Chemistry Reference
In-Depth Information
Fig. 2.2
Hydrogen
bonding in (
a
) nylon 11,
and absent in
(
b
) polyethylene
O
H
N
H
N
O
O
H
N
nylon 11
polyethylene
morphology, on the other hand, means that the chains are aligned in some orderly fashion. Generally,
the freedom of molecular motion along the backbones of polymeric chains contributes to lowering the
melting temperature. Substituents that interfere with this motion tend to raise the melting point. For
instance, isotactic polypropylene melts at a higher temperature than does linear polyethylene. If the
substituent is bulky or rigid it raises the melting point because it interferes with molecular motion.
Dipole interactions, as discussed above, have a similar effect [
4
]. A good illustration is a comparison
of poly(ethylene terephthalate) that melts at 265
C with poly(ethylene adipate) that melts at only
50
C. In the first polyester, there is a rigid benzene ring between the ester groups while in the second
one there is only a flexible chain of four carbons.
O
O
O
O
O
2
O
O
O
2
n
n
4
poly(ethylene terephthalate)
poly(ethylene adipate)
This flexibility of the four carbon segment in poly(ethylene adipate) contributes significantly to the
lowering of the melting point.
Linear polymers that possess only single bonds between atoms in their backbones, C-C, or C-O,
or C-N, can undergo rapid conformational changes [
5
]. Also ether, imine, or
-double bonds reduce
energy barriers and, as a result, “soften” the chains, causing the polymer to become more rubbery and
more soluble in various solvents.
The opposite is true of cyclic structures in the backbones, as was shown in poly(ethylene
terephthalate). Actually, cyclic structures not only inhibit conformational changes but can also
make crystallization more difficult. Among the polymers of
cis
-olefins the structures of the pendant
groups can influence the melting point [
6
]. All linear polyethylene melts between 132 and 136
C[
7
].
Isotactic polypropylene, on the other hand melts at 168
C[
8
].
a
n
n
m.p. = 132-136 °C
m.p. = 168 °C
