Chemistry Reference
In-Depth Information
1.0
T=45°C
0.8
0.6
0.4
0.2
0.0
0.2
0.4
0.6
0.8
1.0
j
CH
PI
Fig. 13 Flory Huggins interaction parameters for solutions of linear PI (
circles
,
M
w
23.2 kg/
mol) and of branched PI (
stars
,
M
w
21.6 kg/mol) in CH as a function of polymer concentration
the second term (resulting from the conformational relaxation). This second sum-
mand is determined by the parameters l and z, both of which depend on the
molecular architecture of the polymer: A higher degree of branching leads to a
reduction in the intramolecular interaction parameter l (accumulation of the seg-
ments in a smaller volume) as well as in z (diminished possibilities to readjust to a
changing molecular environment by conformational relaxation). Because of the
negative sign of the second term, these changes lead to larger w values for the
branched polymer. In other words, in the absence of special effects, the thermo-
dynamic quality of a given solvent declines upon an increase in the degree of
branching. Another feature worth mentioning is the observation that the interrela-
tion between the parameters zl and a, established for linear macromolecules
(cf. Fig.
5
), remains valid for branched materials.
Organic Solvents/Linear Random Copolymers
With systems of this type, a new feature comes into play: In spite of the fact that we
are dealing with binary systems, we need three different interaction parameters to
describe the thermodynamic behavior. This makes the modeling considerably more
difficult and is the reason why the present approach requires more adjustable
parameters, and the theoretical understanding is far from being satisfactory.
For reasons outlined in the theoretical section (Sect.
2
)(
8
) the study reported here
uses weight fractions
w
instead of the usual volume fractions
'
. It was carried out for
solutions of poly(styrene-
ran
-methyl methacrylate [P(S-
ran
-MMA)], with different
weight fractions
f
of styrene units, in CHCl
3
, acetone (AC), methyl acetate (MeAc),
and TL at 50
C[
25
]. Analogous measurement for the solutions of the corresponding
homopolymers, PMMA and PS, were also performed for comparison.
