Chemistry Reference
In-Depth Information
to them. complete exclusion of
-butyl branches from the crystal stem. On the other hand, the
1-hexene copolymer forms much thicker lamellae and a broader thickness distribution than the
synthesized polyethylene with butyl branches. Here, the average thickness is 1.5 times larger than
that calculated from the most probable ethylene sequence length obtained from NMR, or for a
theoretical ethylene sequence Length distribution, indicating that the lamellae are composed predom-
inantly of the sparsely branched longer ethylene sequences.
Linear polyethylene single crystals often exhibit secondary structural features that include
corrugations and pleats. It was suggested that the crystals actually grow in form of pyramids, but
that these pyramidal structures collapse when the solvent is removed during preparation for micros-
copy [
42
]. Various investigators described other complex structures besides pyramidal. Typical
among these descriptions are sheaf-like arrays that would correspond to nuclei. Also there were
observations of dendridic growths, of clusters of hollow pyramids, of spiral growths, of epitaxial
growths, of moir´ patterns, etc. [
42
,
43
].
n
2.3.3 Spherulitic Growth
For polymers that
crystallize from the melt
, an important parameter in the characterization of the
two-phase systems, is the weight fraction of the crystalline regions. The degree of crystallinity that
can be reached is dependent on the temperature at which crystallization takes place. At low
temperatures one attains a much lower degree of crystallization than at higher temperatures. This
implies that crystallization remains incomplete for kinetic reasons [
7
].
Normal conditions of cooling of the molten polymer establish the crystalline texture of the
polymer and usually result in formation of very tiny crystals. These crystals are part of a closely
spaced cluster called
spherulite
. The formation of a single nucleus in a polymer cooled below its
melting point favors the formation of another nucleus in its vicinity due to creation of local stresses.
Microscopic examinations with polarized light of many polymeric materials that crystallized from
the melt show the specimen packed with spherulites. Often these appear to be symmetrical structures
with black crosses in the center [
38
]. It is believed [
39
] that these spherulites grow from individual
nuclei. Ribbons of crystallites grow from one spherulitic center and fan out in all directions. Initially
they are spherical but because of mutual interference irregular shapes develop. The diameters of
spherulites range from 0.005 to 0.100 mm. This means that a spherulite consists of many crystalline
and noncrystalline regions. The black crosses seen in the spherulites are explained [
39
] by assuming
that the crystallites are arranged so that the chains are preferentially normal to the radii of the
spherulites. Spherulitic morphology is not the universal mode of polymer crystallization. Spherulitic
morphology occurs usually when nucleation is started in a molten polymer or in a concentrated
solution of a polymer. Spherulitic growth is illustrated in Fig.
2.13
.
The size and number of spherulites in the polymer tends to affects the physical properties. Thus,
the impact strength of polymer films or their flex life usually increases as the spherulite size
decreases. On the other hand, there does not appear to be any correlation between the yield stress
and ultimate elongation and the size of the spherulites.
Rhythmic crystal growth is generally encountered in thin films of semicrystalline polymers. This is
believed to be due to formation of ring-banded spherulites and attributed to the periodical lamellae
twisting along the radial growth direction of the spherulites [
42
]. Recently, Gu and coworkers [
43
]
reported that by using mild methylamine vapor etching method, the periodical cooperative twisting of
lamellar crystals in ring-banded spherulites was clearly observed.
When the melt or the solutions are stirred
epitaxial
crystallinity is usually observed. One
crystalline growth occurs right on top of another. This arrangement is often called
shish-kebab
