Synthesis and solution behaviour of stimuli-sensitive zwitterionic microgels

Abstract

In this study, we have synthesised small aqueous zwitterionic microgels as model systems for amphoteric microgels. Zwitterions exhibit special behaviour due to defined distance between the opposite charges and provide a possibility to incorporate an identical number of positive and negative charges in microgels. Microgels decorated with zwitterionic groups exhibit unique properties compared with their analogues functionalized with separately incorporated oppositely charged co-monomers. In this work, two series of zwitterionic microgels with N-isopropylacrylamide (NIPAm) and N-vinylcaprolactam (VCL) as main monomers and various amounts of [3-(methacryloylamino)propyl]dimethyl(3-sulfopropyl) ammonium hydroxide (referred to as sulfobetaine) as zwitterionic co-monomer have been prepared. We investigated the effect of zwitterionic co-monomer content on the swelling behaviour, particle size, size distribution and volume phase transition temperature of the NIPAm-co-sulfobetaine microgels using various scattering methods, as dynamic and static light scattering and small-angle X-ray/neutron scattering. We found a decrease of particle size with the amount of sulfobetaine in the presence of sodium dodecylsulfate, which is contrary to results published earlier by Das et al. (Chem Mater 20: 7157–7163, 2008) for a similar system of zwitterionic microgels. The decrease of particle size is related to co-surfactant behaviour of sulfobetaine in mixtures with sodium dodecylsulfate. The incorporation of sulfobetaine also increases the polydispersity of the microgels and broadens the volume phase transition.