Chemistry Reference
In-Depth Information
thermal treatment. For a polymer with a nematic phase of Ti = 162
◦
C, the
annealing in the isotropic phase (190
◦
C) for two minutes had brought the Ti
down to 158
◦
C which is four degrees lower than the original. The enthalpy
of isotropization for the annealed sample was only 57% of the original. On
the other hand, if the above-annealed sample was cooled to 120
◦
C and
annealed at this temperature (in its nematic phase) for two minutes the
Ti would recover its original value. Ti was found to increase further with
a longer time of annealing in the nematic phase. The equilibrium value of
Ti = 172
◦
C was achieved only after a few hours of annealing. One comes to
the conclusion after these studies that there is present, a residual nematic
order in the isotropic phase after the isotropization, and there is present,
a residual disorder in the nematic phase after the transition of isotropic-
to-nematic phase. To erase the memory of the order or disorder requires
several minutes or several hours. The measured Ti for the samples of the
same polymer but with different thermal history may scatter about for as
wide as ten degrees and more.
(5) Special attention is drawn to the phenomenon of the monotropic
liquid crystalline phase. The monotropic phase is metastable and comes
to existence only when its related phase has a su
cient extent of super-
cooling. For instance, a crystalline polymer melts at T
C-I
to form only an
isotropic liquid, but when the isotropic liquid is cooled a nematic phase is
formed before crystallization starts. Such a nematic phase is monotropic.
In its DSC heating curve there is only an endothermic melting peak at
T
C-I
but there are two exothermal peaks in the cooling curve. The higher-
temperature exotherm is of the formation of the nematic phase (T
I-N
) while
the lower one is of the crystallization (T
N-C
). If T
N-C
is su
ciently lower
than T
I-N
it should be possible to cool the sample to a temperature which
is below T
I-N
but above T
N-C
. The monotropic nematic phase is obtained.
The isotropization temperature T
N-I
of the phase is then measured by
heating in DSC. The few transition temperatures should have the sequence
of T
C-I
≥
T
N-I
≥
T
I-N
>
T
N-C
which are shown in Figure 4.23.
C
_
T
I
Crystal
( C )
( I )
Isotropic Liquid
_
_
T
T
N
_
T
N
I
C
N
I
Nematic Phase ( N )
Figure 4.23.
A crystalline compound which exhibits a monotropic nematic phase.
