concentration of the dispersed particles; their size, shape, and chemical structure;
and by the conditions of the freeze–thaw process. All these parameters must be
taken into account in order either to prepare certain cryogels/cryostructurates or to
prevent their formation when it is undesirable, as in the case of staving off the
cryocoagulation phenomena in latex dispersions at negative temperatures [85].
2.2 Cryogels Prepared from Monomeric Precursors
The following three points are of principle significance for the preparation of
cryogels from solutions of monomeric precursors:
1. The chosen solvent must crystallize rather than vitrify under the cryostructuring
conditions; otherwise, if the solvent undergoes a glass transition, the unfrozen
liquid microphase will not form and, hence, no cryo-concentrating effects will
take place. This requirement is also valid for any other precursor system to be
gelled cryogenically.
2. The solubility of the monomers should be high enough not only at positive
temperatures, but also in the unfrozen liquid microphase. If the solubility of the
monomer decreases drastically with decreasing temperature, the monomer concentration in the reaction medium becomes insufficient for the formation of a
spatial network; that is, the critical concentration of gelation will not be reached.
Various cryogels have been synthesized by using low molecular weight monomeric precursors, as in the majority of the reported cases, or macromonomers
(macromers) such as the methacrylated derivatives of gelatine and chondroitin
sulfate [86, 87], or poly(vinyl alcohol) [88, 89]. In the latter cases, the properties
of the resulting gel matrices depend, along with common factors such as the
monomer concentration or the freezing conditions, on the molecular weight of
the macromonomer and on the amount of unsaturated groups in its molecule.
3. The crucial problem in the case of cryotropic gelation via free radical polymerization is the performance of the initiator system at negative temperatures.
Chemical initiators such as peroxides that generate primary radicals owing to
thermal decomposition are not suitable for cryopolymerization. Therefore, redox
initiating systems capable of generating radicals at reduced temperatures are
commonly employed [1, 26, 35, 90, 91]. Nonetheless, some rare cases are also
known, where the “high-temperature” radical cryopolymerization of vinyl
monomers has been carried out using thermally decomposed initiators. One
such example is the 2,2
0 -azoisobutyronitrile-initiated copolymerization of styrene and divinylbenzene at 50
C in the medium of crystallized naphthalene
having a crystallization temperature of about 80
C [35, 91, 92]. In the case of
radiation-induced cryopolymerization, the above-discussed problem of the
temperature-dependent activity of chemical initiators is virtually absent. The
efficiency of radiation-induced processes is mainly a function of the applied
radiation dose but not of the temperature, as demonstrated in the pioneering
A Brief History of Polymeric Cryogels
7
and by the conditions of the freeze–thaw process. All these parameters must be
taken into account in order either to prepare certain cryogels/cryostructurates or to
prevent their formation when it is undesirable, as in the case of staving off the
cryocoagulation phenomena in latex dispersions at negative temperatures [85].
2.2 Cryogels Prepared from Monomeric Precursors
The following three points are of principle significance for the preparation of
cryogels from solutions of monomeric precursors:
1. The chosen solvent must crystallize rather than vitrify under the cryostructuring
conditions; otherwise, if the solvent undergoes a glass transition, the unfrozen
liquid microphase will not form and, hence, no cryo-concentrating effects will
take place. This requirement is also valid for any other precursor system to be
gelled cryogenically.
2. The solubility of the monomers should be high enough not only at positive
temperatures, but also in the unfrozen liquid microphase. If the solubility of the
monomer decreases drastically with decreasing temperature, the monomer concentration in the reaction medium becomes insufficient for the formation of a
spatial network; that is, the critical concentration of gelation will not be reached.
Various cryogels have been synthesized by using low molecular weight monomeric precursors, as in the majority of the reported cases, or macromonomers
(macromers) such as the methacrylated derivatives of gelatine and chondroitin
sulfate [86, 87], or poly(vinyl alcohol) [88, 89]. In the latter cases, the properties
of the resulting gel matrices depend, along with common factors such as the
monomer concentration or the freezing conditions, on the molecular weight of
the macromonomer and on the amount of unsaturated groups in its molecule.
3. The crucial problem in the case of cryotropic gelation via free radical polymerization is the performance of the initiator system at negative temperatures.
Chemical initiators such as peroxides that generate primary radicals owing to
thermal decomposition are not suitable for cryopolymerization. Therefore, redox
initiating systems capable of generating radicals at reduced temperatures are
commonly employed [1, 26, 35, 90, 91]. Nonetheless, some rare cases are also
known, where the “high-temperature” radical cryopolymerization of vinyl
monomers has been carried out using thermally decomposed initiators. One
such example is the 2,2
0 -azoisobutyronitrile-initiated copolymerization of styrene and divinylbenzene at 50
C in the medium of crystallized naphthalene
having a crystallization temperature of about 80
C [35, 91, 92]. In the case of
radiation-induced cryopolymerization, the above-discussed problem of the
temperature-dependent activity of chemical initiators is virtually absent. The
efficiency of radiation-induced processes is mainly a function of the applied
radiation dose but not of the temperature, as demonstrated in the pioneering
A Brief History of Polymeric Cryogels
7