Chemistry Reference
In-Depth Information
with polymer blends, where both components are accessible for segments of the
other polymer. The meaning of the parameter z, on the other hand, remains
unchanged because molecular rearrangements, similar to those occurring with
polymer solutions and polymer blends, will also take place in low molecular weight
mixtures, due to preferentially interacting sites of the components. According to the
present results, a linear composition dependence of the conformational part of the
interaction parameter should suffice to describe reality.
4.2 Ternary Systems
The material presented so far has demonstrated the ability to model the thermody-
namic behavior of binary systems accurately by means of the present approach. For
the description of polymer solutions, it is normally possible to eliminate one of the
three parameters of (
32
) thanks to a general interrelation between a and zl (
34
). For
polymer blends and mixtures of low molecular weight components, a similar
general simplification is presently not known. Notwithstanding this situation, it is
possible to model the principle features [
27
] of all types of phase diagrams observed
for ternary systems using only two parameters for each binary subsystem.
This section deals with the phase-separation behavior of ternary systems, where
a distinction is made between polymer solutions in mixed solvents (Sect.
4.2.1
) and
solutions of two polymers in a single solvent (Sect.
4.2.2
). Furthermore, the systems
are classified according to the way the thermodynamic properties of the ternary
systems are made up from the properties of the corresponding binary subsystems:
Simplicity
denotes “smooth” changes in the phase behavior of the binary subsys-
tems upon the addition of the third component in its pure form or in mixtures (see
later).
Cosolvency
means that the thermodynamic quality of mixture of two com-
ponents is higher with respect to the third component than expected by simple
additivity, i.e., cosolvency reduces the extension of the two-phase region with
respect to that expected from additivity.
Cononsolvency
, finally, denotes the oppo-
site behavior, i.e., an extension of the two-phase region beyond expectation.
4.2.1 Mixed Solvents
The use of mixed solvents is widespread, because it offers the possibility to tailor
desirable thermodynamic conditions by mixing two liquids with sufficiently differ-
ent qualities in adequate ratios, instead of the often inconvenient or even impossible
variation of temperature. The combination of good solvents with precipitants is the
basis of many industrial processes, like membrane production or fiber spinning. In
order not to go beyond the scope of the present contribution, the following con-
siderations are limited to complete miscibility of the components of the mixed
solvent. There is, however, no particular difficulty to extend the treatment to
incompletely miscible components of mixed solvents.
