Browsing by Author "Gießelmann, Frank"

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Open Access
Amphotropic azobenzene derivatives with oligooxyethylene and glycerol based polar groups
(2015) Tan, Xiaoping; Zhang, Ruilin; Guo, Chunxiang; Cheng, Xiaohong; Gao, Hongfei; Liu, Feng; Bruckner, Johanna R.; Gießelmann, Frank; Prehm, Marko; Tschierske, Carsten
A series of amphiphilic azobenzenes with one to three lipophilic alkyl chains at one end and polar groups with oligooxyethylene (EO) and racemic 3-glyceryl units at the opposite end was synthesized and their thermotropic and lyotropic liquid crystalline self-assemblies were studied by POM, DSC and XRD. Tilted and non-tilted lamellar phases with interdigitated double layer structures (SmCd and SmAd, respectively) were found for the compounds with a single alkyl chain, whereas hexagonal columnar phases were formed by the compounds with two or three alkyl chains. The effect of protic solvents, like formamide, ethylene glycol and water, was investigated for representative examples. For the compounds with the single chain, induction and stabilization of SmA phases were observed, though broad regions of lyotropic SmC phases were retained in most cases. Depending on the structure of the polar group, the hexagonal columnar phases were either removed or drastically stabilized by the solvents. Photoisomerisation of an azobenzene chromophore was also studied.
Open Access
Computer simulation of the electro-optical switching process in ferroelectric liquid crystal cells with bookshelf geometry
(1993) Gießelmann, Frank; Zugenmaier, Peter
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.
Open Access
Counterion effects on the mesomorphic and electrochemical properties of guanidinium salts
(2024) Ebert, Max; Lange, Alyna; Müller, Michael; Wuckert, Eugen; Gießelmann, Frank; Klamroth, Tillmann; Zens, Anna; Taubert, Andreas; Laschat, Sabine
Ionic liquid crystals (ILCs) combine the ion mobility of ionic liquids with the order and self-assembly of thermotropic mesophases. To understand the role of the anion in ILCs, wedge-shaped arylguanidinium salts with tetradecyloxy side chains were chosen as benchmark systems and their liquid crystalline self-assembly in the bulk phase as well as their electrochemical behavior in solution were studied depending on the anion. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS) experiments revealed that for spherical anions, the phase width of the hexagonal columnar mesophase increased with the anion size, while for non-spherical anions, the trends were less clear cut. Depending on the anion, the ILCs showed different stability towards electrochemical oxidation and reduction with the most stable being the PF6 based compound. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest a possible contribution of the guanidinium cation to the oxidation processes.
Open Access
Coupled director and layer reorientation in layer tilted ferroelectric smectic liquid crystal cells
(1993) Gießelmann, Frank; Zugenmaier, Peter
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.
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
Director movement and potential function of surface stabilized ferroelectric liquid crystal cells by electrooptical investigations
(1990) Gießelmann, Frank; Zugenmaier, Peter
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.
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
Electrooptical investigation on the three switching states of a chiral smectic side group polymer
(1992) Gießelmann, Frank; Zugenmaier, Peter; Scherowsky, Günter; Kühnpast, Kurt; Springer, Jürgen
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.
Open Access
Ferroelectric liquid-crystalline side group polymers - spacer length variation and comparison with the monomers
(1993) Kühnpast, Kurt; Springer, Jürgen; Scherowsky, Günter; Gießelmann, Frank; Zugenmaier, Peter
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.
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
Sulfobetaine ionic liquid crystals based on strong acids : phase behavior and electrochemistry
(2024) Lange, Alyna; Kapernaum, Nadia; Wojnarowska, Zaneta; Holtzheimer, Lea; Mies, Stefan; Williams, Vance; Gießelmann, Frank; Taubert, Andreas
A group of new zwitterion based ionic liquid crystals (ILCs) have been synthesized. Depending on the counter anion (mesylate or hydrogen sulfate) the phase behavior of the resulting ILCs is quite different. Mesylate based ILCs show complex phase behavior with multiple phases depending on the alkyl chain length. In contrast, hydrogen sulfate based systems always exhibit Colr phases irrespective of the alkyl chain length. The latter show much larger ILC mesophase windows and are thermally stable up to ca. 200 °C. All ILCs show reasonable ionic conductivities of up to 10-4 S cm-1 at elevated temperatures, making these ILCs candidates for intermediate temperature ionic conductors.
Open Access
Synthesis and characterization of liquid-crystalline side group polymers with benzylideneaniline as mesogenic moiety
(1989) Gießelmann, Frank; Zugenmaier, Peter
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.
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).
Open Access
Untersuchungen zum Mechanismus und zur Modellierung elektrooptischer Schaltvorgänge in ferroelektrischen Flüssigkristallphasen
(1992) Gießelmann, Frank
Gegenstand der vorliegenden Arbeit sind Studien zur Beschreibung der Dynamik und des Mechanismus elektrooptischer Schaltvorgänge in der S*c Phase niedermolekularer und polymerer Flüssigkristalle. Die Thematik wurde durch kombinierte Anwendung von Meß- und Simulationsverfahren bearbeitet. Ausgehend von Messungen der Direktortrajektorien in S*c Phasen während des elektrooptischen Schaltvorgangs liefern die Ergebnisse dieser Arbeit über eine direkte experimentelle Bestätigung bisheriger mechanistischer Vorstellungen hinaus weitere, qualitativ neue Aspekte zum Schaltmechanismus von S*c Phasen. Diese betreffen eine reversible Schichtbewegung in Chevron-Strukturen und den Einfluß konformativer Wechselwirkungen auf den Schaltvorgang bei Seitengruppenpolymeren. Das entwickelte Simulationsverfahren ermöglicht eine realistische und konsistente Beschreibung des elektrooptischen Verhaltens der untersuchten Systeme. Darüberhinaus ermöglicht es eine simultane Bestimmung relevanter Materialkonstanten und liefert technisch wichtige Kenngrößen der Schaltcharakteristik.
Open Access
Untersuchungen zum Mechanismus und zur Modellierung elektrooptischer Schaltvorgänge in ferroelektrischen Flüssigkristallphasen
(1994) Gießelmann, Frank
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.