Chemistry Reference
In-Depth Information
Fig. 2.10
Illustration
of a fringed micelle
200
˚
in
size. Rough estimates from these diffraction studies indicate that the size of crystals, or crystallites,
rarely exceeds a few hundred angstroms. The fact that there is a substantial background of diffuse
scattering suggests that considerable amorphous areas are also present. Because the chains are very
long, it has often been suggested that individual chains contribute to several different crystalline and
amorphous domains. This resulted in a proposal of a composite single-phase structure [
5
], a
fringed
micelle
or a
fringed crystallite
model. This is illustrated in Fig.
2.10
. The fringes are transition phases
between the crystalline and the amorphous regions. Some analytical studies, however, failed to
support this concept.
The proportion of crystalline matter in a polymer is defined as the
degree of crystallinity
. It can be
expressed in volume or in mass. By expressing it in volume, it would be:
cause of the broadening [
5
]. The crystals from the melt are approximately 100
200
X
v
¼ V
c
=ðV
c
þ V
a
Þ¼V
c
=V
degree of crystallinity
where,
V
c
,
V
a
, and
V
represent the respective volumes of the crystalline and amorphous regions and
the total volume.
When expressed in terms of mass, the equation would be:
X
m
¼ m
c
=ðm
c
¼ m
a
Þ¼m
c
=m
where,
m
c
,
m
a
, and
m
are the masses of crystalline, amorphous phases, and the total mass.
2.3.2 Crystallization from Solution
For many polymers, crystal growth can also take place from dilute solutions and from such solutions
they yield
single crystals
. Crystal formations in polymers were studied intensively almost from the
time of recognition of their existence in macromolecules.
Single crystals
of organic polymers were
recognized as early as 1927 [
26
]. It became the subject of intensive investigations after observations
that linear polyethylene can crystallize into single crystals. The observations made on polyethylene
were followed by observations that it is possible to also grow single crystals of other polymers. Some
of them are polyoxymethylene [
30
], polyamides [
31
], polypropylene [
32
], polyoxyethylene [
33
],
