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1.
V ra Hamplov Alexej Bubnov Miroslav Ka par Vladimí ra Novotn Yveta Lhot kov Milada Glogarov 《Liquid crystals》2003,30(12):1463-1469
New chiral liquid crystalline materials containing a keto group and two lactate units attached to the mesogenic core have been synthesized and studied. All the new compounds show the paraelectric SmA, ferroelectric SmC* and antiferroelectric SmC*A phases each over rather broad temperature ranges. The properties of the phases were characterized by DSC, electro-optical and dielectric studies. The spontaneous polarization, tilt angle, helical pitch, and switching times were determined for the polar phases. 相似文献
2.
We report the dielectric relaxation behaviour in the antiferroelectric SmCA* and ferrielectric SmCγ* phases of the antiferroelectric liquid crystal 4-[5-(4-octloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(methoxyphenyl)butanoate which shows an antiferroelectric transition at around 88±0.1°C. In the SmCA* phase, two dielectric relaxation modes have been found, namely the usual antiferroelectric Goldstone mode and another arising from molecular rotation around its short axis. In the SmCγ* phase, one dielectric relaxation mode has been observed due to the ferrielectric Goldstone mode. Dielectric increments and relaxation frequencies of the antiferroelectric and ferrielectric phases are estimated from the fits of the Cole-Cole function of the dielectric spectrum. The dependence of the bias field in the ferrielectric phase is also discussed. 相似文献
3.
An achiral swallow-tailed material, 2-propylpentyl 4-(4'-decyloxybiphenyl-4-carbonyloxy)benzoate, p, showing SmA and SmCalt phases was prepared for mixing (by weight percentage) with an antiferroelectric liquid crystal, (S)-MHPOBC, m, for the study. The binary mixture p15/m85 using (S)-MHPOBC (85%) as a host doped with achiral material (15%) resulted in a phase sequence SmA-SmC*-SmC*A. The electro-optic response of this mixture in the ferroelectric SmC* phase displayed V-shaped switching, while that in the antiferroelectric SmC*A phase displayed a double hysteresis switching. The mixture p85/m15 possessed SmA* and SmC*A phases; V-shaped switching was found in the antiferroelectric SmC*A phase of this mixture. These optical phenomena implied that a binary mixture containing a larger amount of achiral swallow-tailed material and/or possessing relatively lower polarization favours the occurrence of V-shaped switching in the antiferroelectric phase. The results of this work also suggested that thresholdless V-shaped switching in chiral smectic liquid crystals can be achieved by mixing an achiral swallow-tailed material with an antiferroelectric liquid crystal. 相似文献
4.
A. Ezcurra M. A. P rez Jubindo M. R. De La Fuente J. Etxebarria A. Rem n M. J. Tello 《Liquid crystals》1989,4(2):125-132
The complex dielectric permittivity of HDOBAMBC has been measured in the vicinity of the chiral smectic C-smectic A transition as a function of temperature and frequency. The Goldstone mode contribution has been clearly identified and its relaxation frequency has been determined over the whole S*C range. High accuracy tilt angle measurements have also been performed by using a new method which allows its determination in a direct and virtually continuous way. These experimental results together with additional measurements of the polarization and helical pitch have been compared with those of DOBAMBC and analized in the framework of the generalized Landau theory. As for DOBAMBC a reentrant behaviour S*C-SC-S*C has been observed under an electric field near the S*C-SA transition. 相似文献
5.
《Liquid crystals》2000,27(11):1437-1443
In order to study the difference in microscopic orientation of ferro- and antiferroelectric liquid crystalline molecules, we synthesized the partially deuteriated chiral compounds, (S)-(-)-2-methylbutyl 4-(4-(d13)-hexyloxy- and (S)-(-)-2-methylbutyl 4-(4-(d17)-octyloxy-phenyl)benzoates. Fundamental physical properties such as phase transition temperatures, spontaneous polarization and tilt angle were determined. Polarized FTIR measurements were also made to provide information on molecular structure and orientation. 相似文献
6.
