Chemistry Reference
In-Depth Information
stabilize the fragmented ends. The now less viscous rubber can be mixed with
other ingredients of the compound, formed, and finally increased in molecular
weight and fixed in shape by vulcanization.
Figure 1.1
shows plateau regions in which further increases in the degree of
polymerization have little significant effect on mechanical properties. While the
changes in this region are indeed relatively small, they are in fact negligible only
against the ordinate scale, which starts at zero. Variations of molecular sizes in
the plateau-like regions are often decisive in determining whether a given poly-
mer is used in a particular application.
Polyester fibers, for example, are normally made from poly(ethylene tere-
phthalate) (1-5). The degree of polymerization of the polymer must be high for
use in tire cord, since tires require high resistance to distortion under load and to
bruising. The same high molecular weight is not suitable, however, for polymers
to be used in injection molding applications because the polymer liquids are very
viscous and crystallize slowly, resulting in unacceptably long molding cycle
times. Poly(ethylene terephthalate) injection molding grades have degrees of poly-
merization typically less than half those of tire cord fiber grades and about 60%
of the DPs of polymers used to make beverage bottles.
Since the strength of an article made of discrete polymer molecules depends
on the sum of intermolecular attractions, it is obvious that any process that
increases the extent to which such macromolecules overlap with each other will
result in a stronger product. If the article is oriented, polymer molecules tend to
become stretched out and mutually aligned. The number of intermolecular con-
tacts is increased at the expense of intramolecular contacts of segments buried in
the normal ball-like conformation of macromolecules. The article will be much
stronger in the orientation direction. Examples are given in
Section 1.8
, on fibers,
since orientation is an important part of the process of forming such materials.
If the polymer molecule is stiff, it will have less tendency to coil up on itself,
and most segments of a given molecule will contact segments of other macro-
molecules. A prime example is the aromatic polyamide structure:
H
H
N
NC
C
x
O
O
1-23
Dispersions of this polymer in sulfuric acid are spun into fibers which can be
stretched to two or three times their original lengths. The products have extremely
high strength, even at temperatures where most organic compounds are appreci-
ably decomposed.
The range of molecular sizes in a polymer material is always a key parameter in
determining the balance of its processing and performance properties, but these char-
acteristics may also be affected by other structural features of the polymer. This is
