A. Vajda

Ester type banana-shaped liquid crystalline monomers: synthesis and physical properties

New ester-type banana (bent-shaped) monomers, 1,3-phenylene bis[4′-(alken-1-yloxy)-biphenyl-4-carboxylate]s Ia–Vb, with different substituent(s) on the central phenyl ring (H, CH3, Cl or NO2) and alkenyl tails in the side arms (decenyl or undecenyl) were prepared. The analogues IIIa–IVb with 4-chloro- or 4,6-dichloro-substituents exhibited a nematic phase, while Va,Vb with 2-nitro-substituent showed a B7 phase at relatively low temperature. All the compounds were stable, no degradation or polymerization was observed under applied electric fields or heat treatments. Electro-hydrodynamic instabilities were observed in the nematic phase of each sample. In the B7 phase of Vb there was an electro-optical switching in the range 112–114.5 °C with a switching time of about 150 µs at 20 V µm−1 field. At lower temperatures no electro-optical switching occurred, but a ferroelectric-type spontaneous polarization of 22 nC cm−2 could be detected. The bend angle was calculated by the semiempirical CAChe/MOPAC/PM5 method.

Properties of a Homologous Series of Ferroelectric Liquid Crystals

Abstract A homologous series of MBOPEnOBA chiral compounds has heen synthesized and some chemical and physical properties of the components as well as that of a binary mixture has heen studied. Measurements of the spontaneous polarization and the pitch are presented and results of eutectic, dielectric, electrooptic and electromechanical investigations are given.

Mesophase behaviour of binary mixtures of bent-core and calamitic compounds

The mesophase behaviour of binary mixtures of bent-core and calamitic liquid crystals is presented. The nematogenic 4,6-dichloro-1,3-phenylene bis[4′-(10-undecen-1-yloxy)-1,1′-biphenyl-4-carboxylate] (I) was the banana-shaped component. As the calamitic compound ethyl 4′-(9-decen-1-yloxy)-1,1′-biphenyl-4-carboxylate (II), similar to one arm of the bent-core molecule, was used which exhibits smectic phases in a wide temperature range. A total of six mixtures with different compositions were prepared and studied by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction on non-oriented samples. In the mixtures, a nematic phase is not concomitant with smectic A phase, and the temperature range of both phases highly depends on the concentration of the comprising compounds. Lowered melting temperatures have been observed for all mixtures with respect to that of the pure compounds. Unforeseen finding is the induction of a monotropic SmC phase in mixtures with lowest concentration of the bent-core compound. Semi-empirical quantum-chemical calculations have also been performed. Based on the calculated molecular conformation, as well as on collected X-ray diffraction data, a model for a possible self-assembly of the banana-shaped and calamitic compounds is proposed.

Structural properties of the ferroelectric liquid crystal mixtures based on the homologous series of alkoxyphenyl alkoxy benzoates

Abstract The temperature dependence of the layer spacings and the average intermolecular distances were studied in smectic A, C, B and nematic phases by X-ray diffraction on non-oriented samples. Two representatives of 4-n-alkoxyphenyl 4′-n-alkoxy benzoates, furthermore their eutectic binary mixture, and ternary ferroelectric mixtures obtained by the chiral additive of (R)-(1-methylheptyl)-1, 1′-4′-1′-terphyenyl-1, 4′-dicarboxilate used in different concentrations were investigated by small and large angle measurements as well.

Phase diagrams and physical properties of binary ferroelectric mixtures based on a series of chiral α-cyanocinnamate derivatives

We report a detailed investigation of binary mixtures composed of members of the homologous series of (S)-4′-(2-n-alkoxypropanoyloxy)biphenyl-4-yl 4-n-alkoxy- f-cyanocinnamates. The phase transition curves for these mixtures have no minima. On the contrary, eutectic behaviour was obtained if the members of this homologous series were mixed with structurally different chiral cinnamic acid derivatives and other ferroelectric liquid crystal materials. Spontaneous polarization, tilt angle and dielectric constant results were obtained for three of the single compounds and nine ferroelectric mixtures.

Polymorphic crystalline forms of 8OCB

For most alkoxycyanobiphenyls (3OCB, 5OCB, 6OCB, 7OCB and 9OCB) it has been observed that slow or delayed cooling gave rise to a lower melting crystalline polymorphic form which then slowly converted into the higher melting stable crystalline structure. Although under slow evaporation from a solvent, polymorphic crystalline structures were observed also for 8OCB, a polymorphism on cooling like that in other nOCBs has not previously been reported. We have now observed that 8OCB also shows polymorphism, which brings it into line with the other homologues. On keeping the material between 35 and 38 °C after it was cooled from above the melting point of the stable crystal form (55 °C), regular platelets grow usually from one point and fill the whole sample. The texture is stable for weeks at room temperature. Upon heating it shows a melting point at 50 °C, i.e. 5 degrees below the stable crystalline melting point. It is interesting that all platelets are non-symmetric and have the same handedness.

Light-induced spontaneous pattern formation in nematic liquid crystal cells

Optically induced instabilities were observed in nematic liquid crystals sandwiched between a photosensitive layer and a reference plate. The instabilities occurred when the light entered the cell from the reference plate and its polarization was parallel to the director alignment at the entrance face. Two kinds of patterns were detected, a static and a dynamic one. The underlying mechanism of pattern formation is the coupling between the director orientation on the photosensitive plate and the polarization direction of the light on that plate.

Properties of non-symmetric bent-core liquid crystals with variable flexible chain length

New non-symmetrical bent-core mesogens with a variable flexible chain length have been synthesised. The compounds were derived from 3-hydroxybenzoic acid, which is the central unit of the molecules. The tails on both ends were connected by ester functionality, one tail contains a terminal double bond and the other is saturated. The compounds represent novel non-symmetric molecules both in their core and tails. The main features of differential scanning calorimetry, polarising optical microscopy, electro-optical and polarisation current measurements for all studied materials indicate a transition between a single tilted synclinic ferroelectric and an antiferroelectric smectic phase.

The Structure of Mesophases of Binary and Multicomponent Mixtures of some Cholesteric Liquid Crystals

Abstract We present the results of the study of binary and multicomponent mixtures of cholesteryl enanthate with cholesteryl nonanoate and cholesteryl formiate. The aim of the study is the preparation of the system with phase transition temperature close to room temperature. Phase diagrams of binary mixtures are formed on the basis of data obtained by the optical microscopy and DS calorimetry and the shift of phase transition points of order 30–50° with respect to initial components was established. In the case of three- and four-component mixtures, we have identified the presence of enantiomorphic phase transitions of smectic and cholesteric type respectively. X-ray diffraction data have enabled the characterization of the structure of smectic or cholesteric mesophase and the calculation of molecular parameters: apparent length of the molecules l and the average intermolecular distance (D).

Magnetically induced shift of the isotropic–nematic phase transition temperature in a mixture of bent-core and calamitic liquid crystals doped with magnetic particles

We have investigated the influence of doping with spherical magnetic nanoparticles on the mixture of a bent-core and a calamitic liquid crystal. Results showed a reduction of the critical field of the magnetic Fréedericksz transition by more than a factor of two after the doping. Moreover, we give for the first time experimental evidence of the theoretically predicted magnetically induced negative shift of the isotropic to nematic phase transition temperature.