《Liquid crystals》1997,23(3):339-348
Heat capacities of the antiferroelectric liquid crystals 4-(1-methylheptyloxycarbonyl)phenyl4-octyloxybiphenyl-4-carboxylate (MHPOBC) and 4-(1-methylheptyloxycarbonyl)phenyl4-octylcarboxybiphenyl-4-carboxylate (MHPOCBC), have been measured with an adiabatic calorimeter between 350 and 460 K. MHPOBC showed three smectic subphases (ferrielectric C*, ferroelectric C* and a fancy phase C*) between antiferroelectric smectic C* and paraelectric gamma alpha smectic A, while MHPOCBC exhibited only one subphase (smectic C*). These phases are clearly discriminated by the existence of phase transitions. The enthalpies and entropies gained at the respective phase transitions were very small. A much larger phase transition from smectic A to isotropic liquid was also observed in both compounds. A alpha 相似文献
7.
In this work, the high frequency dielectric relaxation process has been studied for the N*, S*A and S*C phases of the mixture ZLI 3654. To this end, time domain spectroscopy (TDS) was used to cover the frequency range from 107 to 1010 Hz. The dielectric parameters obtained show that the high frequency spectrum is mostly influenced by the reorientation of molecules around their long axes. In addition, the latter process seems to be strongly hindered in the S*C phase, because the necessary relaxation time is of the order of 1 ns, whereas for the N* and S*A phases, the relaxation time is remarkably shorter and equal to 0.3 and 0.4 ns, respectively. 相似文献
8.
We investigated the correlation between orientational order and microscopic organization of the molecules in smectic A and chiral (racemic) smectic C phases by means of solid-state C-13 NMR, powder X-ray diffraction, and electrooptical measurements. The compounds under investigation are 4-((S)-2-methyloctanoyl)phenyl 4'-nonylbiphenyl-4-carboxylate ((S)-MONBIC) and its corresponding racemic compound ((S, R)-MONBIC). Static C-13 NMR indicates that: (1) the orientational angle of the tail with respect to the magnetic field decreases slightly both in the SA and S*C phases as decreasing temperature, and (2) the angle of the core with respect to the field decreases in the SA phase but increases in the S*C phase as decreasing temperature. Analysis of C-13 T1 reveals that the dynamic molecular deformation for the core part can occur near the transition. We discuss the dynamic molecular deformation in comparison with the reorientation of the director at the SA to S*C transition. Based on the experimental results, we propose the structural model in which describes the microscopic organization of the molecules in the mesophases. 相似文献
9.
Two members of a chiral tolane series exhibiting a new TGBA phase and a S*C ferroelectric phase have been investigated by means of electro-optical measurements. These materials belong to the optically active series of (R)-4-(1-methylheptyloxycarbonyl)-4'-(4'- alkyloxy-3'-fluorobenzoyloxy) tolanes and are the tetra- and octa-decyloxy derivatives. Due to the large pitch values in these TGBA mesophases, complete unwinding of the helix is found to be possible in thick cells. This also allows measurements of the electroclinic properties, such as the electric field induced tilt angle, response time, and the ferroelectric properties in the S*C phase. 相似文献
10.
J. Schacht I. Dierking F. Gieß elmann K. Mohr H. Zaschke W. Kuczy ski P. Zugenmaier 《Liquid crystals》1995,19(2):151-157
The mesomorphic properties of the N*, S*c and higher ordered smectic phases have been investigated for a homologous series of 4-[(S)-2-chlor-3-methylbutanoyloxy]-4'-(4-n-alkyloxy-benzoyloxy)biphenyls. They have been characterized by optical texture observation, differential scanning calorimetry (DSC), small angle X-ray scattering and electro-optic measurements. The compounds exhibit a strongly twisted cholesteric phase and smectic phases with large spontaneous polarization and tilt angle values. One or two higher ordered, monotropic smectic phases were found which significantly differ in their rotational viscosity. 相似文献
11.
