Intermolecular Interactions and Self-Assembly in Aqueous Solution of a Mixture of Anionic–Neutral and Cationic–Neutral Block Copolymers

We have investigated the self-assembly in dilute aqueous solutions of a mixture of an anionic–neutral block copolymer (AP) and a cationic–neutral block copolymer (MP) by changing the added sodium chloride (NaCl) concentration C S or electrostatic interactions among oppositely charged blocks, by direct observation, optical and electron microscopies, and small-angle X-ray scattering. The ratio of the charged to neutral block chain lengths was ca. 10, and the total copolymer concentration and the mixing ratio (the mole fraction of the MP charge unit in the total charge units) of AP and MP were fixed to be 0.005 g/cm3 and 0.6, respectively. With decreasing C S from 2 to 0 M, we have found reentrant one-phase, two-phase, one-phase transitions in the aqueous solution of the AP–MP mixture. The two-phase to one-phase transition at C S ∼ 0.5 M arises from the competition between the macroscopic phase transition and micellization, which is the first observation in dilute block copolymer solutions. Moreover, we have found a micelle morphology transition from the bilayer vesicle to the cylindrical micelle with further decreasing C S from 0.5 M to lower than 0.05 M.