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performed with CH (less favorable solvent) instead of THF using a molecularly
disperse PS sample require at least one composition-dependent ternary interaction
parameter for their modeling. Indications exist that this complication is due to the
presence of PS molecules differing markedly in their molar mass.
One important consequence of the results presented for solutions of compatible
polymers in a common solvent is this: The suggested idea to prepare homogeneous
polymers films containing both types of macromolecules from joint solutions by
solvent evaporation will probably not work. The reason is that solutions containing
comparable amounts of polymers A and B need to pass the unstable area of the
phase diagram upon the removal of solvent, which means that they inevitably
demix into two phases: one rich in polymer A and the other in polymer B. Despite
the fact that the system enters the one-phase region again as the solvent content falls
below a certain value, the high viscosity of the coexisting liquids will normally
prevent homogenization.
5 Conclusions
The theoretical concepts presented in this chapter and the experimental examples
given for their validity demonstrate how the Flory Huggins theory can be made
practical with reasonable effort. The central features of the approach are the
provision for chain connectivity in dilute polymer-containing systems (by means
of microphase equilibria) and the variability of macromolecules with respect to
their spatial extension (expressed in terms of conformational relaxation after mix-
ing). Both particularities contribute to the Flory Huggins interaction parameters
and are quantified in a second, additive term, which becomes zero for most of the
theta systems. In contrast to the original Flory Huggins theory, the interaction
parameters are no longer independent of concentration; complicated functions
w(
) are sometimes necessary to model experimental data, including minima and
maxima in this dependence. It is therefore no wonder that several parameters are
needed to gather the particularities of a certain system. In many cases, two para-
meters suffice for the quantitative description because of some possible simplifica-
tions and interrelations, as described in Sect.
2
. With complex systems (like water/
cellulose) up to four parameters might, however, be required.
There is one finding that speaks strongly for the validity of the present approach,
namely the fact that several types of phase equilibria can be described quantita-
tively by means of the same set of parameters (cf. the systems
n
-C
4
/1,4-PB and
CHCl
3
/PEO). Another eminent advantage of the present approach is its general
applicability to very different classes of polymers (including branched macromo-
lecules and copolymers of different architecture); furthermore, there is no obvious
reason why it should fail for multicomponent systems.
So far, the extension of the Flory Huggins theory has enabled the modeling of
several hitherto unexplainable anomalous phenomena, like uncommon molecular
weight dependencies of second osmotic virial coefficients, the existence of multiple
critical points for binary systems, or the odd swelling behavior of cellulose in water.
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