kind of spherulites obtained by direct branching growth at low temperatures, called
thetype-II spherulites, as illustrated also in Fig.10.17. The branching growth of
lamellar crystals at low temperatures is related to the Mullins-Sekerka instability of
crystal growth, which appears intrinsic for polymer crystallization. Inside the
spherulites, since the lamellar crystalsgrow along the radial direction, with the
chain axis perpendicular to the radial direction. Such an isotropic structure leads
to zero-amplitude extinction along the long and short axes of the optical
indicatrix (the indicatrix reflects the photodichroics of crystals) of lamellar
crystals. Under the cross-polarized light microscopy, it appears as Maltese-
cross extinction, as illustrated in Fig.10.18. If the long axis of lamellar eclipse
is along the radial direction, the spherulite is defined as the positive spherulite;
while if along the direction perpendicular to the radial, it is defined as the
negative spherulite. In most of cases, the chain axis (the long axis of eclipse)
aligns along the perpendicular direction, thus polymer spherulites are mostly the
negative spherulites. Many aromatic polyesters and polyamides generate the
unusual spherulites at high temperatures,whose Maltese cross rotates 45 degree
under the cross-polarized lights.
- Shish-kebab crystals and fiber crystals
From the thermodynamic point of view, if the stress imposed by an external field
causes the stretching and orientation of the amorphous polymer chains, the confor-
mational entropy will decrease. Accordingly, the entropy change of crystallization
from the oriented state will be reduced. FromTm¼DH/DS, the melting point will
increase, and hence the effective supercooling for the crystallization will be
enhanced, offering the priority to initiate crystallization. From the kinetic point of
view, the stretched and oriented state of polymer chains may not need to wait for a
large-scale rearrangement or long-distance diffusion to enter the crystalline phase.
Therefore, the crystallization can be greatly accelerated. Highly oriented polymer
chains will crystallize into the fiber crystal. The fiber crystal, as the crystal nuclei,
will further induce crystallization of other less oriented polymers (such as the low
molecular weight fractions), which form an array of parallel-oriented lamellar
Fig. 10.18Illustration of (a) the lamellar crystal, (b) the optical indicatrix and (c) the Maltese-
cross extinction in polymer spherulites
206 10 Polymer Crystallization