3.3 Bell-Shaped Temperature Dependence of Cryotropic
Gelation Efficiency
In early studies on the kinetic features of simple model reactions in moderately
frozen systems, it was found that the temperature dependence of the reaction rates is
bell-shaped [62, 63]. This type of temperature dependence is a consequence of
competition between the acceleration effect because of the cryo-concentration and
the deceleration effect due to the decreasing thermal mobility of the reactants and
increasing viscosity in the medium of UFLMP at negative temperatures [54,
63]. The temperature dependence of cryotropic gelation efficiency is, as a rule,
also bell-shaped during the formation of both covalent and noncovalent cryogels
[8]. In the following, such effects are demonstrated for cryogels produced via
chemical crosslinking of macromolecular precursors as well as for the
polymerization-type and noncovalent cryogels.
Figure 13 shows the temperature dependence of the gel-fraction yield during the
formation of chitosan-based cryogels, when this polyaminosaccharide was
crosslinked with various amounts of glutaraldehyde (for the reaction scheme see
Fig. 7a). The results reveal that, in the moderately frozen aqueous medium, the
highest performance of the crosslinking process was achieved in the vicinity of
À25
C, whereas above and below this temperature the gel-fraction yield was
lower [10].
Because the freezing procedure employed (i.e., the thermal history of the
system) influences the cryotropic gelation dynamics, it is reasonable to anticipate
that it can also affect the temperature dependence of the gel formation efficiency.
Indeed, this assumption was confirmed experimentally. Figure 14 is a relevant
example showing the temperature dependence of the initial rate (v 0 ) of SH group
decline in the course of oxidative gelation of thiol-bearing poly(acrylamide)
[28]. Curves 1 and 2 correspond to the reaction systems subjected to conventional
freezing and low-temperature quenching procedures, respectively. The distinctions
are obvious: when the reaction solution is subjected to the low-temperature
quenching procedure, the initial rates become slower over the whole range of
reaction temperatures studied, and the position of the maximum shifts towards a
lower temperature. These results testify once again that the reaction conditions in
moderately frozen systems, especially at the initial stages, differ significantly
depending on the thermal prehistory, i.e., on the freezing procedure employed.
Most probably, one of the main reasons for this effect is the nonequivalence of the
phase states in such differently frozen gelling systems. Nonetheless, the bell-like
character of curves 1 and 2 in Fig. 14 is very similar, thus showing the generality of
the tendencies described by such temperature dependences, irrespective of the
thermal history. At the same time, no differences were observed in v 0 values for
the gelation in solutions without any freezing (solid line in Fig. 14) and initially
frozen in a liquid nitrogen followed by placing in a thermostat with pre-set positive
temperature (dashed line in Fig. 14).
78
V.I. Lozinsky and O. Okay
Gelation Efficiency
In early studies on the kinetic features of simple model reactions in moderately
frozen systems, it was found that the temperature dependence of the reaction rates is
bell-shaped [62, 63]. This type of temperature dependence is a consequence of
competition between the acceleration effect because of the cryo-concentration and
the deceleration effect due to the decreasing thermal mobility of the reactants and
increasing viscosity in the medium of UFLMP at negative temperatures [54,
63]. The temperature dependence of cryotropic gelation efficiency is, as a rule,
also bell-shaped during the formation of both covalent and noncovalent cryogels
[8]. In the following, such effects are demonstrated for cryogels produced via
chemical crosslinking of macromolecular precursors as well as for the
polymerization-type and noncovalent cryogels.
Figure 13 shows the temperature dependence of the gel-fraction yield during the
formation of chitosan-based cryogels, when this polyaminosaccharide was
crosslinked with various amounts of glutaraldehyde (for the reaction scheme see
Fig. 7a). The results reveal that, in the moderately frozen aqueous medium, the
highest performance of the crosslinking process was achieved in the vicinity of
À25
C, whereas above and below this temperature the gel-fraction yield was
lower [10].
Because the freezing procedure employed (i.e., the thermal history of the
system) influences the cryotropic gelation dynamics, it is reasonable to anticipate
that it can also affect the temperature dependence of the gel formation efficiency.
Indeed, this assumption was confirmed experimentally. Figure 14 is a relevant
example showing the temperature dependence of the initial rate (v 0 ) of SH group
decline in the course of oxidative gelation of thiol-bearing poly(acrylamide)
[28]. Curves 1 and 2 correspond to the reaction systems subjected to conventional
freezing and low-temperature quenching procedures, respectively. The distinctions
are obvious: when the reaction solution is subjected to the low-temperature
quenching procedure, the initial rates become slower over the whole range of
reaction temperatures studied, and the position of the maximum shifts towards a
lower temperature. These results testify once again that the reaction conditions in
moderately frozen systems, especially at the initial stages, differ significantly
depending on the thermal prehistory, i.e., on the freezing procedure employed.
Most probably, one of the main reasons for this effect is the nonequivalence of the
phase states in such differently frozen gelling systems. Nonetheless, the bell-like
character of curves 1 and 2 in Fig. 14 is very similar, thus showing the generality of
the tendencies described by such temperature dependences, irrespective of the
thermal history. At the same time, no differences were observed in v 0 values for
the gelation in solutions without any freezing (solid line in Fig. 14) and initially
frozen in a liquid nitrogen followed by placing in a thermostat with pre-set positive
temperature (dashed line in Fig. 14).
78
V.I. Lozinsky and O. Okay