首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 0 毫秒
1.
《Liquid crystals》2001,28(10):1581-1583
A new set of trimesogens consisting of a cholesteryl ester, a biphenyl and an azobenzene mesogenic segment joined together through either odd-even, even-even or odd-odd central alkylene spacers has been synthesized and the liquid crystalline properties investigated. The trimesogens having even-even and odd-odd central alkylene spacers exhibit the undulated twist grain boundary phase (UTGBC*) mesophase over a temperature range of ~40-50°C.  相似文献   

2.
A new set of trimesogens consisting of a cholesteryl ester, a biphenyl and an azobenzene mesogenic segment joined together through either odd-even, even-even or odd-odd central alkylene spacers has been synthesized and the liquid crystalline properties investigated. The trimesogens having even-even and odd-odd central alkylene spacers exhibit the undulated twist grain boundary phase (UTGBC*) mesophase over a temperature range of ~40-50°C.  相似文献   

3.
A novel antiferroelectric liquid crystal, 4-[5-(4-octyloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(4-methoxyphenyl)butanoate (TFMB) showing twist grain boundary phases was found and investigated. In optically active TFMB, a diffuse liquid-liquid transition was observed above the clearing point. TFMB exhibited a stable antiferroelectric smectic C* phase. The relationship between antiferroelectricity and the molecular structure is discussed.  相似文献   

4.
A first oligomeric antiferroelectric liquid crystal, a cyclohexane based trimesogen, was synthesized. It was characterized by microscopic observation, differential scanning calorimetry, helical pitch measurement, X-ray diffraction and electro-optical study. A large domain of antiferroelectric phase (DeltaT 55 C) and a very rich polymorphism (SmC*, SmC*, SmC* , A FI SmC*, TGBA, BP) are exhibited by the trimesogen.  相似文献   

5.
《Liquid crystals》2001,28(7):1121-1125
A new phase sequence: twist grain boundary smectic C (TGBC) to smectic blue phases (BPSm) is observed in a chiral compound (S)- or (R)-1-methyloctyl 3'-fluoro-4'-(3-fluoro-4-hexadecyloxybenzoyloxy)tolane-4-carboxylate. It is the first time that a TGBC phase has been found to occur under smectic blue phases in the absence of the twist grain boundary smectic A (TGBA) phase. These phases are characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray scattering.  相似文献   

6.
A new phase sequence: twist grain boundary smectic C (TGBC) to smectic blue phases (BPSm) is observed in a chiral compound (S)- or (R)-1-methyloctyl 3'-fluoro-4'-(3-fluoro-4-hexadecyloxybenzoyloxy)tolane-4-carboxylate. It is the first time that a TGBC phase has been found to occur under smectic blue phases in the absence of the twist grain boundary smectic A (TGBA) phase. These phases are characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray scattering.  相似文献   

7.
Two types of smectic A and smectic C phases, respectively, have been proved by X-ray and DSC methods for compounds that are terminal non-polar, but consist of tuning-fork shaped molecules. The structural models of the phases are discussed on the basis of steric interaction and of a dense packing. The X-ray patterns of oriented samples point to an undulation of the smectic layers in the case of the smectic A and smectic B phase.  相似文献   

8.
《Liquid crystals》2008,35(3):287-298
We synthesised a series of rod-like mesogens with a (S)-2-methylbutyl-(S)-lactate unit in the chiral chain that exhibited extremely wide temperature ranges in the TGBA and TGBC* phases. TGB phases were identified, based on typical textures in confined samples and in free-standing films, by Grandjean-Cano texture and by NMR studies on a deuterium-labelled isotopomer. A sufficiently high electric or magnetic field transformed the TGBA and TGBC* phases into their respective SmA and SmC* phases, the TGB structures being restored within some 20-30 minutes. Therefore values of the spontaneous polarisation and spontaneous tilt angle, when measured under a sufficiently high field, gave evidence of the properties of the SmC* phase. Temperature dependencies of relaxation frequency, dielectric strength, selective reflection and layer spacing showed anomalies at a certain temperature within the TGBC* phase range. Also, changes in textures, as well as in 2H-NMR spectra, occurred at this same temperature. These results suggest the existence of two TGBC* phases.  相似文献   