《Liquid crystals》2001,28(11):1699-1704
The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical smectic phases. Measurements of the optical rotatory power (ORP) of these phases have elucidated the quality of this method for finding the phase transitions between several phases and for investigating their helical structure. The optical rotatory power as a function of temperature for seven wavelengths of light in the range 441 to 665 nm was measured for different phases of the AFLC material investigated, (R)-1-methylheptyl 4-(4'-n-dodecyloxybiphenyl-4-yl-carbonyloxy)-3-fluorobenzoate, with acronym 12OF1M7. The values of the pitch and the optical anisotropy in the plane of the smectic layers for the FiLC (SmC*FI3) phase and SmC*Aphase have been calculated from the ORP data. The results of the ORP rule out the simple clock model for describing the structure of the SmC* (SmC*I1) and AF (SmC*FI2) phases. The results for these phases can be explained either in terms of the modified Ising model or the highly distorted clock model. The ORP measurements establish the existence of SmC*FI3 found already from dielectric, polarization and polarized IR spectroscopy. 相似文献
12.
《Liquid crystals》1997,22(4):401-407
The switching currents and field-induced apparent tilt angles in an antiferroelectric liquid crystal, (R)-MHPOBC, were measured. The structural differences among different smectic C* subphases may sensitively reflect the field or temperature dependence of the apparent tilt angle. In a thin cell, the apparent tilt angle was found to change in two steps as a function of field strength in the SmC*alpha and SmC* phases: a steep increase at lower fields and a small linear increase at higher fields. The steep increase in apparent tilt angle is divided into two parts via a plateau in the ferrielectric SmC*gamma phase. Stepwise change with a plateau is also seen in the relation of apparent tilt angle versus temperature under various electric fields. Moreover, the apparent tilt angle in the vicinity of the plateau is almost temperature- and field-independent, implying a preferred orientation of the molecules in the ferrielectric state. The influence of the cell thickness on the structural changes was also investigated. 相似文献
13.
The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4'-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmCA* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmCA* and hexatic smectic phases were determined. The dielectric strength at the SmCA* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmCA* and hexatic smectic I* phases. 相似文献
14.
Shyamal Kumar Kundu Y. Aoki B. K. Chaudhuri Corresponding author 《Liquid crystals》2013,40(6):787-790
We report the dielectric relaxation behaviour in the antiferroelectric SmCA* and ferrielectric SmCγ* phases of the antiferroelectric liquid crystal 4-[5-(4-octloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(methoxyphenyl)butanoate which shows an antiferroelectric transition at around 88±0.1°C. In the SmCA* phase, two dielectric relaxation modes have been found, namely the usual antiferroelectric Goldstone mode and another arising from molecular rotation around its short axis. In the SmCγ* phase, one dielectric relaxation mode has been observed due to the ferrielectric Goldstone mode. Dielectric increments and relaxation frequencies of the antiferroelectric and ferrielectric phases are estimated from the fits of the Cole–Cole function of the dielectric spectrum. The dependence of the bias field in the ferrielectric phase is also discussed. 相似文献
15.
《Liquid crystals》2001,28(8):1185-1192
A series of trifluoro-substituted benzoate derivatives: (S)-1-ethylheptyl 4-[4-(4-alkyloxy-3- fluorobenzoyloxy)-3-fluorobenzoyloxy]-2-fluorobenzoates is reported. The short chain members (n = 8 to n = 11) display a direct SmC*A-SmA transition, whereas for longer chains a SmC* phase appears, but no ferrielectric phases are present, and a direct SmCA*-SmC* transition is obtained. The mesomorphic properties were studied by optical microscopy and DSC, and by electro-optical, helical pitch and optical rotatory power measurements. The effect of the number and position of the fluoro substituents, and the influence of the chiral moiety on the mesomorphic behaviour are discussed. 相似文献
16.
