G. Sigaud

Rod-like mesogens with antipathetic fluorocarbon and hydrocarbon tails

Abstract A perfluorinated (or semi-perfluorinated) chain is affixed to various classical mesogenic moieties in lieu of the usual hydrocarbon tail. The consequences of this modification on the mesogenic behaviour are discussed by comparison with the perhydrogenated homologues. Smectic A phases are highly favoured. Their layer spacings show a trend to bilayer stacking in many cases likely to result from microsegregation among incompatible portions of the molecules.

Effect of the chemical constitution of the side-chain on the formation and the structure of mesophases in some polysiloxanes☆

Abstract The synthesis and properties are described for a series of new liquid crystal side-chain polymethylsiloxanes with the same mesogenic rigid core but different substitutents or flexible spacers. The effect of the chemical structure of these polymers upon the polymorphism is studied by means of optical, DSC and X-ray experiments. By comparison with conventional mesogens, the partially bilayered structure of the smectic A phases (i.e. when the layer spacing is between once and twice the length of the side-chain) could be explained, taking into account the asymmetric character of the side-chains.

Enantiotropic nematic reentrant behaviour at atmospheric pressure in a pure compound

Abstract By the way of microscopic observations (contact method) we conclude for the first time to the existence of a reentrant nematic at atmospheric pressure in a pure compound.

X-rays investigations of the smectic A1 - smectic A2 transition

The existence in a binary system constituted of a three phenyl ring cyano compound mixed with TBBA (p=latm) of a smectic A-smectic A transition line has been reported [1]. By microscopic observation, it is impossible to detect this SA-SA transition because the two smectic A, called 1 and 2 are perfectly isomorphous. In fact, this line was first revealed by means of Differential Scanning Calorimetry and then by diamagnetic susceptibility measurements [2].

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.

Blue phases and twist grain boundary phases (TGBA and TGBC) in a series of fluoro-substituted chiral tolane derivatives

A series of fluoro-substituted tolane derivatives: (R)-1-methylheptyl 3-fluoro-4-(3-fluoro-4- n alkoxybenzoyloxy)tolane-4-carboxylates is reported. Some members of this series exhibit the phase sequence: Cr-SmC*-TGBC-TGBA-BPI-BPII-BPIII-I. The blue phases, the TGBA and TGBC phases and the SmC* phase were characterized in detail by microscopic observation, differential scanning calorimetry, helical pitch measurements, X-ray structural analysis and electro-optical study. The blue phases directly next to the TGBA phase were shown to be a new type of blue phase exhibiting smectic ordering. A commensurate TGBC phase with constant number of slabs per pitch was observed.

New chiral thiobenzoate series with antiferroelectric mesophases

Abstract A new series of chiral homologous thiobenzoates with non-chiral end chains ranging from heptyloxy to octadecyloxy has been synthesized and characterized. The mesomorphic properties have been analysed by optical microscopy on pure compounds and mixtures, DSC, and electro-optical and pitch measurements. They display a very rich polymesomorphism including SA, S∗Cα , S∗C, S∗CFI , S∗CA phases with reference to the compound MHPOBC through miscibility tests. Peculiarly noticeable are: (i) two S∗CFI phases sandwiched between S∗C and S∗CA phases in some compounds; (ii) full miscibility between the S∗CA phase and the S∗O phase of the racemic reference compound MHTAC which leads us to question whether the S∗CA or S∗O symbol should be used to designate the antiferroelectric smectic C∗ phase; (iii) odd-even cancellation of the S∗CA phase in relation to both the chiral chain and the alkyloxy chain; (iv) a reentrant sequence, C SC SCA SC SA I, in a pure compound assessed by a phase diagram. The electro-optical ...

Dissymmetric Hexasubstitued Triphenylenes

Abstract Several dissymmetrical hexasubstituted triphenylenes have been prepared. They exhibit a low temperature thermotropic mesophase. It is shown that it corresponds to a columnar mesophase (an hexagonal arrangement of columns of discs). The magnetic properties of these new disc-like liquid crystals are compared with those of the corresponding symmetrical ones.

An experimental system for a nematic-smectic A-smectic C Lifshitz's point

Abstract Binary diagram, enthalpic measurements and some qualitative comparisons with a recent nematic-smectic A-smectic C model seem to agree with the existence of a so-called NAC Lifshitzs point in mixtures of two liquid crystals.

Orientational order and enthalpic measurements on binary mixtures at the N–SA transition: Comparison with McMillan’s models

We have studied binary mixtures which present a nematic to smectic A transition leading continuously from a first order transition to a second order transition. The evolution of the order parameter, close to the N–SA transition is observed by using diamagnetic susceptibility measurements. In addition, the transitional enthalpies were determined. A correlation between those different data is shown. We discuss the results with respect to the rigid molecule model proposed by McMillan to describe the N–SA transition.