Chemistry Reference
In-Depth Information
Table 4.1
Representative Degrees of Crystallinity (%)
Low-density polyethylene
45
74
High-density polyethylene
65
95
Polypropylene fiber
55
60
Poly(ethylene terephthalate) fiber
20
60
Cellulose (cotton)
60
80
crystallize to a greater extent than the branched polymer. However, the degree of
crystallinity and the mechanical properties of a particular crystallizable sample
depend not only on the polymer structure but also on the conditions under which
crystallization has occurred.
Quenching from the amorphous melt state always produces articles with lower
average crystallinities than those made by slow cooling through the range of crys-
tallization temperatures. If quenched specimens are stored at temperatures higher
than the glass transition of the polymer, some segments in the disordered regions
will be mobile enough to rearrange themselves into lower energy, more ordered
structures. This phenomenon, which is known as
secondary crystallization
, will
result in a progressive increase in the average crystallinity of the sample.
For the reasons given, a single average crystallinity level cannot be assigned to
a particular polymer. Certain ranges of crystallinity are fairly typical of different
macromolecular species, however, with variations due to polymer structure, meth-
ods for estimating degree of crystallinity, and the histories of particular specimens.
Some representative crystallinity levels are listed in
Table 4.1
. The ranges listed
for the olefin polymers in this table reflect variations in average crystallinities
which result mainly from different crystallization histories. The range shown for
cotton specimens is due entirely to differences in average values measured by
X-ray, density, and other methods, however, and this lack of good coincidence of
different estimates is true to some extent also of polyester fibers.
Crystallization cannot take place at temperatures below
T
g
, and
T
m
is therefore
always at a higher temperature than
T
g
. The presence of a crystalline phase in a
polymer extends its range of mechanical usefulness compared to strictly amor-
phous versions of the same species. In general, an increased degree of crystallin-
ity also reduces the solubility of the material and increases its rigidity. The
absolute level of crystallinity that a polymer sample can achieve depends on its
structure, but the actual degree of crystallinity, which is almost always less than
this maximum value, will also reflect the crystallization conditions.
4.3.2
Microstructure of Semicrystalline Polymers
When small molecules crystallize, each granule often has the form of a crystal
grown from a single nucleus. Such crystals are relatively free of defects and have
well-defined crystal faces and cleavage planes. Their shapes can be related to the
