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pyridine (as part of the polyamide
43
backbone) moiety. It was demonstrated
in these systems that stable (up to ca. 200
◦
C) mesophases could be formed
if there is a substitutent meta to the benzoic acid. If no meta substitutent is
present, then no mesophase was formed. It was hypothesized that the substi-
tution helps to fill in extra space in the supramolecular aggregate (Fig. 19) thus
helping to stabilize the orientations in the mesogenic complexes.
Fig. 19
The structure of the polymeric complex of benzoic acid derivatives
42
with the
2,6-diaminopyridyl-containing polyamides
43
and a schematic of the proposed molecu-
lar arrangement of its LC phase
All of the supramolecular motifs discussed so far are closed systems, i.e.,
only two components interact. However, there are a number of supramo-
lecular motifs that self-assemble into open linear aggregates. Here, attaching
appropriate side-chains to such motifs offers an alternative way to access side-
chain SLCPs (Fig. 3i) in which the polymeric backbone consists of both cova-
lent and non-covalent bonds. Kato et al. have reported a series of studies with
folic acid derivatives
44
to access thermotropic LC materials (Fig. 20). It was
found that the nature of the side chains plays an import role in the mesomor-
phic behavior of these systems. Molecules with small alkyl chains (
n
= 6, 11)
attached to the glutamic acid residue in the folic acid form smectic phases
with periodic distances consistent with the hydrogen-bonded tape side-chain
SLCPs. Increasing the size of the alkyl chains (
n
= 14, 16) changes the nature
of the hydrogen-bonding assembly from linear tapes to macrocyclic quar-
tets, which in turn yields a discotic mesogenic unit and results in columnar
phases. For the
n
= 11 compound, the smectic phase can be converted into the
columnar phase by the simple addition of alkali metal ions, which are known
to template the formation of cyclic quartets [108, 109]. Further increase in
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