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We report various unexpected polymorphisms of fluid mesophases revealed in pure or mixed cyano or nitro rod-like derivatives.

Some observations on the series of n-alkoxybenzoyloxy cyanostilbenes are reported which give more informations on the behavior of reentrant nematic phases with respect to smectic A phases.

Measurements of pitch variations are performed over the twisted phases of three recent pure polar mesogens exhiting reentrant polymorphism: (α) KN re * S A N*I; (β) KS F * S c * N re * S A N*I ; (γ) S A N re *S are N*I. Numerous and precise values confirm pitch divergence near S A , pitch divergence at each side of cholesteric phase when sandwiched between S A and S are phases. S c * pitch keeps a finite value at S c *-N re * and S F *-S c * transitions. Direct measurements of pitch, performed by Grandjean-Cano method are confirmed by optical activity study: such comparison gives informations about birefringence variations L'article expose les variations du pas dans les phases torsadees de trois mesogenes polaires recents presentant du polymorphisme rentrant: (α) KN re * S A N*I; (β) KS F * S c * N re * S A N*I; (γ) S A N re * S Are N*I. Les valeurs obtenues confirment la divergence du pas a l'approche d'une phase S A et la divergence du pas a chaque extremite d'une phase cholesterique comprise entre deux phases S A et S Are ; le pas de la phase S c * conserve une valeur finie aux transitions S c *-N re * et S F *-S c *. Les mesures directes du pas par la methode de Grandjean-Cano sont confirmees par l'etude de l'activite optique: la comparaison des resultats permet de decrire les variations de la birefringence des diverses phases etudiees

The existence of thermotropic disc-like mesogens has been suggested for years. This idea originates first on some simple geometrical considerations: between unidimensional rigid cores (thermotropic rod-like mesogens) and tridimensional systems (plastic liquid crystals) it was easy to imagine that bidimensional aromatic flat molecules may lead to mesomorphism.

The first example of nematic reentrance in a mixture of a side-chain liquid-crystalline polymer with a low-molecular-weight nematogen is described. In addition, we show by x-ray measurements of the layer spacing in the ${S}_{A}$ phase, that, in this case too, a problem of competing lengths exists, although both compounds are terminally nonpolar mesogens.

Since the discovery of the first disc-like molecule [l]exhibiting a thermo-tropic mesomorphism, several series have been synthesized [2,3,4,5,6] and some structural properties of these new compounds were reported essentially by the way of X-ray experiments [7,8,9]. For further knowledge of these materials, it seems fruitful in particular to precise their behavior under an external magnetic field. Thus we have performed the thermal variations of the diamagnetic susceptibility χ (measured in the direction of the magnetic field, Faraday method [10]) of several compounds of two series synthesized in our laboratory [3,5] and with the general formula

The six-, four-, and three-chain derivatives (1), (3), and (2) of biphenyl have been prepared; (1) and (2) display a columnar mesophase and (3) a nematic phase.

The six-, four-, and three-chain derivatives (1), (3), and (2) of biphenyl have been prepared; (1) and (2) display a columnar mesophase and (3) a nematic phase.