Chemistry Reference
In-Depth Information
temperatures well below the melting point. As the crystallization temperature is
reduced, this rate accelerates because of the effects of increasing concentrations
of stable nuclei. The rate passes eventually through a maximum, because the
colder conditions reduce the rate of conformational changes needed to place
polymer segments into proper register on the crystallite surfaces. When
T
g
is
reached, the crystallization rate becomes negligible. For isotactic polystyrene, for
example, the rate of crystallization is a maximum at about 175
C. Crystallization
rates are zero at 240
C(
T
m
) and at 100
C(
T
g
). If the polystyrene melt is cooled
quickly from temperatures above 240
C to 100
C or less, there will be insuffi-
cient time for crystallization to occur and the solid polymer will be amorphous.
The isothermal crystallization rate of crystallizable polymers is generally a maxi-
mum at temperatures about halfway between
T
g
and
T
m
.
Crystallinity should be distinguished from molecular orientation. Both phe-
nomena are based on alignment of segments of macromolecules but the crystal-
line state requires a periodic, regular placement of the atoms of the chain relative
to each other whereas the oriented molecules need only be aligned without regard
to location of atoms in particular positions. Orientation tends to promote crystalli-
zation because it brings the long axes of macromolecules parallel and closer
together. The effects of orientation can be observed, however, in uncrystallized
regions of semicrystalline polymers and in polymers that do not crystallize at all.
4.3.1
Degree of Crystallinity
High-molecular-weight flexible macromolecules do not crystallize completely.
When the polymer melt is cooled, crystallites will be nucleated and start to grow
independently throughout the volume of the specimen. If polymer chains are long
enough, different segments of the same molecule can be incorporated in more
than one crystallite. When these segments are anchored in this fashion the inter-
mediate portions of the molecule may not be left with enough freedom of move-
ment to fit into the lattice of a crystallite. It is also likely that regions in which
threadlike polymers are entangled will not be able to meet the steric requirements
for crystallization.
Several methods are available for determining the average crystallinity of a
polymer specimen. One technique relies on the differences between the densities
of completely amorphous and entirely crystalline versions of the same polymer
and estimates crystallinity from the densities of real specimens, which are inter-
mediate between these extremes. Crystalline density can be calculated from the
dimensions of the unit cell in the crystal lattice, as determined by X-ray analysis.
The amorphous density is measured with solid samples which have been produced
by rapid quenching from melt temperatures, so that there is no experimental evi-
dence of crystallinity. Polyethylene crystallizes too rapidly for this expedient to
be effective (the reason for this is suggested in
Section 4.3.2.1
), and volume
temperature relations of the melt like that in
Fig. 4.1
are extrapolated in order to
estimate the amorphous density at the temperature of interest. Crystalline regions
