systems are very sensitive to the thermal prehistory, i.e., the temperature profile
used to reach the preset temperature point. For instance, upon freezing of concentrated aqueous PVA solutions, the presence of mobile water molecules was detected
by NMR even at –50
C [78]. In addition, since the relaxation processes in viscous
liquids are very slow, the characteristics of UFLMP (such as its volume during
freezing and defrosting stages) differ significantly, leading to hysteresis phenomena
[76, 79]. Similarly to cryochemical reactions, the gelation rate and the gel-fraction
yield of cryotropic gelation processes, as well as the mechanical characteristics of
the final cryogels, are strongly dependent on the frozen storage temperature and are
also bell-shaped. Such a character of temperature dependences was observed during
the formation of covalent cryogels from monomeric [7, 8, 22, 23, 80–85] and
macromolecular precursors [7, 8, 10, 27–29, 32, 45, 46, 86–88], for some physical
cryogels [7, 8, 34, 44, 48, 89–103] as well as during linear cryopolymerization
processes [49, 72, 104–106].
Based on the concept of UFLMP, the scheme of cryotropic gelation processes
can be depicted by the simple diagram in Fig. 3, which reflects all the essential
features of the systems [107]. When the initial feed containing the monomeric or
polymeric precursors freezes at a moderate negative temperature (Fig. 3a), the
frozen system consists of at least two phases, namely the solvent polycrystals and
the UFLMP where the solutes are concentrated (Fig. 3b). Due to the considerable
increase in the concentration of solutes in these liquid-like inclusions, the gelation
can intensify and result in the formation of a 3D polymeric network within the
volume of this phase. Thus, a cryogel is formed in a solution of the precursors that is
much more concentrated than the initial solution. After thawing of the frozen
system, numerous macropores arise in place of the melted solvent crystals
(Fig. 3c). Hence, the crystals act as a pore template or porogen, whereas the
structure of gel phase (the pore walls) is determined by the initial concentration
of gelling agents and by the number of crosslinks of the respective spatial networks.
2.4 Thawing of the Frozen System
Thawing of the gelation system after incubation in a frozen state for a necessary
time period is the last basic stage of the cryotropic gelation processes. The least
energy-consuming procedure is a simple spontaneous thawing of the frozen system
at an appropriate positive temperature. The conditions of thawing are generally
unimportant as long as the formation of a particular covalent cryogel is mainly
completed before this stage, i.e., if the reactions are virtually stopped and the
gel-fraction yield reaches the maximum possible value. However, it is sometimes
necessary to stop the gelation prior to its completion, e.g., for studying the variation
in gel-fraction yield with reaction time [7, 8, 22, 23, 27, 28, 32, 86]. In this case, fast
defrosting techniques should be employed in order to thaw the reacting system as
quickly as possible, but without overheating. The reactions may also be stopped
either by intense dilution or by addition of a relevant chemical inhibitor, e.g., free
60
V.I. Lozinsky and O. Okay
used to reach the preset temperature point. For instance, upon freezing of concentrated aqueous PVA solutions, the presence of mobile water molecules was detected
by NMR even at –50
C [78]. In addition, since the relaxation processes in viscous
liquids are very slow, the characteristics of UFLMP (such as its volume during
freezing and defrosting stages) differ significantly, leading to hysteresis phenomena
[76, 79]. Similarly to cryochemical reactions, the gelation rate and the gel-fraction
yield of cryotropic gelation processes, as well as the mechanical characteristics of
the final cryogels, are strongly dependent on the frozen storage temperature and are
also bell-shaped. Such a character of temperature dependences was observed during
the formation of covalent cryogels from monomeric [7, 8, 22, 23, 80–85] and
macromolecular precursors [7, 8, 10, 27–29, 32, 45, 46, 86–88], for some physical
cryogels [7, 8, 34, 44, 48, 89–103] as well as during linear cryopolymerization
processes [49, 72, 104–106].
Based on the concept of UFLMP, the scheme of cryotropic gelation processes
can be depicted by the simple diagram in Fig. 3, which reflects all the essential
features of the systems [107]. When the initial feed containing the monomeric or
polymeric precursors freezes at a moderate negative temperature (Fig. 3a), the
frozen system consists of at least two phases, namely the solvent polycrystals and
the UFLMP where the solutes are concentrated (Fig. 3b). Due to the considerable
increase in the concentration of solutes in these liquid-like inclusions, the gelation
can intensify and result in the formation of a 3D polymeric network within the
volume of this phase. Thus, a cryogel is formed in a solution of the precursors that is
much more concentrated than the initial solution. After thawing of the frozen
system, numerous macropores arise in place of the melted solvent crystals
(Fig. 3c). Hence, the crystals act as a pore template or porogen, whereas the
structure of gel phase (the pore walls) is determined by the initial concentration
of gelling agents and by the number of crosslinks of the respective spatial networks.
2.4 Thawing of the Frozen System
Thawing of the gelation system after incubation in a frozen state for a necessary
time period is the last basic stage of the cryotropic gelation processes. The least
energy-consuming procedure is a simple spontaneous thawing of the frozen system
at an appropriate positive temperature. The conditions of thawing are generally
unimportant as long as the formation of a particular covalent cryogel is mainly
completed before this stage, i.e., if the reactions are virtually stopped and the
gel-fraction yield reaches the maximum possible value. However, it is sometimes
necessary to stop the gelation prior to its completion, e.g., for studying the variation
in gel-fraction yield with reaction time [7, 8, 22, 23, 27, 28, 32, 86]. In this case, fast
defrosting techniques should be employed in order to thaw the reacting system as
quickly as possible, but without overheating. The reactions may also be stopped
either by intense dilution or by addition of a relevant chemical inhibitor, e.g., free
60
V.I. Lozinsky and O. Okay