Chemistry Reference
In-Depth Information
Swelling of Cellulose in Water
The water/cellulose system is unique in several ways. First of all it is so far the only
one for which we have observed that the vapor pressures above homogeneous
mixtures depend on the particular manner in which the samples were prepared.
The results reported here [
57
] were obtained by means of thin cellulose films
(approximately 20 25
m
m thick) cast from cellulose solutions in the mixed solvent
LiCl
dimethylacetamide. After careful removal of the components of the mixed
solvent, these films were kept in a surplus of water at 80
C until the weight of the
swollen cellulose film no longer changed. The solvent was then removed stepwise
by vacuum treatment and the resulting samples were kept in the measuring cell of
the HS GC until the vapor pressure no longer changed, which was typically after
1 day. The experimental data are highly reproducible but not identical with the
results of measurements (of equally reproducibility; not yet published) with cellu-
lose films that were cast from a different solvent. From these findings, one is forced
to conclude that at least one set of data does not refer to the macroscopic equilib-
rium of the system. It looks as if the final arrangement of the polymer chains after
total removal of the solvent (e.g., with respect to the degree of crystallinity) could
depend on the chemical nature of the solvent employed for film preparation. Under
this assumption, and in view of the high viscosity of swollen cellulose, one can then
speculate that the molecular environment established upon the removal of a partic-
ular solvent is more or less preserved in the swollen state and permits only the
establishment of local equilibria.
Figure
17
shows the results for a cellulose sample with 2940 segments (defined
by the molecular volume of water) prepared from a solution in LiCl plus dimethy-
lacetamide [
57
]. The most striking feature is the enormously large range that the
Flory Huggins interaction parameter spans as a function of composition. It falls
from w
o
¼
þ
6 (for worse than theta conditions, the typical w
o
values are in the order of
0.6) to a minimum of approximately
3.6 (much less than the lowest values
observed so far) for
'
values around 0.6, and increases again up to
1.7 in the
limit of the pure polymer.
6
Fig. 17 Composition
dependence of the interaction
parameter for water/cellulose
(Solucell 500) at 80
C,
obtained from vapor pressure
measurements [
57
]. The two
phase area is
shaded
. The
curve
was calculated by
means of (
26
) and the
following system specific
parameters: a
4
2
Solucell 500
0
-2
- 4
56.8,
0.0
0.2
0.4
j
Cellulose
0.6
0.8
1.0
n
0.56, z
37.9, and
l
1.34
