The highly oriented polymer chains in the fiber crystal bring high strength to the
fiber materials. The famous Kevlar fiber is high strength aramid fiber produced by
oriented spinning from the liquid crystal phase, which is applied as anti-bullet
textiles. The ultra-high molecular weight polyethylene can be gel-spanned from the
semi-dilute solutions and then be further stretched tens to hundreds folds, to obtain
high strength PE fibers. Beyond the stretch ratio of 200, the Young’s modulus of PE
fibers can be stable at 210220 GPa, and the tensile strength can be as high as
4 GPa.
10.4 Kinetics of Polymer Crystallization
10.4.1 Nucleation of Polymer Crystallization
Polymer crystallization is a typical first-order phase transition, following the nucle-
ation and growth mechanism. Therefore, when isothermal crystallization happens
slowly at high temperatures, one can observe a significantly long incubation period
for crystal nucleation, followed by a self-acceleration process for crystal growth.
This process is illustrated by the time-evolution curve of volume crystallinity in
Fig.10.21.
The existence of an incubation period displays a difficulty to initiate crystalliza-
tion. Thermal fluctuations are demanded for generating crystal nuclei beyond a
critical size. The basic principle is analogous to the initiation of phase separation
from the metastable phase. The new phase is required to overcome the free energy
barrier brought by the rise of surface free energy around its boundary. At the early
stage, the free energy change for crystal nucleation with the change of crystal size
ris illustrated in Fig.10.22. The total free energy change can be expressed as
Fig. 10.21 Illustration of the time evolution curve of volume crystallinity upon slow isothermal
crystallization at a high temperature
208 10 Polymer Crystallization