V. Novotn M. Ka par V. Hamplov M. Glogarov I. Rychetský D. Pociecha 《Liquid crystals》2004,31(8):1131-1141
Compounds with differing numbers of lactate units in the chiral part were synthesized. For all materials, at least two smectic phases were found. In addition to the SmA, the SmC* and/or the tilted hexatic SmI*(F*) phase appear according to the length of the non-chiral alkyl chain. For the shortest non-chiral chain, a direct transition from the SmA phase to the SmI*(F*) phase has been discovered and studied. For compounds with the 2-(S)-methylbutyl alkyl chain and two lactate units in the chiral part the antiferroelectric SmC*A phase occurs. The ferroelectric character of the hexatic phase has been confirmed even just below the SmC*A phase. 相似文献
17.
Guan-Yeow Yeap Michi Nakata Yoichi Takanishi Ken Ishikawa Hideo Takezoe 《Liquid crystals》2013,40(11):1437-1443
In order to study the difference in microscopic orientation of ferro- and antiferroelectric liquid crystalline molecules, we synthesized the partially deuteriated chiral compounds, (S)-(-)-2-methylbutyl 4-(4-(d13)-hexyloxy- and (S)-(-)-2-methylbutyl 4-(4-(d17)-octyloxy-phenyl)benzoates. Fundamental physical properties such as phase transition temperatures, spontaneous polarization and tilt angle were determined. Polarized FTIR measurements were also made to provide information on molecular structure and orientation. 相似文献
18.
《Liquid crystals》1998,24(5):639-645
The synthesis and mesomorphic properties of several homologues of (S)-(+)-1-methylheptyl 4-[4(trans-4-n-alkoxycinnamoyloxy)benzoyloxy]benzoates (series I) and (S)-(+)-1-methylheptyl 4-[4-(trans-4-n-alkoxy-alpha-methylcinnamoyloxy)benzoyloxy]benzoates (series II) are described. The influence of the alpha -methyl group on the different mesophases and the clearing temperatures has been evaluated. Many homologues of both the series show a ferroelectric chiral smectic C (SmC*) phase while the antiferroelectric smectic C phase (SmC*A) is exhibited by compounds of series I. The mesophases have been identified by optical polarizing microscopy, differential scanning calorimetry and miscibility studies. The spontaneous polarization, the helical pitch and the tilt angle of two of the compounds have also been measured. 相似文献
19.
Induced ferroelectric S*C phases are formed by non-chiral SC host phases doped with chiral dipolar guest molecules. In those mixtures the spontaneous polarization Ps and the tilt angle Θ has been investigated as a function of the mole fraction xG of the chiral dopant. In most cases the reduced polarization P0 = PS/ sin Θ has been found to depend linearly on xG. The polarization power which is defined by δP=(∂P0/∂xG)ΔT is discussed in terms of the molecular structure of the chiral dopants. There are systems in which P0(xG) deviates positively from linearity. This behaviour can be understood by considering a local field correction to P0. By assuming a local field of Lorentz type a theoretical relation for P0(xG) has been derived which explains the experimental results. The effect of a local field is considerable if the transverse dipole moment and the polarizability of the chiral dopant are large. 相似文献
20.
The characteristic dielectric behaviour of ferroelectric liquid crystals with a large spontaneous polarization has been studied as functions of the D.C. bias field, frequency, cell thickness and applied pressure. Under the condition in which the contribution of the Goldstone mode is suppressed, a sharp peak in the temperature dependence of the dielectric constant is clearly observed at the transition between SA and S*C phases TSC*sA. The relaxation of the soft mode is observed both in the SA and S*C phases by eliminating the contribution of the Goldstone mode under a D.C. bias field. Another relaxation is also observed in the S*C phase around several kHz in addition to that of the soft mode and the Goldstone mode. The pressure effect on the soft mode was also studied. 相似文献