Poly(N-isopropylacrylamide-co-methacrylic acid) microgel was prepared by the precipitation polymerization method. NMR and TEM were used to analyze microstructure and morphology of the microgel. The stability and zeta potential of the microgel were investigated as a function of electrolyte concentration by dynamic light scattering. Zeta potential was －18 mV in pH＝7 solution at 25 ℃, which decreased gradually with increasing ionic strength and reached an equilibrium value at 0.2 mol/L NaCl concentration, indicating that the surface charge of the microgel had been screened out. Increasing in NaCl concentration caused mainly the microgel particles to shrink. For the microgel in certain electrolyte solution, the heating induced the particles to deswell. As a result, the surface charge density became greater gradually, resulting in the increase of zeta potential. For microgel in 0.2 mol/L NaCl solution, the zeta potential was －12.4 mV at 41 ℃, playing an important role in stabilizing the microgel. The potential of the microgel at higher ionic strength displayed an abrupt transition of trending to zero as increasing the temperature, and the relevant temperature was correspondent with the critical flocculation temperature (CFT). The CFT of the microgel was shifted to lower temperature with the increase of NaCl concentration. At a certain salt concentration, the aggregation rate of the particles at a higher temperature was faster than that at a lower temperature.

Key words: N-isopropylacrylamide, microgel, temperature sensitivity, stability