Crystallization of Polymers. Volume 1, Equilibrium Concepts

4.4 Crystallization from a heterogeneous melt 135
to theory, as indicated by the dashed curve. Thus, basic thermodynamic theory
explains in a straightforward manner the melting temperature–composition relation
of mixtures of chemically identical polymers, with the same chain structure, that
form extended chain crystals.
4.4 Crystallization from a heterogeneous melt
The previous discussion has been limited to blends whose components were completely
miscible in the melt. Also to be considered are binary mixtures whose
components are either completely immiscible or are only partially miscible. A distinction
has to be made again as to whether one or both components crystallize.
In order to analyze the melting temperature–composition relations in such systems
it is necessary to examine some of the basic phase diagrams.(11,13,70,71) A typical
set of such diagrams is given in Fig. 4.10. As a reference diagram Fig. 4.10a
represents a mixture with only one component crystallizing. In this diagram the
melting temperature of the crystallizing component decreases continuously as its
concentration decreases. This curve can be represented either by Eq. (4.6), or the
corresponding expression obtained from equation of state theory.
For a partially miscible mixture the binodial needs to be specified as well as its
relative location on the melting temperature–composition curve. An upper critical
solution temperature (UCST) type binodial is taken as an example. The analyses of
other types diagrams, such as those with either a lower critical solution temperature
or an hour-glass type follow in a similar manner. The parameters that determine the
nature of the binodial are given by Scott.(18) If all else is equal, the nature of the
Fig. 4.10 Schematic representation of some typical phase diagrams for liquid–liquid and
liquid–crystal transformations.