Browsing by Author "Dierking, Ingo"

Now showing 1 - 7 of 7

Open Access
Determination of the rotational viscosity and of the elastic constant from measurements of switching times and threshold voltage on planar oriented Sc phases
(1992) Dierking, Ingo; Gießelmann, Frank; Zugenmaier, Peter; Pelzl, Gerhard; Schiller, Peter
The Freedericksz transition of planar oriented Sc phases was studied experimentally with an electrooptical method and the theoretically predicted reorientation along the Sc tilt cone confirmed. Measurements of the threshold voltage and the switching times lead to the determination of the rotational viscosity and the elastic constant for five selected compounds with positive dielectric anisotropy.
Open Access
Electric field induced domain formation in surface stabilized ferroelectric liquid crystal cells
(1994) Dierking, Ingo; Gießelmann, Frank; Schacht, Jochen; Zugenmaier, Peter
Two types of domains have been observed for S sub(C) ferroelectric liquid crystals in surface stabilized cells (SSFLC) by application of a high electric field with the smectic layers tilted by the amount of the chevron angle with respect to the normal of the rubbing direction in the substrate plane. The layer structure resembles that of a chevron configuration in the plane of the substrate similar to the recently reported stripe-shaped SSFLC structure. The two domain types 'appear' to switch in a reciprocal fashion when applying an AC electric field and observed between crossed polarizers. The temperature dependence of this effect has been investigated and an explanation proposed analogous to a striped texture model.
Open Access
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
(1993) Dierking, Ingo; Gießelmann, Frank; Zugenmaier, Peter; Kuczynski, Wojciech; Lagerwall, Sven T.; Stebler, Bengt
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.
Open Access
The origin of the helical twist inversion in single component cholesteric liquid crystals
(1994) Dierking, Ingo; Gießelmann, Frank; Zugenmaier, Peter; Mohr, Kristina; Zaschke, Horst; Kuczynski, Wojciech
The cholesteric twist inversion by temperature variation of a single component thermotropic liquid crystalline system was experimentally investigated and is explained by the partial twisting power model, evaluating the temperature dependent pitch of five different configurations of a benzoic acid biphenyl ester with two chiral centers. The temperature dependence of 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.
Open Access
Properties of higher-ordered ferroelectric liquid crystal phases of a homologous series
(1994) Dierking, Ingo; Gießelmann, Frank; Kußerow, Joachim; Zugenmaier, Peter
Four members of a homologous series of diarylethane α-chloroester ferroelectric liquid crystals [1] have been synthesized and characterized by polarizing optical microscopy and DSC measurements. Two of the compounds exhibit a twist grain boundary structure (TGB A* phase) or twisted smectic A* phase, shortly before reaching the smectic to cholesteric phase transition. The temperature dependence of the optical tilt angle θ, the spontaneous polarization Ps and the effective rotational viscosity γφ were investigated for the ferroelectric S*C, S*I and S*F phases. It was found that especially the spontaneous polarization and the rotational viscosity increased with decreasing length of the alkyl chain for all three ferroelectric phases. At the phase transition from S*C to S*I, all material parameters determined showed a discontinuous change, giving evidence for a first order transition. At the transition from the S*I to the S*F phase, the material parameters show only small, rather continuous changes.
Open Access
Strong electroacoustic effect in ferroelectric liquid crystal cells
(1992) Gießelmann, Frank; Dierking, Ingo; Zugenmaier, Peter
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.
Open Access
TGB A* state in a homologous series of diarylethane α-chloroester ferroelectric liquid crystals
(1994) Dierking, Ingo; Gießelmann, Frank; Zugenmaier, Peter
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).