Macroscopic properties of mixed chiral smectic liquid crystals

Binary mixtures of chiral and non-chiral smectogens, DOBAMBC and DBHP respectively, exhibit a chiral smectic C phase in two separated concentration regions (about 0-20 and 60-100 mol per cent of DOBAMBC) and a chiral smectic I phase in the region of 60-100 mol per cent of DOBAMBC. For the S*_cphase, the temperature and the concentration dependences of the spontaneous polarization, the tilt angle and the helical pitch were determined. The electric field switching properties of the S*_iphase have been studied. For higher concentrations, the switching is similar to that of the S*_iphase of pure DOBAMBC. When the concentration of DOBAMBC is lowered, bistable switching is observed.     

Analysis of tensides in complex samples with comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry

Multidimensional gas-chromatographical analysis of various tensides of natural or synthetic origin in cosmetic products is demonstrated. Comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry allows the qualitative and quantitative determination of alkyl polyglucosides (AG), fatty alcohol ethoxylates (FAEO), fatty alcohol sulfates (FAS), fatty alcohol ether sulfates (FAES) and cocamidopropyl betaines (CAPB) in shower gel and cleaning agents. The samples were aliquoted in two parts. The first part was silylated, diluted and analysed; then, in order to detect anionic tensides (FAES, FAS) too, the second aliquot was hydrolysed before being silylated for analysis. Because of their amphoteric character, the betaines can only be analysed by gas chromatography after thermal decomposition in the injector, which leads to the corresponding amidoamines among other products.     

Field-induced phase transitions and reversible field-induced inversion of chirality in tilted smectic phases of bent-core mesogens

Three homologous achiral five-ring bent-core mesogens are presented where 4-chlororesorcinol is the central core and the aromatic rings are linked by ester groups. These compounds form smectic phases with a tilted arrangement of the molecules (tilt angle ≈ 45^°). On cooling the isotropic liquid this phase adopts a fan-like texture which shows for two homologues at relatively high electric fields ( 25-35V μm^-1) an antiferroelectric electro-optical response based on the collective rotation of the molecules around their long axes. At lower temperature the application of a sufficiently high electric field leads to a continuous transition into a non-birefringent texture which exhibits randomly distributed domains of opposite handedness. These domains can be reversibly switched into a state of opposite chirality by reversal of the field polarity. This switching is bistable and shows a current response typical for a ferroelectric ground state. The possible mechanism of the field-induced phase transition, of the ferroelectric switching and of the field-induced inversion of the chirality is discussed on the base of XRD, ¹³C- and ¹H-NMR investigations, dielectric and electro-optical measurements.     

The Cayley transform in the numerical solution of unitary differential systems

In recent years some numerical methods have been developed to integrate matrix differential systems whose solutions are unitary matrices. In this paper we propose a new approach that transforms the original problem into a skew-Hermitian differential system by means of the Cayley transform. The new methods are semi-explicit, that is, no iteration is required but the solution of a certain number of linear matrix systems at each step is needed. Several numerical comparisons with known unitary integrators are reported. This revised version was published online in June 2006 with corrections to the Cover Date.     

From laterally branched mesogens to novel twin molecules

Extending a design concept for liquid-crystalline compounds with large lateral substituents, we have synthesized several new variants of twin compounds: lateral-terminal flexible bound, and lateral-lateral flexible bound with and without ring systems in the flexible spacer. X-ray investigations of the nematic phases of several new compounds proved the occurrence of reflections that are incompatible with the simple molecular dimensions. Models in which the two halves and ring-containing spacers of the twins are nearly parallel to one another can explain the experimental facts, assuming an interdigitation of the molecules in the nematic state. Several new compounds form nematic glass phases at about room temperature.     

Phase transitions between smectic A phases as well as smectic C* phases and undulated structures in terminal non-polar compounds

Two types of smectic A and smectic C phases, respectively, have been proved by X-ray and DSC methods for compounds that are terminal non-polar, but consist of tuning-fork shaped molecules. The structural models of the phases are discussed on the basis of steric interaction and of a dense packing. The X-ray patterns of oriented samples point to an undulation of the smectic layers in the case of the smectic A and smectic B phase.     

Mesophase characterization of some long chain vicinal multihydroxy amphiphiles [1,2]

The mesophase characterization of three stereoisomeric multihydroxy compounds, the 1, 2(S), 3(R), 4(R)-, 1, 2(R), 3(S), 4(R)-, and 1, 2(S), 3(S), 4(R)-ico-sanetetrols, as well as of an octadecafuranose derivative has been carried out by means of polarizing microscopy, X-ray diffraction etc. The four multiols with their very long flexible alkyl chains are mono-thermomesomorphic; their single mesophases are miscible with each other, with a typical carbohydrate derivative, and are characterized by X-ray diffraction having a layer structure without order in it. The phase transition data of the three icosanetetrols are strongly dependent on their stereochemistry. The measurements of the refractive indices of one of the tetrols show a typical behaviour known for S_A phases or for lamellar phases, i.e. the sign of the birefringence is positive.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic 'banana phase'. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic 'banana phase' occur.