Our department was established in 1972 to study and understand physical properties and phenomena characteristic for liquid crystals. Most of the research work is carried out in international co-operation. Our activities during the past decades included the following topics (those of current main interest are underlined):
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synthesis of non-chiral and chiral liquid crystals: Compounds with elongated molecules are typical examples of liquid crystals. Several Schiff's base, azo- and ester-type liquid crystals have been synthesized which exhibit nematic and/or various smectic phases. New low molar mass and monomeric ester-type liquid crystals containing different chiral centers have also been prepared which do not possess mirror symmetry, thus they form chiral phases (cholesteric or ferroelectric smectic C*) with helical structure. Recently achiral bent-core (banana shaped) molecules have been synthesized with switchable 'banana' phases.
Structural formula and space filling model of the building unit of the copolymer
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isotope labeling of liquid crystals: 2H-NMR and neutron spectroscopy requires substances labeled by stable isotopes in order to provide additional information on molecular order and dynamics. New methods for deuterium labeling of starting materials and new synthetic procedures have been developed for synthesis of labeled liquid cystals.
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textures and phase diagrams: Mesophases exhibit characteristic textures in polarising microscopy which allows determination of phase transition temperatures and phase sequences. Mixing of liquid crystals allows an adjustment of the physical properties and occassionally can lead to induction or supression of mesohases.
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neutron spectroscopy: Neutrons can be used to determine the structure of both mesophases and polymer-liquid crystal composites, similarly to X-rays. Additionally neutron scattering can provide information on molecular dynamics, especially about the collective behaviour, e.g. phonons or librons characteristic of such partially ordered systems.
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dielectric spectroscopy: The anisotropies of the static dielectric permittivity are characteristic of the orientational order and determine the behaviour of liquid crystals in electric fields. The frequency dependence of the permittivity, the dielectric relaxation, is related to molecular and collective motions. The relaxation frequencies change abruptly at transitions from smectic phases with two-dimensional liquid to those with a two-dimensional crystalline order of the molecules.
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cross-effects in cholesterics: Some of the thermodynamic transport processes become coupled in cholesterics due to their chirality. The existence of two such cross-effects has been demonstrated experimentally. The thermomechanical coupling connects heat conduction and director rotation, while the diffusio-mechanical effect couples concentration gradients with shear flow. These couplings are forbidden and thus missing in systems with a mirror symmetry (e.g. in nematics).
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electrooptical properties: The electric field tends to align the director (the preferred direction of the molecules) of nematics (or cholesterics) according to the anisotropy of the dielectric permittivity. That leads to either a homogeneous or a periodic change of the optical axis and hence to a variation of the light transmittance. The various electrooptical effects are utilized in LCDs (liquid crystal displays) which became widespread in hend-held as well as in desktop devices.
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ferroelectricity and antiferroelectricity in chiral smectics: The symmetry of the chiral, tilted smectic phases (e.g. smectic C*) allows for the presence of a spontaneous polarization which is parallel to the smectic layers, but normal to the director. Depending on the stacking of layers the substances can be either ferro- or antiferroelectric. The ferroelectric interaction between electric field and spontaneous polarization results in fast electrooptical switching.
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electromechanical effects in ferroelectric liquid crystals: Chiral ferroelectric liquid crystals exhibit a linear electromechanical effect similar to piezoelectricity. As a consequence the electrooptical switching is accompanied with mechanical vibration or as a converse effect flow may induce polarization. The vibrations are either due to the coupling between director reorientation (Goldstone mode) and flow or to the field induced variation of the tilt angle (electroclinic effect). Some resonances observed in the electromechancal spectrum are related to the chevron defect structure.
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ferroelectricity in non-chiral, bent core mesogens: Bent core (banana shaped) mesogens may prefer a polar stacking due to sterical interactions which leads to ferroelectric and/or antiferroelectric behaviour even though the molecules are non-chiral. These banana phases form either racemic or chiral domains which can be switched by electric field, thus offering potential for applications.
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bulk instabilities (electroconvection, shear instabilities):Electrohydrodynamic instability in nematic liquid crystals is an electric field induced periodic deformation associated with convection rolls. The patterns appearing are easily tunable (from stationary to chaotic); by two control parameters (voltage and frequency) as well as by changing the material parameters or orientation of the substance. Shear flow of nematic layers may also result in similar roll patterns despite of the different underlying mechanisms. By studying the pattern characteristics and/or the morphological transitions in liquid crystals one can test the validity and predictions of various theoretical models developed for pattern-forming non-equilibrium systems.
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interfacial patterns (solidification, viscous fingering): Interfaces between phases or substances often produce patterns under non-equilibrium conditions. Phase transitions between the nematic and the ordered smectic B phases of liquid crystals are analogous to the classical solidification problem. The morphology of the growing patterns (faceted, butterfly, dendrite) depend on the undercooling and the anisotropies of heat diffusion and surface tension. The apparent anisotropies of thin liquid crystal layers can be effectively varied by changing their orientation. A liquid of small viscosity (or gas) pressed into a liquid of higher viscosity also forms patterned interfaces (viscous fingering). Here morphological transitions can be induced by varying the pressure and/or the viscosity (e.g due to reorientation by electric field). Both phenomena can be well described by numerical simulations.
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light induced reorientation: The electric field of a light wave exerts a torque on the molecules of liquid crystals. That may cause similar effects as the quasistatic electric field, thus may result in a reorientation of the nematic layer. This effect requires moderate light intensities easily achievable by focussed laser beams. This interaction between light and orientation leads to a giant optical nonlinearity. Polarised light can induce anisotropies in thin layers of certain polymers which can orient liquid crystals. This is effect has great promises for the LCD industry..
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influence of light on dyed liquid crystals: Anisotropic dyes become oriented when dissolved in liquid crystals resulting in a pronounced dichroism of the mixtures. Illumination of the dyes may excite them into states with different properties (e.g. in azo dyes a photoisomerization between trans and cis conformations may occur). This contributes to the optical torque on the molecules thus, depending on the dye, reduces or increases the intensity necessary for reorientation of nematics.
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polymer - liquid crystal composite systems: The random orientation of liquid crystals in small droplets distributed in a transparent polymer can be changed by electric field. The refractive indeces of the polymer and the liquid crystal should match for utilization as polymer dispersed liquid crystal displays. Fine polymeric networks can also be created by photopolymerisation in liquid crystals containing only a small amount of suitable monomer. This composite system has promising electro-optical properties too.
