Frank Gießelmann

On the phase sequence of antiferroelectric liquid crystals and its relation to orientational and translational order

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this material and, in particular, about the presence or absence of SmC* in its phase sequence. By means of dielectric spectroscopy and polarizing microscopy, we show that whereas the pure compound lacks the SmC* phase, this phase rapidly replaces the SmC*β subphase through the reduced purity resulting from temperature-induced chemical degradation which is hard to avoid under standard experimental conditions. X-ray investigations furthermore show that this change in phase sequence is coupled to a decrease in translational order. This explains the large variations in the reported phase sequence and electro-optic behaviour of MHPOBC, in particular concerning the SmC*β phase which has been said to exhibit ferro-, ferri- as well as antiferroelectric properties. It is likely that the sensitivity of the AFLC phase sequence to sample purity is a general property of AFLC materials. We discuss the importance of optical and chemical purity as well as tilt and spontaneous polarization for the observed phase sequence and propose that one of the key features determining the existence of the different tilted structures is the antagonism between orientational (nematic) and translational (smectic) order. The decreased smectic order (increased layer interdigitation) imposed by chemical impurities promotes the synclinic SmC* phase at the cost of the AFLC phases SmC*α, SmC*β, SmC*γ and SmC*a. We also propose that the SmA* phase in FLC and AFLC materials may actually have a somewhat different character and, depending on its microstructure, some of the tilted phases can be expected to appear or not to appear in the phase sequence. AFLC materials exhibiting a direct SmA* -SmC*a transition are found to be typical ‘de Vries smectics’, with very high orientational disorder in the SmA* phase. Finally, we discuss the fact that SmC*β and SmC*γ have two superposed helical superstructures and explain the observation that the handedness of the large scale helix may very well change sign, while the handedness on the unit cell level is preserved.

Axial chiral allenylacetates as novel ferroelectric liquid crystals

Liquid crystalline alkane-3,4-dienoates (allenylacetates) have been synthesized. Most compounds incorporate a heterocyclic 1,3,4-thiadiazole ring or a pyrimidine ring as a constituent of the rigid core. These axial chiral allene derivatives were at first obtained as racemic mixtures. Some of them were also synthesized in enantiomerically enriched form by enantioselective synthesis. The compounds were investigated by polarizing microscopy and by differential scanning calorimetry. The three-ring compounds exhibit broad regions of smectic C-phases. The optically active three-ring compounds show broad S c *-phases with moderate values of spontaneous polarization.

TGB A∗ state in a homologous series of diarylethane α-chloroester ferroelectric liquid crystals

Abstract A twist grain boundary (TGB A∗) or twisted smectic A∗ state was observed in two compounds of a homologous series of diarylethane α-chloroester ferroelectric liquid crystals. The phases have been characterized by optical polarizing microscopy and differential scanning calorimetry. Textures of the TGB A∗ state obtained by preparation on a glass slide and in thin homeotropically orienting liquid crystal cells or as free-standing films clearly show the helical structure, whereas preparation in homogeneously orienting LC cells suggests that the helical structure is suppressed by the cell geometry, in a similar way to that observed for S∗c phases in the surface stabilized geometry (SSFLCs).

Coupled Director and Layer Reorientation in Layer Tilted Ferroelectric Smectic Liquid Crystal Cells

Abstract A reversible reorientation of smectic layers in surface stabilized ferroelectric liquid crystal (SSFLC) cells with layer tilted (chevron) geometries was found by electrooptical investigations. The reorientation of the layers superimposes the reorientation of the director along the S* c tilt cone and is suppressed, if the layers are irreversibly fixed in a bookself geometry by high electric fields. Based on a potential density expansion, a dynamical model of a coupled director and layer reorientation is proposed, which leads to excellent agreement with the electrooptical behaviour of the system investigated. The fit of the model to experimental data supplies a simultaneous determination of several material and cell constants. Basic electrooptical parameters, as contrast, switching times and optical overshoot, are reproduced correctly.

