Chemistry Reference
In-Depth Information
one in an interaction parameter
χ
. This parameter characterizes a pair of compo-
nents whereas each
can be deduced from the properties of a single component.
In the initial theory,
δ
was taken to be a function only of the nature of the
components in a binary mixture. It became apparent, however, that it depends on
concentration and to some extent on molecular weight. It is now considered to be
a free energy of interaction and thus consists of enthalpic and entropic compo-
nents with the latter accounting for its temperature dependence.
The Flory
χ
Huggins theory does not predict the lower critical solution tempera-
ture (LCST) phase behavior in which the components phase separate at high tem-
peratures but are miscible at low temperatures. As mentioned, most miscible
polymer solutions and blends require favorable specific interactions (e.g., hydrogen
bonds). And such interactions will diminish as temperature is increased, leading to
phase separation.
Figure 5.2
schematically illustrates the possible phase behavior
of polymer solutions and blends. The reason that the Flory
Huggins theory is not
Two phases
One phase
LCST
T
T
UCST
One phase
Two phases
φ
2
φ
2
Two phases
One phase
T
T
Two phases
Two phases
One phase
φ
2
φ
2
FIGURE 5.2
Schematic representation of various phase diagrams of polymer solutions and blends.
