The study of the temperature-sensitive microgels with different ionic groups on their surfaces

Three kinds of temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels with different ionic groups on their surfaces were synthesized by surfactant-free emulsion polymerization using three kinds of initiators, respectively. These PNIPAM microgels exhibited spherical shape and held good monodispersity, irrespective of the initiators used. For the constant mole concentration of initiators used in the surfactant-free emulsion polymerization, the size of the PNIPAM microgel synthesized utilizing ammonium persulfate (APS) as initiator was smaller than that synthesized utilizing 2.2'-azobis(amidinopropane) dihydrochloride (V50) or 4,4'-azobis(4-cyanovaleric acid) V501 as initiator. The initiators had no influence on the swelling ratio and volume phase transition temperature (VPTT) of PNIPAM microgels. The zeta potential of PNIPAM microgels based on V50 as initiator held positive value, and it decreased markedly as medium pH value increased from 4 to 10. Comparatively, the zeta potential of PNIPAM microgels based on V501 as initiator held negative value, and it raised markedly as medium pH value increased from 6 to 10. The PNIPAM microgels were temperature-sensitive, when the medium temperature exceeded their VPTT, not only their size shrinked, but also their zeta potential increased significantly. These temperature-sensitive microgels will be applied in the field of colloidal crystal and the separation of biomolecules.