Chemistry Reference
In-Depth Information
Table 3.8.
Continued
Transition,
◦
C
Mesogenic moiety together with the connecting
group
CH
3
CH
3
O
O
C
C 196 N 218 I
(Blumstein
et al
., 1982)
3
.
35
(
O
)
C
O
N
N
O
O
O
C
O
O
C 231 N 265 I
(Zhou and Lenz, 1983)
3
.
36
O
C
O
O
C
C
O
O
O
O
C
O
C 220 S 267 I
(Ober
et al
., 1983)
3
.
37
C
O
O
O
C
O
C
range of the mesophase. The same is seen for the polymers
3
.
33
,
3
.
34
and
. The two methyl substituted polymers have lower mesophase
transition temperatures and narrower mesophase ranges than the parent
polymer
3
.
35
.
In addition, the comparison between
3
.
34
and
3
.
35
shows the impor-
tance of the position of substitution in the mesophase stability of the
polymers. The methyl substitutions in
3
.
34
are at the 2-and 2
-positions
relative to the central azoxy group, while that in
3
.
35
are at the 3-
, and 3
-positions. Because the 2,2
-substitutions are in the vicinity of
azoxy and thus have stronger steric interactions with the azoxy group,
they should have deviated the two phenylene rings from coplanarity with
azoxy to a larger extent than the 3,3
-substitutions should have. Thus, the
melting and clearing temperatures as well as the mesophase range of the
2,2
-disubstituted polymer
3
.
33
3
.
34
are significantly lower and narrower than
those of the 3,3
-disubstituted
3
.
35
. Recall the properties of the low mass
and its 2,2
-(ortho-) and 3,3
-(meta-) isomers.
Another interesting result is obtained when the way of connecting
the mesogenic units and the flexible segments is varied. This is shown by the
polymer pairs of
compound
3
.
12
. The end pheny-
lene rings of the mesogenic units are connected to the flexible segments
through
−
O
−
or through
−
CO
−
(Figure 3.5):
A large number of studies show that the former way of connection
helps the formation of the nematic mesophase. For instance, of the four
polymers,
3
.
30
and
3
.
31
, and
3
.
32
and
3
.
33
−
O
−
3
.
30
to
3
.
33
, the two with the connection through
−
CO
−
are liquid crystalline; the other two with the
connection are
