Counterion-Mediated Self-Assembly of Ion-Containing Block Copolymers on the Basis of the Hofmeister Series

A Cl- and I- ion-containing amphiphilic diblock copolymer [PEO-b-P(qVBC-co-St)], IBCP, which comprises the poly(ethylene oxide) block and poly(quaternized 4-vinylbenzyl chloride-co-styrene) is prepared by sequence reactions. Then the counterion-mediated self-assembly behavior in aqueous solution is investigated in detail. The hydrophilicity of anion containing units (qVBC) is controlled by the nature of the different counterions after ion exchange, which is crucial in regulating the self-assembly process. The addition of Br-, I-, and SCN- counterions (based on the Hofmeister series) results in the formation of tighter ion-pairs with cations (i.e., more hydrophobic as the qVBC units), which triggers spheres to vesicles, micro-meter long cylinders and "branched wormlike" aggregates, respectively. Moreover, addition of different amounts of Cl- also causes morphological transition from spheres to long cylinders. And this morphological transition is totally reversible after removing additional Cl-. The adhesive collisions of spherical particles may contribute to the observed sphere-to-cylinder morphological transition.