Chemistry Reference
In-Depth Information
into a stairlike one. The side-step units may include
−
COO
−
,
−
CH=N
−
,
−
N=N(O)
−
, trans
−
CH=CH
−
,
−
CONH
−
, and 2,6-naphthalene. The
molecule will depart one step aside at such units, but a linear contour
of the molecule can be maintained (Figure 3.2).
In such molecules, the crank-shaft type movements shown by Figure 3.3
are possible, rendering the molecules with significant flexibility.
Poly(4-hydroxybenzoic acid),
, is an example of these molecules. In
this case, the ester groups serve as side-steps. Because of the steric repul-
sion, the two phenylene rings on the two ends of
−
COO
−
tend to take
the trans-configuration relative to the CO
−
O bond. The phenylene ring
attached directly to the carbon atom in
−
COO
−
takes approximately a
coplanar configuration with the COO plane. The other ring attached to the
oxygen in
−
COO
−
, on the other hand, deviates from the coplanarity with
the COO plane by an angle of about 67 degrees (Laupretre and Noel, 1991),
required by the steric interactions between the hydrogen at the ortho posi-
tion on this ring and the oxygen of the carbonyl. We see in this example the
combination of linearity with side-steps and the combination of coplanarity
with non-coplanarity in the molecule. With these characteristics, poly(4-
hydroxybenzoic acid)s have on the one hand su
cient linearity and rigidity
of the rod-like nature for formation of liquid crystalline phases, they have
on the other hand much higher flexibility than the corresponding poly(1,4-
phenylene)s. With a comparison between the data in Tables 3.1 and 3.2,
one finds immediately the much lower phase transition temperatures of the
former than of the latter. With five phenylene rings, for example, the axial
ratio of the poly(4-hydroxybenzoic acid) was 6.4, higher than the value 4.8
for the poly(1,4-phenylene). Both the compounds form a nematic liquid
crystalline phase. However, because of the higher flexibility, the melting
point and the isotropization temperature of the liquid crystal phase for
the former are 220
◦
C and 464
◦
C respectively, much lower than that for
3
.
2
Figure 3.2.
“Side-steps” incorporated in a liquid crystal molecule.
Figure 3.3.
“Crank-shaft” movement created at the side-steps.
