Microcalorimetric studies on chemical oscillation of microgels.

By use of isothermal titration calorimetry (ITC) and ultrasensitive differential scanning calorimetry (US-DSC), we have investigated the energy change in the periodic swelling-to-deswelling of thermally sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels containing ruthenium(II) tris(2,2'-bipyridine) (Ru(bpy)3) which is a catalyst for Belousov-Zhabotinsky (BZ) reaction. As temperature increases, the induction period and oscillation period of BZ reaction decrease because the reaction rate increases. However, the oscillation disappears at a temperature above the lower critical solution temperature (LCST) of the microgels since Ru(bpy)3 is trapped in the microgels and cannot react with BZ substrates. As microgel size increases or the cross-linking density decreases, the restriction of polymer networks on Ru(bpy)3 decreases, so that Ru(bpy)3 can readily contact with BZ substrates, leading the oscillation amplitude to increase. In addition, the so-called transient chaos occurs at a low stirring speed, and it wanes with the increasing stirring speed. All the facts indicate that the contact between Ru(bpy)3 and BZ substrates determines the oscillation of the microgels.