covalently crosslinked gel materials that generally possess a wide-pore spongy
morphology.
• Solutions of high molecular weight precursors: Covalent crosslinking, i.e.,
curing of macromolecules either with chemical agents or by irradiation
(gamma-rays, electron beams, UV irradiation, photolysis in the presence of a
suitable photoinitiator) in non-deeply frozen systems, results in chemically
crosslinked highly porous matrices. The characteristics of the porosity of such
gel materials (i.e., their macroporous or supermacroporous sponge-like texture)
are governed by the freezing conditions, by the amount of freezable solvent, and
by the size of porogen particles, namely, solvent polycrystals.
• Solutions of so-called self-gelling polymers: Such precursor systems are capable of
forming physical (noncovalent) gels upon “worsening” of the thermodynamic
quality of the solvent [1, 30] or by the addition of a solute that induces a change
in the conformation of the macromolecular chains, e.g., a protein denaturant [31, 32].
• Solutions of polyelectrolytes containing low molecular weight or polymeric
crosslinking counterions: These precursor systems are able to form sufficiently
stable ionic bridges between the polyelectolyte chains. Such kind of gel formation is a relatively rare variant of the cryotropic gelation case, since the
ion-exchange reactions are fast processes and, thus, it is technically difficult to
freeze the precursor solution prior to its gelation. Therefore, some special
methods must be implemented to overcome the mentioned impediments, that
is, to shift the onset of gelation beyond the freezing of the reaction solution. For
instance, an ionic crosslinker can be introduced in the feed solution in the form
of a solid powdered salt having a negative temperature coefficient of solubility, i.
e., its solubility rises with lowering of the temperature. In this way, the feed can
first be frost-bound, and ionotropic gelling will then occur within the moderately
frozen bulk system [33, 34].
The above classification of the precursor systems is also convenient for an
overview of the “cryogel story,” since different types of polymeric cryogels at the
early stages of their history have been discovered virtually independently. This
situation continued at least until the beginning of 1980s, when general approaches
for the preparation of covalently crosslinked cryogels based on both monomeric
and polymeric precursors were elaborated and patented [35]. The listed variants are
considered here in the same sequence.
However, one important remark should be made first regarding the definitions of
“positive” and “negative” temperatures. In the subsequent discussion, the freezing/
melting point of the feed system is taken as “zero” in the temperature scale;
therefore, the processes under the thermal conditions above this point occur at
positive temperatures, while gel formation below this point occurs at negative
temperatures. Further, in this context, we also use the terms “non-deeply frozen”
or “moderately frozen” to designate frozen systems that are not completely solid at
the corresponding negative temperature, and in which some fraction of unfrozen
liquid microphase still exists. As a rule, such temperatures lie not lower than several
tens of centigrade under the freezing/melting point of the corresponding feed
solution [1, 2].
A Brief History of Polymeric Cryogels
5
morphology.
• Solutions of high molecular weight precursors: Covalent crosslinking, i.e.,
curing of macromolecules either with chemical agents or by irradiation
(gamma-rays, electron beams, UV irradiation, photolysis in the presence of a
suitable photoinitiator) in non-deeply frozen systems, results in chemically
crosslinked highly porous matrices. The characteristics of the porosity of such
gel materials (i.e., their macroporous or supermacroporous sponge-like texture)
are governed by the freezing conditions, by the amount of freezable solvent, and
by the size of porogen particles, namely, solvent polycrystals.
• Solutions of so-called self-gelling polymers: Such precursor systems are capable of
forming physical (noncovalent) gels upon “worsening” of the thermodynamic
quality of the solvent [1, 30] or by the addition of a solute that induces a change
in the conformation of the macromolecular chains, e.g., a protein denaturant [31, 32].
• Solutions of polyelectrolytes containing low molecular weight or polymeric
crosslinking counterions: These precursor systems are able to form sufficiently
stable ionic bridges between the polyelectolyte chains. Such kind of gel formation is a relatively rare variant of the cryotropic gelation case, since the
ion-exchange reactions are fast processes and, thus, it is technically difficult to
freeze the precursor solution prior to its gelation. Therefore, some special
methods must be implemented to overcome the mentioned impediments, that
is, to shift the onset of gelation beyond the freezing of the reaction solution. For
instance, an ionic crosslinker can be introduced in the feed solution in the form
of a solid powdered salt having a negative temperature coefficient of solubility, i.
e., its solubility rises with lowering of the temperature. In this way, the feed can
first be frost-bound, and ionotropic gelling will then occur within the moderately
frozen bulk system [33, 34].
The above classification of the precursor systems is also convenient for an
overview of the “cryogel story,” since different types of polymeric cryogels at the
early stages of their history have been discovered virtually independently. This
situation continued at least until the beginning of 1980s, when general approaches
for the preparation of covalently crosslinked cryogels based on both monomeric
and polymeric precursors were elaborated and patented [35]. The listed variants are
considered here in the same sequence.
However, one important remark should be made first regarding the definitions of
“positive” and “negative” temperatures. In the subsequent discussion, the freezing/
melting point of the feed system is taken as “zero” in the temperature scale;
therefore, the processes under the thermal conditions above this point occur at
positive temperatures, while gel formation below this point occurs at negative
temperatures. Further, in this context, we also use the terms “non-deeply frozen”
or “moderately frozen” to designate frozen systems that are not completely solid at
the corresponding negative temperature, and in which some fraction of unfrozen
liquid microphase still exists. As a rule, such temperatures lie not lower than several
tens of centigrade under the freezing/melting point of the corresponding feed
solution [1, 2].
A Brief History of Polymeric Cryogels
5