and, thus, formation of PVA microcrystallites, the nodes in the final cryogel. As a
result, the temperature dependences of the respective parameters pass through the
maxima, a common trend for the formation of physical cryogels in general.
Summing up the discussion on the influence of favorable and detrimental factors
on cryotropic gel formation, one can see that their competition is the main reason
for the bell-like temperature dependence of the gelation efficiency. This effect is
manifested both for chemically crosslinked cryogels and noncovalent gels, when
either monomeric or polymeric precursors are used, and both in aqueous and
organic media. So, such bell-shaped dependences are a characteristic feature of
cryotropic gel formation. However, the exact position of the corresponding maxima
or minima on the temperature axis depends on the particular cryogelation system.
Therefore, the necessity of preliminary search for the “optimal” temperature conditions for freezing and for frozen storage is evident. Although such preliminary
studies can require time and effort, only this path will result in cryogels with the
best possible properties and structure.
3.4 Generation of Specific Porosity Peculiar to Cryogels
The scheme in Fig. 3 visually demonstrates formation of macroporosity in cryogels
due to the presence of polycrystals of frozen solvent acting as porogens. Depending
on the nature of the cryogel precursors, their initial concentrations, the type of the
solvent used, and the cryogenic processing conditions, it is possible to obtain
cryogels with pores having a cross-section from the submicron range up to several
micrometers (Figs. 5 and 6), or supermacroporous (wide pore) gel matrices similar
to the sponge-like chitosan-based cryogels (Fig. 1) where the cross-section of large
pores ranges from tens to hundreds of micrometers. Certainly, some “intermediate”
variants are also possible. The main characteristics of the porosity of cryogels have
been described in several review papers [8, 9, 107, 111, 114, 130, 148–150, 152,
156–164], and are also considered in [47]. However, certain aspects of
macroporosity generation in the course of freeze–thaw gelation will be discussed
in this section.
We will first consider the reason why the size of pores in the cryogels varies by
two orders of magnitude depending on the synthesis conditions. This is mainly due
to the different size of the pore template (i.e., the frozen solvent polycrystals),
which depends on the amount of freezable solvent under the freezing conditions
employed. For instance, when the initial concentration of the monomeric precursors
is not too high, i.e., less than 10 wt%, the fraction of free solvent freezable at
moderate negative temperatures is large, and the viscosity of the feed solution is not
so high as to markedly inhibit crystal growth. This effect leads to the formation of
large polycrystals and, hence, large pores. On the other hand, when the initial
concentration of the polymeric precursors is high, a greater part of the solvent is
bound to the dissolved macromolecules so that the volume of the freezable liquid is
considerably less than in the previous case, leading to a high solution viscosity that
84
V.I. Lozinsky and O. Okay
result, the temperature dependences of the respective parameters pass through the
maxima, a common trend for the formation of physical cryogels in general.
Summing up the discussion on the influence of favorable and detrimental factors
on cryotropic gel formation, one can see that their competition is the main reason
for the bell-like temperature dependence of the gelation efficiency. This effect is
manifested both for chemically crosslinked cryogels and noncovalent gels, when
either monomeric or polymeric precursors are used, and both in aqueous and
organic media. So, such bell-shaped dependences are a characteristic feature of
cryotropic gel formation. However, the exact position of the corresponding maxima
or minima on the temperature axis depends on the particular cryogelation system.
Therefore, the necessity of preliminary search for the “optimal” temperature conditions for freezing and for frozen storage is evident. Although such preliminary
studies can require time and effort, only this path will result in cryogels with the
best possible properties and structure.
3.4 Generation of Specific Porosity Peculiar to Cryogels
The scheme in Fig. 3 visually demonstrates formation of macroporosity in cryogels
due to the presence of polycrystals of frozen solvent acting as porogens. Depending
on the nature of the cryogel precursors, their initial concentrations, the type of the
solvent used, and the cryogenic processing conditions, it is possible to obtain
cryogels with pores having a cross-section from the submicron range up to several
micrometers (Figs. 5 and 6), or supermacroporous (wide pore) gel matrices similar
to the sponge-like chitosan-based cryogels (Fig. 1) where the cross-section of large
pores ranges from tens to hundreds of micrometers. Certainly, some “intermediate”
variants are also possible. The main characteristics of the porosity of cryogels have
been described in several review papers [8, 9, 107, 111, 114, 130, 148–150, 152,
156–164], and are also considered in [47]. However, certain aspects of
macroporosity generation in the course of freeze–thaw gelation will be discussed
in this section.
We will first consider the reason why the size of pores in the cryogels varies by
two orders of magnitude depending on the synthesis conditions. This is mainly due
to the different size of the pore template (i.e., the frozen solvent polycrystals),
which depends on the amount of freezable solvent under the freezing conditions
employed. For instance, when the initial concentration of the monomeric precursors
is not too high, i.e., less than 10 wt%, the fraction of free solvent freezable at
moderate negative temperatures is large, and the viscosity of the feed solution is not
so high as to markedly inhibit crystal growth. This effect leads to the formation of
large polycrystals and, hence, large pores. On the other hand, when the initial
concentration of the polymeric precursors is high, a greater part of the solvent is
bound to the dissolved macromolecules so that the volume of the freezable liquid is
considerably less than in the previous case, leading to a high solution viscosity that
84
V.I. Lozinsky and O. Okay