M. Mauzac

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.

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.

Ferroelectric liquid crystal siloxane homo and copolymers

Abstract Several new chiral liquid crystal siloxane homopolymers and copolymers have been synthesized and studied as to their mesomorphic and ferroelectric behaviour. Almost all of them exhibit chiral smectic C phases over wide temperature ranges. DSC and X-ray measurements are presented; spontaneous polarizations, response times and tilt angles are also given. Some of the spontaneous polarizations are very high, more than 100 nC/cm2, and the response times are found to be among the fastest for liquid-crystalline polymers, less than 1 ms.

Nematic-nematic modification in side-on-fixed polysiloxanes

Abstract A series of side-chain liquid crystal polymers has been synthesized with mesogenic groups laterally branched to a polysiloxane backbone (the so-called side-on-fixed). An anomaly of the diamagnetic anisotropy in the nematic state was shown to be very sensitive to a number of structural parameters such as the length of the aliphatic tails, the length of the spacer and the fixation rate of the mesogenic groups on the backbone. On the basis of accurate density measurements, microscopic observations and X-ray diffraction experiments on aligned samples, the occurrence of a nematic-nematic transition connected to this anomaly is questionned.

Liquid crystal side chain polysiloxanes containing various proportions of non-mesogenic units

Abstract Recent studies have established that side chain polymeric liquid crystals composed of mesogenic and non-mesogenic side groups keep their liquid-crystalline properties even for a low proportion of mesogens. We show that the detailed structures of three kinds of new diluted liquid crystal polysiloxanes depend on the nature of the co-substituent as well as on the proportion of the silicon sites occupied by the mesogenic groups. Mixtures of these systems with low molar mass liquid crystals were also investigated in terms of compatibility and/or stabilization of smectic A phases.

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.

Some relevant parameters on the ‘jacketed’ effect in side-on fixed polysiloxanes

Abstract New ‘side-on’ attached liquid crystal polysiloxanes have been synthesized with different fixation ratios of side groups and various degrees of polymerization. For these syntheses, a series of random copoly(hydrogenmethyl-dimethylsiloxane)s, used a precursors, has been realized. Both thermal properties and small angle neutron scattering experiments on these polymers show a decrease in the nematic ‘jacketed’ effect as the number of mesogenic groups fixed to the backbone decreases and as the main chain length increases.

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.

Side chain polysiloxanes with fluorinated mesogenic groups. The role of the fluorine content on the properties of the smectic A state

Abstract A series of liquid crystalline polymers has been synthesized in which fluorinated or semi-fluorinated mesogenic moieties are used as side groups linked to a polysiloxane backbone. Clear-cut differences in the structure of the smectic A phase are evidenced between fluorinated polymers and their classical non-fluorinated homologues. In a comparative analysis, we discuss the relevance of the steric hindrance of perfluorinated sequences and of the microsegregation of the backbone in response to these modifications as related to the fluorine content in the side group and to the proportion of mesogenic groups.

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.