Properties of poly(N-isopropylacrylamide) (PNIPAM) microgel particles were examined in mixtures of water and acetic acid at 20deg C using dynamic light scattering in order to investigate the cononsolvency phenomenon. It was found that the cononsolvency exhibited by the PNIPAM-water-acetic acid ternary system results in five regions in response to changes in solvent composition. Upon the addition of acetic acid to the aqueous dispersion medium, the regions appeared successively: volume change from a swollen to a collapsed state, unstable microgel aggregates (in water rich range), precipitation, unstable microgel aggregates (in acetic acid rich range), and swelling. The volume change from swollen to collapsed states starts before the phase-separation composition is achieved. Both volume phase transition and swelling are continuous but undergo in a sharp way. In a very narrow range of solvent composition, the microgel aggregates formed are unstable, while the immiscible phase was found to span a wide range. The degree of swelling is dependent on the crosslinker concentration in the solutions of very higher acetic acid content. The cononsolvency of the mixed water-acetic acid solutions for PNIPAM microgel particles is not only involved in changes of the interactions between their components but also in the partial neutralisation of surface charges of the PNIPAM microgel particles.
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