Liquid Crystals

Chapter 1.4.1 - Mixtures

1.4.1.   Mixtures

A large majority of liquid crystals in current device usage are eutectic mixtures of
two or more mesogenic substances. A good example is E7 (from EM Chemicals),
which is a mixture of four liquid crystals (see Fig. 1.14).

The optical properties, dielectric anisotropies, and viscosities of E7 are very different
from those of the individual mixture constituents. Creating mixtures is an art,
guided of course by some scientific principles.11

One of the guiding principles for making the right mixture can be illustrated by
the exemplary phase diagram of two materials with different melting (i.e., crystal →
nematic) and clearing (i.e., nematic → isotropic) points, as shown in Figure 1.15.
Both substances have small nematic ranges (Ti-Tn and Ti'-Tn'). When mixed at the
right concentration,4 however, the nematic range (Tim-Tnm) of the mixture can be
several magnitudes larger.

If the mixture components do not react chemically with one another, clearly their
bulk physical properties, such as dielectric constant, viscosity, and anisotropy, are
some weighted sum of the individual responses; that is, the physical parameter αm of
the mixture is related to the individual responses’ αi’s by αm = Σciαi, where ci is the
corresponding molar fraction. However, because of molecular correlation effects and
the critical dependence of the constituents on their widely varying transition temperatures
and other collective effects, the simple linear additive representation of the
mixture’s response is at best a rough approximation. In general, one would expect
that optical and other parameters (e.g., absorption lines or bands), which depend


Figure 1.15. Phase diagram of the mixture of two liquid crystals.

largely on the electronic responses of individual molecules, will follow the simple
additive rule more closely than physical parameters (e.g., viscosities), which are
highly dependent on intermolecular forces.

In accordance with the foregoing discussion, liquid crystal mixtures formed by
different concentrations of the same set of constituents should be regarded as physically
and optically different materials

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