Z. Raszewski

Some Physical Properties of Mesogenic 4-(trans-4′-n-Alkylcyclohexyl) Isothiocyanatobenzenes

Abstract Temperature dependences of density, refractive indices, and dielectric constants have been measured for mesogenic compounds of as for some of their cyano-analogues: and . The effects of substituting a phenyl ring by a cyclohexyl ring, as well as of a –CN group by an –NCS group have been analyzed. The results lead to the following conclusions: (i) the above exchange of rings leads to mesogens the long molecular axes of which do not coincide with the axes of the terminal groups, –CN or –NCS; (ii) antiparallel correlation of molecular dipoles found for cyano-mesogens does not occur in the case of isothiocyanato-mesogens.

Liquid-CrystalIine 4-(trans-4′-n-AIkyIcyclohexyI) Benzoates. The Effect of Terminal and Lateral Substituents on the Mesomorphic, Thermodynamic and Dielectric Properties

Abstract From 4-(trans-4′-n-akylcyclohexyl)-benzoyl chlorides four series of esters of the general formula: have been prepared. II, n = 5, Z = H, X = H, F, Cl, Br, I, CHO, CH(OCH3)2, COCH3, CN, N(CH3)2, NO2, NCS, SCN, OH, OCOCH3, CH2CN, CH2—CH2CN, alkyl, alkoxyl, phenyl; IIIa and IIIb, n = 2 − 10, Z = H, X = CN(a) and X = NO2(b); IVa and IVb, n = 2 − 10, Z = CN, X = C5H11(a) and X = C4H9(b).

Refractive Indices of the MHPB(H)PBC and MHPB(F)PBC Antiferroelectric Liquid Crystals

MHPB(H)PBC and MHPB(F)PBC were studied by means of refractometric measurements. The refractive indices: n “ (T), n Á (T) and n I (T) were measured using combined methods, applying both appropriately prepared Abbe refractometer and interference wedge. Values of the n “ (T), n Á (T) for the Sm A*, Sm C* and Sm C A * phases are related to the optical axis perpendicular to the proper smectic planes. The results of the optic measurements were discussed.

The Origin of High Optical Tilt in a Homologous Series of Fluorinated Antiferroelectric Liquid Crystals

The compounds from the fluorinated nF6B (n = 1,2,3,4,5,6,7) homologous series were investigated by means of optical, electrooptical, refractometrical, dielectric, densytometric, X-ray and IR spectroscopy methods. For some compounds being a subject of investigations a computer simulation of molecular properties were done. In our opinion high value of optical tilt Θo angle in comparison with a tilt angle obtained from X-ray measurements can be interpreted in terms of de Vries model of SmA* phases as well as of dimers creation process.

Technical aspects of dielectric spectroscopy measurements of liquid crystals

The dielectric spectroscopy measurements were performed for antiferroelectric liquid crystalline mixture. For this purpose, the cells with ITO electrodes were prepared. It was found that it is not possible to detect some important relaxation modes in Sm A*, Sm C*, and Sm CA* phases. The own cell mode (related to cell properties, i.e., capacity and resistivity) covers the dielectric response of liquid crystalline medium. Dielectric measurements in cells with gold electrodes were done to show all possible relaxations in antiferroelectric liquid crystals (LCs).

Determination of ordinary and extraordinary refractive indices of nematic liquid crystals by using wedge cells

A new accurate and fast interference method for determining ordinary and extraordinary refractive indices of nematic liquid crystals is presented and discussed. The method relies on microscopic measurements of distances between interference fringes appearing in polarised parallel coherent monochromatic light beam transmitted normally to the surfaces through a wedge cell filled with a nematic. Both glass plates confining the cell are coated with a partly transparent thin film of metal which is deposited by evaporation in vacuum. Owing to the multiple reflections between the surfaces and a small edge angle, the interference fringes observed near the wedge apex edge are sharp and equidistant. To apply this method one needs only small amount of an investigated liquid crystal. Basic mathematical formulae and results of an experiment are briefly discussed.

New Orthoconic Antiferroelectrics Useful for Applications

Molecular structures and their relation to the mesogenic behavior as well as physical parameters of orthoconic antiferroelectric smectic phases formed by partially fluorinated esters are reviewed. New multicomponent mixtures with long pitch and their electrooptical performance are demonstrated.

Determination of molecular parameters of the MHPB(H)PBC and MHPB(F)PBC antiferroelectric liquid crystals

Abstract Two investigated antiferroelectric liquid crystals, MHPB(H)PBC and MHPB(F)PBC, were synthesized [1] and some their physical properties were described earlier [2]. Apart of the above, MHPB(H)PBC and MHPB(F)PBC have been studied by means of dielectric, densitometric, refractometric and X-ray measurements. Spontaneous polarization P s and tilt angle θ in ferro- (SmC*) and antiferroelectric (SmCA*) phases of MHPB(H)PBC and MHPB(F)PBC have also been investigated. Transverse α1 and longitudinal α1 molecular polarizabilities have been calculated from the ordinary n 0 and extraordinary n e refractive indices and density ρ of MHPB(H)PBC and MHPB(F)PBC, using Neugebauers, Vuks and Lorentz equations with both Haller and Subramhanyama extrapolation procedures. Effective values of the molecular dipole moment μ have been calculated from electric permittivity ε in the 1 phases of these compounds, using Maier and Meier equation.

Dielectric studies of binary liquid-crystalline mixtures

Abstract Six binary liquid-crystalline mixtures have been studied by means of dielectric, microscopic and refractometric measurements. Induced smectic phases appear for four of them. The effective values of the components p ⊥ and p ∥ of the dipole moment per one molecule of the mixtures have been calculated from values of e ∥ and e ⊥. It has been established that the heterocomplexes formed in the mixtures are weak so that they do not cause any changes in the distribution of molecular charges. The observed deviations from linearity of the dielectric constants of the mixtures are caused either by dissociation of the cyanomesogen dimers or by a non-additive behaviour of the mixture density.

Long Pitch Orthoconic Antiferroelectric Binary Mixture for Display Applications

Orthoconic Antiferroelectric Liq. Cryst. (OAFLC) have been extensively studied due to their unique electrooptical properties [1,2]. Practical implementation of this kind of liquid crystalline medium experiences many obstacles including a number of parasitic effects deteriorating the electrooptical performance. In search for working OAFLC a mixture with the helical pitch longer than the typical cell gap was formulated. Using this mixture near perfect optical uniformity and excellent dark state at the zero electric field applied was obtained. The physical and structural properties of mixture are presented and their parameters are discussed. The electrooptical performance is presented and discussed.