The
macro- and microstructural evolution of water swollen and ethanol
swollen regenerated cellulose gel beads have been determined during
drying by optical microscopy combined with analytical balance measurements,
small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering
(WAXS). Two characteristic length scales, which are related to the
molecular dimension of cellulose monomer and elongated aggregates
of these monomers, could be identified for both types of beads by
SAXS. For ethanol swollen beads, only small changes to the structures
were detected in both the SAXS and WAXS measurements during the entire
drying process. However, the drying of cellulose from water follows
a more complex process when compared to drying from ethanol. As water
swollen beads dried, they went through a structural transition where
elongated structures changed to spherical structures and their dimensions
increased from 3.6 to 13.5 nm. After complete drying from water, the
nanostructures were characterized as a combination of rodlike structures
with an approximate size of cellulose monomers (0.5 nm), and spherical
aggregates (13.5 nm) without any indication of heterogeneous meso-
or microporosity. In addition, WAXS shows that cellulose II hydrate
structure appears and transforms to cellulose II during water evaporation,
however it is not possible to determine the degree of crystallinity
of the beads from the present measurements. This work sheds lights
on the structural changes that occur within regenerated cellulose
materials during drying and can aid in the design and application
of cellulosic materials as fibers, adhesives, and membranes.
The drying behavior
of regenerated cellulose gel beads swollen
with different nonsolvents (
e.g
., water, ethanol,
water/ethanol mixtures) is studied
in situ
on the
macroscopic scale with an optical microscope as well as on nanoscale
using small-angle/wide-angle X-ray scattering (SAXS/WAXS) techniques.
Depending on the cellulose concentration, the structural evolution
of beads during drying follows one of three distinct regimes. First,
when the cellulose concentration is lower than 0.5 wt %, the drying
process comprises three steps and, regardless of the water/ethanol
mixture composition, a sharp structural transition corresponding to
the formation of a cellulose II crystalline structure is observed.
Second, when the cellulose concentration is higher than 5.0 wt %,
a two-step drying process is observed and no structural transition
occurs for any of the beads studied. Third, when the cellulose concentration
is between 0.5 and 5.0 wt %, the drying process is dependent on the
nonsolvent composition. A three-step drying process takes place for
beads swollen with water/ethanol mixtures with a water content higher
than 20%, while a two-step drying process is observed when the water
content is lower than 20%. To describe the drying behavior governed
by the cellulose concentration and nonsolvent composition, a simplified
phase diagram is proposed.
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