Chemistry Reference
In-Depth Information
nonuniformity than the original polymer. Using the SPF/SPF method (circles in
Fig.
13
), the last fraction of every intermediate fraction always shows the highest
value for the nonuniformity. With increasing number of the intermediate fraction
this effects is more pronounced. Applying the SPF/SSF mechanism (crosses in
Fig.
13
), the highest nonuniformity is always observed in the first final fraction of
every intermediate fraction, similar to the results for the SSF/SPF mechanism (stars
in Fig.
13
). Similar to the results discussed above, the SSF/SSF (squares in Fig.
13
)
leads to the most effective fractionation, also according to the molecular weight,
and hence this method can be recommended for the cross-fractionation.
3.2.2
Influence of the Solvent Mixture
The fractionation based on the LLE and the LLE depends strongly on the interac-
tions between the molecules present in the system. In connection with the effectiv-
ity of copolymer fractionation, it must be investigated by how much both solvents
used in the cross-fractionation procedure should differ in terms of solution power
for the copolymer. The quality of solvent can be expressed by the interaction
parameter occurring in the
G
E
model (
5
). Calculations were done using two
different parameter sets, representing a large difference between the interactions
of the solvents 1 and 2 with the copolymer:
w
AP
¼
250 K
p
A
¼
0
g
A
¼
1
(64)
w
BP
¼
350 K
p
B
¼
0
g
B
¼
0
:
15
and representing only a small difference in these parameters:
w
AP
¼
250 K
p
A
¼
0
g
A
¼
1
(65)
w
BP
¼
220 K
p
B
¼
g
B
¼
:
:
0
0
5
The calculation results are depicted in Fig.
14
in terms of the Breitenbach Wolf
plot with respect to the segment number (Fig.
14a
) and with respect to the chemical
composition (Fig.
14b
). The fractionation effect can be analyzed by the slope of the
Breitenbach Wolf plot. If the two solvents differ strongly, the Breitenbach Wolf
plot with respect to the segment number has a larger slope meaning a higher
fractionation effect (Fig.
14a
). However, analyzing the data given in Fig.
14b
for
the fractionation effect according to the chemical composition, the converse can be
concluded. The reason for this finding is the difference in the parameter g
A
for the
solvent 2. Whereas the Flory Huggins parameter w
AP
has an impact on both
fractionations types, the parameter g
A
only has an impact on the fractionation
with respect to the chemical heterogeneity. For practical purposes, the w
AP
para-
meters should have a large difference in order to make sure an effective fraction-
ation according the molecular mass, and the g
A
parameter should be always quite
