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Fig. 60
Schematic representation of the nematic phases formed by multipedes
50
and
51
which have mixed lateral and terminal mesogenic groups
is smectic A, is converted into a chiral nematic phase, see
52
.Acomparison
of the nematic structures of
50
and
51
is shown in Fig. 60.
Interestingly, material
51
has a nematic phase at room temperature, with
a glassification point at -20
◦
C. In comparison 4-pentyl-4
-cyanobiphenyl has
ameltingpointof24
◦
C and a clearing point of 34
◦
C. Furthermore, mechan-
ical shearing of specimens of the nematogenic materials
50
and
51
show that
the samples have low viscosity, indicating that the materials have rheological
properties similar to those of low-molar-mass mesogens.
Fine tuning of material properties therefore can be achieved by using mix-
tures of mesogens attached to the periphery of the scaffold. In addition, the
disordering induced can substantially lower melting points and widen tem-
perature ranges of desirable liquid crystal phases. Furthermore, such mate-
rials are miscible with low-molar-mass materials, and can be used to modify
their physical properties.
3.1
“Janus” Liquid-Crystalline Multipedes
One of the more intriguing and challenging aspects in materials science is un-
derstanding the molecular recognition and self-assembling processes in ma-
terials with diversely functionalized faces or sides, which can yield supramo-
lecular objects that may recognize and select left from right, or top from bot-
tom, as described by de Gennes [93]. For example, Janus grains [100], block
co-polymers in the form of Janus micelles [101], segregated amphiphilic den-
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