A. Seed

Heterocyclic Esters Exhibiting Frustrated Liquid Crystal Phases

Abstract A series of novel heterocyclic esters has been prepared based on selenophene, thiophene and furan units; for comparison the parent, phenyl analogue was also prepared. All the materials except the least linear furan analogue are mesogenic and exhibit the SA, SC*, SC*ferri and SC*anti liquid crystalline phases. In addition, the selenophene material exhibits a higher ordered antiferroelectric phase which has been designated the antiferroelectric smectic I (SI*anti) phase and below this the SI* phase was generated. The synthesis, mesomorphic behaviour and physical properties of the materials are reported.

Resonant x-ray scattering at the Se edge in liquid crystal free-standing films and devices

Resonant x-ray diffraction was carried out at the Se K edge in thick free-standing films of a selenophene liquid crystalline material, revealing detail of the structure of the ferro-, ferri-, and antiferroelectric phases. The ferrielectric phase was shown to have a three-layer superlattice. Moreover, the structure of a lower temperature hexatic phase was established. For the antiferroelectric phase, investigations were also carried out in a planar device configuration. The device allowed resonant scattering experiments to be carried out with and without the application of an electric field and resonant data are compared with electro-optic measurements carried out on the same device.

Electric field-induced layer deformations in the subphases of an antiferroelectric liquid crystal device

The layer structure in the antiferroelectric, ferrielectric, and ferroelectric phases of a liquid crystal device is reported, together with its electric field-induced deformation. The field-free chevron angle is comparable to the steric tilt angle, but differs significantly from the optical tilt angle. A sharp field threshold is observed for the chevron to bookshelf transition in the antiferroelectric phase at 1.3 V/μm, while layer deformations occur at much lower fields (0.3 V/μm) in the other subphases. Models are proposed for the layer deformations.

The Physical Properties of A Series of Antiferroelectric Hetrocyclic Esters

Abstract A series of highly chiral hetrocyclic esters have been studied. The effect that the choice of hetroatom has on the linearity of the molecules and hence the mesomophism of the materials has been investigated. Measurements of the spontaneous polarisation, tilt angle, current response time and rotational viscosities of the materials are reported. At 30°C below the transition from the SmA phase all members of the series show a Ps of ∼100nCcm−2 and a high tilt angle of about 30°. These physical measurements confirm the presence of a higher order antiferroelectric phase in the selenophene derivative in this series.

A possible structural model of the SmC*beta phase

The so-called chiral smectic C-beta (SmC*beta) phase has been reported as distinct from the SmC* phase in several materials that exhibit antiferroelectric liquid crystal mesophases and subphases. The SmC*beta phase is known to be chiral, tilted and to exhibit ferrielectric switching, but no structure had been suggested which explains these effects. This paper presents a possible structure for the SmC*beta phase which can explain the ferrielectric properties. The model is proposed on the basis of complementary optical, electric and X-ray diffraction studies of a chiral liquid crystal. The layer spacing, optical and steric tilt and spontaneous polarization over the temperature range of the SmC*beta phase are described. The complementary experimental techniques used reveal the occurrence of inversion phenomena, on which this model is based.

The synthesis, mesomorphic behaviour and physical properties of a chiral liquid crystal exhibiting an antiferroelectric smectici phase

Abstract A new, highly chiral liquid crystalline material exhibiting two antiferroelectric phases is reported. The material is a phenyl biphenyl carboxylate, and both the synthesis and the physical properties are described in detail. Measurements of the electro-optic switching, spontaneous polarisation, tilt angle and viscosity are presented across the entire mesophase ranges. The optical switching characteristics of the material, together with the current pulse, clearly show that the phase below the antiferroelectric chiral smectic C phase is also antiferroelectric. Physical property measurements indicate that the phase is one of the higher order smectic phases, which is identified as smectic I* by miscibility studies. It is concluded that the phase is the antiferroelectric hexatic chiral smectic I phase.

Antiferroelectricity in novel liquid crystalline materials

Abstract A group of six structurally similar compounds exhibiting antiferroelectric phases have been studied. The physical properties of these materials are described in detail. Measurements of the spontaneous polarisation, tilt angle and rotational viscosity as a function of temperature are given. These results are discussed with reference to the causes and stability of the antiferroelectric SC* phase. They are generally found to support conclusions reached by previous authors.

Conoscopic observations of multiple ferrielectricity in a chiral liquid crystal

Abstract The sequence of antiferroelectric, ferrielectric and ferroelectric phases in the material AS573 (Hull, UK) has been investigated via conoscopy and selective reflection. Evidence was found to support the division of the phase previously labelled SmC*γ into two ferrielectric subphases. A revised phase sequence for the material, based on this study, is presented.

The influence of sulfur on phenyl propiolates for ferroelectric applications

Abstract The synthesis of a novel series of phenyl propiolates is reported. The phase behaviour and electro-optic properties of these materials are compared with attention to the effect of the substitution of the alkyoxy terminal chain for an alkylsulfanyl chain. The phase behaviour of these materials is found to be dramatically affected by the addition of an alkylsulfanyl terminal chain, which results in the removal of the frustrated phases. The physical properties are more affected by a change in chiral centre than by the alkylsulfanyl substitution. However it was found that the tilt angle is decreased by the sulfanyl substitution. This is thought to be due to steric effects.

Electro-Optic and Piezo-Optic Studies of an Antiferro-Ferri-Ferro-Electric System

Abstract 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.