Self-oscillating hydrogel systems can be designed by coupling a responsive
hydrogel sample with oscillating chemical reactions. For example, the Belousov–
Zhabotinsky (BZ) reaction is a well-known oscillating system and the overall
process of the reaction is the oxidation of a substrate such as malonic acid in the
presence of oxidizing agent (bromate) and metal catalyst in an acidic solution [152,
153]. Yoshida and coworkers used poly(N-isopropyl acrylamide) gel in a BZ
reaction to produce a self-oscillating gel system [151, 154]. Crook et al. used
poly(methacrylic acid) (PMAAc) gel in a Landolt oscillating reaction system
consisting of bromate, sulfite, and ferrocyanide ions [155, 156]. Although the actual
mechanism of this bromated oscillation system is complicated, the overall reaction
consists of two main steps: (1) the oxidation of sulfite by bromate and (2) the
oxidation of ferrocyanide by bromate, i.e., [155]:
BrO
À
3 þ 3HSO
À
3 þ H
þ Br
À
þ 3SO
2À
4 þ 4H
þ
ð11aÞ
BrO
À
3 þ 6Fe CN
ð Þ
4À
6 þ 6H
þ Br
À
þ 6Fe CN
ð Þ
3À
6 þ 3H 2 O
ð11bÞ
Thus, the first reaction produces H
+ ions so that pH of the reaction solution
decreases, while the second reaction consumes H
+ ions so that the pH again
increases. In response to the oscillatory pH changes in solution, a weak polyelectrolyte gel such as PMAAc oscillates between swollen and collapsed states.
However, all of the self-oscillating conventional hydrogels are limited in their
size to microscopic dimensions, due to their slow rate of response to external
stimuli. Recently, it was shown that centimeter-sized poly(acrylic acid) (PAAc)
cryogels can be used as a pH oscillator in oscillatory bromate–sulfite–ferrocyanide
reactions [31]. The cryogels were prepared at À18
C from frozen aqueous solutions of acrylic acid (AAc) monomer and BAAm crosslinker. Fast responsive
macroporous PAAc cryogels could be obtained at an initial monomer concentration
C o of 4 % (w/v). In Fig. 20, the diameter D of PAAc cryogels formed at C o ¼ 2 %
is shown as a function of time during three successive deswelling–reswelling cycles
in methanol and water, respectively. The gel diameter D varies between 8 and
2 mm, i.e., the gel exhibits completely reversible cycles. This 64-fold change in the
gel volume during the swelling–deswelling cycles does not induce any crack
formation, and the network structure remains stable. The results thus suggest that
such gel samples are useful materials in oscillating reaction systems.
Indeed, PAAc cryogels coupled with a bromate oscillator oscillated between
swollen and collapsed states [31]. The reactions of bromate, sulfite, and ferrocyanide ions were conducted in an open continuously stirred tank reactor. Four feed
solutions (potassium bromate, sodium sulfite, potassium ferrocyanide, and sulfuric
acid) were supplied continuously to the reactor, during which the pH of the reaction
solution was monitored as a function of time. The flow rate of the feed solutions is
an important parameter in determining the extent of pH oscillations. In Fig. 21, pH
versus time plots are shown for four different reduced flow rates k, defined as the
flow rate of the feed solutions divided by the reaction volume. It is seen that the pH
of the solution oscillates between 6.2–6.9 and 3.2–3.8. The dissociation degree α of
a weak electrolyte relates to pH by:
144
O. Okay and V.I. Lozinsky
hydrogel sample with oscillating chemical reactions. For example, the Belousov–
Zhabotinsky (BZ) reaction is a well-known oscillating system and the overall
process of the reaction is the oxidation of a substrate such as malonic acid in the
presence of oxidizing agent (bromate) and metal catalyst in an acidic solution [152,
153]. Yoshida and coworkers used poly(N-isopropyl acrylamide) gel in a BZ
reaction to produce a self-oscillating gel system [151, 154]. Crook et al. used
poly(methacrylic acid) (PMAAc) gel in a Landolt oscillating reaction system
consisting of bromate, sulfite, and ferrocyanide ions [155, 156]. Although the actual
mechanism of this bromated oscillation system is complicated, the overall reaction
consists of two main steps: (1) the oxidation of sulfite by bromate and (2) the
oxidation of ferrocyanide by bromate, i.e., [155]:
BrO
À
3 þ 3HSO
À
3 þ H
þ Br
À
þ 3SO
2À
4 þ 4H
þ
ð11aÞ
BrO
À
3 þ 6Fe CN
ð Þ
4À
6 þ 6H
þ Br
À
þ 6Fe CN
ð Þ
3À
6 þ 3H 2 O
ð11bÞ
Thus, the first reaction produces H
+ ions so that pH of the reaction solution
decreases, while the second reaction consumes H
+ ions so that the pH again
increases. In response to the oscillatory pH changes in solution, a weak polyelectrolyte gel such as PMAAc oscillates between swollen and collapsed states.
However, all of the self-oscillating conventional hydrogels are limited in their
size to microscopic dimensions, due to their slow rate of response to external
stimuli. Recently, it was shown that centimeter-sized poly(acrylic acid) (PAAc)
cryogels can be used as a pH oscillator in oscillatory bromate–sulfite–ferrocyanide
reactions [31]. The cryogels were prepared at À18
C from frozen aqueous solutions of acrylic acid (AAc) monomer and BAAm crosslinker. Fast responsive
macroporous PAAc cryogels could be obtained at an initial monomer concentration
C o of 4 % (w/v). In Fig. 20, the diameter D of PAAc cryogels formed at C o ¼ 2 %
is shown as a function of time during three successive deswelling–reswelling cycles
in methanol and water, respectively. The gel diameter D varies between 8 and
2 mm, i.e., the gel exhibits completely reversible cycles. This 64-fold change in the
gel volume during the swelling–deswelling cycles does not induce any crack
formation, and the network structure remains stable. The results thus suggest that
such gel samples are useful materials in oscillating reaction systems.
Indeed, PAAc cryogels coupled with a bromate oscillator oscillated between
swollen and collapsed states [31]. The reactions of bromate, sulfite, and ferrocyanide ions were conducted in an open continuously stirred tank reactor. Four feed
solutions (potassium bromate, sodium sulfite, potassium ferrocyanide, and sulfuric
acid) were supplied continuously to the reactor, during which the pH of the reaction
solution was monitored as a function of time. The flow rate of the feed solutions is
an important parameter in determining the extent of pH oscillations. In Fig. 21, pH
versus time plots are shown for four different reduced flow rates k, defined as the
flow rate of the feed solutions divided by the reaction volume. It is seen that the pH
of the solution oscillates between 6.2–6.9 and 3.2–3.8. The dissociation degree α of
a weak electrolyte relates to pH by:
144
O. Okay and V.I. Lozinsky