Colloidal stability of a temperature-sensitive poly(N-isopropylacrylamide/2-acrylamido-2-methylpropanesulphonic acid) microgel.

In this work, a study of the thermally induced flocculation of poly(N-isopropylacrylamide/2-acrylamido-2-methylpropanesulphonic acid) microgel, that is, poly(NIPAM/aMPS) microgel samples is presented. The aggregation process is monitored in continuous by dynamic light scattering measurements. Theoretical explanations are considered and discussed in terms of interaction potentials, paying special attention to osmotic and elastic contributions due to particle interpenetration. The rate of aggregation is studied as a function of temperature, and theoretical calculations of kinetic constants based on model core-shell particle interactions provide very good agreement with experimental results. The reversibility of aggregation is also analyzed. It is found that aggregation is completely reversible only under certain conditions: the temperature of an aggregated sample must decrease under 20 degrees C. These results are experimental evidence of the secondary minimum in the interaction potential where aggregation occurs.