Chemistry Reference
In-Depth Information
presence of linear and branched PI in addition to the usual nonuniformity of molar
masses. The consequences of broad molecular weight distributions of linear poly-
mers for the shape of phase diagrams are not negligible, but are usually consider-
ably less pronounced than nonuniformities with respect to the molecular
architecture of the macromolecules. The reason is that polymers of different
chain length are usually completely miscible, whereas this needs not be the case
for linear and branched macromolecules, as exemplified when dealing with their
solutions in a common solvent.
The strange peninsular of the miscibility gap shown in Fig.
29
is caused by the
fact that the PI sample contains both linear and branched material; neither the
solution of the linear product nor that of the branched polymer in the same mixed
solvent show this particularity [
71
]. It is, however, very probable that particular
interactions between the components of the mixed solvent also play a role in the
occurrence of the anomalous peninsula of the phase diagram. This consideration
rests on the fact that the CH/AC system exhibits an upper critical solution tempera-
ture [
72
]at
29
C. The low mixing tendency of these components might increase
the possibilities of the quaternary system to reduce its Gibbs energy via demixing.
4.2.2 Blend Solutions
Solutions of chemically dissimilar polymers in a common solvent play an important
role in the processing of polymer mixtures, where this is particularly true for
incompatible polymer pairs but also for the production of homogeneous films
consisting of two compatible polymers. Like with polymer solutions in mixed
solvents, one can observe all the deviations from additive behavior discussed
earlier.
Simplicity
The modeled example given in Fig.
30
for this behavior shows the gradual dis-
appearance of a miscibility gap existing between two moderately incompatible
polymers upon the addition of a solvent of comparatively low thermodynamic
quality.
The phase diagram of Fig.
30
looks very similar to the one measured for the
solutions of linear and branched PI in CH and shown in Fig.
31
. For these experi-
ments, the originally synthesized branched material (PI* of Fig.
29
) was to a large
extent freed from the linear components by means of the large-scale method of spin
fractionation [
73
]. Despite the fact that the boundary between the homogeneous and
the two-phase area was only mapped, instead of the usual cloud point measure-
ments, the results of Fig.
31
testify to the existence of shape-induced incompatibil-
ity of polymers. It is remarkable that this phenomenon can be observed for
comparatively low molar masses of the components.
