Chemistry Reference
In-Depth Information
6.3.3.
Thermotropic cholesteric liquid crystalline polymers
In addition to the above mentioned lyotropic cholesteric liquid crystalline
polymers composed of rigid polymers, there is a diversity of thermotropic
cholesteric liquid crystalline polymers which consist of a flexible chain incor-
porated with a mesogenic and chiral units. The thermotropic cholesteric
liquid crystalline polymers are classified into two categories: main chain
and side chain.
6.3.3.1.
Main chain cholesteric liquid crystalline polymers
In main chain cholesteric liquid crystalline polymers, the mesogenic groups
and flexible spacers are linked alternatively. The flexible units contain asym-
metrical carbon atoms which enable the polymers to possess chirality and
thus form cholesteric liquid crystals. By varying the ratio of chiral to non-
chiral parts, the cholesteric temperature range and pitch can be changed.
The cholesteric range depends on the mol fraction of the polymers. A typical
main chain cholesteric liquid crystalline polymer is shown in Figure 6.27.
D-3-methyl-glyoxal is a common chiral spacer. In addition, D-butane-1,
3-diphenol, L-propane-1, 2-diphenol, and their dimers and trimers are used
as the chiral spacers in cholesteric liquid crystalline polymers as well.
Linear spacer (B)
and its mole fraction
(1-
x
)
Chiral spacer (A)
and its mole fraction (
x
)
Mesogenic group M
Figure 6.27. A typical main chain cholesteric liquid crystalline polymer. (From
Watanabe and Krigbaum, 1985.)
