Investigations of novel liquid crystals with dipolar order, understanding of their phase diagram, topological defects and low frequency relaxations responsible for dielectric behaviour.
Liquid crystals (LC) are self-organized systems of organic rod-like, bent-shaped molecules and molecules of transient shape (hockey-stick mesogens) that form phases with dipolar order. In chiral SmC* liquid crystalline phases basically the ferroelectric (FE) or antiferroelectric (AF) phases can occur. They represent the main potentiality for technical applications as displays, displays with a memory, spatial light modulators, etc.
Functional groups inducing photosensitivity or luminescence are introduced to the molecules and the effect of illumination and/or electric field on mesomorphic studied. In the new materials the structure of phases are established and dielectric, electro-optical and optical properties determined with the aim to contribute to better understanding the origin of these phases and phase sequences. The compounds exhibiting „de Vries” behaviour (small layer shrinkage at the phase transition to the FE phase) are studied to understand details of the phase transitions in these materials. Preparation of new hybrid materials made from inorganic nano-particles grafted with organic compounds or mixtures, which are able to self-assemble into liquid crystalline structures, have been started.
Search for new liquid crystalline materials with chiral rod-like molecules with FE and AF structures and twist grain boundary (TGB) phases.
Study of „de Vries“ materials, which may provide a defect-free FE structure in planar cells for display application.
Research of banana-like molecules. Characterization of new type of phases.
Search and study of liquid crystalline monomers to be used as mesogenic parts for design and preparation of dimers and polymers with polar mesophases.
Investigation of dielectric, electrooptical and optical properties and properties of relaxation modes in different type phases.
Chirality, surface interactions, defects in FE and AF phases of confined samples and free-standing films, their relation to observed textures and their behaviour in electric field. Theoretical description.
Photoelectric effect in photosensitive materials and study of luminescent liquid crystalline compounds.
Study of mixtures to optimise physical properties and to stabilize or induce new phases.
Polarizing microscope (NIKON Eclipse E600Pol) with the heating-freezing stage (THMS600 Linkam) used for texture observation, digital photo-camera NIKON COOLPIX 990
(contact: V. Novotná, A. Bubnov, ext. 2134, room 147)
Multifunctional Oven BINDER VD23 for cell preparation/filling
(contact: A. Bubnov, ext. 2134, room 147)
Impedance analysers (Hewlet/Packard 4192A and Schlumberger SI 1260) for dielectric spectroscopy study in the range of 0.1 Hz-30 MHz applied for dielectric spectroscopy measurements
(contact: V. Novotná, A. Bubnov, ext. 2134, room 147)
Memory digital oscilloscopes (Le Croy 3904 and TEKTRONIX TDC70) for spontaneous polarisation and switching time measurement
(contact: V. Novotná, A. Bubnov, ext. 2134, room 147)
Photoelastic modulator PEM 100 by HINDS Instruments, lock-in amplifier SR 844 (25 kHz-200 MHz, PIN diode and function generator (Philips PM 5191) in the birefringence measurements set-up
(contact: V. Novotná, ext. 2134, room 147)
Multifunctional self-assembling materials on nano-level
V. Novotná
CSF 204/11/0723 (2011-2014)
Synthesis and characterization of reactive mesogenic monomers and their utilisation in crosslinked systems
V. Hamplová, A. Bubnov
Bilateral project ASCR-HAS (Hungary) No. 4 (2010-2012)
New functional materials with fluorinated chains and photosensitive groups in mesogenic molecules
A. Bubnov
CSF 204/11/0723 (2011-2014)
Two liquid crystalline compounds with different shape and chirality aspect have been used to prepare binary mixtures. Achiral hockey stick mesogen has been blended with a chiral rod-like mesogen with SmA- ferroelectric (FE) SmC* - antiferroelectric (AF) SmCA* phase sequence. By tailoring the composition we succeeded in preparation of mixtures with unusual mesomorphic behaviour. Several frustrated phases (blue phases, TGBA and TGBC phases) have been induced in a broad temperature and concentration ranges none of them existing in the starting compounds. We expect the frustration comes from a conflict of strong tendency to the smectic ordering imposed by the hockey-stick shape of molecules and a formation of helical structures due to the chirality of the rod-like compound. We demonstrate a new system showing chiral nematic and antiferroelectric phase in its phase sequence. That represents exceptional observation because for rod-like mesogens the low ability of a nematic compound to be organized in layers is considered to be in conflict with the existence of the anticlinic smectic C phase, which, on the contrary, deserves strong layering. The admixture of the hockey-stick molecules helps to overcome this conflict as the bent shape is connected with higher translation order typical for the formation of the AF phase. The observed increase of the spontaneous polarization in mixtures is also a result of the unique interactions between hockey-stick and chiral rod-like molecules. We found that admixture of the hockey stick molecules to a chiral compound considerably changes the molecular interactions due to non-trivial packing of both types of molecules, which have a significant impact on the mesomorphic properties as well as on macroscopic behaviour. [V. Novotná et al., Soft Matter (2012)].
