Supramolecular Interactions in the Formation of Thermotropic Liquid Crystalline Polymers - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 124

last two classes, while the figures show the use of two different complemen-
tary units to form these structures, derivatives in which the supramolecular
units are either self-complementary and/or form the mesogenic unit are also
possible. In this last case, there are two additional types of networks that can
be formed, namely blends of two polymers containing complementary units,
(Fig. 3l) or a copolymer in which the two complementary units are in the
same backbone (Fig. 3m).
There are potentially a large number of supramolecular motifs that can be
designed into a molecule or polymer to allow access to supramolecular liquid
crystals. Figure 4 shows just a selection of some hydrogen bond motifs that have
been utilized in the early development of supramolecular liquid crystals. As
we shall see, some of these motifs, particularly the benzoic acid/pyridine and
amide motifs, have continued to be popular choices in the design of new SLCPs.
2
Recent Developments (2000-)
2.1
Main-Chain SLCPs
Over the last decade the area of supramolecular polymerization, i.e., the self-
assembly of small monomeric units into polymer-like materials through the
use of non-covalent interactions, has received a growing amount of attention.
Conceptually, a simple way to achieve such supramolecular polymers is by the
attachment of appropriate supramolecular motifs onto the ends of a core unit
(Fig. 2a-e). The backbones of the resulting self-assembled polymeric systems
will therefore contain non-covalent bonds, in addition to covalent bonds,
which collectively impart upon the system reversibility (i.e., a dynamic de-
gree of polymerization) and temperature sensitivity. This behavior, in turn,
offers the potential to develop polymeric materials where the melt viscosity
at elevated temperatures is more akin to a monomeric-like state, and thus
allows for the utilization of mild (low pressure and stress) processing condi-
tions. One interesting opportunity offered by such systems is the ability to
self-assemble functional units into processable polymeric materials, which,
for example, exhibit attractive electronic and/or optical properties [55-62].
The properties of such non-covalently bound aggregates have a strong depen-
dence not only on their functional core components but also on the nature
(stability and dynamics) of the supramolecular motifs which control the
self-assembly process. In addition, if the supramolecular motif used in the
assembly of the polymer is asymmetric (i.e., consists of two different com-
plementary units) then the supramolecular polymer, along with any potential
functionality that results, will only be formed when both of these complemen-
tary units are present.