Stimuli-Responsive Zwitterionic Microgels with Covalent and Ionic Cross-Links

We describe a facile method for the synthesis of microgels with covalent and noncovalent ionic cross-links. Microgels were synthesized by copolymerization of various N-vinyllactams with zwitterionic monomer (methacryloyloxy)-ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide in the presence of a cross-linking agent (bis(acrylamide)) in waterin-oil emulsion. Monodisperse colloidally stable microgels with a high amount (>30 mol %) of zwitterionic groups were synthesized. High contents of zwitterionic groups in microgels led to the formation of reversible ionic cross-links along with permanent covalent cross-links generated by bis(acrylamide). The obtained microgels exhibit interesting temperature- triggered swelling/deswelling behavior in aqueous solution. With of the temperature above 10 degrees C, the microgels swell due to the destruction of the zwitterionic cross-links. Above the lower critical solution temperature of poly(N-vinyllactam) chains at T > 32 degrees C, the microgels shrink due to the destruction of the hydrogen bonds and enhanced hydrophobic interactions. The variation of zwitterionic groups and cross-linker concentrations influenced the extent of swelling/deswelling at different temperatures. New doubly thermoresponsive microgels were synthesized using three homologues: N-vinylcaprolactam (VCL), N-vinylpiperidone (VPi), and N-vinylpyrrolidone (VPy). It was shown that the temperature-triggered deswelling of microgels is strongly dependent on the size of the lactam ring.