1 Introductory Remarks
Polymeric cryogels are the gel systems formed via the cryogenic treatment (moderate freezing—frozen storage—thawing) of solutions or colloidal dispersions of
the appropriate precursors [1]. On a microscopic level, moderately frozen molecular or colloid solutions are heterophase systems containing both solids (i.e., the
polycrystals of frozen solvent) and some unfrozen fraction called “unfrozen liquid
microphase” [2], where the solutes are concentrated. Thus, gelation can only occur
within the latter unfrozen regions of the system while the crystals of frozen solvent
act as porogens. Such specific conditions of gel formation are the key factors in
determining the rather unusual heterophase macroporous morphology of the
resulting polymer materials, the whole set of their physicochemical characteristics
and, as a consequence, their operational capabilities. The present volume deals with
the diverse aspects of preparation, properties, structure, and practical implementation of various cryogels based on synthetic organic or inorganic polymers, as well
as on natural biopolymers. Taking into account the fact that the number of the
works published in this field has grown almost exponentially during recent years
(Fig. 1a), it is reasonable to first give a brief historical overview of these gel
systems, which are very interesting both from the fundamental and applied viewpoints. Moreover, the authors of some recent publications devoted mainly to the
applied aspects often seem to be unaware of the pioneering studies and main
scientific sources. It is hoped that this chapter will also contribute to a better
understanding of the developments achieved in cryotropic gel formation over the
past three decades.
In many cases, it is difficult to indicate which exactly was the very first
communication on some experimentally observed phenomenon, especially if it
was discovered many years ago in the pre-electronic era, and the report appeared
in an issue hardly available now, or if it was patented locally in a language not
commonly used. The present brief historical information certainly has no claim to
be an exhaustive review of all the early pioneering publications on cryogenically
produced gel matrices. Nevertheless, we can assert that the term “cryogel” was
most probably applied for the first time in a paper published in 1984 to designate
polymeric materials prepared via chemical crosslinking of macromolecular precursors in moderately frozen organic media [4]. The term was created by combining
“cryo” (from the Greek kryos, meaning frost or ice) and “gel,” thus highlighting the
specific formation conditions for the gels of this family. Besides the mentioned
article, different terms were used for gels formed in frozen systems (mainly,
aqueous ones): cryocoacervates [5], cryocoagulates [6], cryo-concentrated gels
[7, 8], anomalous gels [9], freeze–thaw gels [10], etc. However, since the end of
the 1980s the term “cryogel” has become more and more popular (Fig. 1b).
It should also be noted that several other materials are currently called
“cryogels.” Specific examples are as follows:
2
V.I. Lozinsky
Polymeric cryogels are the gel systems formed via the cryogenic treatment (moderate freezing—frozen storage—thawing) of solutions or colloidal dispersions of
the appropriate precursors [1]. On a microscopic level, moderately frozen molecular or colloid solutions are heterophase systems containing both solids (i.e., the
polycrystals of frozen solvent) and some unfrozen fraction called “unfrozen liquid
microphase” [2], where the solutes are concentrated. Thus, gelation can only occur
within the latter unfrozen regions of the system while the crystals of frozen solvent
act as porogens. Such specific conditions of gel formation are the key factors in
determining the rather unusual heterophase macroporous morphology of the
resulting polymer materials, the whole set of their physicochemical characteristics
and, as a consequence, their operational capabilities. The present volume deals with
the diverse aspects of preparation, properties, structure, and practical implementation of various cryogels based on synthetic organic or inorganic polymers, as well
as on natural biopolymers. Taking into account the fact that the number of the
works published in this field has grown almost exponentially during recent years
(Fig. 1a), it is reasonable to first give a brief historical overview of these gel
systems, which are very interesting both from the fundamental and applied viewpoints. Moreover, the authors of some recent publications devoted mainly to the
applied aspects often seem to be unaware of the pioneering studies and main
scientific sources. It is hoped that this chapter will also contribute to a better
understanding of the developments achieved in cryotropic gel formation over the
past three decades.
In many cases, it is difficult to indicate which exactly was the very first
communication on some experimentally observed phenomenon, especially if it
was discovered many years ago in the pre-electronic era, and the report appeared
in an issue hardly available now, or if it was patented locally in a language not
commonly used. The present brief historical information certainly has no claim to
be an exhaustive review of all the early pioneering publications on cryogenically
produced gel matrices. Nevertheless, we can assert that the term “cryogel” was
most probably applied for the first time in a paper published in 1984 to designate
polymeric materials prepared via chemical crosslinking of macromolecular precursors in moderately frozen organic media [4]. The term was created by combining
“cryo” (from the Greek kryos, meaning frost or ice) and “gel,” thus highlighting the
specific formation conditions for the gels of this family. Besides the mentioned
article, different terms were used for gels formed in frozen systems (mainly,
aqueous ones): cryocoacervates [5], cryocoagulates [6], cryo-concentrated gels
[7, 8], anomalous gels [9], freeze–thaw gels [10], etc. However, since the end of
the 1980s the term “cryogel” has become more and more popular (Fig. 1b).
It should also be noted that several other materials are currently called
“cryogels.” Specific examples are as follows:
2
V.I. Lozinsky