Abstract L'effet de la rotation rapide dans le champ magnetique d'une mesophase lyotrope est etudie pour des angles entre l'axe de rotation et le champ magnetique compris entre 25° et 90°. Le comportement a basse vitesse est analogue au comportement des nematiques thermotropes de Δχ < 0. Pour des vitesses elevees, la force d'inertie rotationnelle prend le pas sur la force magnetique et oriente le directeur perpendiculairement a l'axe de rotation. Pour des vitesses intermediaires, il existe un angle o[ugrave] l'on a equilibre entre les deux forces. Cet equilibre conduit a une distribution du directeur autour de deux directions privilegiees, l'une parallele a l'axe de rotation, l'autre perpendiculaire. Nous avons etudie cet equilibre en fonction de la vitesse de rotation. A l'angle magique, le directeur de la phase est distribue isotropiquement sur tout l'echantillon. Sitot que I'on s'ecarte de l'angle magique, le directeur se distribue autour d'une direction privilegiee. L'etude des bandes de rotation perm...

Internal cyclization of three new phenyldiazene liquid crystals (R is an alkyl substituent with 4, 6 or 8 carbons) with activated methylene group in the ortho position to the diazo linkage was studied .The initial liquid crystals was synthesised and characterized by 1 H NMR, electrospray mass spectrometry and differential scanning calorimetry.The final compound was characterized by 1 H NMR and differential scanning calorimetry.The kinetic of cyclization was studied at different temperatures and followed by reversed phase HPLC and a UV detection.For all the temperatures used, it appeared that the cyclisation was a first order reaction for the three compounds.The Arrhenius plot (ln reaction constant k against 1000/T) gave the mean activation energy of the cyclisation.

Two new phenyldiazene molecules with butyloxy and hexyloxy substituents (denoted iC 4 and iC 6 , respectively) have been synthesized and characterized by 1 H NMR, electrospray mass spectrometry, and differential scanning calorimetry (DSC). Because of the activated methylene group in the position ortho to the diazo linkage, internal cyclization of these molecules can occur when the temperature is increased. The kinetics of the two cyclization reactions, and the influence of temperature, were studied by HPLC on a μPorasil column with hexane–chloroform–THF, 53:38:9 ( ν / ν ), as mobile phase and diode-array detection. Complete kinetic data (reaction order, activation energy) were determined for the two reactions. DSC and hot-stage microscopy of the two cyclization products, denoted fC 4 and fC 6 , respectively, revealed the presence of nematic phases. The gas chromatographic properties of fC 4 and fC 6 were determined by using two fused silica capillary columns coated with iC 4 and iC 6 . fC 4 and fC 6 had remarkable separation capacity for cis and trans isomers, phenols, aromatic compounds, and volatile aroma compounds.

A thermodynamic study of 4 liquid crystals with DOE substituents using gas chromatography are carried out. The 4 compounds have the same mesogenic part; they differ by the partial or total substitution of the terminal chain heptyloxy with 2-(2-methoxyethoxy)ethoxy (DOE). The thermal properties of the 4 compounds are determined by differential scanning calorimetry. The knowledge of the gas chromatographic specific retention volume of a serial geometric isomers obtained by using the 4 liquid crystals as stationary phases allow to calculate their thermodynamic data : activity coeffients, enthalpies and entropies of solution. In the other hand, the support influence on the thermal properties are investigated. The results founds show that this parameter is not negligible but disappear when the coating increases. For silica, the transitions (melting, clarification) are apparent only when the coating is higher than 50%. With Chromosorb P, 5% of coating are sufficient to observe the transitions.

Summary The thermal and chromatographic properties of an equimolecular mixture of two liquid crystals have been investigated. Differential scanning calorimetry showed that the clearing temperature of the mixture is intermediate between the corresponding temperatures for the individual liquid crystals. The melting temperature was lower than the corresponding temperatures for the separate liquid crystals. Polarizing microscopy showed that the liquid crystal phase of the mixture was nematic. The analytical performance of the mixed liquid crystal was investigated by use of different kinds of solute (aromatic hydrocarbons, polycyclic hydrocarbons, phenols, volatile aroma compounds). It seemed that the behavior of the nematic mixture was the same as that of a pure nematic liquid crystal. When separations were performed with the mixture the more elongated isomer ( para for positional isomers, trans for geometric isomers) was the most retained solute.

The thermodynamics of four new liquid crystals were investigated in order to understand their selectivity as stationary phases in gas chromatography. In this case study, liquid crystals with a benzoyloxy azobenzene mesogenic core substituted with heptyloxy (C7) and/or dioxyethylene ether (DOE) groups, were used. The chromatographic separations of linalool and citronellal, and of xylene, tetraethylbenzene and cresol isomers, which were achieved with the liquid crystal stationary phases, have been related to the dissolution thermodynamics of the solutes. The results gave us an insight into the mechanism of the molecular recognition involved in the separation processes.

