Résumé
Poly(N-isopropylacrylamide)-based microgels are soft colloids undergoing a
Volume Phase Transition (VPT) close to ambient temperature. Although widely
employed for fundamental research and application purposes, the modifications
of the microgel internal structure occurring at the VPT are not yet completely
understood, especially concerning the role of electrostatics. Here we study in
detail, both experimentally and numerically, the effect of the addition of
acrylic acid (AAc) co-monomer on the microgel deswelling process. By combining
viscosimetry, light scattering and electrophoresis, we show that the
progressive addition of AAc increases the microgel mass and suppresses the
occurrence of the VPT, progressively shifting the microgel collapse to higher
temperatures. Most importantly, it also highly enhances the two-step deswelling
of these submicron-sized networks, so that the inner core collapses at
temperatures always lower than those marking the transition of the outer
corona. These results indicate that a net increase of the charge density
mismatch between the bulk and the surface of the microgels takes place.
Numerical simulations fully confirm this scenario and clarify the impact of the
charge distribution on the two-step deswelling, with mobile counterions
efficiently screening the charges within the inner core, while leaving more
monomers ionized on the surface. Our work unambiguously shows how electrostatic
interactions influence the behavior of thermosensitive microgels in aqueous
environment close to the VPT.