lactic acid derivatives containing an azo group in their molecular core have been synthesised and their mesomorphic properties established. Compounds with a short aliphatic non-chiral chain exhibited the SmA* phase and ferroelectric (FE) SmC* phase. The monotropic antiferroelectric (AF) phase was found in the homologues with a longer terminal chain. Spontaneous polarisation and tilt angle were measured for the FE and AF phases. Dielectric spectroscopy in the frequency range 1 Hz–10 MHz was carried out and dielectric properties established within the temperature range of the smectic phases. Two high frequency modes were detected in the AF phase. Small-angle X-ray measurements were conducted, and the temperature dependence of layer spacing values is established and discussed.

Anomalous phase sequence in new chiral liquid crystalline materials

We have prepared and studied a series of rod-like liquid crystalline compounds with the molecular core laterally substituted by bromine atom and with the lactate unit in the chiral terminal chain. Microscopic observation under the polarising microscope, x-ray and dielectric spectroscopy results enable us to establish the TGBA–SmA–TGBARE–TGBC phase sequence. The presence of the re-entrant TGBA phase below the regular SmA phase, found for three homologues, is quite uncommon. Results are discussed and compared with the previously studied compounds with regular phase sequences.

Ferroelectric, antiferroelectric and TGB phases in lactic acid derivatives

We prepared a series of rod-like mesogens with two biphenyls laterally substituted by a chlorine atom in ortho position to oxygen atoms in the molecular core, having two lactate units and chiral methylbutyl chain in the chiral part. The sequences of phases and transition temperatures were ascertained by differential scanning calorimetry measurements, polarising optical microscopy, dielectric spectroscopy and electro-optical studies. X-ray diffraction studies were performed, phases confirmed and their structural parameters established. In the studied compounds twist grain boundary (TGBA and TGBC) phases were detected in a rather wide temperature range. In addition, the antiferroelectric SmC*A phase was observed. The properties of this homologue series are compared with those established previously for analogous compounds having the same molecular core and differing only in the number of lactate units in the chiral part.

Nanocomposite of superparamagnetic maghemite nanoparticles and ferroelectric liquid crystal

In this study we present a novel type of hybrid nanocomposite prepared by mixing maghemite nanoparticles and a chiral liquid crystalline compound. The hybrid system exhibits ferroelectric as well superparamagnetic properties. The impact of nanoparticles and the effect of an applied magnetic field on the ferroelectric liquid crystalline properties were established. The ferroelectric properties are conserved up to a concentration of 5.6% of maghemite. The magnetic behavior of the composites is typical for a system of superparamagnetic nanoparticles with inter-particle dipolar interactions and surface spin disorder.

Study of de Vries behaviour of the smectic A*–smectic C* phase transition

Homologue series exhibiting the SmA* and SmC* phases have been studied and the tendency to de Vries behaviour has been described for compounds with decreasing length of non-chiral chain. Dielectric spectroscopy has been performed with the aim to characterize dielectric properties and compare different homologue compounds exhibiting de Vries properties with those showing classical behaviour.

Unusual polymorphism in new bent-shaped liquid crystals based on biphenyl as a central molecular core

Summary Bent-shaped mesogens possessing a biphenyl as a central core have been synthesized and the role of the terminal chain and the orientation of the ester as a linkage group have been investigated. For the studied molecular core we have established that both parameters play an important role for the mesomorphic properties. The polyfluoroalkyl terminal chain supports the formation of mesophases, and the introduction of a chiral lactate terminal chain destabilizes mesophases for the first type of mutual orientation of ester groups, attached to the central core. On the contrary, for the opposite orientation of esters, the terminal chain has no effect on the mesomorphic properties, and columnar phases have been found for all compounds. A unique phase sequence has been found for the mesogen with the fluorinated chain. A generalized tilted smectics, SmCG, have been observed in a temperature interval between two different lamellar SmCP phases and characterized by X-ray and dielectric measurements. The dielectric spectroscopy data are unique and presented for the first time in the SmCG phase providing new information about the molecular dynamics.

Highly tilted smectogens with bromine-substituted molecular core

We have synthesised a series of rod-like mesogens with a bromine atom substituted laterally in the molecular core, which consists of four benzene rings, one biphenyl and two phenyl rings, connected by ester bridges. The terminal chiral chain contains a lactate unit. The physical properties of the compounds have been studied, and their SmA* and SmC* phases have been identified over a wide temperature range. The temperature dependency of the polarisation and tilt angle have been measured, together with the helical pitch and layer spacing, and dielectric spectroscopy has been carried out. On cooling, the tilt angle reaches a relatively high value, up to 45°.

Chiral liquid crystalline compounds with a re-entrant SmA* phase

Unusual mesomorphic behaviour has been found in a series of rod-like liquid crystalline (LC) compounds with two chiral centres in the molecular chain. A re-entrant paraelectric SmA* (SmA*RE) phase has been discovered below the ferroelectric SmC* phase on cooling for one homologue. So far this compound is the only one exhibiting the re-entrancy of the smectic A phase in chiral LC system with structural parameters so close to the upper SmA* phase. X-Ray studies, as well as dielectric spectroscopy, spontaneous polarization and tilt angle measurements have been carried out to characterize studied compounds. We compare various homologues and look for tendencies in physical properties behaviour.

Eutectic behaviour of binary mixtures composed of two isomeric lactic acid derivatives

ABSTRACT Mixing of structurally similar self-assembling materials is a powerful tool to tune their mesomorphic, orientational or dielectric properties. We present design and study of binary mixtures composed of two newly designed chiral lactic acid derivatives with a similar structure but different geometry of the molecular core. One compound possesses a monotropic ferroelectric SmC* phase while another shows a direct isotropic – crystalline phase transition. It has been shown that the binary mixtures possess the eutectic behaviour and the ferroelectric phase temperature range has been extended. Moreover, for certain mixtures induced liquid crystalline phases, namely the cholesteric or TGBA phases, were detected.