Investigations of novel liquid crystals with dipolar order, understanding of their phase diagram, topological defects and low frequency relaxations responsible for dielectric behavior.
Liquid crystals with a dipolar order
In chiral SmC* liquid crystalline phases basicaly the ferroelectric (FE) or antiferroelectric (AF) dipolar order can occur. So far ferroelectricity represents the main potentiality for technical applications as displays, displays with a memory, spatial light modulators, etc. Recently, also a display has been reported based on AF SmC* materials profiting from their higher threshold electric field for switching. On the other hand, thresholdless AF phase has been reported in 1996, which exhibits so called random AF dipolar order.
Besides, another phases with a dipolar order can appear in strongly chiral substances as an intermediate states between the cholesteric and FE phases due to frustration between the layered and twisted ordering. The origin and properties of these phases, know as twist grain boundary (TGB) phases, are not completely understood so far.
In 1996 new polar smectic phases have been discovered in materials composed of banana shaped (bent-shaped) molecules, which can exhibit both AF and FE phases even for non-chiral substances. With these compounds the dipolar ordering is not driven by chirality but by a special molecular form. The number of compounds of this type exhibiting liquid crystalline phases with the dipolar order is rapidly increasing. Their mesomorphic properties are richer than those of classical materials with rod-like molecules.
Research Activities
- Search for new liquid crystalline materials with chiral rod-like molecules and with non-chiral banana-shaped molecules promising new FE and AF structures and TGB phases, or so called „de Vries“ phases, which exhibit no shrinkage of the smectic layer thickness at the phase transition to the FE phase, and thus may provide a defect-free FE structure in planar cells used in display applications.
- 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 confined samples of various dimensions. Chirality, surface interactions and non-linear optical properties in switchable phases composed of bent-shaped molecules. Photoelectric effect in photosensitive materials.
- Study of mixtures of described compounds to optimise FE, switching or properties and to stabilize or induce new phases.
- Defects in FE and AF phases of confined samples and free-standing films, their realtion to observed textures and their behaviour in electric field.
Equipment
- dielectric and electrooptic spectroscopy - impedance analyzer Schlumberger SI1260 (0.01mHz-32MHz), lock-in amplifier SR 530 (5Hz-100kHz), function generator
- apparatus for optical studies based on photoelastic modulator and digital oscilloscope Tektronic
- texture study and optical study of free standing films- optical microscope equipped with camera
Running projects
L. Lejček. GA ČR 202/09/0047 2009-11
Molecular structure and phases of new polar liquid crystals
V. Novotná GAAV ČR IAA100100710 2007-9
New liquid crystals with phases switchable by the electric field
M. Glogarová
MŠMT OC175
2007-10
Multifunctional liquid crystalline materials
M. Glogarová Bilateral project AV ČR –DAAD Germany D7-CZ8/08-09 2008-9
Structure-property relations of long-time stable photoisomerizable
smectic and lyotropic
liquid crystals: new frontiers of electro-optic applications
V. Novotná Bilateral project MŠMT – PMŠ Poland MEB 050818 2008-9
Phases
and structures formed by new bent-shaped molecules
A. Bubnov GAAV ČR IAA100100911 2009-11
New functional materials with fluorinated
chains and photosensitive groups in mesogenic
molecules
A.Bubnov Bilateral project MŠMT – PMŠ Poland MEB 050828 2008-9
New chiral liquid crystals possessing highly tilted smectic phases for optoelectronic applications
Some recent results
Coexistence of surface and volume structures in a sample of a smectic B2 phase with a finite thickness
In a smectic B2 phase of the liquid crystal composed of bent-shaped molecules both uniform anticlinic antiferroelectric structure and twisted structure were observed in the same sample. The twisted structure connects the anticlinic antiferroelectric structure in the sample bulk with synclinic ferroelectric structure near surfaces induced by polar surface anchoring [Novotná V. et al., Journal of Mater. Chem. 16, 2031 (2006))].
A simplified theoretical model was proposed to explain this observation [Lejček L. et al., Liquid Crystals 35, 11 (2008)]. Solutions describing above mentioned structures permit to determine theirs energies. As the energies of uniform anticlinic antiferroelectric structure and twisted structure are comparable, model parameters which are connected with material constants of liquid crystal can be estimated.
Under an applied external electric field the above mentioned structures are deformed. If the field is smaller than the transition field to stabilize ferroelectric structure in whole sample, the ferroelectric anchoring is preferred on one surface while it is not favored on the other one where the antiferroelectric structure remains. Both ferroelectric and antiferroelectric structures are connected by a twist deformation. The details of this solution are shown in Lejček L., Mol. Cryst. Liq. Cryst. 494, 21 (2008).
In the above mentioned ferroelectric and antiferroelectric smectic structures chiral domains can exist. Note that molecules are non-chiral but the arrangement of inclined bent-shaped molecules in smectic layers leads to domains of opposite chirality. In the thin ferroelectric structure near surface domains of opposite chirality are separated by the surface π-twist disclination [Lejček L., Liquid Crystals 36, 907 (2009)]. When the thickness of the ferroelectric structure near surface increases with the field, the surface disclination is transformed into a π-wall stabilized by the field. The solution describing such a π-wall is shown in Lejček L., Ferroelectrics, (to be published).
The purpose of this study is evaluation of influence of the surface anchoring on liquid crystals with bent-shaped molecules which induces structural changes of liquid crystal. Such structural changes can be pronounced by an electric field. These changes can influence electrooptical properties of the sample. Their investigation paves the way to prospective new electrooptical applications.
|
