Chemistry Reference
In-Depth Information
The effectiveness of 2,6-naphthalene moiety in depressing melting
temperature is also demonstrated by copolymers containing 2,6-
dioxynaphthalene or 2,6-naphthoyl units. It was reported,
e.g.
, the melting
point of the polymer of terephthalic acid and 2,6-dihydroxynaphthalene was
only 210
◦
C (Harris, 1981); that of the copolymer of 2 mol 4-hydroxybenzoic
acid with 1 mol of 2,6-dihydroxy-naphthalene and 1 mol of terephthalic acid
was 285 (Calundann, 1980); that of the copolymer of 1 mol hydroquinone,
1 mol of 2,6-naphthoic acid and 3 mol 4-hydroxybenzoic acid was 325
◦
C-
340
◦
C; and that of 1 mol 2,6-naphthoic acid, 1 mol 4,4
-dihydroxybiphenyl,
3 mol 4-hydroxybenzoic acid was 385
◦
C-390
◦
C (Calundann, 1978).
As shown by the above examples, with the substitution and incorpo-
ration of side-step moieties such as 2,6-naphthalene side-steps the melting
point of the fully aromatic polyesters can be depressed to below 400
◦
C. But
because of the high degree of chain rigidity of the fully aromatic polymers,
the clearing point of the liquid crystalline phase of these copolyesters is not
observable before decomposition takes place. In order to further increase
the chain flexibility and realize the clearing temperature of the liquid crys-
talline phase, flexible aliphatic or siloxane segments are often used in the
practice of molecular engineering of liquid crystalline polymers. The flexi-
ble element may be applied as substituent on the aromatic rings as in the
polymers
3
.
25
reported by Berger and Ballauff (1988):
O
O
O
O
C
C
3
.
25
C
n
H
2n
+1
The unsubstituted parent polymer does not melt without decomposition.
The methyl-substituted polymer, n = 1 in
, melts at 381
◦
C. With
longer alkyl substitution, the melting point will be decreased to a larger
extent. When n is 10, the polymer melts in the range of 250
◦
C-310
◦
C and
begins to show the clearing point of the liquid crystal phase (
Ti
3
.
25
400
◦
C).
The clearing point will decrease with a longer alkyl substituent, but the
temperature range of the liquid crystal phase will also narrow because the
extent of depression of the melting point is not as large as
Ti
. The polymer
is no longer liquid crystalline when n is 16 and larger.
The other and most popular way is to copolymerize the flexible segment
into the polymer main chains. De Gennes (1975) predicted that incorpo-
ration of both a rigid and a flexible segment in the repeating unit should
afford semi-flexible polymers exhibiting thermotropic liquid crystallinity,
∼
