Chemistry Reference
In-Depth Information
Not surprisingly, polymers with higher intermolecular attractions develop
more strength at equivalent molecular weight than macromolecules in which
intermolecular forces are weaker. Polyamides (nylons) are characterized by the
structure
H
H
N
R
NC
R´
C
x
O
O
1-21
in which hydrogen-bonding interactions are important and are mechanically
strong at lower degrees of polymerization than hydrocarbon polymers like
polyethylene.
A crude, but useful, generalization of this concept can be seen in
Fig. 1.1
,where
a mechanical property is plotted against the average number of repeating units in the
polymer. The property could be the force needed to break a standard specimen or
any of a number of other convenient characteristics. The intercepts on the abscissa
correspond to molecular sizes at which zero strength would be detected by test meth-
ods normally used to assess such properties of polymers. (Finite strengths may, of
course, exist at lower degrees of polymerization, but they would not be measurable
without techniques which are too sensitive to be useful with practical polymeric
materials.) “Zero-strength” molecular sizes will be inversely related to the strength
of intermolecular attractive forces, as shown in the figure. In general, the more polar
and more highly hydrogen-bonded molecules form stronger articles at lower degrees
of polymerization.
The epoxy prepolymers mentioned above would lie in the zero-strength region
of curve B in
Fig. 1.1
. They have low molecular weights and are therefore
0
200
400 600 800
Degree of polymerization
1000
FIGURE 1.1
Relation between strength of polymeric articles and degree of polymerization. A, aliphatic
polyamides (e.g., 1-6); B, aromatic polyesters (e.g., 1-5); C, olefin polymers (e.g., 1-3).
