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AbstractThe structure of aerosil in a liquid crystal was investigated by atomic force microscopy. Depending on the preparation of the sample different agglomerates of aerosil in the liquid crystalline matrix were detected.

AbstractBinary systems of 1-heptyl-4-(4-pyridyl)pyridinium bromide with different proton acceptors were investigated. The existence of hydrogen bonded liquid-crystalline associates was proven.

AbstractThe phase behavior of ceramides IV, the rigid palmitic acid, the fluid oleic acid and the fluid α-methyl palmitic acid in pure and mixed monolayers at the air/water interface is reported. Like natural oleic acid the synthesized α-methyl palmitic acid forms fluid-expanded films, but they are stable with time. All described mixtures were transferred to hydrophilic surface of a Si-wafer as z-type Langmuir-Blodgett monolayers. The surface morphology of these layers is characterized by atomic force microscopy (AFM). It could be shown that addition of all three fatty acids to ceramides IV changes the morphological properties of the film.

AbstractTerminally alkyl- and alkyloxysubstituted 4,6-dichloro-1,3-phenylene bis[4-(phenyliminomethyl)benzoates] have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy, X-ray diffraction and dielectric measurements. Surprisingly, nematic and smectic phases are observed, although comparable bisesters of resorcinol and monochlororesorcinol exhibit B phases typical for banana-shaped mesogens. It could be proved by NMR investigations in the liquid crystallinen state that the molecules have a nearly rod-like shape. Dielectric measurements indicate a rotation around the molecular long axis as known from calamitic mesogens, which is clearly distinguished from the behaviour of bent molecules. Obviously, substitution of the central phenyl ring of the bent 1,3-phenylene bis[4(4-n-alkyloxyphenyliminomethyl)benzoates] by two chlorine atomas causes a markedly change of the molecular shape resulting in nearly calamitic mesogens. These results are supported by single crystal structure studies, which allow the precise determination of the molecular structure and packing in the solid state.

AbstractDielectric investigations of two diols with different molecular shape are carried out in the MHz-range and discussed.

The phase diagram of 4-n-octyloxyphenyl 4-n-pentyloxybenzoate (nematic) and 2-methylphenylbenzoate (not liqu. cr.) was investigated. In the nematic part of the diagram the complex dielectric constants and the relaxation were evaluated as functions of the temperature and the concentration. In the isotropic state the viscosities were measured as functions of the temperature and the concentration. The results allowed the calculation of the nematic retardation factor, the nematic interaction energy E and the activation energy of the rotation of the longitudinal dipole component. E is only slightly influenced by addition of the not liquid crystalline component. The temperature and concentration dependence of the dielectric anisotropy allows the conclusion, that at a constant temperature difference from the initial clearing in the binary system there are constant values of the retardation factor and the degree of order.

AbstractBinary systems of ketones and aldehydes as proton acceptors with different donors were investigated. The existence of different hydrogen bonded liquid crystalline associates was proven by the induction of the smectic A phase as well as by IR-spectroscopic measurements.

A new class of liquid crystalline side chain polymers using swallow-tailed mesogens was synthesized. Comparative dielectric investigations on these polymers and on the corresponding low molecular weight substances in the frequency range from 10 Hz to 100 kHz should give information about the dynamic behaviour. The results obtained from the monomers agree well with those of other swallow-tailed liquid crystals and can be explained by an antiparallel orientation of the molecules in the SA phase. There are hints to a tendency to an antiparallel orientation of the side chains in the polymers, too.

Dielectric relaxation measurements in the frequency range from 1 Hz to 100 kHz on mixtures of a substituted polyacrylate and 4-n-hexyloxycyanobiphenyl have been carried out. In the mixtures the dielectric absorption properties which can be related to the individual compounds has been detected. The relaxation frequency of the low molecular compound is about 50 to 100 times higher in comparison with the δ-relaxation of the polymer but it exhibits nearly the same high activation energy. From the experimental data it can be concluded that there is a molecular disperse distribution of both compounds in which the dynamics is mainly determined by the excess component, the polymer.

Dielectric relaxation and viscosity measurements in a mixture consisting of one component with a high viscosity (A) and the basic mixture Mi 5 have been carried out. The dielectric relaxation times τA for the reorientation around the short molecular axis show a linear increase with the mole fraction of the high viscous compound xA at constant temperature. τA has been extrapolated to χA → 0 and the relaxation time of A at infinite dilution τA∞ has been estimated. The difference between the relaxation times τMi5 and τA∞ is explained by geometrical factors.

