binding cooperativity rather than to an increase in the number of microcrystallites.
The former is manifested in a substantial increase in the fusion enthalpy, whereas
the latter leads to a moderate increase in the fusion temperature of cryogels, i.e., to a
small change in the entropy factor. At a qualitative level, there is an analogy with
the crystallization of low molecular weight substances upon cooling of their hot
saturated solutions; when the system is cooled slower, less crystals are formed, but
their sizes are larger and the crystalline lattice becomes more perfect [116].
The regime of the thawing stage also influences markedly the macroporous
structure of such noncovalent cryogels. During sufficiently slow heating of frozen
samples, the gel formation occurs at subzero temperatures and, simultaneously, the
re-crystallization of the frozen solvent is intensified, which has an impact on the
porous morphology of the resulting cryogels. This influence is demonstrated by the
micrographs in Fig. 5, which show thin sections of PVA cryogels formed at
different heating rates. The cryogels were prepared from aqueous PVA solutions
frozen under identical thermal conditions but defrosted at heating rates of 0.3
(Fig. 5a), 0.03 (Fig. 5b), and 0.003
C/min (Fig. 5c). Because slowing of defrosting
of the frozen system results in an increase in the rigidity of the corresponding
cryogels (Fig. 4a), it can be suggested that such changes in their mechanical
parameters are caused not only by the strengthening of the gel phase of heterogeneous material, but also by a decrease in the inhomogeneity of its structure, where
the macropores act as defects. This was indeed observed. As seen in Fig. 5, the
slower the thawing of the frozen system, the less pronounced is the orientation of
the texture elements. Most likely, this effect is associated with the re-crystallization
of ice at slow heating of the frozen samples when the system has enough time to
balance the temperature fields, and the processes of mass transfer participating in
re-crystallization become equiprobable in all directions. In this regard, the texture
of the sample shown in Fig. 5b seems to be transient between the clearly unidirectional texture of the rapidly thawed sample (Fig. 5a) and the texture of a very slowly
defrosted sample pierced with intersecting pores (Fig. 5c). In addition, slowing
down of the heating rate at the thawing stage leads to a decrease in the total porosity
of PVA cryogels and in a slight diminution in the average cross-section of the
macropores (see caption to Fig. 5).
Another freeze–thaw procedure having a strong impact on the properties and
porous morphology of noncovalent cryogels such as PVA-based cryogels is the use
of multiple or iterative “freezing–frozen storage–defrosting” cycles. This variant of
cryogenic treatment results in the growth of the crystallinity degree of PVA
cryogels [117–120]. Such thermal cycling is able to increase considerably the
strength of physical cryogels and their heat endurance [111, 121–133]. One of the
reasons for this effect is the fact that the gelation system passes across the aboveindicated subzero temperature zone several times, which is very favorable for such
cryotropic gel formation. Thus, the durations of the several traversals at the subzero
temperature range are virtually summed. Such a “temperature saw” also affects the
pore morphology of the resulting cryogels. This is illustrated by the micrographs in
Fig. 6, which show the effect of the number of cycles on the pore morphology of
PVA cryogels. The second freeze–thaw cycle results in a significant increase both
Basic Principles of Cryotropic Gelation
63
The former is manifested in a substantial increase in the fusion enthalpy, whereas
the latter leads to a moderate increase in the fusion temperature of cryogels, i.e., to a
small change in the entropy factor. At a qualitative level, there is an analogy with
the crystallization of low molecular weight substances upon cooling of their hot
saturated solutions; when the system is cooled slower, less crystals are formed, but
their sizes are larger and the crystalline lattice becomes more perfect [116].
The regime of the thawing stage also influences markedly the macroporous
structure of such noncovalent cryogels. During sufficiently slow heating of frozen
samples, the gel formation occurs at subzero temperatures and, simultaneously, the
re-crystallization of the frozen solvent is intensified, which has an impact on the
porous morphology of the resulting cryogels. This influence is demonstrated by the
micrographs in Fig. 5, which show thin sections of PVA cryogels formed at
different heating rates. The cryogels were prepared from aqueous PVA solutions
frozen under identical thermal conditions but defrosted at heating rates of 0.3
(Fig. 5a), 0.03 (Fig. 5b), and 0.003
C/min (Fig. 5c). Because slowing of defrosting
of the frozen system results in an increase in the rigidity of the corresponding
cryogels (Fig. 4a), it can be suggested that such changes in their mechanical
parameters are caused not only by the strengthening of the gel phase of heterogeneous material, but also by a decrease in the inhomogeneity of its structure, where
the macropores act as defects. This was indeed observed. As seen in Fig. 5, the
slower the thawing of the frozen system, the less pronounced is the orientation of
the texture elements. Most likely, this effect is associated with the re-crystallization
of ice at slow heating of the frozen samples when the system has enough time to
balance the temperature fields, and the processes of mass transfer participating in
re-crystallization become equiprobable in all directions. In this regard, the texture
of the sample shown in Fig. 5b seems to be transient between the clearly unidirectional texture of the rapidly thawed sample (Fig. 5a) and the texture of a very slowly
defrosted sample pierced with intersecting pores (Fig. 5c). In addition, slowing
down of the heating rate at the thawing stage leads to a decrease in the total porosity
of PVA cryogels and in a slight diminution in the average cross-section of the
macropores (see caption to Fig. 5).
Another freeze–thaw procedure having a strong impact on the properties and
porous morphology of noncovalent cryogels such as PVA-based cryogels is the use
of multiple or iterative “freezing–frozen storage–defrosting” cycles. This variant of
cryogenic treatment results in the growth of the crystallinity degree of PVA
cryogels [117–120]. Such thermal cycling is able to increase considerably the
strength of physical cryogels and their heat endurance [111, 121–133]. One of the
reasons for this effect is the fact that the gelation system passes across the aboveindicated subzero temperature zone several times, which is very favorable for such
cryotropic gel formation. Thus, the durations of the several traversals at the subzero
temperature range are virtually summed. Such a “temperature saw” also affects the
pore morphology of the resulting cryogels. This is illustrated by the micrographs in
Fig. 6, which show the effect of the number of cycles on the pore morphology of
PVA cryogels. The second freeze–thaw cycle results in a significant increase both
Basic Principles of Cryotropic Gelation
63