Liquid crystalline textures observed under polarizing microscope in a blue phase and at the cholesteric-TGBC phase transition.
Novel bent-shaped compounds with the naphthalene central core possessing an outer thiophene unit in the lengthening arm were synthesised and physical properties studied using optical polarising microscopy, DSC and electro-optical methods. Structures of mesophases were identified by X-ray diffraction and their structural parameters established. Non-substituted compounds with central core substituted by CH3 or Cl exhibited the B2 phase. The B7 phase has been observed for all compounds with CN-substituted central core. Symmetrical materials with a thiophene unit in both lengthening arms were not liquid crystalline except for CN-substituted compound exhibiting the B7 phase. It was found that the replacement of the benzene for thiophene ring leads to lowering of phase transition temperatures [V. Kozmík et al., J. Mater. Chem. 21, 3153 (2010)].
Chemical formula and model of studied banana-shaped liquid crystalline compound laterally substituted by CN group. Photograph of planar texture in the B7 phase (from right side).
Domains with twisted structures have been established on planar samples in tilted smectic C phase of non-chiral compounds with rod-like molecules laterally substituted by fluorine atom in central part. This evidences separation of molecular conformers differing in the sense of axial chirality and confirms polar C2 symmetry of these domains. A simple model considering polar surface anchoring energy and bulk energy of the twist can account for this finding. Conditions for coexistence of twisted and homogeneous domains are analysed [M. Glogarová. et al., J. Chem. Phys. 133, 221102 (2010)].
A new series of new multifunctional materials with the azo-group in the molecular core has been prepared, which exhibit ferroelectric properties and are photosensitive. With these materials (known as photoferroelectrics) the illumination by visible light can change their electric and optical properties. After the light is switched off their properties are restored to the original state. Such materials offer a series of applications in optoelectronics or photonics. Besides, the studied compounds exhibit a low shrinkage of the smectic layer at the phase transition to the ferroelectric state, which makes them quite unique. Due to this effect specific defects that arise during the sample preparation and significantly degrade their properties can be eliminated. It has been found that variation of the length of the non-chiral chain joined to the molecular core can optimize photoferroelectric properties as well as layer shrinkage of the studied materials [see V. Novotná et al., J. Mater. Chem. 19, 3992-3997 (2009)].
A series of 15 new liquid crystalline compounds have been synthesized and studied composed of non-symmetrical bent-shaped molecules based on a 7-hydroxynaphtalene-2-carboxylic acid, the central naphtalene core being laterally substituted by chlorine or the methyl group. Studies of textures in planar samples, calorimetric study, X-ray diffraction and dielectric measurements enabled to establish mesomorphic properties (ie. phase transitions and sequences of phases) of these compounds. The studied compounds exhibit the antiferroelectric B2 phase and relatively scarce polar columnar phases. With compounds substituted by chlorine the nematic phase has been found. The occurence of the nematic phase together with the columnar one is quite unique. The work contributed to understand relationship between the molecular structure of compounds and their mesomorphic properties [see M. Kohout et al., J. Mater. Chem. 19, 3153 (2009)].
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