Invertible Micelles Based on Ion-Specific Interactions of Sr2+ and Ba2+ with Double Anionic Block Copolyelectrolytes

Abstract

Polyelectrolytes show diverse ion-specific effects with a wide variety of ions. In the present work, we study the solution behavior of diblock copolymers with polyacrylate (PA) and polystyrenesulfonate (PSS) blocks in the presence of the earth alkaline cations Ca2+, Sr2+, and Ba2+. Micellization can be triggered by a variation in temperature. For Ca2+, this micelle formation occurs at high temperatures, while for Sr2+ and Ba2+, it occurs at high and low temperatures with an intermediate temperature regime of single chains. Small-angle neutron scattering on a partly deuterated block copolymer unambiguously revealed that at low temperatures, PSS/M2+ forms the core of the micelles, whereas at high temperatures, PA/M2+ forms the core, effectively allowing to reverse the micelle structure by changing the temperature. This inversion of the micellar morphology, induced by a dual-responsive behavior, can be understood by using isothermal titration calorimetry and elucidating the thermodynamics of cation binding.