Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 42

allel to or slightly tilted with respect to the rotational axis that is normal to
the cylinder, i.e., they are nematic-like, and they are packed together side by
side on the outer surface of the cylinder, in a disordered fashion, as shown in
Fig. 51.
The mesogenic behavior of 43 emphasizes the role played by the oc-
tasilsesquioxane core when comparing with the related side-chain liquid-
crystalline polysiloxanes [95, 96]. It is remarkable that the thermal stability of
the chiral nematic phase is similar in both the polymer and the dendrimer,
suggesting that the cubic core does not perturb significantly the associations
between the mesogens necessary to support the chiral nematic phase.
Similarly, the formation of the columnar phases for dendrimer 43 ,but
not in the side-chain polysiloxanes, indicates that the silsesquioxane core
assists in the interaction of neighboring units resulting in the formation
of the hexagonal and rectangular columnar disordered structures, presum-
ably through segregation of the siloxane cores from the mesogenic units in
distinct columns. These remarkable new mesophase structures, which are
essentially variations of “tubular nematic-columnar phases”, are favored by
the spacer length used to attach the mesogen to the dendrimer core, since
it has been kept relatively short (five methylene units) in order to prevent
full decoupling of the mesogenic motions from the silsesquioxane core and
Fig. 51 Schematic structures of the hexagonal tubular nematic and rectangular tubular
nematic phases. The mesogenic groups are roughly parallel to the columns but their pos-
itions are disordered. Thus the columns effectively act as the directors of the nematic
phase