Controlling the synthesis and characterization of micrometer-sized PNIPAM microgels with tailored morphologies

This Article presents the controlling synthesis and characterization of micrometer-sized, multiresponsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-MAA) microgel particles. By combining semibatch and temperature-programmed surfactant-free precipitation polymerization, we have successfully developed a novel approach to the preparation of temperature- and pH-responsive PNIPAM microgels with a dense-shell (DS), dense-core (DC), or homogeneous (HOMO) structure. We then investigated the interaction between the synthesized microgels and some fluorescent dye molecules using confocal laser scanning microscopy (CLSM). Our results have qualitatively revealed that the cross-linkers and the functional carboxylic groups (-COOH) could be homogeneously distributed, predominately localized inside the core, or concentrated near the surface of the synthesized microgels. Moreover, pH-responsive swelling behaviors of the microgels were investigated and discussed with titration and CLSM data. We found that the swelling capability is strongly dependent on the morphology of the PNIPAM microgel. Besides the absorption of fluorescent molecules, the synthesized microgels also showed a strong affinity for fluorescently labeled polypeptide, even at a relatively high salt concentration.