Thermosensitive Ionic Microgels with pH Tunable Degradation via in Situ Quaternization Cross-Linking

Degradable thermosensitive ionic microgels were synthesized via surfactant-free emulsion polymerization.(SEEP), N-isopropylacrylamide (NIPAm) and 1-vinylimidazole :(VIM) at 70 degrees C with degradable 1,4-phenylene bis(4-bromobutanoate) or 1,6-hexanediol bis(2-bromopropionate) as quaternized cross-linkers. VIM could be,quaternized by 1,4-phenylene bis(4-bromobutanoate) or 1,6-hexanediol bis(2-bromopropionate), leading to the formation of degradable cross-linking network and ionic microgels. Combined techniques of transmission electron microscopy (TEM), dynamic light scattering (DLS), electrophoretic light scattering (ELS), UV-vis spectroscopy, FT-IR spectra, and gel permeation chromatography (GPC) were employed to systematically investigate the sizes, morphologies, and properties of the obtained microgels before and after degradation as well as the degradation mechanism. The obtained microgels were spherical in shape with narrow size distribution and exhibited thermosensitive behavior and controllable degradation. The disintegration of the microgels was confirmed to be resulted from the hydrolysis of ester bonds of the cross-linkers. The degradation rate of the obtained microgels could be regulated by tuning the pH value of niicrogel suspensions. The PNI-Ph series of microgels fabricated with 1,4-phenylene bis(4-bromobutanoate) as the cross-linking agent could gradually degrade even in neutral solution with lifetime S' of 44-53 h, depending On the quaternizaiton ratio. The degradation of PNI-Ph series of microgels experienced two reaction processes, that is, the hydrolysis of ester bonds of the cross-linkers and the oxidation of generated hydroquinone to form benzoquinone. It was also demonstrated that different silica nanostructures could be fabricated by Using such degradable thermosensitive ionic microgels as the template at various temperatures.