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Encyclopedia of Physical Science and Technology EN008C-602 July 25, 2001 20:31

Macromolecules, Structure 899

the pyramid axis and are therefore inclined to the faces of
the pyramid. Another deviation from the idealized form is
often observed in crystals prepared at high degrees of un-
dercooling. Under such circumstances the growth of poly-
mer crystals is not limited to the laying down of chains
in a lateral growth pattern. The formation of multilayered
crystals consisting of several superimposed chain-folded
layers all of equal thickness, and originating from a screw
dislocation, is often observed. Their spiral terraces have a
ziggurat-like form.
FIGURE 43 Models of chain morphology in a single crystal with
(a) regular folding with adjacent reentry and (b) irregular, random Single polymer crystals from solution are mainly of
“switchboard” fold surface. scientific rather than technical interest. In practical use,
semicrystalline polymers crystallize from the melt, and
the dominant form is the spherulite, which is uncommon
the form of a “switchboard” with reentry occurring more for small molecules. Spherulites are aggregates of crys-
or less at random and with loops much larger than the tals with a radiating fibrillar structure. In polymers they
minimal size. Such a model will clearly tend to be fa- are microscopic in size, usually of the order of 100 µm or
vored at low extents of crystallinity. These two pictures less in diameter. In Fig. 45 spherulitic growth in a melt
of the morphology of a single crystal are presented in of isotactic polystyrene, observed under the polarizing
Fig. 43. microscope between crossed polarizers, is shown. Being
Although represented in Figs. 42 and 43 as planar for birefringent the spherulites show up against a dark back-
simplicity, lamellar crystals often take complex forms of ground of molten polymer. They are seen in cross section,
hollow pyramids. Viewed along the c axis such crystals having grown in two dimensions in a film of molten poly-
would appear lozenge-shaped. What is seen under a mi- mer between cover slips, which have a small separation
croscope as a planar lamella may actually represent a compared to the spherulite diameter. In bulk polymer, of
collapsed pyramid, as shown in Fig. 44. In such hollow course, their growth would take place in three dimensions.
pyramids the stems of the molecules remain parallel to The maltese cross pattern is typical, reflecting their bire-
fringent and symmetric nature. In this stage, growth is not
yet complete. When it is complete, the spherulites impinge
on one another with more or less straight boundaries, as
shown in Fig. 46. These spherulites also exhibit a banded
pattern that is quite common.
It is found from the sign of the birefringence and from
X-ray study that the molecular chains are normal to the

FIGURE 44 A single crystal of polyethylene that is actually a FIGURE 45 Spherulitic growth in a melt of isotactic polystyrene
collapsed hollow pyramid as indicated by the pleats in the center. (magnification: 115×).