Chemistry Reference
In-Depth Information
Fig. 14
Examples of dendrons
29a
and
29b
studied by Percec et al. [90] which self-
assemble into columnar structures with electroactive aromatic groups at the core
tronic elements present in their cores. The charge carrier mobilities in these
LC stacks were two to five orders of magnitude greater than those found in
similar amorphous materials. EDA stacks at the center of the columnar li-
quid crystals can be accessed by mixing a donor and an acceptor containing
dendrons or by even mixing amorphous donor (acceptor)-containing linear
polymers with an acceptor (donor) dendron.
2.3
Side-Chain and Network SLCPs
Figure 3f-m shows the variety of ways that can be used to access side-chain
and network SLCPs. For example, one route involves the binding of mono-
topic small mesogenic molecules to sites along a polymer backbone to yield
“grafted” structures (Fig. 3f). Prior to 2000, a number of such polymers had
been reported, for example, using poly(vinyl pyridine) with carboxylic acid
end-functionalized mesogens [42-45, 91-95] or poly(acrylic acid) with imi-
dazole end-functionalized mesogens [96]. Using the latter system as the basis
for their mesogenic polymers, Ober, Thomas et al. developed temperature-
dependent photonic bandgap materials [97]. They used a block copoly-
mer of poly(styrene) (PS, 500 kg
/
mol) and poly(methacrylic acid) (PMAA,
96 kg
mol) (
30
) which phase segregates into hexagonally packed cylinders
of PMAA embedded within a PS matrix. The addition of the imidazole end-
functionalized mesogen
31
(0.6 molar ratio mesogen to acrylic acid repeat
units) to the block copolymer yields a SLCP on account of the hydrogen bond-
ing of the mesogenic molecule to the PMAA block (Fig. 15a). This, along
with the increased volume fraction of the PMAA-LC block, results in films of
this material that form lamellar structures consisting of alternating PS and
PMMA-LC domains, with a periodicity ca. 170 nm, that are orientated parallel
to the surface. The liquid crystal domains show smectic ordering and a broad
smectic-to-isotropic transition from 65-85
◦
C. Without any heat treatment
these films are green in color; however, upon heating into the isotropic regime
(80
◦
C) the films turn red-orange. This irreversible change in color is a conse-
quence of the change in the peak reflectance of about 40 nm which is assigned
to the randomized orientation of the mesogens in the isotropic state chang-
ing the refractive index contrast of the layers. Using similar materials, the
same group demonstrated that SLCPs could undergo orientational switching
/
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