Colloidal behavior of polystyrene-based copolymer microgels composed of a hydrophilic hydrogel layer

Polystyrene (PSt)-based copolymer colloidal microgels consisting of a hydrophilic hydrogel layer, such as poly(methacrylic acid) (PMAA) or poly(N-isopropyl acrylamide) (PNIPAM), were synthesized through emulsifier-free polymerization. The morphology and particle size of the microgel particles were characterized using a combination of TEM and photon correlation spectroscopy (PCS). The microgel particles consisted of a hydrophobic PSt-core and a hydrophilic PMAA and PNIPAM shell. When the microgels were characterized for their pH- and temperature-responsive behaviors by PCS, the hydrodynamic diameter of the microgel particles composed of St/MAA/NIPAM gradually increased below pH 4.5 and above pH 7. LCSTs of PNIPAM in the microgels were barely affected by different molar ratios of St/MAA/NIPAM; in addition, the shrinking transition of the microgels, which were composed of St/MAA/NIPAM, shifted to higher region than did the LCST of PNIPAM. The electrophoretic mobility and zeta potential for the microgel particles were found to be negative, and a marked maximum negative value occurred at around pH 5. The colloidal stability of the microgel particles was very sensitive to the concentration of the KCl solution and to the temperature.