Chemistry Reference
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of this system is the ability of the main-chain liquid crystalline polymers
based on a rod building block to self-assemble into ordered structures with
curved interfaces. Formation of supramolecular columnar and bicontinuous
cubic assemblies in the rod-coil copolymers is in marked contrast to the gen-
eral behavior of conventional liquid crystalline polymers based on rod-like
mesogens and segmented copolymers based on alternating rigid and flexi-
ble segments [97, 98]. Formation of the ordered structures with interfacial
curvature from the main-chain rod-coil copolymers can be rationalized by
considering entropic penalties associated with coil stretching and anisotropic
arrangement of rod segments. Bulky PPO coils induce curvature at the rod/coil
interface (arising from the connectivity of the rod and coils), constraint of con-
stant density, and minimization of coil stretching. At the interface separating
the rod and coil domains in the layered smectic structure, the relatively smaller
area per junction favored by rod block results in chain stretching of the coil
block, which is energetically unfavorable. Therefore, the rod-coil copolymers
self-assemble into bicontinuous cubic or hexagonal columnar structures with
larger interfacial area, instead of a layered smectic structure.
In contrast to this, another strategy for manipulating the supramolecular
structure at constant rod-to-coil volume ratio can also be accessible by vary-
ing the number of grafting sites per rod, which might be closely related to
the grafting density at the interface separating rod and coil segments. For
this reason,
30
,
31
and
32
, with rod-coil repeating units consisting of three
biphenyl units connected by methylene ether linkages as the rod block and
PPO with 13 PO repeating units as the coil block, were prepared [99]. All
of the oligomers are self-organized into ordered supramolecular structures
that differ significantly on variation of the number of repeating units, as
confirmed by X-ray scattering. The molecule
30
shows a bicontinuous cubic
liquid crystalline structure. In contrast, the molecule
31
shows a 2D rectangu-
lar crystalline and a tetragonal columnar (col
t
) liquid crystalline structures,
while the molecule
32
displays a hexagonal columnar structure in both their
solid state and mesophase (Fig. 19). These results show that self-assembled li-
quid crystalline structures, from 3D bicontinuous cubic, 2D tetragonal, to 2D
hexagonal lattices are formed by rod-coil structures that differ only in the
number of repeating units.
This interesting variation of self-assembled structures, at an identical rod-
to-coil volume ratio, can be explained by considering the density of grafting
sites at the interface separated by rod and coil. On increasing the number
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