Unusual behavior of binary mixtures of ferroelectric and antiferroelectric liquid crystals with three chiral centers (original) (raw)
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Spectroscopy Letters, 2008
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.
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.
Physical Review E, 1998
Using a high resolution optical polarimeter, we have measured the temperature dependence of the birefringence and optical rotation in chiral smectic liquid crystals that exhibit antiferroelectric, ferroelectric, and intermediate phases. The temperature dependence of the magnitude of the tilt angle was determined from the birefringence of 4-͑1-methyl-heptyloxycarbonyl-phenyl͒ 4Ј-octylbiphenyl-4-carboxylate ͑MHPOBC͒ and of 4-͑1-methylheptyloxycarbonyl-phenyl͒ 4Ј-octylcarbonyloxybiphenyl-4-carboxylate ͑MHPOCBC͒. Both substances exhibit a crossover of the order parameter exponent from the classical value of ϭ0.5 close to the transition to the tricritical value ϭ0.25 far away. This stresses the importance of the sixth order terms in the Landau free-energy expansion for ferroelectric and antiferroelectric liquid crystals. In addition, a discontinuous behavior in the magnitude of the tilt is observed when crossing the smectic-C ␣ * -smectic-C* or smectic-C* -smectic-C A * transitions, whereas the smectic-A -smectic-C ␣ * transition is continuous. The simultaneously determined optical rotation is used to elucidate the structures and the nature of phase transitions. The results are well explained within the framework of a discrete phenomenological model with nearest and next nearest neighbor interactions between the smectic layers.
Liquid Crystals, 2011
In a binary mixture system of ferroelectric and antiferroelectric liquid crystals whose major component shows an unusual reversed phase sequence of SmC * A (1/2) − SmC * , a new phase with ferrielectric order of five layers has been discovered by the electric field-induced birefringence (EFIB) measurements. The EFIB was measured using a photo-elastic modulator (PEM) set-up and by applying an in-plane electric field to a homeotropic aligned cell filled with the binary mixtures of compounds with ferroelectric and antiferroelectric compounds. The contours of constant birefringence in the electric field-temperature (E-T) phase diagrams clearly indicate a distinct region corresponding to a new phase bordering the four-layer SmC * (1/2) on the low temperature side and SmC * α on the high temperature side. This new phase is unambiguously assigned to SmC * (3/5) whose structure has been calculated by Osipov and Gorkunov.
2003
The temperature and frequency dependent dielectric relaxation behavior of a liquid crystalline ͑S͒-͑ϩ͒-1methylheptyl 4-͓2-͑4-alkoxyphenyl͒ thiophene-5-carbonylthiooxy͔ benzoate system is reported. Interesting successive antiferroelectric-ferroelectric-antiferroelectric ͑AF-FE-AF͒ phase transitions are observed in this system resembling the successive phase transitions observed in crystalline Rochelle salt. The smectic-C* (SmC*) to AF1 phase transition ͑around 103.0°C͒ is first order in nature, predicted from the use of Orihara and Ishibashi theory. It is also found that a contribution of the ferroelectric SmC* phase ordering penetrates even in the antiferroelectric AF1 (SmC A *) and AF2 (SmC A *) phases very close to the SmC*-AF1 and SmC*-AF2 phase boundaries ͑critical regions͒. It is suggested that this type of mixing of AF and FE phases might cause surface induced ferroelectric-or ferroelectric-type ordering near the AF-FE phase transitions. A soft mode with Debye-type dispersion was observed in the SmA phase. The thermal behaviors of dielectric dispersion, absorption, and dielectric strength in different phases are also reported and discussed.
In seeking faster switching processes in ferroelectric liquid crystals, research continues to be directed to the smectic-C* subphases. One of the most intriguing of the subphases observed in antiferroelectric systems is the smectic-C*α phase (SmC*α), which has a helicoidal structure analogous to that of the smectic-C* (SmC*) phase, but is nonetheless a distinct mesophase. The observation of broad SmC*α phase ranges in a series of mixtures facilitates a wide range of experimental studies. In this thesis, the effective rotational viscosity of the SmC*α and the SmC* phases are reported for binary mixtures of a ferroelectric liquid crystal and a chiral dopant. The rotational viscosity was deduced by both electro-optical and dielectric methods. The viscosity is found to be independent of the dopant concentration in the most mixtures, although the polarization and tilt angle decrease with increasing dopant concentrations. The dielectric measurements allow the determination of the types of...
Reentrant Ferroelectricity in Liquid Crystals
Physical Review Letters, 2001
The ferroelectric (Sm C ء ) -antiferroelectric (Sm C ء A ) -reentrant ferroelectric (re Sm C ء ) phase temperature sequence was observed for systems with competing synclinic-anticlinic interactions. The basic properties of this system are as follows: (i) the Sm C ء phase is metastable in the temperature range of the Sm C ء A ; (ii) the helix handedness inverts at both Sm C ء -Sm C ء A and Sm C ء A -re-Sm C ء phase transitions; (iii) the threshold electric field that is necessary to induce synclinic ordering in the Sm C ء A phase decreases near both Sm C ء A -Sm C ء and Sm C ء A -re-Sm C ء phase boundaries. All these properties are properly described by a simple Landau model that accounts for nearest neighboring layer steric interactions and quadrupolar ordering only.
Liquid Crystals, 1997
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 signi® cantly. The values of spontaneous polarization are mostly increased compared with both non-substituted and methoxy substituted compounds.