Colloidal copolymer microgels of N-isopropylacrylamide and acrylic acid: pH, ionic strength and temperature effects

Aqueous colloidal microgels have been prepared, based on poly(N-isopropylacrylamide) [poly(NIPAM)], cross-linked with bisacrylamide, containing 5% w/w acrylic acid (AAc) as a comonomer. Transmission electron micrographs of the microgels show that the copolymer microgels are monodisperse spheres. The size of the microgel particles containing 5% AAc has been studied, using dynamic light scattering, as a function of pH (3-10), ionic strength (10-4-10-1 M NaCl) and temperature (20-75°C). The hydrodynamic diameter of the copolymer microgels decrease both with increasing ionic strength (at pH 6 and 25°C) and reversibly with increasing temperature at pH values of 2.6, 3.4 and 6.5. However, under isothermal conditions in 10-3 M NaCl at 25°C, the hydrodynamic diameter increases in going from pH 3.3-9.4. In addition, the temperature-induced conformational changes in the polymer chains have been followed using high-sensitivity differential scanning calorimeiry (HSDSC). A comparison is made with the behaviour of poly(NIPAM) microgel particles, not containing AAc. Explanations are offered to account for the pronounced difference in the physico-chemical properties observed.