摘要:

The ferroelectric properties of induced smectic C* phases with novel chiral dopants were investigated which are characterized by a rigid molecular core directly bearing chiral centres and a transverse dipole. A strong influence of the achiral host phase on the magnitude and the sign of the spontaneous polarization was observed. The host effect is discussed in terms of an extended microscopic model considering hard-core interactions of the dopant molecules with their surrounding host phases

ISSN:1058-725X    

DOI:10.1080/10587259508033565

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The light scattering measurement and the polarized microscopic observation of field induced phase transition between antiferroelectric (AF) and ferroelectric (F) phases have been performed in a uniformly aligned cell. The light scattering is confirmed to be diffraction by the periodical change in refractive index in the stripe domains of AF and F phases. In F → AF phase transition, the nucleation of AF domain occurs at interfaces between liquid crystal and spacer edges and AF domain grows away from spacers. On the other hand, in AF → F phase transition, discrete F domains homogeneously appear and grow throughout the cell. The rotation of smectic layer is also observed under the application of an asymmetric voltage. This rotation is originated from the asymmetry of the polarity reversal of the applied voltage.

ISSN:1058-725X    

DOI:10.1080/10587259508033566

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The dielectric dispersion in the ferroelecric and antiferroelectric phases of a new liquid crystal (S)-4-(1-methylheptyloxycarbonyl)-phenyl 4-(4-dodecyloxybenzoyloxy)benzoate has been investigated in the frequency range from 10 Hz to 10 MHz. The behaviour of the soft mode in SmA and the Goldstone mode in SmC* have been determined. Two dielectric relaxations have been recognized in the antiferroelectric SmC*Aphase : the first one between 5 kHz and 50 kHz and the second one, also observable in the ferrielectric SmC*FIphase, around 1 MHz. In SmC*FI, a drastic change in the behaviour of the low frequency response has been found depending on whether the sample was heated or cooled. A sign reversal of the helical pitch has been determined inside the SmC*FIphase, leading to low pitch values of opposite sign in SmC*Aand SmC*.

ISSN:1058-725X    

DOI:10.1080/10587259508033567

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We report on the uniform alignment and continuous ferroelectric switching of a twisted smectic C* liquid crystals (LCs). In contrast to surface stabilized ferroelectric LCs, a twisted smectic C* (Sm C*) structure exhibits essentially no threshold field and produces intrinsic gray scales. For constructing such a twisted structure, the symmetry in molecular tilt with respect to the rubbing axis is externally imposed by the surface forces. The symmetry required for producing a uniformly twisted structure is that the twist angle between two rubbing axes on the substrates must be exactly twice of the molecular tilt. One important parameter is the existence of the smectic A state in the phase sequence, which facilitates to the uniform growth of the smectic layers. We perform numerical simulations to obtain the director profiles and the resultant electro-optic modulation as a function of the electric field in the twisted Sm C* structure. It is found that the experimental data agree well with the numerical results.

ISSN:1058-725X    

DOI:10.1080/10587259508033568

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The phase transition from the SmA to the lower-temperature SmC* or SmC phase in a restricted geometry of a planar cell is studied theoretically, considering a polar coupling of the ferroelectric liquid crystal to the surfaces. The λ-line in a (2L, T) phase diagram, whereTis the temperature and 2Lis the cell thickness, is determined numerically. The critical temperatureTcof the SmA ⟷ SmC* (SmC) phase transition exibits nonmonotonic dependence on the cell thickness 2L. Two tricritical points are found on the λ-line, which could be candidates for the Lifshitz points.

ISSN:1058-725X    

DOI:10.1080/10587259508033569

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In a discrete phenomenological description of tilted smectic phases, the order parameter, which describes the magnitude and the direction of the tilt of the molecules, is defined for each smectic layer. The free energy expansion in terms of order parameter takes into account interactions between nearest and next nearest smectic layers, and allows for competing interactions. By minimizing the free energy of the system the structure of the frustrated Sm C*αphase is obtained.

ISSN:1058-725X    

DOI:10.1080/10587259508033570

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A detailed study of a novel material exhibiting antiferro-, ferri-, and ferroelectric phases has been undertaken. The tilt angle, spontaneous polarisation and the dynamics of the helical unwinding have been examined as a function of temperature and voltage in all the phases of interest. The material exhibits large tilt angles (∼31°) and values of spontaneous polarisation (∼100nCcm−2) in cases where the helix is totally unwound. The behaviour of tilt angle as a function of applied electric field is examined in the ferri- and antiferro-electric phases. A stepwise increase in tilt angle as a function of applied voltage in the ferri-electric phase is presented, as well as unusual behaviour in the antiferro-electric state. In addition, a preliminary investigation of the change in spontaneous polarisation as a function of pressure is presented. It is shown that the dependence of spontaneous polarisation in the ferroelectric phase is relatively weak.

ISSN:1058-725X    

DOI:10.1080/10587259508033571

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The SmA, SmCα* and SmC* phases of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl) phenyl 4‘-octyloxybiphenyl-4-carboxylate (MHPOBC) have been studied by means of simultaneous measurements of the dielectric constant and the electroclinic coefficient under a dc bias electric field. From the three dimensional representation of the electroclinic coefficient as a function of dc electric field and temperature, it is confirmed that there is no helix with a long pitch in the SmCα* phase and the electric field-temperature phase diagram for these phases is obtained. In the SmA phase near the phase transition temperature, the temperature dependences of the dielectric constant and the electroclinic coefficient are analyzed in terms of the phenomenological theory of Landau type and the parameters appearing in the theory are determined.

ISSN:1058-725X    

DOI:10.1080/10587259508033572

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Phase diagram of the ANNNI model with the third neighbour interaction is obtained to clarify the ferrielectric phases appearing in the sequences of phase transitions of certain ferroelectric smectic liquid crystals. While various mesophases are ferrielectric, those with wave numberq= 3/8 and 3/10 come out to exhibit the second order phase transition from antiferroelectric phase to ferrielectric one with decreasing temperature. The phases withq= 4/13 and 4/11 are the candidates for the ferrielectric phases FIHand FILobserved just above and below the SmCy*, respectively.

ISSN:1058-725X    

DOI:10.1080/10587259508033573

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

It is known that ferroelectric liquid crystal cells vibrate mechanically under the effect of ac fields. The vibrations are mainly due to the backflow and electroclinic effects. For small director oscillations the effect is mainly linear and goes with the frequency of the field. For larger oscillations higher harmonics appear too. We studied the practically interesting case when, due to rectangular excitation, the director switches fully between two stable positions. In the experiments bookshelf textures of (R)-MHPOBC and FLC6430 (from Hoffmann La Roche) were used. The accelerations of the cover plates due to the reversal of the field were detected in three orthogonal directions. The switches cause strong vibrations of the plates (lasting less than a millisecond,. acceleration ∼10m/s2). The vibration is largest at the SmCα*-Sm A phase transition indicating that the dominating effect is electroclinic in origin.

ISSN:1058-725X    

DOI:10.1080/10587259508033574

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor