Liquid Crystalline Assembly of Rod–Coil Molecules - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 92

Fig. 19 Mesophase of the rod-coil multiblock molecules depending on rod-coil repeating
units
of rod-coil repeating units, the density of grafting sites at the interface will
be increased due to an increase in the average number of coils grafted to
arod,whichresultsinstrongentropicpenaltyassociatedwithcoilstretch-
ing at the rod-coil interface. To reduce this coil stretching, a bicontinuous
cubic structure of the monomer would break up into 2D cylindrical domains
in which less confinement and deformation of coil segments occur. These re-
sults demonstrate that systematic variation of the number of repeating units
in the rod-coil multiblock oligomers can provide a strategy to regulate the li-
quid crystalline phase, from bicontinuous cubic, 2D tetragonal columnar, to
2D hexagonal columnar structures.
Picken et al. recently reported on the phase behavior of a series of rod-coil
multiblock copolymers comprised of alternating poly( p -phenylene terephtha-
lamide) as a rod building block and polyamide blocks as a coil part [100].
When the mole fraction of rod parts exceeds 0.5 these polymers show lyo-
tropic liquid crystalline structure in concentrated sulfuric acid solution. The
critical concentration for the formation of a nematic phase increases with
increasing fraction of the flexible fragments in the block copolymer, and
coupling of flexible chains to rod-like oligomers increases the stability of the
liquid crystalline phase. This means that the liquid crystallinity involves in-
duced orientation of the flexible polyamide coils. The incorporation of aramid
blocks in the copolymer induces stretching of the flexible coils, and this
stretching will make the copolymer stiffer.
4.2
Side-Chain Rod-Coil Copolymers
A novel strategy for manipulating the supramolecular structure can also be
accessed by converting the rod-coil monomer into a side chain polymer. Lee