of the swelling behavior of cryogel matrices in different solvents can be an indicator
of the affinity of the capillary system of cryogels with respect to the tested liquids.
The apparent decrease in CCG is also characteristic during the formation of
polymerization-type cryogels. This phenomenon was first observed upon comparison of the critical concentrations of vinyl and divinyl comonomers, namely
acrylamide and N,N
0 -methylene(bis)acrylamide, for the formation of poly(acrylamide) gels and cryogels [7, 8, 19–23]. At room temperature, the CCG for this
gelation system lies in the vicinity of 2 wt% and also varies depending on the ratio
of vinyl and divinyl components [137–139]. However, when the polymerization
occurs in a moderately frozen medium, wide-pore sponge-like poly(acrylamide)
cryogels can be prepared using at least half the concentration of the comonomers [7,
19]. The physicochemical features of such gelation systems are considered in more
detail in [47]. Here, it is reasonable to emphasize only that the effect of an apparent
decrease in CCG is common for the formation of any type of chemically
crosslinked cryogels, and this effect is mainly determined by the increase of
precursor concentration in the volume of UFLMP as the solvent freezes out.
The question is whether the same decrease in CCG is also inherent in the
formation of physical cryogels. The answer in most cases is obviously, yes. For
instance, the gelation process of aqueous solutions of locust bean gum was investigated both at positive and negative temperatures. Locust bean gum is a polygalactomannan gum consisting of poly(1,4-β-D-mannose) main chain and 1,6-α-Dgalactose pendant chains with a mannose to galactose ratio of 1:4 [140]. Solutions
(1–4 wt%) of locust bean gum remain fluid during prolonged storage at positive
temperatures and they transform into very weak gels only after 2–3 months
[141]. However, cryogenic treatment of the same feed systems produces cryogels
within a short period of time [100, 102, 115, 142–144]. Thus, the effect of CCG
decrease was clearly demonstrated for such cryogels. They are thermoreversible,
i.e., the cryogels can be melted by heating and they form again after a new freeze–
thaw cycle. The morphology of the cryogels based on locust bean gum is heterogeneous; they resemble soft sponges as long as the initial polysaccharide concentration is lower than 2.0–2.5 wt%. Such sponge-like cryogels exclude free liquid
under a slight compression. Although cryogenic treatment of more concentrated
locust bean gum solutions also yields gel materials, they do not exclude free liquid
under a moderate pressing. However, the latter gels are similar to the physical PVA
cryogels in that they are also macroporous because, during cryotropic gel formation, ice polycrystals act as porogens and macropores of various sizes and architecture remain in the cryogel body after thawing.
There are also other examples of the manifestation of a CCG decrease during the
formation of noncovalent cryogels. The precursors (i.e., polysaccharides, proteins,
synthetic polymers) together with the conditions of gel formation processes and the
appearance of the final material are listed in Table 2. These data demonstrate the
universal character of the CCG decrease in both aqueous and organic media during
cryotropic gelation as compared to conventional gelation. The best studied among
the processes of the formation of noncovalent cryogels is the cryotropic gelation of
PVA solutions. The reason for this is twofold: First, the final “products”, i.e., the
Basic Principles of Cryotropic Gelation
71
of the affinity of the capillary system of cryogels with respect to the tested liquids.
The apparent decrease in CCG is also characteristic during the formation of
polymerization-type cryogels. This phenomenon was first observed upon comparison of the critical concentrations of vinyl and divinyl comonomers, namely
acrylamide and N,N
0 -methylene(bis)acrylamide, for the formation of poly(acrylamide) gels and cryogels [7, 8, 19–23]. At room temperature, the CCG for this
gelation system lies in the vicinity of 2 wt% and also varies depending on the ratio
of vinyl and divinyl components [137–139]. However, when the polymerization
occurs in a moderately frozen medium, wide-pore sponge-like poly(acrylamide)
cryogels can be prepared using at least half the concentration of the comonomers [7,
19]. The physicochemical features of such gelation systems are considered in more
detail in [47]. Here, it is reasonable to emphasize only that the effect of an apparent
decrease in CCG is common for the formation of any type of chemically
crosslinked cryogels, and this effect is mainly determined by the increase of
precursor concentration in the volume of UFLMP as the solvent freezes out.
The question is whether the same decrease in CCG is also inherent in the
formation of physical cryogels. The answer in most cases is obviously, yes. For
instance, the gelation process of aqueous solutions of locust bean gum was investigated both at positive and negative temperatures. Locust bean gum is a polygalactomannan gum consisting of poly(1,4-β-D-mannose) main chain and 1,6-α-Dgalactose pendant chains with a mannose to galactose ratio of 1:4 [140]. Solutions
(1–4 wt%) of locust bean gum remain fluid during prolonged storage at positive
temperatures and they transform into very weak gels only after 2–3 months
[141]. However, cryogenic treatment of the same feed systems produces cryogels
within a short period of time [100, 102, 115, 142–144]. Thus, the effect of CCG
decrease was clearly demonstrated for such cryogels. They are thermoreversible,
i.e., the cryogels can be melted by heating and they form again after a new freeze–
thaw cycle. The morphology of the cryogels based on locust bean gum is heterogeneous; they resemble soft sponges as long as the initial polysaccharide concentration is lower than 2.0–2.5 wt%. Such sponge-like cryogels exclude free liquid
under a slight compression. Although cryogenic treatment of more concentrated
locust bean gum solutions also yields gel materials, they do not exclude free liquid
under a moderate pressing. However, the latter gels are similar to the physical PVA
cryogels in that they are also macroporous because, during cryotropic gel formation, ice polycrystals act as porogens and macropores of various sizes and architecture remain in the cryogel body after thawing.
There are also other examples of the manifestation of a CCG decrease during the
formation of noncovalent cryogels. The precursors (i.e., polysaccharides, proteins,
synthetic polymers) together with the conditions of gel formation processes and the
appearance of the final material are listed in Table 2. These data demonstrate the
universal character of the CCG decrease in both aqueous and organic media during
cryotropic gelation as compared to conventional gelation. The best studied among
the processes of the formation of noncovalent cryogels is the cryotropic gelation of
PVA solutions. The reason for this is twofold: First, the final “products”, i.e., the
Basic Principles of Cryotropic Gelation
71