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Lyotropic liquid crystal properties of Polycatenar Mesogens

Final Activity Report Summary - LYOPOMS (Lyotropic Liquid Crystal Properties of Polycatenar Mesogens)

The project addressed fundamental issues about how solvents affected the way in which liquid crystals might organise. In general terms, two main classes of liquid crystal are known, namely thermotropic, where the order of the solid state was destroyed by the action of temperature, and lyotropic, where it was destroyed primarily by solvent.

This project sought to investigate the lyotropic properties of a certain class of thermotropic system, i.e. polycatenar liquid crystals. Initial work by us, elaborated during the previous visit of Dr Smirnova, had shown that added polar solvents tended to associate with polar parts of the molecules studied, while non-polar solvents associated with non-polar parts. There was a sense in which this might appear unsurprising, but it did mean that we could emphasise the contribution to the liquid crystal properties of either the polar or non-polar part of the molecule. Nevertheless, more detailed examination showed that the precise nature of the solvent had a much more profound effect so that particular behaviour could be induced using small, highly polar solvents, in which chromonic behaviour was found.

These first systems were distinct as they contained a metal atom at the centre, which formally carried a charge, making the different parts of the molecule (polar and non-polar) quite distinct. The major part of the current project then concentrated on metal-free systems where the difference in polarity between the core of the molecule and its periphery was not so great. What we expected to find was behaviour similar to that in the metal-based systems, yet what we discovered was quite different. Surprisingly, for the majority of the studies liquid crystals non-polar solvents were unselective in the part of the molecule with which they associated and therefore acted to dissolve them. Thus, it was only when the core of the molecules was made polar using either fluorine atoms or cyano groups, or when the system was electronically conjugated, that any effect of solvent was seen.

This finding was very important. It was often said that organisation in liquid crystal phases was driven by a local-scale microsegregation between the polarisable, aromatic parts of the molecule and the non-polar chains. This microsegration as a driving force for formation of layered phases was not entirely consistent with what we observed.