corresponding reactions prior to freezing of the feed, looks more promising. As a
rule, for fast-reacting systems this goal is reached simply by chilling the initial feed.
Some examples of the procedures employed for the preparation of polymerizationtype cryogels and cryogels via chemical crosslinking of the polymeric precursors
can be found in [19–25] and [10, 26–32], respectively.
The above considerations are mainly related to the preparation of covalent
cryogels, where chilling the initial solution prior to the addition of either initiator
or crosslinking agent does not lead to the gelation per se. However, when dealing
with the preparation of physical cryogels, especially with systems undergoing a fast
sol-to-gel transition at positive temperatures (such as aqueous agarose solutions),
the situation is complicated. In such physically gelling systems one must decrease
the self-gelation rate of the precursors to shift the gel-point to longer reaction times
so that the gel will form within the volume of the unfrozen liquid microphase. As
discussed in Sect. 2.4 of [1], the preferable way is to use specific solutes capable of
slowing down gelation, e.g., alkaline additives for the preparation of wide-pore
agarose cryogels [33, 34].
Moreover, the feed compositions capable of producing ionically crosslinked
cryogels are even more sophisticated due to the very high rates of ionic reactions.
Besides, such reactions are only slightly sensitive to the temperature. Therefore,
cooling the initial feed containing dissolved polyelectolytes and necessary
crosslinking counterions cannot sufficiently decelerate ionotropic gel formation
prior to freezing of the system. Possible ways for bypassing such difficulties are
also discussed in Sect. 2.5 of [1]. In short, in terms of the composition of the
precursor solution, only two variants are relevant. The first method is to use a
crosslinking agent with a negative temperature coefficient of solubility, e.g., solid
salts exhibiting increased water solubility as the temperature is decreased
[7, 8]. The second method, namely the so-called internal gelation approach, is to
induce gelation via auxiliary agents capable of gradual solubilization of the
dispersed salt particles containing crosslinking counterions [35, 36]. Whereas the
former variant has already been realized in the preparation of Ca-alginate-based
cryogels [37, 38], no cases of the implementation of the latter variant are known so
far to the authors of this chapter.
2.2 Freezing of the Feed
Freezing of the initial precursor-containing system is the crucial step for both the
subsequent cryotropic gel formation per se and the properties of the resulting
cryogels. This is due to several simultaneous processes that are launched during
the transformation of the feed liquid into a frost-bound solid. These processes
strongly alter the concentration and thermal mobility of the dissolved/dispersed
gelation components, the volume of the unfrozen liquid fraction, its viscosity, and
its polarity. Moreover, depending on the freezing conditions, they also influence the
size and shape of the growing solvent polycrystals, thus biasing the macroporous
54
V.I. Lozinsky and O. Okay
rule, for fast-reacting systems this goal is reached simply by chilling the initial feed.
Some examples of the procedures employed for the preparation of polymerizationtype cryogels and cryogels via chemical crosslinking of the polymeric precursors
can be found in [19–25] and [10, 26–32], respectively.
The above considerations are mainly related to the preparation of covalent
cryogels, where chilling the initial solution prior to the addition of either initiator
or crosslinking agent does not lead to the gelation per se. However, when dealing
with the preparation of physical cryogels, especially with systems undergoing a fast
sol-to-gel transition at positive temperatures (such as aqueous agarose solutions),
the situation is complicated. In such physically gelling systems one must decrease
the self-gelation rate of the precursors to shift the gel-point to longer reaction times
so that the gel will form within the volume of the unfrozen liquid microphase. As
discussed in Sect. 2.4 of [1], the preferable way is to use specific solutes capable of
slowing down gelation, e.g., alkaline additives for the preparation of wide-pore
agarose cryogels [33, 34].
Moreover, the feed compositions capable of producing ionically crosslinked
cryogels are even more sophisticated due to the very high rates of ionic reactions.
Besides, such reactions are only slightly sensitive to the temperature. Therefore,
cooling the initial feed containing dissolved polyelectolytes and necessary
crosslinking counterions cannot sufficiently decelerate ionotropic gel formation
prior to freezing of the system. Possible ways for bypassing such difficulties are
also discussed in Sect. 2.5 of [1]. In short, in terms of the composition of the
precursor solution, only two variants are relevant. The first method is to use a
crosslinking agent with a negative temperature coefficient of solubility, e.g., solid
salts exhibiting increased water solubility as the temperature is decreased
[7, 8]. The second method, namely the so-called internal gelation approach, is to
induce gelation via auxiliary agents capable of gradual solubilization of the
dispersed salt particles containing crosslinking counterions [35, 36]. Whereas the
former variant has already been realized in the preparation of Ca-alginate-based
cryogels [37, 38], no cases of the implementation of the latter variant are known so
far to the authors of this chapter.
2.2 Freezing of the Feed
Freezing of the initial precursor-containing system is the crucial step for both the
subsequent cryotropic gel formation per se and the properties of the resulting
cryogels. This is due to several simultaneous processes that are launched during
the transformation of the feed liquid into a frost-bound solid. These processes
strongly alter the concentration and thermal mobility of the dissolved/dispersed
gelation components, the volume of the unfrozen liquid fraction, its viscosity, and
its polarity. Moreover, depending on the freezing conditions, they also influence the
size and shape of the growing solvent polycrystals, thus biasing the macroporous
54
V.I. Lozinsky and O. Okay