Chemistry Reference
In-Depth Information
Fig. 27 Cosolvency as a
result of unfavorable
interactions between
components 1 and 2. The
numbers of segments of the
different components are
given at the
corners
of the
phase diagram and the
characteristic parameters for
the binary subsystems are
indicated on its
edges
.
Open
symbols
composition of
coexisting phases,
closed
symbols
critical points,
shaded areas
unstable
regions [
27
]
N
P
=
1000
0.00
1.00
0.25
0.75
0.50
0.50
0.75
0.25
1.00
0.00
0.00
0.25
0.50
a = 0.35
z
= 0
0.75
1.00
N
1
=
1
N
2
=
1.3
applies to sufficiently unfavorable 1/2 interactions; in their absence the miscibility
gap would extend from one binary subsystem to the other throughout the ternary
system, i.e., this would be an example of simplicity.
The reason for the complete miscibility of the polymer with mixed solvents
containing comparable fractions of their components shown in Fig.
27
lies in the
adverse interactions between them. Within a certain range of compositions, the
ternary system can avoid these unfavorable contacts between components 1 and
2 by inserting a polymer segment between them and forming homogeneous
mixtures.
Cononsolvency
The creation of a miscibility gap by mixing two favorable solvents was reported a
long time ago [
70
] and many examples have been described since. Figure
28
shows a typical modeling of this behavior. For that purpose, we assume that
the components 1 and 2 are markedly better solvents for the polymer P than
in the case of cosolvency, and that they mix in a combinatorial manner (
g
12
¼
w
12
¼
;0).
The reason why the present combination of parameters for the binaries leads to a
miscibility gap for the ternary system lies in the particularly favorable interactions
1/P and 2/P as compared with the more or less “neutral” interactions 1/2. Under
these conditions, the Gibbs energy of the total system can be lowered by phase
separation such that the polymer-lean phase contains practically low molecular
weight components only and that many favorable 1/P and 2/P contacts can be
formed in the polymer-rich phase.
