Wiktor Piecek

Homologous series of fluorinated antiferroelectric liquid crystals: a comparative study of physical properties and the electro-optical performance

Recently the investigation in the field of LCD technology has been boosted by the availability of new materials. The new group of fluorinated mesogens has been recently synthesized in Poland. In this work we have been focused on investigations of properties of materials having the broad temperature range of antiferroelectric phase. Such key factors as apparent tilt angle and structural properties and electro-optical performance have been studied by means of electro-optical and X-ray methods. The bend molecular shape and the presence of fluorinated tail has been assumed as a factor leading to an extremely high apparent optical tilt. The length of the fluorinated chain seems to affect th mesogenic behavior.

Determination of nematics material parameters by measurements of Fredericksz transitions in crossed electric and magnetic fields in wedge cell

Wedge cells of the wedge angle of order of few milliradians were used to measure the threshold voltages for the Freedericksz transition in electric and magnetic fields crossed. A nematic liquid crystal, PCB, filling cells was of planar orientation enforced by the treatment of the flat boundary plates, covered with poly(amic acid) PM9. A system of interference fringes appeared in each cell placed in normally incident light between analyzer and polarizer crossed. In the vicinity of each fringe a cell could be considered as a flat-parallel one and hence it was equivalent to the system of flat cells of different precisely determined thickness. Threshold voltages, parameterized by magnetic field magnitudes were interpreted as the eigenvalues of the boundary eigenvalue problem for the operator of the second derivative; the interaction between the nematics and the substrates was assumed very strong with the anchoring direction parallel to the cell boundaries. The resulting formulae were used to determine the anisotropy of diamagnetic susceptibility and the splay elastic constant of PCB after the threshold magnitudes measured. The estimates of material parameters agreed well with those determined by the composite method.

Local field of light wave in nematics

Information on anisotropy of the local field of light wave in nematics is obtained mainly from refractivity measurements. Such investigations, supplemented by those concerning thermal dependence of density (rho) (T), allow for determining optical polarizability anisotropy of nematogenic molecules, as well as their orientational order parameter. On the basis of extensive experimental data, this paper discusses the following problems: applicability of approximate relations between refractive indices of nematic liquid crystals (NLCs) and their molecular parameters for describing optical properties of nematic mesophase has been estimated; anisotrpoy of the local field factor Fk has been investigated, as well as its influence on calculated anisotropy of molecular polarizability. Influence of various extrapolation procedures on calculated molecular parameters of the NLCs has been investigated.

Sign and magnitude of the spontaneous polarization in ferroelectric Sm C

For many years scientists have tried to study `improper spontaneous polarization occurred in ferroelectric liquid crystals. In this work we present our theoretical and experimental results concerning behavior of the spontaneous polarization PS of several compounds with the `ordinary spontaneous polarization as well as PS with sing inversion.

X-ray diffraction study of LC films: generalization of Moncton-Pindac method for studies of internal diffraction maximum using reflected beam on thin film with only one free surface

The chosen results of investigations of internal and external diffraction maximum, using a transmission beam to freely suspended `thick and `thin films of smectic LC, are presented. On the basis of these results the way to the generalization of the Moncton-Pindac method was illustrated. Our generalization of the Moncton-Pindac method can be distinguished from the other methods by study of internal diffraction maximum using reflected (instead of transmission) beam. It is important that the film placed on a heated plate has only one free surface. In commonly used methods, the investigated film has two free surfaces. Some results of diffractometer measurements are presented as examples of studies using the generalized Moncton-Pindac method. Possible experiments using this method, advantages, and inconveniences are discussed.

Modulators for MWIR detectors with liquid crystals

In this paper a two liquid crystal (LC) modulators for mid-wave infrared radiation (MWIR) are presented. A two electrooptical effects (EOE) in liquid crystalline structures have been utilized for MWIR modulation: electric field induced cholesteric - nematic (Ch-N*) phase transition (ChN mode) and switching of the twisted-nematic (TN) structure (TN mode). At the Ch-N* mode an intensity modulation depth was of order of 15% but there wasn’t a dark state. In case of the modulation induced at TN mode was near full. These modulators are quite slow, switching times are order of a few hundreds of milliseconds for Ch-N* mode electrooptical effect and dozens of minutes in case of TN mode.

12.1: High Birefringence LCs for Color-Sequential LCDs

High birefringence (0.3<Δn <0.4) and relatively low viscosity liquid crystal mixtures containing isothiocyanato tolane and isothiocyanato terphenyl liquid crystals are developed. The response time of 1.52 ms using a 1.6-μm TN cell is demonstrated.

Determination of same changes in rotational potentials between ferro-and antiferroelectric phases

Rotational potential of molecule in smectic phases, introduced by Zeks, includes two important parts: dipolar as well as quadrupolar ones. The first part is valid for chiral molecules only, whereas the second one is important for chiral and achiral molecules. The question arises: Whether shape of rotational potential changes when the phase transition between SmC* and SmCA* appears? In our opinion, close to the temperature of phase transition (FLC-AFLC) magnitudes of both, dipolar and quadrupolar parts change. Quadrupolar coupling is more important for antiferroelectric phase, while dipolar factor overwhelms quadrupolar one, for ferroelectric phase.

Determination of liquid crystal material parameters by composite method

The practical applications of the composite method of determination of nematic liquid crystal material parameters (namely the splay and bend elastic constants, the anisotropy of diamagnetic susceptibility and the boundary tilt angle) is presented in the article. The director field value at the nematics cell boundaries is described as a function of the torque transmitted from the deformed bulk, modeling the coupling between the nematics and the substrate. The method is based on approximate solving the coefficient inverse problem. The applied procedure of determination of material parameters may be divided into three stages.

Rotational potential of a chiral molecule around its long molecular axis in the Sc* phase of 4-(2'-methylbutyl)-4'-n-octylbiphenyl-4-carboxylate

Chiral 4-(2-methylbutyl)-phenyl-4-octylbiphenyl-4-carboxylate (CE8) has been studied by dielectric, densitometric, refractometric, and x-ray measurements. On the basis of these measurements the effective value of the molecular dipole moment ((mu) ), the angle between the vector of the molecular dipole moment and the long axis of molecule ((gamma) ) as well as the transverse ((alpha) t) and longitudinal ((alpha) 1) molecular polarizabilities have been calculated. In addition the sign and the magnitude of spontaneous polarization (Ps), the tilt angle ((Theta) ), and the helical pitch (gamma) have been measured for the same liquid crystal CE8. Knowing the molecular parameters (mu) , (gamma) , (alpha) t, (alpha) 1, and (Theta) from the experiment and the molecular structure from computer investigation, the spontaneous polarization Ps has been calculated as the vector sum of the components of molecular dipole moments (mu) perpendicular to the tilt direction. To do it the rotational potential given by B. Zeks has been used. According to this approximation, the measured spontaneous polarization Ps is induced by hindered rotation of the molecules around their long axes and is expressed by molecular parameters (mu) , (gamma) , (alpha) t, (alpha) 1, tilt angle (Theta) and the hindered rotation parameter (eta) . The hindered rotation parameter for CE8 shows that dipolar ordering in this smectic C* phase is rather weak in comparison to the strong orientational order of long molecular axes of the same phase.