Self-oscillating pH-responsive cryogels as possible candidates of soft materials for generating mechanical energy

This research highlights the use of poly(acrylic acid) (PAAc) cryogels as a pH oscillator in oscillatory bromate-sulfite-ferrocyanide reactions. The cryogels were prepared from frozen aqueous solutions of acrylic acid monomer and N,N′-methylenebis(acrylamide) crosslinker at -18°C. Fast responsive macroporous PAAc cryogels were obtained at or below 4 w/v % initial monomer concentration. SEM images of PAAc cryogel networks exhibited a heterogeneous morphology consisting of pores of sizes 10¹ μm, typical for macroporous networks formed by the cryogelation technique. PAAc cryogels coupled with the bromate oscillator oscillate between swollen and collapsed states, during which a cyclic three-fold change in the gel volume was observed. The results show that, due to the fast response rate of cryogels, macroscopic gel samples can be used as a pH oscillator which would provide generation of significant amount of mechanical energy.