Chemistry Reference
In-Depth Information
The sample is first fractionated into intermediate fractions in one solvent + non-
solvent system (solvent mixture 1). This is followed by further fractionation
of these intermediate fractions by another solvent + nonsolvent system (solvent
mixture 2).
3.2.1
Influence of the Fractionation Strategy
Using the SSF (Fig.
3b
) and SPF (Fig.
3a
) techniques, copolymers can be fractio-
nated using the cross-fractionation method by combining the two basic types for
fractionation in solvent mixtures 1 and 2. This situation results in four different
fractionation strategies: SSF/SPF, SPF/SSF, SPF/SPF, and SSF/SSF, where the
solvent mixture is changed for the fractionation of the intermediate fractions. The
first question arising in this situation is which strategy should be used in order to
optimize the fractionation efficiency. To answer this question, calculation of all
four strategies were performed.
In these simulations, it was assumed that both solvent mixtures are made only
from one solvent. The applied parameters in the
G
E
model (
5
) for the solvent 1 are:
r
A
¼
1
w
AP
¼
250 K
p
A
¼
0
g
A
¼
1
(62)
and for the solvent 2:
r
A
¼
1
w
AP
¼
220 K
p
A
¼
0
g
A
¼
0
:
5
:
(63)
During the simulation, the original polymer was fractionated into five interme-
diate fractions using the solvent 1 (
62
), whereas every intermediate fraction was
further divided into five final fractions using the solvent 2 (
63
). The temperatures
for every fractionation step were selected in such a way that in every fraction the
same amounts of polymer were present. The polymer feed concentration, expressed
in segment fractions, was 0.01. Figure
12
depicts the calculated mass-average
chemical composition of every fraction using the four different fractionation
strategies. The fractions numbered 10 15 represent the chemical composition of
the final fractions, obtained from the first intermediate fraction. The fractions
numbered with 20 25 represent the chemical composition of the final fractions,
obtained from the second intermediate fraction, and so on. If the fractionation in
solvent 1 is carried out using the SPF mechanism (circles and crosses in Fig.
12
), the
mass-average chemical composition of the intermediate fractions decrease with the
number of the fraction. Under this circumstance the fractionation in the solvent 2 can
be performed using SPF (circles in Fig.
12
) or SSF (crosses in Fig.
12
). Except for
the final fractions from the first intermediate fraction, where the SPF mechanism
leads to a decrease in the
y
W
values and the SSF to increasing values for this
quantity, in all other final fractions the
y
W
value increases by SSF and decreases by
SPF. If the fractionation in the solvent 1 is carried out using the SSF mechanism
