STRUCTURAL STUDY OF THE NEMATIC AND HEXAGONAL COLUMNAR PHASES OF WIRE SHAPED SELF-ASSEMBLIES OF THERMOTROPIC MESOGENS

The nematogenic properties of some substituted diamides originate in their ability to self-assemble in wires, owing to the hydrogen bonds that can link one to each other. An analysis of the X-ray diffraction patterns allows us to give a more accurate picture of the molecular organization of diamides in nematic and columnar hexagonal mesophases. The mean intermolecular distances can be measured and the corresponding ''unit cell'' volume is compared to the molecular volume deduced from dilatometry experiments. In the nematic state, the unit cell volume is 14% larger than the molecular one, while in the hexagonal columnar phase the unit cell volume varies from 1.05 to 0.95 times the molecular volume between the nematic transition and the solidification. This unexpected behaviour could be explained assuming two kinds of sites. Major part of the molecules are inserted into a quasi infinite simple wire, being bound on each side to a hemi-strand. Besides a small amount of molecules are bound to three different neighbors, being thus located on nodes which link together three hemi-strands. Taking into account the features of the scattering function, and its evolution with temperature we describe the disorder induced by each defect and give an estimation of the node concentration in the mesophases.