3. The so-called “carbon cryogels” prepared by swelling of certain crosslinked
organic polymers, e.g., swelling of phenol–formaldehyde resins in crystallizable
organic solvents such as benzene, followed by freezing of the swollen matter and
finally freeze-drying to obtain a macroporous polymeric texturate, which is then
subjected to high-temperature carbonization with the formation of activated
carbon (charcoal) possessing specific macroporosity [14–16]. It is also obvious
that such final materials (i.e., “carbon cryogels”) are not gels at all, because gels
are 3D networks constructed from organic or inorganic polymers and necessarily
contain immobilized low molecular weight solvate liquid (i.e., water in the case
of hydrogels).
4. Some authors have named the polymeric matrices fabricated via conventional
freeze-drying by the term “cryogels.” Certainly, freezing of polymeric solutions
or colloidal dispersions causes solid–liquid phase separation, and the subsequent
sublimation of the solidified solvent crystals “fixes” the system thus structured
[17, 18], but no gelation occurs during these consecutive steps. Therefore, it is
more correct to call such freeze-dried polymeric matrices “cryostructurates” or
“cryotexturates” rather than “cryogels.”
Taking into account the above considerations, one can conclude that the polymeric and biopolymeric materials mentioned above are not related directly to
proper cryogels and that they are probably named by such a term rather
accidentally.
2 Types of Initial Systems Capable of Being Precursors
for the Preparation of Cryogels
Numerous studies on various aspects of cryotropic gelation phenomena have
demonstrated that cryogels can be prepared, similarly to the traditional gels formed
at positive temperatures, from precursor systems that fall into the following categories (see review articles [1, 18–29]):
• Colloidal dispersions: Freezing of colloid sols and the resulting cryoconcentration effects cause reinforcement of the interparticle interactions, leading to the formation of tight particle-to-particle contacts. These contacts are
stabilized either by the cohesion forces or, if some reactive auxiliary substances
are present, by the formation of chemical links between the grains of particulate
matter. It seems likely that the freeze–thaw structuring and gelling of colloid
systems is the oldest example of cryotropic gelation and has been known for
many decades, especially by people who deal with the frozen storage of foodstuffs (some particular instances are given below).
• Solutions of monomeric precursors: Chemically or radiation-induced
crosslinking polymerization or polycondensation in moderately frozen aqueous
or organic media (depending on the chemical nature of the monomers) results in
4
V.I. Lozinsky
organic polymers, e.g., swelling of phenol–formaldehyde resins in crystallizable
organic solvents such as benzene, followed by freezing of the swollen matter and
finally freeze-drying to obtain a macroporous polymeric texturate, which is then
subjected to high-temperature carbonization with the formation of activated
carbon (charcoal) possessing specific macroporosity [14–16]. It is also obvious
that such final materials (i.e., “carbon cryogels”) are not gels at all, because gels
are 3D networks constructed from organic or inorganic polymers and necessarily
contain immobilized low molecular weight solvate liquid (i.e., water in the case
of hydrogels).
4. Some authors have named the polymeric matrices fabricated via conventional
freeze-drying by the term “cryogels.” Certainly, freezing of polymeric solutions
or colloidal dispersions causes solid–liquid phase separation, and the subsequent
sublimation of the solidified solvent crystals “fixes” the system thus structured
[17, 18], but no gelation occurs during these consecutive steps. Therefore, it is
more correct to call such freeze-dried polymeric matrices “cryostructurates” or
“cryotexturates” rather than “cryogels.”
Taking into account the above considerations, one can conclude that the polymeric and biopolymeric materials mentioned above are not related directly to
proper cryogels and that they are probably named by such a term rather
accidentally.
2 Types of Initial Systems Capable of Being Precursors
for the Preparation of Cryogels
Numerous studies on various aspects of cryotropic gelation phenomena have
demonstrated that cryogels can be prepared, similarly to the traditional gels formed
at positive temperatures, from precursor systems that fall into the following categories (see review articles [1, 18–29]):
• Colloidal dispersions: Freezing of colloid sols and the resulting cryoconcentration effects cause reinforcement of the interparticle interactions, leading to the formation of tight particle-to-particle contacts. These contacts are
stabilized either by the cohesion forces or, if some reactive auxiliary substances
are present, by the formation of chemical links between the grains of particulate
matter. It seems likely that the freeze–thaw structuring and gelling of colloid
systems is the oldest example of cryotropic gelation and has been known for
many decades, especially by people who deal with the frozen storage of foodstuffs (some particular instances are given below).
• Solutions of monomeric precursors: Chemically or radiation-induced
crosslinking polymerization or polycondensation in moderately frozen aqueous
or organic media (depending on the chemical nature of the monomers) results in
4
V.I. Lozinsky