3.4 Structure of PVA Cryogels at Different Length Scales
The three-phase model adopted to describe the fraction of crystalline PVA in
freeze–thaw hydrogels from X-ray diffraction analysis is in agreement with numerous studies.
For instance, in 1986, Yokoyama et al. investigated the morphology and structure of PVA freeze–thaw hydrogels (5–15 wt% of PVA), using a variety of
techniques including X-ray diffraction, scanning electronic microscopy (SEM),
and light optical microscopy [49]. They showed, by SEM analysis, that these gels
have a porous structure, where the pore size increases from approximately 1 to
10 μm as the polymer concentration decreases from 15 to 5 wt%. They proposed a
three-phase model to describe the porous structure of these gels (Fig. 11). The
model consists of a water phase with a very low PVA concentration, an amorphous
phase in which each PVA chain is swollen by water, and a PVA crystalline phase
that partially prevents the motions of the amorphous chains. According to this
model, the pores are mainly occupied by a polymer-poor phase that forms an
interconnected continuous network, meandering through polymer-rich regions.
The ice crystals are accommodated in the polymer-poor phase during the freezing
step. The polymer-rich regions, in turn, are interconnected and build up the 3D
network scaffold of the gels delimiting the pores. These regions consist of a swollen
amorphous phase and PVA crystallites where the amorphous portions of chains
connect the crystalline domains acting as junctions. Formation of the crystalline
crosslinks ensures high dimensional stability and gives elastic properties to these
gels.
The porous structure of PVA hydrogels is already imprinted in the first freeze–
thaw cycle. This was shown by Fergg et al., using confocal laser scanning microscopy (CLSM) in the fluorescent mode [59], in the case of PVA hydrogels obtained
by imposing a single freeze–thaw cycle (À15
C for 24 h, then room temperature
for 3 h). The advantage of using CLSM is that water does not need to be removed
from hydrogels prior to examination. In the case of SEM analysis, dehydration
procedures are always required before performing the observations, with the consequent drawback that the native morphology of gels might be significantly altered.
Selected CLSM micrographs (extracted from [59]) of PVA hydrogels are shown in
Fig. 12.
The pore or mesh size in these cryogels increases from %2 to 7 μm with
decreasing PVA concentration. The CLSM analysis also indicates a tight interconnection of the pores all over the macroscopic samples, a uniform size of pores in the
bulk, and no preferential structural orientation [59].
The complex architecture of PVA hydrogels, which includes a polymer-poor
phase filling the pores and interconnected polymer-rich regions building the 3D
network, made up by crystalline crosslinks connected by PVA chains belonging to
the swollen amorphous regions (Fig. 11), has also been confirmed by small angle
neutron scattering (SANS) measurements. Typical SANS profiles from as-formed
PVA hydrogels obtained by subjecting a deuterated water solution of 11 wt% PVA
Kinetic Analysis of Cryotropic Gelation of Poly(Vinyl Alcohol)/Water. . .
179
The three-phase model adopted to describe the fraction of crystalline PVA in
freeze–thaw hydrogels from X-ray diffraction analysis is in agreement with numerous studies.
For instance, in 1986, Yokoyama et al. investigated the morphology and structure of PVA freeze–thaw hydrogels (5–15 wt% of PVA), using a variety of
techniques including X-ray diffraction, scanning electronic microscopy (SEM),
and light optical microscopy [49]. They showed, by SEM analysis, that these gels
have a porous structure, where the pore size increases from approximately 1 to
10 μm as the polymer concentration decreases from 15 to 5 wt%. They proposed a
three-phase model to describe the porous structure of these gels (Fig. 11). The
model consists of a water phase with a very low PVA concentration, an amorphous
phase in which each PVA chain is swollen by water, and a PVA crystalline phase
that partially prevents the motions of the amorphous chains. According to this
model, the pores are mainly occupied by a polymer-poor phase that forms an
interconnected continuous network, meandering through polymer-rich regions.
The ice crystals are accommodated in the polymer-poor phase during the freezing
step. The polymer-rich regions, in turn, are interconnected and build up the 3D
network scaffold of the gels delimiting the pores. These regions consist of a swollen
amorphous phase and PVA crystallites where the amorphous portions of chains
connect the crystalline domains acting as junctions. Formation of the crystalline
crosslinks ensures high dimensional stability and gives elastic properties to these
gels.
The porous structure of PVA hydrogels is already imprinted in the first freeze–
thaw cycle. This was shown by Fergg et al., using confocal laser scanning microscopy (CLSM) in the fluorescent mode [59], in the case of PVA hydrogels obtained
by imposing a single freeze–thaw cycle (À15
C for 24 h, then room temperature
for 3 h). The advantage of using CLSM is that water does not need to be removed
from hydrogels prior to examination. In the case of SEM analysis, dehydration
procedures are always required before performing the observations, with the consequent drawback that the native morphology of gels might be significantly altered.
Selected CLSM micrographs (extracted from [59]) of PVA hydrogels are shown in
Fig. 12.
The pore or mesh size in these cryogels increases from %2 to 7 μm with
decreasing PVA concentration. The CLSM analysis also indicates a tight interconnection of the pores all over the macroscopic samples, a uniform size of pores in the
bulk, and no preferential structural orientation [59].
The complex architecture of PVA hydrogels, which includes a polymer-poor
phase filling the pores and interconnected polymer-rich regions building the 3D
network, made up by crystalline crosslinks connected by PVA chains belonging to
the swollen amorphous regions (Fig. 11), has also been confirmed by small angle
neutron scattering (SANS) measurements. Typical SANS profiles from as-formed
PVA hydrogels obtained by subjecting a deuterated water solution of 11 wt% PVA
Kinetic Analysis of Cryotropic Gelation of Poly(Vinyl Alcohol)/Water. . .
179