9.
《Liquid crystals》1999,26(9):1351-1357
The anticlinic smectic CA phase belongs to the class of tilted smectic phases with an azimuthal angle alternating from one direction (theta=0) to the other (theta=pi) in successive layers. It occurs in general at lower temperature than the uniformly tilted smectic C phase, but may be obtained directly from the untilted smectic A phase. We use the chiral nCTBB9* series synthesized in this laboratory, in order to obtain a phase transition as close as possible to second order, as revealed by DSC. We measure the temperature behaviour of the birefringence and of the optical rotatory power across the transition in order to characterize the tilt angle. We finally study the optical response to a periodic electric field which excites separately the smectic C* and C*A soft modes. The main conclusion is that the only order parameter governing the critica Al behaviour of the phase transition is the tilt angle theta, as we get common divergence of both soft modes at the same temperature. This confirms previous high resolution calorimetric studies by Ema et al. that saw in MHPOBC an initial mean-field second order phase transition when the tilt appears, followed by sharp first order restructuring transitions between the tilted subphases.  相似文献   

10.
The anticlinic smectic CA phase belongs to the class of tilted smectic phases with an azimuthal angle alternating from one direction (theta=0) to the other (theta=pi) in successive layers. It occurs in general at lower temperature than the uniformly tilted smectic C phase, but may be obtained directly from the untilted smectic A phase. We use the chiral nCTBB9* series synthesized in this laboratory, in order to obtain a phase transition as close as possible to second order, as revealed by DSC. We measure the temperature behaviour of the birefringence and of the optical rotatory power across the transition in order to characterize the tilt angle. We finally study the optical response to a periodic electric field which excites separately the smectic C* and C*A soft modes. The main conclusion is that the only order parameter governing the critica Al behaviour of the phase transition is the tilt angle theta, as we get common divergence of both soft modes at the same temperature. This confirms previous high resolution calorimetric studies by Ema et al. that saw in MHPOBC an initial mean-field second order phase transition when the tilt appears, followed by sharp first order restructuring transitions between the tilted subphases.  相似文献   

11.
We propose a simple phenomenological model which is able to account for the various twist grain boundary (TGB) phases, including the recently discovered undulating twist grain boundary-C* (UTGBC*) phase. In the UTGBC* phase, the smectic C* (SmC*)-like blocks and the grain boundaries separating them undulate to form a two-dimensional square lattice perpendicular to the TGB helix axis. We treat the grain boundaries separating adjacent smectic blocks as interfaces with an anisotropic interfacial tension. At moderate chiral strengths we find a TGBA-TGBC-SmC* sequence. As the chiral strength is increased this goes to the sequence TGBA-UTGBC*-SmC*. Such sequences have been observed experimentally.  相似文献   

12.
We propose a simple phenomenological model which is able to account for the various twist grain boundary (TGB) phases, including the recently discovered undulating twist grain boundary-C* (UTGBC*) phase. In the UTGBC* phase, the smectic C* (SmC*)-like blocks and the grain boundaries separating them undulate to form a two-dimensional square lattice perpendicular to the TGB helix axis. We treat the grain boundaries separating adjacent smectic blocks as interfaces with an anisotropic interfacial tension. At moderate chiral strengths we find a TGBA-TGBC-SmC* sequence. As the chiral strength is increased this goes to the sequence TGBA-UTGBC*-SmC*. Such sequences have been observed experimentally.  相似文献   

13.
X-ray diffraction, D.S.C. and miscibility studies have been performed on inary mixtures whose constituent compounds exhibit large values of spontaneous polarization. These studies have led to the observation of a tricritical point for the smectic C*-smectic A transition. The origin of this tricritical behaviour is discussed in relation to the range of the smectic A phase and the large spontaneous polarization.  相似文献   

14.
X-ray diffraction, D.S.C. and miscibility studies have been performed on inary mixtures whose constituent compounds exhibit large values of spontaneous polarization. These studies have led to the observation of a tricritical point for the smectic C*-smectic A transition. The origin of this tricritical behaviour is discussed in relation to the range of the smectic A phase and the large spontaneous polarization.  相似文献   