Comparative gas chromatographic applications of two new liquid crystals called LCa and LCb and their equimolar mixture LC(a+b) were investigated. The thermal properties of LCa, LCb and LC(a+b) were established with differential scanning calorimetry (DSC) and polarizing microscopy. Differential scanning calorimetry of LC(a+b) showed that the melting or clearing temperature was intermediate between the corresponding temperatures of the pure compounds. Polarizing microscopy showed that the liquid crystal phase of A + B was nematic. The chromatographic separation abilities LCa, LCb and LC(a+b) were studied using fused silica capillary columns. Interesting analytical performances were obtained: isomeric separation of aromatics, polyaromatics, phenols.

The internal cyclization of a new phenyldiazene liquid crystal (called A), with an activated methylene group in theortho position to the diazo linkage, has been studied. The kinetics of cyclization were studied at different temperatures and followed by HPLC. Separations were performed on a 30 cm×0.4 cm silica column withn-heptane-tetrahydrofuran-acetonitrile, 190∶20∶5 (v/v), as mobile phase. The Van't Hoff plot (of ln reaction constantk against 1000/T) gives a mean activation energy of 101.3±2.1 kJ mol−1. The analytical properties of A and the final compound B during the decomposition were investigated by gas chromatography on home-made glass capillary columns coated with A. The retention times of the solutes tested became constant when the B/A ratio reached 5, which corresponds to 83% cyclization. The nematic phase of B has interesting properties enabling the separation of the isomers of decalin, the positional isomers of diethylbenzenes and phenols, and some polyaromatic hydrocarbons and their derivatives.

The analytic performances of three stationary phases 'liquid crystal' to side chain have been presented. Mesogenic compounds studied in this work are: - 3-CH3: 2-(3-methylbenzyloxy)-3-n-hexyloxy-4-(4-chlorobenzoyloxy)-4'-(4-methyl benzoyloxy)-azobenzene; - 4-CH3: 2-(4-methylbenzyloxy)-3-n-hexyloxy-4-(4-chlorobenzoyloxy)-4'-(4-methyl benzoyloxy)-azobenzene; - 3-Cl : 2-(3-chlorobenzyloxy)-3-n-hexyloxy-4-(4-chlorobenzoyloxy)-4'-(4-methyl benzoyloxy)-azobenzene. The originality of these three compounds is essentially due to the presence of a substituted benzyloxy group, more voluminous, which is close to the n-hexyloxy flexible chain. Retention properties of these new liquid crystals have been characterised by the capillary column treated and impregnated according to the dynamic method. The position, ortho or para, of the methyl group on benzyloxy does not interfere significantly on the retention mechanism. In the case of the replacement of the methyl group by a halogen atom (chlorine), we noticed a radical change of the retention properties. (C) 2002 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.

A lateral crown ether fragment can be introduced on symmetrical mesogens containing only three aromatic rings. The replacement of the terminal alkoxy chains by chains containing oxyethylene units decreases the melting and clearing temperatures allowing one to obtain nematic compounds near room temperature. These compounds dissolve LiBF4 salt only to a small extent, but the nematic arrangement, is thereby destroyed. The carbon chemical shift anisotropy and the local C-H bond order parameters were obtained for the nematic phase for the crown ether fragment and the terminal chains. This study indicates that the crown ether average conformation changes insignificantly on decreasing the temperature. The lateral crown ether protrudes markedly from the core with the consequence that the molecular shape is far from rod-like in geometry.

In this work thermodynamic and surface properties of two liquid crystals substituted with polyethylene oxyde (POE) chains are considered. The first of them is N,N'diphenyl-[4 -(2,3,4-tri(2-(2-metoxyethoxy) ethoxy) benzylidene)imine] piperidine (LC1) and the second is 2hydroxy-3-methyl-4-[4-(2-(2-butoxyethoxy)ethoxy)]-4'-[4-(2-butoxyethoxy)ethoxy)styryl] azobenzene (LC2). The POE chains increase the polarity and induce an amphiphilic character of these compounds. The inverse gas chromatography was used to determine excess functions of selected solutes infinitely diluted in the liquid or nematic LC1 and LC2. The formation of multilayers by LC1 and LC2 at the air/saturated aqueous NaCl interface was observed using the Langmuir balance technique. Results obtained show that the hydrophilic character of the compounds may play an important role in the mechanism of the molecular recognition during chromatographic separations.

Short terminal polyoxyethylene chains may be introduced into mesogens containing lateral substituents such as two hexyloxy chains or a crown-ether fragment. These compounds have a large nematic range near room temperature and can dissolve large amounts of the salt LiBF4 without destroying the nematic arrangment. In the nematic phase, the ions are ordered and the quadrupolar splitting associated with these ions can allow this ordering to be monitored. Increasing the salt concentration does not seem to change the ordering of the individual ions. The 13C thermal evolution of the field-induced chemical shift in the oxyethylene (OE) units with or without salt does not show any real difference, indicating that the interaction between the ions and the mesogen is small. This means that there is very little change in conformation in the OE unit when adding salt to the mesogen.

