3 Specific Effects of Gel Formation in Moderately Frozen
Systems
3.1 Apparent Decrease in the Critical Concentration
of Gelation
One of the principal conditions for conventional gelation at positive temperatures is
that the concentration of the precursors in the feed has to exceed a certain boundary
value, called the critical concentration of gelation (CCG) [2]. The same is also true
for cryotropic gel formation. In the early systematic studies on the main factors
influencing the properties of freeze–thaw gels, it was found that cryogels can be
obtained at initial precursor concentrations considerably lower than the critical
values needed for gel formation at positive temperatures. Such a decrease in CCG
is inherent in the formation of both chemically crosslinked cryogels [19, 20, 26, 27]
and noncovalent gels [12]. This effect has been observed for cryostructuring in
moderately frozen aqueous systems [20, 26, 27] as well as in organic systems
[27]. Figures 7 and 8 represent typical examples of such an effect. These data
concern covalent-type gelation in “solvent–polymeric precursor–crosslinkingagent” systems, namely crosslinked chitosan gels in an aqueous medium (Fig. 7)
and crosslinked poly(styrene) gels in nitrobenzene medium (Fig. 8). Figure 7a
shows the scheme of the crosslinking reaction of chitosan using glutaraldehyde as
a crosslinker. In Fig. 7b, c, the gel-fraction yield is plotted against the initial
chitosan concentration and the initial molar ratio of aldehyde to amine groups,
respectively. The two curves represent data for chitosan gels formed at À8 and
24
C. It is seen that the crosslinking of chitosan with glutaraldehyde at À8
C
results in the formation of cryogels at about one third of the initial polymer
concentration compared with the formation of conventional gels at 24
C. Moreover, there is also an approximately threefold difference in the critical crosslinking
agent concentration in terms of the molar ratio of aldehyde to amine groups
(Fig. 7c). The reason for this phenomenon is the cryo-concentration effect
(Fig. 3), since the gel phase of cryogels forms within the UFLMP, which is more
concentrated than the initial solution. Therefore, the decrease in CCG in cryotropic
gel formation is an apparent decrease due to the confinement of the reacting species
to the microreactor UFLMP [8]. Qualitatively, the same trend was observed during
the synthesis of covalent cryogels in organic media, which is exemplified by the
crosslinking of poly(styrene) using 4,4
0 -xylylene-dichloride via the Friedel–Crafts
reaction (Fig. 8a). Such cryogels possess a wide-pore spongy texture and can be
obtained in moderately frozen nitrobenzene (T 0 ¼ +5.5
C). In Fig. 8b, the
gel-fraction yield is plotted against the polymer concentration for gels prepared at
relative temperatures ΔT of À19
C and +28
C. The critical polymer concentration
at ΔT ¼ À19
C is six times lower than that at 28
C. Thus, the data in Figs. 7 and
8 demonstrate the universal character of the effect under discussion in both aqueous
and organic media.
66
V.I. Lozinsky and O. Okay
Systems
3.1 Apparent Decrease in the Critical Concentration
of Gelation
One of the principal conditions for conventional gelation at positive temperatures is
that the concentration of the precursors in the feed has to exceed a certain boundary
value, called the critical concentration of gelation (CCG) [2]. The same is also true
for cryotropic gel formation. In the early systematic studies on the main factors
influencing the properties of freeze–thaw gels, it was found that cryogels can be
obtained at initial precursor concentrations considerably lower than the critical
values needed for gel formation at positive temperatures. Such a decrease in CCG
is inherent in the formation of both chemically crosslinked cryogels [19, 20, 26, 27]
and noncovalent gels [12]. This effect has been observed for cryostructuring in
moderately frozen aqueous systems [20, 26, 27] as well as in organic systems
[27]. Figures 7 and 8 represent typical examples of such an effect. These data
concern covalent-type gelation in “solvent–polymeric precursor–crosslinkingagent” systems, namely crosslinked chitosan gels in an aqueous medium (Fig. 7)
and crosslinked poly(styrene) gels in nitrobenzene medium (Fig. 8). Figure 7a
shows the scheme of the crosslinking reaction of chitosan using glutaraldehyde as
a crosslinker. In Fig. 7b, c, the gel-fraction yield is plotted against the initial
chitosan concentration and the initial molar ratio of aldehyde to amine groups,
respectively. The two curves represent data for chitosan gels formed at À8 and
24
C. It is seen that the crosslinking of chitosan with glutaraldehyde at À8
C
results in the formation of cryogels at about one third of the initial polymer
concentration compared with the formation of conventional gels at 24
C. Moreover, there is also an approximately threefold difference in the critical crosslinking
agent concentration in terms of the molar ratio of aldehyde to amine groups
(Fig. 7c). The reason for this phenomenon is the cryo-concentration effect
(Fig. 3), since the gel phase of cryogels forms within the UFLMP, which is more
concentrated than the initial solution. Therefore, the decrease in CCG in cryotropic
gel formation is an apparent decrease due to the confinement of the reacting species
to the microreactor UFLMP [8]. Qualitatively, the same trend was observed during
the synthesis of covalent cryogels in organic media, which is exemplified by the
crosslinking of poly(styrene) using 4,4
0 -xylylene-dichloride via the Friedel–Crafts
reaction (Fig. 8a). Such cryogels possess a wide-pore spongy texture and can be
obtained in moderately frozen nitrobenzene (T 0 ¼ +5.5
C). In Fig. 8b, the
gel-fraction yield is plotted against the polymer concentration for gels prepared at
relative temperatures ΔT of À19
C and +28
C. The critical polymer concentration
at ΔT ¼ À19
C is six times lower than that at 28
C. Thus, the data in Figs. 7 and
8 demonstrate the universal character of the effect under discussion in both aqueous
and organic media.
66
V.I. Lozinsky and O. Okay