be created from the solutions of polyelectrolytes by their crosslinking using counterions capable of forming low-dissociating ionic bonds between the charged
groups of such macromolecular precursors [7, 8]. In all these cases, the procedure
of cryogel preparation includes the following basic stages:
1. Preparation of the feed system
2. Freezing of the feed
3. Incubation of the system in a frozen state
4. Thawing of the frozen system
The conditions of each stage, including the properties of the particular precursors, affect more or less significantly the efficiency of cryotropic gel formation
and, thus, the properties and structure of the resulting cryogel materials. So, it
seems reasonable to discuss step-by-step the factors that are of importance for such
freeze–thaw gelling systems.
2.1 Preparation of the Feed System
At first sight, preparation of the initial feed containing the precursors of the future
cryogel is a rather simple and routine procedure. This is a necessary step for the
preparation of any gel material both at positive and negative temperatures. However, definite peculiarities of the cryotropic gel formation generally dictate certain
requirements for the preparation of the initial system. For instance, one should
minimize the conversion of the precursor to the gel before freezing the feed.
Otherwise, if the gel-point is attained prior to the start of solvent crystallization,
the destruction of the primary 3D network thus formed may occur by the creation of
a polycrystalline phase. Such a process is especially detrimental in quickly forming
chemotropic gels because their covalently linked nodes do not allow the polymeric
chains to be slid apart easily by the growing solvent crystals. These effects, in turn,
can even result in splitting of the weakest covalent bonds in the structure of the
spatial polymeric network, especially, as shown by Jellinek and Fox [18], when the
crystallization fronts are moving rapidly.
There are two ways to “outrun” gelation of the still-unfrozen feed system. The
first method is to freeze the feed as quickly as possible where the gelation reactions
have already begun. The other method is to reduce the initial rate of such reactions,
e.g., by chilling the feed to just above its freezing point before adding the initiator
(the case of polymerization-type cryogels) or the crosslinking agent (the case of
covalent cryogels formed via crosslinking of macromolecular precursors). The
former way, such as the flash-freezing technique using liquid nitrogen, has two
significant drawbacks. First, the initial solution may undergo a glass-transition
instead of the required solvent crystallization so that no cryo-concentration effect
can be generated. Second, when the solvent crystallization does occur, the size of
the very quickly formed crystals is small, and so is the cross-section of the pores in
the resulting cryogel. Therefore, the latter approach, i.e., decreasing the rate of the
Basic Principles of Cryotropic Gelation
53
groups of such macromolecular precursors [7, 8]. In all these cases, the procedure
of cryogel preparation includes the following basic stages:
1. Preparation of the feed system
2. Freezing of the feed
3. Incubation of the system in a frozen state
4. Thawing of the frozen system
The conditions of each stage, including the properties of the particular precursors, affect more or less significantly the efficiency of cryotropic gel formation
and, thus, the properties and structure of the resulting cryogel materials. So, it
seems reasonable to discuss step-by-step the factors that are of importance for such
freeze–thaw gelling systems.
2.1 Preparation of the Feed System
At first sight, preparation of the initial feed containing the precursors of the future
cryogel is a rather simple and routine procedure. This is a necessary step for the
preparation of any gel material both at positive and negative temperatures. However, definite peculiarities of the cryotropic gel formation generally dictate certain
requirements for the preparation of the initial system. For instance, one should
minimize the conversion of the precursor to the gel before freezing the feed.
Otherwise, if the gel-point is attained prior to the start of solvent crystallization,
the destruction of the primary 3D network thus formed may occur by the creation of
a polycrystalline phase. Such a process is especially detrimental in quickly forming
chemotropic gels because their covalently linked nodes do not allow the polymeric
chains to be slid apart easily by the growing solvent crystals. These effects, in turn,
can even result in splitting of the weakest covalent bonds in the structure of the
spatial polymeric network, especially, as shown by Jellinek and Fox [18], when the
crystallization fronts are moving rapidly.
There are two ways to “outrun” gelation of the still-unfrozen feed system. The
first method is to freeze the feed as quickly as possible where the gelation reactions
have already begun. The other method is to reduce the initial rate of such reactions,
e.g., by chilling the feed to just above its freezing point before adding the initiator
(the case of polymerization-type cryogels) or the crosslinking agent (the case of
covalent cryogels formed via crosslinking of macromolecular precursors). The
former way, such as the flash-freezing technique using liquid nitrogen, has two
significant drawbacks. First, the initial solution may undergo a glass-transition
instead of the required solvent crystallization so that no cryo-concentration effect
can be generated. Second, when the solvent crystallization does occur, the size of
the very quickly formed crystals is small, and so is the cross-section of the pores in
the resulting cryogel. Therefore, the latter approach, i.e., decreasing the rate of the
Basic Principles of Cryotropic Gelation
53