15.
A series of smectic C* liquid-crystalline (LC) block copolymers were successfully synthesized via the living anionic polymerization of polystyrene with optically active methacrylate monomers containing (S)-2-methylbutyl 4-(4-hydroxyphenylcarbonyloxy)-biphenyl-4′-carboxylate mesogens. These materials are the first reported smectic C* block copolymers. Anionic polymerization in tetrahydrofuran (THF) at −70°C leads to LC block molecular weights of approximately twenty repeating units. The number-average molecular weight of the polystyrene block was varied from 7000 to 20000 to adjust the composition in the block copolymers. Differential scanning calorimetry and optical microscopy indicate that the smectic C* phase is present in the systems over broad temperature ranges.  相似文献   

16.
Dielectric properties of a novel liquid crystal with direct transition from isotropic to twist grain boundary (TGB) phase were studied. Measurements were performed over a wide range of temperature in the isotropic phase, TGBA, TGBC and SmC* phases. The Arrhenius plot of the relaxation frequency of the molecular process around the short axis shows a variation in the activation energy in the isotropic phase. This is attributed to smectic-like fluctuations in the isotropic phase. The dielectric relaxation strength in the TGBC phase has anomalous behaviour in the vicinity of TGBC-SmC* transition. The single cellective process observed in the TGBC phase splits into two processes in the SmC* phase.  相似文献   

17.
Thirty eight compounds composed of achiral banana-shaped molecules belonging to three new homologous series have been synthesized and their mesomorphic properties investigated. The mesophases have been characterized using a combination of polarized light microscopy, differential scanning calorimetry, miscibility methods and X-ray diffraction measurements. All the compounds reported exhibit mesomorphic properties. Most of the homologues in the three series show B1 and B2 phases and the transition temperatures of these are lower in the fluoro substituted compounds. Some of the lower homologues of the fluoro substituted derivatives also show the nematic phase.  相似文献   

18.
《Liquid crystals》2000,27(12):1613-1623
Thirty eight compounds composed of achiral banana-shaped molecules belonging to three new homologous series have been synthesized and their mesomorphic properties investigated. The mesophases have been characterized using a combination of polarized light microscopy, differential scanning calorimetry, miscibility methods and X-ray diffraction measurements. All the compounds reported exhibit mesomorphic properties. Most of the homologues in the three series show B1 and B2 phases and the transition temperatures of these are lower in the fluoro substituted compounds. Some of the lower homologues of the fluoro substituted derivatives also show the nematic phase.  相似文献   

19.
《Liquid crystals》1999,26(12):1787-1796
Two homologous series of chloro and bromo derivatives of S,S-3-methylpentanoic acid were synthesized and studied. For some of these compounds an inversion of helix twist sense in the smectic C* phase was observed. Dielectric dispersion measurements showed a pronounced anomaly of the Goldstone mode relaxation frequency and strength close to the helix inversion point. Moreover, a qualitative difference in behaviour of the mode in samples with planar and with homeotropic alignment was detected; this is due to surface interactions.  相似文献   

20.
Modifying the position and numbers of lateral fluorine substituent is a common method to design and adjust the mesophase of liquid crystal compounds. Here, a series of 2-(2,2?-difluoro-4?-alkoxy-1,1?-biphenyl-4-yl)-5-substituted benzoxazole with both non-polar (H, CH3) and polar (NO2) groups (coded as nPF(2)PF(2)Bx) is synthesised and characterised. All of the compounds show a conspicuous inter-ring twist angle of 38° compared with corresponding reference compounds I and II which are calculated by density functional theory method, and it is interesting to note that the final compounds nPF(2)PF(2)Bx show only nematic mesophase during heating or cooling. Meanwhile, the UV-vis absorption bands and photoluminescence emission peaks both display remarkable blue-shifted. The aforementioned results reveal that lateral difluoro substituents play a key role to stable the nematic mesophase by increasing the dihedral angle of biphenyl.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号