nucleation contains the lowest free energy barrier, and thus be the fastest, so it is not
easy to be observed as the rate-determining step at the crystal growth front.
Primary crystal nucleation can be further separated into two special scenarios.
The spontaneous crystal nucleation from homogeneous bulk polymer phase is
called homogeneous nucleation. In contrast, heterogeneous nucleation is the crystal
nucleation on a specific location of the foreign substrate, and thus does not require
the generation of a new surface on the contact plane. The foreign substrate can be
air bubbles, dusts, incompatible solid or liquid, incomplete molten crystallites,
walls of the vessel, or stirring sticks. As illustrated in Fig.10.24, the total surface
free energy can be reduced by the use of the foreign substrate. The incomplete
molten crystallites of the same species can by-pass the primary nucleation process
and directly generate crystal growth. Such a process is calledself-seeding process.
In the practical processing of polymer materials, the nucleation agent is added to
supply as many as possible nuclei, and thus to control the sizes of crystallites to be
smaller than the wavelengths of visible lights. Hence, the materials will not have
strong scattering of visible lights, and appear transparent. The typical example is
Fig. 10.23 Illustration of three basic situations of crystal nucleation. Fromlefttorightare primary
nucleation in the bulk polymer phase, secondary nucleation on the smooth growth front, and
tertiary nucleation at the terrace of the growth front
Fig. 10.24 Illustration of total surface free energy change in homogeneous nucleation and
heterogeneous nucleation
210 10 Polymer Crystallization