The effect of pressure on the liquid crystal properties of two new dilaterally substituted nematogens has been studied. The method employed involves measurement of the thermal pressure variation of a sample under isochoric conditions. The pressure-temperature phase diagrams were determined. As for unsubstituted compounds, the nematic phase is stabilized upon application of pressure. The values of the (partial derivativeT/partial derivativeP)(NI) slopes of the clearing lines for these nematogens are, however, significantly higher than those generally characterizing rod-shaped nematics. The entropy separation method was used to estimate the constant volume and the volume-dependent terms of the entropy changes at the nematic-isotropic transition. The values of these contributions were determined on the basis of thermobarometric data showing that the transition entropy is strongly volume-dependent. This suggests that the higher values of the (partial derivativeT/partial derivativeP)(NI) slopes observed for these compounds can be related to the rearrangement of the lateral flexible chains in the nematic phase when the pressure increases, leading to a decrease of the excluded free volume caused by the bulky core of the molecules.

A bifurcated and an open chain ( linear) can be introduced laterally into mesogens containing four rings in the main core. The racemic compounds give a narrow nematic range. The X-ray crystal structures of two isomeric compounds show that the bifurcated chain is not aligned along the core as the linear one is. This unfavourable arrangement may be the cause of the deleterious effect of the bifurcated chain on the nematic range.

Molecules containing the 2-phenylindazole core present liquid crystalline properties even if the two terminal chains do not point along the same axis. 13CNMR in the liquid crystalline phase shows that the molecular long axis is nearly aligned with the para-axis of the phenyl moiety of the 2-phenylindazole core. This implies that the first fragments of the chain belonging to the indazole moiety do not lie along the molecular long axis. To promote liquid crystal properties, this chain needs to possess at least six carbon atoms.

A lateral flexible ring can be introduced on a mesogenic core containing four rings. Lateral aliphatic rings containing up to 12 atoms have been synthesized. Despite this large lateral protruding substituent, a large nematic range is obtained. A compound with a benzo-15-crown-5 lateral ring has been successfully obtained and shows the possibility of designing nematic liquid crystals with a crown ether moiety at a lateral position.

Thermotropic ionic nematics with moderate transition temperatures are obtained by dissolving alkali metal salts within nematogens bearing a crown ether moiety; in these media, the ions show a significant 'apparent ionic order' as measured by quadrupolar NMR spectroscopy.

A new bonded liquid-crystal stationary phase for high-performance liquid chromatography is presented. Synthesis entailed reaction of γ-benzyl-L-glutamate-N-carboxyanhydride with aminopropyl LiChrospher 100 NH2. The new stationary phase was characterized by solid-state13C NMR and elemental analysis. Its analytical properties were tested with polynuclear aromatic hydrocarbons (PAH) and phenols. PAH were chromatographed with hexane-chloroform mixtures as mobile phases and were eluted in order of increasing ring number. Total resolution of the 22 PAH studied was achieved by gradient elution. Phenols were chromatographed with a mixture of chloroform and methanol as mobile phase. Small amounts of methanol have a substantial effect on the separation of the phenols. Use of 0.5% (v/v) methanol enabled the separation of 19 phenols and phenol derivatives.

Mesogenic compounds containing four rings in the core usually have very high melting points. However, when two identical lateral benzyloxy groups are introduced on the same side of one of the central rings, the melting point is lowered dramatically and a large nematic range is retained. This range is affected by the bulkiness of the para-substituents in the lateral rings. Methyl groups can be introduced in the ortho- or meta-positions with a consequent decrease in the melting temperature without much affecting the nematic range. These compounds exhibit a rich solid polymorphism which is certainly related to the effect of the conformations of the lateral substituent on the molecular arrangment in the solid phase. Some preliminary NMR experiments on the nematic phase indicate that the molecular long axis coincides with the core axis, whereas the para-axis of the lateral fragment makes an angle close to the magic angle with respect to the molecular long axis.

The crystal structures of two mesogenic compounds having two neighbouring lateral alkoxy chains in the central part of the molecules have been solved. In both structures, the common four ring central core is extended and quite rigid with a total length close to 28.5 Å. All the molecules are strictly parallel because of the P1 space group; the molecular arrangement is characteristic of a nematogenic system. In these structures, the lateral alkoxy chains are folded back to the core; they exhibit slightly different conformations and are quite disordered. The 13C chemical shift of the first -OCH2- within the lateral chains can probe the mean conformation of the chain in the nematic phase. This chemical shift is independent of the compound. Nevertheless, in the solid phase, this chemical shift is dependent on small geometrical changes due to the influence of the oxygen in the neighbouring lateral chain. In addition, temperature cycling of the sample leads to different crystalline solid forms as evidenced by DSC and NMR studies.