Chemistry Reference
In-Depth Information
For practical purposes, the possibility of predicting the thermodynamic behavior
of random copolymers in a given solvent from knowledge of the corresponding
homopolymers would be extremely helpful. The present results demonstrate that
this is a difficult task and that the choice of the particular solvent plays a decisive
role. For all systems under investigation,
w
w varies considerably with the composi-
tion of the mixture. With one exception [CHCl
3
/P(S-
ran
-MMA) and
f
0.5] the
dependencies
w
w(
w
) of the copolymers do not fall reasonably between the data
obtained for the corresponding homopolymers. In most cases, the incorporation of a
small fraction [
25
] of the monomer that interacts less favorably with a given solvent
suffices to reduce the solvent quality for the copolymer, approximately to that for
the worse soluble homopolymer. Figure
14
shows an example for which this effect
is particularly obvious.
In terms of the
w
w values measured for a given constant polymer concentration,
the polar solvents CHCl
3
, AC, and MeAc are expectedly more favorable for PMMA
than for PS, whereas the nonpolar TL is a better solvent for PS than for PMMA. The
shape of the functions
w
w(
w
) varies considerably. For AC/PMMA and MeAc/PS,
w increases linearly and for AC/PS more than linearly, whereas it decreases linearly
for CHCl
3
/PS. With three of the systems, one observes minima in
w
w(
w
), namely for
TL/PMMA, TL/PS, and CHCl
3
/PS; only MeAc/PMMA exhibits a maximum. On
the basis of (
32
), this diversity of composition influences is easily comprehensible if
one keeps in mind that the composition dependence of Flory Huggins interaction
parameters are made up of two separate contributions. The normally nonzero
parameter n of the first term of this relation (which is primarily determined by the
differences in the shapes of monomeric units and solvents molecules) leads to a
nonlinear composition dependence of
w
w, where the magnitude of this contribution
increases as the absolute values of the parameter a rise. The second term of (
32
)
adds a linear dependence, quantified by the parameter zl. In agreement with the
great diversity of the systems concerning the functions
w
w(
w
), all three parameters
of the present approach may be positive, negative, or zero.
¼
1.00
PMMA
P(S-
ran
-MMA)
f
= 0.5
0.75
w
c
Fig. 14 Composition
dependence of the Flory
Huggins interaction
parameters [based on weight
fractions
w
P
, cf. (
8
)] for the
solutions of PS, PMMA, and
of a random copolymer
containing 50 wt% of these
monomers in TL at 50
C[
25
]
0.50
PS
0.25
50°C
0.0
0.2
0.4
0.6
0.8
1.0
w
P
TL
P
