Chemistry Reference
In-Depth Information
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Storage effects
: Two modes of storage effects exist. One is the negative
mode in which cholesteric liquid crystals, doped with a small amount of
conducting impurity, are negative in dielectric anisotropy. This is caused
by the turbulence of the liquid crystal director field and is called conductive-
mode display. The other storage effect is the positive mode. The cholesteric
liquid crystals have a positive dielectric anisotropy. It is a dielectric-type
display. The current interest — the polymer stabilized cholesteric liquid
crystals (Doane
et al.
, 1995) is based on this display mode.
Both displayed images can be kept for days' or months' long without the
presence of an electric field. From the energy consumption point of view,
such storage effects have a great advantage for displays. In addition, the
display of such kind has a wide view angle which is an important feature
for information board displays.
In the above applications, cholesteric liquid crystals need to be sealed
between two glass plates or in the form of micro encapsulates. However,
cholesteric liquid crystalline polymers can easily form thin films or be coated
on substrates.
6.3.2.
Lyotropic cholesteric liquid crystalline polymers
To form cholesteric liquid crystalline polymers, one either polymerizes
cholesteric monomers or mixes low molecular mass cholesteric liquid crystals
with polymers. In the latter case, two components may be mixed homo-
geneously or in such a way that the polymers act as a matrix while the
small molecular mass cholesteric liquid crystals are in droplets, known as
the polymer-dispersed liquid crystals (PDLC) (Doane
et al.
, 1988) or the
nematic curvilinear aligned phase (NCAP) (Fergason, 1985). In addition,
there are many polymers in nature exhibiting the cholesteric phase such as
PBLG, cellulose, DNA,
etc
.
The rod-like PBLG is the most well studied cholesteric liquid crys-
talline polymer, which is a kind of poly-peptides forming
α
-helices, shown
in Figure 6.19. In such a structure, the neighboring twist sites are connected
by hydrogen bonds in every four peptide bonds between the neighboring
-C=O- and -NH-. The side chains -CH
2
CH
2
-COOCH
2
C
6
H
5
extend out
radically. It exhibits the cholesteric phase in solutions,
i.e
., lyotropic liq-
uid crystalline polymers. The solvents can be dioxane, dichloromethane,
trichloromethane,
etc
.
