H. Gasparoux

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.

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.

Synthesis and Properties of Some Liquid Crystalline Polysiloxanes

Abstract Series of side chain polymethylsiloxanes are synthesized by systematically varying the spacer length, the inter-ring linkage and the free tail in the mesogenic moieties. The mesomorphic properties are studied by optical microscopy, differential scanning calorimetry and X-ray scattering. General properties are established.

Role of Molecular Parameters on the Miscibility of Side-Chain Polymer Solutes

Abstract Binary phase diagrams between a liquid crystalline side chain polymer and a low molecular weight nematogen have been analyzed with regard to their miscibility in the nematic state. Phase separa tion is frequent in these solutions and varying the chemical structure of the components has strong in fluence on the critical temperature. In addition, we observe that these mixtures are likely to produce induced smectic A phases the stability of which is similarly influenced by modifications of the molecular parameters.

Effect of Disc-Like Solute Size on the Orientational Order of a Nematogen

Abstract We observe that the addition of hexa-n-alkoxy triphenylene “disc-like” solutes in a nematic solvent affects the nematic order but that this perturbation is independent of the solute chain length. Some arguments concerning the chain arrangements with respect to the triphenylene ring are proposed.

Investigation of the twist viscosity coefficient γ1 divergence near the nematic-smectic a transition

Abstract We discuss the nature of the N→SA transition as a function of the ratio TNA/TNI. Experimental evidence are given by correlations between dynamic twist viscosity coefficient γ1 and calorimetric measurements.

Experimental tricritical point nematic-smectic A in a two components-system

Abstract Entropic and magnetic measurements give evidence for a tricritical nematic-smectic A point, at 1 atm, in a system involving the mixture of two mesomorphic Schiffs bases : p-heptyloxybenzilidene-p′-aminofluorenon and p-cyanobenzilidene-p′-octyloxyaniline.

Phase separation in nematic and smectic a phases

Abstract Until recently no example was known with a miscibility gap separating two non-ordered mesophases (i.e.: nematic, smectic A or C) of the same symmetry. In most cases of binary solutions it was observed that either the phase diagram is of the simple eutectic type with quasi ideal behaviour of the mixtures or the liquid crystalline phases are destabilized at the benefit of the isotropic solution (Figure 1). These properties of easy miscibility among mesogens have been long and widely applied at different stages of the study and application of liquid crystals.