morphology of the resulting cryogels. In other words, freezing of the initial
molecular or colloidal solution of the precursors affects not only the gelation
efficiency, but also the structural characteristics of the final cryogel.
The key parameters of the freezing process are temperature, cooling rate,
spontaneous or, if special procedures are employed, directed heat sink [39, 40],
the ability of the initial liquid system to be supercooled, the presence of specially
added or accidental (e.g., specks of dust) germs of crystallization, the thermal
conductivity of the material, and the properties of the inner surface of the vessel
containing the feed to be frozen. In this respect, it is also necessary to define the
notions cooling/chilling rate and freezing rate as these two parameters are not
identical, especially when the ambient temperature is not too low. The cooling/
chilling rate can be controlled rather strictly, for instance, by using precision
programmable cryostats, whereas the freezing rate corresponding to the rate of
solvent crystallization depends on many factors, particularly on the supercooling
depth. For instance, in the case of slow chilling of dust-free feed solutions
containing dissolved polymeric precursors, the supercooling effect can be very
pronounced [8, 41]. This effect then results in an initial high rate of crystallization,
and at high crystallization rates the size of the forming solvent crystals is small. If
there is no supercooling, then the lower the freezing temperature, the smaller the
size of solvent crystals [42, 43] and, hence, the smaller the cross-section of
macropores in the resulting cryogels (see [8, 22, 23, 29, 34, 44–46] and also Sect.
4.1 in [47] of this volume). However, supercooling leads to a deviation from the
above-indicated regularity. One such example is given in Fig. 1, which shows the
micrographs of chitosan cryogels prepared in frozen aqueous systems at À10, À15,
and À30
C [10]. The quantitative data obtained from the images, namely the
number and weight averages of the pore diameters and the coefficient of polydispersity of the pores, are listed in Table 1. The results show that the average diameter
of large pores in the cryogel formed at À10
C is smaller than in cryogel prepared at
À15
C (compare Fig. 1a with b). The reason for such an effect is exactly the
supercooling phenomenon.
In some cases, a spatial polymer network may form while the gelling system is in
the supercooled state. Such a nonequilibrium state was reported both during
chemical gelation processes like crosslinking polymerization as well as during
chilling-induced noncovalent gelation. The primary ordinary gel thus formed in
the supercooled state of the gelation system will be subjected to physical stresses
upon the start of the freezing process due to the movement of the crystallization
fronts. Such an effect has been registered upon very slow chilling (0.003
C/min) of
100 g/L poly(vinyl alcohol) (PVA) aqueous solutions [48]. It was shown that a
low-melting physical gel with a fusion temperature of ~21.5
C forms before
freezing of the system. In contrast, the equiconcentrated cryogels formed via
conventional freezing, i.e., without a significant supercooling effect, had fusion
temperatures of the order of 70–75
C.
Several approaches have been proposed in order to exclude the influence of
supercooling on the formation of different cryogels. These approaches include
either special freezing profiles or the use of crystallization initiators. The former
Basic Principles of Cryotropic Gelation
55
molecular or colloidal solution of the precursors affects not only the gelation
efficiency, but also the structural characteristics of the final cryogel.
The key parameters of the freezing process are temperature, cooling rate,
spontaneous or, if special procedures are employed, directed heat sink [39, 40],
the ability of the initial liquid system to be supercooled, the presence of specially
added or accidental (e.g., specks of dust) germs of crystallization, the thermal
conductivity of the material, and the properties of the inner surface of the vessel
containing the feed to be frozen. In this respect, it is also necessary to define the
notions cooling/chilling rate and freezing rate as these two parameters are not
identical, especially when the ambient temperature is not too low. The cooling/
chilling rate can be controlled rather strictly, for instance, by using precision
programmable cryostats, whereas the freezing rate corresponding to the rate of
solvent crystallization depends on many factors, particularly on the supercooling
depth. For instance, in the case of slow chilling of dust-free feed solutions
containing dissolved polymeric precursors, the supercooling effect can be very
pronounced [8, 41]. This effect then results in an initial high rate of crystallization,
and at high crystallization rates the size of the forming solvent crystals is small. If
there is no supercooling, then the lower the freezing temperature, the smaller the
size of solvent crystals [42, 43] and, hence, the smaller the cross-section of
macropores in the resulting cryogels (see [8, 22, 23, 29, 34, 44–46] and also Sect.
4.1 in [47] of this volume). However, supercooling leads to a deviation from the
above-indicated regularity. One such example is given in Fig. 1, which shows the
micrographs of chitosan cryogels prepared in frozen aqueous systems at À10, À15,
and À30
C [10]. The quantitative data obtained from the images, namely the
number and weight averages of the pore diameters and the coefficient of polydispersity of the pores, are listed in Table 1. The results show that the average diameter
of large pores in the cryogel formed at À10
C is smaller than in cryogel prepared at
À15
C (compare Fig. 1a with b). The reason for such an effect is exactly the
supercooling phenomenon.
In some cases, a spatial polymer network may form while the gelling system is in
the supercooled state. Such a nonequilibrium state was reported both during
chemical gelation processes like crosslinking polymerization as well as during
chilling-induced noncovalent gelation. The primary ordinary gel thus formed in
the supercooled state of the gelation system will be subjected to physical stresses
upon the start of the freezing process due to the movement of the crystallization
fronts. Such an effect has been registered upon very slow chilling (0.003
C/min) of
100 g/L poly(vinyl alcohol) (PVA) aqueous solutions [48]. It was shown that a
low-melting physical gel with a fusion temperature of ~21.5
C forms before
freezing of the system. In contrast, the equiconcentrated cryogels formed via
conventional freezing, i.e., without a significant supercooling effect, had fusion
temperatures of the order of 70–75
C.
Several approaches have been proposed in order to exclude the influence of
supercooling on the formation of different cryogels. These approaches include
either special freezing profiles or the use of crystallization initiators. The former
Basic Principles of Cryotropic Gelation
55