does not have any chemical impact on the process of interest. However, a significant acceleration of cysteine oxidation occurs in the presence of polymer when the
temperature is decreased to À15
C (curve 4 in Fig. 11b). Since the water solubility
of cysteine is low, the slow rate of the reactions in the frozen solution at À15
C is
due to the fact that the system is in the post-eutectic state and the amount of cysteine
dissolved in UFLMP is very small. However, the presence of a hydrophilic
unreactive polymer carrying bound water molecules increases the solubility of
both cysteine and oxygen so that their concentrations in UFLMP increase and,
hence, the reaction rate increases. This example also demonstrates one of the
possible approaches for performing the reactions with thermally instable and
insufficiently soluble low molecular weight substances. The presence of an inert
polymeric additive during cryogelation protects the precursors from thermal
decomposition and accelerates the reactions due to the cryo-concentration effect.
The acceleration of gel formation and the non-equivalence of its dynamics,
depending on the thermal prehistory of moderately frozen reaction systems, were
also observed during the synthesis of polymerization-type cryogels. One typical
example is the formation of poly(acrylamide) gels starting from acrylamide monomer and N,N
0 -methylene(bis)acrylamide crosslinker in aqueous solutions with
using of redox initiator systems (Fig. 12a). In Fig. 12b, c, gel fraction versus
reaction time plots at various gelation temperatures are shown for reaction solutions
frozen by conventional and low-temperature quenching procedures, respectively
[23]. At positive temperatures, lowering the reaction temperature from 25 to 13
C
causes a decrease in the rate of ordinary gel formation, and thus results in a lower
gel-fraction yield. This is expected. However, performing the reactions in a moderately frozen system leads to a faster gelation as well as to a higher yield
(Fig. 12b). The freezing mode, i.e., the thermal history of bringing the system to
the reaction temperature, also affects the course of cryotropic gel formation
(cf. Fig. 12b, c), thus pointing to the non-equivalence of the phase states in such
differently frozen polymerizing systems. Similar trends were also observed by use
of NMR during the formation of poly(acrylamide) cryogels [50], as well as during
the linear cryopolymerization of acrylamide in moderately frozen aqueous media,
where different freezing procedures (conventional freezing, flash freezing, and
low-temperature quenching) were employed [65].
The acceleration of gelation over a certain range of negative temperatures is also
inherent during the formation of physical cryogels. This fact was already partially
touched upon in the discussion of the data in Table 2, for instance, regarding the
gelation time of aqueous solutions of locust bean gum or PVA. The same effect was
also observed in maltodextin- and β-glucan-containing systems [101, 145, 146], i.e.,
in systems where the noncovalent self-gelation at positive temperatures proceeds at
a rather slow rate. Probably, somewhat similar events do take place in fast-gelling
systems such as aqueous solutions of agarose, amylopectin, amylopectin–amylose
mixtures, or concentrated solutions of gelatin. However, because of their rapid selfgelation before freezing, the additional acceleration due to the presence of UFLMP
can hardly be detected.
76
V.I. Lozinsky and O. Okay
temperature is decreased to À15
C (curve 4 in Fig. 11b). Since the water solubility
of cysteine is low, the slow rate of the reactions in the frozen solution at À15
C is
due to the fact that the system is in the post-eutectic state and the amount of cysteine
dissolved in UFLMP is very small. However, the presence of a hydrophilic
unreactive polymer carrying bound water molecules increases the solubility of
both cysteine and oxygen so that their concentrations in UFLMP increase and,
hence, the reaction rate increases. This example also demonstrates one of the
possible approaches for performing the reactions with thermally instable and
insufficiently soluble low molecular weight substances. The presence of an inert
polymeric additive during cryogelation protects the precursors from thermal
decomposition and accelerates the reactions due to the cryo-concentration effect.
The acceleration of gel formation and the non-equivalence of its dynamics,
depending on the thermal prehistory of moderately frozen reaction systems, were
also observed during the synthesis of polymerization-type cryogels. One typical
example is the formation of poly(acrylamide) gels starting from acrylamide monomer and N,N
0 -methylene(bis)acrylamide crosslinker in aqueous solutions with
using of redox initiator systems (Fig. 12a). In Fig. 12b, c, gel fraction versus
reaction time plots at various gelation temperatures are shown for reaction solutions
frozen by conventional and low-temperature quenching procedures, respectively
[23]. At positive temperatures, lowering the reaction temperature from 25 to 13
C
causes a decrease in the rate of ordinary gel formation, and thus results in a lower
gel-fraction yield. This is expected. However, performing the reactions in a moderately frozen system leads to a faster gelation as well as to a higher yield
(Fig. 12b). The freezing mode, i.e., the thermal history of bringing the system to
the reaction temperature, also affects the course of cryotropic gel formation
(cf. Fig. 12b, c), thus pointing to the non-equivalence of the phase states in such
differently frozen polymerizing systems. Similar trends were also observed by use
of NMR during the formation of poly(acrylamide) cryogels [50], as well as during
the linear cryopolymerization of acrylamide in moderately frozen aqueous media,
where different freezing procedures (conventional freezing, flash freezing, and
low-temperature quenching) were employed [65].
The acceleration of gelation over a certain range of negative temperatures is also
inherent during the formation of physical cryogels. This fact was already partially
touched upon in the discussion of the data in Table 2, for instance, regarding the
gelation time of aqueous solutions of locust bean gum or PVA. The same effect was
also observed in maltodextin- and β-glucan-containing systems [101, 145, 146], i.e.,
in systems where the noncovalent self-gelation at positive temperatures proceeds at
a rather slow rate. Probably, somewhat similar events do take place in fast-gelling
systems such as aqueous solutions of agarose, amylopectin, amylopectin–amylose
mixtures, or concentrated solutions of gelatin. However, because of their rapid selfgelation before freezing, the additional acceleration due to the presence of UFLMP
can hardly be detected.
76
V.I. Lozinsky and O. Okay