F. Hardouin

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].

Unusual Property in a Novel L-C Side Chain Polysiloxane Series

Abstract A new series of polymethylsiloxanes with laterally grafted mesogens has been synthesized. For compounds with short spacers, the diamagnetic anisotropy variations in the nematic state present an anomaly which suggests a nematic-nematic transition.

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.

New TGBA series exhibiting a SC∗SASA∗N∗ phase sequence

Abstract A new series with a chiral tolane core has been synthesized. These materials belong to the optically active series: 3-fluoro-4-[(R) or (S)-1-methylheptyloxy]4′-(4″-alkyloxy-3″-fluorobenzoyloxy)tolanes (nFBTFO1M7). For the first time, the helical SA∗ phase or TGBA phase is found in all of the derivatives from heptyloxy to octadecyloxy. The SA-SA∗-N∗ phase sequence is observed in several compounds with short chains and the SC∗-SA-SA∗-N∗ phase sequence is obtained with the decyloxy derivative. The TGBA phase has filament or cholesteric textures. The helical pitch of the TGBA phase is short for the octadecyloxy derivative and is compared with that for different chain lengths as a function of temperature. The layer spacing in the TGBA phase shows that it is incommensurate with the fully elongated molecular length.

Dimesogenic compounds consisting of a cholesteryl moiety and an aromatic mesogen interconnected through a central pentamethylene spacer

New non-symmetric dimesogens consisting of a cholesteryl moiety and an aromatic mesogenic unit interconnected through a pentamethylene spacer have been prepared. The aromatic units consist of two phenyl rings linked by carboxy, oxycarbonyl, ethylene, ethynylene, azo or Schiffs base groups. The mesomorphic and transitional properties have been investigated by using optical microscopy, differential scanning calorimetry, helical pitch measurements, and electric field and miscibility studies. The structures of the mesophases have been studied using X-ray diffraction. Several kinds of smectic packings are observed depending on the nature of the linking group and an attempt to understand the origin of the different phase structures at a molecular level is made.

Anomalies of periodicity in TGB structures in new liquid crystal dimers

Non-symmetric liquid crystal dimers consist of two different mesogenic units linked through a polymethylene flexible spacer. Our previous studies have shown that dimers containing a cholesteryl moiety as one of the mesogenic groups and a Schiffs base unit as the second, exhibit a rich polymorphism and that several types of smectic packing are obtained depending on the molecular parameters: specifically, a smectic periodicity similar to the molecular length and an intercalated structure with a smectic parameter lower than half the molecular length can be obtained. The competition between these two incommensurate lengths can induce two-dimensional phases and/or an incommensurate smectic phase in which the two smectic periodicities coexist over a long range. Small modifications of the molecular structure can significantly influence the phase sequence. Here we have replaced the Schiffs base by a tolan unit and the terminal alkyl chains by alkoxy chains. As a result, anomalies of periodicity are also observed in this new dimeric series, but they occur mainly in TGB structures.

Dimesogenic compounds with an electron-attracting terminal group

Non-symmetric dimesogens composed of a classical aromatic mesogenic unit linked to a cholesteryl moiety by a flexible spacer form several types of smectic periodicities: one is connected to the cholesteryl length and the other to the length of the associated dimesogens. In some peculiar cases, anomalies of periodicity resulting from the competition between these incommensurate lengths are observed through the occurrence of two-dimensional modulated phases or incommensurate low ordered smectic phases (Sic). As part of our continuing effort to understand the influence on the smectic arrangement of the molecular parameters of such non-symmetric dimesogens, new homologues with a cholesteryl unit linked by a pentamethylene spacer to an aromatic mesogenic moiety bearing different electron attracting terminal groups have been prepared. For these compounds, only the periodicity resulting from the associated dimesogens is observed. Nevertheless, an incommensurate smectic phase can be induced by mixing one of these compounds with another appropriate dimesogen. Molecular mechanics calculations suggest that the origin of the different smectic structures is strongly connected to the repartition of electrostatic potential along the dimesogen.