Chemistry Reference
In-Depth Information
azeotrope
5
½
M
1
1
r
2
2
(9-22)
½
M
2
1
2
r
1
or from
Eq. (9-14)
1
r
2
2
ð
f
1
Þ
azeotrope
5
(9-23)
2
2
r
1
2
r
2
1) only if
r
1
and
r
2
are simulta-
neously both either greater or less than one. (If
r
1
5
Note that
f
1
is physically meaningful (0
#
f
1
#
r
2
5
1, all values of
f
1
are
azeotropic compositions.) Since the case o
fr
1
.
1,
r
2
.
1 is unknown in free-
radical systems, the necessary conditions for azeotropy in such copolymerizations
is that
r
1
,
1.
Figure 9.2
includes some representative copolymer-feed composition curves
for
r
1
varying with
r
2
constant at 0.8. Azeotropic feed compositions containing
appreciable quantities of both polymers can be achieved only when the reactivity
ratio values are close to each other.
Equation (9-23)
calculates the feed composition that yields an invariant copol-
ymer composition as the conversion proceeds in a batch polymerization. Note that
comonomer ratios that are near but not equal to the estimated azeotropic value
may produce copolymers whose compositions are constant for all practical pur-
poses. The permissible range of feed compositions for which this “approximate”
1,
r
2
,
1.0
0.8
1.0
0.5
0.1
0.6
0
0.4
0.2
0
0.2
0.6
Mole fraction m
1
in feed, f
1
0.4
0.8
1.0
FIGURE 9.2
Relation between instantaneous feed composition f
1
and corresponding polymer
composition F
1
for indicated values of r
1
with r
2
5
0.8. Azeotropic compositions exist at
r
1
5
0, f
1
5
0.17; r
1
5
0.1, f
1
5
0.18; r
1
5
0.5, f
1
5
0.29; r
1
5
1.0, no azeotrope [
Eq. (9-23)
].
