Crystallization of Polymers. Volume 1, Equilibrium Concepts

2.2 Nature of the fusion process 29
Fig. 2.3 Specific volume–temperature relation for linear polyethylene samples. Samples
initially crystallized at 131.3 ◦ C for 40 days. Unfractionated polymer, Marlex-50 ; fraction
M n = 32 000 . (From Chiang and Flory (5))
characteristics of the unfractionated polymer that was illustrated in Fig. 2.2 and
a molecular weight fraction, M = 3.2 × 10 4 crystallized under extreme isothermal
conditions, is given in Fig. 2.3.(5) The melting temperatures are clearly defined
and are the same for both polymers. It is evident in Fig. 2.3 that the melting of
this fraction, (open circles), is appreciably sharper than the whole polymer. For
the fraction, 80% of the transformation occurs over a 2 ◦ C range. Over the same
temperature interval there is only a 35–40% change in the polydisperse polymer.
Molecular weight fractions appear to be able to develop a more perfectly developed
crystalline state with a concomitant sharper fusion process. However, studies
have shown that, depending on molecular weight, the use of fractions can also introduce
complexities into the fusion process. Figure 2.4 illustrates the change in
crystallinity level with temperature for fractions whose molecular weights range
from 3.3 × 10 3 to 1.55 × 10 6 .(6) These samples were isothermally crystallized and
never cooled subsequent to the initiation of fusion. The two lowest molecular weight
fractions melt very broadly. This result is easily explained by the high concentration