Intermolecular Interactions and Self-Assembly in Aqueous
Solution of a Mixture of Anionic–Neutral and Cationic–Neutral
Block Copolymers
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Abstract
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 <i>C</i><sub>S</sub> 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/cm<sup>3</sup> and 0.6, respectively. With decreasing <i>C</i><sub>S</sub> 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 <i>C</i><sub>S</sub> ∼ 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 <i>C</i><sub>S</sub> from
0.5 M to lower than 0.05 M