Chemistry Reference
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side groups in its center shows an unusual supramolecular structural in-
version, from perforated layers to discrete bundles, while maintaining a 3D
hexagonal superlattice (Fig. 15). This phase transition on heating is most
probably due to larger entropic contribution to the free energy associated
with coil stretching [27, 85]. This indicates that the introduction of methyl
side groups into a rod segment leads to the transformation of a 3D hex-
agonal perforated lamellar structure into a 3D hexagonally organized discrete
bundles. This notable feature is that the incorporation of alkyl side groups
into the center of a rod segment generates the structural inversion from or-
ganized coil perforations in rod layers to organized discrete rod-bundles in
a coil matrix, while maintaining a 3D hexagonal superlattice. It is also re-
markable that this structural inversion, retaining a 3D hexagonal superlattice,
occurs directly without passing through any intermediate structures in a re-
versible way by changing the temperature. This abrupt structural change
in rod-assembly may offer an attractive potential for use in supramolecular
switch and thermal sensor.
3.3
BAB Rod-Coil-Rod Triblock Copolymers
Kato et al. reported on rod-coil-rod molecules consisting of rigid mesogenic
cores and flexible PEO coils [86]. The small triblock molecule (
26
)wasob-
served to exhibit smectic A liquid crystalline phase as determined by a com-
bination of optical polarized microscopy and differential scanning calorime-
try. The incorporation of LiCF
3
SO
3
into the rod-coil-rod molecules shows
significant mesophase stabilization. X-ray diffraction patterns revealed that
complexation of
26
([Li
+
]
[EO] = 0.05) drastically reduces the layer spacing
from 44 to 23
A
. This decrease is thought to be due to the interaction of the
lithium salt with the ether oxygen, which results in a more coiled conform-
ation of the PEO coil. Ion conductivities were also measured for complexes
forming homeotropically aligned molecular orientation of the smectic phase.
Interestingly, the highest conductivity was observed for the direction parallel
to the layer (Fig. 16). However, the conductivities decrease in the polydomain
sample, which disturbs the arrangement of ion paths. These results suggest
that the self-organized rod-coil salt complexes can provide access to a novel
strategy to construct ordered nanocomposite materials exhibiting low dimen-
sional ionic conductivity.
Recently, rod-coil-rod triblock copolymers based on polydimethylsilox-
ane and polypeptide were reported by Rodriguez-Hernadez and cowor-
kers [87]. In similar to rod-coil diblock copolymer with poly [poly(
/
-benzyl-
l-glutamate)], this triblock copolymer shows double hexagonal structure.
The hexagonal array formed by
γ
-helices remains stable at high temperatures.
However, at a higher organization level the second hexagonal structure is
lost at temperatures exceeding 160
◦
C. Moreover, this higher-level hexagonal
α
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