Nguyen Huu Tinh

Occurrence of Reentrant Nematic and Reentrant Smectic A Phases in Mesogenic Series

Abstract A systematic comparative study of several series of chemically neighbouring compounds apt to give a reentrant mesomorphism is proposed. Experimental data concerning the various reentrant phases are summed up. From these observations various factors favourable to the occurence of the reentrance are revealed. Thus, the chemical composition, the sense and the location of dipoles in the molecules, the reduced temperatures, the length of the rigid core are discussed as relevant parameters.

Structural features of the hexagonal phase of phasmidic liquid crystals

Abstract Eighteen compounds having the same molecular architecture, a rod-like aromatic core with four to six long aliphatic terminal groups, which all possess a hexagonal mesophase, have been studied by X-ray diffraction. From the lattice constants of these mesophases, we have deduced some information about the molecular organization. We discuss the curvature and the density of the interface between the aromatic and the paraffinic moieties, the orientational disorder and the packing of the rod-like central parts of the molecules. The occurrence of smectic, oblique or cubic mesophases in the same series underlines the similarities with lyotropic polymorphism and justifies a description of the molecular organization in terms of interfaces.

Invited Lecture. Smectic A polymorphism from low to high molar mass systems

Abstract The polymorphism of smectic A phases in low molecular weight (L.M.W.) liquid crystals is associated with strong anomalies in the period defining the layers. The smectic A phases of liquid-crystalline comb-like polymers also have various modes of spacing. However, the behaviour of these polymer phases shows some peculiarities compared with L.M.W. compounds, especially due to the main chain which takes part in the smectic arrangement. To specify further the SA polymorphism in high molar mass systems, the use of side chain polymers with partial fixation appears to be very promising: either taking advantage of better compatibility and lower viscosity in order to describe binary diagrams with L.M.W. mesogens, or through the partial insertions of long polar side groups known to generate anomalies of periodicity as for L.M.W. compounds.

Polysiloxanes substituted with forked mesogens

Abstract Mesomorphic properties including structural aspects are investigated in a homologous series of forked side chain polymers: the mesogenic units, which are conventionally attached to a polysiloxane backbone via a flexible spacer, possess two long aliphatic tails at their extremities as in forked polycatenar molecules. Several polymers with different proportions, x, of such mesogenic units have been studied. The influence of this parameter is especially interesting on the structure of the mesophases: the layer spacings of the fluid smectics observed at high temperature vary irregularly as a function of x. Microsegregation into sublayers is observed for low values of x while ordered phases with lamellar or/and columnar features appear at low temperatures.

Cyano and Nitrobenzyloxy Derivatives: New Polymorphism Aspects

Abstract Several polar molecules with three benzene rings and having the general formula: have been synthesized. All these compounds are built with the flexible –CH;0– linkage and provide rather low temperature mesomorphic phases easily accessible for physical investigations. The striking features of these six series are: • The cyanobiphenyl series provides a rather classical polymorphism. • The nitrobiphenyl series shows another example of the re-entrant nematic phase and furthermore a very spectacular transition Nre → SG or H with complex spatial structuring. • The cyanobenzoate series provides examples of the so-called SA1 AAd , SA2 S[Ctilde] phases and also an interesting SAd-SA2 transition directly observable under the microscope. • The nitrobenzoate series not only exhibits a classical re-entrant nematic phase, but also the inverted sequence K N SA I in a pure compound. • The cyano-Schiffs base series presents the first example of a SB hexatic phase in polar systems. • The nitro-Shiffs base series...

Phase behaviour of liquid-crystalline copoly(dimethyl-methylhydrogeno)siloxanes substituted with long polar side groups

Abstract The liquid-crystalline properties and structural characteristics of copoly(dimethyl-methylhydrogeno)siloxanes substituted with different proportions of long mesogenic cyano groups are investigated. Comparison is made with the phase behaviour of the chemically related low molecular weight molecules. Some of the unusual features of the polymorphism of these polar compounds are preserved. The compatibility of these copolymers with their low molar mass parents is tested through the study of binary phase diagrams.

Reentrant Phases in New Polar Cyclohexane Series

Abstract A homologous series of 4′-alkoxyphenyl 4″-cyanophenyl, 1,4-cyclohexanedicarboxylates in which the alkoxy group is C1 → C12 has been prepared. Interphase transitions between solid, mesomorphic and isotropic phases were studied by hot stage microscopy. The nonyloxy compound is the first cyclohexane derivative to exhibit the reentrant phenomenon with the sequence K-Scre-Nre-Sc-SA-N-I. A reentrant nematic phase is also observed in the binary mixture of the octyloxy and decyloxy derivatives. A plot of the nematic-isotropic transition temperatures against the number of carbon atoms in the alkoxy chain shows the usual odd-even effect. The behaviour of this new series is compared with that of the corresponding benzenic homologues.

Heat capacity of the discotic mesogen, 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene, between 17 and 397 K and phase transitions in the solid and liquid states

Abstract The heat capacity of the discotic mesogen, 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene whose purity was better than 99.3 mole per cent has been measured with an adiabatic calorimeter between 17 and 397 K. Five solid-solid phase transitions were observed in the crystalline state at 105.90, 220.86, 233.91, 330.81, and 337.10 K. The melting and clearing points were 340.27 and 371.17 K, respectively. The sum of all of the solid-to-solid phase transition entropies amounted to 44.87 J K−1 mol−1 (25.8 per cent of the total transition entropy). These solid-solid phase transitions can be ascribed to successive conformational melting of the alkyl-chains attached to the central triphenylene core. The present result strongly supports the prediction that a rich solid polymorphism is a necessary condition for favourable formation of the columnar structure in the discotic mesophases. The entropy change at the phase transition from the columnar mesophase to the isotropic liquid was very small in comparison to that...

Ferroelectric SC* Phase in Some Benzoate Series

Abstract Several homologous benzoate series which exhibit ferroelectric smectic SC* phases have been synthesized. The used chiral acid chains were prepared from commercially available α-amino acids. The series allow us to determine the influence of the size of the substituent of the chiral carbon on the existence and the stability of the SC* phase. They also give some compounds or mixtures which display high spontaneous polarization.

On New Mesomorphic Sequences in Some Polar 2 or 3 Chlorodibenzoates

Abstract Four series of 4-alkoxyphenyl 4′cyano or nitrobenzoyloxy 2″ or 3″-chlorobenzoates have been synthesized and various mesophases have been identified. A discussion of the relationships between the general molecular structure (relative direction of the different permanent dipoles) and the polymorphism is given. These series provide some interesting sequences including the first example of a SA2–SAd transition directly observable through a microscope.