Chemistry Reference
In-Depth Information
3. Aromatic rings:
Most liquid crystal compounds consist of two or more aromatic rings. Those
aromatic rings can be a totally saturated cyclohexane, a cyclocyclohexane,
an unsaturated biphenyl, terphenyl, or combinations of them. Usually, the
longer the ring, the higher the melting temperature.
4. Linking group:
The linking group makes an important contribution to the phase transition
and physical properties such as the birefringence. The following linking
groups have been well studied
(1) Saturated groups, such as ethylene (C
2
H
4
)
(2) Esters
(3) Unsaturated groups containing a double bond or a triple bond such as
stilbene, azoy, schiff base, tolane or acetylene, and diacetylene.
5. Lateral group:
By substituting the hydrogen in the 2-, 3-, or 4-position of a phenyl ring by
cyano, fluoro, or chloro polar group, one can modify the physical properties
of liquid crystals. In most cases, the lateral substitution will broaden the
molecule, thus reducing lateral attractions and lowering the nematic and
smectic phase stability. Not only do the nature and size of the substitution
affect the liquid crystal properties, but also the position of the group can
have a significant effect.
6. Chiral center:
Replace the terminal group by a chiral center and the chiral nematic and
smectic phases can be obtained.
The liquid crystal molecules or monomers are generally 2-4 nm in length
and 0.4-0.6 nm in width. Experiments and theories demonstrate that for
compounds showing the liquid crystal phase, their axial ratio or ratio of
length to width (or to diameter) must be greater than a certain value,
approximately 4.
1.3.2. Discotic liquid crystals
Disc-like molecules may also exhibit the liquid crystal phase. de Gennes pre-
dicted this possibility in his famous topic “The Physics of Liquid Crystals”
