monoclinic crystal series are the most common, occupying about two-third of the
total. The trigonal, the tetragonal and the hexagonal crystal series in sum occupy
about one-fourth. The remaining triclinic crystal series contains about one-seventh.
A careful reader may find that the sum of the above fractions has already been larger
than one. This is due to the presence of the phenomenon ofcrystal polymorphism.
For instance, isotactic polypropylene containsacrystal (monoclinic, the most
stable),bcrystal (hexagonal) andgcrystal (trigonal). The same species of polymers
exhibit different types of crystal structures because of either thermodynamics or
kinetics reasons. On the one hand, at higher temperatures, thermal motions can
change the effective shape of helical conformations, such as oval to circle in the
case of orthorhombic to hexagonal transition in PE at high pressures. On the other
hand, at lower temperatures, the crystallization kinetics may favor the metastable
crystal structures with a larger growth rate. Different types of crystal structures
exhibit different physical properties. Often, they can be controlled by adding
nucleation agents to initiate a specific form of crystal structures.
10.3.3 Folded-Chain Lamellar Crystals
According to Rule No. 1, polyethylene chains tend to form the most stable all-trans
conformation in the crystal. In practice, however, such an extending of chain
conformation could not easily be realized. The crystallization process often chooses
the metastable folded-chain conformation to form the lamellar crystals.
In 1930, Herrmann et al. proposed thefringed-micelle modelshowing that the
crystalline regions serve as physical crosslinking for high elasticity (Herrmann et al.
1930 ), as illustrated in Fig.10.11, in order to explain the good elasticity of LDPE
(showing X-ray diffraction peaks).
In 1957, owning to the invention of Ziegler-Natta catalysts to produce sequence-
regular polyethylene, Keller prepared the lamellar single crystals of polyethylene
from dilute solutions, and observed that the lamellar thickness with its direction
Fig. 10.11 Illustration of the metastable polymer conformation in the crystalline regions. From
lefttorightare the fringed-micelle model, the lamellar crystal with adjacent chain folding, the
switchboard model and the variable-cluster model
200 10 Polymer Crystallization