Synthesis and volume phase transition of poly(methacrylic acid-co-N-isopropylacrylamide) microgel particles in water

A series of lightly cross-linked, nearly monodisperse, and random copolymerized microgel particles, based on methacrylic acid (MAA) and N-isopropylacrylamide (NIPAM) of various content, were synthesized in aqueous solution with sodium dodecyl sulfate at pH = 3.1 +/- 0.2, and 70 degrees C. The polymerization kinetics, the colloidal stability, and the size of the microgel particles varied with the comonomer composition. The volume phase transitions of the MAA/NIPAM copolymer microgel particles with different PMAA contents in very dilute aqueous solution (C < 1 x 10(-5) g/mL) were investigated by laser light scattering (LLS), as a function of both pH at 25 degrees C, and temperature at two different pH values of 3.4 and 7.5. To our knowledge, the volume phase transition of such submicron-sized hydrogels sensitive to multiple environmental changes was studied for the first time by LLS. The equilibrium swelling ratio, the critical transition pH, and the critical transition temperature (T-c) were strongly dependent on the comonomer composition. For the pH-sensitive transition, an increase in the PMAA content increased the swelling extent and enhanced the critical transition pH. For the thermal sensitive transition, an introduction of PMAA enabled us to adjust the hydrophobicity of the microgel network chains by varying the solution pH. At low pH, an increase in the highly collapsed hydrophobic PMAA content decreased the T-c, reduced the collapsing extent, and made the transition less sharp. At higher pH (7-10), an increase in the strongly hydrophilic, highly charged PMAA content raised T-c but unexpectedly reduced the collapsing extent and broadened the transition temperature region, which are contrary to the results observed in other weakly charged PNIPAM gels. The possible explanation related to a shell-enriched PMAA with a high charge density and a core-enriched in neutral PNIPAM for the MAA/NIPAM copolymer microgel with low PMAA content is presented.