bonds, are promoted to yield small crystalline nuclei. Crystallization can proceed
further as the solution remains at low temperatures for longer times. The crystallites
serve as crosslinks to hold the 3D network structure. By addition of organic solvent,
the PVA solution is prevented from freezing, even at temperatures below 0
C. This
permits PVA crystallization to proceed without a significant volume expansion.
Therefore, the resulting gel contains pores smaller than 3 μm, giving rise to a
transparent gel [1, 64].
In the case of PVA gels prepared by freezing and thawing techniques in the
presence of ethylene glycol, Berghmans et al. suggested a two-step mechanism [66]
involving a liquid–liquid phase separation in the first step (evidenced from the fact
that the gel becomes opaque) followed by PVA crystallization in the polymer-rich
phase in the second step.
Extensive studies have also been performed on PVA gels using mixtures of
dimethyl sulfoxide and water as solvent (60/40 v/v) [67–73]. It was shown that gels
prepared by freezing below –20
C are transparent. Their properties depend on the
ratio of dimethyl sulfoxide to water. It was also noted that gelation occurs below –
20
C without phase separation. However, above this temperature, LL phase
separation plays an important role for the gelation process, giving rise to
opaque gels.
Kanaya et al. used wide and small angle neutron scattering and light scattering
experiments to provide extensive information concerning the structural organization of PVA gels formed in mixtures of DMSO and water at various length scales
[67–73]. The presence of small PVA crystallites acting as crosslinks for the 3D
network of these gels was confirmed. The crystallites have a sharp surface,
presenting an average size of %7 nm and average distance of about 15–20 nm.
Trieu and Qutubuddin also investigated the structure of freeze–thaw PVA gels
obtained from aqueous DMSO solutions [74, 75]. The authors characterized the
gels by using freeze-etching and critical point drying SEM techniques. A higher
porosity was observed at the surface than in the bulk of the gel.
It is worth noting that, regardless of the kind of solvent (pure water and/or water
mixtures with other solvents), features common to PVA gels obtained by cryotropic
treatments are the presence of a complex porous architecture that includes
macropores filled with a polymer-poor phase and meandering polymer-rich regions.
The tight interconnection of these pores allows for almost unhindered diffusion of
large and small molecules [1, 4–6, 62, 63]. The polymer-rich regions are
interconnected and constitute a biphasic 3D network consisting of a swollen
amorphous PVA phase and PVA crystallites (Fig. 14). In all cases, although
hydrogen bond interactions play a key role in creating the physical junctions of
the network, the high dimensional stability and mechanical strength achieved by
these gels is due to the presence of crystalline crosslinks. In particular, the size and
amount of PVA crystalline aggregates in freeze–thaw PVA hydrogels play important roles in gel performance because the dimensional stability, toughness, strength
to external stresses, and thermal stability of PVA cryogels are critically dependent
on these parameters.
184
C. De Rosa et al.
further as the solution remains at low temperatures for longer times. The crystallites
serve as crosslinks to hold the 3D network structure. By addition of organic solvent,
the PVA solution is prevented from freezing, even at temperatures below 0
C. This
permits PVA crystallization to proceed without a significant volume expansion.
Therefore, the resulting gel contains pores smaller than 3 μm, giving rise to a
transparent gel [1, 64].
In the case of PVA gels prepared by freezing and thawing techniques in the
presence of ethylene glycol, Berghmans et al. suggested a two-step mechanism [66]
involving a liquid–liquid phase separation in the first step (evidenced from the fact
that the gel becomes opaque) followed by PVA crystallization in the polymer-rich
phase in the second step.
Extensive studies have also been performed on PVA gels using mixtures of
dimethyl sulfoxide and water as solvent (60/40 v/v) [67–73]. It was shown that gels
prepared by freezing below –20
C are transparent. Their properties depend on the
ratio of dimethyl sulfoxide to water. It was also noted that gelation occurs below –
20
C without phase separation. However, above this temperature, LL phase
separation plays an important role for the gelation process, giving rise to
opaque gels.
Kanaya et al. used wide and small angle neutron scattering and light scattering
experiments to provide extensive information concerning the structural organization of PVA gels formed in mixtures of DMSO and water at various length scales
[67–73]. The presence of small PVA crystallites acting as crosslinks for the 3D
network of these gels was confirmed. The crystallites have a sharp surface,
presenting an average size of %7 nm and average distance of about 15–20 nm.
Trieu and Qutubuddin also investigated the structure of freeze–thaw PVA gels
obtained from aqueous DMSO solutions [74, 75]. The authors characterized the
gels by using freeze-etching and critical point drying SEM techniques. A higher
porosity was observed at the surface than in the bulk of the gel.
It is worth noting that, regardless of the kind of solvent (pure water and/or water
mixtures with other solvents), features common to PVA gels obtained by cryotropic
treatments are the presence of a complex porous architecture that includes
macropores filled with a polymer-poor phase and meandering polymer-rich regions.
The tight interconnection of these pores allows for almost unhindered diffusion of
large and small molecules [1, 4–6, 62, 63]. The polymer-rich regions are
interconnected and constitute a biphasic 3D network consisting of a swollen
amorphous PVA phase and PVA crystallites (Fig. 14). In all cases, although
hydrogen bond interactions play a key role in creating the physical junctions of
the network, the high dimensional stability and mechanical strength achieved by
these gels is due to the presence of crystalline crosslinks. In particular, the size and
amount of PVA crystalline aggregates in freeze–thaw PVA hydrogels play important roles in gel performance because the dimensional stability, toughness, strength
to external stresses, and thermal stability of PVA cryogels are critically dependent
on these parameters.
184
C. De Rosa et al.