Transition temperatures, both principal relaxation times and respective activation enargies have been found for the first five members of the pentylphenyl - alkoxythiobenzoate homologous series. It has been found that the transition temperatures exhibit a weakly descanding odd-even effect whereas all dynamic quantities increases almost continuously with the number of carbon atoms in the alkoxy chain.

Results of dielectric relaxation studies of two members of 4, 4'-n-alkoxyazoxybenzene homologous series, 3.OAOB and 5.OAOB, are presented. On the basis of dielectric spectra obtained for parallel (ε∥*) and perpendicular (ε⊥*) orientations of the samples with respect to measuring electric field the following problems are discussed: (i) the sign change of dielectric anisotropy when frequency increases from radio to microwave range; (ii) influence of the local field factor on the dipole correlation time in the nematic phase, and (iii) mechanism of molecular reorientations taking into consideration the dielectric spectra of three substances having different electric structures. It was concluded that the main molecular process contributing to the high frequency dielectric spectra of the nematic phases is the reorientation around the long axis of molecule as a whole, characterized by the relaxation time τ⊥DR ≅ 85 ps, whereas the QNS method reflects the internal reorientations of two molecular moieties around N - phenyl bonds with the correlation times τQNS = (4 ÷ 9) ps.

Four cyano-substituted liquid crystals of different length to width ratio have been added to a nematic basic mixture. Their influence on the dielectric relaxation process observed parallel to the nematic director was investigated in the frequency range from 0.1 to 100 kHz. The decrease of the relaxation frequency by increasing molecular length has been correlated to the molecular geometry as well as to the increasing activation energy.

The geometrical anisotropy in the shape of swallow-tailed ST liquid crystalline molecules results in an extremely strong tendency of an antiparallel order in the short range. Dielectric measurements on a binary system of a ST and a double ST compound show that even at a mole fraction of 0.5 this effect can be observed. The strong deviations from the statistical distribution of the directions of the molecular long axis is caused by the asymmetric repulsive forces and the dipole-dipole interaction.

Swallow-tailed mesogens of liquid crystalline side group polysiloxanes show a very strong tendency to form an antiparallel order in the short range and to connect in this way different oligomer strands - like a molecular zip-fastener. To check the strength of such a molecular cooperation swallow-tailed side groups of oligomers were exchanged stepwise by laterally aryl substituted mesogenes. The gradual destruction of the zip-like gearing caused by this variation can be observed by dielectric measurements. Comparing investigations show that the tendency to form antiparallel package is greater for the oligomers under discussion than in mixtures of the corresponding monomers.

Stabilization and destabilization of liquid crystalline phases in mixtures of derivatives of phenol and cyclohexanole with pyrrolidines and pyridines were investigated by phase diagrams. Whereas in mixtures with pyrrolidines a strong destabilization of the liquid crystalline state was found, mixtures with pyridines stabilize LC phases. The results are interpreted by 1:1 associates formed by hydrogen bridges and the molecular shape of these complexes.

The formation of induced liquid crystalline phases by hydrogen bonds between nitrogen in pyridine derivatives (P) and the OH group in various carboxylic acids (A) was investigated by phase diagrams, X-ray studies and dielectric measurements. In mixtures with mono- and bifunctional pyridines respectively, increasing clearing temperatures and induced smectic A phases were found. Bifunctional non-liquid crystalline acids as components in mixtures with bifunctional pyridine derivatives give only compounds in the solid state. The data are interpreted by the formation of stable associate AxPy which acts as a new compound.

The relaxation of the parallel permittivity component has been observed in the liquid crystalline substance of swallow-tailed molecules with extra CN-side group. The data has been examined with help of the many-body cooperative model and the results compared with some earlier ones.

Liquid-crystalline 1,3-dioxane derivatives incorporating a lateral cyano-group in position 5 were synthesized. The liquid-crystal phases between the transition temperatures were determined and compared with those of related materials. In spite of the lateral CN-dipole, the new compounds exhibit only small negative or even positive anisotropies of the dielectric constant. The Δε-values are only moderate due to compensation of dipole moments.

Liquid crystalline r-2-t-5-disubstituted c-5-cyano-1,3-dioxanes have been synthesized. The melting temperature of the new substances are increased in comparison to simpler 2,5-disubstituted 1,3-dioxanes, whereas the clearing temperatures are of the same order of magnitude. These compounds exhibit large negative dielectric anisotropies. Some of the 2-cyclohexyl-substituted derivatives have Sc-phases.

