Liquid Crystals

Chapter 1.4 - Mixtures and Composites

1.4.   MIXTURES AND COMPOSITES

In general, temperature ranges for the various mesophases of pure liquid crystals are
quite limited. This and other physical limitations impose severe shortcomings on the
practical usage of these materials. Accordingly, while much fundamental research is
still performed with pure liquid crystals, industrial applications employ mostly mixtures,
composites, or specially doped liquid crystals with tailor-made physical and
optical properties. Current progress and large-scale application of liquid crystals in
optical technology are largely the result of tremendous advances in such new-material
development efforts.

There are many ways and means of modifying a liquid crystal’s physical properties.
At the most fundamental level, various chemical groups such as bonds or atoms
can be substituted into a particular class of liquid crystals. A good example is the
cyanobiphenyl homologous series nCB (n=1, 2, 3,…). As n is increased through
synthesis, the viscosities, anisotropies, molecular sizes, and many other parameters
are greatly modified. Some of these physical properties can also be modified by
substitution. For example, the hydrogen in the 2, 3, and 4 positions of the phenyl ring
may be substituted by some fluoro (F) or chloro (Cl) group.11

Besides these molecular synthesis techniques, there are other physical processes
that can be employed to dramatically improve the performance characteristics of liquid
crystals. In the following sections we describe three well-developed ones, focusing
our discussion on nematic liquid crystals.

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