Chemistry Reference
In-Depth Information
nematic phases are speculated by Wang and Warner (1987). Depending
on the sign (positive or negative) of the order parameters of the two, the
nematic phase can be N
I
(+
,
−
), N
II
(
−
,
+) or N
III
(+
,
+). Whether or
not a mesophase is formed and the type of the mesophase is determined
by the relative tendency or strength of the three components in forming or
interfering in the order of the mesophase.
If no flexible segments are used to connect a non-mesogenic main chain
with mesogenic groups, main chain flexibility seems to be essential for the
formation of a mesophase. Taking polymethacrylates and polyacrylates as
examples, the main chain of the former is less flexible than that of the latter
as can be justified by the glass transition temperatures. Thus, if the meso-
genic moiety is not very strong in the tendency of the mesophase formation,
there is more of a chance for the more flexible polyacrylates to form a liquid
crystalline phase. The effect may be demonstrated by poly(biphenyl acry-
late),
(Maganini
et al
., 1974). The biphenyl group is a weak mesogenic unit. More
flexible main chains are more desirable for the formation of a liquid crys-
talline phase. The polyacrylate has a lower glass transition temperature and
does form a smectic phase, but the less flexible polymethacrylate does not.
3
.
42
(Bresci
et al
., 1980), and poly(biphenyl methacrylate),
3
.
43
H
CH
2
C
COO
(g 110 S 280 I).
3.42
CH
3
CH
2
C
COO
(g 170 I).
3.43
with the mesogenic units of a schiff base structure (Frosini
et al
., 1981).
The polyacrylate is a smectic liquid crystal from 93
◦
Cto258
◦
C. The
polymethacrylate does not form any liquid crystalline phase.
The same was observed for the following two polymers
3
.
44
and
3
.
45
H
CH
2
C
CH=N
COO
(g 93 S 258 I).
3.44