AbstractThe structure of aerosil in a glassy state of liquid crystalline polymer was investigated using the method of atomic force microscopy. Rod-like agglomerates of aerosil were found. Average agglomerate size, anisotropy rate and inter-agglomerate distance were estimated.

AbstractSmectic phases are obtained by self-organization of proton acceptors and donors which fulfil the two conditions: a suitable pKs-value of the carboxylic acid and a molecular shape of the associate which is able to form mesophases. Following this concept associates with smectic B phases and at the first time the crystalline SEmodification were designed. The existence of the phases was proven by calorimetric, microscopic and X-ray investigations.

AbstractThe irreversible electro-optical response (memory effect) in suspensions of liquid crystals (LCs) and aerosil was investigated. Thereby the influence of the intermolecular interaction in liquid crystals, of the aerosil concentration and its surface treatment and of electric field parameters on the efficiency of the memory effect was studied. The experimental results are discussed in the frame of a model where the filled nematic LC is considered as a system of aerosil agglomerates in a liquid crystal matrix stabilised by elastic forces.

The destruction of the intercalated smectic A structure of a laterally bulky branched 1,4-phenylen dibenzoate A by addition of a rodlike cinnamate B was studied by the phase diagram as well as by calorimetric, x-ray and dielectric measurements. The thermal stability of the SA phase of compound A and its phase transition enthalpy decrease with increasing concentration of B. Furthermore, the destruction of the short range order of A is characterized by increasing layer distances, decreasing dipolar correlation and an increasing dielectric relaxation frequency only of component A.

Comparative dielectric investigations on polymers with crossing mesogenes have been carried out in the frequency range from 1 Hz to 10U kHz. The spacer length and the mesogenic parts of the main chain as well as the dipole of the crossing units have been altered.

Results of dielectric studies of two members of 4-n-pentylphenyl-4-n-alkoxytiobenzoate (nS5) homologous series, namely 5S5 and 6S5, are preseted and discussed in terms of molecular motions.

New derivatives of α and β- naphthol have been synthesized all of which have glass transitions near to room temperature. The former are isotropic liquids, the latter are monotropic nematic with probable biaxiallty in the glassy state. The compounds have been investigated by calorimetry, viscosimetry, dielectric and X-ray measurements.

Some chiral 2,5-disubstituted 1,3,4-thiadiazole derivatives, (S)-2-methylcarboxylates and (S)-2-alkoxypropionates were prepared. The liquid crystalline and ferroelectric properties of these materials were investigated.

About 50 novel liquid crystalline 2,5-disubstituted 1,3,4-thiadiazole derivatives incorporating a carboxylic group or anoxymethylene unit have been prepared. The dependence of the liquid-crystal transition temperatures of this new class of compounds on diverse central linkages, chain lengths, alicyclic structural units and lateral substituents has been studied systematically. Most of the compounds exhibit broad smectic C mesomorphic ranges.

Measurements of the complex dielectric permittivity for the isotropic, nematic and smectic phases (SA and SC) of the 4-n-pentylphenyl-4-n-octyloxythiobenzoate (8S5) have been performed in the radio frequency range and at one microwave frequency of 9.38 GHz. The dielectric anisotropy of 8S5 is positive in both the radio and microwave frequency regions. However, in the smectic C phase there is a change of sign of the dielectric anisotropy which might be connected with dipole-dipole correlations. The low frequency relaxation process exhibits a Debye-type behaviour in all liquid-crystalline phases. The temperature dependence of the dielectric relaxation time will be discussed in terms of the Arrhenius law.

Dielectric relaxation studies of the isotropic and nematic phases of 4,4'-n-diethoxyazoxybenzene (PAP) have been carried out in the radio and microwave frequency ranges. In the nematic phase two substantially different relaxation processes, with relaxation times τ‖ ∼ 10-8 sec and τ‖ ∼ 10-11 sec, have been detected. They are connected with the reorientational motions of molecules around their short and long axes, respectively. The dielectric permittivity extrapolated to very high frequencies is markedly greater than the squared refractive index. This indicates that there might be additional relaxation process in the infra-red region. The relaxation spectrum of the isotropic phase is broad and the most probable relaxation time τ0 = 2 · 10-11 sec.