Chemistry Reference
In-Depth Information
3. A nonlinear liquid crystal
O
O
O
O
N
N
O
n
n
Many liquid crystal polymers tend to exhibit multiple mesophases at different pressures and
temperatures [
57
]. When heated, these polymers will go through multiple first-order transitions.
Such transitions are from more ordered to less ordered arrangements. This is referred to as the
clearing temperature
with the last one resulting in isotropic melts.
A lyotropic liquid crystalline aromatic polyamide, sold under the trade name of Kevlar, is an
example of such a polymer that is available commercially:
O
H
O
N
N
H
n
Kevlar
The polyamide forms liquid crystals in sulfuric acid solution from which it is extruded as a fiber.
After the solvent is removed, the remaining fiber possesses greater uniform alignment than would be
obtained by mere drawing. This results in superior mechanical properties.
There are polymers, however, that exhibit liquid crystalline behavior, but are very high melting and
insoluble inmost common organic solvents. This is a drawback, because suchmaterials are hard to process.
A preparation of new liquid crystal polymers with bilaterally linked mesogens in main-chain was
reported [
57
]. Such materials exhibit biaxial fluctuation in the nematic phase. This is interesting
because most commonly encountered polymeric liquid crystals have mesogens linked at their ends to
the polymer backbone by flexible spacers. On the other hand, liquid crystal polymers with mesogens
linked bilaterally by flexible spacers are not common and only a few examples were reported [
58
].
One such material can be illustrated as follows:
O
O
O
n
O
O
O
C
6
H
13
O
O
OC
6
H
13
O
