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Séminaire Chimie ED459

The self-assembly behaviour of linked disc-rod mesogens

Prof. Georg H. Mehl (Department of Chemistry, University of Hull, UK)

publié le , mis à jour le

Le Jeudi 12 Juin 2008 à 13h45
Salle de cours SC-16.01 (UM2)

For thermotropic liquid crystalline systems, molecular shape and mesophase structure is in most cases closely linked. Rod-shaped mesogens tend to form layered (smectic) liquid crystal phases showing 1-dimensional (1D) positional order ; disc-shaped mesogens order into columnar structures with 2D order. Due to these different forms of self assembly behaviour, mixtures of rod and disc shaped mesogens tend to phase separate.

The combination of two incompatible units that phase separate at the sub-molecular level in one molecule, is a classical tool to build up complex geometries in the condensed phase, such as those found for surfactants (polar head groups, non polar tails) or for partly fluorinated materials. Most well known is the effect for block-copolymers, where either different size and/or flexibility of the components can lead to very complex self-organisational behaviour. This approach can also be extended to the combinations of rod and disc shaped molecules.

A particularly attractive aspect of disc-rods geometries is that both moieties can carry individual functions, which allows for the design of complex self-assembly schemes. In addition, both groups can be tailored to contribute independently to the organisation in the material, opening routes towards the rational design of a large number of new (bilayered) mesophases. The material presented here will show the effects of a variation of the structure of the disc shaped part of the molecules as well as the structure and number of the rod shaped part of the molecules on the liquid crystalline phase behaviour. The synthesis of the materials, as well as the investigation of the LC phase behaviour by X-ray diffraction over the LC phase range, optical polarizing microscopy and calorimetric methods will be described. The observation of the formation of smectic phases with alternating layers of discs and rods will be shown and correlation between chemical structure and LC properties will be discussed.


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