Chemistry Reference
In-Depth Information
The nature of such adventitious nuclei has not been clearly established. The grow-
ing crystal has to be able to wet its nucleus, and it has been suggested that the
surfaces of the effective heterogeneities contain crevices in which crystalline
polymer is trapped.
The control of nucleation density can be important in many practical applica-
tions. A greater number of nucleation sites results in the formation of more
ordered regions, each of which has smaller overall dimensions. The average size
of such domains can affect many properties. An example is the transparency of
packaging films made from semicrystalline polymers. The refractive indexes of
amorphous and crystalline polymer domains differ, and light is refracted at their
boundaries. Films will appear hazy if the sizes of regions with different refractive
indexes approach the wavelength of light. Nucleating agents are sometimes delib-
erately added to a polymer to increase the number of nuclei and reduce the
dimensions of ordered domains without decreasing the average degree of crystal-
linity. Such agents are generally solids with colloidal dimensions, like silica and
various salts. Sometimes a higher melting semicrystalline polymer will nucleate
the crystallization of another polymer. Blending with small concentrations of iso-
tactic polypropylene (
T
m
.176
C) improves the transparency of sheets and films
of polyethylene (
T
m
.115
137
C), for example.
4.3.2.2
Crystal Lamellae
Once nucleated, crystallization proceeds with the growth of folded chain ribbon-
like crystallites called lamellae. The arrangement of polymer chains in the lamel-
lae has some resemblance to that in platelike single crystals which can be pro-
duced by precipitating crystallizable polymers from their dilute solutions. In such
single crystals the molecules are aligned along the thinnest dimension of the plate.
The lengths of extended macromolecules are much greater than the thickness of
these crystals and it is evident that a polymer chain must fold outside the plate
volume and reenter the crystallite at a different point. When polymer single crys-
tals are carefully prepared, it is found that the dimensions are typically a few
microns (1
10
2
6
m)
μ
m
5
1 micron
5
for
the length and breadth and about
0.1
m for the thickness. The thickness is remarkably constant for a given set of
crystallization conditions but increases with the crystallization temperature.
Perfect crystallinity is not achieved, because the portions of the chains at the sur-
faces and in the folds are not completely ordered.
There is uncertainty about the regularity and tightness of the folds in solution-
grown single crystals. Three models of chain conformations in a single crystal are
illustrated in
Fig. 4.4
.
Folded-chain crystals grow by extension of the length and breadth but not the
thickness. The supply of polymer segments is much greater in the melt than in
dilute solution, and crystallization in the bulk produces long ribbonlike folded
chain structures. These lamellae become twisted and split as a result of local
depletion of crystallizable material and growth around defect structures. The
μ
