Chemistry Reference
In-Depth Information
Taking poly(1,4-phenylene terephthalate) as an example, the basic con-
stitutions of this polymer is the regularly positioned 1,4-phenylene rings and
the ester linkages. Because of the very high structural regularity and lack of
flexible elements, it is highly crystalline with no melting before decomposi-
tion. Thus no mesophase is formed on heating to melt the sample. The most
convenient and very effective way to bring down the melting temperature
has been the introduction of a substituent group on one of the two rings
in the repeating unit as demonstrated by the polymers
3
.
18
(Jackson and
Kuhfuss, 1976) and
3
.
19
(Payet, 1979):
O
O
m.p. 370
◦
C
3.18
O
O
C
C
CL
O
O
O
O
C
C
m.p. 343
◦
C
3.19
In these two cases substitution of an H of the ring with a chlorine atom
or a phenyl group resulted in a remarkable decrease in the melting point.
Both polymers are thermotropic liquid crystals and are melt. Because of
the larger size of the phenyl substituent, polymer
has an even lower
melting point and can be melt-spun to form high strength fibers. A simple
substitution such as that in
3
.
19
, not only has the consequence
of broadening the molecule and separating the neighboring chains, but also
brings in certain irregularities for the chain. One type of the irregularities is
shown by
3
.
18
and
3
.
19
where two types of incorporation of the phenyl substituted
1,4-phenylene moieties are demonstrated for the polymer
3
.
20
. Obviously
in the chain there is a distribution of these two types of bonding. In other
words, this polymer, though generally considered to be a homopolymer, is
actually a random copolymer consisting the two different building blocks.
The substitution thus has a higher eciency in decreasing crystallinity and
transition temperature than one might have realized if only the broadening
and separation effects are considered.
3
.
19
O
O
O
O
O
O
C
C
O
O
C
C
3
.
20
