ago [15–19], cryogels have attracted intense attention only in the last 10 years due
to their extraordinary properties [20–22]. For instance, they do not display undesirable properties such as brittleness, which is commonly observed for macroporous
gels formed by phase separation polymerization. Cryogels are very tough and can
withstand high levels of deformation, such as elongation and torsion; they can also
be squeezed almost completely without any crack propagation. The cryogelation
technique is based on the natural principle that sea ice is less salty than sea water,
i.e., the rejection of brine from freezing salt solutions. This principle is a consequence of the insolubility of the salts in ice compared to their excellent solubility in
water. In cryogelation reactions, the reaction solution, generally containing the
monomers and the initiator, is cooled below the freezing point of the system; since
the monomers and the initiator will be enriched in the unfrozen microzones
surrounded by solvent crystals, the polymerization reactions only proceed in
these unfrozen regions containing a high concentration of monomer. The increased
monomer concentration in the unfrozen reaction zones (i.e., cryo-concentration) is
the main characteristic of the cryogelation reactions and is responsible for the
extraordinary properties of cryogels. A macroporous structure in the final material
appears due to the existence of solvent crystals acting as a template (or porogen) for
the formation of the pores. The removal of template (e.g., ice) is achieved by simply
holding the cryogel at temperatures above the solvent freezing point. This is another
advantage of the cryogelation technique over the phase separation technique, where
the latter requires extensive extraction of the gel to remove the porogen. In contrast
to the cauliflower-like microstructure of macroporous gels formed by phase separation, cryogels exhibit a regular assembly of large pores of 10
0 –10
1
μm in size
separated by dense pore walls of several micrometers in thickness (Fig. 1b) [23].
This chapter discusses the conditions for formation of macroporous gels by the
cryogelation technique. It also discusses how and why the properties of gels
significantly alter upon transition from conventional gelation to the cryogelation
regime. The formation and structure–property relationships of cryogels starting
from monovinyl–divinyl monomers, as well as from linear polymer chains, are
reviewed using examples from the recent literature. Some novel cryogels based on
DNA, silk fibroin, poly(acrylic acid), and several types of rubbers with a wide range
of tunable properties are also presented in details together with their applications.
2 Preparation and Characterization of Cryogels
Cryogels are mainly prepared by crosslinking polymerization of monomers or by
crosslinking of linear polymers in frozen solutions. The most commonly used
monomeric and polymeric precursors, the crosslinkers, the general preparation
conditions, and characterization techniques are summarized in this section.
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O. Okay and V.I. Lozinsky
to their extraordinary properties [20–22]. For instance, they do not display undesirable properties such as brittleness, which is commonly observed for macroporous
gels formed by phase separation polymerization. Cryogels are very tough and can
withstand high levels of deformation, such as elongation and torsion; they can also
be squeezed almost completely without any crack propagation. The cryogelation
technique is based on the natural principle that sea ice is less salty than sea water,
i.e., the rejection of brine from freezing salt solutions. This principle is a consequence of the insolubility of the salts in ice compared to their excellent solubility in
water. In cryogelation reactions, the reaction solution, generally containing the
monomers and the initiator, is cooled below the freezing point of the system; since
the monomers and the initiator will be enriched in the unfrozen microzones
surrounded by solvent crystals, the polymerization reactions only proceed in
these unfrozen regions containing a high concentration of monomer. The increased
monomer concentration in the unfrozen reaction zones (i.e., cryo-concentration) is
the main characteristic of the cryogelation reactions and is responsible for the
extraordinary properties of cryogels. A macroporous structure in the final material
appears due to the existence of solvent crystals acting as a template (or porogen) for
the formation of the pores. The removal of template (e.g., ice) is achieved by simply
holding the cryogel at temperatures above the solvent freezing point. This is another
advantage of the cryogelation technique over the phase separation technique, where
the latter requires extensive extraction of the gel to remove the porogen. In contrast
to the cauliflower-like microstructure of macroporous gels formed by phase separation, cryogels exhibit a regular assembly of large pores of 10
0 –10
1
μm in size
separated by dense pore walls of several micrometers in thickness (Fig. 1b) [23].
This chapter discusses the conditions for formation of macroporous gels by the
cryogelation technique. It also discusses how and why the properties of gels
significantly alter upon transition from conventional gelation to the cryogelation
regime. The formation and structure–property relationships of cryogels starting
from monovinyl–divinyl monomers, as well as from linear polymer chains, are
reviewed using examples from the recent literature. Some novel cryogels based on
DNA, silk fibroin, poly(acrylic acid), and several types of rubbers with a wide range
of tunable properties are also presented in details together with their applications.
2 Preparation and Characterization of Cryogels
Cryogels are mainly prepared by crosslinking polymerization of monomers or by
crosslinking of linear polymers in frozen solutions. The most commonly used
monomeric and polymeric precursors, the crosslinkers, the general preparation
conditions, and characterization techniques are summarized in this section.
108
O. Okay and V.I. Lozinsky