Chemistry Reference
In-Depth Information
) of Fig.
17
, it is essential to use (
26
) and not
(
33
) because the intramolecular interaction parameter l deviates strongly from the
usual value of 0.5. This observation is conceivable considering the fact that
cellulose is not noticeably soluble in water under the prevailing conditions, which
means that isolated polymer coils should be widely collapsed. Under these condi-
tions, the average volume fraction F
o
of the segments within the realm of such a
macromolecule will become very high and so consequently will l [cf. (
27
) and
(
17
)]. The evaluation of the present data yields l
For the modeling of the function w(
'
1.34. All other parameters
required for the modeling of the measured Flory Huggins interaction parameter
also lie well outside the normal range. The leading parameter a of the first term of
(
26
) is positive and very large like with the example of multiple critical points
discussed in the previous section (Sect.
4.1.1.4
). However, this time the large value
is not only due to the chemical dissimilarity of the components, but is also caused
by very favorable intersegmental contacts (H-bonds) that must be broken upon the
insertion of a solvent. In agreement with the general interrelation of the parameters
zl and a, this adverse contribution via a is counteracted by a comparable advanta-
geous conformational relaxation via z. The unique behavior of the water/cellulose
system is also demonstrated by the value of n, which is negative, in contrast to
almost all other polymer solutions studied so far. The only negative value of similar
magnitude was observed for the butane/1,4-polybutadiene system [
50
], which also
exhibits a large solubility gap. One might therefore speculate that the pronounced
self-association tendencies of the components (due to the unfavorable mutual
interaction) causes effective surface-to-volume ratios [cf. (
24
)] that differ consid-
erably from those expected on the basis of the molecular shapes of the components.
A further, immediately obvious particularity of the present system is the anoma-
lous swelling behavior of cellulose in water, as shown in Fig.
18
. To the author's
knowledge it is the only case where a high molecular weight polymer takes up more
of the pure coexisting liquid than does a sample of lower molar mass.
The results shown in Fig.
18
demonstrate that the miscibility gap of cellulose and
water, predicted from the vapor pressures measured above the homogeneous mixture
¼
Fig. 18 Composition of the
phases coexisting for the
system water/cellulose at
80
C as a function of the
number of segments
N
of the
polymer determined in
swelling experiments [
57
].
Symbols
indicate experimental
data; the
solid line
was
calculated by means of (
26
)as
described in the text; the two
phase area is hatched. The
dashed line
(normal behavior)
was calculated bymeans of the
original Flory Huggins
theory, setting the interaction
parameter equal to 0.54
0.3
80°C
0.2
0.1
0.0
2000
3000
4000
N
