K. Flatischler

Parameter characteristics of a ferroelectric liquid crystal with polarization sign reversal

Abstract The compound shows the unusual behaviour of sign reversal in the spontaneous polarization P versus temperature. This was first reported by Goodby, Chin, Geary and Patel (1). We have now a fairly complete set of data concerning polarization, tilt angle, optical response time, helical pitch and dielectric constant. These parameters are discussed, in particular concerning their temperature behaviour, in order to elucidate the mechanism behind the polarization reversal and its physical significance.

Synthesis and Electro-Optical Properties of Some Ferroelectric Liquid Crystals Derived from Lactic Acid

Abstract We have synthesized two series of liquid crystals, viz. 4′-(ω-(2-ethoxy) propoxy)alkoxyphenyl 4-alkoxybenzoate, 2, (abbreviated Cm-Cn-OEt*) and 4′-(ω-(2-chloro)propoxy)alkoxyphenyl 4-alkoxybenzoate, 3, (abbreviated Cm-Cn-Cl*). Compounds belonging to the latter mainly exhibit smectic A and B phases while the former show enantiotropic smectic C* at or near room temperature as well as A and smectic phases of higher order. The magnitude of the spontaneous polarization is moderate (–4 nC/cm2). There is no alteration in sign for the spontaneous polarization and for the helical pitch as function of the n value. The Cm-C3-OEt* and Cm-C5-OEt* compounds are exhibiting unusually long helical periodicity for a pure compound (30–50 μm). Compounds of series 2 also show a close relationship between spontaneous polarization and tilt angle giving a practically temperature independent ratio P/θ, making the definition of a material constant Po = P/θ meaningful.

Linear electrooptic effects in the chiral nematic phase

Abstract A linear electro-optic effect is described in the unwound (infinite or sufficiently long pitch) state of the chiral nematic phase N, and attributed to the electroclinic effect. It is similar to the effect already known in the smectic A phase. In particular, it is characterized by the same speed and its induced tilt is a linear function of the applied field. We have, so far, been able to induce tilts of up to several degrees in this mode. An interpretation of the effect is given, based on the symmetry of the N phase in combination with the aligning property of the surfaces. A similar electro-optic effect of exceptionally large amplitude but slower and much more complex has been found in cylindrical N domains near the clearing point. An induced angle of deflection well over 45 degrees, corresponding to switching angles in excess of 90 degrees can be observed in this case.

Thin Ferroelectric Liquid Crystal Cells with Surface-Induced Alignment: Optical Characterization and Electro-Optic Performance

Abstract Different surface treatments commonly used for nematics have been tested for alignment effect on ferroelectric smectic C* liquid crystals. Thin samples of thickness around 2 μm are used in order to elastically unwind the helix present in the chiral smectic materials. Electro-optic measurements are made as a function of temperature and the viscosities are deduced. Response times and frequency cut-offs are presented for four different ferroelectric compounds. For some conditions bistability is observed. Different methods for obtaining good alignment are discussed in view of the reported observations.

Surface Induced Alignment Transition in a Nematic Layer with Symmetrical Boundary Conditions

Abstract The inner surfaces of a cell of conventional type were covered with an SiO, aligning layer evaporated at an angle α = 60° and subsequently treated with lecithin in order to achieve symmetrical boundary conditions. The alignment of a nematic liquid crystal layer with negative dielectric anisotropy (Δe < 0) was found to be homeotropic below a critical temperature Tc, (low temperature range). Above that particular temperature the homeotropic alignment abruptly transforms into a planar one, which remains with increasing temperature up to the clearing point TNI (high temperature range). The alignment transition was found to be reversible, and is attributed to a packing change of the lecithin layer with the temperature. A simple model based on the different temperature dependence of the anchoring strengths W H, (T) and W P, (T), characterizing the homeotropic and planar alignment respectively, is proposed to explain the surface induced alignment transition.

Smectic layer switching using a field-induced N*-C* phase transition

Abstract In a chiral smectic material having an N* to C* transition, the smectic state can be induced by an applied electric field several degrees in the cholesteric state above TC. At a certain critical field strength the smectic layers grow out along one of two directions determined by the tilt angle of the C* material and of the sign of its ferroelectric response and thus selected by the sign of the applied field.

Sign reversal of the electroclinic coefficient in the smectic B* phase

Abstract In the B phase of a chiral epoxy compound and its mixtures with an achiral liquid crystal material, a sign reversal in the electroclinic coefficient is observed. The temperature where the coefficient vanishes has been found to be almost independent on the concentration of the chiral molecules. A simple explanation is proposed in terms of conformational changes in the chiral molecule. Supported by molecular mechanics calculations we believe that the anti conformation dominates in the A and C phases, whereas some of several gauche conformations gain, weight in the B* phase and become predominant below a certain temperature.

Measurements of rotational viscosity in four ferroelectric (C*) liquid crystals

Abstract The rotational viscosity was measured in the C* phase of four liquid crystal compounds with varying structures and macroscopic characteristics. The different properties include high/low polarization-viscosity, 2.order and 1.order ferroelectric transition (“small” tilt and 45° tilt substance) and polarization reversal. Different measurement methods were used and compared.

Electroclinic Effect in Some Side-Chain Polysiloxane Liquid Crystals

Abstract The electroclinic effect in the chiral smectic A phase has been investigated in side-chain polysiloxane liquid crystals. The amplitude of the electro-optical response in the smectic C* phase was studied, and the tilt angle determined by studying the response at very low frequency. The electroclinic effect shows the same general behaviour as in low-molecular weight systems, with a divergent electroclinic coefficient near Tc and a field-independent response time of about 10 microseconds a few degrees from Tc

Identification of the liquid crystal phases of some chiral Schiff bases

The mesophases of eleven chiral Schiff bases are identified by miscibility examinations, using the contact method. Some of these compounds exhibit medium pitch (2-3 μm) chiral smectic C mesophases ...