THE ORIGIN OF THE HELICAL TWIST INVERSION IN SINGLE-COMPONENT CHOLESTERIC LIQUID-CRYSTALS

Abstract The cholesteric twist inversion by temperature variation o f a single component thermotropic liquid crystalline system was experimentally investigated and is explained by the partial twisting power model, evaluating the temperature dependent pitch o f five different configurations of a benzoic acid biphenyl ester with two chiral centers. The temperature dependence o f the twisting power and the cholesteric pitch for several stereoisomeric compounds can in first approximation be predicted by the partial twisting powers of the individual chiral centers determined from the partially racemic configurations.

Ferroelectric liquid-crystalline side group polymers - spacer length variation and comparison with the monomers

Abstract The phase behaviour and the ferroelectric properties of liquid-crystalline side group polyacrylates containing an (R)-4″(1-methylheptyloxy)-4′-biphenylyloxycarbonyl-4-phenoxy moiety are compared with the properties of their monomers. A S*C phase polymorphism including monolayer and bilayer structures was found for the polymers. Some of the S*C subphases, consisting of smectic mono- or bi-layers, do not exhibit ferroelectric properties. These unusual properties do not exist for the corresponding monomeric acrylates. Therefore, one has to take into account that the polymer chain exerts a strong influence on the phase behaviour and the ferroelectric properties.

Director movement and potential function of surface stabilized ferroelectric liquid crystal cells by electrooptical investigations

Abstract Electrooptical investigations on a ferroelectric liquid crystal (DOBAMBC) were carried out on a well aligned sample in a 2 μm cell. On the basis of these measurements the complete average director movement during a switching process caused by an electric field can be calculated and will be discussed. The thermodynamic potentials at the SA∗-SC∗ phase transition were determined from the experimental results with a modified Landau theory. They serve as a basis for a discussion of the phase transition and the influence of the surface orientation. Some preliminary results were obtained concerning the electrooptical behaviour of the SH∗ phase.

MESOGENIC BEHAVIOR OF OPTICALLY ACTIVE ALLENES : A NEW CLASS OF FLC

Abstract Some optically active liquid crystalline 1,3,4-thiadiazole derivatives incorporating differently substituted allenic moieties providing an axis of chirality have been synthesized. The compounds were investigated by polarizing microscopy, by differential scanning calorimetry and by electrooptical investigation. Thereby a common ferroelectic behavior was found in the case of two derivatives whereas a more complex Sc*-polymorphism was found in the case of the other derivatives. We propose the occurrence of an antiferroelectric, a ferrielectric and a ferroelectric phase.

Dielectric properties of a surface stabilized ferroelectric liquid crystal in cells of various thickness

Abstract Measurements of the complex dielectric permittivity have been performed in the frequency range of the collective director modes (101–106 Hz) in the vicinity of the S*C↔S*A phase transition of the compound 4-[(R)(-)-(2-chloro-3-methylbutanoyloxy)]-phenyl-[4n-decyloxy]benzoate and revealed that a dielectric relaxation process due to collective director fluctuations is active in the helically unwound S*C as well in the S*A phase, if the formation of the S*c helix is suppressed. A twisted and bent director configuration (splayed state) occurs in thin, planar oriented samples with various cell gaps (2–27 μm). The observed mode of Debye type was characterized as a function of temperature and cell thickness. By a decrease of the cell thickness the dielectric increment is lowered in the S*C phase but enhanced in the S*A phase. This Debye mode is only in part suppressed by the application of a bias electric field. With decreasing cell thickness its relaxation frequency decreases in the S*A phase, but incr...

Computer simulation of the electro-optical switching process in ferroelectric liquid crystal cells with bookshelf geometry

The electro-optical properties of a ferroelectric liquid crystal in thin surface stabilized cells with bookshelf geometry were studied with respect to the temperature and to the frequency with alternating amplitude and waveform of the electric field applied. Using a previously proposed model of director reorientation, the measurements were simulated, and a consistent description of the electro-optical behaviour of the system achieved. The fit of the simulations to the measurements provided material constants for the compound investigated and these are discussed.