initial solution causes a loss of solubility and therefore precipitation/coagulation of
the polymer, no crosslinked product can be obtained after thawing of the system,
especially when the concentration of the crosslinking agent is relatively low. In
addition, a rather important parameter is the molecular weight of the polymeric
precursor, which determines both the initial viscosity of the feed solution prior to
freezing and the viscosity in the reaction zone, i.e., within the volume of the
unfrozen liquid microphase. Due to the generally high molecular weight of the
polymeric precursors, very high viscosity of the reaction zone significantly reduces
the segmental and translational mobilities of the polymer chains, thus preventing
the occurrence of gelation reactions. Therefore, some preliminary experiments are
often required to select the optimum molecular-weight characteristics of the
corresponding polymeric precursor [1, 175, 176].
Cryogels prepared by chemical crosslinking of proteins were the first cases that
exploited a scheme for producing immobilized biocatalysts. These were microbial
or plant cells entrapped in a spongy carrier composed of serum albumin or gelatine
cured with glutaraldehyde or formaldehyde, respectively [177–181]. In these early
works, the factors influencing the gelation process and the properties of the final
cryogels were not studied. Specific features inherent in this kind of cryotropic
gelation and its mechanisms were basically established later using polymers chemically simpler than proteins, namely homopolymers or plain AB-copolymers, where
the process of interest is not sophisticated by the numerous secondary interactions.
Such a “modeling” approach found a series of significant effects that turned out to
be characteristic for the formation of various cryogels. For instance, it was shown
that cryogels in both aqueous [50, 182] and organic [4] media can be prepared at
considerably lower initial concentration of precursors as compared to their gelation
at positive temperatures. Thus, the effect of an apparent decrease in the critical
concentration of gelation is inherent in the gel formation processes occurring in the
non-deeply frozen reaction systems [1, 91]. The reason for such an effect is the
cryo-concentrating phenomenon, which makes the concentration of the gelling
agents considerably higher than that in the initial liquid feed. The same phenomenon generally causes the acceleration of cryochemical reactions in moderately
frozen solutions over a certain range of negative temperatures [2, 183, 184] and is
also observed during cryotropic gelation through covalent crosslinking of macromolecular precursors with suitable crosslinking agents, (see, e.g., [1, 91]). For
instance, the oxidation of SH groups in thiol-containing poly(acrylamide) induced
by water-dissolved air oxygen and leading to the formation of disulfide-crosslinked
3D polymeric network was at least five times faster in a frozen system at À15
C
than in a solution at +15
C. Moreover, the gel-point was reached about 1 h after
freezing of the feed solution in the former case, whereas in the latter case the time
was about 1 day [52].
Examples of the majority of the reported covalent cryogels prepared from
macromolecular precursors are given in Table 2. These data show that such
cryogels based on natural and synthetic polymers can be synthesized by chemical
crosslinking or by irradiation techniques in frozen aqueous, organic, or mixed
water–organic media. Thus, the chemical structure, physico-chemical properties,
16
V.I. Lozinsky
the polymer, no crosslinked product can be obtained after thawing of the system,
especially when the concentration of the crosslinking agent is relatively low. In
addition, a rather important parameter is the molecular weight of the polymeric
precursor, which determines both the initial viscosity of the feed solution prior to
freezing and the viscosity in the reaction zone, i.e., within the volume of the
unfrozen liquid microphase. Due to the generally high molecular weight of the
polymeric precursors, very high viscosity of the reaction zone significantly reduces
the segmental and translational mobilities of the polymer chains, thus preventing
the occurrence of gelation reactions. Therefore, some preliminary experiments are
often required to select the optimum molecular-weight characteristics of the
corresponding polymeric precursor [1, 175, 176].
Cryogels prepared by chemical crosslinking of proteins were the first cases that
exploited a scheme for producing immobilized biocatalysts. These were microbial
or plant cells entrapped in a spongy carrier composed of serum albumin or gelatine
cured with glutaraldehyde or formaldehyde, respectively [177–181]. In these early
works, the factors influencing the gelation process and the properties of the final
cryogels were not studied. Specific features inherent in this kind of cryotropic
gelation and its mechanisms were basically established later using polymers chemically simpler than proteins, namely homopolymers or plain AB-copolymers, where
the process of interest is not sophisticated by the numerous secondary interactions.
Such a “modeling” approach found a series of significant effects that turned out to
be characteristic for the formation of various cryogels. For instance, it was shown
that cryogels in both aqueous [50, 182] and organic [4] media can be prepared at
considerably lower initial concentration of precursors as compared to their gelation
at positive temperatures. Thus, the effect of an apparent decrease in the critical
concentration of gelation is inherent in the gel formation processes occurring in the
non-deeply frozen reaction systems [1, 91]. The reason for such an effect is the
cryo-concentrating phenomenon, which makes the concentration of the gelling
agents considerably higher than that in the initial liquid feed. The same phenomenon generally causes the acceleration of cryochemical reactions in moderately
frozen solutions over a certain range of negative temperatures [2, 183, 184] and is
also observed during cryotropic gelation through covalent crosslinking of macromolecular precursors with suitable crosslinking agents, (see, e.g., [1, 91]). For
instance, the oxidation of SH groups in thiol-containing poly(acrylamide) induced
by water-dissolved air oxygen and leading to the formation of disulfide-crosslinked
3D polymeric network was at least five times faster in a frozen system at À15
C
than in a solution at +15
C. Moreover, the gel-point was reached about 1 h after
freezing of the feed solution in the former case, whereas in the latter case the time
was about 1 day [52].
Examples of the majority of the reported covalent cryogels prepared from
macromolecular precursors are given in Table 2. These data show that such
cryogels based on natural and synthetic polymers can be synthesized by chemical
crosslinking or by irradiation techniques in frozen aqueous, organic, or mixed
water–organic media. Thus, the chemical structure, physico-chemical properties,
16
V.I. Lozinsky