Solvent-induced transformations of assemblies of structured ionizable block co-polymers

Micelles formed by ionizable co-polymers are governed by both van der Waals and electrostatic forces. Slight tweaking of the solvent characteristics often drives large structural transformations. Here, the effects of modulating the electrostatic characteristics of solvents on polymeric assemblies formed by a co-polymer with ABCBA topology and an ionizable polystyrene sulfonate center (C) tethered to polyethylene propylene (B) end-capped by t-butyl styrene (A) are probed by small angle neutron scattering in cyclohexane/propanol solutions. With increasing propanol fraction, the spherical core-shell micelles, with the ionic block in their core, elongate and transition into large "swarms." Surprisingly, as propanol becomes the major component of the solvent, reentrant spherical assemblies with a smaller polystyrene sulfonate core are formed. The propanol partitions across the interface between the core and the corona, affecting the polymer distribution in the corona and the core-corona boundary's curvature, resulting in morphing of the shape of assemblies.