Glassy Mesophases and Ferroelectric Properties of Chiral Low Molar Mass Liquid Crystals (original) (raw)
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The introduction of chirality, i.e., the lack of mirror symmetry, has a profound effect on liquid crystals, not only on the molecular scale but also on the supermolecular scale and phase. I review these effects, which are related to the formation of supermolecular helicity, the occurrence of novel thermodynamic phases, as well as electro-optic effects which can only be observed in chiral liquid crystalline materials. In particular, I will discuss the formation of helical superstructures in cholesteric, Twist Grain Boundary and ferroelectric phases. As examples for the occurrence of novel phases the Blue Phases and Twist Grain Boundary phases are introduced. Chirality related effects are demonstrated through the occurrence of ferroelectricity in both thermotropic as well as lyotropic liquid crystals. Lack of mirror symmetry is also discussed briefly for some biopolymers such as cellulose and DNA, together with its influence on liquid crystalline behavior.
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The introduction of chirality, i.e., the lack of mirror symmetry, has a profound effect on liquid crystals, not only on the molecular scale but also on the supermolecular scale and phase. I review these effects, which are related to the formation of supermolecular helicity, the occurrence of novel thermodynamic phases, as well as electro-optic effects which can only be observed in chiral liquid crystalline materials. In particular, I will discuss the formation of helical superstructures in cholesteric, Twist Grain Boundary and ferroelectric phases. As examples for the occurrence of novel phases the Blue Phases and Twist Grain Boundary phases are introduced. Chirality related effects are demonstrated through the occurrence of ferroelectricity in both thermotropic as well as lyotropic liquid crystals. Lack of mirror symmetry is also discussed briefly for some biopolymers such as cellulose and DNA, together with its influence on liquid crystalline behavior.
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Structural, electro-optical, and dielectric investigations of a ferroelectric liquid crystal (FLC) exhibiting the chiral smectic C phase (SmC Ã ) are reported. This study has been performed on the pure FLC showing the SmC Ã -SmA-N Ã phase sequence and having high spontaneous polarization and a large relaxation frequency. We have determined the Goldstone rotational viscosity and the twist elastic constant in the SmC Ã phase from the experimental data of helical pitch, tilt angle, polarization, dielectric strength, and relaxation frequency of the Goldstone-mode relaxation. An Arrhenius-type behavior of the Goldstone rotational viscosity was obtained, and the corresponding activation energies were evaluated.
Structural studies on different types of ferroelectric liquid crystalline substances
Structural studies of ferroelectric liquid crystalline substances of different molecular structure have been reviewed. The discussion of the results deals mainly with the structure of chiral liquid crystals forming the smectic mesophase with ferroelectric order, as the research on ferroelectric liquid crystals, due to their high potential for application in electro-optics and photonics, has recently become one of the most attractive fields. Based on data of polarizing optical microscopy and X-ray diffraction obtained for unoriented samples we have identified the type of the mesophases, the temperature of the phase transitions and outlined the phase diagrams. An analysis of the relation between the molecular conformation and different types of phase transitions has been performed. The layer spacing and the average intermolecular distances have been determined for all studied phases from the positions of the small and the large angle diffraction peaks, respectively.
Journal of Thermal Analysis and Calorimetry, 2005
Thermal properties of a homologous series of ferroelectric liquid crystals S-(-)- [4-(2-n-alkoxy-propionyloxy)] biphenyl-4'-[n-alkoxy-(3,5-dimethyl)]benzoate have been investigated by polarizing optical microscopy and differential scanning calorimetry. The mesophases were identified and confirmed by X-ray too. Three binary mixtures were prepared from the individual homologues. In one of the mixtures (Mix1), the ferroelectric SmC* phase has broadened and became enantiotropic. This mesophase remained monotropic in the other two mixtures (Mix2, Mix3). The chiral nematic N* phase did not appear in Mix1, but remained monotropic for the other two mixtures. Two molecular parameters, the layer spacing and the average intermolecular distance have been calculated from the X-ray results for the homologues and their mixtures. An intercalated tail-to-tail packing of molecules was found both in the single compounds and their mixtures resulting in the layer spacing about half of the molecular length of the single compounds.
Effect of multilactate chiral part of liquid crystalline molecule on mesomorphic behaviour
Journal of Molecular Structure, 2008
Mesomorphic behaviour of several series of materials derived from the lactic acid has been studied and their physical properties have been compared and summarized. These compounds with the same molecular core differing by the linkage group of the non-chiral chain possess a broad variety of liquid crystalline phases: the paraelectric SmA, the ferroelectric SmC * , the antiferroelectric SmC * A , the re-entrant SmC * re , the ferroelectric hexatic phases and a monotropic low temperature phase. The antiferroelectric phase occurs in materials with two lactate groups only, where the ether or the keto group is used as linkage between the non-chiral chain and molecular core. The studied homologues with one, three or even four lactate groups do not exhibit the antiferroelectric phase, but possess a broad range of the SmA and SmC * phases down to room temperatures. The presence of the keto group increases the temperature range and values of the spontaneous polarisation of polar liquid crystalline phases. Molecules with the ether or keto group have a different distribution of electron densities, which seems to be responsible for the difference in the mesomorphic properties. The relationship between the molecular chain structure and mesomorphic properties of lactic acid derivatives with particular molecular core forming the polar mesophases are discussed.
Ferroelectric ordering and electroclinic effect in chiral smectic liquid crystals
Physical Review E, 2004
Ferroelectric ordering, the electroclinic effect and chiral smectic C (SmC*) -smectic A (SmA*) phase transitions in thin planar ferroelectric liquid crystal (FLC) cells are studied by means of linear electrooptic and second harmonic generation techniques. The ferroelectric switching is detected in biased FLC cells by measuring azimuthal dependences of linear and nonlinear responses. The applied DC-electric field rotates the FLC symmetry axis with initial and final orientations in the cell plane. Comparative studies of the switching behavior in reflection and transmission allows to distinguish the contributions from the bulk and the sub-surface layers of the cell. The analysis of temperature dependence shows the existence of a strong surface coupling. The temperature dependent nonlinear polarization shows a critical behavior corresponding to the superfluid model.