
g-phase, at the expense of the commonly encountered
a-phase.
For further information on current thinking and theoret-
ical approaches to crystallization, the reader is referred to
recent reviews by Phillips [32,33].
39.1.2 Case Study Using Isotactic Polypropylene
Figure 39.4 shows several chart recorded bulk crystalliza-
tion traces as a function of temperatures for isotactic poly-
propylene (iPP, M
w
¼ 257,000). Avrami’s analysis takes
the lower values of each plot; i.e., before impingement.
The half-time is the point where half of the intensity is
reached. Figure 39.5 presents Avrami’s analysis for differ-
ent crystallization temperatures, where it can be seen that all
plots have similar slope, n, and different intercepts, k,in
this case.
Examples of the change in spherulite radius with time for
selected temperatures are shown in Fig. 39.6, where it
can be seen that linear growth rates result. Plots of growth
rate versus temperature for iPP can be seen in Fig. 39.7.
When the data are analyzed using the Hoffman–Lauritzen
equation, Fig. 39.8, it is seen that iPP shows the Regime
II–Regime III transition, previously identified by several
groups of workers [7–10]. In these analyses the values of
T
0
m
and U
were 186.1 8 C and 1,500 cal/mol, respectively.
The effects of different values of the thermodynamic
variables on the analyses and on the regime transition tem-
perature have been explored. Variation in T
0
m
has the great-
est effect on the shape of the secondary nucleation plot, but
does not significantly alter the regime transition tempera-
ture. A small change of the values of U
and T
g
simply
causes the curve to move up or down without changing its
shape.
The Regime II–III transition is envisioned as the point at
which the rate of surface spreading becomes less than
the rate of secondary nucleation. Surface spreading, for
an adjacent reentry system, is essentially a reeling-in pro-
cess dependent on the reptational ability of the polymer
chain.
The slopes of the secondary nucleation plots can be used
to estimate the fold surface free energies of the two poly-
mers. In order for these calculations to be carried out it
is necessary to have estimates of the parameters which
appear within Eq. 39.5. The equilibrium melting point
has to be determined in separate experimentation (see
Chapter 11).
The values of ss
e
can be determined from the slope of
the lines. Regime II and regime III give ss
e
¼ 562 and
678 erg
2
=cm
4
, respectively. In order to proceed further it is
necessary to estimate s independently. One way to do this is
to use the Hoffman modification of the Thomas–Stavely
relation [24].
s ¼ 0:1Dh
f
ffiffiffiffiffiffiffiffiffi
a
0
b
0
p
: (39:6)
Values of s have been calculated as 11:5 erg=cm
2
for iPP.
Substitution of these values into the determined values of
−0.2
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30 35
Light intensity
Time (min)
124 °C 126 °C 132 °C 134 °C
FIGURE 39.4. Isothermal bulk crystallization traces as a function of temperature.
CRYSTALLIZATION KINETICS OF POLYMERS / 629