noncovalent PVA cryogels, possess outstanding mechanical and thermophysical
properties. Second, owing to the very simple structure of the polymer itself, they are
very convenient model systems for studying the fine mechanisms of cryotropic
noncovalent gelation [8, 111, 112, 127, 129, 130, 148–152]. The shift in CCG upon
the formation of PVA cryogels is also very remarkable. Even concentrated (10–
16 wt%) aqueous and DMSO solutions of PVA (the latter must be highly deacylated
PVA) do not transform to gels at room temperature for many days, whereas the
freeze–thaw cycle produces viscoelastic cryogels that can be fused only when
heated up to 60–80
C. Similarly to the formation of chemically crosslinked
cryogels, the main reason for such a decrease in CCG is the cryo-concentration of
the gel precursors, strengthening the polymer–polymer interactions owing to higher
overlapping of the polymer coils in the more concentrated medium of UFLMP.
3.2 Acceleration of Gel Formation Over a Certain Range
of Negative Temperatures
One of the important consequences of the cryo-concentration effect is an acceleration of cryotropic gel formation processes within a certain range of negative
temperatures as compared with conventional gelation [8]. Since the rate of the
second or higher order reactions is proportional to a positive power of the concentration of the reactants, a freezing-induced increase in the precursor concentration
within the UFLMP accelerates cryogelation reactions [54, 62–64]. The reaction
scheme in Fig. 10a illustrates the oxidative gelation of a thiol-derivative of poly
(acrylamide) in the presence of water-dissolved air oxygen acting as a crosslinking
agent [28, 153, 154]. Since the number of free SH groups decreases during the
course of interchain coupling through the formation of disulfide crosslinks, the
evolution of SH group concentration with time reflects the process dynamics.
Figure 10b shows the variation in the amount of residual SH groups in 2 wt%
aqueous polymer solutions at positive (15 and 25
C) and negative (À15 and
À25
C) temperatures. Two arrows (Fig. 10b) indicate the vicinity of the gel points
for the unfrozen and frozen gelation systems. The rate of the decrease in thiol group
content (i.e., the crosslinking rate) increases and the gel point shifts to shorter times
in the moderately frozen polymer solutions as compared to unfrozen solutions.
Remarkable is the shortening of the gelation time, which differs by a factor of about
22 in favor of cryotropic gel formation.
The thermal prehistory of such cryogenically gelling systems also influences the
process dynamics. The kinetic data in Fig. 10b were obtained by conventional
freezing of the feeds, i.e., without low-temperature quenching, as mentioned in
Sect. 2.2. When the same reaction solutions are subjected to the low-temperature
quenching technique to freeze the feeds quickly, one obtains the plots illustrated in
Fig.10c. Here, the decrease in the thiol group content of the polymer is shown
during the course of the first 2 h of the reaction. Note that the zero time in these plots
Basic Principles of Cryotropic Gelation
73
properties. Second, owing to the very simple structure of the polymer itself, they are
very convenient model systems for studying the fine mechanisms of cryotropic
noncovalent gelation [8, 111, 112, 127, 129, 130, 148–152]. The shift in CCG upon
the formation of PVA cryogels is also very remarkable. Even concentrated (10–
16 wt%) aqueous and DMSO solutions of PVA (the latter must be highly deacylated
PVA) do not transform to gels at room temperature for many days, whereas the
freeze–thaw cycle produces viscoelastic cryogels that can be fused only when
heated up to 60–80
C. Similarly to the formation of chemically crosslinked
cryogels, the main reason for such a decrease in CCG is the cryo-concentration of
the gel precursors, strengthening the polymer–polymer interactions owing to higher
overlapping of the polymer coils in the more concentrated medium of UFLMP.
3.2 Acceleration of Gel Formation Over a Certain Range
of Negative Temperatures
One of the important consequences of the cryo-concentration effect is an acceleration of cryotropic gel formation processes within a certain range of negative
temperatures as compared with conventional gelation [8]. Since the rate of the
second or higher order reactions is proportional to a positive power of the concentration of the reactants, a freezing-induced increase in the precursor concentration
within the UFLMP accelerates cryogelation reactions [54, 62–64]. The reaction
scheme in Fig. 10a illustrates the oxidative gelation of a thiol-derivative of poly
(acrylamide) in the presence of water-dissolved air oxygen acting as a crosslinking
agent [28, 153, 154]. Since the number of free SH groups decreases during the
course of interchain coupling through the formation of disulfide crosslinks, the
evolution of SH group concentration with time reflects the process dynamics.
Figure 10b shows the variation in the amount of residual SH groups in 2 wt%
aqueous polymer solutions at positive (15 and 25
C) and negative (À15 and
À25
C) temperatures. Two arrows (Fig. 10b) indicate the vicinity of the gel points
for the unfrozen and frozen gelation systems. The rate of the decrease in thiol group
content (i.e., the crosslinking rate) increases and the gel point shifts to shorter times
in the moderately frozen polymer solutions as compared to unfrozen solutions.
Remarkable is the shortening of the gelation time, which differs by a factor of about
22 in favor of cryotropic gel formation.
The thermal prehistory of such cryogenically gelling systems also influences the
process dynamics. The kinetic data in Fig. 10b were obtained by conventional
freezing of the feeds, i.e., without low-temperature quenching, as mentioned in
Sect. 2.2. When the same reaction solutions are subjected to the low-temperature
quenching technique to freeze the feeds quickly, one obtains the plots illustrated in
Fig.10c. Here, the decrease in the thiol group content of the polymer is shown
during the course of the first 2 h of the reaction. Note that the zero time in these plots
Basic Principles of Cryotropic Gelation
73