Chemistry Reference
In-Depth Information
its latent liquid crystal phase that no such phase can be possibly observed.
The substitution on the central ring with a bulky vinyl group gives the com-
pound
3
.
11
. To the researcher's pleasant surprise, by this substitution the
melting point has been depressed to 88
◦
C, a value well below the clearing
point of the latent liquid crystal of the parent molecule. A nematic liquid
crystalline phase is indeed formed, the clearing point of which is 95, lower
but very close to the prediction of Ballauff and Flory.
It is also interesting to mention that the compound
is the first
published example that has lateral but no terminal substitutions and yet
forms a liquid crystal phase. By and large, the studies on substitution
effect have shown that while the terminal substitutions are favorable to the
thermal stability and the formation of a liquid crystal phase, the lateral
substitutions are not. The larger the lateral substitution, the more harm
it can do to the thermal stability of the liquid crystalline phase. A typical
and very convincing example for this idea is given by homologues of the
well known para-substituted azoxyanisole
3
.
11
3
.
12
:
3
.
12
CH
3
O
N=N(O)
OCH
3
Para-azoxyanisole is the terminally substituted homologue. It melts at
118
◦
C and forms a nematic phase. The clearing temperature is 135
◦
C.
However, its laterally substituted meta- and ortho-forms both are non-
liquid crystals.
The meta-form melts at 51
◦
C, the ortho-form melts
at 80
◦
C.
In addition to the negative effect the lateral substitution may have on the
thermal stability of a liquid crystal phase, one more important aspect has
to be considered, for liquid crystal polymers especially when high strength
and high modulus material are the goals. Thus,
e.g.
, if a fibre is made of
the polymer, the tensile strength is a function of the number of polymer
chains per unit cross section. Obviously, the lateral substitution and thick-
ening of the chains will result in a lower strength. Besides, crystallinity
can be affected to a great extent. A lower crystallinity often means a lower
mechanical property and a lower heat deflection temperature. All the neg-
ative aspects have to be taken into consideration when lateral substitution
is used to bring down the melting points.
In this section we have discussed several structural aspects of liquid
crystal compounds. However, these are not the only aspects that have a
significant impact on the formation and property of liquid crystals. For
