Untersuchungen zum Mechanismus und zur Modellierung elektrooptischer Schaltvorgänge in ferroelektrischen Flüssigkristallphasen

Im Zeitalter der modernen Mikroelektronik hat sich der Einsatz von nematischen Flüssigkristallen in optoelektronischen Anzeigeelementen (LCDs, "Liquid crystal displays") zu einer wichtigen Schlüsseltechnologie entwickelt. Mit steigenden Anforderungen werden die Grenzen dieser konventionellen Flüssigkristalltechnologie deutlich. Die heute favorisierte Lösung dieser Probleme liegt in der Entwicklung von Displays auf der Basis ferroelektrischer Flüssigkristalle

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

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

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

Coupled director and layer reorientation in layer tilted ferroelectric smectic liquid crystal cells

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

Synthesis and characterization of liquid-crystalline side group polymers with benzylideneaniline as mesogenic moiety

Two new thermotropic liquid-crystalline side group polymers were synthesized, characterized and compared with the liquid-crystalline monomeric analogues. Some packing features of these polymeric liquid crystals are discussed. Investigations were carried out by differential calorimetry (D.S.C.), polarization microscopic observation and X-ray diffraction on non-aligned and magnetic field-aligned samples in the wide and small angle region. The synthesized polymers contain as mesogenic moiety a benzylideneaniline group which is attached in the 4 position via a hexamethylene spacer to a poly-methacrylate backbone. The benzylideneaniline group is substituted in 4′ position with an ethoxy or butoxy group (PEt or PBu). The monomeric analogues are denoted MEt and MBu. The two polymers show a phase sequence crystalline-smectic A-nematic-isotropic. The liquid-crystalline temperature range is observed between 90 and 150°C. The monomeric MEt exhibits only a monotropic nematic, MBu an enantiotropic nematic and a smectic A phase

Strong electroacoustic effect in ferroelectric liquid crystal cells

A strong electroacoustic effect of a ferroelectric liquid crystal cell was observed with a 1 MV/m electric square field in the frequency range from 3 kHz to 30 kHz. The sound emitted can be heared with the naked ear and easily detected with a simple microphone. The electroacoustic effect vanishes in the chiral SA-phase

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

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)

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

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

Electrooptical investigation on the three switching states of a chiral smectic side group polymer

Electrooptical properties of a polyacrylate with 1,3-dioxolane-4-carboxylic acid as chiral building block terminally attached to phenylpyridine mesogenic moiety and linked via a C11 spacer were studied. Results showed that the formation of 3 switching states for the polymer occurred upon conformational interactions between the side groups and the main chain

Investigations of the structure of a cholesteric phase with a temperature induced helix inversion and of the succeeding S*C phase in thin liquid crystal cells

Investigations of 4-[(S, S)-2, 3 epoxyhexyloxy]-phenyl-4-(decyloxy)-benzoate by polarizing microscopy, the Cano-Grandjean method, optical rotation dispersion and UV-VIS spectroscopy reveal a cholesteric phase with temperature induced reversal of the helical twist. Switching time experiments in the S*C phase show that the intrinsic helix can be unwound reversibly and irreveribly by application of electric fields of different strengths