Chemistry Reference
In-Depth Information
volume ratio. In particular, increasing the length of the rod segment should
disturb the assembly of the rigid rod segments into discrete bundles due to
larger rod-to-rod interactions. Rod-coil molecules
15
,
16
,and
17
have an
identical rod-to-coil volume fraction (
f
rod
= 0.22). As mentioned above, the
triblock molecule
15
based on three biphenyl units exhibits a tetragonal mi-
cellar liquid crystalline phase. In great contrast, the rod segment of
16
based
on longer chain length self-assembles into a honeycomb-like layered liquid
crystalline phase (HC) as a lower temperature mesophase in which hexag-
onally ordered perforations within a layer are filled by coil segments [78].
These layers, in turn, are stacked spontaneously in an ABAB fashion to gen-
erate a 3D hexagonal order. A DSC heating trace of
16
shows a crystalline
melting transition at 136
◦
C, followed by a birefringent liquid crystalline
phase that undergoes transformation into another liquid crystalline phase
at 157
◦
C. On heating to 157
◦
C, the honeycomb-like mesophase transforms
into a 3D tetragonal micellar liquid crystalline phase. On slow cooling from
the isotropic liquid, the formation of fern-like domains growing in four di-
rections with an angle of approximately 90
◦
, which coalesce into a mosaic
texture, could be easily observed using polarized optical microscopy, indicat-
ing the presence of a 3D tetragonal mesophase.
Further increasing the length of rod segment suppresses the formation
of a 3D tetragonal mesophase, while inducing only a honeycomb-like liquid
crystallinephaseasinthecaseofthemolecule
17
.Theseresultsindicate
that the self-assembled 3D liquid crystalline phase changes significantly from
organized rod bundles in a coil matrix (tetragonal structure) to organized
coil perforations in rod layers (honeycomb structure) on increasing the rod
length. This direct structural inversion is also accompanied by changing
temperature. Therefore, changing temperature produces an effect similar to
varying the molecular length. This example proves that the molecular length
in rod-coil systems also has a large impact on the organized structure formed
by self-assembly of rod-coil molecules.
The opposite way to modulate the supramolecular structure can be pro-
vided by variation in the coil structure while maintaining the rod segment
constant. The influence of cross-sectional area of coil segment upon the
self-assembly behavior were explored by ABA type coil-rod-coil molecules
15
and
18
that have identical coil volume fraction (
f
coil
= 0.78) relative to
mesogenic rod segment, but different coil segments, i.e., PPO and PEO, re-
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